TRAPPED CRACKS AT INDENTATIONS .1. EXPERIMENTS ON YTTRIA TETRAGONAL ZIRCONIA POLYCRYSTALS

Indentation radial cracking has been examined in a series of yttria-te tragonal zirconia polycrystals (Y-TZP) with a range of grain sizes. Fo r all materials, two distinct types of cracks were found to exist. Abo ve a threshold indentation load, short cracks of variable length were observed, ''trapped'' in a compressive, contact-induced transformation zone. Above a second threshold, longer, ''well-developed'' cracks of more consistent length formed, extending beyond the zone boundary. The stochastic nature of the formation of both types of cracks, and the e xistence of the transformation zone in general, generate many artefact s in toughness estimates made using unmodified indentation fracture me chanics analyses.