ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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strain rates in excess of 10

-1

. Video extensometers, where a camera records the displacement of marks on a

specimen through a glass window, can also be used with temperature chambers.

References cited in this section

2. J.D. Wittenberger and M.V. Nathal, Elevated/Low Temperature Tension Testing, Mechanical Testing,

Vol 8, Metals Handbook, ASM International, 1985, p 36

10. R. Viswanathan, Damage Mechanisms and Life Assessment of High Temperature Components, ASM

International, 1989, p 62

12. Y.V.R.K. Prasad and S. Sasidhara, Hot Working Guide: A Compendium of Processing Maps, ASM

International, 1997, p 9

14. “Standard Test Methods for Elevated Temperature Tension Tests of Metallic Materials,” ASTM E 21-

92, Annual Book of ASTM Standards, 1994

15. R.E. Bailey, R.R. Shiring, and H.L. Black, Hot Tension Testing, Workability Testing Techniques, G.E.

Dieter, Ed., American Society for Metals, 1984, p 73–94

16. Gleeble

3800 System, Dynamic Systems Inc., Poestenkill, NY, May 1997

17. D.N. Tishler and C.H. Wells, An Improved High-Temperature Extensometer, Mat. Res. Stand., ASTM,

MTRSA, Vol 6 (No. 1), Jan 1966, p 20–22

Hot Tension and Compression Testing

Dan Zhao, Johnson Controls, Inc.; Steve Lampman, ASM International

Hot Compression Testing

Hot compression testing is also relatively easy to perform because of its simple specimen geometry (e.g., a

cylinder). Testing machines and accessories are similar to those for hot tensile testing except the pull bars and

grips are replaced by pushing anvils and platens. The anvils and platens can be made of stainless steel, tungsten

carbide, TZM (Ti-Zr-Mo alloy), ceramics, or carbon.

Details on the applicable temperature ranges of anvils and platens are provided in the article “Testing for

Deformation Modeling” in this Volume. The flat and parallel of platens should be within 0.0051 mm (0.0002

in.) (Ref 18). To improve parallelism, adjustable platens (bearing blocks) can be used. A drawing of such

blocks can be found in Ref 19. Using a subpress, as suggested in the ASTM standards (Ref 18, 20), is very

difficult with the limited space inside the furnace.

Specimen. The simplest specimen geometry is a cylinder. The aspect ratio (height to diameter) is usually

between 1 and 2. An aspect ratio that is too high can cause the specimen to buckle, while one that is too low can

increase friction even if lubricant is applied (see the article “Uniaxial Compression Testing” in this Volume).

Typical specimen diameter is 10 to 15 mm (0.394–0.591 in.), depending on microstructure. For a cast alloy

with coarse grains, large specimens are necessary. Subscale specimens can also be used for fine grain structure.

In general, the specimen size must be representative of the material being tested. Other types of specimens,

such as those with square or rectangular cross sections, can also be used, depending on the purpose of the tests.

For example, a plane-strain compression specimen can have a rectangular cross section (Ref 19).

Lubrication. For testing at elevated temperatures, water-base graphite, graphite sheet, boron nitride solution,

glass-base lubricant, and molybdenum disulfide may be used (Ref 13, 21). The lubricants can be applied to the

top and bottom ends of the specimen. They can also be applied to the platens at the same time to increase the

effectiveness of lubrication. To retain the lubricant, grooves can be machined into the ends. Detailed specimen

and groove dimensions can be found in the article “Testing for Deformation Modeling” in this Volume.

Temperature Control. As mentioned for hot tensile testing, a thermocouple that just touches the specimen does

not provide an accurate temperature measurement unless the specimen is soaked at the nominal testing

temperature for some time. To accurately measure the temperature of a specimen, thermocouples can either be

welded to the specimen or inserted into a small hole drilled into the specimen. Compression-testing specimens

are usually larger in diameter than tensile specimens, so a small hole drilled into the specimen to insert

thermocouples may have little impact on the stress-strain curves. However, a hole may induce false cracking in

a workability test, especially for brittle materials such as intermetallic compounds.

To determine the uniformity of temperature within the specimen, three thermocouples may be used to measure

the top, bottom, and center temperatures of a dummy specimen as a function of time. If the temperatures are

identical, only one thermocouple is necessary during testing. If it takes some time for the entire specimen to

reach the set temperature, this procedure can also be used to determine the necessary soaking time.

To ensure the correct microstructure or specimen condition right before the compression testing commences, a

specimen soaked at the testing temperature for the specified soaking time should be quenched and examined to

determine the starting microstructure. It is essential that the platens be at the same temperature as the specimen.

A temperature difference between the platens and the specimen results in a deformation gradient and, therefore,

barreling of the deformed specimen (Ref 22).

Data Reduction and Temperature Correction. Load and displacement data are acquired from testing. To reduce

the data into true stress and true strain, deformation is assumed homogeneous. Correction for the elastic

deflection of the machine needs to be taken into account. True stress is simply the load divided by

instantaneous cross-sectional area, which can be calculated by assuming constant volume in the specimen. For a

cylindrical specimen, true stress, σ, is calculated as (Ref 19):

(Eq 1)

where P is load, A is cross-sectional area, D and D

are the instantaneous and initial diameter of the specimen,

respectively, and h and h

are the instantaneous and initial height of the specimen, respectively. If friction is

significant, the average pressure, , required to deform the specimen is greater than the flow stress of the

material, σ:

(Eq 2)

where μ is the Coulomb coefficient of friction. The true strain, ε is given by:

(Eq 3)

Deformation heating occurs inevitably during testing, especially at high strain rates. Because isothermal stress-

strain curves are desired for analysis, correction for deformation heating is necessary. The procedure for the

correction can be found in the article “Testing for Deformation Modeling” in this Volume.

References cited in this section

13. M. Thirukkonda, D. Zhao, and A.T. Male, Materials Modeling Effort for HY-100 Steel, NCEMT

Technical Report, TR No. 96–027, Johnstown, PA, March, 1996

18. “Standard Methods of Compression Testing of Metallic Materials at Room Temperature,” ASTM E 9-

89a, Annual Book of ASTM Standards, 1994

19. A.T. Male and G.E. Dieter, Hot Compression Testing, Workability Testing Techniques, G.E. Dieter, Ed.,

American Society for Metals, 1984, p 51–72

20. “Standard Practice for Compression Tests of Metallic Materials at Elevated Temperatures with

Conventional or Rapid Heating Rates and Strain Rates,” ASTM E 209-65, Annual Book of ASTM

Standards, 1994

21. M.L. Lovato and M.G. Stout, Compression Testing Techniques to Determine the Stress/Strain Behavior

of Metals Subject to Finite Deformation, Metall. Trans. A, Vol 23, 1992, p 935–951

22. M.C. Mataya, Simulating Microstructural Evolution during the Hot Working of Alloy 718, JOM, Jan

1999, p 18–26

Hot Tension and Compression Testing

Dan Zhao, Johnson Controls, Inc.; Steve Lampman, ASM International

References

1. W.D. Nix and J.C. Gibeling, Mechanisms of Time-Dependent Flow and Fracture of Metals, Flow and

Fracture at Elevated Temperatures, R. Raj, Ed., American Society for Metals, 1985, p 2

2. J.D. Wittenberger and M.V. Nathal, Elevated/Low Temperature Tension Testing, Mechanical Testing,

Vol 8, Metals Handbook, ASM International, 1985, p 36

3. W.F. Brown, Jr., Ed., Aerospace Structural Metals Handbook, Metals and Ceramic Information Center,

Columbus, OH, 1982

4. Structural Alloys Handbook, Metals and Ceramic Information Center, Columbus, OH, 1973

5. D. Zhao, “Deformation and Fracture in Al

Particle-Reinforced Aluminum Alloy 2014,” Ph.D. thesis,

Worcester Polytechnic Institute, 1990

6. MIL-HDBK 5, 1991

7. Metals Handbook, American Society for Metals, 1948, p 441

8. The Making, Shaping, and Treating of Steel, United States Steel, 1957, p 822–823

9. F.T. Sisco, Properties, Vol 2, The Alloys of Iron and Carbon, McGraw-Hill, 1937, p 431

10. R. Viswanathan, Damage Mechanisms and Life Assessment of High Temperature Components, ASM

International, 1989, p 62

11. S.L. Semiatin and J.J. Jonas, Formability and Workability of Metals: Plastic Instability and Flow

Localization, American Society for Metals, 1984, p 1–5, 93–97

12. Y.V.R.K. Prasad and S. Sasidhara, Hot Working Guide: A Compendium of Processing Maps, ASM

International, 1997, p 9

13. M. Thirukkonda, D. Zhao, and A.T. Male, Materials Modeling Effort for HY-100 Steel, NCEMT

Technical Report, TR No. 96–027, Johnstown, PA, March, 1996

14. “Standard Test Methods for Elevated Temperature Tension Tests of Metallic Materials,” ASTM E 21-

92, Annual Book of ASTM Standards, 1994

15. R.E. Bailey, R.R. Shiring, and H.L. Black, Hot Tension Testing, Workability Testing Techniques, G.E.

Dieter, Ed., American Society for Metals, 1984, p 73–94

16. Gleeble

3800 System, Dynamic Systems Inc., Poestenkill, NY, May 1997

17. D.N. Tishler and C.H. Wells, An Improved High-Temperature Extensometer, Mat. Res. Stand., ASTM,

MTRSA, Vol 6 (No. 1), Jan 1966, p 20–22

18. “Standard Methods of Compression Testing of Metallic Materials at Room Temperature,” ASTM E 9-

89a, Annual Book of ASTM Standards, 1994

19. A.T. Male and G.E. Dieter, Hot Compression Testing, Workability Testing Techniques, G.E. Dieter, Ed.,

American Society for Metals, 1984, p 51–72

20. “Standard Practice for Compression Tests of Metallic Materials at Elevated Temperatures with

Conventional or Rapid Heating Rates and Strain Rates,” ASTM E 209-65, Annual Book of ASTM

Standards, 1994

21. M.L. Lovato and M.G. Stout, Compression Testing Techniques to Determine the Stress/Strain Behavior

of Metals Subject to Finite Deformation, Metall. Trans. A, Vol 23, 1992, p 935–951

22. M.C. Mataya, Simulating Microstructural Evolution during the Hot Working of Alloy 718, JOM, Jan

1999, p 18–26

Tension and Compression Testing at Low

Temperatures

Robert P. Walsh, National High Magnetic Field Laboratory, Florida State University

Introduction

THE SUCCESSFUL USE of engineering materials at low temperatures requires that knowledge of material

properties be available. Numerous applications exist where the service temperature changes or is extreme.

Therefore, the engineer must be concerned with materials properties at different temperatures. Some of the

typical materials properties of concern are strength, elastic modulus, ductility, fracture toughness, thermal

conductivity, and thermal expansion. The lack of low temperature engineering data, as well as the use of less

common engineering materials at low temperatures, results in the need for low-temperature testing.

The terms “high temperature” and “low temperature” are typically defined in terms of the homologous

temperature (T/T

), (where T is the exposure temperature, and T

is the melting point of a material (both given

on the absolute temperature scale, K). The homologous temperature is used to define the range of application

temperatures in terms of the thermally activated metallurgical processes that influence mechanical behavior.

The term “low temperature” is typically defined in terms of boundaries where metallurgical processes change.

One general definition of “low-temperature” is T < 0.5 T

. For many structural metals, another definition of

low temperature is T < 0.3 T

, where recovery processes are not possible in metals and where the number of

slip systems is restricted. For these definitions, room temperature (293 K) is almost always considered a low

temperature for a metal with a few exceptions, such as metals that have melting temperatures below 700 °C

(indium and mercury). In a structural engineering sense, low temperature may be one caused by extreme cold

weather. A well-known example of this is the brittle fracture of ship hulls during WWII that occurred in the

cold seas of the North Atlantic (Ref 1). For many applications, low temperature refers to the cryogenic

temperatures associated with liquid gases. Gas liquefaction, aerospace applications, and superconducting

machinery are examples of areas in engineering that require the use of materials at very low temperatures. The

term cryogenic typically refers to temperatures below 150 K. Service conditions in superconducting magnets

that use liquid helium for cooling are in the 1.8 to 10 K range.

The mechanical properties of materials are usually temperature dependent. The most common way to

characterize the temperature dependence of mechanical properties is to conduct tensile or compressive tests at

low temperatures. Depending on the data needed, a test program can range from a full characterization of the

response of a material over a temperature range, to a few specific tests at one temperature to verify a material

performance. Many of the rules for conducting low temperature tests are the same as for room temperature

tests. Low-temperature test procedures and equipment are detailed in this article. The role that temperature

plays on the properties of typical engineering materials is discussed also. Important safety concerns associated

with low-temperature testing are reviewed.

Reference cited in this section

1. E.R. Parker, Brittle Behavior of Engineering Structures, John Wiley & Sons, 1957

Tension and Compression Testing at Low Temperatures

Robert P. Walsh, National High Magnetic Field Laboratory, Florida State University

Mechanical Properties at Low Temperatures

In general, lowering the temperature of a solid increases its flow strength and fracture strength. The effect that

lowering the temperature of a solid has on the mechanical properties of a material is summarized below for

three principal groups of engineering materials: metals, ceramics, and polymers (including fiber-reinforced

polymer, or FRP composites). An excellent source for an in-depth coverage of material properties at low

temperatures is Ref 2.

Metals. Most metals are polycrystalline and have one of three relatively simple structures: face-centered cubic

(fcc), body-centered cubic (bcc), and close-packed hexagonal (hcp). The temperature dependence of the

mechanical properties of the fcc materials are quite distinct from those of the bcc materials. The properties of

hcp materials are usually somewhere in between fcc and bcc materials. The general aspects of temperature-

dependent mechanical behavior may be discussed using the deformation behavior maps shown in Fig. 1(a) and

1(b). The axes of these graphs are normalized for temperature and stress. Temperature is normalized to the

melting temperature, while stress is normalized to the room temperature shear modulus, G (Ref 2).

Fig. 1 Simplified deformation behavior (Ashby) maps (a) for face-centered cubic metals

and (b) for body-centered cubic metals. Source: Ref 2

The behavior characteristic of a pure, annealed fcc material is shown in Fig. 1(a). The small increase of yield

strength that occurs upon cooling is characteristic of the fcc behavior. The ultimate strength, which is shown as

the ductile failure line, increases much more than the yield strength on cooling. The large increase in ultimate

strength coupled with the relatively small increase in yield strength in fcc materials results from ductile, rather

than brittle, failure (Ref 2).

Figure 1(b) illustrates the classic bcc behavior. The large temperature dependence of the yield strength, the

smaller temperature dependence of the ultimate strength, and a region where the specimen fails before any

significant plastic deformation occurs should be noted (Ref 2).

The previous discussion is for pure annealed metals. Engineering alloys may behave somewhat differently, but

the trends are relatively consistent. Solid solution strengthening typically increases yield and ultimate strengths

of the fcc alloys while giving the yield strength an increased temperature dependence. The temperature

dependence of the ultimate strength is still greater than that of the yield strength, allowing the alloy to maintain

its ductile behavior. The ultimate tensile strengths of the fcc metals have stronger temperature dependence than

those of bcc metals. Austenitic stainless steels have fcc structures and are used extensively at cryogenic

temperatures because of their ductility, toughness, and other attractive properties. Some austenitic steels are

susceptible to martensitic transformation (bcc structure) and low-temperature embrittlement. Plain carbon and

low alloy steels having bcc structures are almost never used at cryogenic temperatures because of their extreme

brittleness. Cases of anomalous strength behavior have been reported where a maximum strength is reached at

temperatures above 0 K. These cases are unique and usually involve single crystal research materials or very

soft materials, although yield strengths of commercial brass alloys are reported to be higher at 20 K than at 4 K

(Ref 2).

Ceramics. Ceramics are inorganic materials held together by strong covalent or ionic bonds. The strong bonds

give them the desirable properties of good thermal and electrical resistance and high strength but also make

them very brittle. Graphite, glass, and alumina are ceramics used at low temperature usually in the form of

fibers that reinforce polymer-matrix composite materials. The high temperature (~77 K) superconducting

compounds are ceramics that pose challenging problems with respect to using brittle materials at low

temperatures.

Polymers and Fiber-Reinforced Polymer (FRP) Composites. Polymers are rather complex materials having

many classifications and a wide range of properties. Two important properties of polymers are the melting

temperature, T

, and the glass transition temperature, T

, both of which indicate the occurrence of a phase

change. The glass transition temperature, the most important material characteristic related to the mechanical

properties of polymer, is influenced by degree of polymerization. The T

is the temperature, upon cooling, at

which the amorphous or crystalline polymer changes phase to a glassy polymer. For most polymers at

temperatures below T

, the stress-strain relationship becomes linear-elastic, and brittle behavior is common.

Some ductile or tough polymers exhibit plastic yielding at temperatures below T

. The T

represents the

temperature below which mass molecular motion (such as chain sliding) ceases to exist, and ductility is

primarily due to localized strains. Suppression of T

helps to produce tougher polymers. The strong temperature

dependence of the modulus is a distinguishing feature of polymers compared to metals or ceramics.

Fiber-reinforced polymer composites are used extensively at low temperatures because of their high strength-

to-weight ratio and their thermal and electrical insulating characteristics. The FRPs tend to have excellent

tensile and compression strength that increases with decreasing temperature. Reinforcing fibers commonly used

in high-performance composites for low-temperature applications are alumina, aramid, carbon, and glass.

Typical product forms are high-pressure molded laminates (such as cotton/ phenolics and G-10) and filament-

wound or pultruded tubes, straps, and structures. Although the FRP composites have desirable tensile and

compressive strengths, other mechanical properties such as fatigue and interlaminar shear strength are

sometimes questionable. Two good sources of properties of structural composites at low temperature are Ref 4

and 5.

References cited in this section

2. R.P. Reed and A.F. Clark, Ed., Materials at Low Temperatures, ASM, 1983

4. M.B. Kasen et al., Mechanical, Electrical, and Thermal Characterization of G-10CR and G-11CR Glass-

Cloth/Epoxy Laminates Between Room Temperature and 4 K, Advances in Cryogenic Engineering, Vol

28, 1980, p 235–244

5. R.P. Reed and M. Golda, Cryogenic Properties of Unidirectional Composites, Cryogenics, Vol 34 (No.

11), 1994, p 909–928

Tension and Compression Testing at Low Temperatures

Robert P. Walsh, National High Magnetic Field Laboratory, Florida State University

Test Selection Factors

Tensile and compression tests produce engineering data but also facilitate study of fundamental mechanical-

metallurgical behavior of a material, such as deformation and fracture processes. If obtaining engineering data

is the objective and the materials application is at low temperature, the designer must be sure that mechanical

properties are stable at the desired temperatures. One important factor related to low-temperature testing is that

the low temperature may cause unstable brittle fracture behavior that tensile or compression tests may fail to

reveal. The cooling of materials, especially bcc metals and polymers, can cause the materials to undergo a

ductile-to-brittle transition. This behavior is not unique to steel but has its counterpart in many other materials.

Brittle fracture occurs in the presence of a triaxial stress state to which a simple tensile or compression test will

not subject the material. Brittle fracture is caused by high tensile stress, while ductile behavior is related to

shear stress. A metal that flows at low stress and fractures at high stress will always be ductile. If, however, the

same material is retreated so that its yield strength approaches its fracture strength, its behavior may become

altered, and brittleness may ensue (Ref 1). If the materials application is at low temperature, the designer must

be sure that mechanical properties are stable, because the possibility of brittle fracture requires modification of

the design approach.

If the material in question is a new material or a material for which little or no low-temperature data exist,

screening tests that can assess susceptibility to brittle fracture are advisable. Two such screening tests are

Charpy impact tests and notch tensile tests. Conducting Charpy or notch tensile tests at various temperatures

can detect a ductile-to-brittle transition over a temperature range. Ultimately, if the fracture toughness of the

material is an issue, fracture toughness testing should be performed.

The intended service condition for the material should influence the test temperature and the decision to

perform tensile or compressive tests. It is good practice to determine the properties while simulating the service

conditions. Of course, life is not always this simple, and actual service conditions may not be easily achieved

with an axial stress test at a given temperature.

Tensile testing is the most common test of mechanical properties and is usually easier than compression testing

to conduct properly at any temperature. The compressive and tensile Young's moduli of most materials are

identical. Fracture of a material is caused by tensile stress that causes crack propagation. Tensile tests lend

themselves well to low-temperature test methods because the use of environmental chambers necessitates

longer than normal load trains. Pin connections and spherical alignment nuts can be used to take advantage of

the increased length for self-alignment purposes. For most homogeneous materials, stress-strain curves obtained

in tension are almost identical to those obtained in compression (Ref 6). Exceptions exist where there is

disagreement between the stress-strain curves in tension and compression. This effect, termed “strength

differential effect,” is especially noticeable in high-strength steels (Ref 7).

There are times when compression testing is required such as when the service-condition stress is compressive

or when the strength of an extremely brittle material is required. The second case is true for almost all polymers

at cryogenic temperatures as they become extremely brittle, glassy materials. The fillet radius of a reduced-

section tensile-test sample can create enough of a stress riser that the material fails prematurely. Stress

concentrations, flaws, and submicroscopic cracks largely determine the tensile properties of brittle materials.

Flaws and cracks do not play such an important role in compression tests because the stress tends to close the

cracks rather than open them. The compression tests are probably a better measure of the bulk material behavior

because they are not as sensitive to factors that influence brittle fracture (Ref 3). A brittle material will be

nearly linear-elastic to failure, providing a well-defined ultimate compressive strength. The following table lists

competing factors that influence the test method choice, many of which are generic while some are specific to

conditions associated with low-temperature testing.

The temperature at which to run the test can be a simple determination such as when mechanical properties data

for a material at the

Tension Compression

Advantages

Common

Good for modulus and yield strength

Self-aligning

Well-defined gage section

No grips

No stress concentration in sample

design

Good for ultimate strength of brittle

materials

Easy sample installation

Good for modulus, yield, ultimate, and ductility parameters

Inexpensive sample cost

Disadvantages

Sensitive to specimen design

End effects (friction/constraint)

Sensitive to alignment

Not always good for ultimate

strength

Difficult to test brittle materials and composites where machining

reduced section is not plausible

Need containment for fractured

material

proposed service temperature are not available. Other cases are not so straightforward, and the temperature

choice should be based on cost and the ability to provide conservative results. Sometimes, a material is to be

used at a cold temperature, but testing it at room temperature will yield conservative data that are sufficient for

the application. For many 4 K applications, conservative properties can be measured at 77 K in a simpler, more

economical test. The degree of strengthening that will occur upon cooling from 77 to 4 K is much less than that

which occurs from 295 to 77 K. When there is doubt about the applicability of data from tests at a temperature

other than the service temperature, testing should be done at the service temperature. Good practice is to test

above, below, and at the service temperature for a more complete understanding of the material behavior.

The relative costs and difficulty of the tests are important. Tests conducted in liquid media are simpler to

perform, in general, than intermediate temperature tests that require temperature control. Below is a list of

testing media and their associated temperatures (Ref 2).

Substance Temperature, K

Bath type

Ice water

273 Slush

Isobutane

263 Liquid at BP

Carbon tetrachloride

250 Slush

Propane

231 Liquid at BP

Trichloroethylene

200 Slush

Carbon dioxide

195 Solid

Methanol

175 Slush

n-pentane

142 Slush

Iso-pentane

113 Slush

Methane

112 Liquid at BP

Oxygen

90.1 Liquid at BP

Nitrogen

77.3 Liquid at BP

Neon

27.2 Liquid at BP

Hydrogen

20.4 Liquid at BP

Helium (He

)

4.2 Liquid at BP

Helium (He

)

3.2 Liquid at BP

All temperatures given at 0.1 MPa (1 atm). BP, boiling point

Some of these substances are more common, cheaper, or easier to handle than others. The most commonly used

substances in mechanical tests are ice water, CO

/methanol slush, liquid-nitrogen (LN

) cooled methanol, LN

and liquid helium (LHe). Obvious hazards are associated with the use of oxygen and hydrogen, and they should

be avoided if possible. Safety issues concerning the use of cooled methanol, LN

, and LHe are discussed

subsequently in this section. Cost of the cryogenic medium is also an issue. Since LN

is common and readily

available, its cost is relatively low. LHe, on the other hand, is about a factor of ten times as expensive as LN

Liquid neon is sometimes used because it is easy to handle and its liquid boiling point temperature is relatively

close to that of liquid hydrogen, but it can be 20 to 40 times as expensive as LHe. The sublimation temperature

of dry ice (CO

) is 195 K, and it can be used to cool a methanol or propanol bath with relative ease. Many of

these bath cooling techniques are tried and true methods that require some practice to perfect but are usually

inexpensive and simple ways to control test sample temperature. Low-temperature control can also be

accomplished with electronic temperature control systems that utilize heaters and a cooling medium. Electronic

temperature control systems are described in the following section.

References cited in this section

1. E.R. Parker, Brittle Behavior of Engineering Structures, John Wiley & Sons, 1957

2. R.P. Reed and A.F. Clark, Ed., Materials at Low Temperatures, ASM, 1983

3. L.E. Neilsen and R.F. Landel, Mechanical Properties of Polymers and Composites, Marcel Dekker, NY,

1994, p 249–263

6. E.P. Popov, Mechanics of Materials, 2nd ed., Prentice-Hall, NJ, 1976

7. J.P Hirth and M. Cohen, Metall. Trans., Vol 1, Jan 1970, p 3

Tension and Compression Testing at Low Temperatures

Robert P. Walsh, National High Magnetic Field Laboratory, Florida State University

Equipment

Low-temperature tensile and compression tests can be performed on electromechanical or servo-hydraulic test

machines with capacities of approximately 50 to 100 kN. The 100 kN machine is preferable for high strength

materials such as steels or composites but of course larger or smaller capacities can be used as necessary. Direct

tension and compression tests usually require a simple ramp function that is possible on the more economical

electromechanical (screw-drive) test machine. Computer controlled servo-hydraulic test systems are versatile

and can perform a variety of tasks as well as direct tension and compression tests.

To facilitate the low-temperature requirement, the test machine must be equipped with a temperature-controlled

environmental chamber. One consideration for the suitability of the machine for low-temperature tests is the

ease with which a low-temperature environmental chamber can be implemented. The physical characteristics of

the test machine come into play, such as the maximum distance between crossheads and load columns.

A major factor to consider for cryogenic tests is the cryostat. “Cryostat” is a general term for an environmental

chamber designed for cryogenic temperatures and can be as simple as a container (dewar) to hold a liquid

cryogen. Cryostats designed for mechanical testing have the added requirement of providing structural support

to react to tensile or compressive forces that are applied to the test material. Typically, a load frame is designed

as an insert to a dewar. Since a dewar is a vacuum-insulated bucket to hold liquid, it is not advisable to have a

hole in the bottom for pull-rod penetration because it introduces a leak potential for liquid, vacuum, and heat.

The closed-bottom feature of a cryostat necessitates that the applied load and reacted load be introduced from

the top. Cryostats are described further in the section “Environmental Chambers” below.

The simplest method to introduce the load path from the top on a servo-hydraulic machine is to use a machine

that has the hydraulic actuator mounted on top of the upper crosshead. Hydraulic machines with this

configuration are available, and the arrangement does not restrict normal use of the machine. Figure 2 shows a

servo-hydraulic test machine equipped with a mechanical test cryostat. The screw-drive type test machine is

usually accommodating and should have a movable lower crosshead with a through hole for the load train.

References 8 and 9 give details of the design of cryostats for mechanical test machines.