Role of matrix/reinforcement interfaces in the fracture toughness of brittle materials toughened by ductile reinforcemen
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I.
INTRODUCTION
THE interface between matrix and reinforcements plays an important role in the mechanical performance of composites. It is commonly accepted that a relatively weak interface is desirable for improving fracture toughness of brittle materials reinforced by ceramic fibers, t~-41 The reason for this is that such an interface when present in the path of an advancing crack would fail locally and blunt the crack. Otherwise, if the interface is strong, the propagating crack is unlikely to "see" the fiber and little improvement in toughness will be realized. This toughening concept has become a dogma in design of brittle materials to be reinforced by ceramic fibers. However, when the reinforcement is a ductile fiber with a high strain to failure, the requirement for the interfacial bonding may be different. This is because occurrence of fracture for the two different fibers is based on different criteria. For a brittle fiber, the occurrence of fracture is simply based on the magnitudes of the maximum tensile stresses which are enhanced by the presence of stress concentration. In contrast, since ductile fibers can yield locally by dislocation slip and thus blunt the crack, the fracture occurs only after exhaustion of ductility of the fiber in the presence of triaxial stresses at the crack tip (particularly in plane strain). As such, there may be different requirements for the interfaces in ductile-fiberreinforced brittle matrix composites. The responsibility for the ductile phase toughening has been attributed to bridging of intact ligaments of the ductile phase behind the advancing crack tip. t5-~2] Based on this mechanism, the increased toughness of the composites in the case of small-scale bridging can be related L. XIAO, Research Assistant, and R. ABBASCHIAN, Chairman and Professor, are with the Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611. Manuscript submitted November 12, 1991. METALLURGICAL TRANSACTIONS A
to the work of stretching and fracturing the ductile phases, AG, by the following equation: t6,9j AG = V:
o'(u) du
[11
where u is the crack opening, o-(u) the nominal stress on the ligament, u* the crack opening at the end of the traction zone, and V/the area fraction of reinforcements on the crack plane. It is clear that the increased toughness depends on the stress-displacement function of the ductile ligament, o-(u), which in turn relies on the extent of decohesion at the matrix/reinforcement interface and can be determined independently using a simple tensile test on a single constrained ductile reinforcement, t7'~3-161 These experiments indicated that a high work of rupture of ductile reinforcements was encouraged by a partial decohesion at the interface, t7,13-15j Deve et al., I161 however, found that whether or not extensive decohesion was desirable for a high work of rupture depended on the work-hardening capability of the reinforcements, suggesting that contributions of decohesion were complicated and interdependent with other material pr
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