A novel unified constraint parameter based on plastic strain energy

Constraint, defined as the resistance of a component against plastic deformation, has a significant effect on the materials’ fracture toughness. A loss of constraint can mean a higher fracture toughness. The standard test geometries such as C(T) specimens are designed to have high levels of constraint thus ensuring that a lower bound toughness is measured. To get a more accurate estimate of the integrity of low constraint components, it is necessary to study an approach of quantifying the constraint level and evaluating it on failure load. Currently, widely accepted crack-tip constraint parameters are validated using specimens with uniaxial loading such as C(T) and SEN(B). However, a large number of industrial equipment experiences loading modes other than uniaxial. The multi-axiality has been considered an important effect in various standards. Load multiaxiality can cause a change in constraint thus fracture toughness. In this paper, a novel unified constraint parameter λ based on the plastic strain energy was proposed. A series of uniaxial and biaxial bending experiments made of BS1501-224 28B steel were conducted at −160℃. A large amount of previous fracture test data of C(T) and SEN(B) specimens were used to support the study. The elastic–plastic parameter Q and the unified parameters φ, A_d* were calculated as a comparison. It is shown that the newly suggested parameter λ not only can characterize the constraint of C(T) and SEN(B) specimens but is effective in characterizing the multi-axiality effect while traditional constraint parameters fail to do so.