Fatigue and fracture in materials used for micro-scale biomedical components

Some biomedical components involve the use of materials in microscopic quantities, i.e. in section sizes which are of the same order of magnitude as microstructural features in the material, such as grains. The mechanical behaviour of the material may be different when used in these quantities, compared to its behaviour in macroscopic amounts. An example of a microscopic component is the cardiovascular stent. To ensure the integrity of the stent during deployment and subsequent use, the designer must be able to simulate possible failure modes, i.e. monotonic fracture and fatigue, and the effect of stress concentrations. We carried out tests on specimens of 316L stainless steel, with and without stress concentrations. We found a significant size effect, in which the behaviour of these microscopic specimens was different from that of larger, macroscopic specimens. Microscopic specimens had lower tensile strengths and higher ductility. Under cyclic loading, the material's behaviour at large numbers of cycles was independent of specimen size, but the microscopic specimens were inferior at smaller numbers of cycles to failure. Fatigue limits for the notched specimens could be predicted using an existing theory (the Theory of Critical Distances) but parameter values were different at the macro- and micro-scale. Thus, data from conventional, macroscopic specimens cannot be used to predict the behaviour of this material when used for microscopic components. Mechanical working and annealing strongly affected the tensile strength and ductility, but had no significant effect on fatigue behaviour.