Ductile shear failure

Shear fracture has been identified as a failure mode in clamped plate naval sandwich structures subject to high-intensity loadings. Motivated by this, along with recent experimental evidence concerning the effect of loading configuration on the strain at fracture, a two part study on shear fracture is pursued. First, a numerical and experimental study of shear-off in flat-nosed projectile impact is presented. The study illustrates the use and limitations of critical-strain failure models. The effects of mesh size on the resolution of the predicted strain distribution and the plastic dissipation associated with penetration are addressed. The second part of the study presents a modification to the Gurson metal plasticity model that accounts for damage in shear. The modification is motivated by recent experimental and theoretical evidence pointing to the limitations of characterizing the failure strain in ductile fracture solely on the basis of stress triaxiality. Additional motivation is found in the Gurson model's inability to account for damage development under straining when the mean stress is zero unless void nucleation is introduced. As such, the original model excludes shear softening due to void distortion and inter-void linking. Consequently, the original formulation effectively excludes the possibility of shear localization and fracture under conditions of low triaxiality if void nucleation is not invoked. The newly modified model incorporates a parameter capable of distinguishing between axisymmetric and shear dominated stress states. The extension retains the isotropy of the original Gurson model by making use of the third invariant of stress to distinguish shear dominated states. The importance of the extension is illustrated by shear localization study over the complete range of applied stress states, clarifying recently reported experimental trends. Finally, the extension is applied to numerical simulations of quasi-static punch out experiments. These simulations illustrate that, in contrast to the original model, the modified model is capable of capturing the low-triaxiality failure process observed in these tests. Additionally, the simulations provide a way of calibrating the single new parameter in the modified model. The extension opens the possibility for computational fracture approaches based on the Gurson model to be extended to shear-dominated failures such as projectile penetration and shear-off phenomena under impulsive loadings. The study is concluded with promising preliminary results of simulations using the modified model to simulate the response of a blast-loaded clamped beam.