Ductile fracture and failure criteria of HY-100 steel weldments

The multiaxial failure behavior was investigated for two hot-rolled HY-100 plate steels as a function of plate chemistry, test temperature, heat treatment, and specimen orientation with respect to the plate's rolling direction. Axisymmetric un-notched and notched tensile specimens of base-plate material, simulated weldment-heat-affected zones, and weld metal were tested to develop a failure limit diagram to represent the stress triaxiality-equivalent plastic strain combinations at failure initiation. The failure data were found to be strongly dependent on the orientation of elongated MnS inclusions with respect to the applied tensile axis, but failure was not dramatically affected by the heat-treatments or decreasing test temperatures.; For long-transverse oriented specimens, where the elongated axis of the MnS inclusion is normal to the applied tension, failure data indicate that between stress triaxialities of {dollar}sim{dollar}0.95 and {dollar}sim{dollar}1.40 two distinct mechanisms dominate failure initiation. Global void-growth dominated damage accumulation results in failure initiation at low-to-intermediate imposed stress triaxialities {dollar}(sigmasb{lcub}rm m{rcub}/ barsigma < 1.05);{dollar} this failure mode is characterized by large failure strains, an exponential relationship between failure strain and stress triaxiality, large damage levels at failure initiation, and fracture surfaces characterized by equiaxed ductile microvoids. In contrast, a void-sheet instability triggered by localized deformation dominates failure initiation under high imposed stress triaxialities {dollar}(sigmasb{lcub}rm m{rcub}/ barsigma > 1.05).{dollar} Void-sheet induced failure is characterized by small failure strains that are weakly sensitive to the imposed stress triaxiality level, small damage levels at the onset of failure, and a layered, ridge/trough fracture surface morphology associated with linking of elongated MnS-nucleated voids.; Void-sheet induced failure initiation is interpreted in terms of a deformation localization process in which an instability (shear localization) triggers linking by the void sheet process of elongated and aligned MnS-voids. A significant observation is that a very small volume faction (V{dollar}sb{lcub}rm f{rcub} sim{dollar} 0.00015) of MnS inclusions is sufficient to initiate void-sheet induced failure.; In the case of rolling-direction oriented specimens, as well as specimens containing lower sulfur and calcium treatments, global void-growth damage accumulation dominates failure initiation. This failure mode also dictates failure initiation in weld metal specimens. In these cases, the absence of elongated inclusions oriented normal to the tensile axis precludes the development of the void-sheet mode of failure. Rather, failure involves damage (voids) accumulating primarily by the void growth process; void nucleation and linking appear to have secondary roles in this ductile fracture process.; Testing performed at {dollar}-{dollar}85{dollar}spcirc{dollar}C and {dollar}10sp{lcub}-3{rcub}{dollar} s{dollar}sp{lcub}-1{rcub}{dollar} on long-transverse oriented specimens, as a means of simulating high strain-rate ({dollar}10sp3{dollar} s{dollar}sp{lcub}-1{rcub}{dollar}) failure behavior at 25{dollar}spcirc{dollar}C, exhibited failure behavior consistent with that at 25{dollar}spcirc{dollar}C and 10{dollar}sp{lcub}-3{rcub}{dollar} s{dollar}sp{lcub}-1{rcub}{dollar}. Failure involved void-sheet linking of elongated MnS- voids with no apparent evidence of global void-growth dominated damage accumulation. Small and isolated brittle (cleavage) regions were also observed in some specimens. This observation is significant in that it suggests high strain-rate failure may involve mixed ductile + brittle (cleavage) fracture.