Micromechanics and fatigue crack growth in an alumina-fiber-reinforced magnesium alloy composite
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INTRODUCTION
M E T A L matrix composites are promising materials for a variety of structural components because of their high specific strength and modulus. At present, structural design approaches using continuous fiber composites require incorporation of off-axis fiber plies to improve transverse strength, thereby improving the resistance to multiaxial loading. Although the response of these materials to off-axis loading, as well as the behavior of crossplied composites, has been developed, a poor understanding of the fracture characteristics of these materials still exists. Use of composites in structural applications has suffered from the lack of fracture predictability. Although numerous studies have been performed on fracture mechanisms in metal matrix composites, predictive capability has been hampered by a lack of knowledge of the specific interactions between the crack and the various components of the composite. This is especially true for the complex fiber-matrix interfacial region with various stress states. The presence of the interface introduces additional mechanisms of crack growth retardation, compared to those for unreinforced materials. Composite fracture mechanisms include the sudden fracture of a fiber, propagation of a fiber crack into the matrix, and crack growth along the interface. Since current design criteria assume that structural components contain notches, preexisting flaws, or cracks when put into service, a knowledge of subcritical crack growth is necessary. When coupled with the high anisotropy in mechanical properties of continuous fiber composites, the multiaxial stress state induced by a crack can cause
interfacial decohesion at relatively low applied stresses. Tetelman ~zj showed that materials with high-strength fibers can achieve good toughness by promoting interfacial splitting without sacrificing transverse strength. Hancock and Shaw Izj found for boron-fiber-reinforced aluminum alloys that the resistance to crack propagation could be optimized by altering the degree of bonding at the fiber-matrix interface. Thus, for applications subject to cyclic loading, it may be desirable to optimize the strength of the interface, making it weak enough to debond ahead of fatigue cracks, but not so weak that transverse tensile strength is unacceptable. This paper reports the results of a study to develop quantitative relationships between the growth of fatigue cracks and the characteristics of the matrix, fiber, and interface for a composite of the magnesium alloy ZE41A reinforced by A1203 fibers. This material has potential applications in weight-critical aerospace and automotive components, such as helicopter transmission housings, bearing supports, aircraft structural members, connecting rods, pistons, suspension arms, and axle housings. Objectives of the work were to determine the local sequence of fracture events near the tip of a growing fatigue crack and to quantify strains when these critical events occurred. The experimental technique used involved cyclic loading of precra
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