Fracture mechanics is a relatively new field of study, having been around for about one hundred years. Although complete understanding of fracture in homogeneous materials is still being developed, the rapid development and application of modern engineered composites has spurred the advancement of fracture mechanics research toward composite materials. 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. Much is known about how these materials behave individually, but as they are applied in situations historically reserved for metals the composites are subjected to the same loadings and must be tested to characterize their performance in similar failure modes. 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 available for a wide range of force capacities. The test fixtures below are designed to ASTM specifications for testing the fracture toughness of composite materials in mode I, mode II, and mixed mode crack propagation.