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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 characterisation of HAPEX bioactive composite

Author: TON THAT, Peter

Year: 2000

Supervisor(s): Liz Tanner

Failures to mechanical components are frequently caused by fatigue. Unlike many conventional engineering components, implants in the body are subjected to complex multidirectional loading patterns, thus fatigue not only occurs under axial fully or partly reversed loading, but also under torsional loading.
This study concerns with the fatigue characterisation of HAPEX, a biocomposite of 40-volume % hydroxyapatite reinforced polyethylene that is currently used for minor load bearing applications. Monotonic test results indicated that HAPEX is stronger in compression than in torsion or tension. Uniaxial fatigue testing at 37° C in saline solution established S-N curves for both axial and torsional loading. Fatigue limits between 37% and 25% of the ultimate strengths of the material were established. Biaxial fatigue showed that the addition of torsion to axial loading significantly reduced the fatigue life. Although when torsion at 25% of the ultimate strength was applied in addition to axial loading at 25% of the ultimate tensile strength, the fatigue life remained above 1.5 million cycles. Out-of-phase loading was less detrimental to the fatigue life of the material than in-phase. Additionally, compression only-torsion fatigue (0-50%UTS25%USS) did not cause failure. Fatigue damage was monitored by increases is dissipated energy and reductions in modulus . The threshold number of cycles at which damage starts to accumulate in the composite was found to be load dependent.

The long term properties of HAPEX were also studied, which demonstrated that immersion in physiological environment reduced the fatigue life of the composite. However, still no failure was observed for 25UTSUSS fatigue at 1.5 million cycles. Immersion for 3, 6 or 12 months appeared to have negligible effects on the fatigue life. Other experimental techniques used to characterise the material included density measurement, which showed no difference between different material conditions; gel chromatography characterisation, which showed a reduction in the molecular weight due to irradiation and microscopy of the fractured surfaces revealed failure in fully reversed fatigue dominated by the lower tensile properties. Biaxial fatigue failure was dominated by shear mechanisms consisting of interlocking and abrasion features of mating surfaces. There was also evidence of interfacial failure and fatigue striations.