ASTM D3410_Standard Test Method for Compressive Properties of Polymer Matric Composite Materials

ASTM - D3410

-Symbols to know:

Summary of Test Method:

A flat strip of material having a constant rectangular cross section, as shown in the specimen drawings of Figs. 1-4, is loaded in compression by a shear force acting along the grips. The shear force is applied via wedge grips in a specially-designed fixture shown in Figs. 5-7. The influence of this wedge grip design on fixture characteristics is discussed in 6.1. 4.2 To obtain compression test results, the specimen is inserted into the test fixture which is placed between the platens of the testing machine and loaded in compression. The ultimate compressive stress of the material, as obtained with this test fixture and specimen, can be obtained from the maximum force carried before failure. Strain is monitored with strain or displacement transducers so the stress-strain response of the material can be determined, from which the ultimate compressive strain, the compressive modulus of elasticity, Poisson’s ratio in compression, and transition strain can be derived.

Required Equipment:

Micrometers and Calipers —A micrometer with a 4 to 7 mm [0.16 to 0.28 in.] nominal diameter ball interface or a flat anvil interface shall be used to measure the specimen thickness. A ball interface is recommended for thickness measurements when at least one surface is irregular (for example, a coarse peel ply surface which is neither smooth nor flat). A micrometer or caliper with a flat anvil interface shall be used for measuring length, width and other machined surface dimensions. Accuracy of 0.0025 mm [60.0001 in.] is adequate for thickness measurements, while an instrument with an accuracy of 0.025 mm [60.001 in.] is adequate for measurement of length, width and other machined surface dimensions.

Compression Fixture —The fixture uses rectangular wedges and allows for variable width and thickness specimens. A sectional schematic and photographs of the fixture are shown in Figs. 5-7. Each set of specimen wedge grips fits into a mating set of wedges that fits into the upper and lower wedge housing block assemblies. By using wedges of different thicknesses, specimens of varying thickness can be tested in this fixture.

Specimen Alignment Jig —Compression test results generated by this test method are sensitive to the alignment of the specimen with respect to the longitudinal axis of the wedges in the test fixture. Specimen alignment can be accomplished by using an alignment jig or gage block that mechanically holds the specimen captive outside the fixture housing blocks (as shown in Fig. 8), or by using a custom jig or machinist’s square for a specimen inserted into wedge grips already in the fixture housing blocks.

Strain-Indicating Device—Longitudinal strain shall be simultaneously measured on opposite faces of the specimen to allow for a correction as a result of any bending of the specimen and to enable detection of Euler (column) buckling. Back-to-back strain measurement shall be made for all five specimens when the minimum number of specimens allowed by this test method are tested. If more than five specimens are to be tested, then a single strain-indicating device may be used for the number of specimens greater than the five, provided the total number of specimens are tested in a single test fixture that remains in the load frame throughout the tests (see Note 5), that no modifications to the specimens or test procedure are made throughout the duration of the tests, and provided the bending requirement of 11.9.1 is met for the first five specimens.

Bonded Resistance Strain Gauges—Strain gage selection is a compromise based on the procedure and the type of material to be tested. Strain gages should have an active grid length of 3 mm [0.125 in.] or less (1.5 mm [0.063 in.] is preferable). Gage calibration certification shall comply with Test Methods E251. When testing woven fabric laminates, gage selection should consider the use of an active gage length which is at least as great as the characteristic repeating unit of the weave. For stain gauges resistances of 350 ohms or higher are preferred. Use the minimum possible gage excitation voltage consistent with the desired accuracy (1 to 2 V is recommended) to further reduce the power consumed by the gage.

Friction stuff — Emery cloth can be used as the interface between the grip and the coupon.

Testing Machine:

Testing Machine Heads —The testing machine shall have two loading heads, with at least one movable along the testing axis. 7.3.2 Fixture Attachment—Typically the upper portion of the fixture is attached directly to the upper crosshead, and a flat platen attached to the lower crosshead is used to support the lower portion of the fixture. The platen should be at least 20 mm [0.75 in.] thick. The fixture may be coupled to the testing machine with a joint capable of eliminating angular restraint.

Drive Mechanism —The testing machine drive mechanism shall be capable of imparting to the movable head a controlled displacement rate with respect to the stationary head.

Force Indicator —The testing machine force-sensing device shall be capable of indicating the total force being resisted by the test specimen. This device shall be essentially free from inertia-lag at the specified rate of testing and shall indicate the force with an accuracy over the force range(s) of interest of within 61 % of the indicated value.

For when atmosphere conditions suck:

Conditioning Chamber —When conditioning materials in other than ambient laboratory environments, a temperature-/ moisture-level controlled environmental conditioning chamber is required that shall be capable of maintaining the required relative temperature to within 63°C [65°F] and the required relative vapor level to within 65 %. Chamber conditions shall be monitored either on an automated continuous basis or on a manual basis at regular intervals. 7.6\
Environmental Test Chamber —An environmental test chamber is required for test environments other than ambient testing laboratory conditions. This chamber shall be capable of maintaining the gage section of the test specimen within 63°C [65°F] of the required test temperature during the mechanical test. In addition, the chamber may have to be capable of maintaining environmental conditions such as fluid exposure or relative humidity during the test (see 11.4).

If the Material is extremely stiff:

Extensometers —Extensometers shall satisfy, at a minimum, Practice E83, Class B-2 requirements for the strain range of interest, and shall be calibrated over that strain range in accordance with Practice E83. For extremely stiff materials, or for measurement of transverse strains, the fixed error allowed by Class B-2 extensometers may be too large. The extensometer shall be essentially free of inertia lag at the specified speed of testing.

Sampling and Test Specimens:

Sampling—Test at least five specimens per test condition unless valid results can be gained through the use of fewer specimens

Geometry:

The test specimen shall have a constant rectangular cross section with a specimen width variation of no more than 1 % and a specimen thickness variation of no more than 2 %. Look to table 1 for more info

Table 2 for actual size recommendations

and Table 3 for Recommended specimen thickness as a function of expected modulus and ultimate compressive stress in the direction of force application for gage lengths of 12, 20, and 25 mm [0.5, 0.75, and 1.0 in.] using an assumed value of Gxz of 4 GPa [600 000 psi] (Gxz can be determined using Test Method D5379/D5379M).

This can be found using the Equation below:

Panel Requirements:

Panel Fabrication —Control of fiber alignment is important. Improper fiber alignment will reduce the measured properties. Erratic fiber alignment will also increase the coefficient of variation. Suggested methods of maintaining fiber alignment are discussed in Section 6. The panel preparation method used shall be reported.
Machining Methods —Specimen preparation is extremely important. The specimens may be molded individually to avoid edge and cutting effects or they may be cut from panels. If they are cut from panels, precautions shall be taken to avoid notches, undercuts, rough or uneven surfaces, or delamination caused by inappropriate machining methods. Final dimensions should be obtained by precision sawing, milling, or grinding. Mold or machine edges flat and parallel within the specified tolerances.
Labeling —Label the specimens so that they will be distinct from each other and traceable back to the raw material, and in a manner that will both be unaffected by the test and not influence the test.

Procedure:

Parameters To Be Specified Before Test:

The compression specimen sampling method, specimen type and geometry, and if required, conditioning travelers.
The compressive properties and data reporting format desired. NOTE 8—Determine specific material property, accuracy, and data reporting requirements prior to test for proper selection of instrumentation and data recording equipment. Estimate operating stress and strain levels to aid in transducer selection, calibration of equipment, and determination of equipment settings.
T he environmental conditioning test parameters.
If performed, the sampling method, specimen geometry, and test parameters used to determine density and reinforcement volume.

General Instructions:

Report any deviations from this test method, whether intentional or inadvertent.
If specific gravity, density, reinforcement volume, or void volume are to be reported, then obtain these samples from the same panels as the test samples.
Condition the specimens, either before or after strain gaging, as required. Condition travelers if to be used.
Following final specimen machining and any conditioning, but before the compression testing, determine the specimen area as A=w×h at three places in the gage section and report the area as the average of these three determinations in mms.
Apply strain gages (or extensometers) to both faces of the specimen.
Loading Rate—It is desired to maintain a constant strain rate in the gage section. Select the strain rate so as to produce failure within 1 to 10 min from the beginning of force application. Suggested strain rates are: Strain-Controlled Tests—A standard strain rate of 0.01 min/1. Constant Head-Speed Tests—A standard crosshead displacement of 1.5 mm/min [0.05 in./min].
Test Environment—Condition the specimen to the desired moisture profile and, if possible, test under the same conditioning fluid exposure level.
Store the specimen in the conditioned environment until test time, if the testing area environment is different than the conditioning environment.
Monitor test temperature by placing an appropriate thermocouple within 25 mm [1.0 in.] of the specimen gauge section. Maintain the temperature of the specimen. Taping thermocouple(s) to the test specimen (and the traveler) is an effective measurement method.

Fixture Installation:

Ensure that the sliding surfaces of the fixture wedges, guide rods, and bearings are flat (wedges), polished, lubricated, and nick- and corrosion-free.
Inspect the parallelism of the platens and the condition of the mating surfaces of the wedge housing blocks. Correct if needed.
Place the lower wedge housing block on the lower platen. Attach the upper wedge housing block to the upper crosshead or insert it into the upper wedge housing holding fixture, centered over the lower wedge housing block. While the load cell may be connected to either crosshead as required, the entire assembly must be centered on the line of action of applied force.
Move a crosshead to close the distance between the two housing blocks while guiding the bearing guide rods into the mating bearing of the companion housing block. The lower housing block can be fitted with guide rods long enough to allow the rods to remain in the bearings while the wedge/ specimen assembly is loading into and out of the housing blocks.

Specimen Insertion:

If necessary, move the testing machine crosshead to open the distance between the two housing blocks so that both upper and lower wedge grip assemblies may be accessed.
If specimen alignment is to be performed with the grip/specimen assembly outside the fixture housing blocks (see 7.2.2), perform this procedure. Place the completed grip/ specimen assembly into the lower housing block and close the distance between the housing blocks. The ends of the wedge grips should be even with each other following insertion into the housing blocks to avoid inducing a bending moment that results in premature failure of the specimen at the grips. When using an un-tabbed specimen, a folded strip of medium-grade abrasive cloth between the specimen faces and the grip jaws (grit side toward specimen) may provide a non-slip grip on the specimen without jaw serration damage. When using tabbed specimens, insert the specimen so that the grip jaws grip the entire length of the tab.
If the specimen is to be aligned with the wedge grips in the fixture housing blocks, raise the lower jaws within the lower housing assembly so that grip-faces open to allow specimen insertion. Place the specimen between the grips such that the entire grip length will contact the grip faces when closed. Center the specimen from side to side and then lower the grips, lightly clamping the specimen. Arrange any pre-attached transducer lead-wires as required.
If necessary, free the upper wedge grips so that they are in the fully open position. Moving the crosshead, close the distance between the housing blocks and guide the upper end of the specimen into the opening between the upper wedge grips. Stop the head and zero the force on the testing machine.
Manually close the upper grips to check specimen vertical displacement. As with the lower grips, when the upper grips are closed onto the specimen the entire grip length should be in contact with the wedge grip faces. If necessary, adjust the head position and repeat this step.
Keeping the grips closed onto the specimen, slowly close the distance between the housing blocks by moving the crosshead while watching the force indicator. Stop the crosshead when the specimen begins to take a compressive force. The application of a small amount of initial compressive force, followed by immediate removal, may be helpful in seating the fixture grips before the test. This preload should be kept to a minimum, in no case more than 5 % of the ultimate force for the material, and use of the technique shall be recorded in the test results.

Transducer Installation:

If the strain transducer(s) other than strain gages are to be used, attach them to the specimen at the mid-span, mid-width location. Attach the strain recording instrumentation to the strain gages or other transducer(s) on the specimen. Remove any remaining preload and zero the transducer(s).

Loading:

Apply the force to the fixture at the specified rate until failure while recording data.

Data Recording:

Record force versus strain (or displacement) continuously or at frequent regular intervals. If a transition region or initial ply failures are noted, record the force, strain, and mode of damage at such points. If the specimen is to be failed, record the maximum force, the failure force, and the strain (or transducer displacement) at, or as near as possible to, the moment of failure. NOTE 13—Other valuable data that can be useful in understanding testing anomalies and gripping or specimen slipping problems include force versus crosshead displacement data and force versus time data.
A difference in the stress-strain or force-strain slope from opposite faces of the specimen indicates bending in the specimen. For the elastic property test results to be considered valid, percent bending in the specimen shall be less than 10 % as determined by Eq 2. Determine percent bending at the midpoint of the strain range used for chord modulus calculations (Table 4).
The same requirement shall be met at failure strain for the strength and strain-to-failure data to be considered valid. This requirement shall be met for all five of the specimens requiring back-to-back strain measurement. If possible, a plot of percent bending versus average strain should be recorded to aid in the determination of failure mode.
Rapid divergence of the strain readings on the opposite faces of the specimen, or rapid increase in percent bending, is indicative of the onset of Euler (column) buckling, which is not an acceptable compression failure mode for this test method. Record any indication of Euler buckling. Example of Euler Buckling below.

Acceptable Failure Modes:

Failure Identification Codes—Record the mode, area, and location of failure for each specimen. Choose a standard failure identification code based on the three-part code shown in Fig. 9 below. A multimode failure can be described by including each of the appropriate failure-mode codes between the parentheses of the M failure mode. For example, a typical gauge section compression failure for a [90/0]ns laminate having elements of Angled, Kink-banding, and longitudinal Splitting in the middle of the gage section would have a failure mode code of M(AKS)GM.
Acceptable Failure Modes—The first character of the Failure Identification Code describes the failure mode. All of the failure modes in the “First Character” Table of Fig. 9 (above)) are acceptable with the exception of end-crushing or Euler buckling. An Euler buckling failure mode cannot be determined by visual inspection of the specimen during or after the test, therefore it must be determined through inspection of the stress-strain or force-strain curves when back-to-back strain indicating devices are used.
Acceptable Failure Area—The most desirable failure area is the middle of the gage section since the gripping/ tabbing influence is minimal in this region. Because of the short gage length of the specimens in this test method, it is very likely that the failure location will be near the grip/tab termination region of the gage section. This is still an acceptable failure area. If a significant fraction (>50 %) of the failures in a sample population occurs at the grip or tab interface, reexamine the means of force introduction into the specimen. Factors considered should include the tab alignment, tab material, tab adhesive, grip type, grip pressure, and grip alignment. Any failure that occurs inside the grip/tab portion of the specimen is unacceptable.

Calculations:

Compressive Stress/Ultimate Compressive Stress— Calculate the ultimate compression strength using Eq 3 and report the results to three significant figures. If the compressive modulus is to be calculated, determine the compressive stress at each required data point using Eq 4 below.

Compressive Strain and Ultimate Compression Strain—If compressive modulus or ultimate compressive strain is to be calculated, determine the average compressive strain at each required data point using Eq 5 and 6, respectively, and report the results to three significant figures.

Compressive Modulus of Elasticity:

Compressive Chord Modulus of Elasticity—Select the appropriate chord modulus strain range from Table 4. Calculate the compressive chord modulus of elasticity from the stress-strain data using Eq 7. If data are not available at the exact strain range end points (as often occurs with digital data), use the closest available data point. Report the compressive chord modulus of elasticity to three significant figures. Also report the strain range used in the calculation. A graphical example of chord modulus is shown in Fig. 10 below.

The recommended strain ranges should only be used for materials that do not exhibit a transition region (a significant change in the slope of the stress-strain curve) within the recommended strain range. If a transition region occurs within the recommended strain range, then a more suitable strain range should be used and reported.

Compressive Modulus of Elasticity (Other Definitions)—Other definitions of elastic modulus may be evaluated and reported at the user’s discretion. If such data are generated and reported, report also the definitions used, the strain range used, and the results to three significant figures. Test Method E111 provides additional guidance in the determination of Modulus of Elasticity.

Compressive Poisson’s Ratio By Chord Method— Select the appropriate Poisson’s ratio strain range from Table 4 (above, under Data Recording). Determine (by plotting or otherwise) the transverse strain (strain in the plane of the specimen and perpendicular to the applied force), εt , at each of the two longitudinal strain range endpoints (measured parallel to the applied force), εl . If data are not available at the exact strain range endpoints (as often occurs with digital data), use the closest available data point. Calculate Poisson’s ratio in the appropriate strain range by Eq 8 (below) and report to three significant figures. When determining Poisson’s ratio, match the transverse strain with the appropriate longitudinal strain. For instance, match output from a single transverse strain gage with the output from the single longitudinal gage mounted in an adjacent location on the same side of the specimen. If back-to-back transverse gages are used, average their output and compare to the average longitudinal strain.

Compressive Poisson’s Ratio (Other Definitions)— Other definitions of Poisson’s ratio may be evaluated and reported at the user’s discretion. If such data are generated and reported, report also the definitions used, the strain range used, and the results to three significant figures. Test Method E132 provides additional guidance in the determination of Poisson’s ratio.

Transition Strain—Where applicable, determine the transition strain from either the bilinear longitudinal stress versus longitudinal strain curve or the bilinear transverse strain versus longitudinal strain curve. Create a best linear fit or chord line for each of the two linear regions and extend the lines until they intersect. Determine to three significant figures the longitudinal strain that corresponds to the intersection point and record this value as the transition strain. Report also the method of linear fit (if used) and the strain ranges over which the linear fit or chord lines were determined. A graphical example of transition strain is shown in Fig. 10 (above, under Compressive Cord Modulus of Elasticity).

Statistics—For each series of tests calculate the average value, standard deviation and coefficient of variation (in percent) for each property determined.

Things that should be noted

Factors that influence the compressive response and should therefore be reported include the following: material, methods of material preparation and layup, specimen stacking sequence, specimen preparation, specimen conditioning, environment of testing, specimen alignment and gripping, speed of testing, time at temperature, void content, and volume percent reinforcement. Properties, in the test direction, that may be obtained from this test method include:

5.1.1 Ultimate compressive strength,

5.1.2 Ultimate compressive strain,

5.1.3 Compressive (linear or chord) modulus of elasticity,

5.1.4 Poisson’s ratio in compression, and

5.1.5 Transition strain.

Things that directly affect the test

The surface finish of the mating surfaces of the wedge grip assembly. Since these surfaces undergo sliding contact they must be polished, lubricated, and nick-free.
Compression strength for a single material system has been shown to differ when determined by different test methods. Such differences can be attributed to specimen alignment effects, specimen geometry effects, and fixture effects even though efforts have been made to minimize these effects.
Compression modulus, and especially ultimate compressive stress, are sensitive to poor material fabrication practices, damage induced by improper specimen machining, and lack of control of fiber alignment. Fiber alignment relative to the specimen coordinate axis should be maintained as carefully as possible,
The data resulting from this test method has been shown to be sensitive to the flatness and parallelism of the tabs.
A high percentage of grip-induced failures, especially when combined with high material data scatter, is an indicator of specimen gripping problems.
Excessive bending will cause premature failure, as well as highly inaccurate modulus of elasticity determination. Every effort should be made to eliminate bending from the test system. Bending may occur for the following reasons: (1) misaligned (or out-of tolerance) grips or associated fixturing, (2) improper installation of specimen, or (3) poor specimen preparation.
Premature failures and lower stiffness's are observed due to edge softening in laminates containing off-axis plies. Because of this, the strength and modulus for angle-ply laminates can be underestimated. For quasi-isotropic laminates and those containing even higher percentages of 0° plies, the effect is less.