Keywords: fatigue crack growth, fatigue crack propagation, bone cement, PMMA, polymethyl methacrylate, overloads, variable amplitude loading, retardation, plasticity induced closure, crazing, orthopaedics, crack growth rate
Effects of intermittent overloads on fatigue of PMMA bone cement
PMMA bone cements are widely used in orthopaedics, but questions remain as to their long term durability. Although fatigue of bone cements has been widely studied, the effects of variable amplitude loading are not fully understood. This paper presents a novel method using a crossover experimental design, in which a crack is propagated under ΔK control and a series of different overload patterns are applied. This method allowed accurate and repeatable measurements of the effects of overloads on crack growth. It was found that single overloads every ten to 100 cycles caused a pronounced increase in the crack growth rate compared to Paris Law predictions; there was no evidence of retardation as would be expected in most materials. Overloads may fracture intermittently propagating crazes and help the crack past micro-structural obstacles, allowing disproportionately faster crack growth. The observed increase in crack growth rate over a wide range of different overload intervals and for single and block overloads could be accounted for by an eight-fold increase in crack growth rate for approximately fifty cycles after each overload.