Cyclic Fatigue Mechanisms in Partially Stabilised Zirconia

Abstract

Cyclic fatigue crack growth rate and crack resistance curve testing were undertaken on 6 different grades of Mg-PSZ. The width of the transformation zone at the flanks of the cracks was determined using Raman spectroscopy and, combined with R-curve toughening values, used to ascertain the level of crack-tip shielding during cyclic fatigue crack growth and hence the crack-tip stress intensity factor amplitude. By normalising the crack-tip stress intensity factor amplitude with the intrinsic toughness of the material, it was found that the cyclic fatigue threshold stress intensity factor was independent of the extent of crack-tip shielding and a function of the stress intensity factor at the crack tip. In situ SEM observations of cyclic fatigue revealed crack bridging by uncracked ligaments and the precipitate phase. Under cyclic loading the precipitate bridges were postulated to undergo frictional degradation at the precipitate/matrix interface with the degree of degradation determined by the cyclic amplitude. Acoustic emission testing revealed acoustic emissions at three distinct levels during the loading cycle: firstly, near the maximum applied stress intensity factor caused by crack propagation; secondly, at the mid-range of the applied stress intensity factor attributed to crack closure near the crack tip, presumably as a result of transformation induced dilation; and thirdly, intermittently near the base of the loading cycle as a result of fracture surface contact due to surface roughness at a significant distance behind the crack tip. Crack closure near the crack tip due to dilation is proposed to significantly reduce the crack tip stress intensity factor amplitude and hence the degree of cyclic fatigue.