The effect of pre-strain and strain-path changes on ductile fracture: experiment and computational modeling

Based on experiments and computational modeling, this study examines the failure behavior of structural steel, HY-100, which has been pre-strained at a high stress triaxiality and subsequently failed at a lower stress triaxiality. Both tensile tests of circumferentially notched specimens and the associated fractography show that even a small pre-strain at high stress triaxiality promotes an extension of the low ductility, ‘void-sheet’ mode of failure to lower stress triaxialities. Thus, there is a decrease in the failure strain compared to that if the material is deformed only at the lower stress triaxiality. These results imply that the pre-strain damage nucleates elongated voids whose growth is critical to the development of void-sheet failure. Micro-mechanical modeling using finite element analysis confirms that localization of plastic flow should occur between elongated ‘hole shaped voids’, despite their rather small initial cross-section size (2.5 μm) and comparatively large spacing (70 μm). Furthermore, employing a local failure criterion, the computational analysis predicts failure strains which are in good agreement with those observed after the pre-strain and strain-path change.