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NASA Contractor Report 180900

Sensors for Ceramic Components

in Advanced Propulsion Systems

Summary of Literature Survey and Concept Analysis

Task 3 Report

Prepared by

W.H. Bennethum

L.T. Sherwood

G E ,4ircraft En,,,ines

,4 dvam'ed Technolo,k'y OI wrati°"

Cincinnati. Ohio 45215

August 1988

Prepared for

National Aeronautics and Space Administration

Lewis Research Center

Contract NAS3-25140

National Aeronautics and

and Space Administration

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NASA Contractor Report 180900

Sensors for Ceramic Components

in Advanced Propulsion Systems

Summary of Literature Survey and Concept Analysis

Task 3 Report

!

|

Table of Contents

Section

1.0

2.0

Introduction

Surface Temperature Measurement

2.1 Summary Listing of Candidate Techniques

2.2 Concept Description and Analysis - Thin Film Thermocouples

2.2.1 Thermoelectric Elements

2.2.1.1 Candidate Material Characteristics

2.2.1.2 Materials Compatibility

2.2.1.3 Noble Metal Sensing Elements

2.2.1.4 Silicon Carbide Sensing Elements

2.2.2 Insulating Materials

2.22.1 Candidate Insulating Material Characteristics

2.3 Concept Description and Analysis - Radiation Pyrometry

2.3.1 Concept Description

2.3.2 Types and Characteristics

2.33 Effect of Reflected Radiation

2.3.4 Adaptability to Ceramic Materials

2.3.5 Long Wavelength Pyrometer

2.4 Concept Description and Analysis - Resistance Thermometry

2.4.1 Platinum Resistance Thermometer

2.42. Resistance Temperature Detectors

2.5 Concept Description and Analysis - Laser-Induced Fluorescence

2.5.1 Concept Description

2.5.2 Current Capabilities

2.5.3 Candidate Phosphors and Upper Temperature Limits

2.5.4 Phosphor Application to Test Part

2.5.5 Recommendations

2.6 Concept Analysis and Description - Noise Thermometry

2.6.1 History

2.6.2 Theory

2.6.3 Discussion

2.6.4 Adaptability to Gas Turbine Environment

2.7 Concept Description and Analysis - Fiber Optic Temperature

Sensors

2.7.1 Techniques Based on Variability of Fiber Optic Properties

2.7.2 Collected Energy Techniques

2.8 Concept Description and Analysis - Ultrasonic Thermometry

2.8.1 Concept Description

2.8.2 Resonant Frequency Technique

2.8.3 Pulse Echo Technique

2.8.4 Adaptability to Ceramic Materials

2.9 Concept Description and Analysis - Raman Measurement

Techniques

iii

Page

1

3

3

3

3

3

7

11

12

17

17

22

22

22

23

24

25

29

29

29

3O

3O

31

31

32

32

32

32

32

33

35

35

35

36

37

37

37

37

38

39

t

pRBOEDING PAGE BLANK NOT FILMED
Table of Contents (Continued)

Section

3.0

2.10

2.11

Strain

3.1

32

3.3

3.4

3.5

29.1 Concept Description

2.9.2 Adaptability to Ceramic Materials

Concept Description and Analysis - Temperature

2.10.1 Description

2.10.2 Recommendations

Surface Temperature Sensing Concepts Proposed for Development

2.11.1 Selection Criteria

2.11.2 Long Wavelength Radiation Pyrometer

2.11.3 Ceramic or lntermetallic Thin ledm Thermecouples

2.11.4 Platinum-Rhodium Alloy Thin Film Thermocouple

Measurement

Concept Description and Analysis - Applied Sensors - NonopticaJ

3.1.1 Resistance Strain Gages

3.1.1.1 Current Capabilities

3.1.1.2 Potential Capabilities

3.1.2 Capacitance Strain Gages

3.1.2.1 Concept History

3.1.2.2 Ctmront Developments

3.1.2.3 Potential Capabilities

Concept Description and Analysis - Optical Discontinuity Methods

3.2,1 Marion Method (Attached Targets - Optical Follower)

3.2.2 The Sharpe Method (Reflection Interference from Target

Indentations)

3.2.3 Simpson-Welch Method (Applied Pattern Duty Cycle

Variation)

Concept Description and Analysis - Surface Pattern Distortion

Methods

3.3.1 Moire Methods

3.3.1.1 Introduction

3.3.1.2 Theory

3.3.13 Discussion

3.32 Optical F'dtering Methods

3.33 Conclusions

Concept Description and Analysis - Holographic Methods

3.4.1 Holographic Interferometry

3.4.1.1 Concept Description and History

3.4.1.2 Limitations

3.4.2 Electronic Speckle Pattern lnterferometry

3.4.2.1 Concept Description and History

3.4.2.2 Applicability to High Temperature Applications

Concept Description and Analysis - Laser Speckle Methods

3.5.1 Concept Description

3.5.2 Speckle Photography

3.5.3 Dual Beam Doppler Interferometer (Optra, Inc.)

tV

Page

39

39

40

40

40

40

40

41

42

43

44

44

44

44

45

45

45

46

46

46

47

47

48

49

49

49

50

51

52

53

53

53

53

55

56

56

58

59

59

60

63
Table of Contents (Concluded)

Section

4.0

5.0

3.6

3.7

3.8

3.9

Concept Descriptions and Analyses- Miscellaneous Optical

Displacement Measuring Techniques

3.6.1 Blade Tip Deflection Sensors

3.6.2 Laser Doppler Methods

Concept Description and Analysis- X-ray Diffraction Methods

3.7.1 Concept Description

3.7.2 Current Status

3.73 Future Prospects

Concept Description and Analysis- Photoelastic Methods

3.81 Introduction

3.82 Theory

3.8.3 Discussion

Concepts Recommended for Future Development

3.9.1 D_on

3.9.2 Spectral Density Indication

3.9.3 Optical Tracking- Attached Targets

3.9.4 Dual Beam Doppler Interferometer

Heat Flux

4.1 Background Information

4.2 Concept Description and Adaptability to Ceramic Materials

4.3

4.2.1

4.2.2

4.2.3

4.2.4

4.2.5

4.2.6

Heat

4.3.1

4.3.2

Embedded Thermocouple

Gardon Gage

Slug Calorimeter

Surface-Mounted Sensor

Indirect Measurement

Radiation Type Heat Flux Sensor

Flux Sensors Recommended for Further Development

Surface-Mounted (Differential) Sensor

Thin Film Surface Mounted (Absolute) Sensor

References

Appendix- Excerpts from NAS3-25140 Statement of Work

Page

64

64

65

66

66

69

70

71

71

71

72

72

72

74

75

75

77

77

78

78

78

81

81

83

83

83

83

85

86

99
List of illu.. ns

Figure

1.

3.

4.

5.

6.

7.

°

.

10.

11.

12.

13.

14.

15.

16.

Weight Gain (Loss) Due to Oxidation for Candidate Conducting Materials at

300ff F in Air..

Recession Rate for Platinum, Rhodium, and Iridium.

Comparison of Oxidation Rates for Two High-Purity Forms of SiC.

Resistivity Versus Temperature - High Temperature Insulators.

Electrical Conductivity Versus Temperature - High Temperature Insulators.

Resistivity Versus Temperature - Pyrolytic Boron Nitride and Silicon Nitride.

Error Due to RefleXed Radiation for Fixed Target Temperature of 200ff'F as

a Function of Reflector Surface Temperature.

Calculation Procedure for Plot of Error Versus Reflector Temperature

(r, gure 7).

Error Due to Emittance Uncertainty at Two Levels of Emittance as a Function

of Temperature.

ESPI Optical Schematic.

Coordinates and Angles of X-ray Measurements on a Plane Surface.

Embedded Thermocouple Heat Flux Sensor Concept Schematic.

Gardon Gage Concept Schematic.

Slug Calorimeter Concept Schematic.

Surface Mounted Heat Flux Sensor Using Plastic or Kapton for Thermal

Barrier.

Surface Mounted Heat Flux Sensor for High Temperature Applications.

Page

8

8

15

19

20

21

26

27

28

57

68

80

80

82

82

84

vt
i

List of Tables

Table

1.

2.

.

4.

5.

6.

.

.

Surface Temperature Measurement Concept Characteristics.

Candidate Conducting Materials for Temperature Sensors Applied to Ceramic

Substrates.

Candidate Sensor Materials - Thermal Expansion Match to _ 30%.

Thermal Expansion Coefficients of Some Insulators and Conductors.

Thermocouples Based on Noble Metals.

Candidate Insulating Materials for Temperature Sensors Applied to Ceran_c

Substrates.

Average Normal Spectral Emittance at Two Wavelengths (From Reference

2-45)

Heat Flux Sensor Concept Characteristics.

Page

4

6

9

i0

12

18

25

79

vii

1.0 Introduction

This report describes work performed by GE

Aircraft Engines for NASA Lewis Research

Center in support of the "Sensors for

Ceramic Components in Advanced Propulsion Systems" program (NASA Contract No.

NAS3-25140). The work effort of this

program is organized under several major

categories as follows.

Task 1 - Survey of Sensor Concepts

Task 2 - Analysis of Sensor Concepts

Task 3 - Review and Report

Task 4 - Sensor System Design and Test Plan

Formulation

Task 5 - Fabrication of Sensor System

Task 6 - Performance of Tests and Analysis

of Results

These tasks are described in detail in the

Statement of Work portion of NASA contract NAS3-25140. Pertinent excerpts from

this document are included in Appendix A.

This report is a summary of Tasks 1 and 2.

A Literature Survey was conducted to identify journal articles, conference proceedings,

books, and vendor information on various

methods of surface temperature, strain, and

heat flux measurement. We have made

extensive use of the Dialog Information Services Inc. data bases. The most useful data

base has been the Aerospace Dataspace,

which is an on-line version of two publications: International Aerospace Abstracts

(IAA), and Scientific and Technical

Aerospace Reports (STAR). We have also

used Dialog's Searchable Physics Information Notices (SPIN) and the National Technical Information System (NTIS) data bases.

Numerous journal articles and other sources

of relevant information have been

assembled.

We have attempted to focus our attention on

concepts which are appropriate for use in the

hot section of a gas turbine engine, where

nonintrusive or low profile characteristics

are often necessary to avoid aerodynamic

perturbations and interference with the

parameter being measured. In cases where

sensor concepts with potential for functioning in a gas turbine hot section could not be

found or were limited in quantity, we have

selected alternate concepts and have

attempted to describe the nature of the

limitations regarding use in operating test

engines. In addition, we have attempted to

rate concepts on the basis of chemical, metallurgical, and thermal compatibility with typical ceramic materials capable of operating in

a combustion gas (oxidizing) environment.

Since specific ceramic materials have not

been identified, we have used typical characteristics for the types of ceramic materials

which may be used in future engine

components.

In addition to reviewing the literature, we

have discussed concept details with

authorities in the field to gain an understanding of capabilities and limitations with

regard to gas turbine hot section application

on ceramic components. In some cases,
where data were not available or were contradictory, we have conducted laboratory

tests to evaluate feasibility.

The process of looking for sensing concepts

adaptable to high temperature ceramic

materials operating in a hot gas stream has

resulted in elimination of numerous concepts which appeared inappropriate for practical application in such an environment, and

has influenced our perspective regarding

specific sensor configuration. For example,

in the area of thermoelectric surface

temperature measurements, considerable

attention is given to thin film sensors, but

application of bonded wire elements is not

discussed because of the temperature and

aerodynamic perturbations caused by typical

bonded wire sensors. ALso, concepts have

been limited to those capable of functioning

in an oxidizing env/ronment. Refractory

metal alloys of tungsten, rhenium, and

molybdenum are not suitable for use at

elevated temperatures in an oxidizing atmosphere without having an impervious protection coating capable of surviving thermal

soaking and cycling. Such a coating could not

be found in the literature and is not considered feasible in the near future. Hence,

these materials have not been reviewed or

considered for future development.

For purposes of comparing the relative

merits, it is useful to group sensor concepts

into two broad categories: (1) non/ntrusive,

where no physical sensing element is

attached to the surface at the point of

measurement; (2) applied sensors, which do

require application of the sensor at the point

of measurement.

Nonintrusive sensors have many advantages,

including avoidance of aerodynamic or sensing element perturbations, but are limited in

some respects (namely, line of sight), which

makes it necessary to include applied sensors

as part of a comprehensive review of

measurement techniques. Criteria used to

select and rank concepts included the

following.

. Environmental survivability in gas turbine hot section flow path characterized

by:

• Turbulent, variable density, high

velocity combustion gases

• Oxidizing atmosphere

• High vibration and thermal shock

environment

2. Compatibility with component material

. Minimum sensor interference with component functionality or sensed

parameter

. Component modifications required to

accept sensing element must not significantly reduce functionality or life of

the component

5. Accuracy, resolution

6. Fabricability

7. Measurement system cost

8. Operating temperature range

9. Life expectancy.

Two reports which cover a range of sensing

techniques with slightly different perspectives (from this document) are

References 1-1 and 1-2.
2.0 Surface Temperature Measurement

2.1 Summary Listing of

Candidate Techniques

For purposes of quick reference, a listing of

the characteristics of the surface temperature techniques with some potential for use

on high temperature ceramic materials is included in Table 1. Some of the information

in these tables is strongly dependent on the

specific measurement problem and should

not be generalized. The rationale for doing

so here is to aid in comparing the various

techniques. It is strongly recommended that

a thorough understanding of the advantages

and problems associated with any specific

measurement system be achieved by reviewing the discussion in the following sections of

this document and the references associated

therewith.

2.2 Concept Description and

Analysis - Thin Film

Thermocouples

2.2.1 Thermoelectric Elements

2.2.1.1 Candidate Material

Characteristics

Table 2 contains a detailed tabulation of high

temperature conducting materials for sensor

elements. The data were collected from

References 2-1 through 2-34. These

materials can be broadly grouped into two

categories: (1) intermetallics and (2) metals.

The intermetallics, except for SiC and ReSi2,

have high resistivities and positive temperature coefficients of resistance (TCR). The

metals generally have lower resistivities

(- 10X) as well as positive temperature coefficients of resistance. Both groups of

materials have low thermoelectric potentials

except for the intermetallics - SiC, CrSi2,

ReSi2, and B4C. All of the materials have

melting points over 3000* F (except TiSi2)

and range from very good to poor resistance

to oxidation at 3000 ° F. Several of the intermetallic materials have excellent oxidation

resistance (SiC formed by Chemical Vapor

Deposition and MoSi2). The noble metals

are less oxidation resistant. Oxidation or

recession rate data reported in the literature

for noble metals ranges over several orders

of magnitude at the same temperature (discussed in a later section of this report).

The resistance of intermetallic materials to

oxidation depends on a number of factors associated with a particular test specimen

(purity, density, composition, microstructure, type of native oxide formed, and test

conditions). As a group, these materials are

more resistant to oxidation at elevated

temperatures than metals. Many intermetallies grow a stable native oxide on the surface

when oxygen is present at high temperatures.

This oxide layer protects the underlying

material and greatly slows the rate of weight

gain with time (parabolic rate kinetics).

Some examples include:

1. MoSi2 and SiC -SiO2 Protective

Oxide

2. TaAl3 -Al203 Protective

Oxide

3. ZrBe13 -BeO Protective

Oxide
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Table 1. Surface Temperature Measurement Concept Characteristics (Concluded).

Sensor Des_p*ion

EnWoNnemud RoeoWkm, mn

1300 75 25 _ion4 >0.1 2

1600 26 10 Hksne

_ and In_- 1300

medicMoys leoo

sc,u_2.ws2

IddkJm-Rhodlum 1300

1000

Tu_;_-"_--".-n2"_'_'-Jm 2000

AL*--

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m___,-p_l _ 1500

AooulUc Tmmoml_ None

Flxw OpUc So_x 1700

Pokukmu¢ T)pe)

R4_CI

TMm_mw -(P_ t I00

F_ 1300

Raman Scan_n9 ?

P._mL_iS

76 60 None

26 20 4onll

76 10 Nono

10 1 None

3 t CsNollOn

rl_:

25 50 None

50 5O

10 N_A

0.2 NO No No )olnt I Low

_,0. I 3 0. I No No No )oln! 1

>0.1 3 0.2 No No No _4n; 1

1.0. I 2 0.2

0.5 *2 ,1

No NO NO _ 1

y_ Yi NO Polnl 6

? ? ? NO No Yes Bulk ?

N_

Low

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WH_

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• Vo4alie Ckklo UmU

Tecql_lUm

Know_

• QIddallon EIIICs May Unt

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25 50 bolaled born S_

10 50 4one

0.01 0.1 0.3

I 0.1

5 0.5

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No No Yes

Point 5

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Sum

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• (_Idall_ Wil Cau_ Ddll

• L:',,;,,_ R--_ oi u-_,,_a_

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• Thin Fire S_,-,_,,-

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• F_. NO4Wel

Delirled

• For Nonreqxmslve

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O_

Table 2. Candidate Conducting Materials for Temperature Sensors Applied to Ceramic Substrates.

Material

MoSi2

S. Kanthal

(MoSi2)

WSi 2

cr3s_

CrSi 2

TaSi2

TISi2

ReSi 2

SIC

SIC

B4C

Ta/U 3

Platinum

Rhodium

Iridium

Rhenium

Tungsten

Method

of Preparation

Vac. Hot Press

Extruded

Hot Press

Hot Press

Hot Press

Thin Film

CVD

CVDC/)

Wire

W_

Wire

Wim

Thermal

Melting Expansion

Density, X-ray Point, Coefficient,

% Density °F 10"6pF

99.0 6.24 3720 4.9

6.24 3992

92-100 9.86 3840 4.6

92 6.43 3218 7.13

9.08 4300 4.9

96 4.04 2750 6.9

3596

99+ 3.22 4712 2-2.4

100 2.6

2.52 4442 1.73

6.33

100 21.45 3223 4.9

100 12.41 3571 4.6

100 22.65 4429 3.8

9.68 5756 3.9

100 19.3 6098 2.5

Thermoelectrical

Potential

(pV/degree)

-3.0

Resistivity,

R.T., p ohm-cm

90 to 100

Temperature

Coefficientof

Res_v_y0),

ppmPF

+6380

Oxidation Resistance-Air

Wt. Change

mg/cm2/hr

+0.3

Temperalure,

OF

3000

-0.2

+200

+14

+5.2

56.6

70

600

35-45

13-16

+4083

+2910

+2930

+3320

+4630

+0.004

<+012

+0.015

+0.24

+0.075

3092

3000

2500

3000

2190

+174

-106(4)

-351

+80

-3.3

+2.5

+1.2

+1.5

-7000

105-107

16 x 10 5

0.3 - 0.8

10.0

4.7

4.7

5.48

Reported to be Oxidation Resistant

at 3272°F (6)

+0.025 / 2912

2500A SiO2-16 hr at 3000°F (7)

+2166

+2572

+2372

+10

+0.07 (1)

-4.08(2X5)

•.S.5('ZXS)

•63.4 (2X5)

> 1000

300

2000

2597

3000

3000

3000

2552

1790

(1) Oxide flakes on cooling

(2) Formation of a surface oxide which is volatile, porous and nonproteclive (linear rate kinetics)

(3) TCR of thin films is typically about 500 of bulk

(4) Beta-SiC crystal, n-lype: Proceedings Conference on SiC, 1959

(5) R.B. Kaplan et al., "Oxidation Protection of Rhenium Thrustors for 2480°K Cyclic Operation by Means of CVD Coatings," Ultramet, 7/30/86

(6) R.B. Kaplan, privale communication

(7) Raytheon CVD SiC lest bar-etched step measured at GE Evendale (SiO 2 surface film alter 10 hours at 3000°F in air furnace)

I I I I I l I I I Ii I I I I I I I
Metals do not form a stable, protective oxide

and show a continuous weight loss with time

at elevated temperatures (linear rate

kinetics).

Figure 1 shows a comparison of weight gain

and weight loss at 1650" C in air for platinum

group metals (platinum, rhodium, and

iridium) and several intermetallic compounds (SIC, MoSi2, WSi2, and TaSi2).

Noble metals and alloys react with oxygen at

high temperatures to form volatile oxides.

The recession rate or loss of metal depends

on a number of factors such as specimen size

and form, test furnace configuration, and

flow rate of air past the test specimen. Many

investigators report widely different recession rates for the same material. Figure 2

illustrates this variability. The data reported

by R.B. Kaplan (Ultramet, Reference 2-2)

was measured in a furnace with a cool wall

temperature and an air flow rate of 2

feet/minute. The data from the Phillips

(Reference 2-3) paper was measured under

static air conditions. The Ultramet data was

used to derive Figure 1 since the test conditions more closely approximate jet engine

environments.

Figure 1 shows that MoSi2 and SiC are the

most stable materials at 3000* F in air by

several orders of magnitude. This is due to

the self passivating or native oxide layer

(SiO2) that grows on the surface and protects

the underlying material from further oxidation. The silicide materials cited in

Reference 2-4 probably do not represent the

best oxidation resistance that might be

obtained with purer and denser forms made

by CVD techniques. MoSi2 represents about

80% of the material in Super Kanthal heating elements manufactured by Kanthal Furnace Products. Kanthal Super 33 has a

maximum element temperature rating of

1800" C (3270* F) for air operation. The

material is reported to be ductile at temperatures above 1200" C. Oxidation resistance of

Kanthal heating elements is "due to a thin

adhesive protective layer of SiO2 on the surface. The thin protective layer does not flake

offwhen the element is cooled down in spite

of the great differences in coefficient of

expansion" (Reference 2-5).

Figure 1 shows the effect of a CVD HfO2 top

coat applied to iridium by Ultramet. The

recession rate of iridium with a top coat was

reduced to less than 3% of the metal loss of

a bare wire (Reference 2-2). If a similar coating could be developed for platinum and

rhodium alloy films (which have lower recession rates than iridium), then the recession

rates might be reduced to .01 mg/cm2/hour at

3000 ° F.

2.2.1.2 Materials Compatibility

Thermal Expansion Match

Matching the thermal expansion coefficients

(TEC) between film and substrate becomes

more important as the ratio of film thickness

to substrate thickness increases. Differences

in TEC's can produce high shear stresses at

the interface between film and substrate

when the structure undergoes a temperature

change. Separation can occur if the shear

stress exceeds the strength of the bond.

The choice of substrate and film combinations narrows as the TEC match required

becomes closer. This is illustrated in Table 3

which lists some selected candidate insulators and conductors for high temperature

sensors (candidate materials were selected

from a more extensive listing of TEC's in

Table 4). The column on the right in Table 3
100(

I00

(0.01

_= (01

11oo)

SIIIcidos end Carbides

wsi 2

Ta_ 2

Platinum

Figure 1. Weight Gain (Loss) Due to OxidaUon for Candidate Conducting Materials at 3000 °F in Air.

10(

1(

1

0.1

f-

.g

:_ 0.01

Q

m

0.001

0.0001i

1500 3000

pla_u_m.. Jk

2000 2500

Temperln.'F

"RB K_n et IL. Ulrlmlt. NAS3-24868. July 30. 1966

"WL Pltlllpl. Tranl. ASM. 1964. Vol 57

Figure 2. Recession Rate for Platinum, Rhodium, and Iridium.
Material

SiC (CVD)

MoSi 2

Tasi2

Iridium

Rhodium

Platinum

SiO2

AIN

Si3N4

/u2o3

HfO 2

BeO

MgO

Table 3. Candidate Sensor Materials - Thermal Expansion Match to ± 30%.

Thermal

Expansion

Coefficient,

OFx 10-6

4.6

4.9

ThermalExpansion

CoefficientMatch,

:I_30=/o

MoSi2, TaSi2, Rhodium, Platinum, AI203, HfO 2, BeO

Tasi2, WSi 2, Rhodium, Platinum, AI203, HFO 2, BeE)

4.9

3.8

4.6

4.9

0.28

2.3-3.0

3.3

4.3

5.0-5.5

5.0

7.8

MoSI2, WSI 2, Rhodium, Platinum, AI203, HFO 2, BeO

SiC, MoSI 2, TaSi 2, WSI 2, Rhodium, Pla_num

AIN, SIsN4, AI203, HfO 2

MoSi2, WSI2, Ta,St2, Platinum, Iridium, AI203, HfO 2, BeO

MOSI2, Wsi2, Tasi2, Rhodium, Iddum, AI203, HfO 2, BeO

None

SiC (CVD), IrMlum, SI3N4

SIC (CVD), IrMlum, AIN, AI203

MoSI2, TaSi2, Wsi 2, Rhodium, Platinum, HfO 2, BoO

MoSi 2, TaSI=, WSi 2, Rhodium, Platinum, AI.zO3, BoO

MoSi=, TaSi 2, WSi 2, _m, Ratinum, AI203, HfO 2

None
Table 4. Them'm/ExpansionCoefficients of Some Insulatorsand Conductors.

Material

sio_

py_yti,: C=.g,hit=

Si3N4 - Reaction Bonded

Silicon

Boron Carbide

Carbm--Cm't:_

Si3N4 - Hot Pressed

SiC - F'meGrain, Reaction

Sinmred

Isotropic Boron Nitrid=

Zk,_oa (67% 7-'O2, 33% SiOz)

SiC - Bonded with Si3N4

SiC- CVD

Tungsten

Molybdenum

ZrO=

AIN

_coniem

Si_'_4- CVD

Tantalum

Mull/_ (76%A1203, 23% SiO2)

Iridium

Chrom/um

AI_-99+%, Sin=red

AhO3-99+%, Sint=ed

AhO3- CVD

Rhodium

Pl=inum

THO2

BeO

Titanium

M_O

Gold

AI203 - Sputtered

Nickel

Incon¢1700

Re_41

304 Sminl¢= Sw=I

Copper

Aluminum

TemperatureRange,oF

68 to2280

0 =o2550

392

0 to 2550

Oto 2550

0to 2&._0

0 to 2550

0 to 2550

68 to 2190

68 to 3000

77to 1830

77 to 1470

68m2550

to 2550

m2550

68m lll2-Deposit*t250_

(ChangeswithTem_mm_ _)

700 to 1400

1800

_.ent

(_ x 10%

0.28

0.55

1.4-1.8

1.73

1.73

1.9

2.0

2.O4

2.24

2.3

2.4

2.6

2.5

2.7

3,06

3.0

3.1

3.28

3.6

3.7

3.8

3.8

4.3

4.5

4.4 to 4.9

4.6

4.9

5.28

5.3

5.3

7.8

7,9

8.1

8.4

8.8

9.3

9.6

9.8

13.2

10
shows the TEC match to within 30% of the

materials in the left-hand column.

Film Adherence-Design Considerations

The adherence of films deposited on a particular substrate depends on a number of

interrelated factors, some of which are listed

below.

Type of Substrate - Oxide films generally bond chemically to oxide surfaces and

exhibit strong adhesion. Metal films may

not form chemical bonds to an oxide surface unless the metals are oxygen-active.

Oxygen-active metals such as Cr, Ti, Al,

Ta, and Si form strong bonds to oxide

surfaces.

Substrate Surface Roughness - Thin

films deposited on polished surfaces may

not form strong bonds. Good adhesion

will depend on the film and substrate

material, deposition method, and interfacial stresses produced. If the substrate

surface is texturized or roughened by

physical, chemical, or mechanical

methods, a wider combination of film

materials will bond fairly well to the

substrate.

Film Thickness - Generally, the thinner

the film, the better it will adhere to a

substrate.

Surface Contamination - Clean surfaces

are a prerequisite for the development

of strong bonds between film and

substrate.

Film Stress - Films under high tensile

stress will crack and craze, and films

under high compressive stress will

buckle if the stresses exceed the bond

strength or elastic limit of the material.

Control of film stresses depends on

many variables (type of material, deposition methods and conditions during

deposition, substrate temperature, TEC,

etc.).

Thermodynamic Considerations

At elevated temperatures, materials in

intimate contact with each other may interact to form new compounds or lower melting

eutectics. Early failure of a sensor system

could result from an incompatible combination of materials.

Selection of material combinations for sensor systems should take into account possible

reactions at elevated temperatures.

2.2.1.3 Noble Metal Sensing

Elements

Published data for high temperature, air

environment thin film thermocouples

(T/C's) have primarily used Pt-10%Rh versus Pt (ANSI, type S). These metal films are

usually greater than 2 _.m thick. This work has

generally been limited to applications on

metallic engine hardware where the

temperatures have been less than 1100 ° C.

This thermoelectric combination is well

suited for this range, but does not seem adequate for the higher temperature requirements of this program. The maximum

working temperatures, as suggested by Samsonov and Kislyi (Reference 2-1), for some

thermocouples of interest are listed in Table

5, column 2. Another source (Reference 2-6)

suggests that Pt-30%Rh versus Pt-6%Rh and

Pt-40%Rh versus Pt-20%Rh are acceptable

in air intermittently to 1800 ° and 1850 ° C,

respectively. This implies that the survivability of thin film thermocouples would

be slightly higher with higher percentages of

rhodium.The references cited were based on

11
Table 5. Thermocouples Based on Noble Metals.

I

Thermocouple

Pt/Ptl0%Rh

Pt/Pt8%Re

Pt8%RWI:130%Rh

Pt20%Rh/Pt40%Rh

RWRh8%Re

Ir/_t40%lr

Ir40%RWPt40%Rh

Max. Working

Temp,, C

1600

1800

1800

1800

1900

2OOO

lg00

Maximum

Working Temp.

for Thin Film

TC's, ° C

4OO

1500

1550

wire thermocouples and need to be derated

for use as thin film sensors. Estimated maximum working temperatures, assuming compatible substrates and topcoats to retard

oxidation, are listed in Table 5, column 3.

Based on our experience with thin-film (type

B) thermocouples, the maximum working

temperature for the noble metal thermocouples would probably be less than 1600 ° C.

2.2.1.4 Silicon Carbide Sensing

Elements

Fabrication Considerations

• Dopin_Met_o_ n/C)

CVD SiC Films - CVD SiC can be doped

during format/on by introdudng controlled

amounts of the appropriate gases. According

to R.B. Campbell (private communication),

SiC can be doped to achieve a resistivity of

about 0.1 ohra-cm and possibly as low as 0.01

ohm-era with nitrogen (n-type) and

aluminium (pty_). The filmsare grown in

an ambient atmosphere of nitrogen or

aluminum trichloride. The doping level can

be controlled by the amount of these gases

added in the vapor phase. Campbell noted

that doping after the material has been

formed is very difficult because of the low

diffusion rates of dopants. For example,

aluminum will diffuse to a depth of only 0.1

to 0.2 mm after 3 hours at 2000* C.

Polyerystalline beta-SiC films (Reference

2-7) were prepared by CVD methods and

doping was accomplished by adding

trimethyl-aluminium and diborane during

film formation.

A beneficial effect of low diffusion rates

should manifest itself during high temperature operation of a thin film SiC T/C under

severe environmental conditions. More

stable thermoelectric outputs should be

obtained compared to thin film metal T/Cs,

which typically have high diffusion rates for

metallic impurities that could alter

calibration.

The Raytheon CVD SiC test specimen that

was used to measure emf characteristics on

this program was undoped. It has a room

temperature resistivity of about 160 ohm-cm.

If this material were deposited as a 5 gm thick

film, the resistance of the T/C leg might be

too high for the measurement instrumenta12
tion to record accurate data. However,

doping the material to 0.1 ohm-cm will result

in a resistance value of 4000 ohms/inch (of

leadpath) at room temperature (5 _m thick

film x 0.05 inch wide). The resistance of SiC

decreases by about an order of magnitude for

each 100 ° F increase in temperature. Therefore, at 1500 ° C, the portion of the T/C leg in

the hot zone will have a resistance about 27

ohms/inch. This value is comparable to thin

film metal T/C's.

Sputtered SiC Films - SiC films with widely

varying properties have been made by sputtering techniques. Beta-SiC (Reference 2-8)

was sputter deposited from silicon and

carbon targets onto a substrate maintained

about 700* C. The films were deposited on

sapphire substrates and had p-type conductivity with a resistivity of - t ohm-cm.

Sublimation grown SiC (Reference 2-9) was

post-deposition doped with nitrogen by, ion

implantation to lxl019 molecules/cm 3 to

produce n-type material. Ion implantation

could also be used to dope sputtered SiC

films after deposition.

Ion implantation (Reference 2-10) was used

to dope SiC crystals. Nitrogen implantation

and annealing produced n-type conductivity

in the implanted areas that were originally

p-type crystals.

It should be feasible to dope sputtered SiC

films by adding nitrogen or diborane to the

sputtering gas (argon) to produce "as

deposited" n- or p-type films. Sputtering

from a predoped target material should also

produce the desired conductivity type.

Deposition on a heated substrate (700* C)

and postdeposition annealing appear to be

necessary to achieve high quality, polycrystalline films.

• Potential Problem Areas

Areas that will require investigation to establish a viable high temperature intermetallic

sensor technology include the following.

Although SiC and MoSi2 as well as other

silicide films have been made by sputtering and CVD methods, no literature

reports have been found on the application or development of these materials

as thermoelectric elements. A report of

the use of sputtered SiC films as RTD

sensors (Reference 2-11) claims an

operating temperature limit of 400' C.

The properties of CVD SiC (oxidation

resistance, thermoelectric output, etc.)

discussed previously in this report are

based on bulk or "free standing"

specimens. Deposition of this material

as thin films (1 to 10 mm thick) on the

candidate ceramic engine parts to fabricate a sensor system on the surface of

these parts may result in different

properties.

• Methods for lead attachment to the SiC

films must be developed.

Feasibility of a high output SiC p-type

versus SiC n-type (thermocouple) has

been demonstrated. These results must

be translated into a thin film structure

using CVD and/or sputtering

techniques. Doping and patterning

processes must be developed.

The stability and maximum temperature

limits of thin film SiC thermoelectric

T/Cs must be established in high velocity

combustion gas environments.

• Methods of fabricating insulating films

(base coats and interlevel insulation)

13
compatible with SiC and silicide thermocouple elements must be identified

and tested to determine operating

temperature limits on conducting

substrates.

Oxidation Characteristics of SiC

Silicon carbide is available in a number of

forms, including materials fabricated by

sintering, hot pressing and CVD, or pyrolytic

deposition. These different forms of the

same basic material compound vary widely in

their density, microstructure, and purity. As

a result, the oxidation resistance and maximum service temperature in air can range

from 1300 ° to over 1600 ° C. The following is

an analysis of the expected maximum operating temperature of SiC thin film T/C's under

oxidizing conditions.

Some forms of SiC are highly resistant to

oxidation up to about 1650 ° C. No quantitative data have been found for CVD

SiC. This material can be made with high

purity and density and should be a good

candidate for thin film thermocouples

deposited on ceramic engine parts.

Assuming CVD SiC oxidizes at a rate

comparable to that reported for MoSi2

(Reference 2-12) at -1650 ° C (0.004

mg/cmZ/hour), then an SiO2 film would

form on the surface at a rate of only

250A/hour. By comparison, an iridium

(Reference 2-3) film would be reduced

in thickness at a rate of 8000A/hour at

1650 ° C without an effective passivating

cover coat.

Minamoto (Reference 2-13) reports that

a carbon/SiC T/C showed essentially

linear and stable output to 1800 ° C in

flowing argon. A total of 14 cycles from

room temperature to 1800 ° Cwere made

following a stabilization heat treatment

at 2100" C.

SiC/SiC thermocouples (Reference 2-

14) are reported to have a life of 50 hours

at 1700" C under oxidizing conditions.

Samsonov et al. (Reference 2-1) report

that SiC/SiC thermocouples may be used

for lengthy work in oxidizing gaseous

media at temperatures up to 1500" to

1750" C; stable readings are obtained up

to 50 hours.

Lauenko et al. (Reference 2-15) studied

the oxidation of SiC to 1500" C in air.

They self bonded SiC (99.5% purity) and

recrystallized SiC (99.9% purity/19%

porosity). The self bonded material

oxidized at a rate of 0.33 mg/cm2/hour,

and a bubbled aluminosilicate glass layer

formed on the surface. It was concluded

that the resistance to oxidation of SiCbased materials is not governed by

porosity but by the content of calcium,

magnesium, and aluminum in the

materials.

Schiroky (Reference 2-16) reports that a

smooth, bubble free oxide film forms

on CVD SiC at temperatures below

1700" C. At 1735" C he reports a

dramatic increase in bubbles in the oxide

film and the formation of tunnel-like pits

in SiC. (SiO2 "melts" at 1728" C).

• Oxidation ofRayt, heon CVD SiC Test Bar

Figure 3(a) shows a plot of the expected

oxidation rate or increase in the thickness of

a surface layer of SiO2 with time at 1650" C

for a Raytheon CVD SiC test bar. The SiO2

film thicknesses were estimated to be 2500A

after 10 hours and 25,000A ( __.1000A) after

60 hours. This estimate was obtained by

14
ORIGINAL PAGE IS

OF. POOR QUALITY

100,000

J_

I.--

.<

d

IO,OOO

1,00(_

0 50 100 150 200 250

Time, hours

(a)Oxidation Rates at 2732 and 3000 ° F

10,000

1,000

Time, hours

(b) Extrapolation Method

Figure 3. Comparison of Oxidation Rates for Two High-Purily Forms of SiC.

15
measuring an etched step using a Tencor

profilometer stylus instrument. Although the

surface of the SiC bar was polished with

diamond abrasive prior to the furnace test,

considerable surface roughness was

recorded by the Tencor instrument. This

accounts for the uncertainty in the film thick=

hess. Figure 3(a) illustrates how the data in

Figure 3(b) were extrapolated beyond

60 hours. These measurements were made

by GE during Task 2 of this program.

The furnace test included a bar made from

sintered SiC (Norton). After 10 hours at

1650 _ C, this bar showed severe corrosion

(formation of a bubbly glassy layer on the surface). The Raytheon CVD SiC bar appeared

to be unchanged except for some spotty

glassy patches. This material apparently was

deposited from previous contamination in

the furnace or else it came from within the

Norton SiC test bar.

Comparison of Noble Metal and Silicon

Carbide Characteristics

A great deal of information is available in the

literature on SiC. This material is used extensively in industrial applications as a structural

ceramic and as furnace heating elements.

Over the past 25 years, a growing interest has

developed in SiC semiconductor films for

integrated circuit applications. Research is

underway to produce single crystal films for

high temperature electronics and other

applications (References 2-17 through 2-21).

This extensive body of information will

benefit the development of SiC films for high

temperature thermoelectric thermometers

on this program.

A somewhat analogous situation exists for

MoSi2 because of its industrial use for above

1650 _ C furnace heating elements (Super

Kanthal). However, this material does not

have the mechanical or semiconducting

properties of SiC.

In addition, silicon carbide is considered to

be primary candidate for high temperature

sensors because of the following

characteristics:

Operation in Air - SiC forms a selfpassivating, nonvolatile surface oxide

(SiO2) that limits further oxidation by

restricting diffusion of oxygen. We

believe the potential exists for T/C

operation in air to above 1650" C for

extended periods of time (50 hours or

more).

Oxidation of platinum metals is not a

self-limiting process. The metals form a

volatile surface oxide in air and lose

weight continuously as a function of time

and temperature. An effective top coat

will be needed for higher temperature

operation of thin film T/Cs.

Thermodynamic Stability - SiC films

should be compatible with all of the candidate ceramic engine parts (substrates)

at high temperatures due to free energy

of formation considerations. Since many

of these materials contain silicon or

silicon additives and/or impurities, the

formation of platinum silicides with low

melting points is a potential failure

mode for thin film platinum based T/Cs

deposited on silicon-bearing substrates.

Adherence - SiC can develop an SiO2

interface oxide during film growth that

will produce a stable and strong bond to

the substrate. Sputtered platinum alloys

do not adhere well to nonmetallic substrates because the alloys do not form a

strong and stable interfacial oxide.

16
The thermoelectric output of SiC is

about 15 times greater than aType KT/C

and about 70 times greater than a Pt6%Rh T/C. A high output SiC T/C will

accurately detect very small temperature changes. For example, such a device

will produce a signal of about 60

microvolts to detect a temperature

change of 0.1" C compared to only 2

microvolts for a Type K T/C.

The following is from Reference 2-22:

"The thermoelectric power of a material

depends on the sum of the potential energy and the kinetic transport energy (for

one kind of carrier, i.e. electrons in the

conductor). In semiconductors, the

absolute value of both these energies are

additive and the thermoelectric power is

large. In metals, the potential energy is less

than zero, and differs little from the

kinetic transport energy. Therefore, their

sum is small and the thermoelectric power

by comparison is small. The absolute

thermoelectric power of semiconductors

usually exceeds that of metals by at least

an order of magnitude."

Catalytic Effects - Platinum thermocouple elements exposed to incompletely burned gases that exist in engine

environments can produce catalytic

burning on the T/C surface leading to a

local increase in temperature. Therefore, the temperature measured does

not represent the actual engine temperature. A SiC T/C with a surface oxide layer

of SiO2 is not expected to induce

catalytic burning.

Thin Film T/C Instabilities - Structural

and thermoelectric instabilities caused

by contamination, loss of material, and

catalytic effects are expected to be less

of a factor for SiC T/C's due to the SiO2

native oxide and the low diffusion rates

of dopant elements at high temperatures.

2.2.2 Insulating Materials

2.2.2.1 Candidate Insulating Material

Characteristics

Table 6 shows high-temperature insulating

materials that may be suitable as base coats,

diffusion barriers, or top coats in a thermoelectric sensor system. The data was

obtained from many different sources. These

materials can be grouped into two

categories: (1) oxides and (2) nitrides.

All of the materials have melting points over

1650" C (3000 ° F) and resistivities at 538 ° C

(1000. F) in the 1010 ohm-cm range (except

SiO2 and HfO2). As Table 6 shows, the resistivity of all materials decreases rapidly with

increasing temperature. Between 1371 ° C

(2500* F) and 1650" C (3000* F), both oxides

and nltrides have resistivities in the range

from 106 to 10a ohm cm. SiO2 has a lower

resistivity (7 x 103 at 2500 ° F).

Figure 4 shows a plot of resistivity versus

temperature for most of the insulators listed

in Table 6. Figure 5 shows a similar chart

from a different reference source

(Reference 2-23). Figures 4 and 5 and Table

6 show that the resistivities of all of the

insulator compounds are essentially the

same at about 3000* F. As pointed out by

Bauer and Bates (Reference 2-23), enhanced

emission and gas conduction effects are very

significant at high temperatures and can

introduce considerable measurement error

(shunt conducting paths). It appears, therefore, that no definitive insulator material

17
TaMe 6. Cand/date Insulating Materials for Temperature Sensors Applied to Ceramic Substrates. From

References 2-23 through 2-32.

Mlledd

Trot $OmldO

_. Pudly

99.7

12 lan 99.9

/_d)s

*J2O_

MoO

BeO

_O 2

H_2

Si3N4 CVD

SaoonNHdde(Odde_ Redmn_: W_

BN CVO I Mil

BN CVO keg.Data

Boron mride (Oxlde_on Remind: We_

NN

99.5

M_

X-_/ Point,

Den,_y _ 'F

3.966 3659

3.965

3.56 5072

3.03 4622

2.32 3142

9.68 5O31

3.16 3452

hi Gakt - 0.000167 mo/¢m 2/hour a12822 .i=)

>99.9 1O0 2.25 4946

< Gain - 3.0 mg/cm2 4tour el 2200 .F)

Thennd

Expansion

Coellicienl,

10 "6PF: 1000

4.3 8 x 1010

8.1 Ix1010

7.0 4x 1012

5.3 1 x 1013

0.28 2 x 10 7

-5,5 2 x 10 8

10

3.3 8 x 10

I x 1014

2.3-3.0

RomMy, oM_om.'F

1500

_x 10 8

2 x 107

8x 10 9

2x 1010

4 x 105

6

2000

5 x 106

4x10 7

7x10 7

Sxl0 4

1 x 10 3 x 104

2x108 6x10 6

2x10 I0 lx107

lx1011 5x108

2_0

3x10 5

2x 10 6

4x10 5

7x 10 3

6x106

3x 10 6

3OO0

5x10 4

8 x 10 4

2x 104

sx lO4

5x10 4

I I I I 1 I I I I I I I I I I I I I I
"'"'_BeO

10'_ I%Mo° ,._ .

Sputtered _'_ 1_ ":_'_'_----- Pyrolytic BN

10 8

s_o_---_\._ cvo%___ I"',, ,0, \.j s,n,_,.,-__.

--._. A,_ _ _ \.

104

0 500 1000 1500 2000 2500 3000

Temperature, ° F

Rgure 4. Resistivity Versus Temperature - High Temperature Insulators. Data is from References 2-23 through

2-32.

t9
il

-4

-6

-8--

-10 --

-¢=

15

UJ

-12 m

-14 m

-16

-18 I I

2000 1000

Temperature, ° C

Figure 5. Electrical ConducUvity Versus Temperature - High Temperature Insulators. From

Reference 2-23.

2O
selection can be made for 3000 ° F sensor

structures based on materials with the

highest resistivity at 3000 ° F.

The oxidation resistance of all of the

materials in Table 6 is very high, with the

notable exception of boron nitride (BN).

Figure 6 shows a plot of pyrolytic boron

nitride (PBN) (Union Carbide, Boralloy)

and CVD Si3N4 resistivity versus temperarare. Comments are also made regarding

oxidation rates. The PBN rate increases from

a negligible value at 1382" F to 0.05

mg/cm2/minute at 2200* F. The oxidation

rate ofCVD Si3N4 is reported to be 0.000167

mg/crn2/hour at 2822* F (Reference 2-24). At

these temperatures, a protective surface film

of SiO2 probably forms in a manner similar

to MoSi2 or SiC. The surface of PBN, by comparisort, is converted to B203, which is a low

melting glass. Eventually, all of the BN

changes to B203.

1016

1014

E 1012

,o

E

e.-

o

>..

_- 10

•-_ 10

108

106

_ Measured

)_ Pyrolytic BN - 1 rail Thick

X_ Measured in Air I

_'C" Direction

i_ _ --PBN Has NegligibleOxidation to 1382" F in Air

____ OxidaUon Rate Increases to 0.05 mg/cm2/min

\. \ ata =2200"

From Several _ _ _ / .

so. ces I \ ", "% l BN

....... I ,_ X xN_ _/" (Mfg. uata,l

l / I_ _D I_ "O" Direction

CVD (l:_y'rolytic)Si., N,L_" _---_----._I NC) i

F'_ of Operating in'All" "_ "_ J _-_ Union Carbide

Temperatures < 2763 ° F -_,,,, _1_,_

_o4 I I I 1 I I I I I I I I I I I I II I I I I I I

0 500 1000 1500 2000 2500 3000

Temperature, * F

Figure 6. Resistivity Versus Temperature - Pyrolytic Boron Nitride and Silicon Nitride.

21
2.3 Concept Descrlptlon and

Analysis- Radiation

Pyrometry

2.3.1 Concept DescrlpUon

During the past 20 years, optical pyrometry

has gained widespread acceptance as an

invaluable technique for surface temperature measurement in hostile environments in

general and for turbine blade measurements

in particular. A radiation pyrometer

measures the radiance of the surface it views.

In general this radiation consists of both

radiation emitted by the target surface and

radiation reflected off of it.

Most pyrometers used in turbine engine

applications are spectral devices; that is, they

measure one or more narrow radiation

bands. The spectral radiance of real bodies

may be calculated by multiplying the spectral

radiance of a blackbody (derived from

Planck's equation) by the emittance of the

target surface. Emittance is a function of

various factors including the chemical composition of the material the surface condition, the angle of emission, the wavelength,

and the temperature.

2.3.2 Types and Characteristics

Spectral band pyrometers measure radiation

only in a narrow wavelength band (practically

at one specific wavelength) so that the "effective wavelength" (Reference 2-35) is essentially independent of temperature. If the

spectral emittance of the specimen surface is

known at the effective wavelength, then the

difference between the true temperature and

the temperature determined by the

pyrometer can be calculated.

Wide band or total-radiation pyrometers

measure what is often termed the brightness

temperature, which is defined as that

temperature at which a blackbody would

emit the same radiant flux as the specimen

surface at the effective wavelength of the

pyrometer. They measure a much broader

spectral range than that measured by narrow

band pyrometers. The effective wavelength

of these pyrometers is temperature dependent, so that numerical methods are required

to accurately determine surface temperature

from radiation measurements made with

these instruments. Before the silicon cell

became widely used in the 1960's, total radiation pyrometers using thermopile detectors

were the predominant industrial pyrometer.

These devices are sensitive to more than 90%

of the radiation emitted from the target, so

that the curve of measured radiation versus

temperature follows the Stephan-Boltzman

curve. Actual temperature can be calculated

if the total emittance of the specimen surface

is known.

Numerous pyrometers measure radiation

from more than one spectral band. The most

widely used of these pyrometers is the twocolor, or ratio, pyrometer (Reference 2-36)

inwhich the temperature is determined from

the ratio of the radiation measured at two different wavelengths. In principle, these

instruments are insensitive to emittance as

long as the emittance of the specimen is the

same for both wavelengths. The assumption

of nonvarying emittance at the individual

wavelengths is reasonable for many

materials, and for special cases, problems can

be minimized by making the two bands close

to each other.

Instruments based on intensity ratio at two

wavelengths are usually more sensitive to the

effect of radiation reflected from the

22
specimen surface. This has been

demonstrated experimentally in our

laboratory and verified by discussions with

manufacturers of commercial instruments.

Considerable effort has gone into using multiple wavelength pyrometers to reduce the

effect of emittance on temperature measurements using radiation pyrometry. Svet

(Reference 2-37) and Quinn and Compton

(Reference 2-38) suggested that it is possible

to calculate temperature from spectral

radiance measurements made at multiple

wavelengths without prior knowledge of

emittance value. These techniques involve

assuming a functional dependence between

spectral emittance and wavelength and then

using the radiance data to determine adjustable parameters and temperature by solving

simultaneous equations. Babelot et al.

(Reference 2-39) described a multiwavelength pyrometer that was designed to

achieve a spatial resolution of 100 _t and a

time resolution of 10 ns. Using the data from

six spectral bands, they claimed they could

determine the functional dependence of

emittance on wavelength. Hunter et al.

(Reference 2-40) presented a method of

determining temperature by using a multichannel analyzer to measure 200 spectral

bands simultaneously. Assuming only that

emittance was a smooth function of

wavelength, they were able to calculate

temperature without prior knowledge of

emittance. They demonstrated that their

technique was accurate to within 1% when

measuring the temperature of platinum up to

1724 K. Neither of these instruments was

capable of compensating for the effect of

reflected radiation.

Other investigators have expressed doubt

that multi-wavelength pyrometry is a practical method of eliminating errors caused by

varying emittance. Coates (Reference 2-41)

showed that in the absence of precise

knowledge of the relationship between emittance and wavelength, it is impossible to

estimate the uncertainty associated with

temperature measured with an n-wavelength

pyrometer, where n > 3. He showed that

even if the spectral dependence of emittance

is accurately known, calculated temperatures

are increasingly sensitive to errors in the

radiance measurement as the number of

channels, n, increases. Since accurate

knowledge of emittance of solids as a function of wavelength is generally unavailable,

Coates believes that multichannel pyrometry

has little to recommend it. Nordine

(Reference 2-42) concurred with Coates and

concluded that measuring three or more

spectral radiances had little practical value

without unprecedented increases in the

precision and accuracy of spectral radiance

measurements.

2.3.3 Effect of Reflected Radiation

Most multiwavelength pyrometers are

designed to enable surface temperature

measurements to be made without prior

knowledge of emittance. Knowledge ofemittance is not in general sufficient to permit

calculation of true surface temperature from

the radiance measurement. Radiation

reflected from other hot surfaces into the

entrance aperture of the pyrometer can

cause significant errors in temperature

measurements. Atkinson and Strange

(Reference 2-43) reported errors in

pyrometer temperature measurements of

turbine blades of 75 K at an actual surface

temperature of about 1300 K. Because

ceramics generally have lower spectral emittahoe at a wavelength of 1 _rt than oxidized

metal surfaces, the problems associated with

23
reflected radiation can be expected to be

more pronounced for ceramic turbine parts.

A method for dealing with reflected radiation using a dual wavelength band pyrometer

is based on the fact that radiation from a high

temperature source will affect the two bands

differently (Reference 2-43). It is assumed

that the primary source of reflected radiation

is the combustion gas and that the target is

surrounded by parts operating at temperatures not much different from itself. In addition, the amount of energy from the hot gas

reflected from the target surface is a function

of complex geometry factors which are difficult to calculate but which may affect the

wavelength bands equally. Equations were

derived for these assumptions and used to

calculate reflected radiation errors based on

estimated combustion gas temperature. Two

bands within the operating range of a silicon

detector were used to evaluate the concept

on a laboratory test strip (using a tungsten

filament lamp as the reflector source) and a

rotating turbine blade row in an operating

gas turbine engine.

Results of these tests were promising.

However, Atkinson and Strange's assumptions may not be applicable to ceramic

materials, especially those which show large

changes in emittance as a function of

wavelength in the wavelength range of the

detector.

The concept of identifying reflected radiation by examining the polarization characteristics of total (emitted and reflected)

radiation from a target surface is described

by T.P. Murray (Reference 2-44). The instrument was developed to measure the surface

temperature of metals with large variations

in emittance without specific prior

knowledge of the actual target surface

emittance. It involved examining the combined radiance from a target surface which

included emitted energy due to its own

temperature plus energy reflected from a

blackbody source at known temperature by

means of a rotating polarizing analyzer. The

polarization of the normally unpolarized

blackbody radiation caused by reflection

from the target was 90 ° to the emitted radiation from the target surface. Relationships

were derived showing that the temperature

of the blackbody source was equal to the target surface whose emittance was unknown

when the a.c. component of the signal from

the rotating analyzer was zero.

The polarization characteristics on which

Murray relied are not generally found in diffuse radiators typical of most ceramic

materials. We believe, therefore, that the

probability of successful isolation of

reflected from emitted radiation using

polarization techniques appears to be low.

The problems associated with reflected

radiation can be minimized by limiting the

radiance measurements to wavelengths in

which the specimen has a high emittance. A

suitable spectral range in which most

ceramics have high values of emittance and

in which the combustor gas stream has low

absorption is found between 8 and 10 _m.

2.3.4 Adaptability to Ceramic

Materials

Noncontact sensing, unlimited temperature

range, and maturity of the technology make

radiation pyrometry a strong candidate for

use in measurement of ceramic surface

temperatures. Effects of unknown or variable surface emittance and transmittance

and reflected radiation from surrounding

components and combustion gases are

major obstacles in obtaining accurate

24
measurement in a gas turbine hot section

environment.

Most historical applications of radiation

pyrometers in gas turbine engines have been

related to measurement of surface temperature of metallic turbine blades. Typical blade

materials have relatively high (0.85 to 0.9)

emittance values after a short time at operating temperature and do not change significantly over time. The effect of radiation

reflected from nearby surfaces and combustion gases, which flucuates widely, is minimized by the high emittance of metallic

superalloys in the wavelength band of silicon

detectors (0.4 to 1.2 _a).

Ceramic materials are difficult to categorize

with respect to emissivity. Typical emittance

values for some ceramic materials are listed

in the following table at two wavelengths.

Table 7 illustrates that some ceramics closely

approximate metallic superalloys in emittance characteristics, but others do not. For

those materials with very low emittances,

errors due to reflected radiation will be

unacceptably large unless a method for

accounting for it is incorporated in the

measuring system.

Table 7. Average NormaI Spectral Emittance at Two

Wavelengths (From Reference 2-45).

Mater_ _

Alumina 0.10 0.85

Zirconia (coating) 0.10 0.85

Silicon Nitride 0.82 0.98

Silicon Carbide 0.90 0.95

2.3.5 Long Wavelength Pyrometer

One method of reducing the effect of

reflected radiation is to select a wavelength

band where emittance values are as high as

possible. For example, it is possible to reduce

the observed reflected energy on a zirconia

coated surface from 90% to 15% by shifting

the detector wavelength from 1 to 8 _rn.

Although long wavelength pyrometry does

not solve the problem of reflected radiation

entirely, it leaves us in approximately the

same position with respect to ceramics as we

are in now with the metallic superalloys and

silicon detectors. In addition to the higher

emittance of ceramics at 8 to 10 _m, long

wavelength pyrometry may reduce the effect

of reflected radiation in a second way. The

relatively steeper slope of the Planck radiation curve at short wavelengths means that, if

the reflected component is a given fraction

of the total radiation emitted from the

specimen surface, then it will have a correspondingly greater effect at short

wavelengths than at long wavelengths.

A comparison of error caused by reflected

radiation on a target surface is shown in

Figure 7 for two combinations of wavelength

and emittance (1 _rn and 0.3; 8 _rn and 0.8).

A target surface temperature of 1366 K

(2000* F) is assumed for this calculation. A

sketch of the target and reflector arrangement and definition of terms is included in

Figure 8.

Use of a long wavelength detector does not

solve the problem of determining emittance

of the target surface. Knowledge of emittance is still necessary to permit calculation

of temperature from measured radiant energy. Emittance measurements can be made on

the target surface over a range of temperature and spectral bands prior to installation

25
900

800

700

600

U.

O

500

400

300

2O0

100

13

m

]¢

/'1 t--

w u

_. -1.0

Target Emittance - 0.3

;L -7.9

Target Emittance = 0.8

/

0 1200 1600 2000 2400 2800 3200

Reflector Temperature, o F

Figure 7. Error Due to Reflected Radiation for Fixed Target Temperature of

2000" F as a Function of Reflector Surface Temperature. See Figure 8 for

calculation procedure.

26
Figure &

/ _/1" B / / / / / TargetSurface

'__ Reflector

Pyromete

W-r

wr

wl

T I =

Error =

T I =

T B =

T_ =

E B =

E R =

FR_B =

Wr =

we =

WR =

W_ =

C 1 =

C2 =

X. =

EBWB+ (1 - EB)(FR_B)(ERWR)

EBW_

C_.-s

e<C_,_ -1

c2

+1

TI-T B

Indicated Target Temperature

Target Surface Temperature = Constant 1366 K (2000 °

Reflector Surface Temperature

Target Emittance = 0.3 atllJm; 0.8 at 81zrn

Reflector Emittance = 1.0

View Factor

Total Spectral Radiance

Spectral Radiance Emitted from Target

Spectral Radiance Emitted from Reflector

IndicatedSpectral Radiance from Target

Planck's First RadiationConstant

Planck's Second Radiation Constant

Wavelength

F)

CalculationProcedure for Plot of ErrorVersusReflector Temperature (Figure 7)

27
of the component into the test vehicle.

Where this is not practical, it is possible to

make emittance measurements on the

installed component by measuring the

reflected energy from an excitation pulse at

the specific wavelength for which the detector has been calibrated. This technique has

been demonstrated in our laboratory using a

prototype instrument described in

Reference 2-46.

The error caused by target surface emittance

uncertainty at two wavelengths is plotted as

a function of temperature in Figure 9. At 1

and 7.9 u.m, errors of 0.1 and 0.05 respectively are assumed. Justification for the higher

uncertainty at the lower emissivity is based

on larger variability in emittance data at

lower emittance levels (Reference 2-45) This

is believed to be realistic for situations where

emittance for a typical target surface is

measured in a laboratory environment and

then used to calculate temperature from target surfaces exposed to effects of time at

temperature in a gas turbine flowpath. If this

assumption is not Valid, and emittance unce rtainties are the same at all wavelengths, then

larger errors in temperature will be caused at

the longer wavelengths.

There are some special problems created by

the use of long wavelength detectors which

should be considered.

180

160

140

U.

120

i...I 100

6o

60

4O

I I I I 0_

O ;. - 1.0 /

Actual Emittance. 0.35

Calibrated (Assumed) Enittance - 0.25

_ ). - 7.9

/

2O

800 1200 1600 2000 2400 2800 3200

Temper'alum. • F

Figure 9. Error Due to Emittance Uncertainty at Two Levels of EmiCance as

a Function of Temperature.

28
Sensitivity: The relationship between

radiant energy and temperature in the 8

_n region reduces the sensitivity compared with that near 1 _'n. This will

result in reduced resolution and a lower

signal-to-noise ratio, or may increase the

minimum target area.

Detector Performance: Many detectors

suitable for use at 8 _ are less

convenient than silicon. Cryogenic cooling of the detector or chopping of the

optical signal may be required.

Optical view path materials are much

more limited for 8 _rn systems. Quartz

and sapphire are not suitable. Special

optical materials will be required. Fiber

optic coupling may not be practical.

2.4 Concept Description and

Analysis - Resistance

Thermometry

2.4.1 Platinum Resistance

Thermometer

Numerous thermometers are based on the

temperature variation of resistance of pure

metals or metal oxides. Since the 1920's, the

platinum resistance thermometer (PRT) has

been used as a standard from cryogenic

temperatures to over 900 K. Crovini

(Reference 2-47) and Quinn and Compton

(Reference 2-48) reviewed platinum resistance thermometry, primarily in regard to its

use as a temperature standard. The latter

paper also evaluates measuring circuits used

in platinum resistance thermometry.

Evans and Burns (Reference 2-49) reported

data on the stability of platinum resistance

thermometers at temperatures as high as

1400 K. They used disks of synthetic

sapphire, silica, and other materials, spaced

along a central Pt rod. Lengths of Pt wire

about 40 mm long were threaded through

holes in the disks and welded in series at their

ends, forming a "bird cage" resistor. In some

cases, their thermometers showed resistance

drifts corresponding to temperature changes

no larger than a few hundredths of a degree

after several hundred hours at 1400 K.

Although valuable as a calibration

instrument, the high-precision platinum

resistance thermometer is a fragile and delicate instrument which does not have direct

applicability to measurement of component

surface temperature in turbine hot sections.

2.4.2 Resistance Temperature

Detectors

There is a class of resistance thermometers

known as resistance temperature detectors

(RTD's) that are intended for industrial use.

In these devices, a platinum or platinum alloy

resistance element is embedded in ceramic

or glass. This gives the resistance sensor

robustness, but at the same time reduces its

stability compared to that of the PRT. There

are two main reasons for temperature

instability of RTD's. First, differences in

thermal expansion between the resistor and

the matrix in which it is embedded can introduce strains in the element which may cause

the resistance to change. Second, the matrix

material or impurities therein can react with

the sensor, causing calibration shift. The contamination problem limits available

industrial RTD's to temperatures less than

about 1200 K (Reference 2-50). Thin film

configurations of RTD's can be expected to

increase problems associated with contamination of the sensor as well as making

the sensor more susceptible to stress. For

these reasons, GE does not believe that

platinum alloy RTD's are good candidates

29
for high temperature measurements in the

hostile combustor environment.

Metal oxide semiconductors, or thermistors,

are widely used as industrial resistance

temperature elements. They typically consist

of mixed oxides of nickel, manganese, and

cobalt which have a negative temperature

coefficient of resistance. For temperatures

above about 600 K, more refractory oxides

are required. The oxides of rare-earth

elements have been used up to about 1000 K,

while devices based on yttria-doped zirconia

are available for temperatures up to about

1300 K. Anderson and Stickney (Reference

2-52) tested high purity alumina and mullite

sensing elements made from solid rods to

1833 K, with good success. They suggest that

an upper temperature limit of 1944 to 2500

K may be possible using MgO or BeO sensing elements. However, we could find no

additional references to successful use of

ceramic element RTD's at high

temperatures.

Sensing elements fabricated from ceramic

materials have potential for operating at

temperatures well above 1400 K. The lack of

information in the literature concerning

stability of these sensors is not encouraging.

The problems of lead integrity and sensor

stability can be expected to become more

severe in the hostile combustor environmerit. These factors lead to the conclusion

that ceramic element low profile RTD's for

surface temperature measurement in turbine engine hot sections have some potential, but will require considerable effort and

have a relatively low probability of success.

3O

2.5 Concept Description and

Analysis - Laser-induced

Fluorescence

2.5.1 Concept DescripUon

Remote temperature measurement by laserinduced fluorescence (LIF) has been well

established for gas and surface temperature

measurements (References 2-52 through 2-

55). In many applications, LIF techniques

offer a relatively simple analytical technique

that is well suited for a variety of laboratory

and industrial applications.

Certain fluorescent materials, known as thermographic phosphors, can be used as

temperature indicators in either of two

modes (Reference 2-56). Both the ratio of

selected emission line intensities and the

lifetime of a particular line have been used

to determine temperature. The ratio

approach is a rather more complex

experimental procedure and consequently

has been used less for surface temperature

measurements. More commonly, temperature is determined by measuring the fluorescent decay time of the phosphor. In many

instances, luminescent decay follows the

Arrhenius relation: _._y Time Constant is

proportional to e "_'_'''J, where T is the

temperature, E the activation energy, and k

is Boltzrnann's constant. The lifetime of such

an excited state can be measured by exciting

the luminescent material with pulsed radiation of an appropriate energy, thereby causing it to fluoresce as it decays back to the

ground state. By monitoring the decrease of

the fluorescent intensity with time after the

excitation has ceased, the lifetime of the

excited state can be deduced. The decay time

of the fluorescent material is observed to

decrease with increasing temperature.

Hence, an empirical correlation of the decay
time with temperature can provide the basis

for a temperature measurement system.

When using LIF of thermographic phosphors to make surface temperature measurements, it is highly desirable, although not

strictly necessary, that the decay of at least

certain spectral lines originate from a single

component process so that the decay follows

the Arrhertius relation. In such cases, the

decay time can be determined by measuring

the fluorescent intensity at two instants after

the phosophor has been excited. It is even

more important, however, that the decay at a

given temperature be highly reproducible,

that it not be a function of excitation intensity, and that there be no hysteresis with

respect to either prior excitation or prior

thermal exposure.

2.5.2 Current Capabilities

An active development program, funded by

the Air Force and the Department of Energy,

is underway at Los Alamos National

Laboratory to explore using LIF of rare earth

phosphors to measure surface temperatures

in hostile environments (Reference 2-57).

This group has determined that the 612 nm

spectral line of Eu-doped yttria is a good candidate for the temperature range in which

they are interested (up to 1500 K). At 1500 K,

the lifetime of this line is approximately 10

ns, or at the limit of the resolution of the signal precessing equipment used by the Los

Alamos group. By 1700 K, the decay time is

expected to be less than 1 ns, a reasonable

lower limit for a practical fluorescent decay

measurements (Reference 2-58), although

significantly shorter lifetimes can be

measured using single photon counting

methods (References 2-59 and 2-60). Even if

suitable instrumentation were available to

measure 1 ns decay times, Eu-doped yttria

may not provide sufficient signal intensity for

measurements at temperatures significantly

higher than 1500 K.

Developmental work with LIF of high

temperature thermographic phosphors has

proceeded beyond the stage of questions

concerning feasibility: it is feasible. Significant questions remain regarding the

extent to which LIF methods can be applied

in combustor environments. An outstanding

question concerns the maximum surface

temperature at which fluorescent measurements of thermographic phosphors can be

made. As discussed above, LIF measurements cannot currently be expected to

measure surface temperatures above

1700 K, and 1500 K may in fact be a practical

limit for Eu-doped yttria.

2.5.3 Candidate Phosphors and

Upper Temperature Limits

A significant increase in the measurable

temperature using thermographie phosphors

can only be expected from other phosphors

(or other spectral lines of Eu-doped yttria).

Little research has been done to determine

whether or not candidates for higher

temperature measurements exist. It is

known, however, that Eu-doped yttria

fluoresces up to its melting point near 2700 K

(Reference 2-61); whether or not it exhibits

a useful thermographic effect at such

temperatures is not known. There are other

thermographic phosphors whose properties

warrant investigation at temperatures

exceeding 1500 K. For example, Gd-doped

yttria has spectral emission lines that are not

quenched at 1300 K and may be a candidate

phosphor for temperature measurements

over 1500 K. For theoretical reasons, Eudoped scandium oxide appears to be an

attractive high temperature phosphor,

although no references to its use could be

31
found. There is room, therefore, for

optimism that suitable phosphors exist to

measure temperatures higher than 1500 K.

More experimental evidence is needed to

answer this question definitively.

2.5.4 Phosphor Application to Test

Part

An area of concern related to use of LIF of

thermograph/c phosphors in hostile environments concerns adherence of the phosphor

to the test surface. Although LIF surface

temperature measurements may be considered noncontacting* they require that the

phosphor be applied to the test surface.

Several methods have been explored for

applying the phosphor, including sputtering,

E-beam vaporization, plasma spray, and

flame spray (Reference 2-62). Other

methods such as chemical vapor deposition

may also have application. Satisfactory bonding for high temperature metallic turbine

parts has been only partially successful, and

this problem may limit the application of LIF

temperature methods in hostile environments.

2.5.5 RecornmendaUons

LIF of thermographic phosphors is a promising method of surface temperature measurements for selected applications. It is probably

too complex for routine measurements,

however. We believe that it may f'md important use as a calibration tool for other surface

temperature methods.

2.6 Concept Analysis and

Description - Noise

Thermometry

2.6.1 History

Noise thermometry was introduced by Garrison and Lawson in 1949 (Reference 2-63).

It is based on measuring the noise voltage,

current, or power generated across a resistor

to infer the temperature of the resistor. It has

the significant advantage of being

independent of environmental conditions

and of the properties of the material of the

resistor. In addition, Johnson noise thermometers are linear devices that measure

absolute temperature. Although the principles of noise thermometry have been established for decades, only in the past fifteen

years has practical use been made of the

technique.

2.6.2 Theory

The mean square noise voltage Vn of a passive device is given by:

Vn 2 = 4kTR (hf/kt) (I)

e(h_'O - 1

where h and k are Planck's and Boltzman's

constants, respectively, T the absolute

temperature, f the frequency, and R is the

res,_stance. For ratios of f/T of less than about

10' Hz/K, equation (1) may be approximated

by:

Vn 2 = 4kTR (2)

For a given frequency bandwidth M, the rms

voltage Vn, and noise current In are given by

<Vn> = [4kTR(A0]m (3a)

32
< In > = [4kT(Af)/R]I/2 (3b)

By multiplying equations (3a) and (3b), the

noise power is obtained:

< Pn > = 4kT(Af) (3c)

which demonstrates that, in principle, noise

power is independent of the value of the

resistance and is linear with respect to

temperature. In fact, noise thermometers

can measure either noise voltage, current, or

power.

It should be noted that these equations apply

only to the sensor: they do not take into consideration interfering signals from such

sources as the noise pickup by the cables,

internal noise generated by the signal

processing, or Johnson noise from connecting leads.

Another important expression for noise thermometry, the Rice equation, describes the

statistical error associated with noise

measurements. Because noise is a randomly

fluctuating quantity, it must be time averaged

in order to measure it. The rms fractional

uncertainty in the measured temperature,

(AT)W, averaged over a time interval t is

given by:

_,T/T = (4Aft) "112 (4)

Obviously, longer integration times and

wider bandwidths will reduce the

unavoidable statistical error.

2.6.3 Discussion

An attractive feature of noise thermometry

is its ability to measure absolute temperature

without reference to a calibration table: the

calibration of the sensor is inherently linear

and the noise is effectively zero at absolute

zero. Compared with thermocouples, noise

thermometers are reportedly less sensitive to

shunting through the insulator and diffusion

from the sheath into the sensor. A general

analysis of the effect of insulator losses on a

noise thermometer was reported by Crovini

and Actis (Reference 2-64). They reported

that the shunting conductance from BeO

insulation around a 100 ohm sensor caused

less than 2 ° C error in a 1800 K temperature

measurement; alumina insulation caused

about four times as much error. Because

Johnson noise is a property common to all

materials, noise thermometers are able to

use a much wider selection of sensor

materials than are thermocouples. Brixy

(References 2-65 and 2-66) has investigated

doped lanthanum oxide sensors for noise

thermometry up to 1800 K and concluded

that they offer advantages over traditional

metallic resistors at high temperatures and in

harsh environments. First, Brixy noted that

ceramic resistors were more durable than

metallic resistors at high temperatures.

Second, their low temperature coefficient of

resistance facilitated matching the

impedence of the sensor with the transmission cable over a wide temperature range.

Noise thermometers should also be subject

to considerably less drift than thermocouples. As such, if a suitable thermocouple

is not available for a particular application,

noise thermometry is an attractive alternative.

One of the main reasons for the growth of

noise thermometry has been improvements

in signal processing that permit accurate

measurement of noise signals which are typically less than a microvolt at a few

nanoamperes. Because of the low signal

levels, more than one measurement is

generally required for accurate temperature

33
measurement. A variety of signal processing

methods have been used in noise thermometry (Reference 2-67). The cross correlation technique, introduced 20 years ago

by Brophy (Reference 2-68), simultaneously

measures noise voltage with two amplifiers.

Signals from the two amplifiers are then correlated so that the uncorrelated noise from

the amplifiers and signal cabling is

eliminated. Correlation techniques are,

however, probably too slow for general use

in the gas turbine environment. Brixy

(Reference 2-69) developed a four-lead system to eliminate the noise generated by

cables, while Klempt analyzed sources of uncertainty in cross correlating noise thermometers (Reference 2-70).

In recent years, Johnson noise power thermometry (JNF_) has received considerable

attention. As the name suggests, JNPT

involves determining the noise power, from

which temperature can be inferred using

equation (3c) above. In essence, JNPT

involves measuring both the open circuit

noise voltage and the closed circuit noise current using separate current and voltage sensitive amplifiers. It is, however, necessary

that the gains and signal bandwidths of the

amplifiers be known.

One of the earliest noise power thermometers was described by Borkowski and

Blalock (Reference 2-71) and was refined by

Blalock et al. (Reference 2-72). Their instrument was designed to measure temperatures

up to 1000 K and employed a 100 ohm

platinum resistor as the noise source. While

making two measurements complicates the

instrumentation, noise power measurement

allows an absolute temperature measurement that is independent of the resistance of

the sensor (see above). This is helpful in a

hostile environment in which the sensor

resistance may vary because of erosion

and/or oxidation. Unfortunately, the open

circuit voltage and short circuit current cannot be measured simultaneously on the same

sensor, so that cross correlation techniques

cannot be used to eliminate noise from the

amplifiers and transmission cables. The

noise power thermometer is, therefore,

inherently sensitive to external noise pick-up

and noise generated by connecting cables,

and probably requires a more "quiet"

environment than is generally available in

industrial applications.

Despite fundamental virtues, significant

practical obstacles must be overcome before

noise thermometry can become widely used

in industrial applications. The low signal

levels produced by sensors make the

problem of eliminating noise from nonthermal origins challenging. Spurious noise

signals in lead resistances and electronic signal processing components have seriously

hampered noise thermometry in practical

applications. A quantitative analysis of

design considerations of preamplifiers in

noise thermometry can be found in

Reference 2-71. Tasman (Reference 2-73)

noted that other researchers reported virtually eliminating noise generated from the

transmission line by using a cross correlation

technique. The noise voltage from one sensing resistor was sensed by two identical

amplifiers in two branch circuits. Because the

noise generated from one branch was not

correlated with that in the other branch, the

spurious noise output tended toward zero

with increasing integration time.

Like thermocouples, noise thermometers

are subject to errors caused by leaky insulation. For this reason alone, they probably

cannot greatly extend the temperature limits

achieved by thermocouples. Because noise

34
thermometry does not rely on specific

material characteristics that can change over

time, they retain their calibration better than

thermocouples when exposed to long operattag times. Further, a much wider range of

materials can be considered for noise thermometers than for thermocouples so that it

can probably be adapted better to hostile

environments.

2.6.4 Adaptability to Gas Turbine

Environment

Even if practical obstacles to noise thermometry are overcome, this technique will

have limited usefulness in combustor

environments. First, the requirement of

attaching the sensor to the specimen surface

will limit the hostile environments in which

noise thermometers can be applied. Second,

noise sensors probably do not lend themselves to thin film configurations except in

environments free of electromagnetic interference (EMI). The requirement for shielding the sensor from EMI is likely to make

noise thermometry a more intrusive technique than thin film thermocouples. Third,

noise thermometers are relatively slow

devices that require averaging in both space

and time. Typically, the noise signal integration periods exceed 10 seconds, although

larger signal bandwidths may reduce this

appreciably.

With impetus from both the nuclear industry

and the space program, noise thermometry is

an active phase of development. While

experts in the field believe that problems

associated with noise thermometry are not

insuperable, this technique will probably

require more sophistication of the user than

do traditional methods such as pyrometry or

thermocouple thermometry.

2.7 Concept Description and

Analysis - Fiber Optic

Temperature Sensors

Fiber optic devices for measurement of

temperature can be categorized according to

the specific sensed parameter. One type of

fiber optic sensor collects energy from a local

zone (usually the tip) and converts the

measured energy to temperature using techniques similar to those previously discussed

with regard to radiation pyrometers.

Another type of sensor uses the temperature

dependent propagation characteristics of the

fibers to sense temperature by means of

interferometric techniques. Neither type is

ideally suited for surface temperature

measurement because both require physical

attachment or insertion into a surface cavity

at the point of measurement.

2.7.1 Techniques Based on Variability

of Fiber Optic Properties

Numerous temperature measuring systems

based on interferometers consisting of fiber

optic legs have been proposed (References

2-74 through 2-75). In these devices,

temperature variations produce phase changes in the optical signal passing through the

fibers by varying the propagation constants

of the fibers. Although they are not ideally

suited for surface temperature measurements because they generally must be

attached to the surface, some of them could

be adapted for surface measurements.

Fiber optic interferometers can be classified

into two categories: single fiber and dual

fiber interferometers. Single fiber interferometers consist of one fiber propagating

two modes that exhibit different phase delays

in response to temperature changes. Dual

fiber interferometers (Reference 2-74) use

two fibers, each of which carries one mode.

35
One of the fibers is exposed to the test conditions while the other is kept at constant

temperature and acts as the reference. Single

fiber interferometers are inherently

advantageous for operation in hostile

environments for two reasons. First, they are

much simpler in construction. Second, since

both modes of the interferometer are subject

to identical conditions of pressure, stress,

and vibration, these devices offer greater

common mode rejection for environmental

factors (Reference 2-75). In essence, single

fiber interferometers respond only to forces

that affect the two modes differently.

Corke (Reference 2-76) demonstrated a

temperature sensor, the "polarimetric interferometer," based on the temperature

induced change in birefringence of a single

highly birefringent silica fiber. Temperature

changes affect the magnitude of internal

lateral stresses and, therefore, the fiber is

birefringent. It is expected that neither background blackbody radiation nor spurious

internally generated radiation significantly

affect the temperature sensor because these

sources are incoherent (Reference 2-76).

Corke claimed that his polarimetric

temperature sensor was suitable for use in

hostile environments such as turbine engine

hot sections. Since his experiments were

restricted to temperatures less than 500 K,

this prediction may be optimistic. According

to one of Corke's co-researchers, David

Jackson, it is questionable whether polarization preserving fibers can be produced to

work at combustor temperatures. Until a

material is found that exhibits the desired

birefringent properties at elevated temperatures, the polarimetric sensor does not warrant serious consideration for use in turbine

hot sections. It has been reported that

sapphire does exhibit birefringence at high

temperatures (Reference 2-77). Yttriaalumina-garnet (YAG) may exhibit desired

birefringence up to about 1650 K.

Dunphy and Meltz (Reference 2-78)

describe a twin core fiber optic sensor

wherein the variability in coupling properties

between the cores is temperature (and

strain) dependent. By exciting one core and

measuring phase shift between the cores at

two wavelengths, it was possible to extract

temperature and strain data at temperature

levels up to 650 K with a sensing length of

6mm.

It should not be concluded, however, that

material limitations are the only constraint

on polarimetric temperature sensors.

Challenging, although not necessarily

insuperable, problems with respect to signal

processing, the need for optical components

in the vicinity of the sensor to recover phase

information may limit the remote use of

these sensors. These difficulties, and

approaches to their resolution, are discussed

in Reference 2-79.

2.7.2 Collected Energy Techniques

This concept is based on measurement of

radiant energy transmitted from a sensing

zone (usually located near the tip of a fiber

optic device) to a detector (typically located

at the cold end). If the sensing zone of the

fiber is configured as a blackbody cavity, the

temperature level can be related to

measured energy by Planck's equation. Dils

(Reference 2-80) describes a system of this

type which uses an opaque coating applied on

the outer surface of a sapphire rod to form

the blackbody cavity (for measurement of gas

temperature). Such a configuration has

potential for use up to 2200 K, although

problems associated with leakage of radia36
tion through the fiber and coating durability

have not yet been fully resolved.

Surface temperature could be sensed by embedding the tip of the fiber optic device in a

surface cavity at the point of measurement.

We have demonstrated the (mechanical)

feasibility of attaching a gold clad quartz

fiber optic device on the surface of a metal

turbine blade using flame sprayed alumina.

This method appears to be satisfactory up to

the limit of the gold quartz fiber optic.

2.8 Concept Description and

Analysis - Ultrasonic

Thermometry

2.8.1 Concept Description

Ultrasonic therrnometry makes use of the

temperature dependence of the velocity of

sound in gases, liquids, or solids (Reference

2-81). The velocity with which sound

propagates through a particular material is a

function of the physical state of the material,

its density, and other physical parameters.

For solids, which support both compression

and shear, the velocity of sound is directly

proportional to the square root of a stiffness

modulus M divided by the density. The

velocity of sound through the solid depends

on the type of wave propagated as well as the

state of the material (Reference 2-82).

2.8.2 Resonant Frequency Technique

Several types of ultrasonic thermometers

have been developed. In 1972, Benjaminson

and Rowland (Reference 2-83) reported a

thermometer using changes in the resonant

frequency of a quartz crystal to measure

temperature. This device, which was

developed into a commercial product by the

Hewlett Packard Company, offered

extremely high temperature resolution and

excellent stability. Bell, working in England,

developed several practical high temperature sensors based on measuring the

resonant peaks of crystalline materials

(References 2-84 and 2-85). Using a sapphire

tuning-fork sensor, Bell was able to measure

temperatures over 2200 K, even in oxidizing

atmospheres. Although such techniques can

measure high temperatures with excellent

resolution and stability, they are limited in

their application by the required isolation of

the resonator from mechanical contact at all

points except nodal points. Therefore, they

are not considered candidates for field

measurements in hostile environments.

2.8.3 Pulse Echo Technique

Numerous ultrasonic thermometers have

been developed based on the measurement

of the propagation time of an ultrasonic wave

in a refractory wire sensor (References 2-82

and 2-86 through 2-89). Virtually all such systems operate in the pulse-echo mode, the

theory of which has been well documented

for nearly 20 years (References 2-90 and

2-91). In the pulse echo technique, a

magneto-strictive sensor introduces

ultrasonic pulses into a transmission line.

The sound pulses are partially reflected at

discontinuities in the wire and at the end of

the wire. Typically, the time delay between

echoes measures the ultrasonic propagation

time between well-defined points in the sensor. From the measurement of propagation

time between points a known distance apart,

a line average temperature over the length of

the sensor can be determined. Temperature

profiles along lengths of wire a meter or

longer also can be deduced from ultrasonic

measurements. Sensors are typically a few

37
centimeters long with a total response time

comparable to that of thermocouples.

In practical thermometers using the pulseecho technique, the wire sensor is situated in

aprotective sheath. Although the pulse-echo

method is generally less sensitive to

mechanical pertubations than the resonance

technique, mechanical contact between the

sensor and the sheath has been a problem in

ultrasonic thermometry. While light contact

pressure can generaUy be tolerated, high

pressures can disturb or even entirely block

the echo signal. Particularly at elevated

temperatures, the sensor may actually attach

to the sheath in a phenomenon known as

"sticking." Tasman (Reference 2-92) reviews

several methods by which the problems

associated with sticking can be addressed and

suggests that to a large degree the problem

has been successfully addressed in numerous

applications. Contact problems could be

more severe in the high vibration environments, however.

Although in principle sensors could be fabricated of virtually any material, practical sensors must survive their intended

environment and should have a large change

in sound velocity over the temperature range

of interest. Sensors for measuring high

temperatures have been made of rhenium

(Reference 2-93), thoriated tungsten

(Reference 2-94), and tungsten-rhenium

(Reference 2-95), among other materials.

This flexibility in terms of sensor materials

permits materials capable of withstanding

oxidizing environments to be used as sensors.

Unfortunately, the speed of sound generally

varies less with temperature in ceramics than

in metals. Ceramic sensors would, therefore,

probably be less sensitive than are metal sensors. Nevertheless, sapphire or ceramics are

candidate sensor materials for pulse-echo

ultrasonic thermometers operating in hostile

oxidizing environments up to 2200 IC

2.8.4 Adaptability to Ceramic

Materials

Applying ultrasonic thermometry to ceramic

materials would require attaching a sensing

element to the component surface so that the

sensor is thermally coupled and acoustically

isolated from the surface. Metallic materials

provide greater variation of acoustic velocity

as a function of temperature than typical

nonmetals, but may not be practical due to

thermal expansion mismatch or chemical

incompatibility with ceramic substrates.

Small diameter sapphire is a candidate for

high temperature applications.

In a review of ultrasonic thermometry,

Lynnworth (Reference 2-82) distinguishes

two broad categories of sensors. The first

category, of which the pulse-echo technique

is an example, inserts a sensor whose

ultrasonic velocity is to be measured into a

thermal enclosure. These methods require

attachment of the sensor to the specimen surface. The second of Lyrmworth's categories

used "the medium itself as the sensor." Successful development of an "intrinsic sensor"

could open wide-ranging applications for

ultrasonic thermometry. An intrinsic

ultrasonic thermometer could, in principle,

use a laser to acoustically excite the target

surface. It may also be possible to detect the

acoustic signal remotely. Promising studies

along these lines have been conducted by

Rockwell International and indicate potential for contaoaess measurement of surface

temperature in hostile, high temperature

environments. We have been unable to

obtain specific information about these

studies. Remote methods of generating

ultrasonic pulses and of measuring the echo

38
appear to have significant potential for use

on high temperature ceramic materials.

However, the necessary technology is not

well developed at this time.

Ultrasonic thermometry in general, and the

pulse-echo technique in particular, should

benefit from advances in instrumentation

that permit routine measurements of

nanosecond time intervals. Improved signal

processing instrumentation should allow use

of coherent detection schemes to demodulate coded pulse trains. Such methods could

reduce the expertise required of operating

personnel as well as allowing valid information to be derived from noisy signals. Given

the high temperatures (over 2200 K, according to reports summarized in Reference 2-

95) that pulse-echo techniques can measure

with errors less than 2%, ultrasonic thermometry must be regarded as a candidate for

high temperature measurements in hostile

environments.

2.9 Concept Description and

Analysis - Raman

Measurement Techniques

2.9.1 Concept Description

The Raman effect occurs when light

illuminates a material containing molecules

that can undergo a change in polarization as

they vibrate. When Raman scattering is

present, light is scattered not only at the frequency of the incident light, but also at frequencies corresponding to the sum and

difference of this frequency and the

molecular vibrational frequencies. Thus, the

Raman spectrum occurs as a series of discrete frequencies, shifted symmetrically

above and below the frequency of the exciting radiation. The spectral lines that are most

frequently studied are those on the low frequency side, the Stokes lines, which are more

intense than the corresponding anti-Stokes

lines on the high-frequency side.

Raman scattering techniques have been used

extensively to measure temperatures of combustion gases from turbine engines

(Reference 2-96). As far as could be determined from a literature survey, however,

little work has been done to apply Raman

spectroscopy to surface temperature

measurements.

2.9.2 Adaptability to Ceramic

Materials

It has been shown (Reference 2-97) that for

certain ceramic materials, such as silica, the

ratio of the intensities of the Stokes and antiStokes lines is a function of the temperature

of the surface from which the light is scattered. Therefore, the Stokes to anti-Stokes

intensity ratio may provide the basis of a surface temperature measurement.

In one important respect, ceramics are good

candidate materials for surface Raman

spectroscopy: because incident light

penetrates ceramic surfaces, signal intensity

may be sufficiently high to permit good

measurements. Numerous questions surround the use of Raman techniques in combustor environments, however. Whether or

not sufficient signal intensity will exist at

temperatures exceeding 1200 K is not yet

known. The answer may depend on the

material; wide bandgap materials such as

silicon carbide may have useful signal intensifies at combustor temperatures.

Even if laser induced Raman scattering will

produce sufficient signal intensity, numerous

questions need to be addressed. Pulse-topulse laser fluctuations, possible phonon

39
"hot-spots," and laser induced strain effects

may all significantly reduce measurement

accuracy (Reference 2-98). Experimental

data is needed to establish whether or not

Raman spectroscopy can be seriously considered for temperature measurements at

combustor temperatures and for which

specific materials it is feasible.

2.10 Concept Descrlptlon and

Analysis - Temperature

Sensltlve Coatlngs (for

Determlnatlon of Maxlmum

Temperature)

2.10.1 DescrlpUon

This type of temperature measurement is

accomplished by applying a temperature sensitive material to the surface by coating or

molecular diffusion. After exposing the part

to temperature, it is examined to detect

changes in the coating which can be related

to the maximum surface temperature to

which the part had been exposed.

Goodman (Reference 2-99) describes a

method for applying a surface layer of the

radioisotope Kr_ to the surface of a part.

Subsequent heating of the part causes

release of the material. After thermal soakingtcycling is completed, the part is heated

under controlled conditions. When the

temperature reaches the maximum operating level, additional material will be

released in measurable quantities.

A description of the characteristics of more

conventional temperature indicating paints

is covered in Reference 2-100. These

materials are applied as surface coatings and

undergo chemical reactions which cause

color changes at discrete temperature levels

up to 1623 IC

Kasanof and Kimmel (Reference 2-101)

describe a series of coatings which melt at

specific temperature levels, producing a

phase change which can be detected by visual

inspection following exposure. These

materials are available in smaller

temperature increments and are usually

easier to interpret than color change

coatings.

2.10.2 RecommenclaUons

Temperature sensitive coatings have a

limited usefulness because they provide an

indication of maximum temperature and

usually require removal or partial disassembly for visual access. They are included

herein because of usefulness where temperature profiles over surface areas are required.

Accuracy is limited by incremental change

levels which vary widely for the paints.

Kryptonates require careful processing

during application and controlled post test

calibration.

2.11 Surface Temperature

Sensing Concepts

Proposed for Development

2.11.1 S¢ieeUon C_erla

As outlined in the Introduction section of this

document, additional effort associated with

NAS3-25140 includes design, fabrication,

test, and evaluation of three surface

temperature sensing systems suitable for use

on high temperature ceramic materials. The

process of reviewing the literature and identifying which concepts to pursue has been

4O
complicated somewhat by the general nature

of the task. The selection process would have

been easier in many respects if the requirements had been more specific.

We have observed that measurement technology tends to become more specialized as

temperature levels increase. Substrate

material properties and compatibility with

sensor interfaces becomes more critical at

higher temperatures. A concept which works

well for one application may not work at all

for another, seemingly similar, one because

of a subtle difference.

Therefore, to a large extent, our choice of

which sensing concepts to continue into te

next phases of this contract are based on our

perception of future sensor requirements

and the specific ceramic materials which will

be used for coatings and components in

advanced gas turbine engines. These perceptions are based on historical measurement

needs related to hot section component

temperature requirements such as: rotating

and stationary airfoil cooling effectiveness;

hardware life expectancy; thermal transient

stress.

We have weighted our selection criteria in

favor of concepts which will be applicable to

near term measurement needs based on current ceramic coating and structural material

capabilities. Although the benefits will be

realized sooner, the upper temperature

limits will be reduced from the program

Work Statement goals.

The concepts we propose tO pursue include

the following:

Long Wavelength Radiation Pyrometer:

The use of this device is based on the fact

that alumina and zirconia ceramics have

much higher emittances at wavelengths

greater than 8 _rn than they do in the

visible region of the spectrum. This

allows a significant reduction in corrections for reflected radiation errors.

Ceramic or Intermetallic Thin Film

Thermocouples (SIC, MoSi2, WSi2):

These have high potential for operation

up to 1600" C, are compatible with

silicon based substrates and have high

thermoelectric output.

Platinum-Rhodium Alloy Thin Film

Thermocouples: These are compatible

with alumina ceramics. The relatively

low anticipated upper temperature limit

(1400"-1500" C) is balanced by the

mature technology associated with

application of these sensors.

2.11.2 Long Wavelength Radiation

Pyrometer

The thrust of effort in this area will be to

evaluate the effectiveness of long, narrow

band wavelength detectors (8 _m) in reducing reflected radiation errors from ceramic

materials which have low emittance at short

wavelengths (1 van). Such an evaluation will

include setting up an experiment with known

reflection component geometric effects and

testing alumina and/or zirconia target

materials in a combustiongas environment.

It is anticipated that this will be accomplished

using a commercially available instrument

which will not have the short time response

required for a rotating turbine blade

measurement. We will then compare

requirements with capabilities of existing

detector types and viewing optics to determine if a system with microsecond time

response is feasible.

41
The basic reason for using the long

wavelength approach as outlined in another

section of this report is to reduce the correction for reflected radiation to 10%-15% of

the total energy at the detector (from 85%-

90% at 1 _an).This makes the task of correcting for reflected energy much less critical

with respect to target surface temperature

accuracy. Negative factors related to long

wavelength pyrometers include lower energy

sensitivity, slower time response, and more

limited optical view path materials selection.

Comparing the impact of the negative factors

with the advantages of higher target surface

emittance will be part of the evaluation effort

devoted to this concept.

2.11.3 Ceramic or Intermetallic Thin

Film Thermocouples

This concept was selected because of its

applicability to silicon based substrates (such

as silicon nitride and silicon carbide) which

are not compatible with platinum at elevated

temperatures on the basis of chemical inertness or thermal expansion.

The materials of interest include silicon carbide and silicides of molybdenum, tantalum,

and tungsten. All have a similar oxidation

protection mechanism which is based on the

formation of a surface layer of Si02 which

reduces additional oxidation and should be

effective up to 1600 ° C.

Although the thermoelectric properties of

these materials have been known for many

years, relatively few applications have been

used and virtually all of these have included

large sensing elements made from bulk

material and are inappropriate for the

anticipated needs of this program. The technology for manufacturing thin film thermocouples of these materials is not well

developed and will require some attention in

this program, the goals of which are as

follows:

Select thermocouple element materials

based on available application techniques and anticipated probability of

success.

Fabricate sensors on silicon carbide

coupons with and without top coats for

oxidation protection.

Test coupons in ovens and high velocity

combustion gases to determine thermoelectric stability and upper temperature limit.

• Fabricate sensors.

• Calibrate and test in laboratory ovens.

Since the fabrication of ceramic thermocouples in a thin film configuration is not

a well developed technology, we anticipate

some problems in fabricating adherent films

and modifying them to establish thermoelectric properties. We do have the ability to

sputter silicon carbide (SIC) and MoSi2 but

do not have in-house capability to dope films

after application (films of pure SiC do not

have thermoelectric properties and must be

modified by application of impurities to be

used as thermocouple elements). Preliminary testing indicates chemical vapor deposition (CVD) forms of SiC are superior to

sputtered films. Doping of CVD SiC may be

less practical than applying the CVD in the

as doped form. A significant part of coupon

and sensor fabrication effort may be subcontracted to firms with specialized equipment and expertise in this area.

42
2.11.4 Platinum-Rhocllum Alloy Thin

Film Thermocouple

This concept was selected because we think

it can be used for near term applications on

alumina (or other relatively high expansion,

non-silicon based ceramic substrates) with

upper temperature limits between 1300 ° and

1650" C. The goals of this effort include the

following:

• Select an alloy for best life and thermoelectric stability.

Conduct coupon tests to determine

upper temperature limit and calibration

stability as a function of thickness in

oven tests and a combustion gas stream.

• Evaluate the effectiveness of top coats in

extending upper temperature limits.

• Fabricate sensors.

• Calibrate and test in laboratory ovens.

A great deal of work has gone into development of technology related to application of

sputtered platinum-rhodium alloy thermocouples. We plan to use that technology base

in this program. We have in-house facilities

suitable for sputtering and plan to use them

as much as possible.

43
3.0 Strain Measurement

3.1 Concept Description and

Analysis - Applied Sensors -

Nonoptical

3.1.1 Resistance Strain Gages

Resistance strain gages are currently the

primary sensing devices for gas turbine hot

section strain measurement. In Reference 3-

1, Perry and Lissner describe historical

development, theory, and practical applications of gages, including attachment techniques, excitation/readout circuits, and

interpretation of data. Because strain gages

respond to changes in temperature as well as

changes in strain of the surface to which they

are attached, quantitative interpretation of

gage output requires isolating the strain

effects from the temperature effects.

Dynamic strain measurement, which

involves measuring a time varying signal

whose time constant is short compared with

the thermal time constant of the substrate on

which the gage is mounted, is less temperature dependent than static strain measurements. Lead wire effects, thermal expansion

mismatch, and metallurgical transformations

are not usually significant for dynamic

measurements. Static strain is sensitive to all

these effects and is more difficult to measure

at elevated temperature.

3.1.1.1 Current CapablllUes

Nickel-based alloy wire filament grids,

applied to metal substrates with ceramic

cements or flame sprayed alumina (FSA),

are currently used to 1280 K for dynamic

measurements as described by Weise and

Foster (Reference 3-2). Static strain

measurements using FeCrAl grid elements

are discussed by Lemcoo (Reference 3-3) to

1366 K, Stetson (Reference 3-4) to 977 K,

and Wu et ai. (Reference 3-5) to 1073 K.

Static measurements using FeCrAI gage elements have significant accuracy problems

and are reported to be cooling rate and strain

level dependent (Reference 3-6). Hulse et al.

discuss development of a palladium-chrome

alloy for static measurement to 1250 K in

Reference 3-7. This alloy has a high tempera=

ture coefficient of resistance which makes

accurate strain measurement dependent on

precise temperature measurement.

The evaluation of a number of alternate gage

alloys for static strain measurement up to

1273 K is discussed by Brittain, Geslin, and

Lei in Reference 3-8. Both metallic and nonmetallic alloys, including B4C and SiC, have

been tested.

Unpublished work at GE and others regarding the use of platinum alloys is ongoing, but

does not look promising compared with

FeCrAI alloys. The upper temperature limit

for dynamic gages applied with ceramic

cements or FSA appears to be in the vicinity

of 1500 IC Limits are set by fatigue strength

and oxidation of the grid and adherence of

the bonding agent.

Due to the aerodynamic disturbance caused

by surface buildup of wire grids applied with

ceramic cements or FSA (typically 0.4 ram),

significant effort has been directed toward

the development of thin film strain gages

with insignificant thickness. Gages made

from nickel-chrome films on alumina

44
insulating films are currently used (at GE) up

to 920 K for dynamic strain measurements.

3.1.1.2 Potential Capabilities

Potential for extending upper temperature

limits for dynamic resistance gages beyond

1500 K is based on identifying sensing element materials with stable resistance characteristics and adequate fatigue strength at

high temperatures. The best candidates for

this application are refractory metals (such as

W-Re alloys) with an adherent, impervious

oxidation protective film, or ceramic elements (such as SIC). The current outlook is

less than promising for either approach.

At present, precision static strain measurement capability with reasonable long-term

stability is limited to approximately 920 K

using PtW alloy gages in an Inconel sheath as

described in Reference 3-9. Use of FeCrAI

gages for higher temperatures is possible at

great sacrifice in accuracy and stability.

Potential for extending service temperature

above 1073 K decreases exponentially as a

function of temperature.

3.1.2 Capacitance Strain Gages

3.1.2.1 Concept History and Current

Status

During the past 15 years, attached

capacitance strain gages have become widely used to measure static strain up to about

1100 K where long-term stability is a major

consideration. Most commercial gages are

based on designs from either the Boeing Co.

(marketed by Hitec Products, Ayer, Massachusetts) or by the Central Electricity

Research Laboratory and the G.V. Planer

Co. in the United Kingdom (this gage is

marketed by Gael Tech Ltd. of Isle of Skye,

U.K.). In the CERL gage, strain is related to

the separation between capacitor plates,

whereas in the Boeing gage strain is a function of the overlap between a pair of sliding

plates. The change in capacitance is

measured as a surface deforms, and the surface strain is related to this capacitance

change. Descriptions of each of these gages

may be found in Reference 3-10. Both gages

require attaching a relatively bulky sensor to

the test surface. The maximum working

temperature of these gages is about 1350 K.

Several problems have made using

capacitance strain gages at elevated

temperatures a difficult task. Stray

capacitance is a major source of spurious

readings for capacitance probes. Because

capacitance gages detect changes in

capacitance of less than 0.5 pf, stray

capacitance of less than 0.01 pf may induce

significant errors in displacement measurement. The capacitance plates, lead wire, and

cables are all subject to stray capacitance

which can interfere with the direct

capacitance between the plates. Three terminal capacitors, in which one plate is

shielded with a conductor, can be used with

capacitance bridges to minimize the effects

of stray capacitance. Techniques for measuring capacitance from strain gages in the

presence of stray and lead capacitance are

discussed in Reference 3-11. In addition, the

high temperature portion of the lead wire is

often a source of electrical noise and therefore should be kept as short as possible.

Both the Boeing-Hitec and CERL-Planer

gages use air as the dielectric between the

capacitance plates. A now abandoned effort

by the Hughes Aircraft Co. to develop high

temperature strain gages used solid

dielectrics such as mica, silica, and alumina

(Reference 3-12). The developer of the

Hughes gage believed that solid dielectrics

45
were more stable than air at temperatures

approaching 1400 K and offered better possibilities for attaining even higher temperatures [Gillette, O.L., Batelle Memorial

Insitute (Retired), personal communication].

3.1.2.2 Current Developments

Current developments in capacitance strain

gage are oriented toward developing miniature, low profile devices capable of

withstanding hostile environments. For

example, Hitec Products is developing a

rugged Boeing type capacitance gage, about

0.25 inch high, which is capable of withstanding turbulent environments up to about 1350

K (Wnuk, S., Hitec Products, Inc., personal

communication). Work on thin film versions

has been reported by G.V. Planer and United

Technologies. Unfortunately, thin film

capacitance gages, even if practical, will

probably involve a tradeoff with upper

temperature limit (the Planar program

envisions a useful upper temperature of less

than 1000 K (Reference 3-13). They will also

probably require shielding and so will be

more intrusive than, for example, thin film

thermocouples.

Although we have found no attached

capacitance gages capable of operating at

temperatures exceeding about 1400 K,

researchers at Southwest Research Institute

(SWRI) have developed a biaxial

capacitance extensometer to measure strains

on the interior of tubular elements

(Reference 3-14). The SWRI gage uses

extension rods to connect a capacitance sensor to the specimen surface. The relative displacement of the extension rods is measured

outside the hot zone with a standard

capacitance-type strain gage. Difficulty in

maintaining a stable temperature distribution along the extension rods limits the

accuracy of strain measurements. Although

the gage reported in Reference 3-14 was only

used up to 1200 K, SWRI has used a similar

extensometer to measure strains at temperatures up to about 1500 K. They are currently

working to extend the upper temperature

limit to over 1900 K and expect to

demonstrate the ability to measure strain

ranging from less than 50 microstrain to over

1% in the near future. It should be noted,

however, that the SWRI design is intended

for use in a laboratory; it almost certainly

does not have application in hostile environmerits.

3.1.2.3 Potential Capabilities

The capacitance strain gage appears to have

significant undeveloped potential. As a

laboratory device, it may lend itself to strain

measurements over a wide dynamic range at

temperatures up to 1900 K, albeit in

restricted experimental setups. The

capacitance gage may be useful in a combustor environment up to 1400 I4. The use of

solid dielectrics probably warrants further

investigation.

3.2 Concept Description and

Analysis - Optical

Discontinuity Methods

Several strain measurement techniques have

been developed based on tracking the movement of light reflected from or emitted by

materials attached to the specimen surface.

Although the fact of attaching a marker to

the sample is a significant restriction, particularly in an engine environment, these

methods are generally simpler, both in terms

of the equipment required and in terms of

the data interpretation, than are holographic

or speckle based methods.

46
3.2.1 Marion Method (Attached

Targets - Optical Follower)

Robert Marion, then of Sandia National

Laboratory, developed a method of measuring strain by optically tracking small targets

attached to the specimen (Reference 3-15).

Marion used an argon ion laser to irradiate

ceramic targets whose reflectance differed

from the specimen and tracked their position

with a digital line scan camera in which the

field of view was imaged by a lens onto a

photodiode array. The photodiode array was

then scanned electronically to produce an

analog pulse train in which the amplitude of

each pulse was proportional to the light

intensity on the corresponding diode. The

pulses before or after a dark-to-light transition on the specimen surface were electronically counted to determine the position of a

target.

The system built by Marion and his

coworkers was used for a project that has

been abandoned, and the equipment is no

longer intact. The system demonstrated a

very large dynamic range; it was capable of

measuring strains from less than 50

microstrain to several percent, from room

temperature to over 2800 K. Although a

complete analysis of the strain measurement

capabilities seems not to have been performed, the method demonstrated the

capability of measuring strains with an

accuracy of about __.4 microstrain with a gage

length of as small as 2.5 mm.

The system as developed by Marion used thin

film tantalum carbide (TaC) targets as well

as nodules of graphite cement. The thin film

targets did not provide sufficient contrast for

routine use, although film targets were not

thoroughly investigated. Target survivability

and contrast with the substrate can be

expected to be a major constraint on the

Marion method. Neverthless, we believe that

film targets can be developed that should

adhere to a wide variety of ceramic substrates. The techniques of plasma spray and

physical vapor deposition that have been

used successfully in attaching thermal barrier

coatings should be useful in applying targets

for strain measurement by Marion's technique. Targets of thermographic phosphors

could also be used if contrast were a limiting

factor.

We believe that Marion's method of measuring strain has considerable potential as a

laboratory tool. It can be used to measure

static strain or dynamic strain with a frequency response limited only by scan rate of the

photodiode array. The conceptual simplicity

of the method and the relative ease of data

interpretation make it a good candidate for

routine use.

3.2.2 The Sharpe Method (Reflection

Interference from Target

Indentations)

The Sharpe method uses a laser based interferometer to measure in-plane displacemerits and strains at elevated temperatures

over very short gage lengths (100 _.m)

(References 3-16 and 3-17). The method

involves pressing two small diamond-shaped

indentations into the specimen and

illuminating them with a laser. The indentations act as sources of reflected

monochromatic light from a coherent source

and so form Young's fringes. This fringe pattern is monitored to measure the relative

motion between the indentations. Sharpe

claims the method can be used to measure

strains with an accuracy of ---3% _.* 150

microstrain, so that a measured strain of

1.5% would have an uncertainty of __ 600

microstrain.

47
As with any interferometric method, rigid

body motions and rotations may cause

serious errors in strain measurements using

this method. For example, rigid body motion

normal to the surface introduces spurious

fringes that appear identical to strain

induced fringes. Stability and proper alignment of test rigs is therefore important to

achieve the quoted accuracy of the Sharpe

method. Furthermore, although the matter

appears not to have been seriously studied,

we would expect this technique to be sensitive to convective currents which introduce

phase changes in the fringe pattern. Nevertheless, Sharpe has reported using the

method successfully up to temperatures of

1000 K. The upper temperature limit at

which the method can be used is not yet

established.

The Sharpe method is a useful laboratory

measurement device capable of measuring

strain in real time. Significant improvements

as suggested by Sharpe (Reference 3-18) may

be attainable with more development of the

method. It is highly urdikely, however, that

the method can be used in a combustor

environment or if indentations used to

generate the reflected interference fringes

would be acceptable for many ceramic

materials.

3.2.3 Simpson-Welch Method

(Applied Pattern Duty Cycle

Variation)

Recently, Simpson and Welch (Reference 3-

19) introduced a method of measuring radial

strain of a rotating disk. The Simpson-Welch

method involves photoetching a pattern of

light and dark regions on the disk and

monitoring changes in the pattern of light

and dark regions with a stationary optical

probe. For best results, the probe would

include a laser coupled with appropriate

optics to focus an intense small diameter spot

on the pattern.

As the disk rotates, the output of the optical

probe fluctuates high/low, depending on

whether or not the probe is positioned over

the light or dark area of the disk. Deformation caused by centrifugal forces results in a

change in the width of the pulses, or the duty

cycle, from the optical probe. Simpson and

Welch selected a spiral shaped pattern so

that the duty cycle sensed by the probe was a

linear function of the radial position of the

pattern relative to the probe. In this way, the

change in the observed duty cycle provided a

direct measure of the total radial expansion

of the disk up to the radius of the point being

measured. Simpson and Welch established

that their method was capable of measuring

strains as low as 1000 microstrain; optimizing

their system should allow good measurements at less than 500 microstrain. The

method should be capable of measuring percent level strains without difficulty.

A complete error analysis for the effect of

rigid body motion on surface strain measurements by the Simpson-Welch method has not

been performed. Translation of the entire

disk along the axis of rotation should have no

effect on a system using a collimated light

source. Bending of the disk would affect the

detector output, since the projected image of

the pattern would be distorted with the outer

edges of the pattern appearing closer to the

pattern center. Surface tilt would produce a

selective foreshortening of the pattern in the

direction of the tilt. A correction for this

error could be made if the degree of surface

tilt were known.

Testing of the Simpson-Welch method has

only been performed on room-temperature

disks at low pressure. The method should,

48
however, be useful at pressures higher than

ambient because it is less sensitive to turbulence and fluctuations in refractive index

in the sight path than are methods based on

optical interference. Applying the method to

high temperature environments would

require another method of applying the pattern to the test part. Simpson and Welch

applied a photographic emulsion to the test

surface that would not hold up to high

temperatures. One intriguing approach

would be to substitute a pattern of thermographic phosphors; fluorescent radiation

from laser excited phosphors would then

substitute for reflected radiation. Thin films

of materials with different reflectance than

the substrate material could also be used (see

the Marion technique above).

It should also be possible to extend the

Simpson-Welch method to the measurement

of strain in a prespedfied direction on a stationary surface by using a scanning excitation

system. To our knowledge, there have been

no experimental investigations using this

approach.

In conclusion, the Simpson-Welch method of

measuring strain could be employed for high

temperature strain measurement and large

strain levels under a rather restricted set of

conditions: measurement of radial strain of

rotating disks. This could be valuable for

laboratory evaluation of materials. Extending the approach to more general applications would require extensive modifications,

to the extent that it may not be a practical

general strain measurement techhnique.

3.3 Concept Description and

Analysls - Surface Pattern

Dlstortlon Methods

3.3.1 Moire Methods

3.3.1.1 Introduction

Moire fringe patterns are formed when two

gratings with slightly different pitch or orientation are superimposed. To obtain displacements or strains from moire fringes, it is

necessary to know the relationship between

two gratings (a specimen grating attached to

the test surface and a fixed reference grating), the recording system, and the object

under study. Moire methods can be divided

into three general categories:

Intrinsic, or in-plane moire, methods can

be used to determine the displacement

field in a given plane. It is a powerful

general technique for investigating a

wide variety of problems in solid

mechanics. In-plane moire methods

have been used in laboratory studies of

fracture mechanics (Reference 3-20),

thermal stress studies (Reference 3-21),

dynamic problems (Reference 3-22),

vibration studies (Reference 3-23), and

strain analysis (Reference 3-24).

Intrinsic moire methods use a specimen

grating attached directly to the specimen

surface and a reference grating remote

from the surface. The specimen and

reference gratings normally are superimposed by imaging one onto the other

using a lens. The specimen grating follows the deformations of the surface

underneath, while the reference grating

is rigid and so has an invariant pitch and

spatial orientation. Although numerous

techniques have been developed for

attaching the specimen grating to the

49
surface, specimen grating survivability

has been described as the main impediment to the use of intrinsic moire

methods in hostile environments

(Reference 3-25).

Moire techniques traditionally used

noncoherent sources of illumination and

were limitedtogratingsof40 lines/mm,

although higher frequency gratingscan

be used to increasesensitivity.Diffractioneffectsmake finegratings(over 100

linesper mm) difficultto use, although

much higher frequency gratings have

been used in low temperature applications (References 3-26 and 3-27).

Coherent illumination is used to

generate more fringesfrom high density

gratings.Moire methods usingcoherent

radiation suffer from poor fringe

visibilitywhen the specimen surface

radiates at the wavelength of the

illumination.

Shadow or projection moire methods

can be used to measure out-of-plane

deformation and shape of an object

(Reference 3-28). Shadow moire

methods do not use physical gratings

attached to the specimen surface;

instead the specimen grating is formed

as the shadow of the reference grating is

cast on the specimen surface. The

specimen gratingisdistorted by out-ofplane movement of the surface and so

forms moire fringes with the reference

grating. Reference 3-29 discusses

various methods of casting the shadow

and viewing the moire fringes. Because

the specimen grating is formed optically, shadow moire methods are not

considered good candidates for use in

hostile environments.

Reflection moire is applied to specular

surfaces in which a virtual image of the

reference grating serves as the specimen

grating. Reflection moire methods have

been used primarily to determine slope

and curvature, not for strain measurement.

Moire methods generally have the advantage

of being f3dl field methods of strain deterruination. With moire, it is possible to obtain

three displacement components simultaneously and so to determine the complete

strain tensor of the specimen surface

(Reference 3-25). Using fringe multiplication techniques, the sensitivity of moire can

approach that of classical interferometry

with a resolution of less than 10 microstrain.

3.3.1.2 Theory

Moire fringes are the interference pattern

created when two gratings with slightly different pitch or orientation are superimposed.

The relationship between the fringe pattern

and the displacement field has been

reviewed by Theocaris (Reference 3-30) and

Durelli and Parks (Reference 3-31).

To measure in-plane deformation, a physical

grating is usually applied to the specimen surface. When stressed, this specimen grating

deforms and optically interferes with the

undeformed reference grating, causing

"moire fringes" to appear. Each fringe corresponds to the locus of points of equal displacement in a direction normal to that of the

reference ruling. Assuming there is no rigid

body displacement or rotation between the

two gratings, the displacement u in a direction normal to the grid lines of the associated

grating is given by:

u = (5)

5O
where N is the fringe order and p is the pitch

of the deformed grating. If strain information

is desired, the displacement must be differentiated either numerically or graphically

to calculate strain. For sufficiently small displacements, the strain fields are given by:

ey= OU/OX

5,= 0u/0y (6)

_¢ = lf2(0u/oy + au/ox)

where _y and ex are the components of strain

in the x and y directions, _ is the shearing

strain, and u,v are the displacements in the x

and y directions, respectively. Various

methods of differentiating moire fringe patterns are discussed in Reference 3-32. At

best, the process of differentiation is tedious;

at worst, it is a source of significant error. It

should be noted that moire is not limited to

analysis of small deformations. For finite displacements, the strain-displacement relation

includes higher order displacement derivatives than are present in equation (6) above,

and the calculations are correspondingly

more complex.

3.3.1.3 Discussion

The need to apply a grating to the specimen

surface severely limits the use of moire

methods in high temperature environments.

In many field situations, practical considerations preclude applying a grid to the test surface. Even in laboratory situations, grating

survivability is a major impediment to the use

of moire methods in high temperature

studies (Reference 3-25). Cloud (References 3-21 and 3-25) used a ceramic paint as an

adhesive for a nickel mesh grating of 40 lines

per millimeter to produce a grid capable of

surviving up to 1650 K on metal superalloys.

He believes that continued development of

gratings should improve their durability

(Cloud, G., Michigan State University, personal communication).

Unfortunately, rigid body rotations between

specimen and master gratings may produce

fringes independent of strain. It is important

to avoid or account for this movement when

using moire techniques to measure strain.

Even in the absence of rigid body motions,

moire has troubling limitations as a method

of measuring strain. In the case of uniaxial

stress, the distance between successive moire

fringes becomes the "effective gage length"

and the pitch of the grating determines the

resolution of the measurement. Since small

deformations produce few fringes with wide

spaces between them, large errors can result

from differentiating these displacement fringes to determine strain. Moire has, therefore,

been used primarily to measure relatively

large deformation strains.

Considerable effort has been made to

increase the sensitivity of moire techniques.

One solution is to use a specimen grating

with a higher pitch, even though increasing

pitch generally reduces the fringe contrast.

Bowles et al. formed a replica specimen grating of approximately 600 lines per mm

(Reference 3-33), although these gratings

are for low temperature use only. It is unlikely that replica gratings suitable for high

temperature moire can be increased by more

than a factor of two or three from the traditional 40 lines per millimeter.

The sensitivity of moire analysis can be

increased by more than an order of magnitude by techniques known as fringe multiplication. Fringe multiplication occurs when

the reference grating pitch is an integral multiple of the specimen grating pitch, so that the

number of moire fringes is determined by the

51
higher pitch. Twenty years ago, Post formulated three techniques of fringe multiplication (References 3-34 and 3-35). These

methods used a relatively coarse specimen

grating coupled with a fine reference grating

to effectively multiply the number of fringes

by as much as a factor of 20. Unfortunately,

the techniques used by Post require methods

of applying and recording the gratings that

are not well suited for high temperature

applications.

Several years ago, Post introduced a techn/que he named "moire interferometry" that

uses gratings formed by the interference of

two coherent beams (References 3=26 and 3=

27). Post refers to gratings generated by op =

tical interference as "virtual" gratings and

has produced them with 2400 lines/ram or

more. By applying a replica of a photographic

image of the interference of coherent beams

to a specimen surface, very high frequency

(2000 lines/mm) real specimen gratings can

be achieved. Because thermal currents

severely reduce the fringe visibility of virtual

reference gratings, and because of temperature limitations of the specimen gratings, this

concept is not well suited to high temperature, ambient pressure environments. It may,

however, be possible to generate good

quality virtual gratings in a high-temperature

vacuum, although moire interferometry has

not been attempted under such conditions.

3.3.2 Optical Filtering Methods

Several researchers have proposed optical

filtering techniques in conjunction with

moire to accomplish fringe multiplication

and reduce noise (References 3-36 through

3-38). Optical filtering also can be applied to

the diffraction pattern from a specimen grating in the absence of a reference grating.

These methods may be considered

52

extensions of moire, even though they do not

use a reference grating.

In 1971, Boone made one of the first

attempts to use spatial filtering of the diffraction pattern generated by a grating applied to

a specimen (Reference 3-39) to measure

strain. The image that was reconstructed

from the filtered pattern was composed of

light and dark areas in which the boundary

between these areas represented the locus of

points of equal strain. Boone also showed

how the magnitude of the strain could be calculated by translating a suitable mask used as

a filter. He cited several advantages of his

technique as compared to classical moire.

First, the absence of a reference grating

eliminated problems associated with grating

mismatch. Second, the method gave information about the strain field in the form of

equal lines of strain and, therefore, avoided

the error-prone process of differentiating

displacement data. Third, for a given grating

pitch, the sensitivity of the method exceeded

that of classical moire. In addition, the sign

of the deformation was determined. Boone

applied his technique to measuring percent

level strain of low temperature steel and

appears not to have considered its use at

elevated temperature. His technique was not

suited to real time measurement as it relied

on a photographic process to record the

image of the grating.

Recently, Burger and his coworkers at Texas

A&M University have developed a method

of measuring strain, dubbed the Spectral

Density Indication (SDI) method, that is

based on evaluating the Fourier transform of

a coarse grid (or any quasi-regular feature)

on a specimen surface (Reference 3-40). In

the SDI method, the spatial frequency of the

grid is measured at both unstressed and

stressed conditions and changes in the
frequency spectrum of the grid are then

related to strain. The transform of the grid is

made on-line from a digitized image

recorded by a charge coupled device camera

so that the method is capable of calculating

fuU-fleld strain in real time. Although the

SDI method measures displacement of the

grid pattern, the calculations used to determine strain are less prone to error than is the

process of differentiating fringes.

The SDI method reportedly is significantly

more sensitive than moire methods for a

given specimen grating pitch: Burger et al.

used a grid of 8 lines/mm to achieve a resolution of 200 microstrain (Reference 3-40).

The use of coarse grids is a major advantage

since such grids can be made more durable

than the fine grids used in moire interferometry. The inventors suggest that SDI

can be used for accurate strain measurements from about 10 microstrain to several

percent (Reference 3-41). Because the SDI

method has not been tested at elevated

temperatures, it is not possible to fully assess

the usefulness of SDI for measuring strain of

high temperature ceramics. The team at

Texas A&M plans to investigate high

temperature applications in the near future.

3.3.3 Conclusions

Moire methods of measuring strain have

exerted a strong appeal for over a quarter

century. The inherent advantages of moire

include an essentially noncontacting

capability and a wide dynamic range. Moire

techniques are an established means of

measuring percent level strains at ambient

temperature. They are particularly useful

where qualitative measures of strain are

desired. In recent years, Post and others have

extended in-plane moire to the high sensitivity domain in laboratory studies. Despite

predictions that moire would find its way

onto the factory floor, widespread use of

moire outside the laboratory remains

elusive. Difficulties associated with maintalning the relative spatial orientation of

specimen and reference grids and in differentiating moire fringes to calculate strain

are challenging hurdles for which no simple,

generally applicable solution is available.

Although researchers in the field remain

optimistic that moire can be applied to high

temperature strain measurements, there has

been little progress to date in measuring

strains at temperatures exceeding 1300 K.

This lack of progress thus far is partly a

reflection of the lack of concerted effort to

apply moire to such difficult environments.

Techniques analogous to moire, but in which

the reference grating is eliminated, may

provide a more promising avenue for high

temperature strain measurement. One such

technique is the Spectral Density Indication

method, which we believe should be

evaluated for high temperature applications.

3.4 Concept Description and

Analysis - Holographic

Methods

3.4.1 Holographic Interferometry

3.4.1.1 Concept Description and

History

Holography is a two-stage imaging process

for recording and wavefront reconstruction.

The illumination is usually by a

monochromatic laser light source. The

photographic record of the interference pattern of the reference beam modulated by the

object beam of the object surface is called a

hologram. The process of recording and

reconstruction of the resulting images can be

analytically described.

53
In holography, the illumination from the

laser is divided into two beams, one to

illuminate the object and the other for use as

a reference beam. The scattered object wave

is optically superimposed with the reference

beam and recorded on the photographic

plate. The recorded image can be

reconstructed by illuminating the processed

and exactly repositioned hologram with a

reference beam identical to the original

reference beam.

Holograms can be classified according to the

way of forming the object and reference

waves or according to the type of recording

procedure (Reference 3-42). Based on the

position of the object, a hologram can be

classified as a Fresnel, Fraunhofer, or Image

Plane hologram. A Fresnel hologram is one

in which the hologram is recorded in the near

field diffraction region. A Fraunhofer

hologram is recorded when the object is

located at the focus of the imaging lens. An

image plane hologram results when the

object is in the plane of the hologram or is

focused onto it (Reference 3-43). The resulting phase and distribution of the hologram

are identical with that of the object.

Superposition of two holograms of an object

in different strain states or the hologram of

the object in initial strain state with the scattered object wave in a second state in real

time, results in an interference pattern. This

technique is known as Holographic Interferometry (HI). The resulting interference

pattern is the mapping of the displacements

of the object surface and is due to the changes in phase of points on the object surface

as the surface is displaced. There is, therefore, a one-to-one correspondence between

the same object point under the different

strain states. Interference from noncoincident points does not occur, as the diffracted

54

waves from these points exhibit random

phase differences.

One of the most significant developments in

holography has been the application of HI to

the analysis of surface deformation.

Developed in the 1960's, HI attracted considerable interest because, unlike classical

interferometry, it could be used to analyze

optically rough surfaces. Holographic interferometry has been extensively reviewed in

several publications (References 3-41, 3-44,

3-45, and 3-46).

Holographic interferometry has been Used

for two decades to measure small static and

dynamic strains (Reference 3-47 and 3-48).

For static strain measurement, the interference pattern is recorded at particular

instants of time, while for dynamic strain,

recording is averaged out over a period of

time.

Based on the method of recording, HI can be

categorized as either a double exposure or a

real time method. In the double exposure

method, different strain states of the object

surface are illuminated by a pulsed laser and

are recorded on the same plate so that upon

reconstruction the interference pattern can

be studied. In real time HI, a hologram of the

initial state of the surface is compared with

the stressed state in real time. The two techniques are similar in terms of experimental

setup, although the double exposure method

freezes the interference pattern and therefore results in a better image. Double

exposure HI can be used to measure relatively large periodic displacements [on the order

of 1000 times the wavelength of the light

source (Reference 3.44)]. However, the real

time technique has other advantages, such as

relative ease of compensation for rigid body

movements and its ability to lend itself to
television imaging and electronic data

processing.

Power and Stetson conducted one of the earliest studies of vibrating objects using

holographic interferometry (Reference 3-

49). They proposed recording a multiple-exposed hologram with a single reference

beam. The object beam consisted of a set of

scattered object waves corresponding to the

successive positions of the vibrating object.

On reconstruction, the hologram will simultaneously reconstruct the superposition of all

the recorded object waves, each properly

weighted by the fractional duration or time

average of the total exposure time of the object field. Because the object is allowed to

vibrate during exposure, this method is an example of Time-Averaged Holography. The

resulting image displays the contours of the

vibrational mode shapes and amplitudes.

The Powell-Stetson technique has several

significant shortcomings that limit its application. Because the brightness of the fringes decreases with increased amplitude, it is

difficult to analyze surfaces whose amplitude

of vibrations exceeds several times the

wavelength of the scattered radiation. Furthermore, it is limited to studying nodal

vibrations only. Last, because displacements

are averaged over a vibration cycle, it is not

possible to obtain information on the relative

phases of vibrations of different points on the

surface.

Improvements and applications of the

PoweR-Stetson method have been reported

by several workers (Reference 3-42). In the

study of vibrations, these methods allow

determination of the vibrational modes and

amplitudes over a frequency spectrum.

3.4.1.2 Limitations

Sensitivity to rigid body motions have

generally restricted the use of holographic

methods to experimental setups in which the

test object is interferometrically stable. It is

necessary to ensure that translation or rotation does not occur between the exposures of

the hologram. Heflinger, et al. (Reference 3-

50) showed that the requirement for rigid

body stability could be relaxed somewhat in

HI with an optical configuration termed

coincident illumination and viewing (CIV) in

which the paths of illumination and viewing

are coincident. This configuration is reportedly capable of obtaining fringes in the

presence of rigid body motions a few times

the wavelength of the reflected light.

Although HI is in principle a very sensitive

technique for the measurement of surface

displacement and strain analysis, in practice,

making quantitative measurements with it

can be very demanding. In addition to the

requirements for specimen stability, HI is

susceptible to interference from changes in

the optical path length between exposures.

Although holography has been successfully

performed on surfaces significantly hotter

than 1250 K (Reference 3-51), HI is not well

suited for high temperature measurements

in air where thermal gradients can introduce

spurious fringes that can in turn introduce

large errors in displacement measurement. It

probably can be used effectively when the hot

specimen is in a vacuum, although very little

use has been made of HI in measuring

material properties at high temperature. In

addition, HI is much more demanding of the

user than is real time "TV holography,"

which is discussed below (Electronic Speckle

Pattern Interferometry). Although the use of

high resolution photographic plates enables

HI to have greater resolution to HI than live

recording methods, this factor will rarely dic55
tare the use of double pulsed or real time HI

when other real time methods are available.

In our opinion, speckle techniques (which

are discussed below) are better candidates

for routine displacement and strain measurement than is HI.

3.4.2 Electronic Speckle Pattern

Intellerometry

3.4.2.1 Concept Description and

History

Electronic Speckle Pattern Interferometry

(ESPI) is a technique that allows speckle pattern correlation fringes to be observed in real

time. It was developed independently about

15 years ago by three groups. The first paper

was published by Butters and Leendertz at

Loughborough University in England

(Reference 3-52); this group is credited with

most of the early development in ESPI.

Macovski et al. from the U.S. described their

work with TV holography shortly after

Butters' pioneering paper (Reference 3-53),

while an early patent on the ESPI design was

filed by Schwomma in Austria (Reference 3-

54). The principles of ESPI have been amply

discussed in a monograph by Jones and

Wykes (Reference 3-55) and will not be

detailed here. ESPI fringe patterns have

been studied theoretically and have been

shown to be similar to those obtained by

holographic interferometry (Reference 3-

56).

ESPI can be considered as an image plane

holographic system with an in-line reference

wave in which the recording and reconstruction steps are performed by video recording

and electronic processing. The in-line configuration is necessary to allow the television

camera to record the pattern resulting from

the interference of the object and reference

waves. Instead of recording a hologram, the

initial state is recorded in a video storage

device and compared in real time to a

deformed state. The two images are

electronically subtracted to produce the

fringe pattern. The ESPI system uses a closed

circuit television in which the target of the

vidicon tube of the TV camera replaces the

photographic plate used in holography. In

contrast to traditional holographic systems,

ESPI involves imaging the test surface onto

the detector plane. Figure 10depicts a simple

ESPI system. The light from a laser is divided

into two beams by a beam splitter. One beam

is expanded to illuminate the test surface

while a lens images this surface on a

television camera via mirror M1. The

reference wavefront is mixed with the image

wavefront to produce an interferogram

whose intensity is converted into a corresponding video signal by the photoelectric

action of the camera. The resulting video signal is then high pass filtered and full wave

rectified in a process that is analagous to

wavefront reconstruction in holography

(Reference 3-57). The processed video signal is then converted to a reconstructed

speckled image of the test surface on the

television monitor.

It is necessary in ESPI that the optical system

be interferometrically stable during image

scanning. This imposes a stringent stability

requirement when continuous wave lasers

are used as light sources. Jones and Wykes

(Reference 3-58) analyzed speckle interferometry theoretically and determined that

speckle correlation would not be maintained

if rigid body displacements exceeded about

100 microns or out-of-plane rotations

exceeded 0.001 radian between exposures.

The exact values are a function of the focal

length of the lens and the particular optical

configuration employed. However, use of a

pulsed laser in which the illumination is

56
Laser

!

Beam

Splitter

/,_ _ Beam

Reference _"x::;;_'e-_u_n_i_n

Beam

L

'i

I

I

i,.ol

k,

Dig_

Video

Frame

Store

ImageSubtraction

Video StoreTape I

Figure 10. ESPI OpticalSchematic.

delivered in a very short time enables interference hinges to be observed on relatively

unstable structures. For example, Cookson

et al. (Reference 3-59) used a Q-switched

ruby laser to obtain ESPI fringes from an

object moving at 2 m/s. Alternatively, a high

speed, low light camera such as a Charge

Injection Device (CID) camera can be used

to obtain exposures over very short time

intervals. In any case, the movement between

any two exposures must be sufficiently small

that decorrelation of speckle fringes does not

result.

Although ESPI methods lend themselves

most readily to analysis of periodic vibrations, they can be used to measure both inplane and out-of-plane displacements and

strains. The configuration illustrated in

Figure 15 was intended to detect out-ofplane movements. For in-plane measurements, the object is illuminated

symmetrically by two plane waves so that no

reference wave is used. In-plane measurements of displacements in a pre-specified x

direction are, in principle, iusensitive to

interferences from displacements in the

orthogonal y and z directions as long as the

following condidous are met: (1) the wave

fronts are plane, (2) the surface is fiat, and

(3) the angles of incidence of the two beams

are equal and opposite (Reference 3-60).

Denby and Leendertz (Reference 3-61)

applied ESPI techniques to in-plane strain

measurements at room temperature and

57
found them most useful in determining strain

distributions. In comparing strain measurements made with strain gages with those

made using ESPI, they found that the two

methods agreed within about 5% at about

150 microstrain. Although Denby and

Leendertz only examined conditions in

which the direction of the principal stresses

were known, they propo_d an ESPI method

using two sets of illuminating beams in which

all the principal strains could be determined.

One technique that could extend the

capabilities of ESPI involves the use of shearing interferometry (References 3-62 and 3-

63). Shearing interferometry is a speckle

interferometric method that is sensitive to

derivatives of surface displacements instead

of the displacements themselves that ESPI

and other speckle interferometric techniques typically measure. A major advantage of

shearing methods is that it relaxes significantly the requirements for vibration

isolation because rigid body translations do

not change the displacement gradient

(Reference 3-63). Its disadvantages include

the requirement for higher levels of illumination and the fact that fringe interpretation is

not as straightforward. By coupling shearing

interferometry with ESPI recording and data

processing methods, it may be possible to

evaluate strains in the presence of random

vibrations, although we know of no published

data to confirm this.

Another way in which ESPI methods could

be enhanced for some applications would be

to replace the analog image processing used

by the television camera with digital image

processing. Digital image processing, in

which the intensity of the image is convened

into digital form based on the grey level of

the illumination, has several advantages over

analog processing. Automation of fringe

58

analysis can be obtained using digital imaging technique (References 3-64 and 3-65). In

digital techniques, the speckle size does not

govern the image resolution, so better fringe

visibility and accuracy can be expected. With

the image already in digital form, further

processing such as addition, subtraction, and

averaging of images can be performed with

relative ease. Use of low fight level, fast time

response cameras should allow a large reduction in the exposure time and a concomitant

reduction in the time interval between

exposures.

3.4.2.2 Applicability to High

Temperature Applications

There has been very little effort made to use

ESPI for high temperature strain measurements. This is probably due in part to the difficulty of recording good speckle patterns

through air which is disturbedby thermal turbulence. When air turbulence is sufficiently

severe, complete decorrelation of the

speckle patterns can result. ESPI strain

measurements would be much simpler if the

test piece were in a vacuum environment. It

has been established, however, that ESPI

techniques can be used to observe and

measure deformation and crack growth of

very hot objects in air. For example, Lokberg

et ai. (Reference 3-66) observed fringes from

deformation of Kanthal specimens heated by

resistance heating to about 1800 K and suggested that, with better f'fltering of background radiation, ESPI techniques should be

useful at temperatures exceeding 2200 K.

Nevertheless, strain determination in the

presence of convective currents can be

expected to be a difficult challenge for ESPI

or any purely optical technique.

In addition to the problem posed by thermal

gradients in the sight path, speckle interferometry at temperatures above 1300 K
faces difficult challenges. Oxidation, contamination, and other changes in the surface

microstrncture during the measurement will

result in decreasing fringe contrast. Changes

in the surface structure greater than about

1/8 wavelength of the illuminating light can

be expected to cause difficulties for ESPI

measurements. Additional difficulties may

be encountered from multiple internal

reflections from translucent materials such

as ceramics which depolarize the reflected

radiation and reduce the visibility of ESPI

fringes. Good quality ESPI fringes on

ceramics have been obtained at room

temperature, however (Babb, J., Ealing

Electro-Optics, personal communication).

Further experimental investigations are

needed to evaluate the usefulness of ESPI

techniques as a measurement tool at high

temperatures. High temperature tests of a

commercial ESPI system are in the planning

stage (Maddux, E., WFAPL, personal communication). Initial high temperature testing

will concentrate on the more tractable

problem of vibration analysis; high temperature tensile and compressive strain will

probably not be investigated until 1989 or

1990. Nevertheless, if the problems associated with high temperature ESPI

measurements can be successfully addressed, it could be an extremely powerful

technique. The potential of a contactless

method for which an entire strain field of a

large surface can be analyzed in real time

warrants close attention to developments in

the field of high temperature ESPI measurements. Developing ESPI methods to

measure strain at high temperatures can be

expected to be a difficult task.

59

3.5 Concept Description and

Analysis - Laser Speckle

Methods

3.5.1 Concept Description

Laser speckles are the result of multiple

interference of reflected light when an optically rough surface is illuminated by a

coherent laser beam. Based on the imaging

system employed, a speckle pattern is classifted as an objective or subjective speckle

(Reference 3-67). An objective speckle pattern is formed in the region in front of the

coherently illuminated object or in the

region behind a coherently illuminated diffuser. On the other hand, if light from the

specimen surface is imaged by an optical system, the resulting speckle pattern is known

as subjective speckle. The spatial frequency

of an objective speckle pattern is determined

by the roughness of the surface, whereas that

of subjective speckles is controlled by the

imaging system.

Speckle dimension is related to the average

distance between two bright speckles and is

usually called average diameter of the

speckle. It is described by the wavelength, x,

and the angle subtended by the scattering

area on the screen (Reference 3-68). This

can be written for objective speckle as:

= (x)UD (7)

and for subjective speckle pattern

_-(s) - 1.22 (1 + M) (x) F (8)

where _(o) and _r(s) are the average diameter

of the resulting speckle, L the distance

between the surface and observation plane,

D the size of the object, M is the magnification, and F the f number of the imaging system. The resulting speckle pattern acts as a
naturally printed grid on the surface. When

two such patterns are superimposed, the

resulting intensity distribution depends only

on the relative phases of the individual patterns. By measuring the correlation of the

speckle patterns in two states, a change of

phase is detected which can be related to surface displacements.

Leendertz (Reference 3-69) was one of the

first to use the speckle phenomenon to

measure displacements. He used a double

illumination method to measure in-plane

displacements in real time. This was later

extended to the "frozen fringe" technique by

Archbold et al. (Reference 3-70). Duffy

(Reference 3-71) showed that nonlinear

recording properties of the recording film

along with optical filtering could be used to

obtain correlation fringes. The technique

developed by Duffy and by Archbold et al.,

known as speckle photography, has found a

number of applications. For example, Barker

and Fourney (Reference 3-72) used speckle

photography to study displacements at the

interior points of a transparent object. Hung

and Hovanesian (Reference 3-73) have

applied the technique to a generalized three

dimensional field and have also proposed

speckle shearing interferometry techniques

to obtain slope contours directly.

Speckle metrology can be divided into two

broad categories: (1) those involving

photographic recording, and (2) those using

other methods. Photographic recording

techniques can be subdivided according to

methods used to extract the desired information from the photographs as described in the

following section.

3.5.2 Speckle Photography

Speckle photography and speckle interferometry are both powerful optical techni60

ques for measuring surface displacements.

These methods make use of interference

fringes produced by the scatter of coherent

radiation from an optically rough surface in

both initial and displaced positions. Stetson

(Reference 3-74) proposed categorizing

speckle metrology based on the correlation

between the images formed by speckle patterns on the initial and displaced surfaces. If

there exists a region in the speckle pattern of

the image of the initial position that is always

well correlated with that of the displaced surface, Stetson proposed naming the process

speckle photography. If the fringes derived

from fluctuations in the correlation of the

speckle patterns between the two images,

Stetson named the process speckle interferometry. Interferometric methods require

distinguishing between parts of the field

where the speckle is correlated and those

parts where it is not; the methods require that

individual speckles in the image plane are

displaced by no more than their own

diameter. Speckle interferometry is, therefore, limited to the measurement of small

lateral displacements unless the speckle size

is made large by reducing the lens aperture.

In speckle photography, correlation methods

are used to measure the displacement of a localized area of a speckle pattern where the

speckle shift is greater than the speckle size

(Reference 3-75). Operationally, speckle

photography differs from speckle interferometry by the absence of a reference

beam at the recording stage.

In speckle photography, a surface is

illuminated with a divergent beam of light

and is photographed. After the object has

been deformed, the film is exposed again,

resulting in a double exposed negative from

which correlation fringes can be extracted by

optical processing to determine displacement of the surface (References 3-76 and 3-
77). The speckle photograph, or specklegram, consists of speckles from an

undeformed exposure and the same speckles

from the deformed exposure. In the absence

of interfering phenomena, the distance

between each speckle pair is directly proportional to the in-plane displacement of the target surface. Analysis of the photograph is

performed by directing a laser beam at normal incidence through the specklegram to

generate the Fourier transform of the

photograph. This transform is in the form of

a diffraction halo modulated by a pattern of

equispaced parallel fringes similar to

Young's fringes. These fringes are aligned

perpendicular to the displacement and have

a fringe spacing that is inversely proportional to the magnitude of the displacement.

Provided that the image movement between

exposures is greater than one speckle

diameter, and that the speckle patterns

remain correlated with each other, the

speckle pattern on the photographic film will

scatter a beam of light into adiffraction halo.

The interference generated by the two sets of

speckles creates parallel equispaced fringes

in the halo, the number of which is proportionai to the in-plane displacement of the

point. The direction of the fringes is perpendicular to the direction of the displacement

vector. Therefore, after measuring the spacing and direction of the fringe patterns at

closely spaced points on a photograph, the

complete 2D deformation field can be

analyzed. The speckle diameter on the test

surface can be shown to be approximately

M0,)F, where M is the magnification, F is the

f number of the lens, and (),) is the

wavelength of the scattered radiation.

Lateral movements greater than M0_)F are,

in principle, measurable by speckle photography. Therefore, with an f/4 lens used at unit

magnification, the surface movement must

be greater than about 3_n; with a 10X magnification, movements greater than about 30

_rn are measurable.

Accurate fringe counting is a major challenge

in speckle photography for which several

techniques have been proposed. The first,

proposed by Burch and Tokarsky, involves

examining the photograph point by point to

determine the corresponding surface displacement. A second method, proposed

independently by Butters and Leendertz

(Reference 3-78) and Archbold et al.

(Reference 3-70) uses a spatial filtering technique. Duffy (Reference 3-71) used a double

circular aperture to interpret fringes as a

moire pattern. The goal of several

researchers in the field has been to develop

an automatic computer-based method of

analyzing fringe patterns. For example,

Kaufmann et al. (Reference 3-79) proposed

a semiautomated hybrid readout system

wherein a photodiode array scanned a fringe

pattern which was later processed in a computer using digital filtering. Sciammarella

(Reference 3-80) described a low-cost

automatic fringe analysis in which the

scanned profile is processed on-line by computers using digital filtering programs.

Ansari and Ciurpita (Reference 3-81)

developed a fully automated microcomputer-based system for analysis of speckle

photographs.

Measurement of surface displacement using

speckle photography is subject to several significant sources of error which have been discussed in detail in Archbold and Ennos

(Reference 3-70), and Archbold, Ennos, and

Virdee (Reference 3-82). It is important that

the focus in the image plane not vary over the

field of view. For flat surfaces, this may occur

when the surface is imaged by a large aperture lens that introduces aberrations causing

81
the image to be curved. Parts of the speckle

patterns are then defocused and are, therefore, sensitive to surface tilt as well as to inplane displacement. ,_u'chbold, Ennos, and

Virdee (Reference 3-82) showed that this effect could be minimized by using a long focal

length lens so that only a narrow field of view

is employed.

Compared to interferometric techniques,

speckle photography is less sensitive to

motion of the surface in the direction of the

line of sight. Axchbold and Ennos calculated

that the depth of focus AZ within which

speckles would remain correlated in the

object plane was given by

AZ-- 2M2F2(x) (9)

where M is magnification, F is the f number

of the camera objective lens, and _, is the

wavelength of the illuminating light.

Archbold and Ennos found, somewhat to

their surprise,that the experimentally determined correlation was maintained for at least

four times the AT. For and f/4 lens with a 7x

magnification, they found a depth of focus of

about 8 mm, instead of the predicted 2ram.

Nevertheless, restrictions on rigid body

motion in the line of sight direction severely

limit the applications of double exposure

speckle photography outside the laboratory

environment. Even under optimal conditions in which a flat object is evaluated,

uncertainties in strain measurement of 100

microstrain are likely to occur; errors are

likely to be larger when the specimen is not

flat or when surface tilt or rigid body motions

occur. For example, Archbold found that a

movement of 50 _an in the line of sight direction gave rise to an apparent strain of 300

microstrain (Reference 3-83). Surface tilt

also gives rise to apparent strain that, at least

for small angles, is proportional to the tilt

angle. Typically, a one milliradian tilt would

induce an apparent strain of up to 300

microstrain.

In speckle photography the displacement of

the object must be such that the displacement of the speckle pattern in the recording

plane is greater than the speckle size in that

plane (Reference 3-84). For in-plane

measurement, the smallest displacement

that can give rise to fringes is given by

Drain > Os/M or

Dmin > 0.5(x)(F) (10)

where Qs is the speckle size and M is the

magnification of the viewing system and

Drain is the smallest measurable displacement. For a lens with a numerical aperture of

2, the minimum speckle diameter is about 0.5

micron. It has been shown, however, that

when the resolution-element diameter is

small object tilt and out-of-plane displacement give rise to apparent in-plane displacements so that errors occur except when the

motion is purely in-plane (Reference 3-84).

The practical application of double exposure

speckle photography is restricted to measuring displacements over a range of little more

than a factor of 10, a significant limitation for

strain measurements. Smith and Stetson

(Reference 3-85) reported an interferometric readout system for specklegrams

that addressed the problems of limited

dynamic range and sensitivity to rigid body

movements. This technique, called

heterodyne speckle photogrammetry, uses

separate photographic plates to record

exposures of the specimen before and after

deformation. Stetson claimed that this technique could improve the accuracy of fringe

measurement by between one and two

orders of magnitude. The specklegrams were

62
then analyzed in a heterodyne interferometer to provide a precise measurement

of differential displacement. Stetson

(Reference 3-86) successfully used this system to measure biaxial strain distributions in

a laboratory furnace up to about 1150 K. Stetson reported agreement within 10% between

measured strain and strain calculated on the

basis of material properties. Accurate determination of mechanical strain required

measuring the surface temperature of the

specimen and compensating for thermal

gradients in the atmosphere surrounding the

test piece, however.

Stetson (References 3-87 and 3-88) also

attempted to use speckle photogrammetry to

measure static strain in a combustor test rig.

Results were less satisfactory than in oven

based tests. He concluded that turbulence in

the optical sight path could cause decorrelation in of the speckle patterns. Turbulence

was particularly troublesome at pressures

exceeding 3 atmospheres at 1000 K. At

higher temperatures, turbulence would

probably reach unacceptable levels at lower

pressures. The effect of turbulence in the

optical path could be significantly reduced by

simultaneously taking multiple photographs

from different angular positions and subtracting out effects due to distortion in the

sight path.

Speckle photogrammetry also is susceptable

to errors caused by out-of-plane movement

and surface tilt of the specimen surface.

Although a thorough analysis of these factors

on strain measurements by speckle

photogrammetry has not been performed,

Stetson found that rigid body movements

severely restricted strain measurements

made in a eombustor gas stream.

3.5.3 Dual Beam Doppler

Interferometer (Optra, Inc.)

Laser Doppler Anemometry (LDA) has long

been used to measure the velocity of particles

in a gas. The principles of LDA are well

known and are amply discussed in the literature on the subject. The measurement principle of LDA is based on detecting the

Doppler frequency shift of light scattered

from a moving object. Often, the Doppler

shift is detected by optically heterodyning the

scattered light with the incident radiation so

that the output from a suitable photodetector is related to the Doppler shift, which is a

linear function of the particle velocity. Alternatively, the particle can pass through a

moving fringe pattern generated by intersecting frequency shifted beams. The output

of the photodetector is then processed

electronically in real time to yield the particle

velocity irrespective of direction.

To determine both velocity and direction of

a particle, a particle passes through a sensing

area consisting of the intersection of two frequency shifted beams. The intersecting

beams form a moving fringe pattern so that

when a stationary particle is inside the sensing area, it scatters light that is frequency

modulated by the difference frequency af between the intersecting beams. The light scattered by a moving particle may be modulated

by a frequency greater than or less than af,

depending on whether the motion of the particle is in the same direction as, or opposite

to, that of the fringe motion. Particle velocity,

in this ease, is proportional to the magnitude

of the difference between the modulation

frequency of light scattered by the moving

particle and the frequency of light scattered

by a stationary particle. Particle displacement can be determined by integrating the

velocity over time.

63
Optra, Inc. of Peabody, MA, has recently introduced an optical displacement and strain

measuring system that can be understood as

an implementation of LDA principles

(Reference 3-89). If a sensing area created by

the intersection of frequency shifted laser

beams is formed on adiffusely reflecting surface, it irradiates small spots which are part

of the natural surface geometry. The displacement of these spots can be determined

by using IDA techniques to determine the

average of the motion of the spots. For strain

measurement, two pairs of intersecting

beams form two sensing areas on the target

surface; the distance between the sensing

areas determines the gage length. Strain is

determined by measuring the relative displacement of the sensing areas (Reference 3-

90).

Although a thorough assessment of the

Optra system's response tO rigid body movements has not been performed, both

theoretical analysis and limited experimental

evidence suggests that the technique is relatively insensitive to out-of-plane rigid body

motions so long as the sensing areas remain

focused on the target surface. For a current

prototype system, the depth of focus is f'Lxed

at 5.1 cam, suggesting that rigid body movements of 2.5 mm are tolerable. According to

the manufacturer, rigid body translations

along the line joining the two sensing

volumes (the "x" axis) can be tolerated as

long they do not exceed about five times the

displacement due to strain. In-plane movement perpendicular to the "x" axis should not

cause errors more than one fringe (typical

fringe spacing is 8 _rn).

The Optra system has not yet been evaluated

in terms of its response to changes in the

index of refraction between a heated sample

and the receiving optics, although this can be

64

expected to contribute to measurement

errors. Measurements of thermal expansion

coefficients have been made in air up to

about 1900 K, with little observable effect

from interference from thermal gradients

surrounding the specimen, according to the

manufacturer. Evaluation of a prototype system used as an extensometer in an elevated

temperature vacuum environment at Oak

Ridge National Laboratory indicated that

the systems agreed well with theory which

indicated a resolution capability of 3.5

microstrain (Liu, K., Oak Ridge National

Laboratory, personal communication).

3.6 Concept Descriptions and

Analyses - Miscellaneous

Optical Displacement

Measuring Techniques

3.6.1 Blade Tlp Deflectlon Sensors

Blade tip deflection measurements have

been an effective tool in detecting vibratory

motion of shroudless turbine rotor blades

and in inferring blade stress. Tip deflection

measurements can provide stress amplitude

and spectral information comparable to that

obtained from dynamic strain gages if

measurements are made at a sufficiently high

sampling rate. According to the well-known

Nyquist criteria, this requires that the sampling frequency be at least twice that of the

highest frequency vibration in the system. In

practice, this may require over two dozen

probes mounted circumferentially around

the engine casing, a prohibitive task for most

developmental work.

McCarty and Thompson (Reference 3-91)

developed ablade tip deflection system using

optical fiber light probes and demonstrated

that they could extract the amplitude and frequency of blade vibrations that were non-
integral multiples of engine speed. They

validated their blade tip deflection data by

comparing the results obtained with the tip

deflection system with strain gage data. GE

has an ongoing effort to reduce the number

of probes needed to obtain the tip amplitude

and frequency information and also to

reduce the time needed to sample the data.

A parallel effort is in progress to speed up the

data processing to the point where the stressrelated amplitudes could be determined in

less than one second. This would provide the

stress safety monitoring capability that is

needed on developmental engine testing.

As is the case for strain gage measurements,

prior knowledge of blade characteristics is

required to correlate tip deflections and

maximum blade stress. Blade tip deflection

versus stress must be obtained from

laboratory tests of representative blades.

Finite element analysis then permits calculation of blade stresses from the tip deflection

data. It has shown that finite element

methods can be used to model entire

blade/disk assemblies. It should be possible

to perform the finite element analysis before

engine testing so that quasi-real-time stress

measurements from tip deflection measurements are possible. Tip deflection methods

used to date have lent themselves only to

steady state analysis.

We believe that there is significant unrealized potential in blade tip deflection

measurements. It should be possible to apply

the methods used so far on compressor and

fan blades to turbine blades. Furthermore,

recent work at GE has shown that both

integral and nonintegral vibration amplitude

and frequency data can be acquired in as little as one revolution of the blade. This would

permit tip deflection methods to determine

transient blade stresses. Although blade tip

deflection methods are restricted in terms of

the engine parts to which they can be applied,

they address what is probably the single most

important stress measurement in turbine

engine development.

3.6.2 Laser Doppler Methods

This measurement concept is based on use of

an optical heterodyne technique for detecting the Doppler frequency shift between

backscanered laser light and the laser excitation beam. The frequency shift is proportional to velocity of the target surface in the

direction of the excitation beam. Integration

of the velocity signal can be used to obtain

displacement. Information which describes

frequency and displacement can be used to

calculate strain if a suitable model of the

component has been defined.

Although limited in scope to nonrotating

components where vibration displacement

amplitude and frequency can be related to

strain, this concept has some significant

advantages:

Nonintrusive, noncontacting sensing

where vibratory displacement normal to

the surface is obtained by integrating the

measured velocity along the excitation

axis.

Existing technology is available and has

been used for low temperature applications. Feasibility for high temperature

measurements is promising.

An example of where such a technique may

be usable would be measurement of

vibratory strain in an exhaust nozzle liner

panel where finite element modeling

methods had been developed to relate displacement to strain.

65
Characteristics of a commercial system

which has been used for vibration measurements on low temperature applications are

as follows (Reference 3-92):

Measuring Distance (Instrument to Target): 1.2 to 20 m

Target Spot Size: < 0.1 mm/m

• Response Time: 1 _s

• Wavelength: 633 nm

• Frequency Range: DC - 740 kI-Iz

• Amplitude Range: 10 .8 to 1 M

Problems associated with detection of backscattered light from high temperature

ceramic materials include the following.

Optical windows may interfere with

backscattered light.

Background interference from emitted

target radiation may cause adverse

effects.

Pleydell (Reference 3-93) describes an alternate technique based on a Laser Doppler

(Michelson) Interferometer wherein

coherent detection is used to extract the

Doppler shift in reflected light from an

unprepared target surface with motion along

the beam axis. This system is based on focusing the reference beam on the detector surface so that its size is smaller than the speckle

size in the reflected pattern from the target.

This optimizes coherent detection by allowing it to occur only in an area of the detector

common to both the reflected reference

beams.

66

3.7 Concept Descrlptlon and

Analysls - X-ray Dlffractlon

Methods

3.7.1 Concept Descrlptlon

Stress analysis by x-ray diffraction (XRD) is

based on measuring changes in the lattice

spacings of crystalline planes. These changes

are referred to as lattice strains. XRD stress

determination requires correlating changes

in lattice strain with uniaxial or biaxial stresses. This correlation requires measuring the

x-ray elastic constants (defined as (1 + _,)/E

and _,/E, where i, and E are Poisson's ratio and

Young's Modulus, respectively) in a direction normal to the lattice planes (Reference

3-94). Because only elastic strains result in

changes in the lattice spacing, XRD

measures only elastic strains. When the elastic limit is exceeded, further strain results in

a disruption of the crystal lattice and XRD

techniques do not apply.

X-ray diffractionmethods have been used

primarily to measure residual strains, both

because of the lack of alternative nondestructive methods capable of measuring

residuals and because of the difficulties in applying x-ray methods. In principle, however,

there is no reason why strains from applied

stresses cannot be evaluated. Because of the

recognized importance of residual stress

measurement in both manufacturing and

design, this discussion will focus on residual

stress/strain determination. Much of the discussion also applies to applied stress,

however.

Metallic and polycrystalline ceramic

materials consist of innumerable randomly

oriented crystalline grains. When such

materials are stressed, the interatomic distance of the crystal lattice changes proportionally with the applied stress (within the
elasticlimits of the material). The relationship between the wavelength x of the x-ray

beam, the angle of diffraction 2 0, and the lattice spacing d is given by the Bragg equation:

nx = 2dsin {a} (11)

where n is the diffraction order (see Figure

ll(a). This equation states that for a given

wavelength and diffraction order, the diffraction angle 20 is a simple function of the lattice spacing. Any change in the lattice spacing

d results in a corresponding change in the diffraction angle.

The essentials of XRD applied to stress and

strain analysis can be illustrated by considering tensile stress measurements. Placing a

specimen in tension results in a Poisson's

ratio contraction, which in turn reduces the

lattice spacing and so increases the diffraction angle 20. If the specimen is rotated

through a known angle _, as shown in Figure

1 l(b), the tensile stress present in the surface

will result in an increase of the lattice spacing oriented at an angle _,with respect to the

surface, and a corresponding decrease in 20.

By measuring the change in the angular position of the diffraction peak for at least two

values of 4, the stress in the plane of the incident and diffracted x-ray beams can be calculated. To measure strains in different

directions, the specimen is rotated about its

surface normal to make the direction of interest coincide with the diffraction plane.

The fundamental equation describing the

relationship between lattice spacing and

biaxial surface stress is given by

dO¥ - dO = (2_-) (_1 cos2 0 + _2 sin2 ¢)

• 2 V

dO s,n _ - -_- dO (G1 + 0"2)

(12)

where oa and o-z are the principal surface

stresses in the plane of the surface

(Reference 3-95), do is the unstressed lattice

spacing, and d4,o is the spacing between the

lattice planes measured in the direction

defined by 4, and ¢ (see Figure l lc). Equation 11 provides the basis for the most widely used techniques of XRD stress/strain

measurement. If the lattice spacing is a linear

function of sin2{q,} and if the x-ray elastic

constants are known, then the stress can be

determined if the lattice spacing is measured

for any two angles {¢}. This method, called

the two-angie technique, has been thoroughly investigated by the SAE (Reference 3-96)

and is the primary method of applying XRD

to stress analysis in the United States. An

extension of the two-angle technique, called

the sin2{¢} method, is similar to the twoangle technique except that the lattice

spacing is determined for numerous .an_les

{¢}. The primary advantage of the sin {4}

technique is to establish the linearity of the

lattice spacing as a function of sin2{¢}, thereby demonstrating that the technique is a

feasible way to measure strain.

Equation 12 shows that XRD stress analysis

requires that the x-ray elastic constants,

defined in terms of Poisson's ratio and

Young's modulus, be known. It is important

that the elastic constants used are not bulk

values, but refer to the values for the specific

crystallographic direction normal to the

planes used for stress measurement. Bulk

values of Young's modulus and Poisson's

ratio are an average over all possible directions in the crystal lattice and may differ significantly from the values for a specific

direction. The elastic constants are determined by measuring the change in the

separation of the crystallographic planes

used for strain measurement as a function of

applied stress. Prevey's method (Reference

67
\

__ _/=0

D_acted N_ Ray

Normal

(a)

\\

¥

N

Figure 11. Coordinates and Angles of X-ray Measurements on a Plane Surface.

68
3-94) involves placing a sample of the

material of interest upon a diffractometer

mounted in a load fixture. Known stresses are

applied to the sample and the lattice spacing

is measured as a function of the applied

stress. The slope of a plot of the change in lattice spacing as a function of applied stress is

constant for a given material and diffraction

peak, and is proportional to the elastic

properties of the material in the appropriate

crystallographic direction.

Certain materials, including polycrystalline

ceramics, may exhibit a nonlinear relationship between lattice strain and sin {_}. It has

been found, for example, that the presence

of stress gradients and stress components

perpendicular to the specimen surface

makes application of the sin" {,t,} method difficult (Reference 3-97). Experts disagree

over whether or not reliable data can be obtained using the sin 2 {,} method in the

presence of stress gradients and orthogonal

stress components. Pelter and Lode

(Reference 3-98) proposed an extension of

the sin 2 {,} method, the x-ray integration

method (RIM), that permits accurate determination of the entire strain field of polycrystalline materials in the presence of

orthogonal stresses and stress gradients.

Marion and Cohen also developed a technique to measure lattice strains in materials exhibiting a nonlinear dependence of lattice

strain on sin 2 {,} (Reference 3-99). The

Marion-Cohen method has not yet been

widely adopted, although Prevey has shown

that it leads to essentially identical results as

the two angle and sin 2 {,} methods for a

variety of materials (Reference 3-94). Unfortunately, the methods designed to eliminate

the requirement of a linear dependence of

lattice spacing on sin e {t} also require

precise knowledge of the unstressed lattice

spacing. Because the value of unstressed lattice spacing is generally unknown, these

methods probably will have limited application.

3.7.2 Current Status

There has been very little effort to date in applying x-ray diffraction methods to high

temperature strain measurement, although

facilities are available for such studies. Commercial equipment is available from

manufacturers such as Rigaku Denki (U.S.

Headquarters in Danvers, MA) and Technology for Energy Corporation (TEC) of

Knoxville, TN, that permits residual stress

analysis up to about 1800 K in air. The

former, which uses a rather conventional diffractometer that scans a range of angles with

a single detector to obtain a diffraction peak,

may require over 1 hour to determine each

peak. TEC supplies a relatively new detector

developed by James and Cohen (Reference

3-100) called the position-sensitive proportional counter (PSPC). With this detector,

data are obtained simultaneously over a

range of diffraction angles 20 so that a diffraction peak is obtained without moving the

detector. The only required motion is

moving either the specimen or the diffractometer to obtain diffraction peaks at several

values of ,. The time required to obtain diffraction peaks is typically less than a tenth

that required by conventional diffractometers.

The High Temperature Materials

Laboratory (HTML) at Oak Ridge National

Laboratory has recently brought on line a

facility to perform high temperature x-ray

diffraction studies on ceramics. This new

facility, which uses a PSPC detector, should

facilitate determination of essential material

properties. Although not specifically intended for measurement of stress and strain,

it may be possible to evaluate elastic

69
constants at high temperature and to investigate XRD methods in high temperature

stress analysis.Although there is no fundamental reason why lattice strains cannot be

measured in polycrystalline ceramics, XRD

measurements will probably be more complicated on ceramics than with metals. For

example, specimens with large grain sizes or

preferred grain orientations are probably not

well suited for x-ray diffraction because such

materials provide few irradiated grains. Furthermore, single phase materials are

generally the best candidates for x-ray

analysis, whereas most structural ceramics

contain polycrystalline or amorphous phases.

In these materials, lattice strains are primarily determined by a small fraction of one

phase only, so that care must be taken in

drawing conclusions about the entire

specimen. The extent to which the presence

of multiple phases limits x-ray analysis has

not been established.

Finally, diffraction peak broadening caused

by variations in crystallite size will probably

be a more important factor in ceramic

materials than with metals. Although peak

broadening caused by strain in the crystal lattice can be separated from broadening

caused by reduction in the crystallite size, the

method is a rigorous one involving Fourier

analysis of the diffraction peak profile and

collection of sufficient data to precisely

define the shape of the entire diffraction

peak. The method is highly dependent upon

the precision with which the tails of the diffraction peak can be separated from the

background intensity. Even if accurate strain

measurements can be made on ceramics, the

relatively high Young's modulus of ceramics

means that strain measurements on ceramics

can be expected to be less sensitive than with

metals.

7O

3.7.3 Future Prospects

X-ray measurements at high temperatures

pose challenges beyond those encountered

at room temperature. Ovens limit both the

access of the x-ray beam to the sample and

the angles at which diffracted radiation can

be analyzed. For extensive high temperature

strain analysis, it would be desirable and perhaps necessary to design a heating system

that allows better access than do existing

ovens. Laser heating has been suggested as

one way in which specimens could be heated

while preserving access to the specimen for

x-ray sources and detectors. More fundamental problems may make x-ray diffraction analysis at high temperatures difficult.

For example, thermal diffuse scattering of

the diffracted x-rays can be expected to result

in a loss of signal intensity, although the extent to which this may limit strain measurements can only be determined by

experimentation. In general, strain measurement by x-ray diffraction needs to establish

itself at room temperature before high

temperature measurements are attempted.

Before x-ray diffractioncan be considered

for high temperature strain analysis, certain

fundamental material studies must be made.

The x-ray elastic constant must be measured

for each material of interest over the range

of relevant thermomechanical conditions.

Using bulk or theoretical values of this constant has been shown to produce large errors

in residual stress measurements, even at

room temperature (References 3-94 and 3-

101). The facilities at Oak Ridge might be a

logical place to pursue the required studies.

Despite more than two decades of research

and development, x-ray diffraction methods

cannot be regarded as an established technique for measuring strain, even at room

temperature. Studies to date have focused on
residual stress/strain measurement for which

traditional x-ray diffraction methods are best

suited by virtue of the long time required to

resolve peaks. We believe, however, that xray diffraction is probably suitable for strain

measurements at room temperature. Its applicability to high temperature strain

measurements cannot be evaluated until further research is done. There is room for optimism that high temperature strain

measurements can be made under:

laboratory conditions by highly trained personnel. A 3- or 4-year development program

that employed experts in the field of x-ray diffraction could probably determine how

effective a tool XRD would be in measuring

strain of high temperature ceramics.

3.8 Concept Description and

Analysis - Photoelastlc

Methods

3.8.1 Introduction

Certain transparent materials exhibit a

phenomenon known as double refraction, or

birefringence, in which incident light is

resolved into two components having planes

of vibration parallel to the directions of the

principal stresses. The component waves

generally have distinct velocities (that is, the

material has distinct indices of refraction for

each wavefront) so that the relative phase

between the component wavefronts changes

as they traverse the material. By allowing the

component waves to optically interfere after

traversing the bifringent material, the relative phase of the components can be

measured by standard fringe counting techniques. Photoelastic stress measurements

are based on measuring the relative retardation of the optical components derived from

polarized light that has traversed a birefringent layer called a photoelastic coating

(Reference 3-103). In photoelastic materials,

the birefringence (that is, the difference in

the indices of refraction) is a function of the

strain at each point of the material. Stress and

strain measurements using photoelastic

coatings have been introduced commercially by Measurements Group, Inc. of Raleigh,

NC. Although restricted to low temperatures

by virtue of temperature limits of the

polymeric coatings used, commercial

photoelastic methods are capable of measuring strains over the range from about 100

microstrain to several percent.

3.8.2 Theory

When a beam ofpolarizedlightpropagates

througha stressedbirefringentcoatingof

thicknesst,the lightdivides into two

polarizedbeams thatpropagateatdifferent

velocities. Itmay be shown thattherelative

retardationsbetween thetwo beams isgiven

by

8= t(Nx-Ny) (13)

where Nx and Ny are the indices of refraction

in the x and y directions. The birefringence

(Nx - Ny) is proportional to the difference of

the principal strains Ex and Ey, or

(Nx- Ny) --k(Ex -Ey) (14)

where k is the strain optic material constant,

a physical property of the material analagous

to the gage factor of resistance strain gages.

Combining the above equations yields the

basic equation for the measurement of strain

using birefringent sensors:

Ex- Ey = 05)

In many applications, light passes through

the birefringent material, reflects off of a diffusely reflecting substrate surface, and passes

71
through the birefringent material a second

time. An instrument known as a photoelastic

polariscope is then used to measure the

retardation. It may be shown that the image

obtained by a circular polariscope is unaffected by the direction of the principal strains

and that the intensity I of the light emerging

from the polariscope is given by:

I = A sin2[ (b/x)] (16)

where _,is the wavelength of the light and A

is a proportionality constant. The light intensity, therefore, becomes zero when

8 = Nx (17)

where N, the isochromatic fringe order, is an

integer.

Once 8 is known, the difference in the prindpal strains is obtained by:

Ex-Ey - 5/(kt) - N_/(Kt) -= Nf (18)

where f is a proportionality constant. In

theory, therefore, the principal strain difference can be obtained by counting

isochromatic fringes.

3.8.3 Discussion

In practice, it is often difficult to measure the

retardation accurately. Recently, a method

called Spectral Contents Analysis (SCA) has

been proposed to overcome many of the

limitations associated with conventional

techniques (Reference 3-104). In the SCA

method, the reflected light is analyzed with a

spectophotometer so that the intensity of

numerous spectral bands is measured. The

retardation is then calculated from the

spectral intensity measurement.

Strainoptic Technologies, Inc. of North

Wales, PA, is currently exploring the

feasibility of using SCA for high temperature

strain measurement as part of an SBIR

project with NASA Ames. They have used

fused quartz as a sensor material up to

1300 K and will investigate several candidate

materials for higher temperatures, some of

which are proprietary. While in principle it

would be possible to operate the SCAsystem

in reflection mode by irradiating the surface

normally and detecting light reflected by the

test surface, in practice this may be difficult.

Sensitivity would probably be low for any

reasonable coating thickness unless a coating

with a high strain optical coefficient is found.

The outstanding problem with high temperature photoelastic measurements involves the

selection of the birefa'ingent coating. The

material must retain its birefringent properties and adhere to the substate at operating

conditions. If the strain-optic coefficient is

temperature dependent, then temperature

compensation is required. Until a satisfactory coating has been identified, successful

application of photoelastic methods to high

temperature strain analysis is problematic. If

such a coating is found, photoelastic methods

appear quite promising as a laboratory tool

for high temperature strain analysis.

3.9 Concepts Recommended for

Future Development

3.9.1 Discussion

No single measurement technique is ideally

suited to all the diverse applications of high

temperature strain measurement. These

applications range from the the relatively

straightforward case of tensile testing in a

materials laboratory to the intractable case

of measuring static strain on a rotating blade

in an operating gas turbine engine. There is

72
currently considerable interest in the former

measurement techniques suitable for determining the stress-strain response of

materials at high temperature. Our recommendations are oriented toward techniques

that have wide application and not toward

those intended for specialized enviroments.

Despite the importance of materials testing

in the laboratory, one of the most important

applications in turbine engine development

is stresststraln determination of hot rotating

parts. Two of the strain measurement techniques reviewed in this report, the SimpsonWelch method (Section 3.2.3) and blade tip

deflection methods (Section 3.6.1), could be

considered for this difficult application.

Based on proprietary data, we believe tip

deflection methods are a very promising

method of determining stress in rotating turbine blades. However, because tip deflection

analysis has little application aside from turbine engine development, we do not recommend it within the context of the present

study.

Point contact gages such as resistance and

capacitance strain gages have been standard

strain measurement tools for over 20 years.

We believe there is room for growth in the

resistance strain gage and that it may be the

preferred measurement tool in a variety of

applications for short term static strain

measurement up to about 1200 K and for

dynamic measurements up to 1450 K.

However, given the problems associated

with gage attachment, temperature compensation, deterioration of insulation resistance,

and material degradation, we have little confidence that the resistance strain gage can

extend beyond these limits. Therefore,

although we believe the resistance strain

gage warrants continued development, it

does not appear promising given a target

temperature over 1600 K.

Like the resistance strain gage, capacitance

strain gages require contact with the test surface. The gage may be mounted directly on

the surface or used as an extensometer with

the capacitance element remote from the

surface. In the former design, most lead wirerelated problems that limit resistance gages

also probably affect capacitance gages.

Incremental improvements in the

capacitance gage may allow this concept to

be used up to 1400 K. We are not optimistic

that direct mounted capacitance gages can

successfully address higher temperatures,

although it may be possible with gages using

dielectrics other than air. As part of a remote

extensometer, the capacitance gage may be

usable to significantly higher temperatures in

a controlled laboratory environment. Nevertheless, we believe that the extensometer

approach will have limited application even

if current efforts to reach 1900 K are successful.

Ideally, strain measuring devices should not

contact the test urface, and indeed much

effort has been devoted to this end. One

promising method of contactless strain

measurement is speckle interferometry. A

real time implementation of speckle interferometry, known as either ESPI (Electronic

Speckle Pattern Interferometry) or as TV

holography, has recently been introduced.

Existing commercial units are not primarily

intended for strain measurement however.

Although ESPI has been used to measure

full-field static and dynamic strains, it suffers

from a problem inherent to all

interferometric techniques: thermal

gradients in the atmosphere surrounding a

specimen surface appear as spurious strains.

In addition, strain measurement in the

73
presence of random vibrations is a major

challenge. Despite the enormous progress

made in developing the ESPI technology,

these fundamental problems restrict its usefuiness. Experts in the field consider strain

measurement in hostile environments to be

a difficult challenge for ESPI, but they are

hopeful that the challenge can be met. We

regard ESPI applied to hostile environments

as a speculative technology and, therefore,

we reluctantly cannot endorse ESPI as one of

the most promising technologies for high

temperature strain measurement.

Moire methods also have been regarded as a

promising contactless method of measuring

full field displacement and strain. Moire

methods provide a mapping of the surface

displacements from which it is possible to

calculate extensional and shearing strains.

Unlike ESPI, moire does not use the natural

features of the specimen surface to diffract

light, but instead requires attaching or

engraving a specimen grating to the test surface. It is, therefore, not totally contactless,

and indeed, specimen grating survivability is

a major constraint on the use of moire

methods in hostile environments. Conventional moire uses coarse specimen grids

(about 40 lines/mm) and is, therefore,

limited to measuring percent level strains.

High sensitivity moire techniques have

dramatically increased the sensitivity of

moire methods so that under favorable conditions moire can measure as low as tens of

microstrain.

Moire techniques have been applied in

numerous ways in displacement and strain

measurement. Several hybrid techniques

couple moire methods with other technologies. One example is holographic moire

(HI), in which moire procedures are applied

to holographic interferometry. Holographic

74

moire can provide sensitive real-time

measurement of strain. As suggested by its

name, HI requires isolation from mechanical

vibrations and is sensitive to thermal

gradients in the atmosphere surrounding the

specimen. Another hybrid technique is

moire interferometry, which combines the

advantages of moire with those of two or four

beam interferometry. Moire interferometry

has been used with very high frequency gratings (up to 4000 lines/ram) to achieve high

sensitivity strain measurement over a very

wide dynamic range. To our knowledge,

moire interferometry has not been applied to

high temperature measurements, and thermal gradients in the atmosphere surrounding

a specimen can be expected to be a difficult

challenge.

Moire methods have progressed considerably over the past 15 years to where they can

reasonably be applied to practical measurements. Unfortunately, the requirement for

attached gratings, the need to maintain alignment between specimen and reference gratings, and, particularly for high sensitivity

moire, the effects of thermal gradients

between the gratings make moire methods

less than ideal candidates for high temperature strain measurement.

3.9.2 Spectral Density Indication

An interesting concept for full-field strain

measurement has recently evolved out of

moire methods. Called Spectral Density

Indication (SDI) by its inventor, it may use

either a coarse grid etched onto the specimen

surface or the natural surface features of a

rough surface. In SDI, the spatial frequency

of a digitized image of the grating in a

stressed state is compared with that in an

initial unstressed state. Eliminating the

reference grating and the need to maintain

the relative orientation of reference and
sample gratings provides SDI with a major

advantage over moire methods. Furthermore, etched gratings can be expected to be

more stable in hostile environments than are

attached gratings. SDI has a very wide

dynamic range: strain from less than 100

microstrain to several percent can be made

from a single optical set up. The effect of

thermal gradients in the atmosphere surrounding the specimen has not yet been

evaluated, but can be expected to be fignificantly less than is associated with interferometry or moire methods. Although the

SDI technique is relatively new and its limitations have not been well defined, we believe

it is a very promising technique for strain

measurement in hostile environments.

Therefore, we recommend SDI for further

development so that its applicability to strain

measurement in hostile environments can be

fully assessed.

3.9.3 Optical Tracking - Attached

Targets

The strain measurement technique

developed by Robert Marion at Sandia

Laboratories over a decade ago was successfully employed on a specific project to

measure uniaxial strain of material

specimens heated to 3000 K. Marion's

system used an optical tracker to measure the

elongation between targets attached to the

specimen surface. Although used primarily

for dynamic uniaxial measurements, it also

can be applied to static measurements. The

requirement of attaching a target to the

specimen surface can be a significant constraint; specifically, the cement nodule targets used by Marion would be a drawback in

many strain measurement applications.

Development of thin film targets would minimize the problems associated with large targets. The relative simplicity of the Marion

method makes it an attractive candidate for

general laboratory use and possibly for

industrial use. We believe that the Marion

technique holds the potential for becoming a

useful method of uniaxial strain measurement of high temperature ceramics given

relatively modest funding.

3.9.4 Dual Beam Doppler

Interferometer

Optra, Inc. has developed a commercial noncontacting extensometer that is suitable for

real time displacement and strain measurement for high temperature material testing.

The Optra extensometer is capable of very

high resolution measurements (less than 10

microstrain) under favorable test conditions.

The upper temperature limit at which strain

can be measured reliably has not been determined. Background radiation seems to

become a significant problem above 1500 K,

although the manufacturer believes that substantially higher temperatures can be

tolerated if optical bandpass filters are used.

Thermal gradients in the sight path also may

introduce measurement errors, although the

manufacturer believes these should not pose

a major problem for steady state measurements. The OPTRA extensometer requires

that rigid body movements be carefully controlled, preferably to within 10 times the

movement due to strain. This stringent

requirement may be relaxed significantly by

optically tracking the rigid body movements.

Such an enhancement could allow this instrument to be used outside of the controlled

laboratory environment.

The Optra extensometer is no panacea for

high temperature strain measurement. Rigid

body movement and surface tilt may cause a

loss of phase correlation and thereby contribute significantly to total measurement

error (Optra refers to such errors as "deadreckoning errors"). In its current state, the

75
Optra systen affords no way to eliminate or

determine the magnitude of dead reckoning

errors, although Optra believes that integrating optical tracking into the system may help

solve this problem. Additionally, the extent

to which multiple reflections from subsurface scattering centers may constrain

measurements on ceramic materials has not

yet been determined. However, the Optra

system is sufficiently advanced that we

believe that it has application in high

temperature strain measurement. Enhancemerits to the existing design may allow the

Optra extensometer to play a major role in

material testing of ceramics in hostile

environments. We therefore recommend the

Optra extensometer as a candidate for further development.

76
4.0 Heat Flux

4.1 Background Information

Heat flux sensors described in the technical

literature can generally be categorized as

steady state or transient measurement

devices. In basic terms, a steady-state device

consists of a means for generating a differential temperature across a thermal barrier

such that temperature difference is proportional to heat flux. Transient devices are

capable of generating an output proportional

to a change in heat flux.

Practical applications for heat flux sensors

related to development of advanced gas turbine engines are generally related to verification of computer models used to generate

input for thermal analyses of hot section

components. A typical environment for both

rotating and stationary components is a combustion gas flowpath. Steady-state heat flux

sensors are suitable for most applications

involving cooled structural components

based on current materials limitations.

Advances in materials temperature cability

may lead to use of noncooled (nonmetallic)

components for which transient heat flux

sensors will be needed.

The requirements for steady-state and transient heat flux transducers for application in

a gas turbine hot section component include

the following.

• Must be capable of surviving in the test

environment

Must generate a signal proportional to

heat flux or change in heat flux with minimum interference with heat flow

• Must not cause significant aerodynamic

interference

• Must be installed in, or be a part of, the

component being tested

Response changes caused by variations

in absolute temperature level must be

repeatable and calibratable

• Must have spatial resolution consistent

with requirements.

Although the literature survey did not

include any references to heat flux sensors

suitable for use on ceramic materials at

temperatures above 1350 K, it did turn up a

significant quantity of useful information

about heat flux sensors for use on superalloy

materials operating at temperatures up to

1250 IC Such sensors are of interest because

they are directly applicable to hot section

components using thermal barrier coatings

over superalloy materials.

The concepts which have been most

thoroughly investigated include the

following:

1. Embedded Thermocouple (normal

gradient sensor)

2. Gardon Gage (lateral gradient sensor)

3. Slug Calorimeter

4. Surface Mounted Sensor

5. Indirect Measurement.

77
General characteristics of various types of

heat flux sensors are compared in Table 8.

4.2 Concept Description and

Adaptability to Ceramic

Materials

4.2.1 Embedded Thermocouple

The concept is usable for both steady state

and transient measurements where the conditional assumption of one dimensional heat

flow is valid (Reference 4-1). Steady-state

heat flux through the wall is a function of

temperature drop across a known wall thickness according to the following relationship.

Q = k/x (T1 - T2) (19)

Where:

Q = Heat Flux

k = Thermal Conductivity

x = Wall Thickness Between T1 and T2

T1 = Wall Temperature, Hot Side

'I"2 = Wall Temperature, Cold Side

Under conditions of transient heat flux, it is

necessary to measure both the differential

temperature across the wall and the absolute

temperature of either wall. The equation

which relates heat flux to temperature and

time is as follows.

Q(t) = k/x[Tl(t + xE/ad)

- T2 (t- x2/6d)] (20)

Where:

t = time

d = diffusity of wall

78

The embedded thermocouple concept

(Figure 12) is directly applicable to metal

parts which can be slotted to accept the sensots. Good results up to 1250 K have been

reported (Reference 4-2). The concept is

adaptable to fabrication as a separate entity

that can be calibrated and subsequently

welded into the component where it will be

used. An alternate fabrication method consists of manufacturing the sensor directly into

the component. This may complicate the

calibration procedures but will avoid compromising the component or its coating by

joining the preassembled sensor.

A serious limitation is related to the need to

have access to the cold side for sensor installation and/or lead routing. This would limit

usefulness because many hot section components of interest (such as airfoils with

internal cooling passages) do not have cold

side access required for sensor installation.

The problem of finding a suitable thermocouple alloy may be serious, depending

on upper temperature capability requirement. As shown in Figure 12, the metal wall

is one element of the differential thermocouple. Choices for the other thermocouple element become more limited as

temperature levels increase.

Adaptability to ceramic materials is severely

limited. It is unlikely that grooves will be permitted for placement of the thermocouples,

as this would lead to structural compromise.

For nonconducting ceramics, the concept of

a differential thermocouple becomes a significant design task.

4.2.2 Gardon Gage

This concept, as illustrated in Figure 13, can

be used for both steady-state and transient

measurements and depends on the radial
I I I 1 I I I I I I I I I I I I I 1 I

Table 8. Heat FluxSensor Concept Characteristics.

galahad

Tempwatum

_¢roesWd

(EnCaxlded

Tunl_mnure

in_dWal

(Garden_)

S_O_

k

TmUe

m_

aani_ (Sudace

Uounwd 11VnRim

Senlw

Sudace Nounled

"nVnFire Semor

Indlmd MeesummenlUes a 81n01eTeml:)walureSemor al I

Spdd_ Loca_n so

Vw'W Rnile _t

Model

D_dW,,vV,,n_

RadlaVonP_rommr

To

Meud Coating Noec_clu_in 9

8ubslrate Ova"Metal Ceramic

Yes Yes No

Yes

No Yes

Temperature

LJn_K

1255

(Type K)

1650

(Ty_ s)

M_bm_n

Pmt)eb4My 6en,lng

al 6ucce_, Ares.

% mm

8O

4O

No 1250 80

(Tin, K)

1650 40

(TypeS)

Yes 1650 50

Yes 1650 40

(Ty_ S)

Yes 1650 50

Yes 1650 50

Only Wleh

Coadn0

No UnVl

1.5

1.5

15.0

1.0

15.0

1.0

50 1.0

Minimum

Wal Rework

Thickness. to

mm Part

1.0 Grooves

forLeads

1.5 _ Fram

Cold Sk/e

to Form

Foil

1.0 Fw

,Senmg AlL1.0 Hot Side

Shalow

Gmove

UOOW.

D._h)

1.o NO

Depends Depends on

on Model Modeland

and Sensor Location

Type Seleclod

No Minimum No

Cold ,5% Accuracy

,o,

oYes Yes-aliHd

onGaliinlinn

Yes

Yes

No

No

No

No

• Study S,-U w

Tr_

(,ee F_re 11)

Yes-Based • SteadySlam or

: on Calballon Transient

(eso Figure 12)

yes_ • TranslenlOnly

0nCalaxd0n (NO Figure 13)

Yes-Based • Steady StaZe

m Ca_mkm (see flOu_ 15)

Yes-Based • Transt_l Only

on Cal_allon

Yes

QuesUanalde

• Requiresa Well

Oeveloped

Model to be

Effective

• L_S_I

Requ_ed

• Swady S_

O0

"n::M

0_,

;x]r-

_0

rrq
Q

0.25 mm Sheath

_ +'-_ . \

+..+

1 mm rain

w+

Junctioml = X

Differential Thermocoupte Formed

Between Alloy A Wires and Sensor

Wall Material

L

\

T

Ab+okde

Temperalure

Figure 12. Embedded Thermocouple Heat Flux Sensor Concept Schematic.

Q

_i • T-- -- Foil Thickness ,.

_ i 7 L___ InsulatedCavity X

/

Differential Thermocouple Formed

Between a Pair of Common Alloy

Leads and Sensor Body

Figure t& Gardon Gage C<xcept Schematic.

80
flow of heat from the center of the sensor

(thin foil disk) to the sidewall heat sink. For

steady-state conditions, the basic relationship between heat flux and differential

temperature is as follows (Reference 4-1):

Q = 41odr 2 (Tt -T2) (21)

Where:

Q = Heat Flux

k = Thermal Conductivity

x = Foil Thickness

r = Foil Radius

T1 = Temperature at Center of Disk

T2 = Temperature at Radius of Disk

For transient heat flux measurements with

frequency components less than 4d/r 2

rad/sec, the relationship becomes

(Reference 4-1):

Q = oc[3/4(dT1/dt) + 1/4(dT2/dt)

+ 4d/r2 fit-T2)

Where"

o = Disk Density

c = Disk Specific Heat

d = Disk Diffusivity

These units are commercially available for

use up to 1300 K and have been evaluated in

References 4-2 and 4-3. Units have been

fabricated as part of a component by machining a flat-bottom hole into the wall from the

cold side leaving a thin foil disk which is

instrumented with a differential thermocouple from the cold side.

The need for cold side access represents a

serious limitation for practical applications.

A thermal barrier coating over the sensor

would reduce the sensitivity by increasing the

effective thickness of the foil disk. Adaptability to ceramic parts is difficult because of

geometry considerations and limited differential thermocouple design problems.

4.2.3 Slug Calorimeter

This concept is based on measurement of

time rate of change of absolute temperature

of a mass with thermal inertia exposed to a

change in heat flux. Figure 14 illustrates the

concept and additional information is

included in References 4-1 and 4-4.

The sensor can be added to or made as part

of the wall of the component. If heat flux

gradients exist along the component wall and

local fluxes are required, a means of thermally isolating sections of the wall around the

sensor is needed.

Cold wall access is necessary, with inherent

restrictions concerning limited usefulness

associated with many components and difficulty in adapting to ceramic components.

Because of interference with the measurement of temperature, the technique is not

practical where a coolant fluid is present.

4.2.4 Surface-Mounted Sensor

This concept consists of two temperature

sensors isolated from each other by a thermal

barrier as illustrated in Figure 15. When such

a device is mounted on a surface through

which heat is flowing, a difference in

temperature across the thermal barrier

proportional to heat flux is generated in

accordance with the equation for embedded

thermocouple heat flux sensors operating in

the steady-state mode. Although most

applications of this technique reported in the

literature are not suitable for high temperature (References 4-5 and 4-6), there is reason

81
Q

l CH°t

old

Thermal Isolation Required.-J 7

to IsolateSensor from Absolute Temperature

Lateral Flux Gradients Sensor

Figure 14. Slug Calorimeter Concept Schematic.

Q

Thermal Barrier

Bonded to Surface

Thermocouple

with One Junction on Each

Side of Thermal Barrier

Figure 15. Surface Mounted Heat FluxSensor UsingPla,_ic or Kapton for Thermal Barrier.

82
for optimism concerning the use of thin film

sensors in a configuration which could

extend the upper temperature limit to the

point where it could be of interest for the current program.

A major advantage of this sensor is that it

does not require access to the cold side. It

does require attachment to the hot side surface and may be thick enough to cause

boundary layer interference unless recessed

in the component surface or used under thermal barrier coatings. Development of thin

film technology is required for successful

application of this concept for temperature

levels beyond 1250 IC

4.2.5 Indirect Measurement

This concept is based not on a direct

measurement of heat flux but on its inference

from temperature measurements and data

matching techniques. This has been the

method most often used in gas turbine

applications in the absence of acceptable

direct heat flux measurement techniques.

Advances in finite element modeling and

inverse conduction codes have significantly

improved this procedure. Measurement of

absolute temperatures at specific locations is

required (Reference 4-4). The concept is

appropriate for both steady-state and transient heat flux modes. This technology is

directly applicable for thermal barrier coated

pans. The adaptation of the concept for use

with ceramic composites operating at high

temperatures depends on the development

of techniques for measurement of surface or

internal temperature at specific locations in

such materials.

4.2.6 Radiation Type Heat Flux Sensor

No reference was found in the literature

regarding this concept. The idea is based on

the fact that some ceramic materials may be

transparent at certain wavelengths and

opaque at others. Considering the case of a

thermal barrier coated blade (100-500 _Tl of

coating on a metal substrate), it should be

possible to measure either the metal substrate surface or the coating surface temperature by selecting two radiation pyrometer

detector wavelengths. If the thermal conductivity and thickness of the coating is known,

it will be possible to calculate heat flux. This

also necessitates a knowledge of surface

emittances at both wavelengths. Temperature change across the coating needs to be

relatively high to allow determination of differential temperature from two absolute

values.

4.3 Heat Flux Sensors

Recommended for Further

Development

4.3.1 Surface-Mounted (Differential)

Sensor

Adaptation of the classical concept of differential temperature across a thermal barrier is shown in Figure 16. This sensor does

not require access to the cold side at the point

of measurement. It does require developmerit of thin film technology beyond current

capabilities. Use of SiC as the sensing element would limit upper temperature to 1850

K in air due to softening of SiO2 which forms

on the surface of SiC. Insulation resistance

may restrict upper temperature limit to much

lower levels, depending on the resistivity of

the substrate. Application and doping of the

SiC films will require extensive process

development.

Interference with boundary layer due to the

sensor application thickness may require

mounting below the normal substrate sur83
Q

5-10 lu_ ---_- :_--.- Top Coat

10 Wn ---_! __vmx, c_m_w. ,,_w_A._..---SiC (type P)

25 I_n _ _....._____________________________ Insulating Film

25

2514rn-_ :.: Insulating Film

10 I_m---_- __4-SiC (type P)

5-10 I_m---_ ____:_:_`_i_i__- Insulating Film

Estimated Transducer Sensitivity = 3 I_V/KW/m 2

Based on:

&T = temperature difference, K

Q = heat flux, 5xl06W/M 2

&x = transducer thickness, 75x10 -s M

k = thermal conductivity, 2.4x10 -2 KW/MK

75x10 "s

&T across (SiC)N = 5000x = 15.625 K

2o4Xl 0-2

Volts output = 15.625 K x 960.-_- 15,000 I_V

Transducer sensitivity = 15,000 FV =3 _V

5,000 KW/M 2 KW/M 2

V

Note: 960 .._.vTherrnoelectdc potential based on measurements between (SiC)N and (SiC)p

K at 1100 K (byGE)

Figure 16. Surface Mounted Heat Flux Sensor for High Temperature Applications.

8,t
face. Sensor interference with heat flux will

depend on thickness and property differences between sensor and substrate.

4.3.2 Thin Film Surface Mounted

(Absolute) Sensor

This concept is based on application of a thin

film temperature sensor on the surface of a

component exposed to transient heat flux

normal to the surface. If the thin film sensor

has insignificant temperature drop across its

thickness and if the component wall can be

considered infinitely thick, then a transient

heat flux can be determined by measuring the

time rate of change of the surface mounted

thermocouple in accordance with the following (Reference 4-1):

1 ,--- . [ T{t) ! f'_'_T(t)- _TO.) _ ,

Q = Heat Flux

t = Time

p = Density of Wall

c = Specific Heat

k = Thermal Conductivity

Dummy Time Variable for

Integration

Although there are specific limits on sensor

and wall thicknesses and transient heat flux

frequency components, this type of sensor

appears to have potential for use on ceramic

components. Access from one side only is

necessary and no rework is required.

4.3.3 Indirect Measurement

Although this is not a specific technique for

direct measurement of heat flux, it does allow

validation of thermal analysis modeling techniques and should be considered for that purpose. This method is applicable to thermal

barrier coated components using embedded

thermocouples in the metal substrate and on

ceramic composite components using surface mounted thermocouples or radiation

pyrometer readings of surface temperature.

85
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98
Appendix- Excerpts from NAS3-25140 Statement of Work

Sensors for Ceramic

Components in Advanced

Propulsion Systems

Task I - Survey of Sensor Concepts

The Contractor shall conduct a survey of sensors that have the potential for measuring

surface temperature, strain, and heat flux on

ceramic components in advanced propulsion

systems. The Contractor shall compile a list

of potential sensor concepts using as a minimum the following approaches: (1) a literature search; (2) consultation with recognized

authorities; (3) adaptation of existing concepts to this application; (4) development of

advanced concepts. Exhibit A shall be used

to define the materials of interest and the

parameter range of application.

Task 2 - Analysis of Sensor Concepts

The Contractor shall perform an analysis of

all sensor concepts identified in Task 1. As a

minimum, the following factors shall be addressed in the analysis:

1. Usable temperature range

2. Accuracy

3. Compatibility with propulsion systems'

environment

4. Compatibility of materials in contact

(i.e., sensor/substrate compatibility)

5. Life expectancy

6. Size

1

8.

99

Cost

Data acquisition and reduction requirements

9. Fabricability

Exhibit A - Materials and Parameter Ranges

to be Considered for Study on Sensors for

Ceramic Components in Advanced Propulsion Systems

Maximum

Materials Temperature

Thermal Barrier Coating,

ZrO_Y203 1650K

Monolithic Ceramic Si3N4 and SiC 1900K

Ceramic Composite Si3N4/SiC 1900K

Carbon/Carbon Composite 2200K

Strain: Strain levels to be measured on these

materials extend to the limits imposed by

fracture.

Heat Flux: Anticipated heat fluxes in advanced propulsion systems may be as high as

5MW/m 2.

Exhibit B - Performance Goals Overall
Overall Measure= Maximum

merit Error Acceptable Target Goal

Temperature

Strain (Static)

Strain (Dynamic)

Heat Flux

+ 5% ofAbs Temp

+ 15% of F.S. Strain

+ 10% of F.S. Strain

+ 10% of F.S. Heat Flux

+ 2% of Abs Temp

+ 10% of F.S. Strain

+ 5% of F.S. Strain

+ 5% of F.S. Heat Flux

Sensor Lifetime Goal: 50% Survival After 50 hr. in Test Environment

I00
_A Report Documentation Page

Na_lon_ Aerc_aul_$ and

I. Report No.

NASA CR180900 I 2. Government ACC._AFFIOn NO.

4. Title and Subtitle

Sensors for Ceramic Components in Advanced

Propulsion Systems - Summary of Literature

Survey and Concept Analysis - Task 3 Report

7. Author(s)

W.H. Bennethum, L.T. Sherwood

9. Performing O_anization Name and _dress

General Electric Company

Aircraft Engines

Cincinnati, OH

12. S_nsoring Agen_ Name and Address

NASA

Lewis ResearCh Center

21000 Brookpark Road

Cleveland, OH 44135

75. Su_lem_taW Notes

NASA Lewis Research Center Project Manager,

and Control Technology Office.

3. Recipient s _-t_-Kxj ilu.

5. Report Date

July 1988

6. Performing Organization Code

8. Perfov_n;n,.3 Organization Report No.

10. Work Unit NO.

535-07-01

11. Contract or Grant No.

NAS3-25140

13. Type o! Report and Period Covered

Contractor Report-lnteri_

April --December 1987

14. Sponsoring Agency Code

Frank G. Pollack, Instrumentation

16. Abstra_

This report summarizes the results of a literature survey and concept analysis

related to sensing techniques for measurement of surface temperature, strain,

and heat flux for (non-specific) ceramic materials exposed to elevated

temperatures (up to 2200 K). Concepts capable of functioning in a gas turbine

hot section environment are favored but others are reviewed also. Recommendations

for sensor development in each of the three areas are as follows:

I. Surface Temperature - Thin film thermocouples Pt-Rh alloys and SiC

- Long wavelength (8-I0 um) radiation pyrometer

2. Strain - Spectral density analysis

- Optical tracking of attached targets

Dual beam doppler interferometer

3. Heat Flux - Surface mounted differential sensor

- Surface mounted absolute sensor

- Indirect technique

17. Key Words (Suggested by Authods))

Propulsion Systems

Ceramics

Sensors

19. Security Classif. (of this report)

Unclassified

NASA FORM 1626 OCT 86

Temperature

Strain

Heat Flux

18. Distribution Statement

Publicly Available

Star Category 35

20. Security Unclassi Glassif. fied (of this page)

21 No of pages

106

22. Price"

"For sale by the National Technical Information Service, Springfield, Virginia 22161

i01
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Technical Center, Building F

i00 Northeast Adams Street

Peoria, IL 61629

combustion Engineering

Attn: John Fishburn

Dept. 9001-2226

Windsor, CT 06095

combustion Engineering

Attn: J. Niziolek

Dept. 9005-0301

Windsor, CT 06095

Cranfield Institute of Technology

Attn: Dr. Robin L. Elder

School of Mechanical Engineering

Cranfield, Bedford MK43 OAL

UNITED KINGDOM

Dantec Electronik

Attn: Dr. Lars Lading

Medicinsk og Videnskabeligt

Maleudstyr A/S

Mile Parken 22, DK-2740 Skovlunde

DENMARK

DFVLR

Attn: Dr. Ing R. Schodl

Inst. F_r Antriebstechnik

Postfach 90 60 58

5000 K_in 90

FEDERAL REPUBLIC OF GERMANY

Douglas Aircraft Company

Attn: J. R. S. Findlay

Intl, Mail Code 36-41

3855 Lakewood Blvd.

Long Beach, CA 90846
Eaton Corporation

Attn: Earl D. Jacobs, President

Electronic Instrumentation Division

5340 Alla Road

Los Angeles, CA 90066

Eaton Corporation

Attn: Dr. Lamont Eltinge

P. O. Box 766

Southfield, MI 48037

FluiDyne Engr. Corporation

Attn: Mat Matsuura, Group Leader

Controls and Instrumentation

5900 Olson Memorial Highway

Minneapolis, MN 55422

Garrett Turbine Engine Co_any

Attn: N. Fred Pratt

M/S 47-40/302-2 C

P. O. Box 5217

Phoenix, AZ 85010

GarrettTurbine Engine Company

Attn: Mr. Joe Allan

M/S 47-451/320-2

P. O. Box 5217

Phoenix, AZ 85010

General Electric Company

Attn: Patrick J. Poinsot, Engineer

Stress &Vibration Instrumentation

1000 Western Avenue, M.Z. 129 KD

Lynn, MA 01910

General Electric Company

Attn: Ronald J. Casagrande-MS 43A

Aircraft Equipment Division

50 FordhamRoad

Wilmington, MA 01887

General Electric Company

Aircraft Engine Business Group

Attn: Gary Poppel, MSH-45

Evendale, OH 45215

General Electric Company

Aircraft Engine Business Group

Attn: Paul W. Mossey

Mail Stop H-78

Cincinnati, OH 45215

General Electric Company

Aircraft Engine Business Group

Attn: E. L. Pauly

Mall Stop H-78

Evandale, OH 45215

General Motors Corporation

Allison Gas Turbine Division

Attn: Mr. Ed Nitka

Speed Code T-12

P. O. Box 420

Indianapolis,, IN 46206-0420

General Motors Corporation

Allison Gas Turbine Division

Attn: David Willis

P. O. Box 420

Indianapolis,, IN 46206-0420

General Motors Corporation

Allison Gas Turbine Division

Attn: Ralph Fox, MS WI6

P. O. Box 420

Indianapolis, IN 46206-0420

General Motors Corporation

Allison Gas Turbine Division

Attn: Dr. Donald R. Zi_,erman

P. O. Box 420

Indianapolis, IN 46206-0420

General Motors Corporation

Allison Gas Turbine Division

Attn: Ken Cross

P. O. Box 420

Indianapolis, IN 46206-0420

HITEC Corporation

Attn: Steve Wnuk

PO Box 790

87 Fitchburg Rd

Ayer, MA 01432

Kulite Semiconductor Products, Inc.

Attn: John R. Hayer

1039 Hoyt Avenue

Ridgefield, NJ 07657

Lewis Engineering Company

Attn: C. B. Stegner

238 Water Street

Naugatuck, CT 06770

104
MassachusettsInst. of Technology

Attn: Dr. Alan Epstein, Rm. 31-266

Cambridge, MA 02139

Medtherm Corporation

Attn: Larry Jones

P. O. Box 412

Hunstville, AL 35804

National Bureau of Standards

Attn: George Burns

Inst. for Basic Research

Washington, DC 20234

National Bureau of Standards

Attn: Ken Kreider, 221-B312

Washington, DC 20234

Naval Air Propulsion Center

Attn: Gaeian J. Mangano

P. O. Box 7176

Trenton, NJ 08628

Naval Air Propulsion Center

Attn: Russ Vizzini/Code PE32

Trenton, NJ 08628-0176

Naval Air Propulsion Test Center

Attn: Joseph De La Cruz

P. O. Box 1776

Trenton, NJ 08628

Naval Air Systems Command

Attn: Andrew Glista/Code 933E

Washington, DC 20361

Naval Post Graduate School

Attn: Prof. R. P. Shreeve

Department of Aeronutics (Code 67Sf)

Monterey, CA 93943

O.N.E.R.A.

Attn: Alain Boutier

29 Ave. de la Division Le Clerc

92320 Chatillon Sous Bagneus

FRANCE

Pennsylvania State University

Attn: Prof. B. Lakshminarayana

233 Hammond Building

University Park, PA 16802

Physical Sciences Dept.

Attn: M. G. Dunn

Arvin/Calspan Adv. Tech. Ctr.

Buffalo, NY 14225

PRC Kentron

Attn: Dr. M. M. Lencoe

P. O. Box 273

Facility Support Complex (FSC)

Edwards, CA 93523-5000

Public Service Elect. & Gas Company

Attn: Dr. Melvin L. Zwillenberg

Research & Development Dept.

80 Park Plaza

Newark, NJ 07101

RdF Corporation

Attn: Frank Hines

23 Elm Avenue

Hudson, NH 03051

Rockwell International

Rocketdyne Division

Attn: Mr. Sarkis Barkhoudarian

6633 Canoga Avenue

Canoga Park, CA 91303

Rockwell International

Rocketdyne Division

Attn: Dr. John C. Lee

6633 Canoga Avenue

Canoga Park, CA 91303

Rosemont, Inc.

Attn: Mr. Larry Miller

Mail Stop F-10

P. O. Box 959

Burnsville, MN 55337

Spectron Development Laboratories

Attn: Dr. Chris W. Busch

3535 Hyland Ave.

Suite 102

Costa Mesa, CA 92626

Stanford University

Attn: Dr. R. J. Moffatt

Asst. Prof., Mech. Engr.

Thermosciences Division

Stanford, CA 94305

105
Stein EngineeringServices, Inc.

Attn: Mr. Peter K. Stein

5602 East Monte Rosa

Phoenix, AZ 85018-4646

Sverdrup (AEDC)

Attn: Paul McCarty

M/S 930

Arnold AFB, TN 37389

Teledyne CAE

Attn: Julie J. Senn

1330 Laskey Road

P. O. Box 6971

Toledo, OH 43612-0971

Teledyne CAE

Attn: Joseph Pacholec

1330 LaskeyRoad

Toledo, OH 43612

Teledyne CAE

Attn: Jim Shortridge

1330 LaskeyRoad

P. O. Box 6971

Toledo, OH 43612-0971

The Univ. of Tennessee Space Institute

Attn: James W. L. Lewis

Applied Physics Research Group

Tullahoma, TN 37388

Thermogage, Inc.

Attn: Charles E. Brookley

330 AlleganyStreet

Frostburg, MD 21534

Thermonetics Corporation

Attn: H. J. Poppendiek

1028 Garnet Avenue

San Diego, CA 92109

TSI Instruments

Attn: Dr. Rajan Menon

P. O. Box 64394

St. Paul, MN 55164

United Technologies Corporation

Pratt and Whitney Aircraft

Attn: M. Carl Williams, MS 116-01

Engineering Division, MS 162-29

400 Main Street

East Hartford, CT 06108

106

United Technologies Corporation

Pratt and Whitney Aircraft

Attn: W. Gilbert Alwang

Engineering Division, MS 162-29

400 Main Street

East Hartford, CT 06108

United Technologies Corp.

Pratt &WhitneyAircraft

Attn: WilliamWatkins, MS 731-36

Main Plant

P. O. Box 109600

West Palm Beach, FL 33410

United Technologies Corporation

Pratt and Whitney Aircraft

Attn: Bill Davies, MS 731-671

P.O. BOx 109600

West Palm Beach, FL 33410-9600

United TechnologiesOptlcal Systems

Attn: Dr. JaimeRoman

P. O. BOx 4181

West Palm Beach, FL 33401

United Technologies Research Center

Attn: Dr. A. Eckbreth

Silver Lane

East Hartford, £T 06108

United Technologies Research Center

Attn: Dr. G. M. Dobbs

Silver Lane

East Hartford, CT 06108

Virginia Polytechnic Institute

and State University

Attn: W. F. O'Brien, Jr.

Mechanical Engineering Dept.

Blacksburg, VA 24061

Williams International

Attn: Chloral L. Shew, MS 4-10

P.O. Box 200

Walled Lake, MI 48088

Williams International

Attn: David Easton

P.O. Box 200

Walled Lake, MI 48088

i