Cryogenic ductile and cleavage fracture of bcc metallic structures – Influence of anisotropy and stress states

The anisotropic cryogenic ductile and cleavage fracture properties of a body-centered cubic (bcc) steel at −196°C have been investigated under a broad spectrum of loading conditions, crossing stress triaxiality range from −1/3 to 1.5, and along three loading directions. Fracture is completely impeded in uniaxial compression tests. Conventional brittle behavior is only observed in high triaxiality conditions for the investigated bcc steel, and on the contrary ductile fracture with shear and void coalescence as underlying mechanisms takes place in low (simple shear) and moderate triaxiality (uniaxial tension) at −196°C. Cleavage fracture occurs after significant plasticity at −196°C under moderate triaxiality conditions during tensile tests using flat-notched specimens under plane-strain tension. For all the stress states, anisotropy has shown a profound influence in ductile fracture, brittle fracture, and, particularly in the transition region mixed with two failure types. It is concluded that the reason for the anisotropic transition of activated failure mechanisms crossing stress states at −196°C is because strain hardening capacity, Lankford coefficients, fracture initiation strain as well as cleavage fracture strength are all dependent on loading orientations. Based on the collected local critical stress and strain variables from finite element simulations using an evolving anisotropic plasticity model, an anisotropic unified fracture criterion revealing the underlying failure mechanisms is developed and demonstrates distinguished predictive capability in describing cryogenic ductile and cleavage fracture properties under different stress states and loading directions.