A strain-gradient, crystal plasticity model for microstructure-sensitive fretting crack initiation in ferritic-pearlitic steel for flexible marine risers

A three-dimensional, strain-gradient, crystal plasticity methodology is presented for prediction of microstructure-sensitive length-scale effects in crack initiation, under fatigue and fretting fatigue conditions, for a ferritic-pearlitic steel used in flexible marine risers. The methodology, comprising length-scale dependent constitutive model and scale-consistent fatigue indicator parameters, is calibrated and validated for representative (measured) dual-phase microstructures under strain-controlled low cycle fatigue conditions. Prediction of the effects of length-scale on fretting crack initiation is based on a three-dimensional, crystal plasticity, frictional contact model to predict fretting crack location and initial growth path, accounting for the effects of crystallographic orientation. The length-scale dependent fatigue and fretting simulations predict (i) significant beneficial effect of reducing length-scale for low cycle fatigue life, (ii) complex cyclically- and spatially-varying effects and differences due to changing contact and grain length-scales, and (ii) that fretting damage generally decreases with decreasing (contact-grain) length-scale.