REFERENCE publications

NAT L INST. OF STAND & TECH

AlllDb 0317D8

SIR 85-3135

Development of an MBS Polymer

Gage for Dynamic Soil Stress

Measurement

Riley M. Chung

Center for Building Technology

Structures Division

Anthony J. Bur

Center for Materials Science

Polymers Division

and

Edward Reasner

Center for Building Technology

Building Materials Division

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Gaithersburg, MD 20899

April 1985

V

w'M. \

^OfAU

wO* %/

f ^ -QC

100

.1156

85-3135

1985

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF 8TANDAR08

NATIONAL BUREAU

OF STANDARDS

LIBRARY

NBSIR 85-3135

p#e> DEVELOPMENT OF AN NBS POLYMER

GAGE FOR DYNAMIC SOIL STRESS

MEASUREMENT

i (si- (& no. sr-3

IQ

Riley M. Chung

Center for Building Technology

Structures Division

Anthony J. Bur

Center for Materials Science

Polymers Division

and

Edward Reasner

Center for Building Technology

Building Materials Division

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Gaithersburg, MD 20899

April 1985

U.S. DEPARTMENT OF COMMERCE. Malcolm Baldrige. Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

DQ

TABLE OF CONTENTS

Page

ABSTRACT ii

LIST OF FIGURES iii

LIST OF TABLES V

NOTATION Vi

ACKNOWLEDGEMENT vii

1. INTRODUCTION 1

2. THE NBS POLYMER GAGE 12

2.1 The Polymer Material 12

2.2 Previous Studies Using the Gage for Dynamic

Pressure Measurement 14

2.3 Gage Design and Calibration for the Present

Study 16

3. GEOTECHNICAL LABORATORY TEST PROGRAM 32

3.1 Soil Used in Testing 32

3.2 Test Variables and Test Program 32

3.3 Test Set Up and Data Recording 36

4. PRESENTATION, INTERPRETATION, AND

DISCUSSION OF TEST RESULTS 5 0

5. CONCLUSIONS 71

6. RECOMMENDED FUTURE STUDIES 74

7. REFERENCES 76

l
ABSTRACT

Polymer gages developed by the National Bureau of Standards (NBS)

have been tested extensively in the NBS Geotechnical Engineering

Laboratory to evaluate their capability and reliability for use

in determining dynamic soil stresses generated by blast loadings.

Penetration of soil grains into the gage surface was found to be

the major concern and a major effort was undertaken to develop

the most appropriate protective covering. Gages were dynamically

loaded to develop their corresponding calibration curves. The

test results indicate that the gage provides a linear relationship between the input stress and the gage output over the stress

range tested. The calibrated linear relationship is not affected

by the gage location, i.e., whether in soil or on a concrete

pedestal; the thickness of the protective covering; the frequency

of the impact load; and the length of the co-axial cable used in

testing

.

11
1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

LIST OF FIGURES

Page

Theoretical Soil Stress Gage Response 8

Stress Distribution over Stress Gage 9

Theoretical Lateral Stress Rotation for Rigid 9

Ellipsoidal Inclusion

Computational Model of Dense Ottawa Sand - 10

Volumetric Behavior

Basic High Modulus Configuration for the 11

SRI Gage

The Polymer Material - Polyvinylidene Fluoride 25

(PVDF)

The Polymer Gage - Bilaminate Construction 26

The Polymer Gage - Bilaminate Construction 27

with PVDF Sheets as a Protective Cover

The Polymer Gage - Bilaminate Construction 28

with Polycarbonate Sheets as a Protective Cover

An NBS Polymer Gage at Its Latest Design 29

Drop Test Set-up 30

Typical Pressure Pulse Response by Polymer 31

Gage and Reference Transducer from The Drop Test

Grain Size Distribution of Florida Sand 44

Compaction Curve of Florida Sand 45

NBS Polymer Gage Test Set-up in The 46

Geotechnical Engineering Laboratory

Close-up View of The NBS Polymer Gage Test 47

Set-up

Schematic Drawing of The Test Cylinder and Its 48

Accessories

Typical Strip Chart Output from Impact Load 49

Testing of the NBS Polymer Gage

NBS Polymer Gage Test Results - Effect of Test 56

Cylinder Insulation

in
20

21

22

23

24

25

26

27

28

29

30

31

32

33

Page

57

58

59

60

61

62

63

64

65

66

67

68

69

70

NBS Polymer Gage Test Results - Bentonite

Layers Used as a Protective Covering with the

Test Cylinder Insulated

NBS Polymer Gage Test Results - Bentonite

Layers Used as a Protective Covering with the

Test Cylinder Not Insulated

NBS Polymer Gage Test Results - Films Used as Protective Cover

NBS Polymer Gage Test Results - Films Used as Protective Cover

NBS Polymer Gage

Cable Length

NBS Polymer Gage

Cable Length

NBS Polymer Gage

Location

NBS Polymer Gage

Location

Test Results -

Test Results -

Test Results -

Test Results -

Photo Negative

Photo Negative

Effect of

Effect of

Effect of Gage

Effect of Gage

Effect of

Effect of

Effect of

Effect of

for

#62

NBS Polymer Gage Test Results - Frequency, Gage PC #49

NBS Polymer Gage Test Results - Frequency, Gage PC #54

NBS Polymer Gage Test Results - Frequency, Gage PC #64

NBS Polymer Gage Test Results - Frequency, Gage PC #58

Summary of NBS Polymer Gages Calibrated

Field Testing

Non-linear Characteristics of The Gage

Exhibited During The Testing of Gage PC

iv
LIST OF TABLES

Page

Table 1 Summary of Test Program 41

Table 2 Gage Types 42

Table 3 Types of Protective Coverings

Experimented with in the Test

Program

43

v
NOTATION

Ar = Aspect ratio; the ratio of total gage thickness

to total gage diameter, T/D.

Egc

Em

Emd

Emc, ms

ER

Ko

t

= Diameter of active area of the gage in mm or inches.

= Hydrostatic piezoelectric constant.

= Total gage diameter in mm or inches.

= Mean soil grain size in mm or inches.

= Gage modulus in KPa or psi.

= Modulus of PVDF material in KPa or psi.

= Modulus of Polycarbonate sheet in KPa or psi.

= Soil Modulus in KPa or psi.

= Modulus of sand under impact loading in KPa or psi.

= Modulus of sand in KPa or psi.

= Modulus ratio; the ratio of gage modulus to soil

modulus, E

g / Enu

= Coefficient of earth pressure at rest.

= Thickness of active area of the gage in mm or

inches.

T = Total gage thickness in mm or inches.

Og = Gage stress in psi.

aa = Axial stress in psi.

v = Poisson's ratio of soil.
ACKNOWLEDGEMENT

The studies presented in this paper were sponsored by The Air

Force Armament Laboratory, Eglin Air Force Base, Florida and The

Engineering and Services Laboratory, Tyndall Air Force Base,

Florida under Contract Numbers DLYV 82-64 and 83-90119. The

support of these two organizations is gratefully acknowledged.

The authors also wish to acknowledge the valuable advice provided

by James R. Holder, Ralph McGuire, and Paul Thompson from these

organizations.

vii

1. INTRODUCTION

The measurement of stresses in a soil mass due to external loads

poses an important challenge to the geotechnical engineering

profession. To properly measure these stresses is even more

critical at present since these measurements are required for

their comparison with numerous analytical methods that can be

used for the prediction of these stresses.

There have been many studies conducted in the development and

evaluation of soil stress gages, including the work by Abbott, et

al. [1], Ingram [12], Hamilton [10], Selig [20], Triandaf il idis

[25], Krizek, et al. [15], Weiler and Kulhawy [26], and others.

Most of their efforts were concentrated on the measurement of

stresses due to static loads. An excellent review, evaluation,

and synthesis of the development of soil stress gages was given

by Weiler and Kulhawy [26],

Measurement of soil stresses due to dynamic loads is also critical for studying problems related to the response of soils and

structures buried in soils subjected to earthquake and blast

loading conditions. Many studies on this subject have been

conducted in recent years. Some examples include the work by

Baum and Kovarna [3], Burcham [6], Keough et al. [13], Labreche

et al. [16], and Tracy [24].

1
A number of factors affect stress gage performance in soils.

Weiler and Kulhawy [26] in their review separated these factors

into three areas. They are: a) stress gage properties and geometry; b) properties of the soil in which the gage is placed; and

c) environmental conditions.

Area (a) includes aspect ratio, A r , the ratio of total gage

thickness to total gage diameter, T/D; modulus ratio, ER ' t ^ie

ratio of gage modulus to soil modulus, Ea / E^ ; gage

deflection and arching; stress distribution over the gage surface; and lateral stress rotation. Researchers have long recognized that a gage with small aspect ratio, when it is emplaced in

soil, should introduce the least disruption in a uniaxial stress

field. A ratio of less than 1/5 was suggested by a Waterways

Experiment Station study in 1944 [22] and this criterion has

been followed by gage designers since. Early researchers also

recognized the fact that a stress gage stiffer than its surrounding soil will result in an over-registration in its measurement and a softer gage will otherwise under-register. Several

studies have been conducted to establish the correlation between

aspect ratio and modulus ratio [22, 17]. A classical study was

done by Loh in 1954 [17] describing the development of an internal stress gage for cementitious materials. Using the theory of

elasticity, Loh developed the relationship between the response

factor, i.e., the ratio of gage stress to axial stress, a /a

g a and the modulus ratio, E

g / E TO , for such materials. This

relationship is modified for the present study by using a typical

2
Poisson's ratio of 0.33 for sandy soil as shown in figure 1.

This figure suggests that we should expect accurate measurement

from a gage if the gage is designed with an aspect ratio not

greater than 1/10 and the modulus ratio not less than unity.

A gage with low values of A r and high values of Ep should also

minimize the development of arching in soil. Arching is the

phenomenon by which normal stresses in soil are changed into

shear stresses, thereby developing the ability of the soil to

support itself when the external support is removed. As a result,

the gage will register virtually zero stresses when arching is

developed at the interface of the gage and its overlying soil.

Normal stress distributions over gage surfaces, examined by Hvorslev [11] and called Terzaghi stress distributions, are shown in

figure 2. Figure 2 also presents the results of elastic analyses

by Monfore [18], and Peattie and Sparrow [19] to assess stress

concentrations around the outer perimeter of a gage. The figure

indicates that a fully active gage face will inevitably cause

over-registration. Thus, it is necessary to use an inactive and

stiff annular rim around the gage to reduce the sensitive area of

the gage. Monfore [ 1 8 ] suggested that the sensitive area of a gage be less than 45 percent of its total area, i.e., d ? / D*

7 <

0.45. Peattie and Sparrow [19] considered it necessary to further reduce this ratio to less than 0.25 to ensure a relatively

uniform stress distribution across the gage suface. Another

factor affecting uniform stress distribution over the gage sur3
face is the possibility of measuring point loads due to the

particulate nature of the soil. From their study, Weiler and

Kulhawy [26] suggested that the ratio of active gage diameter

to mean soil grain size, d/D , had to be 10 or greater to

keep the error from point loadings to less than 3 percent.

The amount of applied lateral stress acting normal to the gage

surface was evaluated by Askegaard [2], His findings were replotted by Weiler and Kulhawy [26] as shown in figure 3. Figure 3

shows that the Poisson's ratio of a soil is the overriding factor

that controls the amount of lateral stress transfer. For

example, a gage with an aspect ratio of 1/10 would result in

about 12 percent of lateral stress transfer for a soil with a

Poisson's ratio of 0.33; whereas the transferred lateral stress

would be less than 5 percent if the soil has a Poisson's ratio of

0.4.

Area (b) affecting stress gage performance includes the properties of a soil itself that will affect gage stress measurement.

Generally, two types of calibration tests have been conducted to

determine the gage response under static loading: The K

Q

condition and the triaxial test condition. Different soil

stress-strain relationships may be obtained from these two test

conditions, resulting in different gage stress measurements. The

method of gage emplacement in soil also affects the gage

response to the imposed load. Taylor [23] introduced the term

"pocket action" to describe the error in registration if the soil

4
surrounding a gage is placed at a different density from the rest

of the soil. Over-registration of the gage will occur if the

surrounding soil is denser; otherwise it will under-register.

Weiler and Kulhawy [26] indicated that an error of this type

amounts to 10 to 20 percent of the actual stress. Nevertheless,

the actual error in measurement is difficult to control during

field operations. Hadala [19] suggested that the simplest placement techniques are the ones most easily reproducible and so

cause the least scatter in results.

In area (c), the gage should be designed to resist corrosion as

well as moisture which the gage will inevitably encounter during

its use in the field. The gage calibration should also include

the possible change of gage response due to the change of temperature of the environment that may occur during the service

life of the gage.

When the gage is designed to be used to measure soil stresses

generated by dynamic loads, other characteristics of the gage

such as natural frequency, inertia force due to the weight of the

gage, and response time should be considered. The natural frequency of the gage should be much larger than the maximum frequency anticipated from loading to allow for reliable stress

measurement. From his study, Triandaf il idis [25] suggested that

the ratio of these two frequencies should be at least three to

five for gage design consideration. A gage which is heavier than

the soil it replaces will result in an additional inertial force

5
during dynamic loading. This effect will be especially critical

for high frequency loading conditions.

The stress-strain characteristics of a soil are affected by both

the magnitude of strain and the stress level developed in the

soil mass by dynamic loads. Figure 4 gives an example of the

effect of the stress level on a stress-strain relationship [14].

From point 2 to point 3 in the figure, soil exhibits a seven

times increase of its modulus as compared to the value from point

1 to point 2 which covers a lower stress range. Studies conducted by Shannon and Wilson and Agbabian-Jacobsen Associates

[21] reported that for small strains associated with shock type

loading, the modulus of elasticity of a soil may be as much as

five to ten times higher than that measured when the associated

strain magnitude is large.

At present, two types of gages are commonly used for in-situ soil

stress measurement during blast loading. The first type was

designed by the Waterways Experiment Station (WES) and is called

the WES-SE gage. The gage is of the diaphragm type with an

aspect ratio of about 0.1. For example, the 34.5 MPa (5000 psi)

version of the gage used in an Air Force sponsored research

conducted by the New Mexico Engineering Research Institute had a modulus of 3.3 GPa [16], The second type, the SRI gage, which is

a high modulus and low aspect ratio stress gage, has been under

extensive development by SRI International and is referred to as

the SRI gage. A detailed description of the gage is given by

6
Keough, et al. [13]. Figure 5 provides a schematic view of the

gage. The gage consists of piezoresistance foil made of Ytterbium (Yb), bonded between thin layers of an insulating material

(Kapton or epoxy). To increase the effective modulus of the

gage, the foil and insulating layers are sandwiched between

relatively thick, high modulus steel plates.

The gage NBS is developing, referred to as the NBS polymer gage,

uses different principles. Principles and information on the

calibration and testing of the gage are presented in the next

three sections.

7
RESPONSE

FACTOR,

a

g

/a

MODULUS RATIO, Er (E g/Em)

Figure 1 Theoretical Soil Stress Gage Response (Ref. 17)

8
%

OF

FREE

FIELD

STRESS

Figure 2 Stress Distribution over Stress Gage (Ref. 11)

a.

Figure 3 Theoretical Lateral Stress Rotation for

Rigid Ellipsoidal Inclusion (Ref. 2)

9
PRESSURE,

(p)

—

MPa

120

110

100

90

80

Point p, MPa K, MPa

1 0.005 0.5 100.0

2 0.020 5.0 300.0

3 0.030 25.0 2000.0

4 0.260 100.0 326.1

5 0.220 0 2500.0

Hydrostatic Behavior Dense Ottawa Sand

I

J

I

I A

/ I

' I

Chattahoochee Sand — dense^

/

/

/

/

0.05 0.10 0.15 0.20 0.2$ 0.30

EXCESS COMPRESSION — p = dvn

- dv

0.35

dvFigure 4 Computational Model of Dense Ottawa Sand

Volumetric Behavior (Ref. 14)

10
Figure 5 Basic High Modulus Configuration for the

SRI Gage (Ref. 13)

11
2. THE NBS POLYMER GAGE

2.1 The Polymer Material

Piezoelectric polymers are a relatively new kind of material for

use in measuring instruments and detecting devices. The principal

advantages of this material are their extremely good sensitivity

as a dynamic pressure detector, and a wide variety of properties

that allow them to be used in situations where conventional

sensing materials are unsuitable. These materials have high

internal damping, high natural frequency, and can be obtained

readily in thin, light, flexible sheets, in contrast to conventional piezoelectric and magnetostrictive materials, which are

stiff, dense, brittle, and relatively resonant. Its flexibility

makes this type of gage particularly attractive for its use in

geotechnical engineering problems associated with dynamic loading

conditions. For example, the gage can be placed behind

retaining walls to study lateral earth pressure distribution due

to earthquake loads, placed underneath pavement or airport runways to determine the stress distribution due to traffic and

impact loads, or located in earth embankments at different orientations to measure stress distributions within the embankment

during earthquake loading. The gage can also be installed in

soil samples in laboratory test experiments to evaluate the

stress distribution within the sample during dynamic testing.

The mechanism of piezoelectric activity in a polymer is complicated. The composition of the polymer, its division into amorphous and crystalline parts, the crystal structure, the presence

12
of surface and space charges and of ionic impurities have all

played parts in different explanations. Schematically, we may

consider the polymer to consist of long chains of identical units

called monomers. In the case of polyvinylidene fluoride (PVDF),

which is the material used in the NBS polymer gage construction,

the chains consist of linked carbon atoms. Each monomer has two

carbons, one joined to two hydrogen atoms and the other joined to

two fluorine atoms. Each monomer has a strong dipole moment.

As the material is received, the dipoles are oriented randomly as

shown in figure 6a. The material is then heated and subjected to

a strong electric field before returning to room temperature with

the applied electric field maintained during the cooling process.

This process results in the alignment of a significant number of

dipoles normal to the plane of the sheet as shown in figure 6b.

The dipoles can be considered to be stiffer than the bonds between dipoles in adjacent chains so that any stimulus that

changes the thickness of the sheet will change the surface density of charge on each surface. A compensatory flow of charges

through the circuit connecting the electrodes forms a measureable

signal as shown in figure 6c.

Two methods are currently used for manufacturing the NBS polymer

gages using PVDF material. One involves the technique of mechanically orienting a polymer sheet, followed by vacuum depositing

aluminum electrodes on the sheet, and finished by poling the

sheet in a high electric field. The other method includes

13
annealing a PVDF sheet, evaporating aluminum electrodes on the

sheet, and followed by poling the sheet in a high electric field.

Detailed descriptions of the gage fabrication are beyond the

scope of this report; however, they can be found in publications

by Bur and Tsao [5] and DeReggi [7] of the National Bureau of

Standards.

2.2 Previous Studies Using The Gage for Dynamic (Impact)

Pressure Measurement

The polymer gage has been used in several studies to measure

dynamic pressure developed by impact loads. In a 1975 study by

DeReggi [7], the polymer gage was designed, constructed, and

calibrated for recording pressure transients developed over the

interface between two bodies as a result of impact. The measurement of interface pressure is of interest in studying injuries

sustained in vehicular crashes. The intended primary response

of the gage is to compression in the thickness direction, which

is produced by either hydrostatic or normal pressure. Two

methods of testing were developed to measure the gage response.

The first method uses a mechanical press for static calibration

of gage output under approximating uniaxial compression. The

second method uses a drop-test machine for the measurement of

gage output under conditions in which pressure and flexure are

simultaneously applied.

Another study by Burcham [6] devoted its effort to evaluate the

following aspects of the gage; a) charge/stress sensitivity under

14
dynamic loading; b) survivability under repeated loading? c)

survivability of thermal excursion, and d) response factor in

sand. Tasks a) through c) were carried out in a series of

dynamic tests using a dynamic test cell. The cell was filled

with hydraulic oil and a plunger was employed to introduce a hydrostatic pressure step with a 10 ms rise time. Task d) was

accomplished by embedding a polymer gage in sand inside a vertical shock tube. The rise time in this test was about 10 s. Results of Burcham's study indicated that the gage showed a very stable charge/stress sensitivity for a hydrostatic pressure

range from 0 to 2000 psi. Compared to a Norwood reference gage

in the same dynamic test cell set up, the difference in pressure

response measured by these two gages was on an average 3 percent.

However, a larger difference was observed from the results of

tests under elevated temperature conditions. A test conducted at

97o F showed that the pressure registered by the polymer gage was

about 19 percent higher than that measured by the Norwood

reference gage. Additional tests following the elevated temperature test indicated that the difference in measurement between

these two gages returned to 3 percent? i.e., the increase in

charge sensitivity due to elevated temperature had no lasting

effect. Shock tube tests conducted by emplacing a polymer gage

in sand resulted in a response factor of 0.963, which was

expected for a gage of this type with an aspect ratio of 0.002.

Burcham concluded further that gage placement for a polymer gage

is not as critical as it is for rigid gages with smaller aspect

ratios, e.g., the WES-SE gage.

15
Two test programs were carried out by Tracy [24] using polymer

gages. One program placed a bilaminated polymer gage in a

Hopkinson Bar test set up to evaluate gage response to various

impact velocities. The other program used a shaking table to

study the gage response to various frequencies and higher g-load

forces. Resistive type strain gages were used as reference gages

during testing. It was found that the polymer gage produces an

output signal, measured in volts, of over 500 times that of the

resistive strain gage, indicating its superior charge/stress

sensitivity. A good correlation between the polymer gage

response and the measured impact velocity, as well as the resistive strain gage output was also established. Results of the

shaking table tests indicated a linear relationship between the

gage output and g-loads for a range of values from 2 g to 21 g,

when the table was shaken at a constant frequency of 2000 Hz.

2.3 Gage Design and Calibration for the Present Study

Polymer gages used in the present study were fabricated in the

laboratory of the Polymers Division of the National Bureau of

Standards. In the current design, the pyroelectric characteristics of the polymer gage have also been considered in developing

a temperature compensated pressure gage. Details of this development are given in References [4a, 4b, 4c, and 4d], It should be

noted that the gage design objective is to develop the gage to

properly measure both air pressures and soil stresses due to

blast loading. The pyroelectric response of the gage is produced

by adiabatic heating of the gage itself and its surroundings due

16
to the imposed pressure pulses. Adiabatic heating of the PVDF

will coincide with its pressure change, i.e., there will be no

time delayof a thermal energy pulse with respecttothepressur

e

pulse. When adiabatic heating of the surrounding medium occurs,

the thermal time constant for diffusion of heat into the gage

will determine its pyroelectric response as a function of time.

If the temperature changes in PVDF and its surroundings are

identical, then there will be no heat transfer and the pyroelectric response will be due to adiabatic heating of the PVDF

only. Calculation for this case shows that the pyroelectric

charge signal is approximately 8 percent of the piezoelectric

charge [8].

In general, adiabatic heating of both PVDF and its surroundings

along with the time dependent nature must be considered. For

the PVDF gages used in this study, the time constant for heat to

diffuse into the gage from the surroundings is approximately 30

ms. Therefore, for pressure measurements in a time frame much

less than the time constant of the PVDF material, temperature

compensation can be achieved by applying an 8 percent correction

factor to the gage signals [4d]. For longer times, the

conduction of heat from the surroundings must be measured in

order to apply the appropriate correction factor to the output

signal of the gage.

This method of temperature compensation was developed using the

following approach: a) measure the temperature change of the PVDF

17
gage using a thermocouple with a fast response time; b) amplify

the thermocouple voltage to equal that generated by the pyroelectric response of the gage; c) add the gage voltage to the amplified thermocouple voltage yielding a corrected gage voltage. A

compensation amplifier was developed specifically for this purpose and was used in all the tests presented later in this report.

Temperature compensation is important if air pressure measurement

is of interest. In an experiment conducted in a triaxial soil

test chamber where the chamber was pressurized to 100 psi r the

temperaturein the air chamber rose from 70° F to a peak of 83° F.

Thechamber temperature returned to 70° F after about 70 minutes

of observation when the chamber pressure was maintained at 100

psi. On the other hand, the thermocouple within the soil sample

which was placed in the chamber indicated only a small rise of

temperature from 70° F to a peak of 72.5° F after 25minutesof

observation. As it will be shown later in this report, the temperature rise of the PVDF gage embedded in soil due to impact

loading is even smaller being a maximum of about 0.6 °F. As a

result, no measureable difference between the corrected and uncorrected output signals from the polymer gage can be found for

the range of stresses (up to about 2000 psi) introduced to the

polymer gage.

Several versions of polymer gages were fabricated. The difference

in each design evolved from the need to improve the gage design

following Geotechnical Engineering Laboratory testing. In all

18
cases, the gage consists of a basic unit called "bilaminate

construction" with a thermocouple (figure 7). It is made from

two sheets of PVDF which are laminated together using epoxy.

Each sheet (0.012 or 0.025 mm thickness) contains an active area

on which aluminum electrodes are deposited. The active area is

10 mm (d) with an overall gage diameter of 15 mm (D). A

copper-constantan thermocouple junction, made of 0.075 mm wire,

is placed between the two sheets and within 2 mm of the active

area of the gage. The overall thickness of the active area of

the gage in a bilaminate construction is approximately 0.1 mm

(t). This gage design will be referred to as Type A in the presentation.

Prior to lamination the electroded regions are made piezoelectr ically active by poling them at room temperature with an electric

field of 2 megavolts per centimeter (MV/cm). The active areas are

then laminated face to face so that the polarization vectors in

each element point in opposite directions. In this bilaminate

pattern, the ground electrodes are on the exterior surface and

the inner electrodes carry the signal potential. The advantage

of this design is that signals generated in the two elements by

bending are opposite in polarity and add to zero.

Other gage designs are given in figures 8 and 9. Figure 8 shows

a Type B gage which is a "bilaminately constructed" gage sandwiched by two additional layers of PVDF of equal thickness to

serve as protection for the inner two layers so that the gage can

be used in environments requiring mechanical ruggedness. The

19
Type C gage is similar to Type B but indium rather than aluminum

is used as an electrode because of its better ductility. Figure

9 shows the final design of the gage. The protective covers used

in this design are 0.125 mm polycarbonate sheets (Type D gage).

The Type E gage is similar to Type D except that 0.250 mm polycarbonate sheets were used. As shown in figure 9, Type D and E

gages use both aluminum and indium as electrodes. The overall

thickness of the gage in its final design is about 0.35 mm (T).

Figure 10 shows a picture of an NBS polymer gage in its latest

design.

A check of the polymer gage in its present design against the

criteria for gage design presented and discussed earlier in this

report is of interest. To do this, we have selected the

following information related to the characteristics of the gage

and its surrounding medium:

The_NB S_Pp_lyme E_gage L

d = diameter of the active area = 10 mm = 0.39 in

D = overall diameter = 15 mm = 0.59 in

t = thickness containing the active elements = 0.100 mm

= 0.004 in

T = overall thickness = 0.350 mm - 0.014 in

E

gp = modulus of PVDF = 106 KPa = 1.5 x 105 psi

Egc = modulus of polycarbonate sheet = 107 KPa

y (used in computation) = 1.5 x 10g pS ^

20
Surrounding medium : Typical modulus values of sandy soils

as used in the computation [27]

Ems = modulus of sand A = 2500 psi (1.7 x 10, KPa) at loose state

= 9000 psi (6.2 x loj KPa) at dense state

= 5700 psi (4.0 x 10 KPa) as an average and typical

value

5 Emd = 57,000 psi (4 x 10 KPa) under impact loading

= Poisson's ratio = 0.33 as a typical value

d50 = mean soil grain size = 0.3 mm = 0.012 in

Parameters required for checking are computed as below:

eR = Egc/Ems = 1.5 x 10

6

/ 5.7 x 10^ =260

Er = Egc/Emd - 1.5 x 10 / 5.7 x 10 =26

(if modulus under impact loading is used)

A^. = T/D = 0.014" / 0.59" = 0.024

d

2

/ D

2 = 0.392

/ 0.59

2 = 0.44 < 0.45

d / D

50 = 0.39" / 0.012" = 33 > 10

Referring to figure l,a gage with an aspect ratio, Ar , of 0.024

and a modulus ratio, Ep ,of either 260 or 26, will warrant a response factor, a g/

a a , of close to unity. The ratio between the

sensitive area of the gage and the total gage area falls slightly

below 0.45, which is satisfactory according to the criterion

recommended by Monfore [18], It should be noted that the dimensions of both d and D in the current gage design were decided

upon for the convenience of soil testing. The active area of the

gage can be made to any shape and any size to fit the specific

21
need for using the gage. Likewise, the total area of the gage

can also vary, reducing the ratio of d /D ' to a valve much lower

than 0.45. The ratio of d/D 50 is 33 for the present gage and for

the sandy soil from Florida used in testing. This value satisfies

the minimum value of 10 recommended by Weiler and Kulhawy [26],

I#

Several types of tests were performed on each gage in the laboratory of the Polymers' Division prior to its testing in the

Geotechnical Engineering Laboratory. Details of these tests are

presented in an NBS report by Bur and Roth [ 4 d ] . These tests are

briefly described below.

Two experimental tests were devised in an attempt to simulate

the onset of a thermal pulse at the site of the gage. The objective of these two tests was to determine the amount of temperature compensation attainable using a specially designed circuit.

In the first test, which is called a water immersion test, a gage

with an encapsulated thermocouple is immersed into a cold water

bath by slapping the broadside surface of a gage against the

surface of the water. In doing so, the surface of the gage in

contact with the water is held at the bath temperature and heat

diffuses in such a way that the gage reaches a temperature equal

to that of the bath. In the second test, which is called a light

flash experiment, the surface of the gage was sprayed with a dull

black paint which serves as a light absorber. A light pulse was

then directed at the surface of the gage and a pulse of thermal

energy was imparted to the surface of the gage which subsequently

22
diffused into the gage. At longer time intervals the gage has an

average temperature which is higher than that of its surroundings. The water immersion test approximates the physical conditions present when the gage is buried in soil, i.e. r the gage

maintains contact with the soil which acts as a thermal reservoir. The light flash experiment approximates conditions relevant

to the gage in air.

Gages were also tested in an oil pressure cell to obtain the

value of the hydrostatic piezoelectric constant,d^ . The oil

bath test set up is shown schematically in figure 11. The value

of d h was evaluated for dynamic pressure pulses having a pulse

3

width of approximately 10 ms and peak pressures between 7 x 10

and 2.1 x 10 4 KPa (1000 and 3000 psi). The presure cell consists of a stainless steel block, 7.5 cm wide, 16 cm long and

15 cm high (3 in. by 6-1/4 in. by 6 in.), with a 1.9 cm bore traversing the 16 cm direction. At one end of the bore, the reference transducer is placed. At the other end, feedthrough connectors carry signals from the PVDF gage and thermocouple,

plunger fits as a piston into a 2.54 cm bore with vertical orientation which intersects with the horizontal bore. In preparation

for measurements, the cell is filled with either a vacuum pump

oil or an alkyl-benzene dielectric oil. Care was taken to remove

all visible bubbles from the oil, but the oil was not degased.

During the test, the cell was placed at the bottom of a vertical

column which guides the drop weight as it falls onto the plunger.

23
The PVDF gage is placed in the oil pressure cell at room temperature and a pressure pulse is initiated by dropping a 16 kg (35

lb) mass through a distance of approximately 30 cm onto the

plunger in the cell. Signals from an accelerometer, which is

attached to the top of the drop weight, the gage, the reference

presure transducer, and the thermocouple are captured individually by a transient recorder signal processor and are used to

compute the hydrostatic piezoelectric constant, d . A typical

plot between the temperature compensated pressure pulse from the

polymer gage and that from the reference transducer as a function

of time are given in figure 12.

24
H F

The Monomer

(c)

Figure 6 The Polymer Material - Polyvinylidene

Fluoride (PVDF)

25
Active area

Hi electrode: Evaporated aluminum

Ground electrode: Evaporated aluminum or silver paint

Figure 7 The Polymer Gage - Bilaminate Construction

(Type A Gage)

26
Figure 8 The Polymer Gage - Bilaminate Construction with

PVDF Sheets as Protective Covers (Type B Gage)

27
Active area 10mm in dia.

Polycarbonate sheet 0.125 to 0.250mm — |— Indium, 10_5mm

Thermocouples

Aluminum, 10“5mm

Indium —Aluminum

V V V DVDF

\ \

Polycarbonate sheet

0.025mm = (—Aluminum — '— Indium

0.125 to 0.250mm

Figure 9 The Polymer Gage - Bilaminate Construction with

Polycarbonate Sheets as Protective Covers (Type

D and E Gages

)

28
Figure 10 An NBS Polymer Gage at Its Latest Design

(April 1985)

29
Drop Test

Plunger

Steel Block

Reference Gage

PVDF Gage

Figure 11 Drop Test Set-up

30
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31
3. GEOTECHNICAL ENGINEERING LABORATORY TEST PROGRAM

3.1 Soil Used in Testing

A sandy soil shipped from Eglin Air Force Base in Florida was

used as the material in the test program and is referred to as

"Florida Sand" in this report. It was planned that the gages be

field tested at Eglin Air Force Base once their calibration and

testing in a laboratory environment were completed. The soil is

reddish brown in color and was slightly moist when it was received with an average natural moisture content of 6.0 percent.

A gradation curve is shown in figure 13 which describes the soil

as being a fine to medium sand, a SP-SM material according to

the Unified Soil Classification System containing a small amount

of fine material. Compaction tests using ASTM test procedure

D698 were conducted to develop the compaction curve shown in

figure 14. The Florida sand has a maximum dry density of 114.5

pcf at an optimum moisture content of 12.5 percent.

3.2 Test Variables and Test Program

The test program carried out in the Geotechnical Engineering

Laboratory is summarized in table 1. The way table 1 is organized and presented also illustrates the developmental nature

of the project. A number of test variables were evaluated in

the program including gage type, gage location, type of protective covering, effect of frequency, effect of cable length, and

effect of mold insulation.

32
A polymer gage was placed under two different conditions within a test cylinder. In the first case, the gage was placed on a concrete pedestal covered by Florida sand to evaluate gage response

when the gage is placed against the exterior wall of a buried

concrete structure. In the second case, the gage was placed on a compacted surface of Florida sand and backfilled by the same soil

to simulate the free field condition for gage response study.

The longer the coaxial cable, the higher the capacitance. The

length of cable connecting the gage, from the test cylinder to

the compensation amplifier, could have some effect on the measured gage response. Concern regarding this problem is certainly

valid considering that only a few feet of cable are normally used

in the laboratory whereas several hundred feet of cable will be

required to carry out the same measurement in a field test set

up. In this test program, we have used a 150 foot long cable to

conduct some tests and compared these results with those obtained

using only 3 feet of cable to determine the possible effects of

cable length.

It was also thought that the high thermal conductivity of the

aluminum test cylinder could affect the amount of heat buildup

inside the soil-gage assembly, thus affecting the soil stress

measurement using the polymer gage. For this reason, tests were

conducted early in the program by having the test cylinder wrapped with and without insulation tape. These tests were conducted

using the Type B gage without any additional protective covers.

33
We should mention here that several Type A gages were used to

start the program. Although this type of gage offers the highest

charge/stress sensitivity, it is also the most vulnearable type

to damage due to indentations on the gage by sand grains. As a

result, all Type A gages were damaged without completing a test.

The types of gages used in testing are given in table 2, which is

a brief summary of that presented earlier. As a result of Type A

gage testing, we decided that the gage surfaces needed additional

protection. The Type B gage, which is a Type A gage with two

layersof PVDF material, was thus developed and tested for its

ruggedness and response. It was found that the added polymer

sheets are not rugged enough to survive the impact loads applied

to the soil/concrete pedestal column. Additional gage protection

was required which led to experimentation with other types of

protective measures as given in table 3, including the use of

dry bentonite material, stainless steel shims, hardened steel

shims, and photo negative films in addition to the 2 PVDF sheets

in the Type B gage design. These protective covers were used to

sandwich the gage in the test set up. Both the stainless and

hardened steel covers did not work well in serving their intended

function. The gages tested using the stainless steel shims were

easily short-circuited during testing. This result is not surprising when the shims were viewed after testing. Numerous

indentations by sand grain penetration were evident on the shim.

Hardened steel shims were also considered unsuitable for gage

surface protection since they are too rigid, making them incom

34
patible with the PVDF material. Results from Test 25 (a) indicated that the gage was short-circuited because the lead of the

gage was cut by the shims at their perimeter during impact load

application.

The use of photo negative film was successful. This led to the

present design using polycarbonate sheets of various thicknesses

for gage protection (Types D and E gages). Most of the gages

tested are Type D, which use two 0.125 mm thick polycarbonate

sheets. The Type E gage design is identical to the Type D gage

except that polycarbonate sheets of 0.250 mm are used as protective covers.

It should be noted that the use of other materials as electrodes

in gage design was also carefully examined. Aluminum was in the

original design; however, it was found that the aluminum is too

brittle under impact loading. Indium, which is similar to aluminum in chemical properties, was thus introduced during gage

fabrication because of its better ductile nature. It was thought

a ductile material should have a better chance to heal after

impact loading, thus improving the longevity of the gage. What

we found was that indium flows too much when it is compressed.

Furthermore, indium does not work as well as aluminum does with

the PVDF material during gage fabrication. To take advantage of

each material, both aluminum and indium are used in the final

gage design, as shown in figure 9 where aluminum is deposited

over the PVDF sheet and is covered by a layer of indium.

35
3.3 Test Set Up and Data Recording

All tests were conducted in a test set up, shown in figure 15

which includes a test cylinder, where the polymer gage is

located, with impact loads being applied with a load piston from

a MTS dynamic testing machine; an electronic control console

which controls input magnitude and frequency of the impact load,

and records the magnitude of specimen displacement; a Gould

amplifier system connected to a multi-channel strip chart

recorder to record all input and output signals from the test. A

closeup view of the test assembly including the test cylinder is

shown in figure 16. It shows a black paint coated gage coming

out of the test cylinder connected to a compensation amplifier

box placed next to the test assembly. The stainless steel cylindrical block is used to raise the elevation of the test assembly

to facilitate testing.

A schematicdrawing is given in figure 17 to show in detail the

test cylinder and its accessories. The assembly consists of a

10 cm by 10 cm base plate with a 2.19 cm diameter groove made in

the middle of the plate to accomodate an O-ring. A test cylinder

(inner ring) 2.19 cm in diameter and 4.10 cm in height is seated

over the O-ring. An outer ring 4.75 cm in diameter is placed

over the test cylinder resting in another groove made on the base

plate. Aluminum is used in constructing these parts. The outer

ring along with foam cut to fill the space between the outer and

inner rings is used to hold the test cylinder in place during

test preparation and testing. A slot approximately 18 mm wide

36
was cut on the surface of the test cylinder to allow for

insertion and extraction of the polymer gage before and after

testing.

In testing a gage on a concrete pedestal, the pedestal was

placed into the test cylinder first. The pedestal was prepared

to have the same height as that of the slot location on the test

cylinder. When protective covering of types 2 through 5 were

used in the experiment, one cover was constructed or placed

immediately over the surface of the concrete pedestal. A gage

was then inserted through the slot and placed over the protective

cover and another layer of protection was then constructed or

placed. For tests using the final gage design, i.e., with polycarbonate sheets used as a protective cover, the gage was simply

inserted through the slot to rest on the pedestal. Florida sand

was carefully poured over the pedestal in small quantities and

compacted by means of the loading rod connected to the load

piston, using the static load application capability of the MTS

test machine. The compacted soil surface was leveled off at the

top of the test cylinder.

To simulate the free field condition, Florida sand was poured

into the test cylinder and compacted in steps until the sand

surface reached the level where the slot is located. The gage

with its protective measures was then placed in the test cylinder

with the rest of the space inside the cylinder being backfilled

by the compacted sand in the same manner as described before.

37
Some words should be said about the density of sand placed in the

test cylinder. At first, we planned to compact the sand at its

natural moisture content to a density of about 105 pcf in accordance with the compactive curve in figure 14. However, we found

that this was very difficult to achieve due to the small amount

of soil needed to be compacted into place. Through trial and

error, we have managed successfully to consistently prepare soil

specimens to a dry density of 92 to 95 pcf at its natural

moisture content.

The loading rod was then lowered to make contact with the compacted soil surface in the test cylinder in preparation for

impact load application (refer to figure 16). The gage was then

connected to the input port on the compensation amplifier through

the use of microdot connectors and a 2 mm diameter co-axial

cable. The thermocouple of the gage was also connected to the

amplifier for temperature measurement and compensation. The

impact load, which consists of a half cycle inverted sine

waveform, was dialed in through the use of the MTS test machine.

The magnitude of the load acting on the soil surface as well as

the displacement of the specimen were recorded on channels 3 and

4 of the strip chart recorder, respectively. Outputs from the

compensation amplifier which include the corrected and uncorrected gage responses with respect to temperature and the

temperature measurement were recorded on channels 1, 2, and 5 of

the strip chart recorder, respectively. A typical strip chart

output from an impact load test is shown in figure 18. The

38
frequency of the inverted sine waveform shown in figure 18 is 50

Hz, which is the frequency employed in most tests. Other frequencies ranging from 5 to 40 Hz were also used to study the

effect of frequency on gage response. We should mention here

that the temperature in this typical plot probably represents the

most significant recorded thermocouple response in all the tests

conducted and was estimated to be at about 0.6 ° F. Also, the

difference in voltage recorded for the corrected and

uncorrected gage signals is difficult to appreciate. Although

the corrected gage responses were used for the plots in this

report, we believe that gages without a built-in thermocouple

should also yield good dynamic stress measurement in a soil

environment.

The sequence of impact load application on each specimen is given

below: An impact load at 50 Hz frequency which generated about

500 psi on the soil surface was dialed in through the control

console of the MTS test machine and a recording such as that

shown in figure 18 was obtained. This represents one data point

on the plots shown in figures 19 through 31 in the next section

of this report. The magnitude of impact loading was increased in

steps of about 200 psi to develop a maximum input stress between

1000 and 1500 psi to study the gage response during loading. The

magnitude of impact loading was then reduced in decrements of 200

psi to an equivalent input stress level of about 20 psi to study

its characteristics during unloading. In most of the tests

conducted, reloading characteristics of the gage were also eva39
luated by again increasing the magnitude of impact loading in

increments back to the magnitude where the test was begun

completing the cycle of testing for an individual specimen. All

of these impact loads were applied at a frequency of 50 Hz.

The possible effect of frequency on gage response was studied on

some test specimens when the above-mentioned loading sequence was

completed. The study was accomplished by varying the frequency

of the half cycle inverted sine waveform at 5, 10, 20, 30, and 40

Hz while maintaining the same dial setting for the impact load

application.

40
Table 1 Summary of Test Program

a/ b/

Gage

No.

Gage

Type

Test

No.

Gage

Location

Type of Protective Cover

with

Cable

without

Cable Remark

AF-31 B 12 on 1 X not

concrete insulated

pedestal

13 n 1 X

n 14 n 1 X insulated

AF-30 B 15 n 2 X

n 16 II 2 X

if 17 n 2 X - 18 n 2 X - 19 n 5 X - 20 n 5 X —

AF-43 C 22 it 5 X — 24 it 5 X - 25 it 5 X - 25(a) n 4 X gage shortcircuited

AF-44 C 25(b) II 3 X

n

PC-49 D 28 n 6 X — 29 II 6 X - 30 n 6 X - 43 n 6 X gage re-epoxyed

PC-54 D 32 n 6 X — 33 ii 6 X —

PC-62 D 34 it 6 X exhibited non35 it 6 X linearity from

36 n 6 X zero to 400psi

37 n 6 X

n

PC-64 D 38 in soil 6 X

39 n 6 X - 40 n 6 X - 42 on 6 X - concrete

pedestal

PC-58 E 44 n 7 X — 45 in soil 7 X -

a/ see table 2

b/ see table 3

41
Table 2 Gage Types

Description

Bilaminate Construction, i.e., 2 layers of PVDF;

aluminum as an electrode

Bilaminate Construction plus 2 layers of PVDF

as a protective covering. Coated with black

paint; aluminum as an electrode

Same as Type B except that indium was used as

an elctrode

Bilaminate Construction plus 2 @ 0.125 mm polycarbonate sheets as a protective covering;

aluminum and indium used as electrodes

Same as Type D except that the polycarbon sheets

used are 0.250 mm in thickness
Table 3 Types of Protective Coverings Experi

mented with in the Test Program

Type Thickness Description

1

2

3

4

5

6

7

No protective covering other than the 2

PVDF sheets used in the Type B gage

design

2.0 mm Two bentonite layers as additional

protection

0.074 mm Two stainless steel shims as additional

protection

0.126 mm Two hardened steel shims as additional

protection

0.182 mm Two photo negative films as additional

protection

0.125 mm Two polycarbonate sheets in Type D gage

design

0.250 mm Two polycarbonate sheets in Type E gage

design

43
PERCENT

PASSING

BY

WEIGHT

GRAVEL SAND

SILT OR CLAY Coarse Fine Coarse | Medium Fine

Diameter

UNIFIED SOIL CLASSIFICATION SYSTEM

U.S. STANDARD SIEVE SIZE

3

" 1 - 1 / 2" 3/ 4" 3/ 8

" 4 10 20 40 60 100 200

GRAIN SIZE IN MILLIMETERS

Figure 13 Grain Size Distribution of Florida Sand

44
CRY

DENSITY,

7

dry

,

ocf

Figure I Compaction Curve of F'iorirJa CanC

45
Figure 15 NBS Polymer Gage Test Set-up in The

Geotechnical Engineering Laboratory

46
Figure 16 Close-up View of The NBS Polymer Gage

Test Set-up

47
To MTS

machine

Unit in Ceti meter

Figure 17 Schematic Drawing of The Test Cylinder

and Its Accessories

48
uncorrected

corrected

time,

ms

temperature

displacement

input

load

gage

response

gage

response

RESPONSE OF POLYMER GAGE TO DYNAMIC LOAD (GAGE ON

CONCRETE PEDESTAL AND BURIED IN SOIL) Test 14, Run 12

0 \2S mS

ill

49

Figure

18

Typical

Strip

Chart

Output

from

Impact

Load

Testing

of

the

NBS

Polymer

Gage
4. PRESENTATION, INTERPRETATION, AND DISCUSSION OF TEST RESULTS

Results from all the tests listed in table 1 are given in figures

19 through 33. All of these figures were presented in the same

format to establish the relationship between input stress in psi

(calculated from the load trace recorded on channel 3 of the

strip chart recorder) and corrected output of the polymer gage in

volts (calculated from the voltage trace shown on channel 1 of

the strip chart recorder).

Tests 12, 13, and 14 were conducted to evaluate the effect of

test cylinder insulation on gage response and their results are

given in figure 19. Tests 12 and 13 were run without insulation

whereas Test 14 was carried out by insulating the inside of the

test cylinder with duct tape. AF Gage #31 is a Type B gage and

no additional protective covering was used in testing. Figure

19 shows that no difference in gage response resulted from the

insulation of the test cylinder. Subsequent tests other than

Tests 15 and 16 were conducted without insulating the test

cylinder

Gage response using bentonite layers as a protective covering are

shown in figures 20 and 21. Both figures also serve as good

examples for the loading sequence in each test specimen. Each

point represents one particular impact load application and each

curve shows the loading-unloading segments of a test. No

reloading was carried out in these four tests; however, it was

50
carried out in the rest of the test program. Results shown in

figures 20 and 21 indicate the sensitive nature of gage response

as a function of input stress when the bentonite material is used

for gage protection. A loop can be traced following the loadingunloading sequence in each individual test. A possible explanation for this observed phenomenon is that the voltage measured

from the gage during initial loading may be envisioned as a gross

measure of the gage response to point loads due to the particulate nature of the dry bentonite soil used as the protective

cover. Only after the bentonite layers have been tightly

compacted to provide uniform contact with the gage surface can

the measured gage response be considered to properly represent

the level of stress on the gage surface (i.e., during the

unloading segment of the curve). By examining the characteristics of the curves shown in both figures 20 and 21, we may

conclude once again that test cylinder insulation has no effect

on the gage response.

We concluded from figures 20 and 21 that the use of thin

bentonite soil for gage protection is not acceptable and other

types of protective measures should be explored. In fact, the

use of a bentonite layer is also not practical for field application even if laboratory test results turned out to be acceptable.

Figures 22 and 23 present test results using photo negative films

as a protective covering. All five tests were conducted by

placing the gage on a concrete pedestal backfilled with Florida

sand. Gage AF #30 is a Type B gage which uses aluminum as its

51
electrode while the electrode for Gage AF #43 is indium. Another

difference in the data presented in these two figures is that in

tests 22, 24, and 25, silicone grease was used in between the

gage and its protective covering. Although there is a small

degree of scatter in the data developed from these tests, a linear relationship between the input stress and the corrected

gage output can be established for both data sets as shown in the

figures. The use of silicone grease to lubricate the interfaces

apparently has no effect on improving the test results.

Favorable results from the tests using photo negative films to

protect gage surfaces led to the final gage design of gage Types

D and E. Four gages. Gages PC #49, #54, #64, and #58, were

tested to determine the calibration factors for their use in

field testing. The first three gages are Type D gages and the

last one is a Type E gage. Results of Gage PC #49 testing are

shown in figure 24. The gage was placed on a concrete pedestal

in all four tests. Tests 28, 29, and 30 were conducted using a 3

foot cable whereas Test 43 was conducted using a 150 foot long

cable. It should also be noted that the gage was delaminated

between one polycarbonate sheet and the PVDF layer at the end of

Test 30. The gage was re-epoxyed and tested again as Test 43 to

detect whether this repair process would cause any change in gage

response. We were very encouraged to observe from figure 23 that

neither the cable length nor the re-epoxy process of the gage has

any effect on the gage response, i.e., a linear relationship can

still be established from all the data points obtained from all

52
four tests. This finding can be further substantiated by the

plot of figure 25 showing the results of Gage PC #54 testing with

and without the use of the 150 foot cable.

An important test variable that had to be investigated is whether

the response of a gage placed in a free field condition would be

different from that of the gage placed against a rigid concrete

surface and covered by soil. Figures 26 and 27 plotted from the

results of Gages PC #64 and #58 testing, respectively, provide

the answer to this question. It can be seen that for the same

load setting dialed in through the MTS test machine, a much

greater stress was developed in the specimen when the concrete

pedestal was present, resulting in a much higher gage response.

In general, the stress generated in the presence of a concrete

pedestal is about 2.5 times of that without a pedestal. Nevertheless, all data points are falling into a narrow band and a linear relationship between the input stress and the corrected

gage output can be retained. This finding is very important

because it means that it is possible to test the gage under one

condition in the laboratory and the relationship established from

that can be satisfactorily used to determine the magnitude of

stress for the other case. The linear relationship established

in both figures 26 and 27 also demonstrates that an increase of

the thickness of the polycarbonate sheets from 0.125 mm in Gage

PC #64 to 0.250 mm in Gage PC #58 does not invalidate this linear

correlation. However, our assessment is that the Type D gage

design which uses a 0.125 mm polycarbonate sheet offers sufficient

53
ruggedness to withstand impact stresses up to 2000 psi achieved

in the test program. Observing the gage surfaces after testing

of the Type D gages indicates that only minor degrees of indentation were noticed on the gage surface, therefore, there is no

need to increase the thickness of the protective covering unless

higher impact load testing planned in the future dictates the

change

Results of the impact load tests conducted on four gages at

various frequencies ranging from 5 to 50 Hz are given in figures

28 through 31. These figures demonstrate the validity of the

established linear relationship between the two variables of

interest, which are not affected by the magnitude of frequency or

the impact load used in testing.

Figure 32 summarizes the test results given in figures 24 through

27. The calibrated results for these four gages are tabulated in

the figure in two different units, i.e., volt/psi and pC/psi, to

facilitate their use in the field, depending on the type of

instruments used for data acquisition.

Finally, results of four tests conducted on Gage PC #62, which is

a Type D gage, are given in figure 33 to illustrate the

occasional non-linear nature of the gage response. The test

numbers are given in accordance with the sequence of testing.

Note that, for all four specimens tested using the same gage,

the gage/stress sensitivity was reduced slightly after each

54
specimen testing. Also, a straight line can be drawn for the

data points from each specimen testing for all data points above

about 400 psi. Below 400 psi, the gage showed non-linear

response, i.e., all data from the four specimens tested merged

toward the origin. The reasons for exhibiting this non-linearity

is unclear. This matter should be investigated in future studies.

55
3 CO

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Test No.

a 12 Test cylinder not insulated

v 13 Test cylinder not insulated

14 Test cylinder insulated

w/o 150' cable

w/o protective covers

x x X X 1 X X JL X

500 1000

INPUT STRESS, oa , psi

1500

Figure 19 NBS Polymer Gage Test Results - Effect of

Test Cylinder Insulation

56
i—i—

—1—i

t

r Gage AF #30

Gage on concrete pedestal

w/o 150' cable

2 at 2mm bentonite layers used as

protective covers

Inside of test cylinder wrapped with

duct tape

50 Hz loading

1 I I I I I L 500 1000

INPUT STRESS, aa , psi

1500

Figure 20 NBS Polymer Gage Test Results -- Bentonite

Layers Used as a Protective Covering with

The Test Cylinder Insulated

2000

57
Figure 21 NBS Polymer Gage Test Results - Bentonite

Layers Used as a Protective Covering with

The Test Cylinder Not Insulated

58
Figure 22 NBS Polymer Gage Test Results - Photo Negative

Films Used as Protective Covers

59
ID CO

Cl h- H -J ZD O

O >

LU LU £ < ho

o

>

Q

LUH

O

LU

QC

QC

O

O

CO

CO

LUQCH

CO2

o

QC

500 1000 1500 2000

INPUT STRESS, oa , psl

Figure 23 NBS Polymer Gage Test Results - Photo Negative

Films Used as Protective Covers

60
Figure 24 NBS Polymer Gage Test Results - Effect

of Cable Length

61
Figure 25 NBS Polymer Gage Test Results - Effect of

Cable Length

62
Figure 26 NBS Polymer Gage Test Results - Effect of

Gage Location

63
Figure 27 NBS Polymer Gage Test Results - Effect of

Gage Location

64
Figure 28 NBS Polymer Gage Test Results - Effect of

Frequency, Gage PC #49

65
D CO

CL J— I- _l => o

o >

UJo

<

h-_l

o

>

Q

UJ

1“o

UJ

QCCCo

o

UJo

<

o

CO

CO

UJ

a:J—CO2

o

CC

Figure 29 NBS Polymer Gage Test Results - Effect of

Frequency, Gage PC #54

66
Figure 30 NBS Polymer Gage Test Results - Effect of

Frequency, Gage PC #64

67
Figure 31 NBS Polymer Gage Test Results - Effect of

Frequency, Gage PC #58

68
Figure 32 Summary of NBS Polymer Gages Calibrated

for Field Testing

69
Figure 33 Non-linear Characteristics of The Gage Exhibited

During The Testing of Gage PC #62
5. SUMMARY AND CONCLUSIONS

The following summary and conclusions can be drawn from the

development and testing of the NBS polymer gage program:

a. The gage, which is made of two thin sheets of PVDF material, a thermocouple, and two thin polycarbonate sheets for gage surface

protection, has been successfully developed to measure reliably

dynamic pressures up to 2000 psi due to impact loads.

b. The gage, in its present design, has an aspect ratio of 0.024

and a modulus ratio of at least 35, which offers excellent dimen

sional and physical requirements for accurate stress measurement.

c. The nature of the gage design further offers the flexibility

and feasibility to build gages with any aspect ratio and any

shape required for specific applications. Numerous areas of

gage application in geotechnical engineering have been identified

to measure stresses due to dynamic loading.

d. Pyroelectric characteristics of the gage material have also

been studied thoroughly to develop a temperature compensation

amplifier to correct the gage signal. The temperature compensation is necessary when the gage is used for air pressure measurement due to blasting. However, test results show that this

correction is not required when the gage is used to measure

dynamic soil stresses since the temperature rise is very small.

This leads to the conclusion that an NBS polymer gage without the

embedded thermocouple can be equally effective in measuring

dynamic soil stresses.

e. The present gage design evolved from an extensive test program in the Geotechnical Engineering Laboratory. Test variables

71
includedin the program were: test cylinder insulation;use of

long co-axial cable; gage location, i.e. whether the gage was

placed on a concrete pedestal or buried in soil; four gage types;

frequency of applied impact load; and seven different measures

used for gage protection. The impact load consisted of a half

cycle inverted sine waveform generated through an MTS load machine.

Gage response to input load through a loading-unloading-reloading

cycle was studied. A linear relationship between the input

stress and the corrected gage response can be established from

these test results as shown in figures 22 through 31. Specific

findings from the test results with respect to each variable are

given as follows:

1) Insulation of the test cylinder to prevent heat dissipation

from the test specimen where the gage was embedded has resulted

in no effect on the gage response.

2) Tests conducted with a 3 foot co-axial cable and 150 foot

co-axial cable have yielded the same results. This means that

the length of cable, which is usually in the order of several

hundreds of feet in field application, should not affect the use

of this gage for proper stress measurement.

3) Surfaces of the gage definitely need proper protection.

Test results showed that the use of bentonite layers as a protective measure are not acceptable as indicated from the plots given

in figures 20 and 21. The use of thin bentonite layers in field

application was also judged not to be feasible.

4) Other types of protective covering such as stainless steel

shims and hardened steel shims were also tested and were found to

72
be unsatisfactory for gage design.

5) The use of photo negative films as the protective cover

resulted in good gage response measurement which led to the

development of using polycarbonate sheets in the final design.

6) In the final gage design, polycarbonate sheets of two

different thicknesses were used. Test results showed that a linear relationship can be established for both cases.

7) The linear relationship between input stress and corrected

gage output holds for the gage whether it is placed on a concrete

pedestal and backfilled by soil, or embedded in soil to simulate

a free field condition. Therefore, the gage needs only to be

tested under either condition to establish the relationship.

8) Frequencies of impact loads applied to the specimen were

varied from 5 to 50 Hz, with the majority of the tests conducted

under 50 Hz loading. The results indicated that frequency has no

effect on the gage response, as shown in figures 28 through 31.

9) One gage was retested after re-epoxying a delamination which

occurred between the PVDF sheet and the polycarbonate sheet. The

test results indicated that the repair process does not affect

the gage response as shown in figure 24.

f. Results of the gage calibration can be summarized in a figure

such as that given in figure 32, which provides the calibration

factors in units of volt/psi or pC/psi to facilitate the use of

the gage for field application.

73
6. RECOMMENDED FUTURE STUDIES

A number of areas have been identified as a result of the present

research and development program with regard to the NBS polymer

gage. Additional studies are warranted to further refine the

gage for its intended use. These areas are given as follows:

a. Effect of soil type: Due to the scheduled field test program,

only the sandy soil from Eglin Air Force Base, Florida, where the

field work was scheduled, was used in testing. Other types of

soils including clay, silt, and silty sand, should be used to

determine whether the NBS gage is sensitive to soil type.

b. Effect of soil moisture content: Florida sand was tested in

its natural condition as it was received. The soil was slightly

moist with its natural moisture content being about 6 percent.

It is quite conceivable that gages may be placed in various

moisture conditions including the saturated condition in its

field application. Thus, laboratory testing of the gage by

placing the gage in soil with various amounts of moisture should

be conducted to evaluate the gage response.

c. Effect of initial placement density: Soil specimens with

densities ranging from very loose to very dense should be prepared along with different gage embedments to study whether the

gage would respond differently with respect to the placed soil

density.

d. Rise time of impact load: Rise time in field blast tests are

anticipated to be in the of order of 1 ms, which is faster than

the frequency of the input impact load signal provided through

the use of the MTS load machine. Tests should be conducted to

74
evaluate the gage response under a faster input signal.

e. High pressure gage testing: A maximum soil stress up to 2000

psi was accomplished in the present program. Gage testing and

calibration for higher pressure measurements, e.g., to 10,000

psi, should be developed. The tasks for this work should include

development of test apparatus, test procedure, and proper protective coverings. It may also require a shot tube test set up to

develop the higher pressure at a faster rise time. NBS at

present does not have shot tube test equipment that can be used

to accomplish this goal.

f. The flexibility in the polymer gage design with respect to its

dimension and shape makes this gage particularly attractive for

use to determine dynamic stress buildup and distribution in a centrifuge test environment. The gage can be miniaturized to a convenient dimension and placed in a soil bucket with almost no

addition of weight to the soil mass. Many aspects of this miniaturization process will need systematic study, including gage

fabrication and shielding, gage connection to the readout system,

and a super fast data acquisition and reduction system through

the slip-rings to accomodate the extremely fast rise time during

a centrifuge blast test set up.

75
7. REFERENCES

1. Abbott, P. S. , Simmons, K. B., Reiff, C. M., and Mitchell,

S., "Recent Soil Stress Gauge Research," Proceedings, International Symposium on Wave Propagation and Dynamic

Properties of Earth Materials, University of New Mexico,

Albuquerque, N. M. , 1967, pp. 221-238.

2. Askegaard, V., "Measurement of Pressure in Solids by Means

of Pressure Cells," Bulletin No. 17, Structural Research

Laboratoary, Technical University of Denmark, Copenhagen,

Denmark, 1963.

3. Baum, N. and Kovarna, J., "Dynamic Soil Stress - Its Calculation and Measurement," Contract report (AFWL-TR-81-89) to

Air Force Weapons Laboratory, New Mexico Engineering

Research Institute, University of New Mexico, Albuquerque,

NM, January 1982, 176p.

4a. Bur, A. J. and Roth, S., "A Polymer Pressure Gage for

Dynamic Pressure Measurements," Proc. Second Symposium on

the Interaction of Non-nuclear Munitions with Structures,

Panama City, Florida, April 1985.

4b. Bur, A. J., and Roth, S., "Measurement of A Piezoelectric

Constant for Poly (vinyl idene fluoride) Transducers Using

Pressure Pulses," Journal of Applied Physics, Vol. 57, No.

1, January 1985, pp. 113-118.

4c. Bur, A. J., and Roth, S., "Development of A Temperature

Compensated PVDF Transducer for Dynamic Pressure

Measurements," Proc. Conference on Electrical Insulation and

Dielectric Phenomena, IEEE Electrical Insulation Society,

October 1984, pp. 423-430.

4d. Bur, A. J., and Roth, S. C., "Development of a Polymer

Pressure Gage with Temperature Compensation," National

Bureau of Standards Report NBSIR 84-2862, Gaithersburg, MD,

December 1983, 31p.

5. Bur, A. J., and Tsao, A. K., "Fabrication of Ultra-Drawn

Thick Film PVDF Transducers," National Bureau of Standards

Report NBSIR 81-2418, Gaithersburg, MD, 1981.

6. Burcham, R. L. , "Evaluation of Piezoelectric Polymer SoilStress Gage," Contract report to Air Force Weapons Laboratory, Eric H. Wang Civil Engineering Research Facility,

University of New Mexico, Albuquerque, NM, July 1974, 18p.

7. DeReggi, A. S., "Piezoelectric Polymer Transducer for Impact

Pressure Measurements," National Bureau of Standards Report

NBSIR 75-740, Gaithersburg, MD, 1975, 35p.

76
8 . DeReggi, A. S., Edelman, S., and Roth, S. C., "Piezoelectric

Polymer Transducers for Dynamic Pressure Measurements,"

National Bureau of Standards Report NBSIR 76-1078, June

1976.

9. Hadala, P. F., "The Effect of Placement Method on the

Response of Soil Stress Gauges," Proceedings, International

Symposium on Wave Propagation and Dynamic Properties of

Earth Materials, University of New Mexico, Albuquerque, NM,

1967, pp. 255-263.

10. Hamilton, J. J., "Earth Pressure Cells: Design Calibration

and Performance," Technical Paper No. 109, Division of Building Research, National Research Council, Ottawa, Canada,

1960.

11. Hvorslev, M. J., "The Changeable Interaction between Soils

and Pressure Cells; Tests and Reviews at the Waterways Experiment Station," Technical Report No. S-76-7, Waterways Experiment Station, Vicksburg, MS, June 1976.

12. Ingram, J. K., "The Development of a Free Field Soil Stress

Gauge for Static and Dynamic Measurements," Instruments and

Apparatus for Soil and Rock Mechanics (STP 392), American

Society of Testing and Materials, Philadelphia, PA., 1965,

pp. 20-36.

13. Keough, D., DeCarli, P., and Rosenberg, J. T., "Development

of a High Modulus, Piezoresistance Gage for Dynamic In-situ

Soil Stress Measurement," Contract report to Defense Nuclear

Agency, Proceedings, Conference on Instrumentation for

Nuclear Weapons Effects, March 30 - April 1, 1982, Vol. 1,

pp. 36-58.

14. Korvana, J., "A Computational Model of Dry Ottawa Sand in a State of High Density," New Mexico Engineering Research Institute, Albuquerque, NM, May 1980.

15. Krizek, R. J., Farzin, M. H., Wissa, A. E. Z., and Martin,

R. T., "Evaluation of Stress Cell Performance," Journal of

The Geotechnical Engineering Division, ASCE, Vol. 100, No.

GT12 , December 1974, pp. 1275-1295.

16. Labreche, D. , Beam, J., Baum, N. , and Boor, D. , "Soil Stress

Gage Placement and Testing," Contract report (AFWL-TR-80-

126) to Air Force Weapons Laboratory, New Mexico Engineering

Research Institute, University of New Mexico, Albuquerque,

NM, March 1981, 66p.

17. Loh, Y. C., "Internal Stress Gauges for Cementitious Materials," Proceedings of The Society for Experimental Stress

Analysis, Vol. XI, No. 2, 1954, pp. 12-28.

77
18. Monfore, G. E., "An Analysis of the Stress Distribution In

and Near Stress Gauges Embedded in Elastic Solids," Structural Laboratory Report No. SP26, U. S. Bureau of Reclamation,

Denver, CO, 1950.

19. Peattie, K. R. , and Sparrow, R. W., "The Fundamental Action

of Earth Pressure Cells," Journal of the Mechanics and

Physics of Solids, Vol. 2, No. 3, April 1954, pp. 141-155.

20. Selig, E. T. , "A Review of Stress and Strain Measurement In

Soil," Proceedings, Symposium on Soil-Structure Interaction,

Tucson, AZ, 1964, pp. 172-186.

21. "Soil Behavior under Earthquake Loading Conditions," Shannon

and Wilson, Inc., and Agbabian-Jacobsen Associates, Report

prepared under subcontract to Union Carbide Corporation for

U. S. Atomic Energy Commission, January 1972.

22. "Soil Pressure Cell Investigation," Technical Memorandum No.

210-1, Waterways Experiment Station, Vicksburg, MS, 1944.

23. Taylor, D. W., "Review of Pressure Distribution Theories and

Earth Pressure Cell Investigation," Chapter 2 in Report to

U. S. Waterways Experiment Station, Vicksburg, MS, November

1945.

24. Tracy, H. A., Jr., "Piezoelectric Polymer Transducers," Contract report to The Air Force Armament Test Laboratory, May

1983, 25p.

25. Tr iandaf ilidis, G. E., "Soil-Stress Gauge Design and Evaluation," Journal of Testing and Evaluation, American Society

of Testing and Materials, Philadelphia, PA, Vol. 2, No. 3,

May 1974, pp. 146-158.

26. Weiler, W. A. and Kulhawy, F. H., "Factors Affecting Stress

Cell Measurements in Soil," Journal of The Geotechnical

Engineering Division, ASCE, Vol. 108, No. GT12, December

1982, pp. 1529-1548.

27. Winterkorn, H. F., and Fang, H. Y. (Editors) , "Foundation

Engineering Handbook," p. 567, Van Nostrand Reinhold

Company, 1975.

78
NBS-’ 14A (REV. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 85-3135

2. Performing Organ. Report No. 3. Publication Date

April 1985

4. TITLE AND SUBTITLE

Development of An NBS Polymer Gage for Dynamic Soil Stress Measurement

5. AUTHOR(S)

Chung, R. M., Bur, Anthony, J., and Reasner, E.

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. AF DLYV 82-64 & 83-90119

8. Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

The Air Force Armament Laboratory, Eg! in Air Force Base, Florida

The Engineering and Services Laboratory, Tyndall Air Force Base, Florida

10.

SUPPLEMENTARY NOTES

J Document describes a computer program; SF-185, FIPS Software Summary, Is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here)

Polymer gages developed by the National Bureau of Standards (NBS) have been

tested extensively in the NBS Geotechnical Engineering Laboratory to evaluate

their capability and reliability for use in determining dynamic soil stresses

generated by blast loadings. Penetration of soil grains into the gage surface

was found to be the major concern and a major effort was undertaken to develop

the most appropriate protective covering. Gages were dynamically loaded to develop

their corresponding calibration curves. The test results indicate that the gage

provides a linear relationship between the input stress and the gage output over

the stress range tested. The calibrated linear relationship is not affected

by the gage location, i.e., whether in soil or on a concrete pedestal; the

thickness of the protective covering; the frequency of the impact load; and the

length of the co-axial cable used in testing.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words t/ se~ cc

Blasting; Dynamic; Gage; Instrumentation; Measurement; Polymer; Soil; Stress

13.

AVAILABILITY

\XJ Uni i mi ted

| | For Official Distribution. Do Not Release to NTIS

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14. NO. OF

PRINTED PAGES

86

15. Price

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