Influence of Reinforcement Content and Diameter on the R‐Curve Response in SiC‐Whisker‐Reinforced Alumina

Abstract

Experimental studies were conducted to examine the fracture resistance or R‐curve response of fine‐grained (1 to 2 μm grain size) alumina‐based composites reinforced with SiC whiskers. A precracked applied moment double cantilever beam (DCB) specimen was employed where initial crack lengths of ≥50 μm were attained by back‐cutting the crack tip wake region. For a given crack length, the results show that the fracture resistance is shifted upward with whisker content in composites reinforced with 0.8 μm diameter SiC whiskers for all crack lengths ≥50 μm. When the whisker content is fixed at 20 vol% but the whisker size is varied, measurements show that the R‐curve response increases with the average diameter of the whiskers for all crack lengths ≥100 μm. These experimental results are consistent with analytical descriptions of the R‐curve contribution for frictional bridging and pullout processes. The frictional bridging component, involving the displacement within an intact but partially debonded bridging whisker, reflects the significant radial compressive stresses acting on the interface due to the larger thermal contraction of the matrix. Because of the large tensile or bending stresses imposed on the whisker during crack opening, frictional bridging is limited to the region immediately behind the crack tip. Pullout of such bridging whiskers can occur when they fracture or separate from the matrix away from the crack; this can occur close to the crack tip but should become more dominant with increasing distance behind the crack tip. Observations of crack wake zones support these conclusions. Both the experimental results and the constitutive models suggest approaches to tailor the R‐curve response of ceramics reinforced by discontinuous fibers (e.g., whiskers or elongated grains).