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The process of fracture in homogeneous and anisotropic composite materials involves three steps : (1) the initiation of a microcrack, (2) the stable growth of this microcrack under increasing load, out to macrocrack size, and (3) the unstable propagation of this crack at a critical stress level. The conditions for each of these processes are reviewed for homogeneous materials and then discussed in detail for fiber composite materials. The conditions for fracture initiation by fiber cracking depend primarily on the length and diameter of the fiber, their perfection, the fiber modulus, the test temperature, and the degree of interaction of the fiber with the surrounding matrix. Matrix and interface strength depend primarily on temperature, strain rate, and interfacial void content (for resin matrices). Unless the matrix is extremely strain-rate sensitive, premature fiber fracture does not lead to instability, and the composite strength is determined primarily by the average fiber strength. Several mechanisms for increasing the matrix toughness are discussed. The toughness associated with unstable, longitudinal crack propagation in composites then is considered in terms of fiber content, matrix toughness, interfacial bond strength, and matrix shear strength. It is shown that in certain composites the toughness decreases with increasing fiber content, and consequently maximum load carrying capacity is achieved at a particular fiber content
fractures (materials), composite materials, fiber composites, mechanical properties, crack propagation, fracture mechanics, evaluation, tests
Tetelman, A. S.
Professor, University of California, Los Angeles, Calif.