Continued advancements in the design and manufacture of engineered composites has allowed composite materials to work their way into the products we use everyday. The most common modern advanced composites are fiber-matrix composites and they can be manufactured with polymer, carbon, metal, or ceramic matrix and an extremely wide range of reinforcement fiber including, carbon, boron, aramid, and glass. Since composites are anisotropic materials, care must be taken in setting up and conducting mechanical tests to ensure proper specimen alignment and load application to the test sample. Specimen alignment and orientation may not be as crucial for non-aligned short fiber composites, as many exhibit quasi isotropic behavior. However, non-aligned short fiber composites should still be carefully aligned and tested in multiple orientations to ensure that manufacture processes do not induce undesired fiber alignment. All aligned fiber composites will exhibit different mechanical properties in different orientations. Since advanced composites are typically manufactured with brittle matrix materials, fracture initiation often occurs and propagates within the matrix. During fracture propagation, reinforcing fibers maintain structural integrity until the crack or series of cracks has grown too large, at which point material failure occurs. In laminated composites, the bond between lamina is often a point of crack initiation. Fracture toughness tests of composite materials are conducted in a similar manner as with metals. Whether testing for interlaminar or translaminar fracture toughness, a crack is artificially initiated in the test specimen to ensure fracture propagation by the desired method. Test methods typically test for mode I, crack propagation by opening, or a mixture of mode I and mode II, crack propagation by crack faces sliding over each other with no relative face opening. Interlaminar fracture toughness is primarily dictated by interlaminar bond strength with fracture propagation theoretically only occurring within the matrix, while translaminar fracture toughness measures fracture propagation through both matrix and fiber phases. In both cases, test specimen preparation and alignment are crucial to ensure uniform loading and proper crack propagation for the mode being tested. ASTM and ISO have developed standard test methods for testing fracture toughness of composite materials. The standards provide methods that can be recreated, ensuring materials are tested in the same manner and conditions and allowing test results validation between manufactures and customers. Popular fracture toughness test methods are ASTM D4762 for polymer matrix composites, ASTM D5528 for mode I interlaminar fracture toughness of unidirectional fiber reinforced polymer matrix composites, ASTM D6671 for mixed mode I TestResources provides all the equipment needed to properly conduct fracture toughness tests of composite materials. Universal test machines, capable of applying both compressive and tensile loads, are excellent for fracture toughness tests. The test fixtures below are designed to ASTM specifications for testing the fracture toughness of composite materials in both mode I and mode II fracture propagation.

Applicable Testing Standards

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