Investigation of slow crack growth in dense hydroxyapatite using the double torsion method

Few investigations devoted to the mechanical properties of hydroxyapatite have shown that these ceramics were sensitive to slow crack growth and fatigue, mainly by so-called static or dynamic fatigue tests. Hydroxypatite was shown to present reduced lifetime under stress. In this work, double torsion tests were performed to investigate slow crack growth behavior of dense hydroxyapatite materials. The goal was here to obtain the crack rate (V) versus stress intensity factor (K-I) law, which is the intrinsic crack propagation law of a given ceramic. The V-K-I curve of hydroxyapatite is composed of three distinct stages, related to a stress corrosion mechanism by water molecules. Crack propagation occurs over a wide range of stress intensity factors, well below the toughness, meaning that hydroxyapatite is highly sensitive to stress corrosion. This could be explained by the fact that hydroxyapatite ceramics contain hydroxyl groups (HAP: Ca-10(PO4)(6)(OH)(2)), favouring water adsorption on the crack surface and thus a strong decrease of surface energy in the presence of water.