NBSIR 79-162

W'

MATERIALS FOR CRYOGENIC WIND TUNNEL

TESTING

R. L. Tobler

Fracture and Deformation Division

National Measurement Laboratory

National Bureau of Standards

Boulder, Colorado 80303

Prepared for:

National Aeronautics and Space Administration

Systems Engineering Division

Langley Research Center

Hampton, Virginia 23365

May 1980

NBSIR 79-1624

MATERIALS FOR CRYOGENIC WIND TUNNEL

TESTING

R. L. Tobler

Fracture and Deformation Division

National Measurement Laboratory

National Bureau of Standards

Boulder, Colorado 80303

May 1980

Prepared for:

National Aeronautics and Space Administration

Systems Engineering Division

Langley Research Center

Hampton, Virginia 23365

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

FOREWORD

The information in this report derives from literature surveys as well . as inputs from NBS scientists with experience in cryogenics. The data

presented are believed to be typical of materials in common forms and

conditions. There is always some uncertainty in material properties, but

errors can be greatly reduced by making sure that the materials used in

design are equivalent in composition and condition to the materials for

which handbook data are available. In this report some materials are classified as unacceptable for general cryogenic applications, based on conventional criteria. However, the "unacceptable" materials may be satisfactory for specific applications or exceptional circumstances, if suitability can be demonstrated by qualification tests.

This document has been prepared for the use of the National Aeronautics

and Space Administration. Responsibility for its further use rests with that

agency. NBS requests that if release to the public is contemplated, such

action be taken only after consultation with the Technical Information and

Publications Division at the National Bureau of Standards.

The author is indebted to L. L. Sparks, F. R. Fickett, J. G. Hust

(Thermophysical Properties Division, NBS) and M. B. Kasen (Fracture and

Deformation Division, NBS) for contributing information from their unpublished

review manuscripts in the areas of instrumentation, electrical properties,

thermal properties, and structural composites, respectively.

TRADE NAME DISCLAIMER

The use of tradenames of specific products in this paper is essential

to facilitate an understanding of the materials described. The use of tradenames for this purpose in no way implies endorsement or exclusive recommendation by NBS. Alternative materials and commercial suppliers are listed in buyers' guides such as the Materials Selector which appears annually in the

Journal of Materials Engineering .

ii i

TABLE OF CONTENTS

Page

1. INTRODUCTION 1 1.1. The National Transonic Facility 1 1.2, Purpose of This Materials Study 2

2. MODEL DESIGN CONSIDERATIONS 3 2.1. Conventional Structural Design Approach 3 2.2. Recommended NTF Structural Design Approach 3 2.3. Fracture Mechanics 3 2.4. Alloy Selection for Fracture-Resistant Design 5

3. CRYOGENIC PROPERTY TRENDS 7 3.1. Tensile Properties 7 3.2. Fatigue Properties 7 3.3* Charpy Impact and Fracture Toughness Parameters 12

3.4. Elastic Properties 12

3.5. Electrical Resistivity and Conductivity 14

3.5.1. Metal Conductors and Alloys 14

3.5.2. Dielectrics (Insulators) 19

3.6. Thermal Expansion and Contraction 19

3.7. Thermal Conductivity 23

3.8. Martensitic Phase Transformation 26

3.8.1, Selection of Stable Stainless Steels 28

3.8.2, Selection of Metastable Stainless Steels 28

4. METALS AND ALLOYS FOR STRUCTURAL APPLICATIONS 30

4.1. Carbon and Alloy Steels 30

4.2. Nickel Alloy Steels 30

4.2.1. Austenitic Fe-Ni Alloys 32

4.2.2. Ferritic Fe-Ni Steels 33

4.2.3. 18Ni Maraging Steels 34

4.2.4. Other Ferrous Alloys 35

4.3. Stainless Steels 35

4.4. Aluminum and Its Alloys 41

4.5. Copper and Its Alloys 46

4.6. Nickel and Its Alloys 48

4.7. Titanium and Its Alloys 50

4.8. Cobalt and Magnesium 53

5. METALS FOR BRAZING AND SOLDERING 54

5.1. Fillers and Fluxes 54

5.2. Brazing 54

5.3, Soldering 56

5.4, Joints 59

6. METALS FOR PLATING AND PROTECTION 60

v
Page

7. NONMETALS 61

7.1. Wood 61

7.2. Glass 62

7.3. Ceramics 68

7.4. Polymers 69

7.5. Filled Polymers 72

8. FOAMS, FILMS, AND FIBERS 75

8.1. Foams 75

8.2. Films 75

8.3. Fibers 75

9. STRUCTURAL COMPOSITE LAMINATES 79

9.1. Industrial Polymer Laminates 79

9.2. Low Pressure Polymer Laminates 79

9.3. Metal Matrix Laminates 84

9.4 Selective Reinforcement 84

10. ADHESIVES, SEALANTS, AND RELATED MATERIALS 85

10.1. Adhesives 85

10.2. Sealants 86

10.3. Potting Compounds 86

10.4. Contact Agents 86

10.4.1- Varnish 86

10.4.2. Grease 87

11. SEALS 88

11.1. Nondemountabl e Seals 88

11.2. Demountable Seals 88

11.2.1- Design Considerations 88

11.2.2. Gasket and 0-Ring Seals 88

11.2.3. Temperature~Assisted Seals 89

11.2.4. Spring and Pressure-Assisted Seals 89

11.3. Seal Materials 89

11.3.1. Elastomers 90

11.3.2. Other Polymers 90

11.3.3. Metals 90

12. FRICTION, WEAR, AND LUBRICATION 92

12.1. Design Considerations 92

12.2. The Necessity of Dry Lubrication 92

12.3. Friction Coefficients 93

12.3.1, Unalloyed Metals 93

12.3.2, Alloys 93

12.3.3, Laminar Solids 95

12.3.4- Polymers 95

12.3.5- Filled Polymers 97

12.3.6- Polymer Films and Coatings 97

12.3.7. Ceramics, Cermets, and Glass 97

vi
Page 13. INSTRUMENTATION 99

13.1. Temperature Sensors 99

13.1.1, Thermocouples 99

13.1.2, Resistance Thermometers 99

13.1.3, Bondable Temperature Gages 102

13.1.4, Thermistors 102

13.1.5, Filled Systems 102

13.1.6- Combined Thermometers 102

13.2. Strain Gage Sensors 102

13.2.1. Gage Selection 103

13.2.2. Gage Factors 103

13.2.3. Apparent Strain 103

13.2.4. Self-Temperature Compensation 103

13.3. Pressure Sensors 104

13.4. Sensor-to-Specimen Contact 104

13. 4.1- Thermal Anchoring 104

13.4.2- Thermal or Electrical Contact Agents 104

13.5- Signal Maintenance 105

13.5.1. Wire Insulation 105

13.5.2. Wire Connections 105

13.5.3. Feed-Throughs 105

13.5.4. Shielding and Grounding 105

13.5.5. Switching 107

14.

SOURCES OF CRYOGENIC DATA 108

14.1. Technical Journals 108

14.2. Monographs 108

14.3. Conference Proceedings 108

14.4. Computerized Data Banks 108

14.5. Abstracting Services 109

15.

REFERENCES 110

MATERIALS FOR CRYOGENIC WIND TUNNEL TESTING

R. L. Tobler

Fracture and Deformation Division

National Bureau of Standards

Boulder, Colorado 80303

A study was conducted to guide the evaluation and selection

of materials and techniques to be used in construction of model

aircraft for cryogenic wind tunnel testing. In this report, the

mechanical, thermal, and electrical property behavior of materials

at temperatures as low as 77 K is briefly reviewed. Metals, structural alloys, non-metals, composites, joining methods, coatings,

sealants, adhesives, contact agents, lubricants, transducers, and

instrumentation for cryogenic applications are discussed. Acceptable structural materials, conductors, and insulators are discussed for service at temperatures in the range 367 to 77 K. Numerous references to handbooks and other cryogenic data sources

are cited as a guide to additional information.

Key words: Cryogenic properties; instrumentation; low temperature

materials; materials properties; metals; non-metal lie materials;

wind tunnels.

1. INTRODUCTION

A wind tunnel is an aerodynamic flow loop for testing model aircraft under simulated

flight conditions to improve the performance and efficiency of full-scale aircraft. One

relevant design parameter is the chord Reynolds number which characterizes the fluid flow

past a structure by relating the velocity, density, and viscosity of the fluid to the

dimensions of the body being tested. Unfortunately, conventional wind tunnels using air at room temperature are not able to simulate the very high Reynolds numbers experienced by

modern aircraft operating in the transonic flight velocity range (Mach 0.8 to 1.2). This

limitation of conventional wind tunnels has severely hampered aerodynamic engineering for

the transonic range.

A unique solution to the problem was demonstrated in 1972 when NASA constructed a prototype (1/3-m square) cryogenic wind tunnel, proving that the necessary high Reynolds

numbers could be obtained using nitrogen gas at temperatures as low as 80 K. In fact, a tenfold increase in Reynolds numbers is possible, owing primarily to the higher density and

viscosity of cryogenic nitrogen compared to room temperature air. Cryogenic wind tunnels

also enable independent control of Reynolds number, Mach number, temperature, and pressure.

Thus, the effects of these variables can be separated and studied for the first time [1,2].

1.1. The National Transonic Facility

To take advantage of the benefits of cryogenic testing, NASA is currently constructing

a National Transonic Facility (NTF). The NTF incorporates a 60-m-long, fan-driven, closedcircuit, high-speed cryogenic wind tunnel capable of generating Mach numbers from 0.2 to

1.2. The 2.5-m square test section accomodates models built on a 1:10 scale, or up to 1.5-

m wingspan. The projected test pressures range from 100 to 900 kPa, while temperatures

from 77 to 367 K may be achieved.
Cooling in this facility will be achieved by evaporating liquid nitrogen into the

tunnel circuit at flow rates of up to 450 kg/s. Low temperature tests will require 3 h,

with a gradual cool-down to 80 K taking 2 h. After reaching thermal equilibrium at the

test temperature, the velocity of the circulating gas will be gradually increased. Severe

impact loading, such as encountered in some conventional blow-down tunnels, will be avoided,

but the static loads imposed on models and structural supports will be up to five times

higher compared with conventional wind tunnels.

1.2. Purpose of This Materials Study

Owing to differential thermal contraction and the embrittling effects of low temperatures, combined with extremely high stress, it is certain that many traditional model

construction materials and fabrication techniques are unacceptable for the newly developed

NTF applications. To date, model design has been based entirely on experience gained at room or elevated temperatures. Reliable design practices must now be developed for the

cryogenic region.

This study was undertaken to provide guidance in the evaluation, selection, and use of

materials for NTF model building. The text is addressed to design engineers who may be

totally unfamiliar with cryogenic technology. Contained here is an evaluation of potentially useful materials and techniques, representative data for specific materials of

interest, notes on acceptable cryogenic practice, and recommendations intended to support

materials selection and the development of design criteria for NTF models and structural

supports.

2
2. MODEL DESIGN CONSIDERATIONS

Wind tunnel models incorporate a broad range of materials and fabrication methods.

Several models may be needed for each proposed design or test temperature range. The

models often have moving or interchangeable parts, and most are instrumented for pressure,

temperature, or strain measurements. As listed in Table 2.1, metals and alloys are the

most widely used construction materials. Wood, glass, ceramics, polymers, and composites

have miscellaneous applications.

2.1.

Conventional Structural Design Approach

Wind tunnel models are traditionally designed by experience, intuition, and stress

calculations using safety factors of 3 on yield stress and 4 on ultimate tensile strength.

Fatigue resistance is evaluated using smooth or notched bar S versus N curves. This

approach assumes stresses are approximately uniformly distributed over a given cross sec- tion, so that safety factors are sufficient to mask differences between the assumed and

actual stress distributions. The approach is best suited to small, ductile components.

2.2.

Recommended NTF Structural Design Approach

The conventional design approach outlined above is adequate for many NTF structures,

particularly secondary structures. However, safety factors of 3 or 4 are restrictive and

impractical for critical primary components (e.g., model stings, balances, and fuselage

struts). In these structures, the design allowable stresses are projected to range up to 700 MPa, as opposed to 193 MPa for conventional models. The higher stresses coupled with

safety factors of 3 or 4 require a performance exceeding the limits of currently available

structural alloys. It is, therefore, recommended that reduced safety factors be permitted

for primary components and that fracture mechanics be employed to ensure structural safety

Safety factors as low as 1.5 are used in some cryogenic aerospace components that are subject to fracture mechanics analysis [3]. Fracture mechanics has not previously been

used in model design, but is recommended for NTF model design since:

1) For many alloys, low temperatures tend to increase the probability of brittle

fracture,

2) High design stresses tend to increase crack initiation and growth rates in struc- tures subject to low-cycle fatigue,

3) Primary structures require high-strength alloys that are flaw sensitive, and

4) Model fractures would be costly in terms of damage to the NTF itself.

2.3.

Fracture Mechanics

The application of fracture mechanics to cryogenic structures is reviewed by McHenry

[4], The goal of fracture mechanics is accurate component life prediction. Typically, a small crack is assumed to exist in a structure at a critical location. The material properties of interest then are the yield stress (a^), the plane strain fracture toughness

(Kj

c

), and the fatigue crack growth rate behavior (da/dN versus AK).

The stress intensity factor, K, is proportional to the magnitude of elastic stresses

at the crack tip. In service, K increases as the load increases, or as the crack grows

larger during fatigue. Failure occurs when the critical value, Kj

c

, is reached. Using

handbook equations for specific component geometries, the maximum flaw size that a cracked

structure can tolerate can be calculated based on the material's Kj value. Fatigue crack

growth rates then enable a prediction of the total number of stress

c cycles (lifetime)

required to propagate an initial or assumed crack to critical size.

The initial assumption that a flaw exists in the structure is made for the sake of

conservative life prediction. The assumption is particularly appropriate for high strength

alloys, welded structures, or structures subject to low-cycle fatigue.

3
Table 2.1. Materials for wind tunnel model construction.

Appl i cations

Materials used in conventional

wind tunnels at 295 K Materials recommendedwind tunnels at for cryogenic77 K

Primary and secondary Carbon steel Nickel alloy steels

structures Alloy steel

Stainless steel

Superalloys

Aluminum alloys

Austenitic stainless

Nickel superalloys

Titanium alloys

Al umi num al 1 oys

steels

Secondary structures Compos i tes Copper al 1 oys

Polyami des

Composi tes Copper al loys

Electrical conductors Precious metals Precious metals

or insulators Copper alloys

Ceramics

Glass

Polyamides

FI uoropl asti cs

Copper alloys

Ceramics

Glass

FI uoropl asti cs

Thermal conductors Copper al 1 oys Copper alloys

or insulators Cerami cs Glass

Elastomers

Polyami des

Reinforced plastics

Fluoroplastics

Ceramics

Glass

Reinforced plastics

FI uoropl asti cs

Lubricants Oils, grease

Precious metals

Dry films

Dry films

Seals Fluoroplastics

Elastomers

Fluoroplastics

Metals

Adhesives, fillers Epoxies Epoxies, modified expoxies

Plati ng Nickel, chromium

Precious metals

Nickel

Precious metals

4
2.4 Alloy Selection for Fracture-Resistant Design

It may be assumed that the fatigue crack growth rates of alloys from a particular base

metal system are nearly equivalent [5]. Initial alloy selection is then based on consideration of yield stress and fracture toughness. As shown in Fig. 2.1, and Kj

c are inversely related, with high strength alloys exhibiting lower toughness. Therefore a proper

balance between these parameters must be obtained for a particular application.

A general discussion of how o and Kj influence the design approach is given by

Crooker and Lange [5]. Wells, Kos^owsky , Eogsdon , and Daniel [6] provide one example of a parametric analysis illustrating alloy selection and life prediction. These references

provide a starting point for engineers seeking familiarity with fracture mechanics and

alloy selection. A figure of merit in alloy selection is the crack size factor,

(K

t /ct ) 2

. This factor is proportional to the critical flaw size for a given material, and

x c y

it accounts in a single term for the effects of both strength and toughness on flaw tolerance [7]. The best material for a given application is the one having the highest crack

size factor, assuming other requirements (e.g., cost, availability, etc.) are satisfied.

The absolute value of critical flaw size must be calculated for each application, because

it depends on the applied stress level and component geometry, as well as on material

properties.

5
100 250

YIELD STRESS, crv (ksi)

150

y200

CM

Figure 2.1. Toughness-versus-strength trend for structural metals

at 77 K.

6

FRACTURE

TOUGHNESS,

K

Tr

(ksi-in
3. CRYOGENIC PROPERTY TRENDS

This chapter describes the expected temperature dependence of mechanical, thermal, and

electrical properties of interest to model design. Selected design parameters, data

trends, data sources, measurement uncertainties, and data estimating techniques are discussed. Cryogenic data handbooks [8-17] are recommended as primary data sources, whereas

Wigley's text [18] is recommended as a basic introduction to cryogenic property behavior.

In this chapter, the aluminum alloy system is used to exemplify the regular cryogenic

behavior of face-centered cubic metals. Specific materials recommendations are given in

subsequent chapters.

3.1. Tensile Properties

ASTM Method E8-69 [19] describes standard procedures for measuring the yield and

ultimate stresses and ductility of metals and alloys in the unnotched condition. The 0.2%

yield stress is a quantitative design parameter, while reduction of area and elongation

values are qualitative guides to brittle fracture resistance. The uncertainty in yield

stress values owing to material variability and measurement errors is typically less than

± 10%.

Tensile property data are compiled in several handbooks [8-13,17], Materials generally show increased strengths at low temperatures (Fig. 3.1), but ductility may increase

or decrease. Cryogenic effects on metals and alloys are summarized by Wigley [18], Conrad

[20], and Wessel [21]. Face-centered cubic (fee) metals and alloys show desirable behavior

at low temperatures, whereas body-centered cubic (bcc) or hexagonal close packed (hep)

metals are embrittled. The classical trends for an fee metal are as shown in Fig. 3.2 for

aluminum: reduction of area is high, elongation increases, there is a modest increase of

yield stress and a greater increase of ultimate strength. As a result, austenitic stainless steels and alloys of copper, aluminum, and nickel are widely used for cryogenic

service.

In contrast to fee metals, bcc and hep metals show a stronger temperature dependence

of yield stress, in conjunction with low temperature embrittlement. Embrittlement usually

takes the form of a ductile-to-brittle transition, which is marked by an abrupt loss of

ductility, impact energy, toughness, and fatigue resistance occurring over a narrow temperature interval. Most commercially available ferritic or martensitic steels, bcc refractory

metals, bcc titanium alloys, tetragonal tin or tin-based solders, and most polymers exhibit

low temperature transitions, which render them useless for cryogenic service. But a few

ferritic and martensitic materials are useful at 77 K, as discussed in section 4.2.2. and

4.2.3.

3.2. Fatigue Properties

Fatigue is a process of cyclic plastic deformation resulting in crack initiation,

crack propagation, and fracture on the final stress cycle. Both crack initiation and

propagation occur in conventional fatigue tests which report stress-versus-cycles-tofailure (S-N) data for smooth or notched specimens. S-N results for some cryogenic

metals and alloys are reported in the literature [22-29].

Under stress-controlled conditions, the fatigue resistance of metals and alloys usu- ally shows improvement at 77 K, compared with 295 K. Exceptions to this behavior are bcc

alloys, which are embrittled at low temperatures, and metastable austenitic stainless

steels, which exhibit martensitic transformation. Under strain-controlled fatigue conditions, a cross-over behavior is typical--e. g. , cycle life at 77 K is usually higher than at 295 K for high-cycle fatigue, whereas the reverse is true for low-cycle conditions

(Fig. 3.3).

In fracture mechanics, fatigue crack propagation is treated separately, as discussed

by Clark [30]. Fatigue crack growth rates (da/dN) are plotted versus the fracture mechanics parameter AK, which is the difference between the maximum and minimum stress intensity

factors produced by the peak loads of the fatigue cycle. Typical data for cryogenic

alloys are shown in Fig. 3.4. In general, temperature effects on da/dN behavior correlate

with crystal structure [31]:

7
YIELD

(o)

OR

ULTIMATE

(•)

STRESS

(MPa)

Figure 3.1. Cryogenic effects on strength.

8

YIELD

(o)

OR

ULTIMATE

(•)

STRESS

(ksi)
DUCTILITY

(Percent)

STRESS

(MPa)

400

Figure 3.2. Cryogenic effects on the tensile properties of an aluminum (fee) alloy.

STRESS

(ksi)
Figure 3.3. Cryogenic effects on the fatigue life of an aluminum

(fee) alloy.

10
FATIGUE

CRACK

GROWTH

RATE,

da/dN

(mm/cycle)

STRESS INTENSITY FACTOR, AK (ksiin

172

)

Figure 3.4. Fatigue crack growth rates for structural alloys at

77 K.

11

FATIGUE

CRACK

GROWTH

RATE,

da/dN

(in/cycle)
1) Fee alloys show reduced rates at low temperature,

2) Hep titanium alloys show no measurable temperature dependence,

3) Bee alloys and ferritic steels show increased rates at low temperatures where

cleavage begins to occur, and

4) Metastable austenitic stainless steels show unpredictable behavior.

Inconel superalloys and stable austenitic stainless steels exhibit the greatest resistance

to fatigue crack growth at 77 K.

Scatter in fatigue data is high. In terms of the value of da/dN at a given AK value,

the interlaboratory variability may be 3:1, while the intralaboratory variability may be

2:1 [32],

3.3. Charpy Impact and Fracture Toughness Parameters

In the case of high strength alloys, uniaxial tensile ductility does not assure a ductile

fracture mode if cracks are present. For example, fatigue-cracked Inconel 718 (fee

structure) fractures in a brittle manner at 295 and 76 K, even though 20% elongation and

20% reduction of area are observed in conventional tensile tests. This crack-sensitive

behavior is typical of all high-strength alloys and is attributed to the fact that cracks

introduce a nonuniaxial stress state locally.

Charpy V-notch (CVN) impact values and notched tensile tests are traditionally used

for the purpose of ranking the toughness of structural materials. But the plane strain

fracture toughness, K

c

, is a more sophisticated toughness parameter that can be used

quantitatively to calculate the allowable stress levels for components in service, in

addition to ranking the toughness of materials.

Kj

c is measured according to ASTM Method E 399-72 [33]. Valid results are achieved

only if the specimen fails under linear-elastic conditions. The uncertainty in valid Kj

c measurements is between 4 and 10% [34], Estimates of Kj

c based on the J-integral are less

accurate, about ± 15%. Moreover, K^. values are highly sensitive to composition, specimen

orientation, and heat treatment. Therefore, designers must be sure that any handbook

values of K^

c are representative of the alloy in service.

Kj

c data for some structural alloys of cryogenic interest appear in some handbooks

[10-12], but no single data source is adequate. Most cryogenic data have only recently

been generated, so the best data sources may be conference proceedings and journal articles. Quantitative predictions of K^

c are unreliable, but a qualitative prediction of

temperature effects on K^

c can be based on alloy crystal structure. Representative

trends are shown in Fig. 3.5. In general:

1) Fee alloys retain relatively high toughness at all temperatures,

2) Hep titanium alloys have relatively low toughness at all temperatures, and

3) Bcc alloys show abrupt transitions from high to low toughness at critical temperatures associated with the onset of cleavage. Charpy impact data usually show a temperature

dependence similar to Kj .

3.4. Elastic Properties

The most useful elastic constant is Young's modulus, which is routinely used to calculate the deflection and buckling resistance of components under elastic stress. Young's

modulus for a material may be measured as the initial linear slope of the engineering

stress-strain curve, but acoustic measurement methods provide greater accuracy. The

engineering values are accurate to ±10%, whereas acoustic measurements are accurate to

± 2%.

Values of E are more commonly quoted, but Poisson's ratio (v), shear modulus (G) and

bulk modulus (B) are also available in many publications cited. If only two of the elastic

constants are known, the others can be calculated from the following equations, assuming

isotropic behavior:

12
Figure 3.5. Cryogenic effects on fracture toughness of structural

alloys.

13

FRACTURE

TOUGHNESS,

K

Ic

(ksi-in
r =

E u 2(1 + v)

B =

E =

3G

1 + G/3B

The elastic properties of metals and alloys are weakly dependent on cold work, heat

treatment, and alloy phase or magnetic transformations. For example, aged maraging steels

show E, G, v, and B, increases of 9, 10, 5. and 0.1%, respectively, as compared with the

solution annealed condition. Material composition is a more important factor. In addition,

porosity in wood or ceramics and fiber orientation in composite laminates may significantly

influence the elastic properties.

Fig. 3.6 compares the Young's moduli for several materials as a function of temperature. The highest values are observed for some commercial ceramics, and the lowest, for

polymers. Composites, metals, and glasses typically are intermediate. As temperature is decreased from 300 to 20 K, a ten-fold increase of E occurs for some polymers, whereas

ceramics, metals, and glasses show no more than a several percent increase. Most materials

exhibit a smooth and continuous modulus increase at cryogenic temperatures, but exceptions

and anomalies are noted for unstable austenitic stainless steels, Invar, and glass.

Anomalies occur in some austenitic stainless steels, owing to magnetic transitions [35].

In Fig. 3.7, the relationships between E, G, v and B are illustrated for a wellbehaved metal, 2219-T8 aluminum [36]. E, G, and B increase at low temperatures, while v decreases. For fee metals, G and E are expected to be proportional at a ratio of about

0.375. For metals and alloys exhibiting regular behavior, the nearly linear behavior

between 100 and 295 K extends to higher temperatures, so that data can be extrapolated to 400 K for engineering purposes.

3.5. Electrical Resistivity and Conductivity

In addition to applications in electrical design, electrical resistivity (p) and

conductivity data are used to characterize the purity of metals and to predict values of

other transport properties which are more difficult to measure, such as thermal conductivity. As shown in Table 3.1 there is an enormous resistivity difference amounting to a factor of 10 25 between metals (conductors) and non-metals (insulators).

3.5.1. Metal Conductors and Alloys

Good electrical conductors are metals of commercial purity or better. The best electrical conductors at 295 K rank as follows: silver, copper, gold, aluminum. The electrical conductivities of metals improve at low temperatures, but not at identical rates. Thus, copper and aluminum rank highest at 76 K, and these two metals are the major competitors for wiring and electrical applications. Silver, platinum, palladium, and gold serve

as contact terminals, but they are too expensive for broader use. Nickel is a fair conductor, finding some applications in electrical instruments.

For pure metals, the decrease in resistivity between 295 and 76 K is rather large,

typically 80%. Structural alloys have much higher resistivities and show smaller changes,

about 20 to 30%, for the same temperature range. Commercial resistance alloys are purposefully designed to be insensitive to temperature, showing changes as small as 0.5%. Re- sistance alloys find applications as precision resistors and strain gage elements, whereas

some pure metals and dilute alloys are used for temperature sensing [37-43].

Representative data for pure aluminum and its alloys are shown in Fig. 3.8. Copper

and its alloys behave similarly. High purity aluminum (99.99%) shows a continuous decrease

14
YOUNG'S

MODULUS,

E

(GPq)

TEMPERATURE (K)

Figure 3.6. Cryogenic effects on Young's modulus of materials.

15

YOUNG'S

MODULUS,

E

(ksi)
POISSON’S

RATIO,

v

MODULUS

(GPa)

Figure 3.7. Cryogenic effects on the elastic constants of an aluminum

(fee) alloy.

16

MODULUS

(I0 6

psi

)
Table 3.1. Resistivities of selected materials [37-41].

Material

s

p @ ~ 295 K (pQcm)p @ 273

(pficm)

K p @ 80 K (pQcm)

p @ 76 K (pQcm)

% change

Pure Metal

s

Si 1 ver 1.61 -- 0.29 -- 82

Copper 1. 70 -- 0.21 -- 88

] Gold 2.20 -- 0.46 -- 79

A1 umi num 2.74 -- 0.25 -- 91

'I Nickel 7.00 -- 0. 55 -- 92

Tin 11.0 -- 2. 10 -- 81

Lead 21.0 4.97 76

j Commercial Metal Conductors

Copper, OFHC -- 1. 559 -- 0.20 87

Aluminum 1100-0 -- 2.67 0.31 88

Nickel TD

j! ““ 6.73 0.74 89

Structural Alloys

70:30 Brass -- 6.65 -- 4.65 30

Beryllium Copper -- 9.21 -- 7.24 21

i| 304L Stainless Steel -- 70.4 -- 51.4 27

Inconel X-750 -- 123 -- 118 4 Invar -- 79. 5 -- 54.4 31. 5 T

i -6A1-4V , ELI 156 137 12

Resistance Alloys

Evanohm (75Ni-20Cr, Al-Cu) 134 -- 133 133 0.5

Karma (76Ni-20Cr, Fe, A1

)

133 -- 132 0.5

Constantan (57Cu-43Ni) 49 -- 48 2 Manganin (84Cu-12Mn-4Ni

)

48 45 6

Insulators

Polyimi de 2 x 10 21 -- >2 x 1023 -- -- Polycarbonate 3 x 1021 -- >2 x 10 23 -- -- Polyethylene terephthal ate 2 x 10 23 -- >2 x 1023 -- -- Polytetraf 1 uoroethylene 3 x 10 19 -- 4 x 10 18 -- -- Pyrex glass

1

•

5 x 10 17 ""

Note: 1 pOcm = 0.394 pQin.

17
RESISTIVITY,

p

(^2

cm)

TEMPERATURE (K)

Figure 3.8. Electrical resistivity of aluminum alloys.

18

RESISTIVITY,

p

(/xttin)
in resistivity as temperature is lowered, until a plateau is reached at about 10 K [38-

40]. Adding even small amounts of impurities or alloy elements rapidly degrades the electrical conductivity such that the plateau value of resistance for commercially pure aluminum (99%) is three orders of magnitude greater than the value for high purity aluminum

(99.99%). With further alloying, the plateau resistance becomes predominant. Thus, heavily alloyed structural materials exhibit high resistivities that are only mildly temperature dependent.

A comparison of alloys is shown in Fig. 3.9 [38]. Titanium alloys have the highest

resistivities; next highest are the heavily alloyed superalloys, stainless steels, brasses,

bronzes, cupronickels, aluminum alloys, and pure metals. Between 295 and 76 K, the tem- perature dependence of p for alloys is gradual and continuous, assuming it is not interrupted by phase transformations, such as occur in some metastable austenitic stainless

steels. The trends reported by Clark, Childs, and Wallace [38] should facilitate prediction or extrapolation of the resistivities for untested alloys.

3.5.2. Dielectrics (Insulators)

The electrical resistivities of polymers and ceramics are so large that measurement is practically impossible because the sensitivity of common current measuring devices is exceeded [41, 42]. Thus, resistivity values for dielectrics at low temperatures are not widely available. But resistivity data for dielectrics are not needed in engineering

design. NTF model testing entails small signal processing as in temperature and strain

monitoring. Therefore, it is sufficient to note that the common electrical insulators

remain good insulators at all temperatures.

3.6. Thermal Expansion and Contraction

The thermal expansion and contraction of materials must be accounted for in any

multicomponent structure operating over a broad temperature range. The length change of a material during heating or cooling depends on its initial length, the temperature change

involved, and its thermal expansion coefficient. Thermal expansion or contraction is described by either the total length change relative to room temperature (AL/L) or by the

coefficient of linear expansion, a, where

The most common measurement technique is the push rod dilatometer, which provides an un- certainty in A L/L typically less than ± 5%.

The thermal contraction of metals at cryogenic temperatures is reported by Arp,

Wilson, Winrich, and Sikora [44], Rhodes, Moeller, Flopkins, and Marx [45], and Clark [46].

Diller and Timmerhaus [47] review the behavior of engineering materials in general. In addition, several handbooks contain cryogenic data [15, 16]. Figure 3.10 compares the

general trends of engineering materials. Polymers have high thermal contractions between 295 and 77 K, with AL/L values about four times greater than typical metals and

alloys and twenty times higher than glass. Low contraction materials include fused silica or high-silica glasses, graphites or glassy carbons, certain glass-ceramics, and Invar [16, 48].

As for Young's modulus, thermal expansion is a structure-insensitive property, strongly

dependent on material composition and related to basic aspects of atomic bonding. Clark

[46] observes that a rather large alloy addition of 10-20% may be needed to produce a 10%

change in the thermal expansion for a particular base metal, and that heat treatment or processing do not have a large effect unless a phase change occurs, as in martensitic

transformation or precipitation hardening. Aluminum provides a good example of regular

behavior in a base metal system for which there are no known anomalies. As shown in Fig.

3.11, the thermal contractions between 293 and 77 K of unalloyed aluminum and a variety of

commercial aluminum alloys agree to within ± 7%.

19
RESISTIVITY,

p

(/x&

cm)

Figure 3.9. Cryogenic effects on electrical resistivity of metals.

20

RESISTIVITY,

p

(/iilin)
THERMAL

CONTRACTION,

AL/L

Figure 3.10. Cryogenic effects on thermal contraction.

21
Figure 3.11. Cryogenic effects on the thermal contraction of aluminum

a 1 1 oys

.

22

CONTRACTION,

L

f^

3

Lj

(Percent)

L293
Although most solids contract on cooling, there are notable exceptions and anomalies:

1) Fused silica undergoes a reversal and begins to expand after cooling below 200 K. 2 Lithium-aluminum silicate is an inorganic material that expands on cooling below

295 K, and

3) Invar is an exceptional alloy that contracts only 0.05% between 295 and 77 K, whereas a 0.3% contraction is typical for steels.

The thermal expansion of untested materials may be predicted by several methods.

First, there is a proportionality between a and specific heat. This will yield an engineering estimate for a at a specified temperature, T, assuming that the specific heat, C , is known at another reference temperature. Second, estimates can be based on data for

v similar materials as suggested by Fig. 3.11. A third approach for alloys is based on the

ratio:

L 77 K

~ L 295 K a 295 K

For many alloys, this ratio falls in the range 170 to 190, so the value of a at 295 K is a useful guide to the total contraction expected between room temperature and 77 K [46].

Unless thermal contraction in cryogenic apparatus is anticipated, a variety of undesirable effects will be encountered. Consequences range from simply a loss of machined

tolerances to the introduction of residual stresses, buckling, or cracking. Fractures may

develop when dissimilar materials are joined and cooled to cryogenic temperatures where

incompatible strains are set up. Self-cracking is common in brittle bulk materials (glasses, polymers) subject to local, rapid cooling. To alleviate problems, thermally induced

strain gradients must be reduced. The overlap of data bands in Fig. 3.10 suggests the

possibility of choosing materials having matching thermal expansion characteristics. Other

approaches include: (1) using plastics filled with low expansion glass, ceramic, or asbestos fibers or powders (2) using low expansion materials, (3) using thin section

construction where possible, and (4) reducing thermal shock by gradual cool-down rates.

3.7. Thermal Conductivity

Hust [49] reviews thermal conductivity and thermal diffusivity considering theory,

measurements, data trends, correlations and predictive techniques. Of particular significance to model designers engaged in heat transfer calculations is one-dimensional,

longitudinal heat flow under steady state. For these conditons thermal conductivity is defined as the proportionality constant, between the heat flow rate (Q) per unit area (A)

and the temperature gradient:

The existing handbooks containing low temperature thermal conductivity data for materials

are particularly valuable since they contain some data from unpublished sources and from

corporate literature [10, 13-15].

Data trends for selected engineering materials are compared in Fig. 3.12. Reduced

thermal conductivity at low temperatures is typical for most materials, but moderate in- creases between 300 and 76 K are observed for high purity metals. At temperatures of

interest for NTF testing, the spread of thermal conductivity values for solids covers four

orders of magnitude. Data for aluminum and its alloys are shown in Fig. 3.13.

Comparison of Figs. 3.12 and 3.9 reveals an inverse ranking of metals as regards

electrical resistivity and thermal conductivity. This correlation does not carry over to nonmetals, however. Metals generally have much higher electrical conductivity than non- metals, but there are certain ordered dielectrics (diamond, sapphire, beryllium oxide) that

have thermal conductivities approaching or exceeding metals.

Thermal conductivity measurements are time consuming and expensive, and no single

23
Figure 3.12. Cryogenic effects on thermal conductivity of materials

24

THERMAL

CONDUCTIVITY,

k

(BTU/ft

h-

°F)
THERMAL

CONDUCTIVITY,

k

(W/(m

K))

Figure 3.13. Cryogenic effects on the thermal conductivity of aluminum

alloys.

25

THERMAL

CONDUCTIVITY,

k

(BTU/fMv°F)
measurement technique suffices for all material and temperature combinations. For precision apparatus and best technique, measurement uncertainties may be pushed as low as + 1%,

although + 10% is more usual, and still higher uncertainties are recorded in specific

cases. Material variability can also be great. For example, the thermal conductivity of

high purity metals such as copper and aluminum may vary by orders of magnitude at temperatures below 76 K. Pure metals and structural alloys exhibit significantly reduced material

variability at 76 K and above.

Predictions based on theory, empirical relationships, or existing data are of particular interest in this area because experimental results for many materials at cryogenic

temperatures do not exist. Interpolated or extrapolated predictions based on existing data

may be uncertain within ±5 to ±50% [49]. Predictions may be based on residual resistivity

or Lorenz ratios in the case of metals and alloys [50], but the only reasonable prediction

scheme for an untested nonmetal is comparison with a chemically similar material.

3.8. Martensitic Phase Transformation

Martensitic transformations occur in some alloys after cooling or deformation at low

temperatures. This transformation consists of a sudden shearing of a group of atoms that

alters the crystal structure locally, with no atomic diffusion. A variety of physical and

mechanical property changes occurs in conjunction with martensitic transformation and these

changes must be anticipated in design.

Reed and Breedis [51] review the subject of martensitic transformations. Most cryogenic metals and alloys (e.g. , copper, aluminum, nickel) are crystal lographical ly stable at

all temperatures and exhibit no martensitic transformation. But austenitic stainless

steels may transform. In austenitic stainless steels, the face-centered cubic phase

(austenite) is metastable, being achieved by the alloy additions of Cr, N i , Mn and other

elements. But the equilibrium structure for iron-based alloys at room temperature and

below is body-centered cubic. Therefore, transformation may occur during service, depending on alloy content, service temperature, stress level, and metallurgical condition.

In susceptible AISI 300 series stainless steels, the fee phase (y) partially trans- forms to hep (e) and bcc (a) martensitic phases. These transformations may occur spontaneously on cooling, or they may be induced by application of stress. In the case of one AISI

304L alloy, about 2% martensite forms on cooling to 77 K, whereas 90% martensite forms with

deformation to the point of fracture [52], as shown in Fig. 3.14. Spontaneous transformations have also been reported in AISI 303, 304, 321, and 347 stainless steels, and repeated

thermal cycling may cause progressive transformation [53]. Among the consequences of

transformation are volumetric expansion, magnetic transition, and anomalies in the temperature dependence of physical and mechanical properties.

Volumetric Expansion . Martensitic transformation in AISI 300 series stainless steels

is accompanied by a volume expansion of about 3%. This expansion can be detrimental in some applications, causing loss of dimensional tolerances, loosening bolts, or creating

residual stress. For example, transformation in a machined stainless steel airfoil could cause surface relief effects that would ruin the aerodynamic surface. Such problems are best overcome using stable alloys that do not transform.

Magnetic Transition . The bcc martensites formed in austenitic stainless steels are ferromagnetic whereas the hep martensites and the original fee phases are paramagnetic

(non-magnetic). Ferromagnetic components may cause problems in magnetic fields. Where

problems could develop, magnetic permeability should be considered. Stable stainless

steels, such as AISI 310, have low magnetic permeability values in the range from 0.001 to 0.009, whereas AISI 304 grades containing several per cent bcc martensite have permeability

values of about 1.0 at 77 K.

Mechanical Effects . Martensitic tranformations in austenitic steels are capable of

influencing the tensile, elastic, fatigue, and fracture properties. Deleterious effects on ductility and toughness at cryogenic temperatures might be expected, since the martensites

formed have non-fee crystal structures. In fact deleterious effects occur in association

with transformation in the AISI 200 series stainless steels, which are not recommended for

cryogenic service (section 4.3.).

26
APPLIED STRESS (ksi)

Figure 3.14. Stress effects on the martensitic phase transformation in an AISI 304L stainless steel.

27
In the case of AISI 300 series stainless steels, however, the effect of martensitic

transformation on mechanical behavior is of little significance for routine base metal

applications. Since the stability of the recommended AISI 300 series stainless steels is relatively high, little or no martensite is formed in the absence of extensive plastic

deformation. With AISI 304, for example, different heats have varying tendencies to transform owing to compositional variations within the material specification, but excellent cryogenic tensile properties are expected with some scatter, as indicated by the data

of Fig. 3.15. Other effects of martensitic transformations on mechanical and physical

properties of austenitic stainless steels are noted in section 4.3.

3.8.1. Selection of Stable Stainless Steels

To avoid any possibility of martensitic transformation, one solution is to choose a fully stable alloy. AISI 310 (Fe-25Cr-20Ni ) and A 286 (Fe-25Ni-14Cr) are examples of

completely stable stainless steels which, owing primarily to high nickel content, can be

deformed to fracture at 77 K or lower with no evidence of phase transformation. These

alloys exhibit regular physical and mechanical property trends with temperature, which are similar to the usual trends for other stable fee metals and alloys. In contrast, anomalous

trends are expected for unstable stainless steels.

3.8.2. Selection of Metastable Stainless Steels

Another approach is to choose metastable alloys that are sufficiently stable for

service conditions in a particular application. Williams, Williams, and Capellaro [54] and

Larbalestier and King [55] compare the relative stabilities of AISI 300 stainless steels at temperatures between 4 and 373 K. Stability increases with total alloy content. Thus,

AISI 316 is more stable than AISI 304, and high nitrogen grades such as AISI 304N are more stable than normal AISI 304 or low-carbon grade AISI 304L. The temperature at which mar- tensite forms spontaneously on cooling (M ) can be predicted to within about 40 degrees K, based on composition [54, 56].

28
DUCTILITY

(Percent)

YIELD

OR

ULTIMATE

STRESS,

cr

(MPa)

Figure 3.15. Scatter in tensile properties owing to phase transformations in AISI 304 stainless steels.

29

YIELD

OR

ULTIMATE

STRESS,

cr

(ksi)
4. METALS AND ALLOYS FOR STRUCTURAL APPLICATIONS

Metals and structural alloys are the most widely used model materials. Castings are used when complex shapes are required, but wrought products are generally favored for

lower cost and superior ductility. Suitability for service at cryogenic temperatures is largely dependent on composition and crystal structure. This chapter discusses metals at temperatures as low as 77 K, concluding that, as primary structural alloys, conventional

carbon and alloy steels generally must be abandoned in favor of certain nickel alloy

steels, austenitic stainless steels, and nonferrous fee alloys.

4.1. Carbon and Alloy Steels

Owing to their non-fee structures, carbon steels such as SAE 1020 (Fe-0. 2C~lMn)

possess totally unacceptable cryogenic ductility and toughness. Such steels exhibit

classical notched bar ducti le-to-brittle fracture transitions at temperatures in the range

of 250 to 375 K depending on material and test variables [57-59]. Above the transition

temperature, ductile failure occurs by void coalescence; below the transition temperature,

brittle failure occurs by cleavage with zero ductility. The Charpy impact energy for a mile carbon steel cleaving at 77 K is only 6.8 J, which corresponds to a K T value of only

11 MPa*m . Since this level of fracture toughness is too low for most structural applications, this material could only be used in rare cases where application does not involve

tensile stress.

Most nonaustenitic steels are likewise brittle at cryogenic temperatures, but a few

are acceptable for service at 77 K. Acceptability is often judged by two fracture toughness criteria:

1) If a transition temperature is clearly identified, the steel must not be

used at a service temperature lower than the transition temperature.

2) If a transition temperature is not obvious (as in high strength or high

carbon steel), Charpy V-notch impact energy should be at least 21 J (15

ft*lbs) at the minimum service temperature.

On the basis of these conventions, the majority of ferrous metals are disqualified

for NTF applications. As structural materials supporting tensile stresses, entire steel

classes are unfit for service at 77 K, as listed in Table 4.1. Many steels favored for

conventional wind tunnel testing (i.e., AISI 4340, and AISI 410) are unacceptable at 77 K. The acceptable steels for tensile applications are discussed in sections 4.2. and 4.3.

4.2, Nickel Alloy Steels

Nickel has beneficial effects on the cryogenic properties of steels. The beneficial

effects include reduced ducti le-to-brittle transition temperatures for ferritic steels, as well as higher toughness at subtransition temperatures. These effects are attributed to favorable influences on the deformation modes and cleavage strength of iron, combined with

a tendency to promote the formation of retained austenite in some alloys. Taking into

account the acceptability criteria of section 4.1, the following limited selection of

nickel alloy steels is recommended as useful for NTF service:

1) low- to medium-strength austenitic Fe-Ni alloys

2) intermediate-strength, ferritic Fe-9Ni steel, and

3) high-strength, martensitic 18Ni maraging steel

All of these acceptable steels have low carbon contents, and high nickel content. Carbon contents higher than 0.2% harden the iron matrix and promote low temperature brittleness. Austenitic stainless steels containing Cr as well as Ni are also acceptable and are discussed in section 4.3.

30
Table 4.1. Suitability of ferrous alloys and steels for tensile structural

applications at temperatures ranging from 367 to 77 K. (Examples are cited parenthetically).

Unacceptable Acceptable

1 . carbon steels (SAE 1005-1095) 1 . austenitic Fe-Ni alloys

(Fe-36Ni, Ni-Span C)

2. high strength low alloy steels

(ASTM A94)

2. ferritic 9Ni steel

(ASTM A353)

3. free machining steels

(1109, 1151, 1211)

3. some 18Ni maraging steels

(200 or 250 grades)

4. carburizing steels

(3310, 4320, 8620)

4. various austenitic stainless steels

(AISI 304, Fe-21Cr-6Ni-9Mn-0.3N)

5. direct hardening steels

(3140, 4340)

6. nitriding steels (N,135)

7. low temperature carbon steels

(ASTM A 514)

8. tool steels (A2, A7, Dl, D7)

9. electrical steels (M4, M8)

10. ferritic stainless steels

(AISI 405, 430)

11. martensitic stainless steels

(AISI 410, 416)

12. most ultrahigh-strength steels

(300M, D6AC)

31
4.2.1 Austenitic Fe-Ni Alloys

a. General Information

Iron alloys with 36Ni or more have the austenitic (fee) structure. Combined with

low carbon content, this structure assures excellent ducitility and toughness at low

temperatures.

b. Preferred A1 Toys

Invar (fee structure) is a relatively low-strength alloy, best known for exceptionally

low thermal contraction. Between 300 and 77 K, Invar contracts only 0.05%, whereas most

metals contract 0.2 to 0.4%. The nominal composition is Fe-36Ni-0.1C-0.35Mn-0.3Si-0.1Al0. 015P-0. 015S. The yield stress of Invar at room temperature is 289 MPa, only slightly

higher compared to that of AISI 304. Invar is ductile, tough, and weldable. It can be

used in the annealed or cold-worked conditions, and is recommended for applications where

high strength is not required, but close dimensional stability is necessary to minimize

thermoelastic stresses. Past cryogenic applications include measurement devices, glass

window seals, bellows-less piping systems, and membrane liners for cryogenic storage

tanks. This is the most widely used low expansion alloy.

Ni-Span C (fee structure) is a medium-strength alloy that is recommended as a substitute for Invar in applications requiring higher strength and intermediate thermal contraction. Ni-Span C is a precipitation-hardenable alloy with a nominal composition: Fe-42Ni0. 06C(max)-5. 2Cr-2. 4Ti-0. 5A1 . Originally developed to achieve a constant Young's modulus

between 227 and 338 K, Ni-Span C is often used for precision springs and bellows [60].

c. Fabrication

Invar's high ductility and toughness render it as difficult to machine as austenitic

stainless steel, but a modified grade (0.2Se) for improved machinabi 1 i ty is available.

Annealing of Invar is performed by heating at 1073 K for half an hour per each 25 mm of

thickness. A special three-step stability anneal for Invar confers minimum thermal expansion:

1) 1063 K, to 1103 K, 0.5 h per 25 mm, water quench,

2) 588 K, 1 h per 25 mm, air cool,

3) 368 K, 48 h, air cool.

Ni-Span C is solution treated and aged at 922 K for 5 h.

Weld procedures and joint designs are similar to those used for AISI 300 series

stainless steels. Invar welds are compatible with stainless steel and nickel alloys. No preheating or postheating is required. Inconel 92 or Hastelloy W are general purpose

fillers, but a modified Invar filler, Fe-36Ni-0. lC-3Mn-0. 0Si(max)-l. 0Ti-0. 01S(max)-0. 02P

(max), more closely matches the base-metal expansion behavior.

d. Availability

Invar is distributed by seven major corporations and is available in cast or wrought

forms: strip, sheet, plate, wire, bar, tube, pipe, and billet forgings. Ni-Span C is less

readily available. Costs for Invar and Ni-Span C are higher compared with stainless

steels owing to higher nickel content.

e. Mechanical and Physical Properties

Tensile: Between 300 and 77 K, the tensile properties of annealed Invar change as follows: yield stress increases from 289 to 620 MPa, ultimate strength in- creases from 552 to 862 MPa, and elongation decreases from 55 to 42%. For the same temperature interval the properties of heat-treated Ni-Span C vary as follows: yield stress increases from 128 to 1549 MPa, and elongation increases

from 24 to 31% [10]

.

32
Charpy Impact: CVN values for Invar and Ni-Span C are high at all temperatures. A typical value for annealed Invar at 77 K is 65 J.

Elastic: Young's modulus for Invar is anomalous, decreasing from 152 to 139 GPa

between 300 and 150 K, then increasing to 141 GPa at 77 K [61]. Data for NiSpan C are not available.

Thermal Contraction: The thermal contraction of Ni-Span C between 293 and 80 K is 0.128%, compared with_Invar's 0.05%. The 1 i near_thermal expansion coefficient

for Invar is 2.5 x 10 6 /K at 293 K and 0.83 x 10 6 /K at 80 K, or about one tenth

that of most steels.

Thermal Conductivity: The thermal conductivity of Invar decreases from 13.8 to 6.2

W/(m-K) between 367 and 77 K [11].

4.2.2. Ferritic Fe-Ni Steels

a. General Information

From the finding that increased Ni content correlates with a progressive reduction of

ducti le-to-brittle transition temperatures, a family of low temperature ferritic steels

was developed containing 2k to 9Ni. The 9Ni grades are acceptable for NTF applications at

77 K, but all other grades in this family have insufficient nickel content and fail by

cleavage at 77 K.

b. Preferred A1 loys

Fe-9Ni is a medium-strength, weldable steel that has been extensively used in cryogenic applications at 77 K since 1947. In NTF structures, 9Ni .steel is recommended as an alternative to AISI 304 stainless steels where a lower cost, higher yield strength, more machinable steel is desired. The nominal composition is Fe-9Ni-0.10C-0.6Mn. Two grades

are available: ASTM A 353 (double-normalized and tempered) and ASTM A 553 Type I (quenched

and tempered). The quenched and tempered grade offers 10% higher strength and slightly

higher toughness.

c. Fabrication

The 9Ni steel is more easily machined compared with austenitic stainless steels, and

is readily formed. Stress relief after cold forming is conducted at 824 to 857 K, for

2 h. Exposure at 643 to 813 K should be avoided or temper embrittlement will result.

Heat treatments are performed by the supplier.

The 9Ni steel is weldable in 50-mm sections or less without preheating or postheating

but there is no matching filler. Gas metal-arc welds use austenitic nickel base fillers

(Inconel 82, 92, 182, or 625). Shielded metal arc welds use austenitic Inco Weld A or Inconel 112. Heat inputs are controlled to maintain adequate fusion and heat-affected

zone properties. Welding is further described in product data bulletins. The mechanical

properties of welds are summarized by Pense and Stout [62].

d. Availability

Several steel corporations produce 9Ni steel in the form of plates, forgings, pipe,

tubing, bar sheet, or rolled sections. Plates are covered by the A 353 and A 553 specifications. Other forms are covered by ASTM specifications A 522, A 333, and A 334.

e. Mechanical and Physical Properties

Mechanical property data, including fatigue and fracture toughness data for ferritic

Fe-Ni steels and welds, are compiled and summarized by Pense and Stout [62]. The properties quoted below are representative of A 553 Type I quenched and tempered 9Ni steel for

temperatures decreasing from 300 to 77 K.

33
Tensile: Yield stress increases from 700 to 1000 MPa, ultimate strength increases

from 800 to 1180 MPa, elongation is nearly constant at 24%, and reduction of area decreases from 67 to 57%.

Charpy: The CVN values decrease from 108 to 47.5 J for the transverse orientation.

Elastic: Young's modulus increases from 186 GPa to 207 GPa, and all of the elastic

properties are reported to be regular functions of temperature [63].

Fracture Toughness: Fig. 3.5 illustrated the temperature dependence of fracture

toughness for 9Ni steel. Typical Kj

c estimates from J-integral tests are 156 to 205 MPa-m^ at 295 K and 164 to 182 MPa-m*5 at 77 K [64-65],

Thermal Conductivity: The thermal conductivity of this steel decreases from 32 to 28

W/(m-K) between 373 and 295 K, and from 28 to 12.5 W/(m-K) between 295 and 73 K.

Thermal Contraction: _The linear thermal expansion coefficient is 10.4 x 10 6 /K at 295 K and 5 x lo 6 /K at 73 K. Total contraction between 295 and 73 K is 0.35%.

Expansion between 295 and 373 K is 0.15%.

4.2.3 18Ni Maraging Steels

Ultrahigh-strength steels having nonausteni tic structures are infrequently used at cryogenic temperatures, since their toughness levels are lower than austenitic alloys. But a need for ultrahigh strength arises in some primary NTF components where the 77 K design stresses are projected to reach as high as 700 MPa. Maximum toughness at the

required yield stress is paramount, and other factors, including cost, are secondary.

a. General Information

Available yield strength-versus-Kj

c data indicate that the 18N i maraging steels offer

superior strength and toughness combinations compared with other high strength steels,

such as quenched and tempered 9Ni-4Co steels, AISI 4340, 300 M, or D6AC. Maraging steels

have a martensitic structure, but the martensite is exceptionally tough owing to composition and low levels of impurities. At 77 K the 18Ni maraging steels remain tough owing to the effects of low carbon and high nickel contents. The 18Ni maraging steels also offer

simpler heat treatments, deeper hardening with less distortion, and better dimensional

stability on heat treating. Disadvantages are the lack of 77 K data and a growing problem

of availability for maraging steels. Since fracture toughness decreases at cryogenic

temperatures, accurate K

T data and fatigue data are required, and additional materials

testing must be performeaat 77 K before these alloys can be safely used.

b. Preferred Alloys

The 18Ni maraging steels have the following compositional ranges: Fe-17 to 19Ni , 3 to 5.2Co, 0.15 to 2Ti, 0.05 to 0.2A1, < 0.03C, < O.lOMn, < 0.01P. Four grades are available, designated 200, 250, 300, and 350, which indicates the nominal yield stress in ksi at room temperature. Specifications covering these steels are MIL-S-46850 (Mi) and

Aerospace Materials Documents specifications 64BL, 64BM, 64BN, 64BP-1, and 64BR-1. To date the 18Ni maraging steels have seen limited cryogenic service, having been used as specimen grips and load frame components for cryostats at 77 K or lower. The 200 and 250

grades are recommended for NTF applications, but the 300 and 350 grades are not recommended

in view of lower cryogenic impact and toughness values.

c. Fabrication

The 18Ni maraging steels are stronger than austenitic superalloys or stainless

steels, and yet easier to fabricate. Machining in the annealed condition is comparable to that of AISI 4340 steel at similar hardness, R 30. After air cooling from the solution

annealing temperature, 1088 K, maraging steelsare hardened in a single 3 h aging cycle at 755 K. No protective atmosphere is needed, just air, and there is no decarburization

problem since the initial carbon content is low.

34
In welding maraging steels the GTA process is preferred, with techniques similar to stainless steels. Preheating or postheating is not recommended. Joint efficiencies of

90% can be expected using matching fillers. Best properties are achieved by welding in

the annealed condition.

d. Availability

The 18Ni maraging steels are furnished as plate, sheet, billet, rod, and bar, in the

mill-annealed condition. Bar is the most common form, but not all forms are available

from every supplier, and availability is decreasing. Costs are high and vary with grade,

size, and quantity desired.

e. Mechanical and Physical Properties

The properties of 18Ni maraging steels have not been extensively characterized at low

temperatures. Some available mechanical properties for three grades are listed in Table

4.2. The 200 grade maraging steel is perhaps the best candidate, showing an increase in

yield stress from 1418 to 1861 MPa at temperatures between 295 and 77 K, in combination

with high ductility, impact and toughness values.

In addition to the data of Table 4.2, Shoemaker and Rolfe [66] and Clausing [67]

report yield stress and fracture toughness data for 200 and 250 grade maraging steels.

Between 295 and 77 K, K T for the 250 grade decreased linearly from 110 to 44 MPa*M

2

. ^ c j. The Kj

c value for the 200 grade steel at 77 K was 126 MPa*m 2

. Nachtigall [25] and Tobler,

Reed, and Schramm [68] report cryogenic fatigue data for 300 grade steels.

4.2.4. Other Ferrous Alloys

Maraging steels containing 20Ni or 25Ni were originally produced in experimental

quantities but they offered no advantages compared with the 18Ni grades and so have fallen

from favor and availability. More recently 12Ni-5Cr-3Mo maraging steels have been

developed but cryogenic data are unavailable.

The quenched and tempered 9Ni-4Co series steels containing 0.20 to 0.45% carbon are probably the closest competitors to 18Ni maraging steels for high strength applications.

The higher carbon grades, such as 9Ni-4Co-0.45C, exhibit inferior impact toughness at 77

K. The lowest carbon grade, 9Ni-4Co-0. 20C, covered by AMS-6523 and AMD 62 CF-1 specifications, is more attractive. The reduction of area for this grade is a respectable 62% at 300 K and 51% at 77 K, and the yield stresses at these temperatures are 1265 MPa and 1654

MPa respectively. Nevertheless, Vishnevsky and Steigerwald [7] indicate that 200 grade

18Ni maraging steel offers superior strength and fracture toughness, as well as simpler

fabrication.

A cast iron that Pearson [69] considers acceptable at 77 K is Ni-resist cast iron

type D-2M. The composition of this material is Fe-2.9C-22Ni-4Mn-2.5Si. The yield stress

at 295 K is 231 MPa. However, cryogenic property data are not available.

Another alternative to consider is PH 13-8Mo or related alloys (section 4.3.5). PH 13-8 Mo in the H 1150-M condition offers a yield stress of 583 MPa at 295 K, or 993 MPa at

77 K, with a 50% reduction of area at 77 K [7], Unfortunately, valid fracture toughness

data are lacking for alloys of this type.

4.3. Stainless Steels

Steels containing 12-30% chromium to resist corrosion are classified as stainless

steels [70]. There are five commercial alloy systems:

1) AISI 200 series (austenitic Fe-Cr-Mn-Ni alloys)

2) AISI 300 series (austenitic Fe-Cr-Ni alloys)

3) AISI 400 series (ferritic or martenistic Fe-Cr alloys)

35
Table 4.2. Mechanical properties of aged 18Ni maraging steels.

Material Test Yield stress Ultimate strength Red. of Young 1 s Poi sson

1 s Charpy impact Fracture temperature area modulus ratio energy toughness, Kj

K MPa MPa % GPa j MPa-m 2

200 grade 295 1418 1461 65 193.7 0.311 49 187

77 1861 1930 60 203.0 0.306 39 86

250 grade 295 1696 1792 -- 195.3 0.308 28 110

77 2206 2275 -- 204.8 0.304 14 44

300 grade 295 1903 1982 55 196.8 0.308 25 -- 77 2344 2475 40 206.3 0.303 12 ““

Note: 1 MPa = 0.145 ksi

1 GPa = 0.145 msi

1 J = 0.737 ft. lb

1 MPa- in'4 = 0.91 ksi

36
4) nitrogen-bearing series (austenitic Fe-Cr-Ni-Mn-N alloys)

5) AISI 600 series (various precipitation hardening alloys).

The AISI 300 series stainless steels are the most widely used cryogenic alloys.

a. General Information

AISI 200 series (austenitic Fe-Cr-Mn-Ni alloys) . The 200 series stainless steels are not recommended for cryogenic service. These alloys were developed for room temperature

service as substitutes for the 300 series steels at a time when nickel was difficult to obtain. Compared to the 300 series stainless steels, the 200 series steels contain less

Ni and more Mn; they are less stable with respect to martensitic transformation and less

ductile at 77 K.

AISI 300 series (austenitic Fe-Cr-Ni alloys) . By far the most widely used cryogenic

alloys are the AISI 300 series stainless steels. These steels contain 16-26% Cr, 6-22%

Ni , not more than 2% Mn, and various other elements in smaller quantities. Nickel tends

to stabilize the austenitic crystal structure, but not all grades in this series retain a fully austenitic structure at low temperatures; small amounts of martensite spontaneously

form in some grades after cooling to 77 K (section 3.8).

In addition to excellent cryogenic properties, the 300 series stainless steels offer

ease of fabrication, extensive service history, and ready availability in all standard

product forms and sizes. The main disadvantages are: (1) higher material and machining

costs compared with aluminium alloys or ferritic 9Ni steel, and (2) complications assoc- iated with sensitization or martensitic transformation in some grades.

The 300 series stainless steels are typically used in the annealed condition, with

yield stresses not exceeding 275 MPa at room temperature. Nitrogenated "N" grades can be

used where higher strength and austenite stability is required, but availability and

service experience is limited for N grades. Cold working is another satisfactory method

of gaining strength, but only for thin sheet.

Sensitization is a problem encountered when stainless steels are heated at 700 to 1088 K for significant time periods. At these temperatures, chromium carbides precipitate

at grain boundaries, ruining corrosion resistance, reducing ductility and toughness, and

lowering stability with respect to martensitic transformation. Sensitization frequently

occurs in the heat-affected zones of welds, for example, and there are two methods of

combating it. One approach is to select stabilized grades such as AISI 321 or 347 that

contain Cb and Ta additions that form carbides in preference to Cr. The second approach

is to select low carbon grades designated "L" or "S". The low carbon grades have lower

yield strengths, but their welds offer higher impact resistance compared with welds of

stabilized Cb/Ta grades.

AISI 400 series (ferritic or martensitic Fe-Cr alloys) . Owing to unfavorable

crystal structures these steels are hard, abrasion resistant, and brittle at cryogenic

temperatures. AISI 440-C steel has been used as a ball bearing alloy at 77 K; but otherwise none of the 400 series steels can be recommended for cryogenic service.

Nitrogen-bearing series (austenitic Fe-Cr-Ni-Mn-N alloys) . The addition of manganese

to austenitic stainless steels helps to stabilize the austenitic structure and increase

the solubility limit for nitrogen, which significantly increases the yield stress. Two types of nitrogen-strengthened stainless steels are available: the N-grade AISI 300

series alloys, or the Fe-Cr-Ni-Mn-N alloys considered here. The Fe-Cr-Ni-Mn-N alloys

typically contain 0.3N, whereas AISI 304N contains 0.10 to 0.16N. There are five alloys

in the relatively new Fe-Cr-Ni-Mn-N series:

1) Fe-18Cr-2Ni-12Mn-0 . 3N

2) Fe-18Cr-3Ni-13Mn-0 . 3N

3) Fe-21Cr-6Ni-9Mn-0. 3N

4) Fe-21Cr-13Ni-5Mn-0 . 3N, and

5) Fe-17Cr-8Ni-8Mn-0. 3N

37
In the annealed condition in which they are used, these alloys offer yield stresses

at least 25% greater than AISI 304N. The preferred grade for cryogenic service is Fe- 21Cr-6Ni-9Mn-0. 3N , because of its relatively high stability, more extensive data base, and

good properties at 77 K. Disadvantages are higher cost and limited availability and

service experience compared to AISI 300 series steels.

AISI 600 series (various precipitation-hardening alloys) . There are more than a dozen steels in this series, classified as martensitic, semiausteni tic , or fully austenitic.

Examples of martensitic or semiausteni tic grades are stainless W(AISI 635), 17-4 PH (AISI

630), 15-5 PH, PH 13-8MO, 17-7 PH (AISI 631), 15-7 PH (AISI 632), AM 350 (AISI 633), and AM 355 (AISI 634). These steels are too brittle in the fully aged conditions for general

applications at 77 K. Therefore, the H temper conditions are mandatory. Even so, cryogenic

applications are rarely recommended. Alloy 15-5 PH in the H 1150 M temper has been used

for valve seats at 77 K, but the CVN values are marginal, ranging from 6.7 to 38 J.

Most of the fully austenitic grades also have inferior cryogenic properties, but one alloy, A-286, is acceptable for cryogenic service [60], Alloy A-286 offers the advantage

of a higher yield stress in the precipitation-hardened condition, compared with annealed

AISI 300 series steels. Disadvantages are high cost, and more difficult fabrication than

the 300 series steels.

b. Preferred Alloys

From the information above, the preferred stainless steels for NTF service are several

steels from the AISI 300 series, Fe-21Cr-6Ni-9Mn-0. 3N, and A 286.

AISI 302 (Fe-18Cr-8Ni-<0. 15C) is the basic 18-8 stainless steel, the usual choice for room temperature service. But AISI 304 (Fe-18Cr-8Ni<0. 08C) is preferred for cryogenic

service since it has lower carbon content. AISI 304 was chosen for construction of the NTF wind tunnel walls.

There are about two dozen steels in the AISI 300 series, each developed for a specific

purpose [72]. Most grades having more than 0.08% carbon are not used at 77 K, but an exception is AISI 301 which has been used as high-strength, cold-worked sheet for aerospace

skin structures at 77 K. More commonly used at cryogenic temperatures are AISI 304, 304L,

316, 310S, and the sensitization resistant grades 321 and 347.

AISI 304L (Fe-18Cr-8Ni<0. 03C) has lower carbon than AISI 304 and is substituted for

welded structures to further reduce sensitization. AISI 316 and 310S are more highly

alloyed than either 304 or 304L and are more resistant to martensitic transformation. In fact, AISI 310S (Fe-25Cr-20Ni<. 03C) is completely stable and forms no martensite when

fractured at cryogenic temperatures. Specifications for the austenitic stainless steels

are listed in Table 4.3. The federal specifications for AISI 300 series stainless steels

are QQ-S-766 and QQ-S-763.

Alloy Fe-21Cr-6Ni-9Mn-0. 3N is covered by ASTM specifications A 412 for plate, sheet,

and strip and A 276 for bars and pipes. The alloy has a relatively high austenite stability compared with AISI 304, and the yield stress at 300 K is about 353 MPa, compared with

200 MPa for AISI 304. Cryogenic applications to date include torque tubes and rotors for

superconducting machinery at temperatures as low as 4 K.

Alloy A 286 has the nominal composition Fe-25Ni-14Cr-2. 2Ti-l. 5Mn-l. 2 Mo-0. 3V-0. 2A1-

0.5Si. Considerable cryogenic data exist for this alloy, which is nonmagnetic and completely stable with respect to martensitic transformation when cooled or stressed at cryogenic

temperatures. The yield stress in the fully hardened condition at 300 K is 550-750 MPa. This alloy is recommended for applications requiring a stable fee structure and significantly higher yield stress than any of the annealed austenitic stainless steels, particularly at room and elevated temperatures.
Table 4.3. ASTM and ASME specifications for various

forms of austenitic stainless steels.

Product Form

ASTM

speci f i cati ons ASME

speci fi cations

Plates A240 SA-240

Forgi ngs A473 SA-182

Bars and sections A479 SA-479

Seamless tubes A213 SA-213

Welded tubes A249 SA-249

Seamless and welded tubes A269 --

Bolts, screws, studs, stud bolts A320 SA-320

Nuts for bolts A194 SA-194

39
c. Fabrication

The high ductility of austenitic stainless steels insures good formability, but

greater force is needed compared with aluminum alloys or plain carbon steels. The austenitic steels can be formed by bending, cold-roll forming, tube forming, drawing, and spinning, but difficulty increases with increasing yield stress. Alloy A- 286 is particularly

difficult to form and machine.

Casting alloys are available and may be used if complex shapes are needed, but cast

products have inferior mechanical properties compared with wrought products. Casting

alloy CF-8 is the analog of the wrought alloy AISI 304. For information on cast stainless

steels, consult Peckner and Bernstein [70], Forging is also possible, but austenitic

stainless steels are generally more difficult to forge than most structural alloys owing

to their relatively high strength at elevated temperatures.

High work hardening and low thermal conductivity make annealed stainless steels more difficult to machine than plain carbon steels. Selection of cutting fluids is important

to extend tool life by lubrication. It is necessary to use heavy feeds, slow speeds, high

quality tool bits, and rigid machines. Guides to machining are available in technical

notes from suppliers.

The austenitic stainless steels are annealed at temperatures between 1227 and 1422 K, depending on composition, or stress relieved at 477 to 700 K. Stress relieving treatments

reduce residual stress or improve dimensional stability. Rapid cooling is specified

because the interval 700 to 1088 K must be avoided to prevent sensitization, and the

interval 1005 to 1162 K must be avoided to prevent formation of brittle sigma phase.

Alloy A-286 is usually solution treated (1172 K) and aged (1005 K, 16 h, air cooled).

The austenitic stainless steels can be soldered using Pb-Sn solders and special

fluxes to break down oxide films. They can also be brazed. Brazing is conducted at 1255

to 1366 K, followed by rapid quench. Alloy A-286 is preferably brazed in vacuum, with

nickel plating to improve wettability.

Except for AISI 303, all austenitic stainless steels are considered weldable [71].

Gas tungsten arc welding is preferred for flat stock, while shielded metal arc is preferred for gages thinner than 1 mm. Oxyacetylene , electron beam, or resistance welding

techniques are also possible. Among the problems encountered in welding are sensitization

and hot cracking. To minimize sensitization, stabilized grades (AISI 321, 347, 348) and

AISI 304L or 308L fillers are often preferred. Fillers are formulated to provide 4 to 10%

delta ferrite to reduce hot cracking.

For alloy A 286, available fillers are Inconel 92, Hastelloy W, and AISI 310, in

addition to matching fillers. For Fe-21-Cr-6Ni-9Mn-0. 3N, type 308 filler can be used, but

it offers lower strength compared with the base metal. N 40W is a matching filler for Fe- 21Cr-6Ni-9Mn-0. 3N , which gives good cryogenic toughness in sections up to 6 mm; Inconeltype fillers are preferred for thicker sections.

d. Availability

The availability of AISI 304, 316, 310 and alloy Fe-21Cr-6Ni-9Mn-0. 3N is unlimited in standard product forms, including sheet, strip, wire, bar, plate, tubing, and forging

billets. Costs for alloys in bar form range up to $2. 3/kg. Costs vary considerably with

form and increase with higher alloy content. Alloy A-286 is available as vacuum arc remelted billets, bars, and wire, in the solution-treated or aged conditions; but not every supplier furnishes all forms, and costs are higher than AISI 300 series steels.

e. Mechanical and Physical Properties

Brickner and Defilippi [72] and Sanderson and Llewellyn [73] review the mechanical

properties of austenitic stainless steels. Extensive mechanical and physical property

data for the recommended stainless steels are available in handbooks and reference books

[74],

40
Tensile: The yield stress of annealed AISI 300 series stainless steels range from

172 to 262 MPa at 77 K. Among AISI 304, 310, and 316, AISI 304 has the lowest

yield strength at 77 K (owing to lower alloy among content), but the highest

ultimate strength (due to greater strain-induced martensite content). AISI 321

and 347 behave similarly to AISI 304. Data for high strength alternatives

including AISI 304N, Fe-21Cr-6Ni-9Mn-0. 3N , and A- 286 are shown in Fig. 4.1.

Using these alloys, 77 K yield strengths of 399 MPa, 353 MPa, and 550-750 MPa

can be achieved expense of ductility.

Tensile data for cold worked AISI 301, 304, 304L, and 310 stainless steel sheet

are also available at cryogenic temperatures [75], The effect of cold-work is to increase strength at the expense of ductility. Cold-work is usually indicated

by the terms quarter, half, or full hard. Data for AISI 310 are reported by

Christian, Gruner, and Girton [76] show a 300 K yield stress of 1158 MPa at 80%

cold-work, compared to 278 MPa for the annealed condition. But elongation for

the 80% cold-worked condition is only 2% at 300 K.

Elastic: For AISI 304 stainless steel, Young's modulus is 1.896 GPa at 300 K. The

77 K amounts to 8%. The effect of temperature between 295 and 77 K is not always regular, but may be anomalous for some austenitic stainless steels, as explained in section 3.4 [77-78].

Impact: Charpy impact values for the recommended stainless steels are high at all temperatures. In general there is a modest reduction of CVN between 300 and 77

K. Representative CVN data for AISI 304 are 150-217 J at 295 K and 122-150 J at

77 K.

Fatigue: Stress-controlled tests show improved fatigue strengths and high fatigue

resistance for smooth bars and notched bars between 295 and 77 K. Fatigue crack

growth rates for AISI 310 and A286 decrease between 295 and 77 K, while for AISI

304 and AISI 316 there is little change in rates measured at these two temperatures [31].

Fracture Toughness: Annealed austenitic stainless steels exhibit ductile fractures

and high toughness, even when deeply cracked. The toughness of annealed alloys

is so high that J-integral methods are necessary to estimate Kj

c values [79-

82]. Representative results for AISI^310S are shown in Fig. 3.5, where toughness estimates increased to 160 MPa-rn^ at 77 K. The toughness-to-yield strength

ratios for annealed austenitic stainless steels are such that significant plastic deformation is expected to precede fracture and a fracture mechanics analyses is generally not required. Fracture toughness decreases for highly coldworked, precipitation-hardened, or welded and sensitized steels. For example,

alloy A-286, in the precipitation-hardened^condition indicates relatively low

toughness values of about 114 to 161 MPa*m^ at 295 K. Since material variability is significant at these lower toughness values, quality control and fracture

mechanics analysis may be appropriate when this alloy is used in critical applications.

Thermal Conductivity: The thermal conductivity of AISI 304 decreases from 16W/(m*K)

at 295 K to 8.2W/(m-K) at 77 K. The values for other stainless steels are indicated in Fig. 3.12.

Thermal Expansion: Thermal expansion coefficients and total contractions for austenitic stainless steels are reported by several investigators [44-47]. The total

contraction for the AISI 300 series steels between 367 and 90 K is typically

0.4%; contraction between 293 and 80 K is about 0.28%.

4.4 Aluminum and Its Alloys

Aluminium and its alloys (fee structure) are excellent cryogenic materials whose

mechanical and physical properties are regular, predictable functions of temperature [83]. Pure aluminum offers high thermal and electrical conductivities. Aluminum alloys offer

high strength-to-weight ratios, low costs, and ease of fabrication. Disadvantages are

41
H 250

TEMPERATURE (K)

Figure 4.1. Yield stress data for stainless steels.

42

YIELD

STRESS,

<ry

(ksi)
higher thermal contraction, lower strength, and lower stiffness than copper, titanium, and

steel alloys.

a. General Information

Wrought aluminum alloys are classified by four-digit numbers followed by temper

designations. The alloy families are:

1000 series - unalloyed aluminum

2000 series - aluminum/copper

3000 series - aluminum/manganese

4000 series - aluminum/silicon

5000 series - aluminum/magnesium

6000 series - aluminum/magnesium/silicon

7000 series - aluminum/zinc

Tempers are designated 0, H, or T to denote the annealed, cold-worked, and aged or heat-treated-condi tions , respectively. The 2000, 6000, 7000, and some 4000 series alloys

are precipitation hardenable. Other alloys can be cold-worked to improve strengths moderately.

b. Preferred Alloys

Most aluminum alloys are acceptable for cryogenic applications [84]. Most commonly

used are the 5083-0, 6061-T6, and 2219-T8 alloys. All three of these are readily welded,

but only 5083-0 alloy can be welded to a joint strength efficiency of 100%. Specifications

for these alloys are listed in Table 4.4.

Alloy 5083-0 has the nominal composition A1 -4. 5Mg-0. 6Mn. It is used in massive quantities as a primary structural alloy for large cryogenic storage tanks. It can be coldworked but is typically used in the annealed condition for maximum ductility and toughness.

Alloy 5083-0 has a 300 K yield stress of 140 MPa.

Alloy 6061-T6 has the nominal composition Al-lMg-0.6Si-0.27Cu-0.2Cr. This alloy is widely used in cryogenic piping systems, offering intermediate strength and ductility,

with a 300 K yield stress of 270 MPa.

Alloy 2219 has the nominal composition Al-6.3Cu-0.3Mn. In the T87 temper this alloy

offers high strength, moderate ductility, and adequate fracture toughness for weightsaving applications. The 300 K yield stress is 388 MPa. Applications for alloy 2219

include airborne cryogenic pressure vessels.

Many other aluminum alloys are also useful for specific purposes. Alloys 2024 and

7075 are recommended for nuts and bolts or bolted components that do not require welding.

Unalloyed aluminum EC is a conductor grade rivaling copper in cost. Commercial purity

1100-0 aluminum offers superior formability and corrosion resistance. Alloy 3003 is used

for heat exchangers and is the most brazable aluminum alloy. Alloy A712 offers the highest

ductility of any casting alloy at 77K. Alloy 7005-T5351 offers the best combination of

weldability, strength, and toughness of any 7000 series alloy. Alloy 2021 offers the

highest strength possible in the aluminum system but is the least ductile. Alloy 2014 is

stronger than 2219, but less weldable.

c. Fabrication

Aluminum alloys are among the easiest alloys to form, forge, cast, and machine. Pure aluminum is soft and gummy when machined, but heat-treated alloys have such excellent

machinability that machined and bolted structures of high strength aluminum alloys are often more economical to produce than welded stainless steels. Heat treatment is usually

performed by the producer. Solution-treatment temperatures are 715 to 750 K for the 7000

series alloys and 760 to 810 K for the 2000 or 6000 series alloys.

43
Table 4.4. Specifications for aluminum mill products.

Alloy Product form Federal ASTM ANSI

5083 Sheet and plate QQ-A-250/6 B209 H38.2

Wire, rod, bar, shapes, tube QQ-A-200/4 B221 H38.5

Forgings and forging stork QQ-A-367 B247 H38.8

2219 Sheet and plate QQ-A-250/30 B209 H38.2

Wire, rod, bar: rolled or cold

f ini shed

— — B211 H38.4

Wire, rod, bar: extruded -- B221 H38.5

Forgings and forging stork QQ-A-367 B247 H38.8

6061 Sheet and plate QQ-A-250/11 B209 H38.2

Wire, rod, bar: rolled or cold QQ-A-225/8 B211 H38.4

Wire, rod, bar, shapes, tube QQ-A-200/8 B221 H38.5

Structural shapes QQ-A-200/16 B308 H38.10

Pipe -- B241 H38.7

Forgings and forging stork QQ-A-367 B247 H38.8

Structural pipe and tube: extruded — B249 H38.16

44
Gas metal arc and gas tungsten arc are the preferred welding processes. Alloy 5083-0

retains full strength in the as-welded condition, and with a tensile strength of 140 MPa

at 300 K, 5083-0 welds offer higher strength than 6061 or 2219 welds unless the latter are heat treated. Postweld heat treatment can restore the strength of 2219 welds to nearly

100% of base me£al values.

d. Availability

Most alloys including 5083, 6061, and 2219, alloys are available as plate, sheet,

strip, bar, and forgings. Newly developed alloys, such as 7005, are difficult to obtain

except as forgings and extrusions. Alloy 6061 is readily available in pipe form or related geometries. The cost of a 2.5-cm thick aluminum alloy plate is about $2/kg.

Costs do not vary as widely for the different grades as they do for stainless steels.

e. Mechanical and Physical Properties

The cryogenic properties of aluminum are regular functions of temperature and stero- typic of an fee system. The temperature reduction from 300 to 77 K is favorable, resulting

in moderately improved tensile, fracture, and fatigue properties.

Tensile: Yield stresses at 300 K range from 48 MPa for unalloyed 1100-0 aluminum to 459 MPa for 2021-T815 alloy. Between 300 and 76 K, 1100-0 aluminum shows a yield stress increase from 48 to 62 MPa, a tensile strength increase from 93

to 191 MPa, an elongation increase from 46 to 52%, and a reduction of area decrease from 88 to 81%. Similar trends are observed for aluminum alloys.

Elastic: Young's modulus for unalloyed aluminum is 69.4 GPa at 300 K. Alloys have

higher values, typically 70 to 78 GPa [85-86]. The increase between 300 and 77

K is typically 8%. The lower elastic moduli of aluminum alloys compared with

steel causes a factor of three greater elastic deflections and 1/3 lower buckling

resistance compared with similar-sized components of stainless steel.

Impact: There is usually little change and sometimes a modest increase of impact

resistance at lower temperatures.

Fatigue: The fatigue resistance of aluminum alloys at room temperature is sensitive

to ambient moisture. Crack initiation and propagation occurs more readily in

humid conditions. Fatigue crack growth rates at 77 K are significantly lower

than at 300 K [87-89]. The notch sensitivity in conventional S-N tests does not increase at lower temperatures. Strain cycling tests on 2014-T6 and 2219-T851

show cross-over behavior, where cycle life is higher at 77 K for lives longer

than 500 cycles and higher at 300 K for lives longer than 500 cycles [25].

Fracture Toughness: Annealed alloys (5083-0) fracture by ductile tearing with high

toughness at all temperatures. The high strength alloys (2219-T8) exhibit

linear-elastic fast fractures, but seldom is there a large decrease in toughness

between 300 and 77 K. The Kj

c values for precipitation-hardened 2000 series

alloys (2014, 2024, 2021, and 2219) range from 20 to 30 MPa-nf5 at 300 K, and

there is typically a 20% increase at 77 K [89-90].

Electrical Resistivity: The electrical conductivity of EC grade aluminum at 300 K is 62% IACS. The resistivity dependence on alloying (shown in Fig. 3.8) is discussed

by Clark [38].

Thermal Conductivity: The thermal conductivity of 1100-0 aluminum is 210 W/(m*K) at 300 K and 280 W/(m*K) at 77 K. Structural alloys including 2014-T6, 2219-T81,

5083-0, 6063-T5 and 7075-T6 have lower values, 100 to 180 W/(m*K) and 50 to 130

W/(m*K) at 77 K. Thus, cryogenic temperatures improve the thermal conductivity

of unalloyed aluminum but decrease the conductivity of alloys.

45
Thermal Expansion: The linear thermal expansion coefficients for aluminums range

between 20 and 24 X 10 6 /K at 300 K and between 8.5 and 10 x 10 6 /K at 77 K. Total contraction for this temperature interval is typically 0.35 to 0.41%,

which is higher than other structural metals.

4.5 Copper and Its Alloys

Unalloyed copper (fee structure) offers higher electrical and thermal conductivities

than other metals. Copper alloys offer medium strength and modulus, easy fabrication, and

convenient joining by soldering or brazing. High costs preclude the use of copper in large quantities, but practical applications exist for small components, mainly in secondary

structures where fabrication or physical property advantages are paramount.

a. General Information

Copper and its alloys are conventionally specified using the three-digit designations.

The alloy systems of primary interest are unalloyed coopers, brasses, bronzes, precipitationhardening coppers, and cupronickels.

There are only two precipitation-hardening alloys: Pd-135 and beryllium coppers.

Other copper alloys must be strengthened by solid solution hardening or cold work. Every

method of strengthening compromises thermal and electrical conductivities. Thus, there

are no high-strength, high-conductivity copper alloys. This fact, combined with rising

materials costs and higher stress requirements has generally forced copper alloys out in favor of less expensive aluminum or ferrous alloys that offer competitive strength, machinability and conductivities in many cases.

b. Preferred Alloys

Large quantities of pure copper are sold for electrical applications. Unalloyed

coppers (100 series) have the highest electrical and thermal conductivities, but they are

soft. NTF applications would use these conductor grades for electrical wiring. The

electrical conductivities of grades 101 and 110 are rated 100% by the International Annealed

Copper Standard (IACS). These commercial grades are better electrical conductors than

aluminum. Most widely used for electrical wiring is electrolytic tough pitch (ETP) copper

110 (99.9%Cu). Oxygen free copper (CDA 101, 99.99%Cu) has even higher purity and conductivity. The yield stress of copper 101 at 300 K is 75 MPa.

Pd-135 is a Cu-0.3Cd-0.3Cr alloy, which offers improved strength compared with unalloyed

copper at a modest sacrifice of conductivity. In the precipitation-hardened condition,

the yield stress at 300 K is 255 MPa and electrical conductivity is 90% IACS. Possible

uses for Pd-135 include high tensile conductor wire or electrical contacts.

Beryllium coppers (grades 170, 172, 173) are based on the Cu-1.8Be composition, with

some additions of cobalt. These precipitation-hardening alloys offer the highest strengths

in the copper system, with 300 K yield stress in the aged condition reaching 1000 MPa, or

18 times the value of unalloyed, annealed copper. However, the electrical conductivity of

beryl ium coppers is only 4-10% IACS. In cryogenic service, beryllium copper has been used

for springs and other special items.

Medium-strength brasses and bronzes are routinely used for small, machinable components

in cryogenic apparatus. Bronze is a broad term covering alloys in the 500 and 600 series.

Phosphor, silicon, and aluminum bronzes offer corrosion resistance, weldability, and good

mechanical strength. Brasses are Cu-Zn alloys, available in three structural types:

1) Alpha brasses (fee, less than 39Zn)

2) Alpha + beta brasses (fee + bcc, 39 to 46.6Zn)

3) Beta brasses (bcc, 46.6 to 50Zn)

46
The beta brasses, unlike other bcc metals, are not brittle at cryogenic temperatures.

Nevertheless, the alpha brasses are more widely favored for cryogenic service. Plain Cu- Zn brasses are numbered in the 200 series. Leaded brasses having Pb additions for improved

machinability' are numbered in the 300 series. The 360 grade is most popular for machining.

Brasses having Sn additions to improve strength are numbered in the 400 series.

The cupronickels (between 5 and 45% Ni , some Fe, and some Mn) offer corrosion resistance and weldability. Nickel silvers (60-65% Cu, 7-30% Ni , and the balance Zn) constitute

an alternative to stainless steel for tubing. Constantan is a generic name for the Cu-40%

Ni alloys, which find uses in heater windings and thermocouples by virtue of their high

temperature insensitive electrical resistance.

c. Fabrication

The cold formability of copper and its alloys is excellent. The best cold-forming

grades have a single-phase fee structure: solution annealed beryllium copper, 70/30

brass, and unalloyed conductor grades 101, 110. The two-phase alloys, such as alpha/beta

brasses, are preferrable for hot working.

Coppers are nearly as easy to machine as aluminum alloys. On a relative scale, free

cutting leaded brass grade 360 ranks 100% machinable, whereas unleaded brass ranks 30%.

Least machinable are the soft conductor grades 101, 110, which work-harden significantly.

The precipitation treatment for beryllium coppers consists of a 1- to 3-h heating at temperatures ranging from 588 to 673 K. Other alloys are typically annealed at temperatures ranging from 588 to 977 K.

Copper and its alloys are readily joined by soft soldering. Brazing is also readily

accomplished, noting precautions. Brazing temperatures will anneal cold-worked base

metals. Therefore, temperature control is needed in critical joining operations where

filler melting and base metal annealing points are close. Certain fluxes must be used to prevent dezincification or loss of Pb in leaded brass. Welding usually involves the gas tungsten-arc process for materials less than 4.8-mm (0.187-in) thick, and the gas metalarc process is best suited for thicker gages.

d. Availability

There are few limitations on availability of copper. The wide variety of product

forms reflects the high formability of copper alloys: wire, tubing, valves, springs,

screws, gaskets, fasteners, seamless pipe, sheet, rods, plates, and bars. Cost is the more severe restriction. Copper is typically twice as expensive as AISI 304 stainless

steel. Copper wire products costs 2 to 8 times higher than aluminum wire, depending on purity of grades. Copper costs are significantly reduced, however, using alloys such as brass with sizable additions of zinc.

e. Mechanical and Physical Properties

Low temperature effects in the copper systems are qualitatively similar to those in

the aluminum system. In terms of mechanical behavior, improvement or little change is expected in tensile, fatigue, and impact behavior as temperature is reduced between 295

and 77 K. Cryogenic data are compiled in the monograph by Reed and Mikesell [91].

Tensile: Between 300 and 76 K, annealed oxygen-free copper, exhibits a stress of

about 66 to 89 MPa with elongations of 53 to 60%. Over the same range, beryllium copper, the strongest copper, exhibits a yield stress of 985 to 1200 MPa

with elongations of 6 to 10% [92]. Cartridge brass is representative of the

majority of copper alloys, which achieve intermediate strength and ductility as

a result of solid solution hardening and cold work.

Elastic: Young's modulus for copper at room temperature is 129 GPa, slightly higher

than values for titanium. The moduli for copper alloys at 295 K varies with

composition, but the temperature dependence between 300 and 77 K is regular,

increasing by about 7 or 8% [93-94].

47
Charpy Impact: CVN values usually increase or remain nearly constant at cryogenic

temperatures. For example, CVN data for annealed oxygen-free copper between 300

and 77 K increase from 71 to 89 J. Aged beryllium copper increases from 6.6 to 9.5 J.

Fracture Toughness: Copper alloys are not commonly used for fracture-critical components. Therefore, Kj

c data are unavailable. Brittle fracture is possible for

heavily cold-workedal loys or for aged beryllium copper. Alloys in the annealed

condition are highly ductile and fracture by gradual tearing [95],

Electrical Resistivity: The electrical resistivity of oxygen-free copper is sensitive

to purity, but representative values are 1.56 pflcrn at 273 K and 0.2 pQcm at 76

K. Higher resistivities and reqular temperature dependences are observed for

alloys [38].

Thermal Conductivity: The 300 K thermal conductivities are highest for oxygen-free

copper 400 W/(m-K) and lowest for German silver, Manganin, and Constantan: 22

to 25 W/(m-K). A temperature reduction to 77 K improves the conductivity of

oxygen-free copper to 550 W/(m-K), but decreases the values for alloys. The 77

K values for German silver, Manganin and Constantan alloys range from 12.5 to 17 W/(m* K)

.

Thermal Expansion: The linear thermal expansion coefficients for unalloyed copper

decrease from 17.7 x 10 6 /K at 294 K to 8.4 x 10 6 /K at 80 K. The corresponding

total contraction is 0.302%. For the same temperature interval, the contractions

for alloys range from 0.245% for Constantan to 0.350% for yellow brass (Cu35Zn)

.

4.6. Nickel and Its Alloys

Nickel and its alloys (fee structure) offer excellent low temperature properties.

Commercially pure nickel is ferromagnetic and corrosion resistant; it is a fair electrical

conductor, and a crucial alloy element in cryogenic steels [96]. Nickel alloys offer high

elastic moduli and medi um-to-high strengths, but costs are high, and fabrication is complicated.

a. General Information

Designations for nickel and its alloys usually consist of generic names and threedigit numbers. Every alloy series contains precipitation hardening as well as non-heattreatable grades. Unalloyed nickel, as well as members of the Ni-Cu, Ni-Mo, Ni-Cr, Ni-CrMo, Ni-Ce-Fe, and Ni-Fe-Cr alloy system are candidates for cryogenic service. The superalloys are of greatest potential for NTF primary structural applications.

Superalloys are precipitation-hardening alloys containing chromium. Although originally developed for elevated temperature service, the superalloys also exhibit excellent

cryogenic properties owing to their austenitic structure. Cryogenic temperatures lead to further strengthening, with little or no loss of ductility and toughness. Previous uses include aerospace pressure vessels at 20 K and torque tubes for superconducting machines

at 4 K. Superalloy metallurgy is complex, however, and good quality control is needed in processing and fabrication.

b. Preferred Alloys

Commercial purity nickel (Nickel 200, 99 . 5%N i ) is a fair electrical conductor, finding

applications in strain and temperature gages as well as liner materials for filament-wound

pressure vessels. The nominal composition of Nickel 200 is 99. 5Ni-0. 08C-0. 18Si-0. 18Mn0. 2Fe-0. 13Cu-0. 005S , plus some cobalt. The yield stress is 144 MPa at 300 K.

The basic Ni-Cr alloy is Inconel 600 (Ni-15. 5Cr-8Fe) which offers properties similar

to those of annealed AISI 304 stainless steel. Inconel 625 (Ni-21.5Cr-2.5Fe-0.2Al-0.2Ti9Mo-3.65Cb + Ta) is a modification which can be precipitation-hardened to a 300 K yield

48
stress of 413 MPa, or twice the value for Inconel 600. Still higher strengths are obtained using Inconel 706 (Ni-16Cr-37. 3Fe-27Al-l. 66T i - 2 . 95Cb&Ta) ,718 (Ni -19Cr-18. 5Fe-0. 9T i

3Mo-5. lCb&Ta) , and X-750 (Ni~15. 5Cr-7Fe-0. 7A1 -2. 5Ti -0. 95Cb&Ta) superalloys.

In view of high strength and toughness, Inconel 718 and Inconel X-750 are currently

the most widely psed superalloys. Inconel 718 offers the highest yield stress, 1172 MPa

at 300 K, whereas Inconel X-750 offers higher toughness at a slight reduction of strength.

Inconel 706 is substituted for these alloys when better machinabi 1 ity and fabricabi 1 i ty is desired. Several other nickel alloys have been used for cryogenic service [102], The

best known Ni-Cu alloy is Monel K-500 (66.5 Ni-28.5Cu-0.13Cu-0.75Mn-l.0Fe); it has been

used in liquid oxygen lines and manifolds where complicated shapes must be formed from

soft sheet, then welded, then aged to higher strength. Flastelloy B ( 61N i - 2 . 5Co-28Mo-lCr5Fe-0.05C) has been used in aerospace applications spanning both high and low temperature

extremes. Hastelloy X (47Ni - 1. 5Co-9Mo-22Crl8. 5Fe-0. 1C) provides better castings, compared

with Inconel X750. Incoloy 903 (38Ni -15Co-3Cb-l. 4Ti-0. 7A1-Fe bal) is known for its relatively low expansion coefficient of 7.2 x 10 6 /K at room temperature.

c. Fabrication

Nickel base alloys, particularly the superalloys, are difficult to fabricate. The

superalloys were designed to resist elevated temperature deformation, so forming temperatures are necessarily high. Casting temperatures are also high, and castings represent

some sacrifice of mechanical properties compared with wrought products, although the

associated heat treatments are often less complicated.

Nickel alloys are also difficult to machine, owing to high hardness and high workhardening rates. Superalloys are best machined in the solution-annealed or hot-worked

conditions. To maintain critical dimensions, the proper procedure is to machine slightly

oversize, age-harden, then finish to size. This anticipates the 0.09% contraction during

precipitation hardening, and better surface finish is obtained. In some cases, electrochemical, discharge, ultrasonic, electron beam, or chemical mill machining may be more

economical than conventional machining.

The most common welding process for nickel alloys is gas tungsten arc, but shielded

metal arc, gas metal arc and oxyacetylene flame are possible alternatives. Components are usually joined in the solution-treated condition, then postweld heat-treated. Weld pro- cesses are similar to those for stainless steels. Weld ductility and toughness are generally inferior to base metal properties, although strengths may be equivalent. Nickel

alloys can also be brazed, but copper, rather than silver, brazing alloys are preferred if

precipitation-hardening treatments are to follow.

Inconel 718 and Inconel X750 are heat-treated as follows: Inconel 718 is solutiontreated (1256 K, air cool) and double-aged (992 K, 8 h, furnace cool; 894 K, 20 h, air

cool). Inconel X-750 is solution treated (1255 K, 1 h, air cool), and double-aged (1144

K, 25 h, air cool/977 K, 20 h, air cool). Delayed hardening gives Inconel 718 better

resistance to postweld strain-age cracking, compared with Inconel X750.

d. Availability

Superalloys are available primarily as aircraft quality sheet or forged bar. Plate

is not available. Owing to high nickel content, difficult forming, machining, and heat

treatments, superalloys are among the most expensive structural alloys.

e. Mechanical and Physical Properties

The Inconel superalloys are the best characterized Ni-base alloys. The existing

fracture mechanics data base [97-101] for Inconel 718 and Inconel X-750 alloys can be

readily translated to NTF applications.

Tensile: Yield stress at 300 K range from 117 MPa for annealed, commercially pure

nickel to 1172 MPa for precipitation-hardened Inconel 718. Between 300 and 77

K, commercially pure nickel shows a yield stress increase of 117 to 137 MPa, a

49
tensile strength increase of 413 to 593 MPa, and an elongation increase of 46 to

52%. For the same temperature range, Inconel 718 shows a yield stress increase

of 1172 to 1342 MPa, a tensile strength increase of 1404 to 1649 MPa, and nearly

constant elongation at 20%.

Elastic: Young's modulus for unalloyed nickel is 225 GPa at 295 K. The increase at

77 K is approximately 6%. Nickel alloys exhibit slightly lower moduli, but

similar temperature dependences [103-104],

Impact: Charpy impact values for nickel-base alloys are only moderately temperature

sensitive, and large decreases at cryogenic temperatures are not observed.

Fatigue: Nickel alloys offer improved fatigue resistance at cryogenic temperatures,

as do other fee metals. Inconel 718 for example has a 77 K fatigue strength of

965 MPa at 10 6 cycles, which corresponds to 60% of the ultimate strength [27],

In strain cycling fatigue Inconel 718 shows improvement at 77 K, compared to 300

K, at all cycle lives between 100 and 106 [25]. Fatigue crack growth rates for

Inconel 718 and X-750 superalloys are shown in Fig. 3.4.

Fracture Toughness: A substantial fracture mechanics data base has been built up for

Inconel X-750 and Inconel 718 base metal and welds [97-101]. Fracture mechanics

analysis is advisable for primary structures since superalloys may fail in a relatively brittle manner when cracks are present in thick sections. Reported^

Kj

c values for Inconel X-750 and Inconel 718 fall in the range 70 to 250 MPa*m 2

at 295 K. At 77 K there is a 7% increase of K T for Inconel 718 and a 10%

Ic decrease for Inconel X-750. Significant variability in «

T can be expected

owing to heat-to-treat variations [98]. Best toughness properties are obtained

in heats not having grain boundary carbides.

Thermal Conductivity: The thermal conductivity of commercially pure nickel increases

from about 78 W/(m-K) at 300 K to 74 W/(m*K) at 77 K. Inconels, Hastelloys, and

Monels show values in the ranges 0.1 to 0.23 at 300 K and 0.058 to 0.16 at 77 K.

Thermal Expansion: Between 293 and 80 K, commercially pure nickel contracts 0.21%,

which is near values for ferritic steels. The contractions for many nickel

alloys are within ± of pure nickel [38],

4.7 Titanium and Its Alloys

Unalloyed titanium and extra-low-interstitial (ELI) Ti-5Al-1.5Sn and Ti-6A1-4V alloys

are qualified for cryogenic applications where higher material and fabrication costs are justified by the physical and mechanical properties obtained. Advantages are excellent

corrosion resistance, low thermal conductivity, low density (about 4/7 that of stainless

steel), and high cryogenic strength. On the other hand, titanium components are relatively

difficult to fabricate, and the medium strengths at room and elevated temperatures are disadvantageous for some applications. The elastic modulus of titanium is higher than that

of aluminum alloys, but lower than that of steels.

a. General Information

Titanium alloys have alpha (hep), beta (bcc), or alpha + beta crystal structures.

Alloys in which the hep phase predominates are useful at cryogenic temperatures, but all beta alloys are brittle at low temperatures and worthless for applications at 77 K. Since

the mechanical behavior is sensitive to purity, the Ti-5Al-2.5Sn and Ti-6A1-4V alloys must

be obtained as ELI grades. Note that normal grade Ti-5Al-2.5Sn and Ti-6A1-4V alloys are obsolete for cryogenic service because their higher impurity levels compromise cryogenic

ductility and toughness. Since cracked ELI titanium alloy components are apt to fail in a brittle manner at cryogenic temperatures a fracture mechanics analysis may be necessary to verify structure integrity.

50
b. Preferred A1 loys

Unalloyed titanium is available in three grades and is used in the annealed condition.

The 295 K yield stress of the three grades are nominally 379 MPa, 488 MPa, and 517 MPa. Selection of one of these three grades of unalloyed titanium should be based on the knowledge that strength increases while ductility decreases with decreasing purity. NTF applications for unalloyed titanium would be those for which strength is not the primary

consideration, as in the construction of aerodynamic surfaces. Unalloyed titanium is less

expensive, lower in strength, and about twice as thermally conductive as the T

i -5A1

2 . 5Sn( ELI ) and T

i -6A1 -4V( ELI ) alloys. Specifications for titanium alloys are listed in

Table 4.5.

T

i -5A1 -2. 5Sn(ELI) and Ti -6A1 -4V( ELI ) alloys are used where high strength-to-thermal

conductivity or strength-to-weight ratios are required. T

i -5A1 -2. 5Sn(ELI) has a singlephase hep structure and is used in the annealed condition. The 295 K yield stress of Ti5A1-2. 5Sn(ELI) is 750 MPa. Past applications specified furnace cooling from the annealing

temperature of 1088 K, but air cooling apparently would confer superior fracture toughness

[105].

T

i -6A1 -4V( ELI ) has a two-phase microstructure consisting predominantly of hep and

some bcc. This alloy can be aged, but is typically used in the annealed condition to maximize toughness for cryogenic service. A recrystallization anneal at 1200 K followed

by furnace cooling results in a 295 K yield stress of 840 MPa. Past uses for T

i -5A1

2.5Sn(ELI) and Ti-6A1-4V (ELI) include large welded cryogenic pressure vessels for aerospace propellant systems, as well as small laboratory cryostat components. For applications at 77 K or above, Ti-6A1-4V (ELI) is more commonly selected because it offers adequate

toughness and higher strength than T

i -5A1 -2. 5Sn(ELI).

c. Fabrication

Titanium alloys are more difficult to machine than most steels, except austenitic

stainless steels. Difficulty in processing arises owing to contamination by atmospheric

impurities. For this reason electron beam welding in vacuum is preferable to gas-metal -arc

or gas tungsten-arc welding. Electron beam welding is more expensive, but it reduces the

risk of contamination and yields more ductile welds.

d. Availability

Unalloyed titanium and Ti -6A1 -4V( ELI ) are readily available in standard product

forms. Billets of Ti - 5A1 - 2 . 5Sn(ELI) are also available in sections 10 cm square or 10 cm

in diameter, but smaller forms may be more difficult to obtain. Prices for ELI grade

alloys range from about $2. 7/kg for plate or billet to about $4. 5/kg for sheet. Unalloyed

titanium may be several dollars per pound cheaper.

e. Mechanical and Physical Properties

The mechanical properties of titanium are strongly temperature dependent and sensitive

to impurities (0, H,N, C, and Fe). Accurate fracture mechanics data are needed in critical

applications.

Tensile: The 295 K yield stresses for the recommended alloys range from 379 MPa for

unalloyed titanium to 140 MPa for T

i -6A1 -4V( ELI ) . Temperature reductions to 77

K approximately double the yield stress, while ductility is moderately reduced.

Elastic: Young's modulus for unalloyed titanium at 295 K is 114 GPa, slightly lower

that of copper. The 295 K Young's moduli for Ti-6A1-4V and Ti-5Al-2.5Sn alloys

are nearly equivalent at 111 GPa. Temperature reductions to 77 K increase the

moduli by 9 or 10% [106].

51
Table 4.5. Titanium alloy specifications.

Applicable specifications

Material Product form Federal ASTM AMS

Commercial Purity

T i tan i um

A 40, 55A Sheet, strip, plate MIL-T-9046, I, A ASTM B 265-72 4902B

A 55, 65A Sheet, strip, plate MIL-T-9046, I, C ASTM B 265-72 4900F

A 70, 75A Sheet, strip, plate MIL-T-9046, I, B ASTM B 265-72 4901G

Ti-6A1-4V, ELI Sheet, strip, plate MIL-T-9046, III, D ASTM B 265-72 4907C

Bar, forgings, rings MIL-T-9047, Comp. 7 ASTM B 381-69 4930B

Ti-5Al-2.5Sn, ELI Sheet, strip plate NA ASTM B 265-72 4909C

Bar, forgings, rings MIL-T-9047, Comp. 3 ASTM B 381-69 4924C

52
Impact: Charpy impact energies for titanium alloys decrease at cryogenic temperatures

A typical CVN value for annealed T

i -6A1 -4V( ELI ) is 19.5 J at 77 K.

Fatigue: Within the data scatter of replicate tests, fatigue crack growth rates for

Ti-5Al-2.5Sn and T

i -6A1 -4V alloys are temperature insensitive between 295 and 76

K providing the stress intensity factors remain at intermediate values, not approaching «

T

. Stress controlled S-versus-N fatigue data show improvement as temperature isreduced between 295 and 77 K.

Fracture Toughness: Flawed titanium alloys are brittle at 77 K, even though the same alloys are ductile in the unflawed or unnotched condition. The critical flaw

sizes at 77 K are small. The Kj

c values for titanium alloys decrease with

decreasing temperatures [107]. Typical Kj

c values for recrystallization annealed

Ti-6A1-4V(ELI) are 100 MPa-m^2 at 295 K and 61 MPa-m*5 at 77 K [108]. Typical

values for annealed, air-cooled T

i -5A1 -2. 5Sn(ELI) alloy are 101 MPa-m 2 at 295 K and 75 MPa-m*5 at 77 K.

Thermal Conductivity: Thermal conductivities of Ti - 6A1 -4V and Ti-5Al-2.5Sn at 295 K are about half the values of unalloyed titanium, stainless steel, and 9%Ni

steel. The temperature dependence of thermal conductivity for T

i -5A1 -2 . 5Sn

(normal grade) shows a decrease from 8.17 W/(m-K) at 300 K to 4.4 W/(m-K) at 80

K.

Thermal Expansion: The linear thermal expansion coefficients for titanium and the

recommended alloys at 300 K are 8.4 to 9.4 x 10 3 /K, which is nearly equivalent

to values for 9%Ni steel. Total contraction between 295 and 90 K is 0.14 to 0.18%.

4.8. Cobalt and Magnesium

Cobalt superalloys (hep structure) have served in some cryogenic aerospace applications but are not recommended for NTF applications in view of high cost, limited availability, poor fabricabi 1 ity, lack of design data, and lack of documented service experience

Magnesium and its alloys were once considered attractive for high strength-to-densi ty

ratios, but Charpy V-notch impact energies for all cast and wrought Mg metals are too low

(1.35 to 9.5 J) for service at temperatures between 295 and 77 K [109-110],

53
5. METALS FOR BRAZING AND SOLDERING

Most metals, including stainless steels copper, nickel, aluminum, and titanium alloys,

can be brazed or soldered under appropriate conditions. The purpose may be to establish

electrical, mechanical, or thermal contacts, or simply to produce a seal.

Brazed and soldered joints are formed by fusing a thin layer of nonferrous filler

metal into a narrow gap between two base metal components. Bonding results from the wet- ting and dissolution of the base metal by the filler, without fusion of the base metal.

The only distinction between brazing and soldering is the temperature reguired. By con- vention, joining at temperatures greater than 725 K is brazing, while joining at lower

temperatures is soldering [111-112].

5. 1. Fillers and Fluxes

For both brazing and soldering, fillers having an eutectic composition are favored

because they melt at relatively low temperatures. Care must be taken if noneutectic

compositions are used because noneutectics melt over a range of temperatures during which

the filler exists as a two-phase solid/liguid "paste." Pasty fillers do not flow readily

into narrow gaps, and leaks or voids owing to lack of penetration may result. Specific

fillers are recommended in the following sections.

The filler must wet the base metals to be joined. Wetting depends on the affinities

of specific materials and is promoted by using fluxes to remove surface oxide films and

protect the heated metals from the atmosphere. Borax or boric acid fluxes are generally

used for brazing. There are three alternatives for soldering: 1) hydrochloric or phosphoric acid fluxes, 2) pastes of petroleum jelly, and 3) rosins or resins. Manufacturer's

recommendations should be followed in selecting fillers and fluxes for specific applicati ons

.

5.2. Brazing

The advantages and disadvantages of welding, brazing, and soldering are listed in Table 5.1. Brazing is preferable to welding where dissimilar metals must be joined con- veniently and rapidly, without fusing the base metals. Brazed joints are stronger, more permanent, and more resistant to thermal cycling than are soldered joints. On the otherhand, the permanency of a brazed joint can be disadvantageous, and care must be taken that

the brazing temperatures (875 to 1150 K) do not damage the base metals. For example,

brazing may anneal copper or aluminum alloys, sensitize austenitic stainless steels, or volatilize the lead in leaded brass. Consequently, it may be impossible to replace a brazed joint without damaging a complex apparatus [113-115].

Brazing is also referred to as hard soldering or silver soldering because the recom- mended brazing fillers are usually silver alloys which are stronger than solders. For most cryogenic applications, brazing alloys with 45 to 72% silver are superior to brazing

alloys containing little or no silver [116]. Silver brazing alloys are available in the

form of sheet, wire, powder, and preforms. Copper, nickel, and gold brazing fillers are also available for special purposes.

The most frequently used commercial silver brazing alloys for cryogenic service are

the 50%Ag-15. 5%Cu-16. 5%Zn-18%Cd and 45%Ag-15%Cu-16%Zu-24%Cd alloys, which are included in AWS A5.8-76 specifications. The cadmium additions create alloys of near-eutectic composition, with melting points in the range 893 to 908 K. These alloys have low viscosity and

flow readily into joints with gaps from 0.025 to 0.127 mm. Seligmann and Sarwinski [117]

report a 0.433% thermal contraction for one such alloy between 295 and 4 K. Thermal con- ductivity is about 60W/(m-K) at 200 K, and 33 W/(m-K) at 77 K [118]. Electrical resistivity at 295 K is about 6 to 10 pQcm, and electrical conductivity is about 18 to 28% IACS. The 295 K thermal expansion coefficients of about 1.8 X 10 5 /K closely match the coefficients of coppers, brasses, bronzes, and AISI 304 stainless steel.

The tensile properties of the Ag-Cu-Zn-Cd fillers are unreported, but the tensile

strengths of Ag-Cu-Zn brazing alloys at 300 K range from 234 to 417 MPa. Reduction in

54
Table 5.1. Comparison of welding, brazing, and soldering.

Weldi ng Brazing Sol deri ng

Advantages High strength

High thermal cycling resistance

Few corrosion problems

Large sections

Joins dissimilar metals

Little or no warping

Thin sections Good appearance

Stronger than soldered joints

Joins dissimilar metals

Little or no warping

Thin sections

Good appearance

Low heat input

Inexpensive

Easy to disassemble

Di sadvantages Base metal is melted

High heat input

Di storti on Expensive

More expensive than soldering

Difficult to disassemble

Higher heat input than soldering

Low thermal cycling

resistance

Corrosive fluxes

Low strengths

55
area is typically 15 to 50%. Good mechanical properties are retained at cryogenic temperatures, since silver and copper have fee structures. Some available data at temperatures

from 295 to 77 K show 20 to 60% improvements in tensile strength of Ag-Cu-Zn brazing

alloys and only minor effects on ductility [119-122].

5.3. Soldering

Compared to brazing alloys, the melting points of solders are relatively low. Therefore, soldered joints are easily formed, disassembled, and reformed without thermal damage

to neighboring apparatus. Solders are also less expensive than brazing alloys. On the

otherhand, soldered joints are weaker than brazed joints, less resistant to thermal cy- cling, and limited in use at elevated temperatures near the melting point. Aluminum,

magnesium, and chromium are difficult to solder, except by means of special techniques and

fluxes. Copper alloys are easiest to solder.

The most common solders are Pb-Sn alloys which melt at temperatures ranging from 456

K (the 37%Pb-63%Sn eutectic) to 599 K (unalloyed Pb). Other solders contain lead, tin,

indium, bismuth, cadmium, mercury, or gallium. Most of these elements are volatile and

toxic, especially Cd, Hg, and Pb. Therefore, care must be taken during melting, and

overheating should be avoio'd.

Tensile, shear, and impact data for solders are reported at temperatures between 20

and 373 K [120, 123-128], In general, the mechanical properties of solders are sensitive

to composition and testing rate. For example, Thwaites and Hampshire [128] show two- or three-fold increases of tensile strength for Pb-Sn solders at 295 K as the testing rate is varied from 0.0083 to 0.83 mm/s. Rate effects are also sizable at cryogenic temperatures

[123],

Lead-tin Solders . Selected mechanical property data for Pb-Sn solders are shown in

Figs. 5.1 and 5.2. All Pb-Sn solders are ductile at room temperature, but tin-rich solders

exhibit low temperature ducti le-to-bri ttl e transitions. The tin-rich solders offer higher

strength and creep resistance for room or elevated temperature service, but their poor low

temperature ductility makes them undesirable for cryogenic service, except in applications

where mechanical properties are unimportant [125].

Lead additions increase the amount of fee phase in solders, while decreasing the

ductile-to-brittle transition temperature. For example, unalloyed tin (tetragonal struc- ture) is brittle at 163 K, whereas 50%Pb-50%Sn solder is brittle at 123 K. Ainsworth

[129] concludes that a lead content of 80% or more is required to retain ductility and

toughness in Pb-Sn solders at 77 K. Nevertheless, 40%Pb-60%Sn solder is frequently used

at 77 K if mechanical performance is not critical.

The strength, ductility, and impact properties of unalloyed lead (fee structure)

continuously increase between 295 and 20 K, but unalloyed lead is a poor solder because it

does not wet many metals. Some tin is needed to promote wetting and compatibility with

rosin fluxes. Consequently, the optimum ductile solders for extreme cryogenic service are alloys such as 90%Pb-10%Sn, 97. 5%Pb-l%Sn-l. 5%Ag, and 93. 5%Pb-5%Sn-l. 5%Ag. These solders

show improved strength and high ductility at temperatures down to 77 K [126].

Although some thermal property measurements for 40 to 60% Pb solders are reported

[16, 117, 130], such data are rare, and the cryogenic thermal properties for most solders

must be estimated from the pure metals.

Bismuth Solders . Solders containing 50% Bi or more expand on solidification, whereas

most metals and alloys contract. Therefore, special uses in sealing applications have

been found for bismuth solders, despite the fact that they are weak and brittle at room

and cryogenic temperatures.

Wood's metal (50%Bi-25%Pb-12. 5%Sn-12. 5%Cd) is a bismuth solder noted for its exceptionally low melting point of about 335 to 342 K. Thus, Wood's metal can be used to form

joints without disturbing neighboring Pb-Sn soldered joints. However, Wood's metal is

56
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(K)
only about half as strong as Pb-Sn solders, and it has a low resistance to thermal cycling.

This solder also has a melting point lower than the 367 K temperature encountered in the

NTF. Seligmann and Sarwinski [117] report a 0.582% thermal contraction for Wood's metal

between 295 and 4 K.

Indium Solders . The most outstanding characteristic of indium is its ability to wet

diverse materials. Thus, indium solders can be used for a variety of gl ass-to-gl ass

,

glass-to-cermanic, and ceramic-to-metal joints. Indium solders acceptable for cryogenic

service are 100%In, 90%In-10%Ag, and 50%In-50%Pb. These solders are noted for melting

points ranging from 416 to 573 K, low strength, excellent ductility at all temperatures,

and higher thermal conductivity compared to Pb-Sn solders.

5.4. Joints

A brazed or soldered joint consists of two base metal components, a narrow diffusion

zone, and a filler zone. Lap or butt joints are common in service, but ring-and-pl ug joints are recommended for research [128],

The mechanical properties of brazed or soldered joints cannot be predicted simply

from the properties of the fillers themselves. Joint properties are a function of base

metal properties, joint geometry, and type of stress applied. Since the filler is con- strained by base metal components, the joint may sustain higher stress than expected for

fillers in bulk form [131].

Temperature cycling frequently causes failure in soldered joints. To minimize this

problem, the solder and base metal thermal expansion coefficients should be matched as closely as possible.

The 300 K shear strengths of soldered joints typically range from 10 to 50 MPa and

are less than or equal to the bulk solder tensile strength. Raising the temperature from

295 to 373 K may decrease the shear strengths of soldered joints by 25 to 80% [128].

Brazed joints give higher strengths, with butt tensile strengths at 295 K ranging as high

as 1100 MPa. Representative shear strengths for brazed copper joints at 77 K range from

123 to 207 MPa, which is equal to or greater than the strengths at 295 K.

As a guide to the mechanical behavior of joints at cryogenic temperatures two experimental studies are noted:

1. Jaffee, Minarcik, and Gonser [124] tested copper tubing joints made with Pb-Sn

solders. The tubes (25.4 mm ID, 1.6 mm wall) were soldered using standard sleeve fittings,

then tested at 300, 244, and 91 K. Regardless of solder type, the joint-breaking loads

increased linearly with decreasing test temperature. For high-tin solders (75%Pb-25%Sn

and 50Pb%-50%Sn) , the low temperature fractures occurred in the joint. For high-lead

solders (97. 5%Pb-2. 5%Sn and 95%Pb-5%Sn), fractures tended to occur in the tubes or fittings. But the breaking loads at a given test temperature were nearly equivalent to and

virtually independent of the solder used.

2. Liebenberg [132] tested single-overlap joints made from 6.25 x 37.5 x 76.2 mm brass plates and a 60%Sn-40%Pb solder. The ultimate shear strength at 295 K immediately

after soldering was 34 MPa (5 ksi). However, room temperature aging for 50 days led to a 25% reduction of shear strength. Ten thermal cycles between 295 and 77 K decreased the

joint shear strengths by an additional 20%. The shear strengths at 295 and 20 K were

nearly equivalent. This apparent temperature independence of shear strength is also

reported for silver-brazed copper joints tested between 295 and 77 K [133].

59
6. METALS FOR PLATING AND PROTECTION

Plating serves functional and aesthetic purposes. Some common cryogenic steels, such

as Fe-9Ni steel, will rust after cryogenic testing unless plated or dried in an air jet to quickly remove condensed moisture. Plating can also be used to improve surface condition

and appearance, prevent galling between nuts and bolts, or embed lead wires and instrumentation.

The satisfactory cryogenic plating metals listed in Table 6.1 have fee structures. Non-fcc metals, such as tin, are brittle at cryogenic temperatures and are, therefore,

avoided. The platings in Table 6.1 have served in aerospace applications at temperatures

to 77 K or lower [134]. In selecting a plating, materials compatibility and cost should

be considered. To avoid corrosion, the plating and substrate should be ranked appropriately on the electromotive series. Gold, platinum, rhodium, and aluminum platings are expensive. Nickel and silver are more commonly used.

Table 6.1. Platings for cryogenic service.

Plating Hardness at 295 K (Rockwell B or C, as noted)

Aluminum <56R

b Copper <97R

b Gold 26 to 77 r b Indi urn NA (very soft)

Lead NA (very soft)

Nickel 76 to 93 r b Platinum NA

Rhodi um 43 to 72 RC

Si 1 ver <88 R b

60
7. NONMETALS

Wood, glass, ceramics, and polymers are discussed in this section. These materials

find special applications in wind tunnel models, but are less commonly used than metals.

7.1. Wood

Wood is a porous natural composite of cellulose and lignin. Wood is rarely used at cryogenic temperatures, but it serves as a structural insulator in special applications.

Most thermal insulators are heavy, except wood and foamed plastics. Foamed plastics,

however, have low strength. Therefore wood may be the best choice where a balance of

strength and thermal insulation is required.

Current applications for wood include cryogenic storage tank and pipeline supports.

In some designs balsa blocks are used to support the entire weight of liquefied natural

gas storage tanks [135]. Plywood facings and balsa frameworks are used in combination

with foamed plastics in modern thermal insulation and support systems for LNG tankage.

Wood frameworks overlayed with fiberglass are used for honeycomb structures. In NTF applications, wood can be used similarly, if it is sealed against moisture and if sizable

thermal contraction can be tolerated.

a. General Information

The mechanical and thermal properties of wood are proportional to density and strongly

dependent on moisture content and orientation. Orientation is specified with respect to the three orthogonal axes: longitudinal (parallel to grain), radial, and tangential. For example, _the linear thermal expansion coefficient for Douglas fir at room temperature is 2.7 x 10 6 /K in the longitudinal direction compared with 27 x 10 6 /K in the tangential or radial directions [136]. Similarly, longitudinal strength may be ten times greater than

radial or tangential strength. The anisotropy can be reduced using plywood.

Moisture in wood significantly reduces room temperature strength. Moisture also causes swelling and warping, and general loss of dimensional tolerances. Moreover, premature failure at low temperatures may result from high water content [136, 137], This is

a problem since cryogenic apparatus will condense moisture from ambient, humidified air.

b. Specific grades

Among the hundreds of species of soft and hardwoods, balsa is unique. The lightest

of woods, balsa is commercially available in densities ranging from 88 to 192 kg/m3. End-grained balsa (the strongest orientation) can support 6.9 MPa while providing lower

thermal conductivity than many other woods, and lower thermal expansion than insulating

plastics. Some cryogenic mechanical properties of balsa are reported by Schwartzberg

[138].

Plywoods are laminates of an odd number of wood plies glued together at angles of up to 90°. Compared with natural wood, plywoods offer several advantages, such as higher

resistance to splitting, reduced anisotropy, and greater dimensional stability in the

presence of moisture.

Impregnated wood is a unique material, well suited for cryogenic environments. In the impregnation process, wood is densified by heating and pressing at high pressures,

then polymerized by irradiation. Impregnation with methyl methacrylate, styrene, ethyl a- crylate, or phenolic resin improves strength while obviating the moisture problem. Impregnated wood can be placed in direct contact with water without losing dimensional stability,

since moisture absorption is less than 1%. After impregnation, tests at room temperature

show improved stiffness (34%), compressive strength (600%), tensile strength (18-86%), and

hardness (350 to 1000%). Toughness usually decreases, however, because irradiation damages some woods, particularly hardwoods. Impregnated wood has been used in bearing insulating block applications at compressive design stresses up to 83 MPa and at shear stresses

up to 6.9 MPa. The primary disadvantage is higher cost [139, 140].

61
c. Mechanical and Physical Properties

The mechanical and physical properties of wood at cryogenic temperatures are poorly

documented, but data at 77 K for balsa, spruce, fir, beech, beech plywood, and impregnated

beech are reported [136, 137, 139, 140]. Representative data trends are illustrated in Figs. 7.1 and 7.2. Dried natural woods generally show improved mechanical properties at cryogenic temperatures. The compressive strengths for longitudinal orientations at 77 K range from 28 MPa for balsa to 158 MPa for beech. By comparison, the compressive strengths

for impregnated and densified beech veneers at 77 K may reach 321 MPa. Adler's results

for impregnated wood between 293 and 81 K indicate approximately a 20% reduction in tensile

strength, shear strength, and impact values [139],

Limited thermal conductivity data for some woods and wood fiber products at selected

temperatures between 367 and 77 K are tabulated by Loser, Moeller, and Thompson [141]. A gradual decrease in thermal conductivity is expected at low temperatures. The thermal

conductivity values for balsa wood decrease from 0.04 to 0.026 W/(m-K) between 201 and

90 K. The thermal conductivities of an impregnated and densified wood are reported to range from 2.6 to 4.5 W/(m*K) at 90 K [140], The average thermal expansion coefficients

for impregnated wood between 77 and 290 K range from 2.5 to 3.5 x 10 6 /K [139].

d. Fabrication

Wood is readily available and easily machined using carpentry or metal-working tools.

At room temperature, glued joints may be stronger than the wood itself. At cryogenic

temperatures, joints may be weaker, owing to mechanical constraint or embrittlement of the

adhesive.

Impregnated wood can be obtained at higher cost in sections up to 102 mm (4-in)

thick. Greater thicknesses are achieved by bonding separate sections. Carbide-tipped

tools are recommended in machining the very hard impregnated woods. Tolerances of

±0.381 mm can be met.

7.2. Glass

Glasses are ceramics manufactured by cooling from the melt at rates too rapid to permit crystallization. Consequently a vitreous (amorphous) structure is produced, intermediate between the cyrstalline solid and liquid states. Silica (Si

0

2 ) is the major

component in commercial glasses, and silica tetrahedra are the basic building blocks of

glass structures, which consist of three-dimensional networks lacking long-range order.

a. General Information

Glasses offer high heat resistance, thermal stability, insulation and transparency.

Cryogenic applications include windows for spacecraft and bubble chambers, vacuum dewars

for cryostats, compressive load-bearing insulators, and reinforcing fibers for composites.

Brittleness is the chief disadvantage of glass. Therefore, applications involving high

tensile stresses or impact loading are avoided. Glasses should be used in the annealed

condition to relieve residual stresses, unless toughening treatments are applied.

Many scientific studies have been performed on glass. Fracture stresses are always

lower in tension than compression, widely scattered, and highly sensitive to flaws or stress concentrators. Other undesirable aspects of mechanical behavior include the phe- nomenon of static fatigue and the poor thermal shock resistance of most grades.

b. Specific Materials

Some common glasses and their compositions are listed in Table 7.1. Many compositions are unacceptable for cryogenic service, owing to poor thermal shock resistance.

E and S glass fibers are recommended for composite reinforcement applications, as discussed in section 8.3. For bulk cryogenic applications, Pyrex is favored because it

has high chemical and thermal shock resistances. Typical applications for Pyrex are

62
Table 7.1. Glass compositions.

Material Composition (wt%)

Si0 2 B 2 O3 Na 2 0 CaO A1 2 0 3 K 2 0 MgO

Fused silica 99.9 _ _ _ _ _ Vycor 96 3 <0.2 - 0.4 <.2 - Pyrex 80-81 12-13 4

- 2 0.4 - Soda-1 ime 60-75 - 12-20 8-20 1

- 3

E glass fibers 54 8.3 <1.2 17.5 14.5 <1.2 4.5

S glass fibers 64-66 ' 24-26 9-11

63
50

0

l 1 1 1 0 0 100 200 300 400 TEMPERATURE (K)

Fi gure 7.1. Cryogenic effects on the compressive strength of woods

.

64

COMPRESSIVE

STRENGTH,

crc(

ksi)
IMPACT

TOUGHNESS

MODULUS

OF

ELASTICITY,

E

(MPa)

(low

temp./

room

temp,

ratio)

Figure 7.2. Cryogenic effects on the impact toughness and Young's

modulus of woods

.

65

MODULUS

OF

ELASTICITY,

E

(ksi)
dewars and windows. As a window material, Pyrex competes with clear plastics [142], The

clear plastics offer superior formability and toughness, and are lightweight, but Pyrex

has superior mar resistance and dimensional stability. Impact values at 77 K are typically 0.3 J for Pyrex and 4 J for Plexiglas.

Vycor and fused silica offer high thermal-shock resistances, but they cost more than

Pyrex. Fused silica glasses are used where minimum thermal expansion is required, as in dilatometer rods. Ultralow-expansion fused silica (ULE 7971) is a unique glass with the

lowest thermal expansion of all.

c. Mechanical and Physical Properties

The physical and mechanical properties of glasses correlate well with silica content. Strength, thermal conductivity, and electrical resistance tend to increase, while thermal

expansion tends to decrease with higher silica content.

Strength: The strength of glass depends on specimen type, surface condition, chemical composition, thermal history, and loading rate as well as the amount of water dissolved in the bulk, absorbed on the surface, and present in the surrounding atmosphere.

Hillig [143] showed that undamaged fused silica filaments may exhibit fracture stresses of

up to 4.7 GPa at room temperature, but much lower values are expected for commercial, bulk

glass. In bulk glass, low tensile strengths from 34.5 to 207 MPa commonly result from

surface defects. This is supported by Ritter and Cooper [144], who demonstrate a sixfold

increase in strength (from 413 to 2480 MPa) after chemical etching to reduce surface

flaws. Therefore, high tensile strengths are theoretically possible for small, carefully

prepared laboratory specimens, but low strengths are realistic for engineering purposes

since commercial glasses always contain microscopic damage introduced during handling or manufacture.

Figure 7.3 depicts the temperature dependence of glass tensile strengths in the range

between 76 and 400 K. Conditions varying from carefully prepared (flame-drawn, chemically

polished fibers) to intentionally abraded (sandblasted) bulk glass are represented [145-

148], The fracture strengths vary widely in magnitude, but in each case the strength

approximately doubles between 295 and 76 K.

Static fatigue: Glass weakens in the presence of moisture, especially at slow loading rates. This results from the hydrolysis of flaw-tip Si-0 bonds, which promotes the

gradual growth of existing flaws under tensile stress. At higher temperatures, the

"static fatigue" process is accelerated and the strength reduction is quite dramatic: dry

strengths are about double the wet strengths at 300 K [149]. Static fatigue occurs at temperatures down to 194 K, but there is little or no effect at 76 K [147],

Elastic modulus : A typical engineering value for Young's modulus of glass at 300 K is 69 GPa (10 7 psi). With increasing temperature between 77 and 367 K, the moduli for

Pyrex and fused silica glasses actually increase a few percent, following a linear temperature dependence [150, 151].

Electrical resistivity: Hirayama and Berg [42] report electrical resistivity data

for selected glasses at temperatures between 245 and 473 K. Resistivities range between

lO 10 and 10 18 ohm*cm, with the highest values for Vycor, then Pyrex, then soda-lime glass.

Thermal expansion: The thermal expansion coefficients of glasses at 295 K vary by over two orders of magnitude, from 0.2 to 90 x 10 7 /K [42, 152-156]. Total contraction

for Pyrex between 295 and 76 K is 0.05%. In most cases the expansion coefficients decrease

monotonically at temperatures down to 76 K, but fused silica glasses undergo a reversal

and begin to expand at temperatures below about 200 K. Thus, ultra-low-expansion fused

silica (ULE 7971), exhibits a negative expansion coefficient of -10 x 10 7 /K at 76 K.

Thermal conductivity : Thermal conductivity values for most glasses in the cryogenic

range are readily available [157-160]. Ratcliffe's data [158] for 22 glasses defines a band of thermal conductivity values ranging from 37 to 86 x 103 W/(m-K) at 123 K to 61 to 146 W/(m-K) at 373 K. Ratcliffe also presents formulas for predicting values for untested

66
FRACTURE

STRESS,

o>

(MPa)

7000 1000

Figure 7.3. Cryogenic effects on the fracture strength of glasses.

67

FRACTURE

STRESS,

<xf

(ksi)
glasses at 373, 273, or 173 K to an accuracy of ± 15%. Measurements at 76 K or lower are tabulated by several authors [157-160].

Desi gn : If glass must be subjected to tensile or bending stresses, every reasonable

attempt should be made to design conservatively. Horton's [146] approach to the design of

pressurized, fused silica windows for space vehicles is exemplary. In that study, numerous flexural tests spanning the el evated-to-cryogeni c temperature service range were statistically evaluated to arrive at design allowables. The test specimens were first

sandblasted to assure conservative baseline data. The design allowable flexural stress obtained was 29 MPa for temperatures between 295 and 76 K. This compares to the average

flexural strength measurements of 89 MPa and 56 MPa at 76 and 295 K, respectively. Ten- sile allowables were taken as 1/2 the value of flexural allowables, which is common procedure for ceramics.

7.3. Ceramics

Ceramics are inorganic materials known primarily for elevated temperature service.

Cryogenic uses are rare, and only miscellaneous uses are anticipated in NTF testing.

Possible applications would be insulators, bearing materials, or fillers for polymers.

a. General Information

Ceramics are typically brittle, weak in tension, fatigue-prone, hard, abrasive, and

difficult to machine. Scatter in mechanical properties is high. On the other hand,

ceramics are relatively inexpensive, strong in compression, corrosion resistant, wear

resistant, electrically insulating, moderately dense, and stiff. Ceramics may be classified as: (1) glasses (2) glass-free ceramics, (3) polycrystall i ne aggregates in a glassy

matrix, and (4) composites such as cermets. Glasses are discussed in section 7.2.; cer- amic fibers have no cryogenic uses.

b. Specific Materials

Glass-free ceramics include mica, alumina (A1 2 0 3 ), magnesia (MgO), beryllia (BeO),

silicon carbide (SiC), titanium carbide (TiC) and boron nitride (BN). These ceramics

consist of crystallites 1 to 50 pm in diameter, produced by hot pressing or sintering of

powders. In cryogenic applications A1 2 0 3 has served as an insulator, A1 2 0 3 and TiC serve

as bearing materials, and all of the above could serve as fillers for polymers.

Polycrystals in a glassy matrix include porcelain, pyroceram, and machinable glass

ceramic (Corning Code 9658). Porcelain is a common load-bearing electrical insulator at room temperature; to reduce thermal shock, pyroceram may be substituted for cryogenic

service.

A promising cryogenic material is the machinable glass ceramic which consists of

finely dispersed mica (~50 vol.%) in a boro-al umi no-si 1 icate glass matrix [161]. Most

ceramics are unmachi nabl e , but this material can be machined to precise tolerances using common metal-working tools. The mica dispersiods cleave easily, aiding chip formation and

conferring machi nabi 1 i ty approaching that of a plain carbon steel. The material is thermal

shock resistant, fully dense, inert, and insulating; it has a thermal expansion comparable

to metals.

Cermets are ceramic-metal composites where the metal serves as the matrix or filler.

Sintered titanium or tungsten carbides bound with cobalt or nickel are used as bearing

materials for cryogenic axial mechanical pump seals to take advantage of their high hard- ness and wear resistance.

c. Mechanical and Physical Properties

The mechanical behavior of ceramics at room temperature is reviewed by Wachtman

[162], but cryogenic data are available for few materials, and ceramics are ignored in most cryogenic handbooks. Alumina is representative of the glass-free ceramics. Crystal

size, purity, and density affect the material properties. The tensile strengths for

68
commercial grades of alumina range from 155 to 310 MPa at 295 K. The strength increase

at 77 K for 96% grade alumina amounts to about 35% [163]. The fracture toughness of

alumina is low (4.3 to 5.2 MPa^m^) at all temperatures between 300 and 77 K [164]. The

elastic and shear moduli are significantly higher (E = 398 GPa at 295 K) compared with

steel, and all elastic properties are regular functions of temperature [165].

High thermal conductivity is an outstanding advantage of some glass-free ceramics.

Beryl 1 i a (BeO) has the highest thermal conductivity of any oxide ceramic, with 300 K values for dense polycrystals approaching 310 W/(m*K) or 75% of the value for copper. For

this reason, BeO is added to thermal contact greases for cryogenic applications. Other

thermally conductive ceramics are SiC, BN, and A£N, which have 300 K thermal conductivity

values of 250, 300, and 660 W/(m-K), respectively. Temperature reduction to 77 K has only

a slight affect on the thermal conductivity [166],

Selected cryogenic data for cermets are shown in Table 7.2 [167],

d. Design

Ceramics are preferably used in compression. Tensile strength and toughness is low

in all ceramics. Scatter in properties is high, and polyaxial stress conditions further

reduce tensile strengths [168]. If tensile stresses are unavoidable, all sources of stress concentration must be minimized or eliminated.

For optimum mechanical properties, fully dense, fine grained, high quality cermanics

should be chosen and carefully handled. Flaws are often introduced inadvertently owing to abrasion or thermal shock caused by rapid cooling. These flaws should be removed by

polishing, and the finest possible surface finish is desirable.

For most ceramics, it is assumed that the flexure strength is twice the tensile

strength. In critical cases, any assumptions should be verified. Proof testing is recom- mended to overcome uncertainties. The most sophisticated design approaches combine sta- tistical analysis with proof testing [169, 170],

7.4. Polymers

Polymers are chain-like molecules in amorphous or semicrystalline arrangement,

having strong covalent intramolecular bonds and weak i ntermol ecul ar bonds. Numerous

structures are possible, and the great diversity of commercial products now available

defies any rigorous classification. The conventional classification distinguishes between

thermoplastics and thermosets. Thermoplastics have huge chain structures with little or

no crosslinking; they soften and can be molded or shaped repeatedly if heated. In con- trast, thermosets are strongly cross-linked, and they cannot be reshaped after the

initial curing. Beyond this, material designations are confusing. Thousands of structural variations may be categorized by a single generic name, and multiple tradenames

exist for similar products. A useful aid is the list of trade designations compiled by

Titus [171].

1. General Information

Polymers offer moderate cost, moldability, machinabil ity, low friction, desirable

insulating properties, and, in some cases, transparency. Unfortunately, bulk applications

at 77 K are severely limited owing to large thermal contraction, low ductility, and poor

thermal shock resistance. This leaves miscellaneous uses that are discussed separately in

various sections of this report: adhesives (10.1), sealants (10.2), gaskets (11.3.1,

11.3.2), coatings (12.3.6), windows (7.2.2), potting compounds (10.3), fibers (8.3.),

films (8.2.2.) and foams (8.1.). The cryogenic properties and uses of polymers are comprehensively reviewed by Landrock [172]. Additional design data can be obtained from

NBS Monograph 132 [17].

One parameter limiting cryogenic utility is the glass transition temperature, T , y defined as the switch-over point between rubbery and glassy behavior. All polymers have

a Tg value higher than 77 K. Consequently there are no cryogenic elastomers, and the 77 K

69
Table 7.2. Some properties of cermets.

Material Flexural strength

GPa Charpy impact energy

J Mean thermal expansionxl0' 6 /K

i coefficient

xl0' 6 /K

@295 K @77 K @295 K @77 K 77 to 295 K 295 to 400 K

WC/Co 2.05 2.16 1.35 1.35 1.87 2.52

WC/Ni 2.32 2.52 — — 1.82 2.52

TiC/Ni, Mo 1.78 1.79 0.34 0.27 2.08 3.70

Note: 1 GPa = 0.145 msi.

1 J = 0.737 ft* lb.

1 K - 1. 8°F

70
ductility of polymers is low. Upper service temperature limits are also encountered,

owing to thermal softening or decomposition. Elastomers and adhesives are two polymer

classes that lose strength rapidly above 340 K. Upper service temperature limits for

polymers are listed by Wood [173],

Nitrogen at low temperatures is deleterious to polymers. PTFE and five other thermoplastics have exhibited crazing (porosity and fibrillation) and/or reduced fracture

stresses when tensile tested in cold liquid or gaseous nitrogen, as compared to vacuum or helium environments [174-176]. Deleterious effects on mylar are observed in gaseous

nitrogen between 50 and 160 K, where the fracture stresses are reduced by up to 25%, com- pared to vacuum [176].

For most cryogenic applications fluorocarbons are the recommended thermoplastics and

epoxies are the recommended thermosets.

Fluorocarbons. The most widely used fluorocarbons for cryogenic service are the Teflons--

polytetrafluoroethylene (PTFE) and fluorinated ethylene propylene (FEP). These materials

are recommended for gaskets and seals (10.0), insulation (13.5.1), antifriction devices

(12.3), and protective coatings.

Both PTFE and FEP are chemically inert and thermally stable between 77 and 367 K. PTFE has a lower friction coefficient and is harder and more wear resistant. FEP offers

processing advantages. The most serious disadvantage of these fluorocarbons is their

tendency to cold flow under static loading, particularly at higher temperatures. If this

is a problem, stronger fluorocarbons may be substituted, such as polychlorotrif 1 uoro- ethylene (PCTFE).

Epoxy resins . Resins are thermosetting polymer systems containing highly reactive threemembered rings of oxygen and carbon which form cross-links when mixed with curing agents.

The most common epoxy resin is diglycidal ether of bisphenol A, for which there exist

hundreds of curing agents, flexibi 1 izers , modifiers, and fillers.

Despite numerous variations all epoxies are classified as three types - rigid at 295 K, semiflexible at 295 K, and flexible at 295 K. Rigid types are strong at 367 K, but

brittle at 77 K [177]. Highly flexible types resist embrittlement at 77 K, but soften

excessively at 367 K. Therefore semiflexible types may be best for general use in NTF applications in the range 77 to 367 K. The selection of epoxy adhesives (10.1), sealants

(10.2), potting compounds (10.3), and composite matrix materials (9.2) are discussed in

the sections noted.

Cryogenic temperatures produce large property changes in polymers. General trends

for a temperature reduction from 300 to 77 K are [172]:

tensile, compressive, flexural strengths,

fatigue resistance '

b. Specific Materials

c. Mechanical and Physical Properties

Young's modulus

creep resistance

thermal contraction

electrical resistance

adhesive strength

thermal conductivity |

variable

ductility

toughness

impact resistance

thermal expansion coefficient

specific heat

71
The tensile properties of bulk thermoplastics are compared in Table 7.3 [178-179].

Ultimate tensile strengths range from 8.5 to 149 MPa (1.3 to 22 ksi) for the polymers

shown. Elongation at break is 7.5% for PTFE at 77 K, and lower for other polymers.

Young's moduli at 300 K for 67 polymers range from 26 to 38 MPa, and, despite a large

stiffness increase at 77 K, polymers remain relatively low stiffness materials. Thermal

contractions for polymers are large (Fig. 3.10). The thermal conductivities of amorphous

polymers show a steady decrease at low temperatures, with values of 0.16 to 0.22 W/(m*K)

at 77 K. Semicrystal 1 i ne polymers show a mild decrease or a mild increase at low temperatures with values between 0.1 and 1 W/(m-K) at all temperatures between 350 and 77 K [180],

7.5. Filled Polymers

Powders or chopped fibers may be added to polymers prior to molding or casting to achieve improved mechanical and physical property combinations. The designer can purchase

filled polymers or fabricate his own. Filled polymers, like fiber-reinforced laminates,

are composite materials. But laminates achieve superior strengths. The primary goal of a filled polymer is usually increased thermal conductivity or decreased thermal contraction,

with perhaps a moderate improvement of strength. Thus, the thermal conductivity of epoxy

can be increased by a factor of 10 by adding powdered aluminum, or total contraction

between 295 and 77 K can be reduced from 1.25 to 0.4% by the adding 75% glass filler.

Fillers for specific purposes are listed in Table 7.4.

Cryogenic uses for filled polymers include antifriction components, ball-bearing

separators, spacers for superconducting magnet windings, and reinforced gaskets and valve

seats. Guides to predicting engineering properties based on filler composition are provided by several authors [181-187],

72
Table 7.3. Tensile properties of bulk thermoplastics.

Material Temp.

K

Glass

transition,

T , K 9

Ultimate strength,

MPa

Elong.

,

%

Youngs modulus,

MPa

Polymethylmethacrylate 300 377 76 9.4 1274

PMMA 77 — 122 4.0 3138

Polystyrene 300 373 30.1 2.6 1274

PS 77 41.2 2.3 1863

Polyvinylchloride 300 355 58 139 980

PVC 77 149 6.1 2350

Polypropyl ene 300 255 33 853 686

PP 77 89 4.7 2450

Polytetrafl uoroethylene 300 223 18.5 530 402

PTFE 77 37 7.5 1370

Polyethylene 300 164 8.5 487 598

PE 77 108 4.3 2450

Note: IMPa = 0.145 ksi

|

73
Table 7.4. Epoxy fillers for polymers.

Purpose Filler

Reduced contraction glass, metals, ceramics,

Improved impact chopped glass, asbestos, aluminum,

fluorocarbon fibers

Abrasion resistance glass, alumina, iron, sand,

silicon carbide

Improved thermal conductivity aluminum, copper

Reduced thermal conductivity glass

Improved electrical conductivity aluminum, copper, silver

Machi nabi 1 i ty aluminum, copper

Extenders clay, silicates, metal salts

Rei nforcement asbestos, glass, wire, fibers

Lubricity graphite or M0 S 2

74
8. FOAMS, FILMS, AND FIBERS

This chapter discusses films, fibers, and foams that offer unusual properties owing

to unique aspects of material form, size, or structure.

8.1. Foams

Foams have a porous, cellular structure that makes them useful thermal insulators by

virtue of their lower thermal conductivity and lower density compared with solid materials.

Polymer and glass foams are available. Polymer foams are easily cut, and they conform

readily to container shapes. Glass foams are stronger and more rigid.

The average thermal conductivities of six unevacuated foams between 300 and 77 K range from 0.033 to 0.055 W/(m*K) [188]. Evacuated foams offer still lower thermal con- ductivities. The thermal contractions of polymer foams between 300 and 77 K are typically

0.8 to 1.8 %. Tensile strengths between 77 and 350 K may be as high as 0.35 MPa [172].

Polyurethane foam retains useful strengths at these temperatures. Sparks [189] provides

an extensive review of the available information on polyurethane and polystyrene foams.

8.2. Films

Films are used for electrical insulation, linings, and vapor barriers, often in com- bination with foams. Desirable films combine high strength and strain capability with

flexibility and insulating properties. High strength enables a reduction in film thickness with resultant increased flexibility.

Commercial films having good electrical insulating properties and acceptable for

cryogenic service are listed in Table 8.1. Polyethylene terephthalate (PET) is used for

cryogenic diaphragms, expulsion bladders, and tank liners. Aluminized PET serves as a thermal radiation shield for insulation systems. At 77 K PET retains 5% elongation at break, an elastic modulus of 100 GPa, and a tensile strength of 303 MPa. Van de Voorde

[178] indicates that polyimide (PI) film is even stronger than PET, but both retain useful

mechanical properties between 77 and 367 K [179, 190].

Javorsky and Popelis [191] present thermal contraction data which are useful to match

films and foams with various substrates. Additional data are presented by Landrock [172]

and Jelinek and Muller [192]. For extremely low thermal contraction, glass cloth/polymer

film laminates are recommended.

8.3.

Fibers

Owing to small size and relative freedom from manufacturing defects, fibers are effective reinforcements for composite laminates and filled polymers. Since fibers con- tain fewer flaws compared to materials in bulk form, fibers approach theoretical strengths

more closely than do bulk materials. Size effects also confer higher strain capability

for fibers. For example, bulk glass fails at 0.1% tensile strain, whereas some glass

fibers are capable of 5% strain.

In the order of value as cryogenic materials, the fibers may be ranked as follows:

1) glass fibers,

2) advanced fibers (graphite, boron, kevlar),

3) polymers,

4) metals, and

5) ceramics

For structural composite laminates requiring strength or stiffness, glass and advanced

(high modulus) fibers are of greatest technological importance. Glass and advanced fibers

are poor conductors of heat, and they exhibit low thermal contractions, which improve the

performance of composites using epoxy matrices.

75
Table 8.1. Tensile properties of polymer films.

Material Temperature Tensi le strength

MPa

El ongati on %

Young 1 s modul us GPa

Polyimide 298 178 44 5.00

PI 77 318 16 6.27

Polycarbonate 298 63 112

PC 77 132 4.5

Polyami de 298 81 21

PA 77 142 3.5

Polyethyl enete rep htha late 298 162 88 4.41

PET 77 255 9.5 8.53

Polyvi nyl chlori de 298 22 156 1.08

PVC 77 93 3.6 2.11

Polyethylene 298 24 525

PE 77 103 4.3

Polytetraf 1 uoroethylene 298 20 480

PTFE 77 42 6.5

Note: 1 MPa = 0.145 ksi

76
In most cases, fibers are manufactured and used as coated strands (bundles of fibers).

The coatings prevent surface damage and enhance compatibility with specific matrix materials. Costs are generally high for quality fibers, and prices vary with weave, yarn, and

finish types. The production, availability, price, and future trends of advanced fibers

are discussed in detail by Kaiser [1931. The following discussion is taken from Kasen

[194-200].'

Glass fibers Glass is the most widely used cryogenic fiber. Advantages are high tensile

strengths up to 3.7 GPa, low cost, availability, low thermal and electrical conductivity,

and low thermal contraction. The most serious disadvantage is low elastic modulus (73 to 86 GPa). The most common glass finish is silane A, which is compatible with polyester,

epoxy, and phenolic matrices.

The recommended glass fiber types are E-glass and S-glass (section 7.2.). Electrical

insulating grade E glass is a general purpose fiber costing about $l/kg for roving (bundles

of twisted strands). Cameron [145] reports the breaking strengths of E glass fibers to be

3.44 GPa at 295 K and 5.65 GPa at 77 K. Specialty grade S-glass is a higher strength

fiber, about 30% stronger than E glass. Two alternatives, S-901 and S-2 glass fibers have

similar mechanical properties, but S-2 glass is cheaper, costing twice as much as E glass.

Graphite fibers Graphite fibers rank a distant second to glass in total cryogenic usage.

Graphite fibers are electrically conductive, extremely stiff, and brittle. Costs are moderate to high. There are three graphite fiber types:

Type I - lowest strength, highest modulus, highest cost

Type II - intermediate strength, modulus, and cost

Type III - highest strength, lowest modulus, lowest cost

For Type III fiber, strengths of about 2.6 GPa are available with a modulus of 225

GPa. For Type I fiber, moduli of about 520 GPa are available at strengths of 1.9 GPa. Thus, the highest graphite fiber strengths are comparable to E glass, but the highest

modulus is 7 to 8 times greater than E or S glass.

Continuous or chopped graphite filaments, fiber flock, and woven cloths are available,

mostly produced from the pyrolytic degredation of polymers. Typically, fibers of about

8 pm are bundled in strands of 1000 to 160,000. Costs range from tens to hundreds of

dollars per kg. One manufacturer sells low priced pitch-base fibers, but the mechanical

properties are inferior.

Kevlar 49 The aramid polymer fiber Kevlar 49 may be considered as a replacement for glass

if a 45% higher fiber modulus and 42% lower fiber density is needed. Of all fibers,

Kevlar 49 offers the highest possible strength-to-density ratio. It offers a high negative

thermal contraction coefficient. However, Kevlar 49 is restricted in use to epoxy resin

matrices, and compressive properties are poor. Prices range from nearly equivalent to S glass to seven times higher.

Boron Boron fibers may be considered if both high strength and high stiffness are re- quired. Boron is as strong as S-glass, but five times stiffer. The elastic modulus at 295 K is 410 GPa, twice the value of steel. Boron also offers extremely high compressive

strength. Unfortunately, owing to a low bend radius, Boron is unsatisfactory for complex

shapes. Another deterrent is high cost (hundreds of dollars per kg).

Polymers Polypropylene, polybenzimidazole, nylon, and other synthetic fibers were studied

to identify relatively strong cryogenic fibers having minimal thermal conductivity [172]. PET fibers can be recommended as useful insulation for electrical conductors since the

strands are flexible enough to bend around small radii. Flexible polymer fibers have also

been used as thermal insulation for cryogenic pipes and transfer lines, and nylon finds

applications in expanding, inflatable bladders.

Metal

s

Metals are the least important reinforcements for composites because metal strengths

and moduli are low, while densities and thermal conductivities are higher compared to glass and the advanced fibers discussed above. Nevertheless, metal "fibers" for molded

77
plastic reinforcement may be formed by cutting fine alloy wires. Stainless steel wires in

diameters as low as 19 pm are readily available.

Ceramics Alumina, silicon carbide, and boron nitride fibers are in developmental stages.

Availability is limited and costs are high. The main advantages of ceramic fibers are their insulating properties at very high temperatures. There are no cryogenic applications.

78
9. STRUCTURAL COMPOSITE LAMINATES

Composite laminates are epoxy or metal matrix materials reinforced with continuous

uniaxial fibers or woven fiber clothes, with fiber volume fractions of about 50 to 70%.

These laminates are capable of high strength-to-weight, strength- to-thermal conductivity,

and stiffness-to-thermal conductivity ratios which cannot be matched by structural alloys. But the use of composites at cryogenic temperatures is hampered by a lack of service

experience, standards, and design data for specific composites. Material costs and property

variability are higher, while design procedures and joints are more complicated with

composites than with structural alloys.

Most laminates are linear-elastic when loaded to failure, with tensile strains ranging from very low in transverse fiber orientations to as high as 5% for some glass reinforced laminates in the longitudinal fiber orientation. A peculiar failure mode occurring

in fatigue is wear-out, which is a process of fiber/matrix debonding associated with

stiffness reduction. Design with composites is thus more difficult than with structural

alloys, which are far less anisotropic.

There are no established procedures or standards for design with composites at cryogenic temperatures. The initial approach should be based on procedures in the Advanced

Composites Design Guide [201] and MIL HDBK 17A [202], with Kasen's reviews [194-200]

serving as a guide to cryogenic property trends. Macromechanical theory provides a basis

for predicting the mechanical properties of proposed layup configurations. Quality assurance is necessary in critical structural applications to account for material variability,

which strongly depends on workmanship during fabrication.

Glass fiber reinforced epoxy is by far the most widely used composite, accounting for

perhaps 90% of all current applications. Cryogenic uses include filament wound aerospace

pressure vessels, load-bearing thermal standoffs and supports, dewar construction, and

electrical/thermal supports for superconducting magnet windings at temperatures as low as 4 K. Glass-epoxy composites are recommended for applications requiring high tensile

strength and toughness, if low stiffness can be tolerated, and if sufficient benefit

accrues in performance and weight considerations compared with alloys. Advanced fibers

are possible substitutes for glass fibers in epoxy laminates for special purposes, as noted in the text. For discussion, structural composites are classified below as industrial

(high pressure) polymer laminates, low pressure polymer laminates, metal-matrix laminates,

and selectively reinforced structures.

9.1.

Industiral Polymer Laminates

Examples of industrial laminates acceptable for cryogenic service are listed in Table

9.1.

Industrial laminates are commercially fabricated at curing pressures up to about 8

MPa. They are relatively inexpensive and available in bulk form as plate, sheet, or bar.

These materials are recommended for non-critical electrical or thermal insulating purposes

at moderate stress levels. Components can be readily machined from bulk stock. As indicated in Tables 9.2 to 9.3, and Fig. 9.1, there is generally a modest improvement in

thermal and mechanical properties at cryogenic temperatures, but the existing specifications cover only room temperature properties. Therefore most industrial laminates are not recommended for cryogenic structural applications subject to high stress, unless quality

control tests are performed. Cryogenic grades of G-10 and G-ll, called G-10CR and G-11CR,

have been developed for more critical cryogenic applications, and some data for these

newely developed grades are shown in Table 9.2 and 9.3.

9.2.

Low Pressure Polymer Laminates

Low pressure laminates are fabricated at low curing pressures (from zero to 2.8 MPa).

With these laminates, designers can custom fabricate structures in situ, excercising

options in fiber and resin selection so as to tailor properties to specific applications.

For example, honeycomb structures or composite overlays can be built up using preimpregnated tape or filament winding as described by MIL HDBK 17A [202], and Advanced Composites

Design Guide [201]

.

79
Table 9.1. Industrial laminates of primary cryogenic interest.

Commercial

standard

MIL-P and LP designation Description

NEMA/ASTM LE MIL-P-15035, Type FBE Cellulose fabric, fine weave phenolic resin

NEMA/ASTM C MIL-P-15035, Type FBM Cellulose fabric, medium weave phenolic resin

NEMA/ASTM G-5 -- Glass cloth, melamine resin

NEMA/ASTM G-10 MIL-P-18177, Type GEE Glass cloth, epoxy resin

NEMA/ASTM G-ll MIL-P-18177, Type GEB Glass cloth, high-temperature

epoxy resin

EH57 , EH57 LP 5094 Beechwood veneer, phenolic

impregnated

80
Table

9.2.

Mechanical

properties

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G-10CR

and

G-11CR

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81
Table 9.3. Thermal properties of G-10 CR and G-ll CR industrial laminates.

Material /direction Thermal contraction

(L2 93

-

l-7 5)/l-2 93 ,%

Thermal conductivity W/(m*K)

295 K 77 K

Warp 0.235 0.860 0.380

G-10CR Fill 0.269 - -

Normal 0.730 0.600 0.275

Warp 0.219 0.730 0.315

G-11CR Fill 0.243 - -

Normal 0.620 0.620 0.275

Note: 1 W/(m*K) = 0. 578 BTU/(ft*hr °F)

82
ULTIMATE

TENSILE

STRENGTH,

<xu

(MPa)

2000

TEMPERATURE (K)

Figure 9.1. Cryogenic effects on the ultimate strength of composites.

83

ULTIMATE

TENSILE

STRENGTH,

cru

(Ksi)
Glass fibers are used for high strength, low modulus laminates, whereas graphite or boron fibers are used for high modulus laminates. In either case, epoxies are the recom- mended matrix materials. The selection of a particular epoxy resin is more important for

low modulus laminates than for high modulus laminates since fibers of lower stiffness

transfer greater strain to the matrix.

Epoxies suitable for cryogenic applications are discussed by Kasen [199], For NTF applications at 367 K, highly flexibilized resins such as NASA Resin 2 are unacceptable

owing to loss of strength above room temperature. Rigid or semirigid epoxies are acceptable but no single formulation can be recommended as clearly superior. Kasen therefore

recognizes two approaches to matrix selection:

1) Select a resin system that has been used previously with composites, and for

which cryogenic data already exist in the literature. (This method may be fruitless since

many tested epoxies are no longer readily available).

2) Base initial selection on cost and availability, then fabricate laminates and

perform screening tests at the temperatures of interest. Screening tests are recommended

in any case, since performance is dependent on fabrication variables, as well as on the

choice of materials and layups.

Based on the materials selected, the laminate properties can be predicted based on the key properties and using macromechanical theory. Representative data for uniaxial ly

reinforced composite laminates are shown in Fig. 9.1. For a description of temperature

effects on tensile, compressive, flexural, shear, fatigue, impact and thermal properties,

the reader is directed to Kasen [194-200] and Hartwig [203].

9.3. Metal Matrix Laminates

Metal matrix composite laminates are in developmental stages. Boron-reinforced 6061

aluminum is the only commercially available metal matrix composite and the only one for

which cryogenic data exist. Potential advantages for this material are higher strengthto-density and modul us-to-densi ty ratios compared with structural alloys, such as AISI

304. Compared with boron-epoxy low pressure laminates, boron-aluminum offers no mechanical

property advantages in the fiber direction, although tests in the transverse direction

show sevenfold superiority in strength and fourfold superiority in modulus. Unfortunately,

the fabrication process for boron-aluminum introduces residual stresses that cause nonlinear

stress-strain behavior. As a result, the stress-strain behavior of crossply boronaluminum composites is less predictable compared with boron-epoxy composites. Therefore

boron-aluminum is currently used only for uniaxial ly loaded structures; NTF applications

are not anticipated.

9.4. Selective Reinforcement

Advanced composites can be bonded to metal components to selectively stiffen or strengthen primary structures [201]. This technique might be applicable in the design of

stings or other NTF model supports. For example, graphite or boron-epoxy overlays or cores in stainless steel shafts should result in a stronger, stiffer shaft while maintaining the simplicity of metal shaft connections [194].

84
10. ADHESIVES, SEALANTS, AND RELATED MATERIALS

This chapter is subdivided into sections on adhesives, sealants, potting compounds,

and contact agents, but many of the materials discussed could serve multiple functions.

10.1. Adhesives

An adhesive is any substance capable of bonding two materials together. Silicones,

varnishes, cements, and other materials are acceptable for light bonding applications, but

this section is concerned with structural adhesives where high strength is the primary

criterion. A structural adhesive is defined as having a shear strength in excess of 6.9

MPa.

Cryogenic adhesives were originally developed for aerospace applications between 20

and 800 K, for bonding metals that could not be welded or riveted, for bonding insulation

or wires, for fabricating honeycomb structures, or to prevent galvanic corrosion between

contacting dissimilar metals. Many structural adhesives are now available that serve as well at 77 K as at room temperature, but no single formulation serves every purpose.

Although polytetraf 1 uoroethyl ene is well known for its antisticking property, Eppinger and

Love [204] created polytetraf 1 uoroethyl ene- to-steel bonds as strong as 55 MPa using a nitrile-phenolic adhesive. Thus, it appears that any solid can be adhesively bonded,

using the appropriate technique.

Epoxies, modified epoxies, and thermosetting polyurethanes are generally recommended

for bonding metals and nonmetals at temperatures in the range 77 to 367 K. Many products

are available as solvent based compounds, the solvent being removed by curing at room or elevated temperatures. Epoxies offer the greatest versatility in bonding many adherends,

with or without fillers or reinforcements. Epoxy-nylons are stronger and tougher than

unmodified epoxies, but more susceptible to moisture-induced degradation, and more sensitive to bond-line thickness. Polyurethanes are stronger in shear, but more difficult to process than epoxies. In any case, the choice of adhesive is influenced by the joint

geometry and type of loading; selection should be based on comparison of lap-shear,

short-beam-shear, butt-tensile, or peel-test data, according to the type of loading anticipated in service [205].

Regardless of the type of adhesive selected, there are proper techniques for optimizing performance. Adhesion is promoted by cleaning and roughening the adherend surfaces, and the use of primers and couplers may be beneficial. Thin layers of adhesive

are generally more resistant to thermal cracking than are thick layers. Peeling resistance is higher if the adhesive surrounds the substrate completely as compared with a local

application. Peeling resistance is also higher if the adhesive is constrained in a narrow gap between two components as opposed to being applied open-faced.

To achieve optimum performance with epoxies, the stress concentrations arising from

differences in elastic moduli and thermal contraction of the adhesives and adherends can

be reduced using fillers or cloth reinforcement [206], Van de Voorde [178], Hamilton,

Greene, and Davidson [207] cite the following examples of commercial filled adhesives with

thermal contraction adjusted for specific metals:

1) Stycast 2850 FT-X3 matches 0FHC copper

2) Stycast 2850 GT matches yellow brass

3) Stycast 2850 FT matches aluminum

4) Stycast 2850 FT + quartz matches AISI 304

More detailed information can be obtained from the literature [203-223]. Commercial epoxy

adhesives are discussed by Hartwig [203] and by Mackowski, West, Samsel , Holdeman, and

Peters [208]. Hillig, Legrand, Kelran, de Tores, and Coblenz [209] identify the better

adhesives for specific adherends and temperature ranges of interest. Froelich and Fitzpatrick [211] evaluate epoxy, varnish, and glass cloth supported adhesives at 77 K, exploring the influence of adherend type and surface preparation. Williamson and Olien

[212] provide 130 abstracts from research articles on adhesives and sealants. There is also a welter of manufacturer's product bulletins to consider. The recommended approach

is to consult this literature as a guide to the proper adhesive type for a particular

application, and then purchase a number of commercial products which can be screened

experimental ly.

85
10.2. Sealants

Sealants are used in model building to fill holes, joints, and surface imperfections.

In cryogenic applications, sealants are also used to waterproof or exclude dirt. Adhesives, solders, platings, and putty-like materials for caulking and fairing are available

for cryogenic service, but published data and experience in this area is lacking.

Epoxies. Araldite XD580 is an example of a putty-like resin which can be finger-formed

before it sets up. This resin has a thermal expansion coefficient near that of copper,

and it presents no cracking problems when used in magnet coils at temperatures as low as 4 K.

Si 1 i cones. The use of RTV silicones as cryogenic sealants is discussed by Screihans and

Robinson [213]. Silicones lose their elastomeric properties at temperatures below 200 K, but they retain significant strength and superior peel resistance compared with polyurethanes.

Rubbers . Rubber-based sealants are also useful at cryogenic temperatures. One commercial

product is pliable after evaporation of the solvent; it can be smeared on to provide

temporary seals, and is easily removed with acetone [214]. Sta Set is good for light

bonds between "stick-resistant" Teflons and metals, glass, or other plastics [215].

10.3 Potting Compounds

Castable potting compounds may be used to embed instrumentation. Sealing wax is used

at room temperature since it can be easily melted and removed. For cryogenic service,

epoxies are recommended. As discussed by Hartwig [203], thermal cracking is a problem in

rigid fully reacted epoxy resins, owing to lack of cryogenic ductility. Therefore, semiflexibilized resins may be best for general use at NTF test temperatures. More experience

is needed in this area.

10.4. Contact Agents

Contact agents are used to improve energy transfer between heater wires, thermocouples, heat sinks, and electrical contacts. Bond strength is not the primary requirement, and strong adhesive properties may actually be undesirable if quick disconnection of

fragile elements is necessary.

Any material with wetting properties can be considered for contact applications, but

the most practical choices are solders, adhesives, varnishes and greases. Varnishes and

greases are discussed below; solders and adhesives were discussed in sections 5.2. and

10.1, respectively.

10.4.1. Varnish

GE varnish 7031 is a common room temperature electrical insulating thermal contact

varnish and it is often used for cryogenic service. This particular varnish is favored

partly by tradition, and there are no published data comparisons with other commercial

varnishes.

GE varnish 7031 can be used at all temperatures between 77 and 367 K. Varnish is more strongly adhesive than greases, yet more easily removed than epoxy adhesives. Commercial varnish contains a solvent which, if allowed to air dry, leaves a flexible resin

which can be softened and removed by soaking in methyl alcohol. Baking produces a thermoset which is as difficult to remove as epoxy adhesives.

The thermal cracking resistance of varnish at cryogenic temperatures in thin layers

compares favorably with adhesives. The thermal conductivity of the varnish is nearly

equivalent to Teflon [219]. Peeling resistance is higher than some well-known epoxy

adhesives [217] but lap shear strengths are reported to be lower, with representative

values of 2.71 MPa at 293 K and 2.04 MPa at 77 K [211].

86
10.4.2. Grease

Greases are typically used as thermal contact agents or as release agents for gaskets

and seals. Greasing a specimen/heat-sink interface may increase the heat flow rate by up

to a factor of ten, but the extent of improvement is difficult to predict. Some thermal

properties for Apiezon N and T greases are reported at cryogenic temperatures [220]. Due

to freezing, some mechanical strength is developed at cryogenic temperatures, but lack of

strength at room temperatures limits some applications.

87
11. SEALS

Seals prevent fluid flow between coupled components for the purpose of maintaining

pressure differences or simply to separate gases or liquids. Nondemountabl e seals are created by welding, brazing, soldering, or adhesive bonding. Demountable seals are cre- ated by pressing a deformable sealing material against couplings to block the interface. 11.1.

Nondemountabl e Seals

Adhesives and alloys for brazing and soldering are discussed in section 5. Thermal

expansion or contraction behavior should be considered when sealing by these methods. For example, bismuth alloys are unique for soldered seals because they expand on solidification, and will not shrink away from the seal interfaces [224], Thermosetting resins can also be used, if thermal cracking can be avoided. To minimize thermal cracking, the

expansion coefficients of the resin and coupling materials must be closely matched (see

section 10. 1. ).

11.2.

Demountable Seals

Demountable seals for ~ryogenic service are reviewed by Robbins and Ludtke [225] and

Daniels [226]. There are two types: static and dynamic. A static seal is one where

there is no relative movement between the seal interfaces during operation. Shifting of

the interfaces during cool-down or otherwise may break the intimate contact of a static

seal and should be avoided. Dynamic seals are most often of the spring type discussed

below.

11.2.1. Design Considerations

The most familiar static demountable seals are 0-ring or gasket seals. In addition

there are temperature-, spring-, or pressure-activated seals, and toggle and boss seals.

Cryogenic sealing by any of these methods requires accounting for some factors not con- sidered in room temperature applications:

1) Limited reusability of the seal material,

2) Failure owing to thermal shock,

3) Distortion of machined interfaces, owing to thermal gradients, and

4) Seal separation owing to differential thermal contraction.

Thermal contraction, causes difficulties in sealing dissimilar materials. Nevertheless, effective seals between diverse materials, including gl ass-to-metal , metal-toplastic, and plastic-to-glass combinations can be obtained using appropriate techniques

[227-267].

Numerous seal designs for specific applications are cited in the literature. Since

a detailed illustration of all possibilities is not possible here, Daniel's [226] approach

to seal design is recommended: (1) determine the performance requirements, (2) screen the

existing sealing concepts or develop new ones, and (3) evaluate the alternatives in terms

of cost, maintenance, and availability of materials.

11.2.2.

Gasket and 0-Ring Seals

Gaskets and 0-rings must be compressed between relatively heavy flanges using a ring

of tightening screws. Effective seals are thereby obtained, but flanges are undesirable

for some applications owing to space, weight, and load considerations. Alternatives are discussed in sections 11.2.3. and 11.2.4.

Gaskets are rectangular in cross section and, therefore, stable when positioned on flat flanges. But flat flanges require accurate machining, since scratches or lack of

planarity may cause leakage. 0-rings are circular in cross section, and they seal over a narrower contact area, requiring less force than gaskets. Grooved or tongue-and-grooved

flanges are used to maintain symmetry as the flange is tightened. Often the groove is

88
made smaller in volume than the 0-ring, so the latter will be partially extruded from the

groove, improving the seal. Teeth opposite the groove will improve stress concentration

so that tightening loads may be reduced. However, metal 0-rings or putty- like materials

may become embedded and difficult to remove from grooved flanges. A flat aluminum spacer

may serve as an alternative to a groove if it is desirable to avoid more expensive machining operations on the flange.

Most seal designs are custom made, requiring experimentation to prove effectiveness.

0-ring and flange materials must be compatible since the seal must function as an integrated

whole. If dissimilar flange metals are used, corrosion must be avoided by choosing metals

closely ranked on the electromotive series, or by using nonmetal 0-rings or gaskets. The

0-rings and gaskets must not be harder than flange materials, or the flanges will be

scratched, causing leaks. If glass is involved, scratches may cause fracture when sealing

loads are applied.

Flange surface finish is sometimes critical. Finishes generally range from 8 to 64

microinches, RMS. The flanges should be cleaned with solvents such as acetone or alcohol,

and if cold-welding (see section 12.3) is relied upon to aid the seal, assembly should be

performed using gloves. Hold-down screws should be tightened gradually and systematically,

and thermal shock can be reduced by slow cool-down or filling rates.

Once in operation at low temperature, most static seals function well for extended

periods, but some seals fail gradually with temperature cycling. The leading cause of

failure is relative motion between the flanges, causing extrusion of the soft sealing

materials or coatings.

11.2.3. Temperature-Assisted Seals

Differential thermal contraction provides one basis for effective seals. Consider

the design of a nylon collar around a brass component: on cooling, nylon contracts more

than brass, and the choking effect creates a seal. A similar effect occurs if aluminum

rings or casings encircle steels of other alloys that contract less at a given temperature.

If two metals are involved, Teflon or soft metal coatings may be applied to the interface

to improve the seal. Stearns [236], Agarwal and Betterton [251] and Vesel [252] describe

thermally assisted seals that function at pressures up to 20 MPa.

11.2.4. Spring and Pressure-Assisted Seals

Spring seals are aluminum, stainless steel, or superalloy backbones with soft metal

(silver) or plastic (PTFE) coatings. The metal backbone provides sealing forces when

compressed, and compensates for dimensional changes during cool-down. The soft coatings

improve sealing characteristics. Spring seals are commercially available in a wide variety of geometries such as "K," "C," or "W" cross sections. These seals are simple,

readily available, and suitable for dynamic applications. Heavy flanges are not required.

Problems include cold flow of soft coatings and accurate load selection.

Pressure-assisted seals are high performance spring seals having an open geometry

(for example, C shaped cross section) so that the sealing forces can be increased by

pressurization from one side. Seals of this type are used in aerospace engines at temperatures from 20 to 800 K and at pressures up to 435 MPa. Since spring seals are adequate

for NTF pressures, the use of pressure-assisted seals is not anticipated.

11.3. Seal Materials

Elastomers, fluorocarbons, hard polymers, and metals are common 0-ring, gasket, and

coating materials. Soft metals and fluorocarbons are the most common choices for cryogenic service. At room temperature, elastomers, metals, and polymers are all very useful,

but there may be some loss of resiliency at elevated temperatures where flow is easier.

89
11.3.1 Elastomers

Elastomers are generally the best materials for 0-rings and gaskets at room temperature

where they are easily deformed, resilient, and reusable. However, every known elastomer

shatters like glass at 77 K. In fact, 200 K is sufficient to embrittle most types [253].

Nevertheless, elastomer 0-rings do find some cryogenic applications. They can be used if the seal is formed at room temperature, highly compressed, cooled, and not further stressed

at cryogenic temperatures. The seals remain effective at cryogenic temperatures only if the elastomer is highly compressed at room temperature; compression ratios approaching 80%

are necessary to build up the restoring forces needed to compensate for the large thermal

contraction of elastomers relative to flanges [254-257].

Elastomers that have been evaluated for cryogenic use include molded rubber or RTV silicones, natural rubber, nitrile rubber, neoprene, and others [239, 256, 257], In one application, an elastomer 0-ring provided a simple reusable seal for the lid of an evacuated

cryostat [258]. In other applications, elastomer seals can withstand more than an hour of

high frequency vibrations at 6.9 MPa and at 20 K, with leak rates less than 2.5 x 10 3 cc, at STP, of helium gas per mm of seal [254].

11.3.2. Other Polymers

Polymers work best as flat gaskets cut from sheet. An advantage of polymer gaskets

is that they prevent corrosion by avoiding contact between dissimilar metals. However,

polymers are harder than elastomers at room temperature. To reduce sealing forces, thin

gaskets are used in compression between one carefully machined flat flange and another

flange having a semi-circular machined ring. One successful design uses only 7J (5 ft- lb)

torque on six 1/4-28 bolts for a 10 mil PET sheet compressed 3 mil using a 2.54 cm diameter

ring; leak rates approach those for elastomer 0-rings [254].

PTFE (soft, inert, and stick-resistant) is most widely used polymer gasket. Teflon's

flow properties are desirable for some applications where harder polymers would simply

rupture. On the otherhand, Teflon's cold flow can result in extrusion or compression set,

causing leaks. This can be overcome using filled or laminated composite gaskets [225].

11.3.3. Metals

Metal 0-rings are commonly used at cryogenic temperatures where brittle elastomers

would be inadequate. Metals can be directly substituted for elastomers in most seal

designs, but resiliency and reusability are sacrificed.

Metal 0-rings can be purchased or fabricated. Material selection should be based on mechanical properties, cost, availability, corrosion resistance, and thermal properties.

Any soft, ductile, fee metal or alloy might be considered. Some specific applications are aluminum [259], copper [245], gold [260], indium [261-265], solder alloys [244], and

stainless steel [241].

Hardness is one of the primary factors influencing material selection. Soft alloys

seal with less force. For this reason harder alloy 0-rings such as 304 stainless steel are frequently plated or coated with soft metals (gold, silver, indium) or Teflon. Noble

metal platings also provide corrosion protection. All of the metals noted above can be

used at cryogenic temperatures down to 77 K, but an upper temperature limit may be imposed

by the lower melting points of some metals.

The most versatile cryogenic sealing metal is indium, which is well suited for

cryogenic gasket, 0-ring, or coating applications. Indium is extremely soft (it doesn't

scratch glass), readily available, and inexpensive compared to other corrosion resistant

metals. Indium 0-rings are easily formed from wire, gaskets are easily cut from sheet,

and coatings are easily plated, soldered, or foil wrapped [265-266]. The strong wetting

and cold-welding tendencies are useful in applications relying on adhesion to improve the

seal [264]. But in some cases, cold welding only makes demounting more difficult and

90
prevents reuse. Then, greasing the O-ring will facilitate removal [263]. Since indium is

soft, flange surface finishes are less critical. A 32 flange finish is generally acceptable [265].

Once tightened, indium 0-ring seals can be cycled hundreds of times between 295 and 4

K [264]. The seal holds if there is no extrusion of indium. If extrusion does occur, it

may be due to the differential thermal contraction and expansion of dissimilar flange

materials [266]. In such a case, intermittent tightening will maintain the sealing force,

but ultimately a seal failure is expected [261].

91
12. FRICTION, WEAR, AND LUBRICATION

Friction is the resistance to movement of sliding surfaces. Wear is the resultant

surface degradation and material loss that occurs owing to mechanical interaction. Although

friction and wear are closely associated, there is no direct relationship. The conditions

minimizing friction for a given sliding couple do not necessarily minimize wear.

Wear is in fact poorly understood. Few "laws" have been formulated, and knowledge

rests largely on empirical observations. Usually, wear decreases with decreasing tem- perature or with increasing surface smoothness and hardness. However, the wear rates of

materials show much greater variability than their friction coefficients, and there are no measurement standards.

Friction is treated by Bowden and Tabor [268, 269]; Bisson [270] reviews cryogenic

behavior. The actual contact area between two sliding bodies is much smaller than the

apparent surface areas. All surfaces are rough on the microscale, so true contact occurs only at the meeting of asperities (high points). Friction results from: (1) the shearing

of junctions formed by contacting asperites, and (2) the deformation or "ploughing" of the

softer material by the harder. These processes evolve heat, particularly at high pressures

or high sliding velocities.

Although friction is sometimes desirable, more often it is a problem that must be overcome by force and/or lubrication. Lubrication consists of establishing an interface

of low shear strength between the moving parts. In addition to reducing frictional drag,

lubricants may also serve to carry away heat, exclude dirt, or prevent corrosion. In thick-film fluid lubrication there is no asperity contact and wear is minimized. At cryogenic temperatures, however, we are usually concerned with boundary lubrication, where

the asperities are in contact but the junctions are surrounded by a solid lubricant.

12.1. Design Considerations

The lubrication of shafts, cams, gears, rollers, and electrical contacts in the inert

cryogenic gases may be more difficult than in conventional air environments, since:

1) Austenitic stainless steels, generally favored for cryogenic cryostructural

applications, have poor friction and wear properties.

2) Metallic cold welding is encouraged in environments that exclude oxygen,

3) Practical cryogenic liquid lubricants do not exist, and

4) Graphite fails to lubricate properly in certain inert environments.

The selection of a method to reduce friction should be based on the performance

requirements of the specific application. A large number of variables affect friction and wear results, among them: sliding velocity, load, chemistry of the environment, type of

motion (continuous or intermittent, sliding, rolling, etc.), part geometry, material

composition, hardness, finish, and cleanliness. Since reported friction data usually

pertain to very specific conditions, preliminary testing may be necessary to verify a choice of materials if friction problems are critical.

Particular consideration must be given to the environment, which often has greater

influence on lubricity than temperature has. Friction and wear studies are typically

conducted by sliding a loaded rider against a rotating disc. To date, most cryogenic

tests have been conducted in vacuum, gaseous nitrogen, liquid nitrogen, or liquid hydrogen

environments. Vacuum, gaseous nitrogen, and liquid nitrogen environments are generally

considered inert, but the liquid provides a superior cooling medium, the effects of which

may be difficult to predict.

12.2. The Necessity of Dry Lubrication

Oils and greases are the most common lubricants at 295 K. But oils and greases

freeze at temperatures between 220 and 250 K, becoming unusable, glass-like solids.

Cryogens, such as liquid nitrogen, also fail to lubricate effectively because their viscosities are orders of magnitude too low. Apparently, the fluorinated polyesters are the

92
only liquids that do have lubricating properties at cryogenic temperatures [271], but

their use requires cumbersome ancilliary cooling and containment systems. Thus, liquid

lubrication at cryogenic temperatures is generally dismissed as impractical. Dry lubrication and design using antifriction materials are the usual alternatives.

12.3. Friction Coefficients

Figure 12.1 compares the self-friction coefficients of selected materials tested in

inert environments. The friction coefficients of metals are relatively high compared with

non-metals. The friction coefficients for metals and alloys at 295 K typically range

between 0.4 and 1.0. The high friction is also associated with poor wear resistance. Both

result from cold-welding.

Cold-welding (or galling) denotes the strong adhesion that occurs when unfilmed

metals come into contact. Normally, metals form surface oxide films that prevent coldwelding. These protective films deteriorate by abrasion during sliding, and they cannot

reform in environments where oxygen is excluded. Under these conditions, cold-welding is encouraged, possibly leading to the seizure of moving parts. Thus, metallic friction is greater in vacuum than in air and greater in liquid nitrogen than in liquid oxygen. For similar reasons, metallic friction in the gaseous nitrogen NTF environment should be more

severe than in conventional wind tunnel environments using air.

12.3.1. Unalloyed Metals

The friction coefficients for 18 metals in inert environments between 4.2 and 600 K are reported by Simon, McMahon, and Bowden [272], Burton, Russell, and Ku [273], and

Bowden and Childs [274]. Our knowledge of metallic friction at cryogenic temperatures is

based primarily on the findings of these three studies. There is general agreement that

metallic friction in inert environments is weakly temperature dependent. The kinetic

friction coefficients are usually about 10 to 16% lower than the static coefficients.

Ductile metals usually exhibit high coefficients, with values possibly approaching 5 or

10, whereas the coefficients of brittle metals are significantly lower and seldom exceed

1.0. Dissimilar metal couples generally produce lower friction than similar metal couples,

and hard riders sliding on soft discs usually give higher friction than soft riders sliding

on hard discs.

12.3.2. Alloys

Friction and wear data for structural alloys at cryogenic temperatures are rare. Apparently, steels are the only structural alloys that have been investigated. Simon et

al. [272] report friction coefficients for a 0.16% carbon steel and an austenitic stainless

steel; Wisander, Maley, and Johnson [275] report friction and wear data for AISI 304 and

440-C stainless steels and SAE 52100 bearing steel at 77 K. All of these steels exhibited

high friction coefficients (nearly 0.4 or greater) at 77 K, and the three steels that were

wear tested exhibited cold welding and tearing at 77 K.

Since steels are preeminent structural materials, a convenient means of reducing

friction would be welcomed. Accordingly, attempts have been made to reduce metallic

friction by means of surface diffusion coatings and plating operations that change surface

properties.

Diffusion Coatings Simon et al . [272] showed that case-hardening lowers the friction

coefficient of a 0.16% carbon steel from 1.3 to 0.6 at 77 K. Hoping for a similar result,

Wisander and Johnson [276] applied commercial nitriding, chromizing, and sulfurizing

treatments to AISI 304 stainless steel. Sulfurizing reduced the friction coefficient from

about 0.4 to 0.2, while reducing wear by two-thirds. In comparison, nitriding led to only

a modest improvement, and chromizing had adverse effects.

Plating It is theoretically possible to improve sliding characteristics by plating a thin

layer of soft metal between two hard alloy surfaces. The soft metal is supposed to provide a low shear strength interface while preventing wear owing to direct contact between

the hard alloys. In practice, however, results are difficult to predict. Chromium plating failed to improve the performance of AISI 304 stainless steel at 77 K [276]. Other

93
1.4

Fi gure

=L

I—z

UJo

Ul

Ll. UJo

o

o

t—o

a

:

1.2

1.0

0.8

0.6

0.4

0.2

0

304 Stainless Steel

Pyrex ( /x s )

3

[ Teflons ( —

1 1 0 100 200 300

TEMPERATURE (K)

12.1. Temperature dependence of self-friction coefficients for

selected materials.

94
tests show that lead, indium, or tin platings can be successful for AISI 440-C stainless

steel in liquid hydrogen at 20 K for limited time periods [277]. For example, ion-plated

lead coatings reduced wear significantly and yielded friction coefficients in the range

0.16 and 0.20, as opposed to 0.57 for unplated AISI 440-C. But Baber, Burton, Chang,

Cuellar, Ku, and Russell [278] report high friction for lead-plated SAE 52100 tool steel

at 77 K. Therefore, it appears that the effects of plating depend on the specific test

conditions.

12.3.3. Laminar Solids

Certain solids having laminar (plate-like) crystal structures are easily sheared

along their basal planes. This accounts for the high lubricity of graphite, boron nitride,

and molybdenum disufide (MoS 2 ). Graphite and MoS 2 are common dry lubricants at 295 K [279], but MoS2 is generally preferable at cryogenic temperatures.

Graphite is less expensive than other solid lubricants. But the lubricity of graphite

in inert environments is poor, since the lubricity depends on the presence of water vapor

or other absorbable gases. Thus graphite fails to lubricate in vacuuum, and difficulties

are encountered in some cryogenic environments where graphite has been shown to cause high

friction and abrasive wear between sliding metals [280, 281].

Graphite is available in film form or as a component in mechanical carbons. In either case, low temperature tests have led to some erratic results. Baber, et al. [278]

report an anomalous spike in the temperature dependence of the static friction coefficient

for sprayed graphite coatings on copper, as shown in Fig. 12.2. Except for this anomaly,

low static friction coefficients were observed in the cryogenic range (p s 0.1). But the

effects of prolonged sliding and wear were not investigated.

Molybdenum disulfide (MoS 2 ), unlike graphite, is less sensitive to the environment.

The reported friction coefficients for MoS2 in air at 295 K range between 0.04 and 0.1. At cryogenic temperatures, both high and low friction coefficients have been reported,

depending on variables that are not completely understood.

MoS2 can be added as a filler to plastics, or it can be used as a film coating for

metals and plastics. When used as a film, MoS 2 can be rubbed onto metal and alloy sur- faces; it may also be sprayed, brushed, or adhesively bonded to metals or plastics.

Bonded films endure longer, especially if the substrate is roughened or chemically etched

prior to coating. On the other hand, the curing temperatures for certain commercial

binders may be too high for some substrate materials.

Although superior static friction coefficients (0.1 to 0.2) are reported in Fig. 12.2

for MoS 2 films on copper, continuous sliding at cryogenic temperatures may cause film

deterioration. Wisander and Johnson [276] observed rapid film deterioration during wear

tests of MoS2 -coated AISI 304 stainless steel at 77 K. The use of commercial MoS2 products in accordance with manufacturer's recommendations should minimize problems.

12.3.4. Polymers

In addition to moldability, machinability, and abrasion resistance, polymers show

little tendency to adhere to metals. Thus nylon has replaced metals in some sliding

applications such as gears, especially where freedom from corrosion is desired. A basic

disadvantage of polymers is inability to dissipate frictional heat. This drawback is most

evident for material in bulk form; it can be alleviated by utilizing filled polymers or

film coatings, as discussed later.

In contrast to metals, the friction of polymers is more sensitive to load, temperature, and sliding speed; such variables may exert pronounced effects [272-290]. For example, Allan and Chapman [282] report sharp increases in the friction of Teflon at 300 K as load is reduced or as sliding velocity is increased; the low friction expected for

Teflon is observed only at loads greater than 2.0 kg and at sliding velocities less than

25 mm/s.

95
TEMPERATURE (K)

Figure 12.2. Effect of lubricating films on the friction of copper.

96
The room temperature static friction coefficients of polymers range from about 0.04

to 0.6. In most cases, the coefficients decrease at low temperatures. But Tabor [285]

and White [290] agree that the static friction coefficient of PTFE shows a step-like

increase from 0.1 at 253 K to 0.2 at 213 K. Outside this step region the coefficient is constant up to 350 K [282]. Friction coefficients in the range 0.06 to 0.13 have been

reported for Teflon at 77 K, but no one has ever reported the complete cryogenic temperature

dependence.

Kharitonova [286] reviews the cryogenic uses of Teflon (PTFE or FEP). Teflons have

the lowest friction coefficients (p> 0.04) of all known solids. Teflons are also exceptional in that they retain several percent ductility at 77 K, whereas other polymers are more brittle at this temperature. PTFE has a lower friction coefficient, is harder, and more wear resistant than FEP. If minimizing friction is not of overriding importance,

other polymers having superior cold-flow resistance can be substituted for Teflons.

Alternatives evaluated by Wisander and Johnson [289] include acetal copolymer, parapolyphenol , polyamide, polyimide, polycarbonate, polyoxymethyl ene , and polychl orotri f 1 uoro- ethylene (PCTFE).

12.3.5. Filled Polymers

Fillers can be blended with polymers prior to molding to achieve improved physical

and mechanical properties. Diverse fillers have been used, including graphite, carbon,

MoS 2 , fibers, glass, pyroceram, ceramics, metals, and metal salts such as CaF 2. Typically, the filler amounts to about 25% of the total composition. Such a composite may

offer significantly improved strength and thermal properties, while the antifriction

properties of the bulk polymer are retained and problems associated with adhesive films

(discussed below) are avoided. Filled polymers have been used as ball bearing separators

[291],

A substantial data base exists to aid the selection of filler compositions for specific

purposes. Friction and wear data for nine different polymers and seventeen filled compositions at 20 K are reported by Wisander and Johnson [289]. Studies devoted exclusively to filled Teflon compositions at temperatures down to 77 K were conducted by Timofeev, Drobinin,

Grushevskii and Antropov [292], and by Wisander, Maley, and Johnson [275]. In the later

study, 22 filled Teflon compositions were tested as rider specimens versus AISI 304 stainless steel. The friction coefficients varied between 0.06 and 0.13 at 77 K. Data are

also presented for tests conducted in gaseous nitrogen at temperatures up to 344 K.

12.3.6. Polymer Films and Coatings

Coatings of PTFE or PCTFE may be applied to metal components to combine the strength

and thermal conductivity of alloys with the antifriction properties of polymers. Wisander

and Johnson [293] report that 0.25 mm coatings of PTFE or PCTFE on AISI 304 stainless

steel specimens give longer wear and lower friction compared with sulfurizing, chromizing,

nitriding, chromium plating, and most solid-film lubricants. The success of a polymer

coating depends on reliable application techniques, and to the long list of variables

already cited as influencing friction and wear results, we must add two new variables —

film thickness and bond integrity.

Coating failures are frequently caused by breakdown of the bond between the film and

substrate. Wisander and Johnson's success was obtained by applying twenty 0.0127 mm thick

Teflon layers and fusing at 644 K. In some cases this method may be impractical because

of the high temperatures involved. An alternative, then, is adhesive bonding with organic

resins, such as phenol ics, epoxies, or silicones.

Regardless of bonding technique, spalling of the film from metal components is prone

to occur since polymers expand and contract with temperature more than their metal substrates. For example, the average expansion coefficients between 77 and 300 K for Teflon

and AISI 304 stainless steel are 86 x 10 6 /°K and 14.7 x 10 6 /°K, respectively. A practical solution was engineered by Wisander and Johnson [293], who formulated a 10% Teflon-40%

epoxy-50% lithium-alumina-silicate film. The epoxy addition improved film adhesion, while

the silicate reduced thermal contraction. The average coefficient of thermal expansion

97
for this film was 28.8 x 10 6 /°K, which is close to values for metals and alloys. Tests

verified that 0. 127-mm-thick coatings of this film on AISI 304 stainless steel gave low

friction (|j = 0.02 to 0.07), low rider wear, and outstanding film life. Another lubricating film for inert cryogenic environments is described by Sliney [294].

12.3.7. Ceramics, Cermets, and Glass

The cold-welding of metals can also be avoided if at least one component of the

friction couple is manufactured using a ceramic, cermet, or glass. These materials are corrosion resistant, wear resistant, and hard. They are also brittle at all temperatures,

and will shatter if overloaded. Therefore, impact loading, high static loading, and high

sliding velocities must be avoided.

Low temperature friction and wear data for these materials are scarce. Simon et al. [272] reported the static friction coefficient for fire-polished Pyrex glass to be constant

at about 0.23 for temperatures between 77 and 300 K. Shynyrev and Drobinin [295] evaluated

tungsten carbides at liquid nitrogen temperatures, reporting excellent wear resistance.

Other materials of potential interest in this category include pyroceram, sapphire, and

bonded oxides, such as A1 2 0 3 .

98
13. INSTRUMENTATION

The successful monitoring of cryogenic tests requires the appropriate selection and

use of temperature, strain, and pressure sensors. In addition, proper technique must be

observed in small voltage signal maintenance.

13.1.

Temperature Sensors

Cryogenic thermometry reviews [296-300] include discussion of three main classes of

temperature sensors for cryogenic service: (1) thermocouples, (2) resistance thermometers,

and (3) filled systems. Representatives from each class are identified in Figure 13.1,

and the performance characteri sties of some common thermometers are compared in Table

13.1.

Copper-Constantan or Chromel-Constantan thermocouples appear to be the best general

temperature sensor for cryogenic wind tunnel applications. Not the least consideration

here is the need for small size and remote reading which thermocouples can satisfy.

Platinum resistance thermometers and bondable temperature gages might also be useful, but

other thermometers are of little or no value for NTF testing for reasons stated below.

13.1.1.

Thermocouples

Thermocouples are small, simple, convenient, fast in response time, readily available, well characterized, and low priced. The ancilliary equipment required for use are

low voltage measuring devices and reference junctions. Of the many possible types of

thermocouple wire combinations, three standard types can be recommended for NTF testing:

Types E, K, and T.

Type T (Copper versus Constantan) . Type T is the traditional choice for service

between 89 and 644 K. Owing to loss of sensitivity, Type T is not recommended for service

below 76 K. Type T has high thermoelectric homogeniety, which means freedom from the

adverse effects of material defects. Type T is the only thermocouple combination for which

the limits of error are established below 273 K. Type E (Chromel versus Constantan) . This combination is recommended by ASTM for

service between 23 and 1144 K. Sensitivity at the low temperature extreme is superior to Types T or K. Homogeniety is satisfactory, as is corrosion resistance in moist environments. Both wire elements have low thermal conductivity that reduces heat conduction to the specimen.

Type K (Chromel versus Alumel) . This combination is recommended by ASTM for service

between 23 and 1533 K. Sensitivity is nearly equivalent to type T, and only about half

that of Type E. Both wire elements have low thermal conductivity and excellent corrosion

resistance in moist environments.

The thermoelectric voltages, sensitivities, and other data for these thermocouples

are given by several authors [301-305], Procedures for installation, instrumentation,

circuitry, and error analysis are treated in reviews [296-300].

13.1.2.

Resistance Thermometers

The platinum resistance thermometer (PRT) is the most accurate and reproducible

thermometer available over a broad temperature range. Using the better commercial PRT's,

wind tunnel temperatures can be measured routinely with high accuracy. Other resistance

thermometers, however, must be dismissed from consideration: carbon and germanium re- sistance thermometers are restricted to service below 100 K, respectively, while copper

and indium resistance thermometers simply lag the PRT in development at the present time.

The use of PRT's at low temperatures is discussed by Sinclair, Terbeek, and Malone

[300]. The primary disadvantages are size and cost. Thermocouple wires are only 0.08 mm

in diameter, but a PRT junction may be 1 x 8 x 10 mm. Ruggedness and reliability vary

widely for commercial products, not necessarily correlating with price. Many configurations

are commercially available, and custom designs can be ordered at additional cost. In general, however, all PRT's are classified as immersion probes or surface sensors.

99
TYPE OF THERMOMETER

Type E Thermocouples

Type T Thermocouples

Type K Thermocouples

Platinum Resistance

Copper Resistance

Indium Resistance

Thermistor Resistance Germanium Resistance Carbon Resistance 02 Vapor Pressure

N 2 Vapor Pressure

Ne Vapor Pressure

H z Vapor Pressure

He 4 Vapor Pressure

4 6 10 30 60 100 300 TEMPERATURE (K)

Figure 13.1. Thermometers and their useful temperature ranges.

100
Table 13.1. Operating characteristics of some selected thermometers.

Class Type Range

K Reproducibility

K Accuracy

K Response times

Liquid-in-glass Pentane 70-300 2 0 . 2

- 0.

5

slow

Thermocouple Copper-constantan 76-644

-1 -2

10

1

- 10 ^ 0.1 ~1

Resistance Plati num 14-900 io

' 3

- 10~ 4 -2 -4

10 - 10 * 0.1 - 10

| Vapor pressure Oxygen 54-155 io

" 2 - io

" 3 0.02 0.1 - 100
Surface sensors are relatively small, typically 2.5 x 13 x 13 mm. Surface sensor geometries make good contact with various component shapes. They also have high custom

design capability, and may be attached by clamping, soldering, welding, or cementing.

The variety of immersion probes is so great that product data bulletins must be

consulted for specific descriptions. The typical immersion probe has a resistance some- where between 100 and 1000 Q. Repeatability at 273 K is typically certified conserva- tively at ± 0.1°C, but several thermal cycles to cryogenic temperatures are needed to establish repeatability. Time response is between 0.1 and 7 s.

Potentiometric voltage determination or direct resistance measurement can be used to measure PRT output. Appropriate bridge circuits are outlined by Harris [306], Stout [307]

and Daneman and Mergner [308], Both immersion probe and surface sensor PRT 1 s are available in versions with built-in bridge circuits, which allow adjustments for improved

interchangeabi 1 ity.

13.1.3. Bondable Temperature Gages

Manufacturers of strain gages (described below) also produce bondable temperature

gages for service between 76 and 513 K. These surface temperature sensors are constructed

like strain gages, but the active elements are usually high purity nickel or nickel-iron

foil grids. Accuracies equivalent to thermocouples can be achieved, but bonded temperature

sensors are not reusable. Read-out methods, installation, stability, error analysis, and

other information are described in product data bulletins.

13.1.4. Thermistors

Thermistors (thermally sensitive resistors) are small, sensitive, and respond in less

than 1 s. Unfortunately, few are designed for extreme cryogenic service, and each is usable only for narrow temperature ranges.

13". 1. 5. Filled Systems

Pentane liquid-in-glass thermometers can be used at temperatures as low as 70 K. These thermometers require no calibration or instrumentation, but severe restrictions on use are imposed by the need for visual accessibility for direct reading. These thermometers may provide quick reference if positioned near wind tunnel viewing portals.

Vapor pressure devices function only for narrow temperature intervals and temperatures lower than 155 K, and are clearly inappropriate for wind tunnel service.

13.1.6. Combined Thermometers

Thermometry capabilities can sometimes be increased by the combined use of thermocouples and resistance thermometers. Sinclair, Terbeek, and Malone [300] refer to several possibilities, including a case where the measurement of small temperature differences was improved using a thermocouple and a PRT.

13.2. Strain Gage Sensors

'

This section is exclusively devoted to electrical resistance strain gages which are directly bondable to machine components. Reference books on strain gages were authored by

Perry and Lissmer [309] and Aronson and Nelson [310], but these books do not discuss

cryogenic applications. Current information is best obtained from the gage manufacturers.

Today's foil strain gages are produced in a wide variety of sizes, patterns, materials, and forms. One vendor boasts 40,000 variations, but not every kind is stocked.

Most gages have a stated strain capability of 1% but this can usually be pushed higher,

sometimes to 3%. Prices for photoetched films are about $4 to $10 each, depending on options. Options include protective encapsulation, solder dots, integrated terminals,

preattached leads, high endurance leads, strippable backings, and sel f- temperature compensation.

102
A typical gage consists of an active alloy element and a dielectric backing, which

provides easy handling as well as insulation. Backing materials include paper, bakelite,

polyimides, epoxies, and reinforced plastic laminates. Active foil elements may be

Constantan, Manganin, Karma, Nichrome, Chromel , platinum, nickel, Elinvar, or Armour D alloys. However, not every wire element and backing combination functions well at cryogenic temperatures.

13.2.1.

Gage Selection

Several gage element and backing combinations acceptable for various cryogenic operating conditions are identified by the manufacturers. "High performance" materials and

construction are recommended for NTF applications since gage costs are minor compared to

the overall research investment. The Karma alloy elements offer good apparent strain

characteristics, stability, and fatigue life at temperatures down to 76 K. Armour D and

Nickrome elements are also satisfactory at 76 K. Constantan is not generally applicable

at cryogenic temperatures. Iso-elastic (Elinvar) elements are usable for dynamic testing

between 76 and 500 K. Bondable terminals provide a stress relief point between the gage

and lead wires, to avoid damaging the gage when lead wires are stressed. Satisfactory

adhesives for bonding strain gages to all types of structural materials and test temperatures are available from the gage manufactures, as are cleaners, solders, applicators,

measuring bridges, informative technical notes, and hardware kits.

13.2.2.

Gage Factors

The gage factor, G, indicates the sensitivity of the active element. Gage factors

for the common cryogenic strain gage elements are approximately 2, but the value varies

slightly with gage size, pattern, substrate, and temperature. In general, precise gage

factors are furnished with commercial products, but it is possible to determine experimentally the temperature dependence of the gage factor using the relation:

r _ AR

G "

eR where R is the gage resistance and AR is the change in resistance due to a strain, e.

13.2.3.

Apparent Strain

Unless precautions are taken, serious errors in strain measurements at cryogenic

temperatures will arise from temperature effects on gage resistance, combined with thermal

contraction of the materials involved. These errors give a false indication of strain,

the magnitude of which can be much larger than the real strain.

Apparent strain must be accounted for to measure the true strain associated with

mechanical stress. If the temperature is constant during the test, the problem is solved

by bringing the gaged component to thermal equilibrium at zero load and then cancelling

the apparent strain electrically. Or, if the apparent strain as a function of temperature

for a gage mounted on a specific test material is published by the gage manufacturer, it can be subtracted from the total indicated strain to obtain the true strain. Alternative

methods of eliminating apparent strain by using "dummy gages" are discussed in the technical notes published by gage manufacturers.

13.2.4.

Self-Temperature Compensation

Apparent strain may also be reduced using self-temperature compensating gages designed to minimize apparent strain when bonded to certain structural materials having

matching thermal expansion and contraction coefficients. Generally, the best temperature

compensation is achieved in the range 250 to 475 K. Detailed discussions of self- temperature compensation and the problem of apparent strain are to be found in technical notes

published by the gage manufacturers.

103
13.3. Pressure Sensors

Pressure-sensing devices for cryogenic service are reviewed by Hayakawa [311] and by

Arvidson and Brennan [312], A number of commercial devices are suitable for NTF applications at pressures up to 900 kPa (9 atm). The simplest means of pressure measurement

under static conditions is the bonded strain gage attached to a diaphragm. Pressure on the diaphragm stretches the gage, altering its electrical resistance. Although strain

gages can be used for dynamic as well as static testing, piezoelectric devices offer

superior frequency response in dynamic testing. These devices can be inserted by means of

probes into the wind tunnel or into the model aircraft themselves.

13.4. Sensor-to-Specimen Contact

The object in bonding sensors to structural components is to achieve true physical

contact, while minimizing the disturbance of the object being monitored.

For example, whenever a thermocouple is attached, a thermal barrier is created at the

interface with the specimen, and heat must transfer between the specimen and sensor. The

two major sources of heat are (1) heat conducted along the wires and (2) Joule heating

(I 2 R) in the resistive thermoel ectric elements. If the thermal barrier at the specimen

interface is significant, htat flow across it will generate a temperature gradient.

Consequently, the thermocouple may register a higher temperature than the true temperature

of the specimen. As discussed in the following sections, thermal anchoring and contact

agents can be used to alleviate the problem.

13.4.1. Thermal Anchoring

Thermal anchoring (also called tempering) of wires to a heat sink whose temperature

is near that of the specimen will allow heat to dissipate before it can affect measurements. Thermal anchoring is important in any cryogenic environment, but particularly in vacuum where there is no surrounding medium to absorb heat. Hust [313], Kopp and Slack

[314] and Allen, Bradford, and Crabtree [315] discuss thermal anchoring in detail. The

required anchoring length is a function of materials, wire size, insulation, temperature,

environment, and the acceptable temperature mismatch of the wire and heat sink.

Thermal anchoring and other heat transfer calculations require knowledge of the

thermal conductivity of wires, insulators, and contact agents (greases, adhesives, and

varnishes). The thermal conductivities of some low temperature thermal contact agents for

various temperature ranges are given by Denner [316], McTaggart and Slack [317], and

Kreitman, Ashworth, and Rechowicz [318]. Denner and McTaggart and Slack also report

thermal conductivity data for some electrical insulators and solders. Using discretion,

engineering estimates of thermal conductivity for untested materials may be based on available data for similar materials.

13.4.2. Thermal or Electrical Contact Agents

Contact agents are frequently used in cryogenics to attach heater wires, thermocouples, and heat sinks, or to improve electrical contacts. In the case of thermocouple

attachment discussed above, electrical isolation must be maintained, preferably using a contact agent that is easily removed and resistant to thermal cracking. Greases, var- nishes, and adhesives can be used, in conjunction with thermal anchoring.

Solders greatly reduce thermal barriers, but solders conduct electricity also. As electrical insulators, adhesives offer adequate thermal contacts with higher strength than

varnishes or greases. Thermal conductivity and other useful data for adhesives and cements

appear in the literature [217, 316, 319]. Thermal barriers can be further reduced by

filling the adhesive with conductors such as silver or beryllium oxide. Kallin [320]

reports a doubling of the thermal conductivity using a 65% epoxy and 35% silver powder

mixture, as compared with the epoxy alone. This mixture was used to attach thermocouples

to unweldable, brittle materials. The addition of silver did not decrease the adhesive

bond strength significantly.

104
13.5. Signal Maintenance

Electrical signals from sensors must be maintained and processed without distortion.

This is important since cryogenic instrumentation relies on low level signals. Wire insulation

compatible with the cryogenic environment must be selected, and the introduction of spurious

voltages must be avoided at wire junctions, electrical feed-throughs, ground current

circuits, and switches.

13.5.1. Wire Insulation

Wire insulation materials include polymers, varnishes, and glass fabric. These

dielectrics retain their insulating properties at cryogenic temperatures. For example,

polymer breakdown voltages are on the order of kilovolts. Of greater concern, therefore,

is mechanical failure. Tensile, bending, or thermal stresses may crack wire insulation

materials at cryogenic temperatures, causing electrical shorts.

Polymers are the most common form of wire insulation, and fluorocarbons are the

recommended polymers. Based on simple mandrel winding tests at cryogenic temperatures,

Teflon emerged superior in flexibility compared to polyethylene and polyvinylchlorides

which are commonly used at room temperature [321]. Teflons and polychlorotrif 1 uoroetylene

are also preferred in aerospace cryogenic wiring applications by virtue of superior chemical inertness over a broad temperature range, as well as low dielectric loss. The FEP

form of Teflon is readily available as spaghetti tubing or skived tape.

Regardless of the type of insulation selected, strain relief precautions should be

taken during wiring. Note also that thin layers of insulation generally offer greater

flexibility since higher strains are developed in thick layers subject to bending.

13.5.2. Wire Connections

Wire connections should be limited to the minimum number practical. If a junction is

made of dissimilar metals, it should not be located in a thermal gradient, or else spurious thermoelectric voltages will be generated. Note that solder junctions form a dissimilar metals situation, even when the two wires soldered are the same. Junctions can be

made by soldering, spot welding, or simply twisting the leads together; all of these

methods have unique applications and can be used successfully under proper conditions.

13.5.3.

Feed-Throughs

It is frequently necessary to route wires through walls where significant temperature

and pressure changes occur in short distances. The two basic types of electrical feedthroughs used in cryogenic systems are the continuous wire and junction types as shown in

Fig. 13.2. If a large temperature gradient exists across the feed-through, the continuous

wire variety is preferable since it avoids a dissimilar metals situation. The specifications of materials, sizes, wire capacity, and pressure rating vary widely in commercial

feed-throughs so that many practical applications can be satisfied.

Figure 13.2 also shows one of the many potting methods which may be used for "inhouse" feed-throughs. The cup and reservoir design is simple and effective as an ambient

temperature seal. If black sealing wax is used in the reservoir, the seal is easy to remake with new wires. Use of a continuous wire type eliminates the necessity of a junction, or at least allows the junction to be made at ambient temperature where large gradients do not exist.

13.5.4.

Shielding and Grounding

Care must be taken with low level voltages to obtain proper shielding or grounding.

A single ground is preferred to avoid loop currents that can introduce spurious voltages

in the signal path. Most modern low level dc amplifiers have excellent 60 Hz rejection;

however, ac pickup may be sufficient to saturate the amplifier, resulting in a steady

shift of output. The recommended procedures for low level dc circuits are:

105
Tubular feed-throughs /

CONTINUOUS JUNCTION

WIRE

(a)

(b)

Figure 13.2. Various types of wire feed throughs used in cryogenic

appl ications

.

106
1) Operate either at floating potential or with one ground, preferably at the

amplifier input.

2) Use extension cables that have a high rejection of electromagnetic noise.

Twisted leads decrease the flux pickup area, and commercially available weave patterns

cause cancellation of pickup noise. The leads should be encased in a shield grounded at only one point. Since normal copper shielding does not adequately shield 60 Hz noise,

cables having ferromagnetic shielding are preferable. Cables and wire junctions can be

placed in iron conduits or boxes to provide additional electromagnetic shielding. Morrison

[322] discusses shielding and grounding in greater detail.

13.5.5. Switching

Switches can be operated at ambient temperatures where problems of spurious voltage

generation are not severe, but they must be of sufficient quality to prevent degradation

of the signal -to-noi se ratio. For manual operation, low-resistance rotary switches in

metal cabinets are satisfactory. For automatic operations, enclosed gold- or silverplated switches are usually satisfactory; they typically have about 1 mV transient noise

and therefore introduce excessive noise if switched faster than about once a second. All switches should be thermally isolated since thermal voltages may be generated at junctions

where wires are attached.

107
14. SOURCES OF CRYOGENIC DATA

In a survey of this kind, it is not possible to describe all materials and parameters

that may be of potential interest for specific applications. New materials and measurements

are appearing at a high rate. To keep abreast of developments, several data sources dealing with cryogenics are described below. Cryogenic handbooks [8*17] have been recom- mended earlier in the text. An additional aid to data retrieval from sources possibly

containing some cryogenic data is the survey by Westbrook and Desai [323] which attempts

to assess the content of the most prominent handbooks, technical compilations, information

centers, technical societies, and 14.1. trade associations.

Technical Journals

Cryogenics (Plenum) is an international journal covering all aspects of low temperature research. Cryogenic Technology (Cryogenic Society of America) and Cryogenics and

Industrial Gases (Business Communications, Inc.) are primarily of technological interest.

Cryogenic Information Report (Technical Economics Associates) surveys current activities

and developments in all fields of cryogenics.

14.2.

Monographs

The International Cryogenics Monographs Series (Plenum) at present consists of eleven

volumes. Of these, Wigley's [18] Mechanical Properties of Materials at Low Temperatures

is of greatest relevance to model designers. The National Bureau of Standards also

publishes monographs devoted to cryogenics. A complete listing of published NBS titles is available from Frizen and Mendenhall [324],

14.3.

Conference Proceedings

Advances in Cryogenic Engineering (Plenum) contains the proceedings of both the

International Cryogenic Materials and Cryogenic Engineering Conferences, which are held

every two years. There are currently 24 volumes. Volume 20 contains an index to all previous volumes.

Other sources include six volumes of proceedings from the International Cryogenic

Engineering Conferences (IPC Science and Technology Press) and seven Special Technical

Publications (STP's) from the American Society for Testing and Materials:

ASTM STP 158 (1954).

ASTM STP 287 (1961).

ASTM STP 302 (1962).

ASTM STP 387 (1966).

ASTM STP 496 (1971).

ASTM STP 556 (1974).

ASTM STP 579 (1975).

Symposium on the Effect of Temperature on the

Brittle Behavior of Metals with Particular

Reference to Low Temperatures.

Low Temperature Properties of High Strength

Aircraft and Missile Materials.

Evaluation of Metallic Materials in Design for Low Temperature Service.

Behavior of Materials at Cryogenic Temperatures.

Fracture Toughness Testing at Cryogenic Temperature

Fatigue and Fracture Toughness - Cryogenic

Temperatures.

Properties of Materials for LNG Tankage.

14.4.

Computerized Data Banks

Bibliographies on specific subjects may be purchased from organizations maintaining

automated data bases and search systems. The following information on data bases is quoted from Fickett [325].

108
1. NTIS - covers abstracts from U.S. government sponsored research by 240 agencies

and their contractors. Dates from 1964. Updated biweekly. Contact: U.S. Department of

Commerce, National Technical Information Service, 5285 Port Royal Road, Springfield, VA

22161. This service also publishes a separate section of its Weekly Government Abstracts

under the title "Materials Sciences". Cryogenic applications are not separately indexed,

although they appear occasionally. Available by subscription.

2. SSIE - summaries of research in progress and recently completed (U.S. and foreign). Dates from 1974. Updated monthly. Contact: Smithsonian Science Information

Exchange, Room 300, 1730 M Street, Washington, D.C. 20036.

3. C0MPENDEX-- Engineering Index data base. Abstracts of published work from 3500

journals, reports and books. Dates from 1970. Updated monthly. Contact: Engineering

Index, Inc., United Engineering Centre, 345 East 47 Street, New York, NY 10017.

4. METADEX - covers abstracts of the international metallurgical literature. Dates

from 1966. Updated monthly. Contact: Metals Information. American Society for Metals,

Metals Park, OH 44073.

5. NASA maintains a computerized data base from its Scientifc and Technical Aerospace

Reports (STAR) abstract journal, but the situation regarding general access is not clear.

The published version of STAR is generally available. Contact: NASA Scientific and

Technical Information Facility, P.0. Box 8757, B.N.I. Airport, Maryland, 21240.

In addition to the above, the Cryogenic Data Center, Thermophysical Properties Division, NBS, Boulder, Colorado, 80303, also prepares tailored bibliographies based on an automated indexing system. The minimum service charge is $35.00. The Cryogenic Data

Center is a prime source of cryogenic information, maintaining a library and world litera- ture file with over 100,000 entries open to the visiting public. Out of print, or hardto-locate reports are included.

14.5. Abstracting Services

The Current Awareness Service (Cryogenic Data Center, NBS, Boulder, Colorado, 80303)

is a biweekly publication listing an average of 200 cryogenic articles per issue. Based

on a scan of 1,000 periodicals, the content is divided into sections on Low Temperature

Physics and Chemistry, Cryogenic Engineering, and Energy. The annual cost is $25.00.

The Review of the Low Temperature Properties of Metals (Metals Properties Council and

Metals and Ceramics Information Center, Battel le-Col umbus , Columbus, Ohio, 43201) also

provides a convenient summary of low temperature research studies appearing in the open

literature and in private reports.

109
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128
BS-114A (REV. S-78)

U.S. DEPT. OF COMM. 2.Gk>v’tAcceWon No.j 3. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 79-1624

4. TITLE AND SUBTITLE

Materials for Cryogenic Wind Tunnel Testing

5. Publication Date May 1980

t- AUTHOR(S) 8. Performing Organ. Report No.

R. L. Tobler

I. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

2. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP) National Aeronautics and Space Administration

Systems Engineering Division

Langley Research Center

Hampton, VA 23365

13. Type of Report & Period Covered

5. SUPPLEMENTARY NOTES

I | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

. ABSTRACT (A 200-word or leas factual summary of moat aighificant information. If document includes a significant bibliography or

literature survey, mention it here.) A study was conducted to guide the evaluation and selection of materials and

techniques to be used in construction of model aircraft for cryogenic wind tunnel

testing. In this report, the mechanical, thermal, and electrical property behavior

of materials at temperatures as low as 77 K is briefly reviewed. Metals, structural alloys, non-metals, composites, joining methods, coatings, sealants, adhesives, contact agents, lubricants, transducers, and instrumentation for cryogenic

applications are discussed. Acceptable structural materials, conductors, and insulators are discussed for service at temperatures in the range 367 to 77 K. Numerous references to handbooks and other cryogenic data sources are cited as a guide to additional information.

1 [1 • KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

| separated by semicolons)

* Cryogenic properties; instrumentation; low temperature materials; materials

properties; metals; non-metallic materials; wind tunnels

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