Crack-bridging Processes and Fracture Resistance of a Discontinuous Fiber-reinforced Brittle Matrix Composite
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Crack-bridging processes and fracture resistance of a discontinuous fiber-reinforced brittle matrix composite Takashi Akatsu,a) Yasuhiro Tanabe, and Eiichi Yasuda Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226, Japan (Received 2 December 1997; accepted 10 August 1998)
A simple bridging model is proposed for the toughening of a discontinuous fiber-reinforced brittle matrix composite, in which the frictional bridging of fibers during, as well as after, the interfacial debonding is considered. The R-curve behavior and the work-of-fracture of the composite can be theoretically predicted by the computation of the bridging model applying material parameters, such as fiber volume fraction, size and shape of fibers, fiber tensile strength, elastic moduli of fibers and matrix, fracture toughness and work-of-fracture of matrix, and frictional shear stress at interface. The experimental result obtained from a SiC-whisker-reinforced Al2 O3 composite confirms the theoretical predictions of the present bridging model. Through the model calculation, the R-curve, crack profile, and bridging stresses of the composite can be estimated correspondingly to the bridging processes.
I. INTRODUCTION
Ceramic fiber-reinforced ceramic composites are expected to be used as structural components exposed to elevated temperatures, because the reliability, fracture resistance, and the flaw tolerance are improved by crack-face fiber bridging.1,2 Comparing with continuous (or long) fiber reinforced composites, discontinuous (or short) fiber reinforced ones have several advantages including easy fabrication, easy densification, low anisotropy, etc. Whisker-reinforced ceramics3–5 and Si3 N4 -based ceramics,6 having the characteristic microstructure with elongated grains, are considered to be in the category of short fiber-reinforced composites. The fracture resistance of short fiber-reinforced composites increases with crack extension (i.e., rising R-curve behavior). On the one hand, the rising R-curve behavior has been utilized to experimentally estimate the distribution of bridging stresses behind the crack tip.7–9 On the other hand, the micromechanics of crack-face fiber bridging has been theoretically developed to examine bridging stresses.10–12 It is important to substantiate the correlation between the micromechanics of fiber bridging and the R-curve behavior observed. Theoretical and quantitative understanding for the correlation between the R-curve experimentally observed and the crack-face fiber bridging processes and mechanisms theoretically derived will give an essential contribution to designing tough and reliable composites. However, this correlation so far proposed has not yet been of enough benefit for materials designing, for numbers of the complex microstructural parameters of the composites have made fuzzy the strategy for materials designing. In this sita)
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