Determination of the Effect of Stress State on the Onset of Ductile Fracture Through Tension-Torsion Experiments

A tubular tension-torsion specimen is proposed to characterize the onset of ductile fracture in bulk materials at low stress triaxialities. The specimen features a stocky gage section of reduced thickness. The specimen geometry is optimized such that the stress and strain fields within the gage section are approximately uniform prior to necking. The stress state is plane stress while the circumferential strain is approximately zero. By applying different combinations of tension and torsion, the material response can be determined for stress triaxialities ranging from zero (pure shear) to about 0.58 (transverse plane strain tension), and Lode angle parameters ranging from 0 to 1. The relative displacement and rotation of the specimen shoulders as well as the surface strain fields within the gage section are determined through stereo digital image correlation. Multi-axial fracture experiments are performed on a 36NiCrMo16 high strength steel. A finite element model is built to determine the evolution of the local stress and strain fields all the way to fracture. Furthermore, the newly-proposed Hosford-Coulomb fracture initiation model is used to describe the effect of stress state on the onset of fracture.