Failure of Alumina in Torsion Tests

Fatigue behavior under cyclic torsion loading shows a complicated dependency between the number of cycles to failure Nf and the height of the applied load σappl. A recently published result is chosen for which three clearly different regions are observed in which the normally straight log(Nf)‐log(σ) plot shows different slopes. Whereas the high‐stress region near the torsion strength can be interpreted as normal mode‐I failure of randomly oriented surface cracks, the steep part may be understood as the consequence of friction degradation at grinding cracks which are under pure mode‐II and mode‐III loading. At lowest loads, quasi‐static subcritical crack growth support has to be expected with its commonly higher load exponents. Fatigue behavior under cyclic torsion loading shows a complicated dependency between the number of cycles to failure Nf and the height of the applied load σappl. Whereas the high‐stress region near the torsion strength can be interpreted as normal mode‐I failure of randomly oriented surface cracks, the steep part may be understood as the consequence of friction degradation at grinding cracks which are under pure mode‐II and mode‐III loading. At lowest loads, quasi‐static subcritical crack growth support has to be expected with its commonly higher load exponents.