A high temperature vacuum extensometer

Ames Laboratory Technical Reports

Ames Laboratory

7-1960

A high-temperature vacuum extensometer

J. R. Bohn

Iowa State University

Glenn Murphy

Iowa State University

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A high-temperature vacuum extensometer

Abstract

An autographic extensometer for use at temperatures up to l000°C is described. Strain information is obtained from within a high vacuum enclosure by a unique application of linear variable differential transformers which provides great versatility in strain magnification and range. Important details of the vacuum test chamber and extensometer are given. Typical high temperature stress- strain records for uranium and tantalum are included to illustrate the performance of the equipment.

Disciplines Chemistry

A HIGH-TEMPERATURE VACUUM EXTENSOMETER

by

UNCLASSIFIED

IS-16 7

Chemistry-General (UC -4) TID 4500, August 1, 1959

UNITED STATES ATOMIC ENERGY COMMISSION

Research and Development Report

A HIGH-TEMPERATURE

VACUUM EXTENSOMETER

by

J. R. Bohn and Glenn Murphy

July 1960

Arne s Laboratory at

Iowa State University of Science and Technology F . H. Spedding, Director

Contract W-7405 eng-82

2 IS-167

This report is distributed according to the category Chemistry-General (UC-4) as listed in TID-4500, August 1, 1959.

Legal Notice

This report was prepared as an account of Government sponsored work.

Neither the United States, nor the Commission, nor any person acting on behalf of the Commission:

A. Makes any warranty of representation, express or implied, with

respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not infringe privately owned rights; or

B. Assumes any liabilities with respect to the use of, or for

damages resulting from the use of any information, apparatus,

method, or process disclosed in this report.

As used in the above, "person acting on behalf of the Commission" includes any employee or contractor of the Commission, or employee of such

contractor, to the extent that such employee or contractor of the Com-mission, or employee of such contractor prepares, disseminates, or

provides access to, any information pursuant to his employment or contract

with the Commission, or his employment with such contractor.

Printed m USA. Price

$

0 .. 50 Available from the

Office of Technical Services

U. S. Department of Commerce

ABSTRACT . • • .

INTRODUCTION .

IS-167

CONTENTS

DESCRIPTION OF THE TEST FACILITY.

TEST CHAMBER ••

EXTENSOMETER •

LOAD SENSING .

. .

. .

. . . .

.

. . . .

.

. . .

.

.

.

.

'

. .

. .

.

RECORDING . . ,

PROCEDURE. .

TEST RESULTS •

CONCLUSIONS AND SUMMARY • • . . • • . • .

,

.

3

Page

5

·!;)·

7

10

11

14

15

17

19

IS-167

A HIGH-TEMPERATURE VACUUM EXTENSOMETER

J. R. Bohn and Glenn Murphy

ABSTRACT

An autographic extensometer for use at

tem-peratures up to l000°C is described. Strain

infor-mation is obtained from within a high vacuum

enclo-sure by a unique application of linear variable dif-ferential transformers which provides great

versa-tility in strain magnification and range. Important

details of the vacuum test chamber and extensometer are given. Typical high temperature stress- strain

records for uranium and tantalum are included to

illustrate the performance of the equipment. INTRODUCTION

Current interest in materials for high temperature applications has

pointed out the need for a reliable means of determining high temperature tensile properties of materials. This is particularly evident when the materials to be tested do not in themselves possess the ability to

with-stand oxidation at temperatures where test information is desired. For

6

such materials contaminating environments must be prevented from coming

r l

in contact with the surface of the specimen during testing. This may be

accomplished by enclosing the specimen in a controlled environment

con-tainer; however, the effectiveness of such a method depends on the

inte-grity of the means used to transmit the necessary strain information from

within the controlled environment enclosure without interfering with the

function of the container. This presents the problem of transferring

rela-tive mechanical movement through the container walls by means of an

appropriate flexible seal arrangement. In any case, no functional

com-promise should be made between the facilities provided for strain

mea-surement and oxidation protection.

An effective method of protecting tensile creep specimens has been

developed which involves encapsulating the specimen in a flexible capsule

capable of withstanding prolonged. exposure in elevated temperature

cor-rosive conditions.* Strain measurements in this case are transmitted

through the capsule walls by means of a compound knife-edge

arrange-ment. This technique can be applied to short-time tensile testing; however,

the cost of a capsule, which is expendable, is usually unwarranted for

tests lasting only several minutes, as contrasted to long time creep testing.

* Bohn, J. R., Uhrig, R. E. and Murphy, Glenn. A method of specimen

corrosion protection for high temperature creep testing. USAEC Report

7

This investigation deals with the development of equipment arid

tech-niques for oxidation protection and strain measurements of specimens

sub-jected to short-time elevated temperature tensile tests. Test results

on uranium and tantalum demonstrate the performance of the equipment

at high temperature, vacuum and strain magnification.

DESCRIPTION OF THE TEST FACILITY

The majority of the system components used in conjunction with the

high temperature vacuum test chamber and extensometer were standard

commercial items. They included a 60, 000-lb tensile testing machine for load application, an x-y recorder for load-strain recording, vacuum

pumping and measuring devices, and temperature indicating and recording

potentiometers. Temperature control was accomplished using an

elec-tronic proportioning furnace controller designed for this specific

applica-tion, where temperature sensing for control was provided by a platinum

resistance thermometer wound integrally with the furnace heater windings.

Temperature measurements were made using chromel-alumel

thermo-couples at four points along the gage length of the specimen. A sketch

of the high temperature vacuum test chamber and extensometer is given

in Fig. 1. Figure 1 serves as schematic illustration of the functional

mechanical components of the system and also provides an index of

CUTAWAY OF FASTENER SEAL 3

EXTENSOMETER MECHANISM ROTATED 90° 14

15

I

~'-....

"1a

19

17

16

5

1-1;;?---28

C'rl""'(ll)~~t----

27

- -

25

24 - - - - j

~ 3 ..

LEGEND

1. Upper cross -head fastener

2. Cross -head fastener vacuum seal

3. Cutaway of fastener seal 4. Chamber extension bellows 5. Test chamber

6. Resistance furnace

7. Specimen adaptor grip (upper) 8. Upper knife edge

9. Lower knife edge 10. Specimen grip (lower)

11. Tensile specimen

12. Tensile load rod

13. Extensometer mechanism

14. Extensometer mechanism - rotated 90

°

15. Vacuum ion gage 16. Oil diffusion pump

17. Nitrogen cooled cold trap 18. Vacuum couplings

19. Alignment bellows

20. Lower test chamber flange

21. Flexible extensometer well arrangement 22. Glass to metal vacuum coupling

23. LVDT assembly

24. Lower cross-head fastener

25. Auxiliary extensometer assembly

26. Thermocouple seal assembly 27. Gage length positioning screws 28. Gage length spacer rods

10

TEST CHAMBER

A vacuum chamber, which provides the corrosion barrier for oxidation

protection, houses the specimen and allied extensometer fixtures. The

chamber is rigidly attached to the lower load application rod which is

mounted in the lower stationary cross-head of the tensile machine. Thus,

the weight of the test chamber does not affect the tare weight of loading.

Elongation of the test chamber during loading is allowed by a bellows which

is attached to the top of the chamber and coupled to the upper cross -head

fastener and load rod through an "0" ring seal. The system is designed

to accommodate specimens of three sizes: 0. 252-in. diameter by 1. 0-in.

gage length, 0. 357-in. diameter by 1. 5-in. gage length and 0. 505-in.

dia-meter by 2.D-in. gage length. The over-all length between cross heads

is maintained constant for specimens of the various gage lengths by

chang-ing the length of the upper specimen grip. The chamber is evacuated

.th~rough an outlet in the side of the chamber which. is coupled to the vacuum

pumping system with a Genco vacuum seal. The strain and temperature

measuring fixtures located on the bottom flange of the chamber are mounted

with fittings incorporating

"0"

iing_ seals. A detachable water -cooled

coil placed around the lower chamber provides cooling for the vacuum

seals. The furnace tube portion of the test chamber is constructed of

inconel and the majority of the other components are type 316 stainless

11

The pumping system, which consists of a mechanical fore pump, an

oil diffusion pump and a nitrogen cooled cold trap, is consistently capable

of producing pressures between 1 and 5 x lo- 6 mm Hg. Pressure

mea-surements are taken in the vacuum line between the cold trap and the test

chamber. The diffusion pump and the cold trap are mounted on the vertical

support screw of the lower cross head. This mounting permits the

dif-fusion pump and cold trap to be swung out of position when not in use.

The flexible coupling in the vacuum line between the cold trap and test

chamber provides easy alignment when setting up a test. A separating

member between the cold trap and the test chamber fixes the length of

the bellows which would otherwise tend to collapse upon evacuating the

system. Figure 2 shows the apparatus less the cooling coil and cold

trap, mounted in the tensile machine.

EXTENSO METER

' .... \_

Strain measurements are transferreq through the vacuum wall of

the test chamber by a unique application of linear variable differential

transformers (LVDT) and appropriate mechanical linkages. The windings

of the LVDT are attached to the exterior of a fl'e~ible well extending thro.ugh

the bottom of the test chamber. The movable core of the LVDT is

posi-tioned on the inside of the well. With this arrangement electrical

[image:15.592.57.759.37.528.2]

...

N

13

by the core through the vacuum wall of the well. The voltage which is

developed in the secondary winding is proportional to the relative positions

of the core and the windings. Two extensometer wells were provided

to maintain symmetry in the forces developed by the bellows (flexible

well coupling) which are resisted by the extensorneter linkages. The

two well arrangement provides the necessary facilities for dual strain

recording applications.

Knife edges which attach directly to the specimen gage length reference

movement and provide the source of strain measurement. Movement

of the lower knife edge is transferred to the bottom flange of the

exten-someter assembly and then to the walls of the extenexten-someter well. A

bellows which is attached to the side of the well and to the bottom of the

test chamber provides flexible vacuum seal and permits the strain

initiated movement of the lower knife edge to be transmitted to the

exterior of the test chamber. This strain movement is developed between

the lower specimen grip and knife edge, a distance of about 3/8 of an inch.

The elongation of a specimen over this distance is small, imposing a

correspondingly small dimensional change in the extensometer well

bel-lows. Movement of the upper knife edge is transferred to the top and

middle flanges of the extensometer assembly, and then to the LVDT core

14

The LVDT core support rod is nonmagnetic and makes no physical

con-tact other than its point of suspension from the middle flange 6£ the

extensometer assembly.

The four· spacer rods which connect the upper and the middle flange

of the extensometer assembly can be replaced to accommodate

speci-mens of varying gage lengths. The same knife edges are used with the

0. 357 -in. diameter, 1. 5-in. gage length and the 0. 505 -in. diameter,

2. 0-in. gage length specimens by increasing the length of the spacer

rods 1/2 in. The 0. 252 -in. diameter, 1. 0 -in. gage length specimens

require a special set of knife edges. Gage length positioning screws

reference the upper and lower knife edgeclinkages at the middle

exten-someter flange in order initially to establish the exact specimen gage

length before setting the knife edges. After the gage length is established

these screws are removed. The hair··pin-shaped spring provides lateral

alignment stability of the extensometer assembly.

LOAD SENSING

An LVDT is attached directly to the load cell of the tensile machine

in parallel with the load indicator linkage. This provides the advantage

of using range and scale switching facilities built into the load indicator,

in addition to those provided in the LVDT measuring circuit.

1 ':'

15

RECORDING

A number of techniques are available for recording the ac output

voltage of an LVDT. Several of the more common methods are: (1)

di-rect ac recording with an ac potentiometer, (2) the use of specially

designed null balance type servo bridges, where the active and

compen-sating legs of the bridge are LVDT's, and (3) de recording of the rectified

output of the LVDT. In this investigation the third method was employed

since it was desired to sense both load and strain with LVDT's which

required x-y recording, thus eliminating the possibility of utilizing

method {2) which is most commonly employed in strain-recording

applications. The circuit diagram shown in Fig. 3 illustrates the

elec-trical treatment given the primary source voltage and the ac output of

the LVDT for de recording. The same circuitry is used for both strain

and load recording. Line voltage is stabilized and transformed to

pro-vide a range of 0 to 10 volts to supply the primaries of the load and

strain sensing LVDT' s in parallel. The LVDT' s used have a linear

range of 0. 125 in. on either side of null; however, the phase sensitive

rectification circuit employed eliminates the null output inherent to the

LVDT and doubles the useful range providing an output signal of a given

polarity over a range of 0. 250 in. The output signal is attenuated for

I AMP. 'o- o--<!"''...P--IAMP. ~ It AMP VARIAC M-500

+ I

~ M-500

: LOAD I L.V.D.T.

.. ________ .J

: t

+ :

: ~M-500

: STRAIN : L.V.D.T.

.. ___

_

____

....

oad- straui measuring circuit

IOK.

HELIPOT Y- RECORDER

20K

RM42R

ALL 10.0.-1~

3 4 5 MV. STEPS

6 L ~STEPS

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_{.,.._}

I

I I

a::..---·_

2.5K-1%

20K

ALL

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VARIABLE

X-RECORDER

2.5K

1%

RM42R

POL. SW.

VARIABLE

'--'

17

versus magnification is linear. A maximum magnification of 2500x is

possible with an input voltage to the LVDT primary of

6.

3 volts; however,

proportionately high magnifications can be obtained by increasing the

input voltage upward to 10 volts.

A bucking voltage in series with the output signal to the recorder is

provided as a means of extending the recording range. The bucking

signal is provided for both the load and strain axes and can be utilized

in two ways, continuously variable or in steps of 7. 5 mv on the x-axis

and 5 mv on the y-axis. These values correspond to full- scale

deflec-tion on the respective scales of the x-y recorder use d.

In calibration tests of the load-strain measuring system, both linearity

and stability showed deviations of less than 1% for periods less than

one-half hour. Instrument warm-up periods of at least an hour are necessary

to as sure these conditions, since temperature compensation of the

LVDT's can not be provided.

PROCEDURE

Setting up a test initially involves the three components shown in

Fig. 4. The specimen is inserted in the lower grip and the lower

knife edge is fastened to it. Then the gage point screws are set after

18

[image:21.595.56.533.71.670.2]

after which the gage point set screws are removed. The four

thermo-couples are attached, the uppe:r load rod is screwed onto the specimen

and the vacuum chamber is placed over the assembly and secured to the

, bottom flange with six cap screws. Next, the seal is made between the

bellows and the upper cross -head fastener. The test chamber assembly

is now ready to be mounted in the tensile machine. The test chamber

assembly is supported in the spherical joint of the lower cross head

only which prevents the tare weight of the assembly from adding to the

load to be placed on the specimen. The remainder of the procedure is a

matter of making the necessary connections, pumping the vacuum and

establishing the desired temperature conditions. Running the test is

routine.

TEST RESULTS

19

Autographic load-strain records for uranium and tantalum have bee,n included in Figs. 5,

6

and 7 as the basis for illustrating the effectiveness

of the testing apparatus. For these tests 0. 357-in. diameter, 1. 5-in.

gage length specimens were used. These specimens have a

cross-sec-tional area of 0. 10 sq. in., hence the engineering stress may be

con-veniently calculated as 10 times the load. Strain as indicated is the

total strain over the

1.

5-in. gage length; thus, unit strain ~s the total

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, j ,../

N N

[image:25.591.8.738.22.558.2]

abscissa are the result of expanding the strain axis, and areo::aused by

the time lapse of full scale pen travel. Recording was terminated as

indi-cated by the notation, "extensometer disengaged". Disengagerr:e nt of the

extensometer occurs when necking-down of the specimen causes uncoupling

of the knife edges.

CONCLUSIONS AND SUMMARY

The elastic properties of materials do not adequately describe the

stress-strain properties of materials which behave in a semi-plastic

manner. In elevated temperature applications it is particularly desirable

to have available the actual stress-strain records for the material.

Re-producible autographic recordings greatly simplifies such a task.

The performance of the test equipment described is considered

ex-cellent for intended applications. It is believed that testing at

tempera-tures greater than l000°C could be achieved by improving the materials

of construction and providing the heat from within the vacuum chamber.

Strain magnifications up to 3000 are possible over a range of 0. 250 in.

High magnification and extended range provides great versatility for

most stress -strain applications. In addition, the provisions for dual

strain sensing provides the advantage of dual range recording. One

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greatly magnify a portion of the stress-strain curve of particular interest.

Another outstanding feature of the strain measuring apparatus is its

ability to give a continuous record to fracture without necessitating