Prediction method for ductile crack initiation and growth resistance for pre-strained steel based on ductile damage model

Abstract

This study aims to develop a numerical simulation-based method for predicting ductile crack growth resistance curve (R-curve) for pre-strained components for rational assessment of ductile crack growth controlling fracture for pre-strained structural component with any plastic constraint. Experiments on R-curve of 3-point bend specimen with a shallow crack for steel pre-strained by 6% over uniform elongation provides that the pre-strain does not significantly reduce ductile crack initiation and growth resistance, even though the pre-strain induces disappearance of uniform elongation and work hardening together with drastic reduction in elongation. Observations of damage evolution in terms of micro-void formation indicate that ductile crack growth behaviors in both virgin and pre-strained steels are based on a micro-void nucleation-controlled ductile fracture mechanism. These experimental results demonstrate that the ductile damage model that we have already proposed for predicting R-curve for virgin steel that exhibits micro-void nucleation-controlled ductile fracture behaviors can be applicable for these virgin and pre-strained steels used. Thus, based on the previously proposed ductile damage model, a simulation-based method to predict the R-curve of pre-strained specimens only from the properties of virgin steel is proposed taking material degradation (change in strength and damage properties) due to pre-strain into account. The applicability of the proposed method is verified by showing the predicted R-curve for pre-strained steel are in good agreement with experimental results.