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ASTM D6641 Combined Loading Compression (CLC) Test for Polymer Matrix Composite Laminates

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ASTM D6641 Combined Loading Compression (CLC) Test for Polymer Matrix Composite Laminates

ASTM D6641, Combined Loading Compression (CLC) Test for Polymer Matrix Composite Laminates, is a protocol that measures the compressive strength of polymer laminates that are subjected to combinations of uniaxial and biaxial loading.

What Is ASTM D6641?

ASTM D6641 establishes a procedure for determining the compressive strength and stiffness properties of polymer matrix composite materials using a combined loading compression (CLC) test fixture. ASTM D6641 applies to flat balanced and symmetric laminates that contain at least one 0° ply. In this application, the standard specimen is untabbed, and so, for strength determination, the laminate is limited to a maximum of 50 % 0° plies. The compressive force is introduced into the specimen by combined end- and shear-loading. ASTM D6641 can be used to test unidirectional composites, multidirectional composite laminates, fabric composites, chopped fiber composites, and similar materials.

Why Use the CLC Test Method?

CLC tests are useful for compression testing of untabbed composite specimens of moderate strengths with compressive strengths up to about ~150 ksi, including cross-ply and quasi-isotropic lay-up laminates, fabric composites, random chopped-fiber composites, and other materials of similar or lower strength.  CLC test fixtures can produce compressive strength and modulus values equivalent to those obtained using tabbed specimens and more complex and expensive test fixtures such as the IITRI, Celanese, and their modified configurations. 

Important Considerations When Testing per ASTM D6641

Unidirectional (0° ply orientation) composites can be tested to determine unidirectional composite modulus and Poisson’s ratio, but not compressive strength.

ASTM D6641 produces compressive property data for material specifications, research and development, quality assurance, and structural design and analysis.

Factors that influence the compressive response include: type of material, methods of material preparation and lay-up, specimen stacking sequence, specimen preparation, specimen conditioning, environment of testing, speed of testing, time at temperature, void content, and volume percent reinforcement.

ASTM D6641 Test Method

The ASTM D6641 CLC test fixture can test untabbed, straight sided composite specimens of rectangular cross section. A typical specimen is 140 mm (5.5 in) long and 12 mm (0.5 in) wide, having an unsupported (gage) length of 12 mm (0.5 in) when installed in the fixture. Gage lengths between 12 mm (0.5 in) and 25 mm (1.0 in) are acceptable. The short 12-mm gauge length provides sufficient space to install bonded strain gauges when they are required. The fixture, which subjects the specimen to combined end- and shear-loading, is loaded in compression between platens. Load-strain data are collected until failure occurs (or until a specified strain level is achieved if only compressive modulus or Poisson’s ratio, or both, is to be determined, and not the complete stress-strain curve to failure).

The specimen may be untabbed (Procedure A) or tabbed (Procedure B), as required. One requirement for a successful test is that the specimen ends do not crush during the test.

Laminate test properties (in the test direction) that can be obtained from ASTM D6641 include ultimate compressive strength, ultimate compressive strain, compressive (linear or chord) modulus of elasticity, and Poisson’s ratio in compression.

Test Machines for Polymer Matrix Composites

A universal test machine, equipped with special composite test accessories, is capable of performing quasi static tension/compression tests, including tensile, peel, adhesion, and flexural testing. TestResources machines includes a Newton universal servo controller and so features a uniform constant rate of extension (CRE), or velocity control mode, constant load rate and constant strain rate. Force measurement complies with ASTM E4.