EVALUATION OF CRACK-PROPAGATION IN HYDROXYAPATITE BY DOUBLE-TORSION METHOD IN AIR, WATER AND TOLUENE

The double-torsion method was employed to study the effect of environm ents on crack-growth law of hydroxyapatite. Three regions (regions 1, 2 and 3) were distinctly observed for measurement in air. In water onl y region 1 was observed. As the temperature increased, the crack-propa gation curves in water shifted to lower applied K-I values. The change of crack velocity with increasing K-I in region 2 was small compared with those detected in regions 1 and 3. It was argued that crack propa gation in region 2 is controlled by the rate of water diffusion toward s the crack tip. Region 3 was observed in air and toluene. Intergranul ar fracture was dominant in the specimen fractured in water, while bot h transgranular and intergranular fracture were observed in air. On th e other hand, the specimens showed only transgranular fracture in tolu ene. Since a low Ca/P ratio phase was observed at grain boundaries of hydroxyapatite, it was deduced that this phase is selectively dissolve d into water.