Mixed‐mode fracture of a glass‐ceramic and an alumina ceramic from inclined Knoop indenter flaws was studied in diametral compression and four‐point‐bend tests. In annealed specimens the directions of extension of the cracks and mode I and mode II stress‐intensity factors at fracture were analyzed and compared to the predictions of a maximum crack‐tip hoop stress theory. Knoop flaws in all cases extended in directions normal to the principal maximum tension rather than in the direction of maximum hoop stress near the crack tip. Mixed‐mode fracture envelopes assessed in experiments, particularly with the diametral‐compression test, showed significant deviation to higher K II values relative to the fracture‐mechanics predictions. As a consequence, an apparent K IIc value assessed in the diametral‐compression test was approximately twice the value of K Ic .
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Shetty et al. (1986) studied this question.
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