The toughness transition behavior of bcc steel results from the strong competition between two possible fracture mechanisms.At low temperatures,cleavage fracture is observed,while ductile fracture happens at elevated temperatures.In the transition phase,ductile fracture is followed by cleavage fracture.Charpy impact toughness properties of bcc steel are predicted with a newly-developed hybrid damage mechanics model.An elastic visco-plastic plasticity model with stress-state dependent yielding and isotropic hardening forms the basis of the new model.Since temperature,strain rate and the materials strain hardening properties are the major influences on the microscopic failure mechanisms,the plasticity core of the model considers these influencing parameters.Thus,failure under adiabatic loading conditions can be predicted.Additionally,the effect of damage on the materials plasticity is described with a scalar damage variable D coupled into the yield potential,and both the cleavage and the ductile fracture mechanisms are considered in the corresponding damage evolution law.Stress triaxiality and normalized Lode angle are the major influencing parameters that the hybrid damage evolution law takes into account.Material parameter identifications and successful model application in terms of Charily impact toughness tests are demonstrated.As the model has got a phenomenological character,its applicability for the safety assessment of engineering structures can be expected.