Failure diagrams for unidirectional
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INTRODUCTION
FATIGUE crack growth in unidirectional fiber metalmatrix composites (MMCs) can proceed in one of three characteristic processes, rE2,31As shown in Figure I ,~3]these failure processes include the following: (1) fracture of fibers located ahead of the crack tip and the linkage of the resulting microcracks with the main crack, lt,4.Sl (Figure l(a)); (2) crack deflection and branching at fiber/ matrix interfaces, followed by extensive interface crack propagation and ultimately fiber f r a c t u r e , I6,71 (Figure l(b)); and (3) fiber bridging of the fracture surfaces in the crack wake with frictional sliding of fiber/ matrix interfaces located within the bridging zone, I8,91 (Figure l(c)). The dominance of individual fracture processes during fatigue crack growth is dictated by the properties of the fiber, matrix, and fiber/matrix interface, all of which vary with individual composite systems. Failure of composites with strong interface and relatively weak fibers is predominantly by fiber fracture, e.g., B4C-B/Ti-6A1-4V t61 and FP/ZE41A. 151In contrast, a weak interface favors interface cracking over fiber fracture in thermally exposed BaC-B/Ti-6A1-4V.[7] Most of the recent MMCs based on SCS-6 fibers exhibit low interface strength but high fiber strength. Failure in these types of composites, which include SCS-6/Ti-6AI4 V , II0"ll'12] SCS-6/Ti-15-3, I13'14"151and SCS-6/Ti-24AI11Nb, 115,16,171generally involves frictional slip along fiber/ matrix interface and bridging of crack surfaces by unbroken fibers in the crack wake. The interdependence of the ,carious fracture processes in MMCs-on the interface and fiber properties suggests that they might lend themselves to graphical representation. Previously, Wells and Beaumontt~81proposed the use of fracture energy maps to represent the sources of fracture toughness in brittle matrix composites. In a fracture energy map, contours of equal fracture toughness K.S. CHAN, Staff Scientist, is with the Southwest Research Institute, San Antonio, TX 78238-5100. Manuscript submitted June 9, 1992. METALLURGICAL TRANSACTIONS A
value were shown together with the corresponding dominant toughening mechanisms originating from either postdebond friction or interfacial energy. More recently, He and Hutchinson~9~ presented a theoretical delamination diagram in which regions of interface deflection and penetration are delineated for a crack impinging an interface joining two dissimilar elastic materials. Schematic failure diagrams showing cracking modes observed in fiber-reinforced MMCs under fatigue have also been suggested by Evans t2~ and Chan and Davidson. t3J With the exception of the analysis by He and Hutchinson, t~9J a quantitative description of the fracture mechanisms and their transition is lacking in these failure diagrams for fiber composites. The objective of this article is to present results of a recent investigation whose goal was aimed at developing quantitative relations for predicting the regimes of dominance of various fracture mechanisms, including fi
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