ASTM - D3410
-Symbols to know:
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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.
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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.
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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
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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:
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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.
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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.
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