Investigation of Crack Growth Process in Dense Hydroxyapatite Using the Double Torsion Method

Abstract

In this work, double torsion tests were performed to investigate slow crack growth behavior of dense hydroxyapatite materials. Crack rate, V, versus stress intensity factor, K I , laws were obtained for different environments and processing conditions. Stress assisted corrosion by water molecules in oxide ceramics is generally responsible for slow crack growth. The different propagation stages obtained here could be analyzed in relation to this process. The presence of a threshold defining a safety range of use was also observed. Hydroxyapatite ceramics appear to be very sensitive to slow crack growth, crack propagation occurring even at very low K I . This can be explained by the fact that their contain hydroxyl groups (HAP: Ca10(PO4)6(OH)2), favoring water adsorption on the crack surface and thus a strong decrease of surface energy in the presence of water. This study demonstrates that processing conditions must be carefully controlled, specially sintering temperature, which plays a key role on V-K I laws. Sintering at 50°C above or below the optimal temperature, for example, may shift the V-K I law towards very low stress intensity factors. The influence of ageing is finally discussed.