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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Fatigue behaviour of hydrostatically extruded HAPEX TM

Author: MCGREGOR, William

Year: 2001

Supervisor(s): Liz Tanner

HAPEX TM is a bioactive bone replacement composite consisting of 40 volume % hydroxyapatite and 60 volume % polyethylene and is used clinically as the shaft of middle ear prostheses. While the material is successful as a low load bearing material, its modulus and strength are insufficient for highly loaded applications. HAPEXTM was produced using three different hydroxyapatite powders and then hydrostatic extrusion was used to orientate the polyethylene matrices, increasing the moduli, strengths and ductilities. The fatigue properties of the extruded composites were assessed in 4-point bending at 37°C in saline and for comparison isotropic HAPEXTM and bovine bone specimens were also tested. The extruded composites did not fracture during fatigue, so failure was defined when the secant modulus declined to 70% of its original value. S-N types graphs of applied stress versus the number of cycles (to reduce the modulus to 70%) were produced. For a given number of fatigue cycles, extruded HAPEXTM was able to withstand approximately three times the applied cyclic stress of the equivalent isotropic material. The particle size distributions and morphologies of the different batches of hydroxyapatite were found to influence the processing of HAPEXTM during hydrostatic extrusion and the achievable actual extrusion ratio. The scatter in the fatigue lives of the hydrostatically extruded HAPEXTM specimens, were mainly due to variations in their initial moduli, which was related to their actual extrusion ratios. The cyclic energy dissipation and tan delta were measured throughout the fatigue process. The effect of temperature and stress on the dynamic creep component during fatigue was studied using an activated model. Differential scanning calorimetry was used to examine the changes in the polymer matrix that occurred upon hydrostatic extrusion. Scanning electron microscopy was used to examine the fracture surfaces of isotropic specimens and sections from the unfractured extruded specimens.