Fatigue failure of coarse‐grained alumina under contact loading

Abstract

Bars cyclically loaded by oppositely concentrated forces via rollers are appropriate test specimens for the determination of fatigue under contact loading. The contact strengths and numbers of cycles to failure under contact loading were determined for coarse-grained alumina. From theoretical considerations, it can be concluded that delayed failure in contact fatigue tests is caused by a reduction of the friction between the crack surfaces due to cyclic loading and the related increase of the effective stress intensity factor until the fracture toughness is reached.

Failure of Alumina in Torsion Tests

“…This may lead to an increasing smoothing of surface roughness and a reduction of the friction coefficient with increasing number of cycles as it was successfully assumed for the interpretation of contact fatigue experiments. [20] Figure 7(a) illustrates the frictional tractions near a crack tip and its degradation with increasing number of cycles.…”

“…[20] It was found: n < 10. -In contrast, normal cyclic fatigue experiments carried out under tensile stresses in bending tests [21] exhibited a strongly higher exponent of n % 24.…”

Failure assessment of alumina in unlubricated unidirectional sliding contact

“…The contact fatigue model described in detail in [1,7] determines the number of cycles to failure for cyclic contact loading by…”

“…Contact strength experiments with the cylindrical contact geometry (two-roller test) show a comparable strength but a reduction of the Weibull exponent m by a factor of about two. In addition, contact fatigue experi-ments show a different fatigue mechanism and a much smaller crack growth parameter n compared to conventional four point bending tests because of a friction effect and inhibited water diffusion in the region of the crack tip due to K I < 0 [1,7]. The prediction of the failure and fatigue behaviour in practical applications requires a generalisation of these experimental results for arbitrary geometries and loading conditions.…”

“…The resulting contact loading causes a multi-axial inhomogeneous stress state with high stress gradients, which have to be considered in the experimental characterisation and failure prediction. A new method for prediction of the contact strength and contact fatigue behaviour of ceramic components under normal, sliding, and cyclic contact loading has been developed [1]. The method is implemented in a fast Contact Design-Tool (CDT), that works independent from FEM methods and can be used to assess the lifetime and the failure probability of the loaded ceramic.…”