Ductile fracture behaviour and modelling of S355 hot-rolled and hot-finished steel sections under tension and shear loads

This study develops a triaxiality-dependent fracture model for hot-rolled and hot-finished S355 structural steel. Tensile tests on dog-bone specimens from universal beams (UB) and square hollow sections (SHS), along with transversally (TN), inclined (IN), and vertically notched (VN) coupons, characterized fracture behaviour under various triaxialities. Two thicknesses and three notch radii were tested per section type. Based on these results, iterative numerical simulations produced a combined tensile–shear fracture model applicable to diverse stress states. Experimental fracture displacements defined equivalent plastic strain and stress triaxiality at the critical location. Unlike existing models that define fracture strain at maximum load and show mesh sensitivity, the proposed approach incorporates ductility via a validated plasticity model, reducing mesh dependency and predicting fracture beyond necking without using a damage index. Results show fracture models must reflect manufacturing process of steel sections, as steel sections of the same grade can exhibit differing ductility due to process-induced microstructural variations. Fracture was shear-dominated at low triaxiality, tensile-dominated at high triaxiality, and mixed in between. Model validation against additional tensile, shear and patterned-plate tests showed close agreement with experiments.