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

If you plan on conducting ASTM D6641 we recommend acquiring the test standard from the standards organization. There are many important details regarding this test method that are provided which are key to achieving successful results. Every standard provides important environmental conditions, test sample preparation and conditioning requirements, sample dimensions and test reporting expectations. It’s critical that you fully understand the standard so that you can set up the test, run the procedure, and report the results requirements.

ASTM D6641 Test Equipment

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.

What Other Standards Apply to Polymer Matrix Composite Testing?

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. The machine 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.

A 25 kN (5,500 lbf) or 50 kN (11,250 lbf) system may be needed for testing glass fiber composites. A 150 kN (33,750 lbf) or 300 kN (67,500 lbf) system is needed for testing carbon fiber composites.

ASTM D6641 Test Machines

313 Family Electromechanical Universal Test Machine

50 kN (11,250 lbf)

313 family dual column load frame with extra height and width options. Even with higher loads this frame has a load accuracy +/- 0.5% of indicated load within load cell rated force range.

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315 Family Electromechanical Universal Test Machine

150 kN (33,750 lbf)

315 family floor standing universal test machines are configured to each customer's needs. This machine is for higher force requirements with a column clearance of 550 mm (21.6 in).

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316 Family Electromechanical Universal Test Machine

300 kN (67,500 lbf)

316 family floor standing universal test machines are configured to each customer's needs. This machine is for higher force requirements such as for testing heavy steel products.

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ASTM D6641 Fixtures

ASTM D6641 Strain & Deflection Measurement

E3542 Clip on Axial Extensometer

E3542 Clip on axial extensometers are the most common solution to measuring strain values that are important to generating modulus and yield data.

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E3442 Clip On Miniature Axial Extensometers

Miniature general purpose clip on extensometers for axial tensile, compression, and cyclic testing. Gauge lengths from 3 mm to 12 mm (0.15 in to 0.47 in) and measuring ranges from 5% to 100% strain.

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ASTM D6641 Controls Software

ASTM D6641 Test Machine Controller Software

ASTM D6641 Test Machine Controller Software | ND6641

Application specific module features preset settings, test calculations, and graphical test report contents.

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ASTM D6641 Test Machine Controller Software

Video Recording and Playback Software Module | TestVideo

High resolution video capture of the test. Playback is time synchronized with load data.

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ASTM D6641 Test Machine Controller Software

ASTM D6641 Digital Micrometer | TestGage

TestGage is used with a digital micrometer or caliper to measure sample thickness (depth) and width. After the user makes measurements, data is transferred to the control software.

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Note: This is not a complete summary of the standard. Anyone planning to conduct this test should read the full standard.