Multiaxial fatigue characterization of self-reinforced polylactic acid-calcium phosphate composite

• Main Author: Mustafa, Zaleha Binti
• Format: Dissertation
• Language: English
• Published: University of Glasgow 2013
• Subjects: TJ Mechanical engineering and machinery

The majority of failures of mechanical components are 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. The fatigue behaviours of self-reinforced poly lactide composite (PLA) of unidirectional PLA fibres in PLA matrix containing tricalcium phosphates (TCP), (PLA-PLA-TCP) produced via pre-pregging technique has been investigated. Quasi-static test results indicated that PLA-PLA-TCP is stronger in tension than in compression and torsion and is significantly influence by moulding temperature. Uniaxial fatigue testing at 37° C in saline solution established S-N (Wöhler) curves for both axial and torsional loading for two moulding temperatures (140°C and 150°C). Compression loading showed significant effect on the axial fatigue behaviour. Biaxial fatigue results showed that the addition of torsion to axial loading significantly reduced the fatigue life. Out-of-phase loading was less detrimental to the fatigue life than in-phase. Fatigue development was evaluated by reduction in secant modulus and increase in energy absorbed. The threshold number of cycles at which damage starts to accumulate in the composite was found to be load ratio and direction dependent. The effects of the degradation process in saline solution on the fatigue behaviour of the composite were also studied at 25% of the ultimate tensile and shear stresses. The specimens were immersed for 8, 12, 16 and 20 weeks periods before testing. The results indicated that even though immersion in saline solution reduced the static and cyclic properties of the composite, it still had good strength retention and comparable to the cortical bone at the end of 20 weeks of degradation period. Microscopy examination on the fracture surface indicated that in uniaxial tension-compression fatigue, the failure was dominated by compression and failed via microbuckling mechanism. Biaxial fatigue failure was dominated by shear mechanisms with evidence of interfacial failure and fatigue striations.