Ductile fracture by void-sheet coalescence in HY-100 steel: A modeling study

The ductile fracture of HY-100 steel at high stress triaxiality commonly occurs by a void-sheet mode of failure. The associated far-field failure strains decrease slowly with increasing stress triaxiality and are sufficiently small to limit ductility. The failure path suggests that a localized deformation instability may be responsible for this form of fracture. Micro-mechanical modeling using finite element analysis has been employed to examine the deformation localization behavior within void arrays based on the observed metallurgical microstructure.; This study examines the localization of plastic flow between both pairs of holes and multi-hole arrays as a function of applied stress state, hole "microstructure," and matrix strain hardening. The computational models based on pairs of holes correctly predict the observed tendency to form intense, localized bands of high strain as stress triaxiality increases and as strain hardening decreases. Thus, high strength alloys with small strain hardening capacity are predicted to be especially prone to void-sheet failure at high stress triaxialities. Evaluations of void growth show that the voids generally act as isolated voids, despite their ability to interact and induce deformation localization between them. A critical ratio of void size to spacing was found to exist for the initiation of void-sheet coalescence.; Image-based multi-hole models based on the observed inclusion microstructure of HY-100 steel show the significance of the critical features (size, spacing, clustering) of the voids initiated at inclusions on deformation localization and failure initiation. A diverse distribution of voids, based on the size and location of MnS inclusions has been analyzed by various model cases. The results show that deformation localization is especially sensitive to the presence of a few large voids spaced within roughly 30 hole diameters of each other and oriented on planes 450 +/- 150 to the maximum principal stress. A high density of small voids generally tends to diffuse deformation but the small voids can promote deformation localization if located between the large voids. Finally, when compared to the deformation behavior within a regular array of voids, these results also show the importance of clusters of voids in inducing deformation localization and fracture.