Micromechanics of fatigue with application to stents

Abstract

This thesis describes the development of a computational framework, based on experimental characterisation and validation, for the microstructure-sensitive modelling of fatigue crack initiation, with a focus placed on its application in stent fatigue design. The framework developed utilises crystal plasticity constitutive formulations for describing slip in individual grains in the microstructure. Coupled with realistic finite element microstructure geometries and microstructure-sensitive parameters, crack initiation is predicted for different loading conditions and microstructures. Extensive experimental testing is carried out for the calibration and validation of the micromechanical framework for the L605 CoCr alloy, typically used in modern stents. Grain structure, crystallographic texture and precipitate content are characterised via microscopy, while low cycle fatigue testing allows calibration of crystal plasticity constants, via comparison of macroscopic hysteresis behaviour, and identification of critical parameter values for prediction of fatigue crack initiation.

The calibrated framework is applied to the assessment of the high cycle fatigue performance of a generic stent design for both 316L stainless steel and the L605 alloy. The importance of size-scale consistency between fatigue predictive techniques and application is demonstrated. The micromechanical framework is also used to redesign the generic stent geometry for the L605 alloy, for which the original geometry is shown to be over-conservative. The framework is later validated for use in the high cycle fatigue regime via comparison of predictions against stress-life data for L605 foil micro-specimens, representative of the stent size-scale.

Studies on the microstructural mechanisms influencing fatigue are also investigated. A study on ferritic steel four-point bending fatigue tests, in which the specific grain topography and crystallographic texture in the notch region of individual specimens is explicitly modelled, highlights the influence of elastic anisotropy and accumulated plastic slip on the location of fatigue crack initiation sites. In another study, a strain-gradient plasticity formulation is adopted to predict, for the first time, the experimentally-observed effect of grain size on the low cycle fatigue behaviour of as-received and heat-treated L605 CoCr material, where higher geometrically necessary dislocation density in fine-grain specimens is shown to play a key role.