A critical evaluation of shear specimen geometry for accurate stress triaxiality control

The forming limit in many non-conventional forming applications is increasingly modelled using continuum damage models. The ductile fracture under proportional loading is dependent upon the deformation stress state. Accurate characterisation of specific stress paths is essential for the development and calibration of reliable damage models in metal forming. Among these, achieving and maintaining a pure shear stress state remains particularly challenging due to the constraints posed by the geometric design and experimental loading. This study presents a novel, robust and systematic framework for evaluating and comparing a range of shear specimen geometries for a high strength steel based on the evolution of stress triaxiality and strain localisation behaviour, through finite element method. It further examines the formation and progressive rotation of shear bands during deformation — an aspect that has received limited attention in existing literature. With the exception of the eccentric notch shear (ENS) specimen, none of the geometries were found to be effective in achieving a pure shear state for the chosen material properties. However, the ENS specimen exhibited further limitations when applied to a high-strength dual-phase steel, including compressive-to-tensile triaxiality transitions and inclined fracture paths. Based on these insights, a novel Half V-Notch shear specimen is proposed. The new geometry achieves a near-zero average stress triaxiality, eliminates compressive triaxiality transitions, and maintains fracture plane alignment parallel to the loading axis, preventing any in-plane rotation. Experimental validation using DP1180 steel further confirmed the specimen's suitability for accurately calibrating shear-sensitive fracture models.