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UBC Theses and Dissertations

A fracture mechanics study of fatigue in a cobalt-base orthopaedic alloy Kannan, Ravi


A fracture mechanics approach was used to investigate the fatigue behavior of a large grained cobalt-base F-75 orthopaedic alloy. Fatigue tests were conducted in air for most of the specimens. The effect of solution heat treatment of the as-cast alloy was investigated in detail; results show that solutioning gives superior fatigue properties to the material. Tests on the as-cast-aged material and solution-treated-aged material were performed for comparison. A 3.5% NaCl solution was used to study the fatigue response of an as-cast and a solution-treated specimen. A comparative test was performed for the solution treated alloy in dry air. Metallographic and fractographic analyses, in conjunction with the fatigue crack growth data, showed that the alloy spends most of its fatigue lifetime in Stage I, exhibiting transgranular crystallographic fracture. Notch geometry and plastic zone sizes were shown to be of significance at near threshold fatigue crack propagation. Crack closure and retardation effects were observed and preliminary work was done to investigate this phenomenon. On the basis of this investigation, one can conclude that simplistic fatigue-life predictions of orthopaedic irtir plants cannot be made without further work.

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