In-situ X-ray CT investigation on micro-void evolution of S355, S690 and S960 steel towards verification on micro-fracture mechanics

Abstract

This study investigated the micro-void evolution in structural steel through the use of in-situ X-ray computed tomography (CT), and specially designed steel coupons were loaded in displacement control to various elongations, and scanned simultaneously to capture formation or growth of micro-voids with a minimum volume of 56 μm3 up to fracture. A CT coupon of each of S355, S690 and S960 steel were investigated to track key stages of micro-void evolution through scanning and imaging at critical deformation points. High-resolution 3D reconstruction technique was employed to conduct detailed morphological analysis of the captured micro-voids, and then, their sizes, locations and distributions in the vicinity of the necking region of the coupons were examined. Numerical models were then established to investigate the influence of stress triaxiality and equivalent plastic strain on the evolution of micro-voids, and applicability of the Rice-Tracey formula to both the S690 and the S960 steel was verified according to the measured void sizes and the predicted stress triaxiality and equivalent plastic strain obtained from the numerical models. Besides, all four void-related parameters of the GTN model were obtained from the in-situ X-ray CT investigation while the other five non-void-related parameters were readily obtained according to existing procedures whenever needed. The integration of CT scanning and imaging, mechanical testing, and numerical modelling highlighted the importance of the role of stress states in micro-void evolution, particularly during necking and fracture initiation. This work elaborates the interaction between structural behaviour of the S690 and the S960 steel coupons and their micro-void evolution, providing a reference for prediction on ductile fracture.