MIXED-MODE FRACTURE CHARACTERIZATION OF HYDROXYLAPATITE-TITANIUM ALLOY INTERFACE

Cantilever beam and four-point bend specimen geometries were used to e xperimentally determine the critical energy release rates for a plasma sprayed hydroxylapatite-titanium alloy (HA-Ti alloy) interface. A loc us of energy release rates as a function of crack tip phase angle was determined where a 0 degrees phase angle represented tensile opening ( mode I) loading and a 90 degrees phase angle represented in-plane shea r (mode II) loading. Energy release rates were found to increase subst antially with an increase in phase angle. An energy release rate of 0. 108 N/mm was determined for a phase angle of 0 degrees (mode I). Energ y release rates of 0.221, 0.686, and 1.212 N/mm were determined for ph ase angles of 66 degrees, 69 degrees, and 72 degrees, respectively. Th e experimental data was matched to a phenomenological model for which crack propagation depended on mode I loading alone indicating that cra ck propagation at the Ha-Ti alloy interface is dominated by the mode I loading component. Therefore, regions of HA coated implants that expe rience compressive or shear loading across the HA-Ti alloy interface m ay be much less likely to debond than regions that experience tensile loading. (C) 1994 John Wiley & Sons, Inc.