ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation
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Fig. 2 A 100 kN capacity test machine equipped with cryostat for low-temperature testing
If the machine is not configured as described in the preceding paragraphs, the machine is relatively
incompatible for cryogenic tests. Cryogenic tests on an incompatible machine require specially designed
cryostats or an external frame system both of which are usually expensive and cumbersome alternatives. Figure
3 shows a schematic of a simple test chamber (canister) for immersion bath tests above liquid nitrogen
temperature. This fixture provides an inexpensive method for conducting tests on conventional machines down
to approximately 100 K.
Fig. 3 Schematic of simple tensile canister from a standard-configuration machine for
low-temperature testing
Environmental Chambers. For low temperature tests, an environmental chamber is a thermal chamber that
contains a gaseous or liquid bath media used to control the low temperature of a test. Sub-room-temperature
environments are obtained with three basic chamber designs: a conventional refrigeration chamber; a thermally
insulated box-container, or a cryostat designed for cryogenic temperatures with vacuum insulation; and thermal
radiation shielding.
Conventional refrigeration covers the temperature range from +10 to -100 °C and could be employed for tests
in this range, much the same as furnaces are used on test machines to achieve elevated temperatures. Although
mechanical refrigeration seems like a logical choice to cool environmental chambers, it is rarely used. This is
probably because of the capital expense and the relative simplicity of other methods.
Commercial environmental chambers designed for use with test machines are available for controlling
temperatures from approximately 800 K down to 80 K. Such chambers use electrical heaters for elevated
temperatures and cold nitrogen gas cooling for sub-room-temperature. The cold nitrogen gas is supplied from a
liquid nitrogen storage dewar. The flow of cold gas determines the cooling power and is controlled at the inlet
with a variable flow valve that is regulated by the temperature controller. These systems are versatile in that a
wide range of test temperature is possible with a single system. Some of the disadvantages are bulkiness, which
can make setup difficult, and that the tests can be time consuming with respect to attaining equilibrated test
temperatures.
Cryogenic temperature tests are conducted in an environmental chamber called a cryostat. Cryostat is a general
description of a low-temperature environmental chamber and can be as simple as a container (dewar) to hold a
liquid cryogen. Reference 10 is an excellent historical perspective on low-temperature mechanical tests that
details a number of cryostat designs, many of which use conventional machines with standard load path
configurations. As mentioned previously, pull-rod penetration through the bottom of a cryostat introduces a
leak potential for liquid, vacuum, and heat and is not recommended for liquid bath-cooled tests. Modern
mechanical test cryostats are typically a combination of a custom designed structural load frame fit into a
commercial open-mouth bucket dewar. Some of the design details of a tensile test cryostat are shown in the
schematic in Fig. 4 and photograph in Fig. 5. The design of the cryostat load frame is driven by engineering
design factors such as cost, strength, stiffness, thermal efficiency, and ease of use. A good design philosophy is
to produce a versatile fixture that can test a variety of specimens over a range of temperatures. The effect of
lowering the temperature on the properties of a material can be evaluated by comparing the baseline room
temperature properties. It is advisable to have the test apparatus capable of testing the material at both room
temperature and cold temperatures. Construction materials used are austenitic stainless steels, titanium alloys,
maraging steels, and FRP composites. For tensile tests, the cryostat frame reacts to the load in compression.
The frame can be thermally isolated with low-thermal conductivity, FRP composite standoffs. For compression
tests, the reaction frame is in tension and is not as easily thermally isolated. The cryostat shown here is easily
converted between the more thermally efficient tensile cryostat and the more robust compressive cryostat (Fig.
6).
Fig. 4 Schematic of a tensile test cryostat
Fig. 5 Tensile test cryostat. The force-reaction posts have fiber-reinforced polymer
composite stand-offs.
Fig. 6 Compression test cyrostat, including heavier force-reaction posts and a tubular
push rod designed to withstand buckling
Cryogen Liquid Transfer Equipment. The supply and delivery of cryogenic fluids require special equipment.
The equipment described here pertains to the use of the two most common cryogen test media, liquid nitrogen
and liquid helium. Both liquid helium and nitrogen can be purchased from suppliers (usually welding supply
distributors) in various quantities that are delivered in roll-around storage dewars. Liquid nitrogen can be
transferred out of the storage dewar into the test dewar with simple or common tubing materials. Its thermal
properties and inexpensive price allow its flow through uninsulated tubes. For example, butyl rubber hose can
be attached to the storage dewar, and the hose will freeze as the liquid passes through. Liquid helium, on the
other hand, is more difficult to handle, and it requires special vacuum-insulated transfer lines. Liquid helium
transfer lines are usually flexible stainless steel lines with end fittings to match the inlet ports of the test cryostat
and the supply cryostat. For both liquid nitrogen and helium, the storage tank is pressurized to enable transfer of
the liquid.
Instrumentation. The minimum instrumentation requirement in any tensile or compression test is that for force
measurement. Typically, forces are measured with the test machine force transducer (load cell) in the same
manner as for forces measured in room temperature tests. During low-temperature tests, precautions should be
taken to ensure the load cell remains at ambient room temperature.
Strain measurements may require temperature dependent calibration. Common strain measurement methods
used are test machine displacement, bondable resistance strain gages, and clip-on extensometers or
compressometers. Also applicable to low-temperature strain measurements but less commonly used are
capacitive transducer methods (Ref 11), noncontact laser extensomers, and linear variable differential
transformers (LVDT) with extension rods to transmit displacements outside of the environmental chamber to
the LVDT-sensing device.
Test machine displacement (stroke or crosshead movement) is a simple, low-accuracy method of estimating
specimen strain. The inaccuracy comes because the displacement includes deflection of the test fixturing plus
the test specimen gage section. Compensating for test fixturing compliance improves accuracy.
Bondable resistance strain gages are for sensitive measurements such as modulus and yield strength
determination. The strain gage manufacturer supplies strain gage bonding procedures for use at cryogenic
temperatures. The overall range of strain gages at cryogenic temperatures is limited to about 2% strain.
Applicable strain gages recommended by strain gage manufacturers have temperature dependent calibration
data down to 77 K. Interest in their use down to 4 K has resulted in strain gage research verifying their
performance to 4 K (Ref 12). A typical gage factor (GF) is 2 for NiCr alloy foil gages and it increases
approximately 2 to 3% on cooling from 295 to 4 K. Thermal output strain signals are a large source of error that
must be compensated for. Compensation is usually accomplished using the bridge balance of the strain circuit
where zero strain can be adjusted to coincide with zero stress. If this is not possible, other steps must be taken
to electrically or mathematically correct the thermal output strain.
Extensometers and compressometers applicable to low-temperature tests utilize strain gages mounted to a
bending beam element. The temperature sensitivity can be determined by calibrating with a precision
calibration fixture that enables calibration at various temperatures. Depending on the accuracy desired, it is
possible to use one or two calibration factors over a large temperature range. A typical strain-gage
extensometer-calibration factor changes about ±1% over the temperature range from 295 to 4 K.
Temperature measurement is done with an assortment of temperature sensors. Reference 2 has a section
devoted to temperature measurement at low temperatures. The most common method of temperature
measurement is to use a thermocouple. Type E thermocouples (Chromel versus Constantan) and Type K
(Chromel versus Alumel) cover a wide range of temperature and can be used at 4 K when carefully calibrated.
A better choice of thermocouple, designed to have higher sensitivity at cryogenic temperatures, is a AuFe alloy
versus Chromel thermocouple. Electronic temperature sensors (diodes and resistance devices) are available
with readout devices that have higher precision than thermocouples. Silicon diodes, gallium-aluminum-arsenide
diode, carbon glass resistor, platinum resistor, and germanium resistor are some of the more commonly used
types of sensors.
Cryogenic temperature controllers that work with the types of temperature sensors named above are available.
The majority of temperature controllers vary heating power and require that the test chamber environment is
slightly cooler than the set-point temperature. The test engineer is responsible for the environmental chamber
and cooling medium of the system. The controllers use the temperature sensors as the feedback sensor to
operate a control loop and supply power for resistive heaters.
Additional Equipment Considerations. Teflon-insulated (E.I. DuPont de Nemours & Co., Inc., Wilmington,
DE) lead wires are advisable at very low temperatures because the insulation will be less likely to crack and
cause problems. Electronic noise reduction can be an issue in low-temperature tests because lead wires tend to
be long. Standard methods of noise-reduction are shielding and grounding. Self-heating and thermocouple
effects are important issues at low temperatures. Precautions should be taken to ensure that thermal effects do
not mask the test data. Strain gage excitation voltages should be kept low. Reference 10 gives the parameters in
terms of power density for calculating excitation voltage to be used for strain gages at 4 K.
References cited in this section
2. R.P. Reed and A.F. Clark, Ed., Materials at Low Temperatures, ASM, 1983
8. G. Hartwig and F. Wuchner, Low Temperature Mechanical Testing Machine, Rev. Sci. Instrum., Vol 46,
1975, p 481–485
9. R.P. Reed, A Cryostat for Tensile Test in the Temperature Range 300 to 4 K, Advances in Cryogenic
Engineering, Vol 7, Plenum Press, NY, 1961, p 448–454
10. J.H. Lieb and R.E. Mowers, Testing of Polymers at Cryogenic Temperatures, Testing of Polymers,
J.V.Schmitz, Ed., Vol 2, John Wiley & Sons, 1965, p 84–108
11. R.P. Reed and R.L. Durcholz, Cryostat and Strain Measurement for Tensile Tests to 1.5 K, Advances in
Cryogenic Engineering, Vol 15, Plenum Press, NY, 1970, p 109–116
12. C. Ferrero, Stress Analysis Down to Liquid Helium Temperature, Cryogenics, Vol 30, March 1990, p
249–254
Tension and Compression Testing at Low Temperatures
Robert P. Walsh, National High Magnetic Field Laboratory, Florida State University
Tension Testing
As at room temperature, tensile tests at low temperature are for determining engineering design data as well as
for studying fundamental mechanical-metallurgical behaviors of a material such as deformation and fracture
processes. The usual engineering data from tensile tests are yield strength, ultimate tensile strength, elastic
modulus, elongation to failure, and reduction of area. The effects of material flaws (inclusions, voids, scratches,
etc.) are amplified in low-temperature testing, as materials become more brittle and sensitive to stress
concentrations. Data scatter tends to increase, and the quantity of tests to characterize a material is usually
greater than that used for room temperature testing. The test engineer must judge when sufficient testing has
been done to provide representative data on a material.
Test fixture alignment is important at low temperatures because of necessarily long load trains. Self-alignment
in tensile tests can be accomplished through the use of universal joints, spherical bearings, and pin connections.
The alignment should meet specifications detailed in ASTM E 1012, “Standard Practice for Verification of
Specimen Alignment Under Tensile Loading.” Strain measurement should be done using an averaging
technique that can reduce errors associated with misalignment or bending stress. Strain measurement equipment
is detailed above in the instrumentation section.
Metals. The standard tensile test method for metals, ASTM E 8, covers the temperature range from 50 to 100 °F
and is used as a guideline for lower temperature tests. The need for engineering data in the design of
superconducting magnets has resulted in the adoption of the tensile test standard ASTM E 1450 for tests of
structural alloys in liquid helium at 4.2 K.
The strain rate sensitivity of the flow stress in metals decreases as temperature is reduced. Typical strain rates in
standard tensile tests are on the order of 10
-5
s
-1
to 10
-2
s
-1
and do not have a pronounced effect on the material
flow stress. The strain rate becomes important in cryogenic temperature tests because of a tendency for
specimen heating causing discontinuous yielding in displacement control tests. Discontinuous yielding is a
subject of low-temperature research of alloys, well described in ASTM E 1450. The localized strain/heating
phenomenon typically initiates after the onset of plastic strain and results in a serrated stress-strain curve.
ASTM test standard E 1450 prescribes a maximum strain rate of 10
-3
s
-1
and notes that lower rates may be
necessary. The strain required to initiate discontinuous yielding increases with decreasing strain rate. If
discontinuous yielding starts before the 0.2% offset yield strength is reached, the associated load drop affects
the estimation of the yield strength. It may be possible to slow the strain rate to postpone the serrated curve
until after the 0.2% offset yield strength is reached and then to increase the rate, not to exceed 10
-3
s
-1
.
Reference 13 reports research on the effect of strain rate in tensile tests at 4 K.
Test specimen sizes are preferably small for low-temperature tests. The common 0.5 in. round, ASTM-standard
tensile specimen is rarely used at low temperature. Tensile specimens should be small due to size constraints
placed by the environmental test chamber, which is designed for thermal efficiency. Standard capacity test
machines (100 and 50 kN) favor small specimens due to high tensile strengths encountered at low temperatures.
A subscale version of the 0.5 in. round that meets ASTM specifications and works well at cryogenic
temperature is shown in Fig. 7(a). A 100 kN force capacity test machine can generate about 3.5 GPa stress on a
6 mm diameter gage section. Figure 7(b) shows a flat, subscale tensile specimen that is also commonly used at
cryogenic temperatures.
Fig. 7 Schematics of tensile specimen commonly used at low temperature. (a) Round. (b)
Flat. Dimensions are in inches (millimeters.) thd, threaded
Polymers and Fiber-Reinforced Polymer (FRP) Composites. Tension tests of FRP composites are governed in
test procedure ASTM D 3039, while polymers and low modulus (<20 GPa) composites are tested using the
guidelines established in ASTM D 638. Neither have specific temperature ranges or limitations.
The problem with testing polymers at low temperatures is the tendency for polymers to be extremely brittle
materials. Test temperatures below room temperature are usually well below the glass transition temperature,
T
g
, of the polymer. The test specimen designs in ASTM D 638 are susceptible to grip failures once the material
is brittle. Traditional strain measurement techniques must be performed carefully. Clip-on extensometers must
mechanically attach to the material, usually producing some sort of stress concentration that may initiate
failure. Strain gages locally reinforce low-modulus material, and the associated error and correction method is
explained in Ref 14. Most tensile tests of polymers show an increase in tensile strength upon cooling from 295
to 77 K and a decrease or constant level of strength with continued cooling to 4 K (Ref 15). One would expect
the strength to continue to increase with decreasing temperature. This anomalous behavior is probably an
artifact of the tensile testing of an extremely brittle material.
Tensile tests of FRP composites at low temperatures are simpler than tests of neat polymers because of the
more rugged sample. One challenge is the gripping of high-strength, unidirectionally reinforced composites.
The convenience of hydraulic wedge grips is not usually an option in low-temperature tests. An example of a
low-temperature tension and compression test program to characterize a unidirectionally reinforced epoxy
composite from 295 to 4 K is described in Ref 16.
References cited in this section
13. R.P. Reed and R.P. Walsh, Tensile Strain Effects in Liquid Helium, Advances in Cryogenic
Engineering, Vol 34, Plenum Press, 1988, p 199–208
14. C.C. Perry, Strain Gage Reinforcement Effects on Low Modulus Materials, Manual on Experimental
Methods for Mechanical Testing of Composites, R.L. Pendelton and M.E. Tuttle, Ed., Society for
Experimental Mechanics, 1989, p 35–38
15. R.P. Reed and R.P. Walsh, Tensile Properties of Resins at Low Temperatures, Advances in Cryogenic
Engineering, Vol 40, Plenum Press, NY, 1994, p 1129–1136
16. R.P. Walsh, J.D. McColskey, and R.P. Reed, Low Temperature Properties of a Unidirectionally
Reinforced Epoxy Fibreglass Composite, Cryogenics, Vol 35 (No. 11), 1995, p 723–725
Tension and Compression Testing at Low Temperatures
Robert P. Walsh, National High Magnetic Field Laboratory, Florida State University
Compression Testing
Compressive properties are of interest to the design and analysis of structures subjected to compressive or
bending stress. Compressive properties include modulus of elasticity, yield stress, deformation beyond yield
point, and compressive strength (assuming that the material does not just flatten).
Compression testing of metals is conducted using the guidelines established in test procedure ASTM E 9,
“Standard Test Methods of Compression Testing of Metallic Materials at Room Temperature.” An application
of low-temperature compression tests is to determine if there is a strength differential effect for a particular
metal. Another case for compression tests is to measure the yield strength of a brittle metal that tends to fail in
tension before the 0.2% offset yield strength is reached.
Polymers are compression tested at low temperatures using ASTM D 695, “Standard Test Method for
Compressive Properties of Rigid Plastics” for guidance. This standard states that it is applicable to tests of
unreinforced and reinforced rigid plastics, including high-modulus composites. Research comparing results for
different compressive test methods on high-modulus composites has shown that ASTM D 695 is acceptable for
modulus determination but not a good method for strength measurement (Ref 17). This direct compression test
method is excellent for determining the strength and modulus of brittle polymers at low temperatures. A
compressive test fixture used for testing polymers and composites at low temperatures is shown in Fig. 6. The
fixture uses a spherical bearing seat on the lower stationary compression platen (dry graphite powder is used as
a lubricant) to adjust platen parallelism. Applying a nominal load with the upper platen aligns the lower platen,
which is then locked into place with three clamps. Figure 8 shows some data from recent low-temperature tests
on a high-strength polymer (Ref 18). The results graphically show the difference between the tension (ASTM D
638) and compression (ASTM D 695) tests of these relatively brittle materials.
Fig. 8 A comparison of the effect of temperature on tensile and compressive properties of
some high strength polymers. A, polyphenylene; B, modified polyphenylene; BF, chopped
fiber-reinforced polyphenylene; and BP, polyphenylene with plasticizer
Compression tests of FRP composites are governed by ASTM D 3410, “Standard Test Method for Compressive
Properties of Polymer Matrix Composite Materials with Unsupported Gage Section by Shear Loading.” This
standard is for high-modulus composites where the elastic modulus of the composite is greater than 21 GPa.
The test standard has three test procedures within it. Procedure A uses a compression test fixture commonly
called the Celanese compression fixture. This test fixture has been applied to low-temperature tests described in
Ref 16. Test procedure B is similar but uses a different test fixture, commonly called the IITRI (Illinois Institute
of Technology Research Institute) fixture. Test procedure C is a sandwich beam test where the beam is loaded
in four-point bending. All three-test procedures are rather compact and well suited for low-temperature testing.
The Celanese and IITRI fixtures are rather heavy and take some time to heat and cool. This is a consideration
when repetitive testing is to be done at low temperature. These two fixtures also have alignment sleeves or pins
that may have a tendency to freeze at low temperatures due to differential thermal contractions and/or due to
ice. The sandwich beam test specimen is rather small and may be well suited for low-temperature tests.
References cited in this section
16. R.P. Walsh, J.D. McColskey, and R.P. Reed, Low Temperature Properties of a Unidirectionally
Reinforced Epoxy Fibreglass Composite, Cryogenics, Vol 35 (No. 11), 1995, p 723–725
17. N.R. Adsit, Compression Testing of Graphite Epoxy, Compression Testing of Homogeneous Materials
and Composites, STP 808, R. Chait and R. Papirno, Ed., ASTM, 1983, p 175–186
18. V.J. Toplosky, R.P. Walsh, S.W. Tozer, and F. Motamedi, Mechanical and Thermal Properties of
Unreinforced and Reinforced Polyphenylenes at Cryogenic Temperatures, Advances in Cryogenic
Engineering, Vol 46, Plenum Press, NY, 1999
Tension and Compression Testing at Low Temperatures
Robert P. Walsh, National High Magnetic Field Laboratory, Florida State University
Temperature Control
Test temperature is controlled by a bath temperature or by controlling the temperature of a gaseous
environment. The temperature of the test specimen should be maintained ±1 K for the duration of the test. For
bath-cooled tests, the temperature of the bath and the specimen should be the same. A potential source for error
is the conduction path or heat sink that the load train provides, possibly causing the specimen to be warmer than
the bath. For tests at temperatures below 77 K (typically 4 K tests), the test cryostat is precooled with liquid
nitrogen as an economical time-saving step.
The temperature of the specimen is usually monitored for tests using gaseous environment temperature control.
The temperature should be measured at the gage section and at the gripped ends to ensure that the temperature
across the length of the specimen is constant. For temperatures above 80 K, the cooling can be a static method,
such as a pool of liquid nitrogen in the dewar below the test fixturing. Test temperatures below the liquid
nitrogen bath temperature are best accomplished through the use of cold helium gas for cooling power. This can
be accomplished with a flow cryostat, where cooling power is regulated by throttling the flow of the cold gas or
liquid. Between the manual regulation of the cooling medium and the regulated heater power, one can obtain
constant test temperatures between 77 and 4 K. Variations of cryogenic temperature control exist such as
cryogenic refrigerators, which can be applied to the temperature control of mechanical tests. Cryocoolers and
other mechanical refrigeration techniques are not commonly used in mechanical tests, due to the initial capital
expense and the relative simplicity of other cooling methods.
Tension and Compression Testing at Low Temperatures
Robert P. Walsh, National High Magnetic Field Laboratory, Florida State University
Safety
Safety in a laboratory or industrial setting is always a concern. Some of the safety issues with respect to low-
temperature testing are common sense issues, while some others are not so obvious. Table 1 describes the most
common safety issues and appropriate solutions for each. Safety issues associated with the use of liquid
hydrogen or liquid oxygen are not dealt with in this article and can be found in Ref 10. In addition to the issues
listed in Table 1, personnel should consider general safety issues related to tension and compression testing at
all temperature ranges.
Table 1 Safety issues associated with the use of liquid cryogens for low-temperature
testing
Safety issue Solution
Liquid cryogens can spill or splash onto the body
and cause freezing of human tissue.
Personnel should wear appropriate clothing and avoid
direct contact with cold parts.
Helium, nitrogen, or carbon dioxide can displace
air in a confined area.
Cryogens should be used only in well-ventilated areas.
Volumetric expansion is extremely high (typically
700–1000 times) when a liquid cryogen vaporizes.
Such expansion is dangerous when it occurs in
closed containers or fixture components with
potential for trapped gas/liquid volumes.
Cryostats must include safety pressure-relief
capabilities; redundancy is necessary because
mechanical relief valves may freeze or malfunction.
Component parts such as tubes or threaded connections
that can trap liquids or gases should be identified, and