Crack Shielding in Ce‐TZP/Al2O3 Composites: Comparison of Fatigue and Sustained Load Crack Growth Specimens

Abstract

Crack shielding stress intensities in in situ loaded compact tension specimens of two types of ceria‐partially‐stabilized zirconia/alumina (Ce‐TZP/Al2O3) composites with prior histories of subcritical crack growth in sustained and tension‐tension fatigue loading were directly assessed using laser Raman spectroscopy. Crack‐tip stress fields within the transformation zones were measured by measuring a stress‐induced frequency shift of a peak corresponding to the tetragonal phase. The peak shift as a function of the applied stress was separately calibrated using a ball‐on‐ring flexure test. Total crack shielding stress intensity was estimated from the far‐field applied stress intensity and the local crack‐tip stress intensity assessed from the measured near‐crack‐tip stresses. The shielding stress intensities were consistently lower in the fatigue specimens than in the sustained load crack growth specimens. The reduced crack shielding developed in the fatigue specimens was independently confirmed by measurements of larger crack‐opening displacement under far‐field applied load as compared to the sustained load crack growth specimens. Thus, diminished crack shielding was a major factor contributing to the higher subcritical crack growth rates exhibited by the Ce‐TZP/Al2O3 composites in tension–tension cyclic fatigue. Calculations of zone shielding considering only the dilatational strains in the transformation zones accounted for 81% and 86% of the measured values in the sustained load crack growth specimens, but significantly overestimated the shielding in the fatigue specimens. Possible reasons for the diminished crack shielding in the fatigue specimens are discussed.