PREDICTION OF INDENTATION-LOAD DEPENDENCE OF FRACTURE STRENGTHS FROM R-CURVES OF TOUGHENED CERAMICS

Rising-crack-growth resistance or R-curves evaluated by direct measure ments of crack lengths associated with indentation flaws in a sintered silicon nitride and a sintered and isostatically hot pressed (HIPed) SiC (whisker)-reinforced alumina were used to predict the correspondin g indentation-load dependence of fracture strengths. Two empirical R-c urve functions, a power law and an exponential function, were fitted t o the R-curve data for the analysis. Fracture strengths were calculate d by a combined numerical and graphical procedure that determined the point of tangency between crack-driving forces and the R-curves. The r esults revealed that the exponential function gave a better prediction of the measured log fracture-strength versus log indentation-load rel ation than the power law. The exponential function also predicted a ne arly linear relation between log fracture-strength and log indentation -load, thus indicating that the apparent linearity of this plot is not adequate evidence to assume a power-law R-curve function. This study, therefore, reinforces the case for R-curve evaluations to be based on direct crack-length measurements rather than strength measurements, b ecause the indentation-load dependence of the fracture strength is not sufficiently sensitive to discriminate between potential R-curve func tions.