Effects of whisker content and dimensions on the R -curve behavior of an alumina matrix composite reinforced with silico
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A study was made of the effects of whisker content and dimensions on the R-curve behavior of an alumina matrix composite reinforced with silicon carbide whiskers. Experiments showed that the rising R-curve behavior is strongly enhanced by the increase in the volume fraction and the size of whiskers. Simulation based on a model of crack face whisker bridgings was made to elucidate the effects on the R-curve of the composite. The validity of the simulation was confirmed by good agreement between the experimental and the calculated R-curves. In addition to the R-curve, the distribution of crack closure stresses as well as crack tip opening displacements was also calculated, which directly reflects crack face whisker bridging processes in the composite. Then the dependence of whisker content and dimensions on the R-curve was analytically discussed taking the bridging processes into account.
I. INTRODUCTION
Knowledge of toughening in ceramics is becoming more and more important because the brittleness of ceramics confines their utility as structural materials. It is well known that remarkable toughening in ceramics has been achieved by the addition of elongated reinforcement phases such as whiskers or short fibers. Previous work has suggested that the toughening in ceramic matrix composites reinforced with ceramic whiskers is caused by crack-face whisker bridging.1–3 As a result of the bridging processes, the increase in a critical stress intensity factor KIc with the increase in crack extension length ⌬a (so-called rising R-curve behavior) is usually observed in composites. Tailoring the microstructure of composites to govern the rising R-curve behavior is important because the initial rising gradient of the curve strongly affects their mechanical properties such as fracture strength, strength reliability,4 thermal shock damage resistance,5 etc. Regarding the microstructural tailoring of composites, it has been pointed out that whisker content and dimensions are easily regulated in comparison with other microstructures related to the whisker–matrix interface. The effects of whisker content and dimensions not on the R-curve but on the fracture toughness and work-of-fracture of composites have been studied.1,6,7 It is, however, rather difficult to extract the R-curve effects
a)
Address all correspondence to this author. J. Mater. Res., Vol. 16, No. 7, Jul 2001
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from those on fracture toughness and work-of-fracture because they reflect only one aspect of the R-curve, which is progressively obtained as a result of cumulative crack-face whisker bridging processes. In addition to the uncertainty of the effect of microstructural change on the R-curve of composites, we have difficulty with conventional analysis8–10 of R-curve to evaluate crack closure stresses b because the analysis is mostly performed by fitting the observed result into an exponential or a polynomial function, which has no sound physical meanings for the bridging. It is emphasized that the b-value
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