Study on the unified constraint parameter for characterizing in-plane and out-of-plane constraint based on the equivalent plastic strain

Background: Constraint can significantly alter the material's fracture toughness. Purpose: In order to increase accuracy of the structural integrity assessment. It needs to consider the effect of constraint on the fracture toughness of nuclear power materials and structures. A unified measure which can reflect both in-plane and out-of-plane constraint is needed. Methods: In this paper, the finite element numerical simulation method was used, a unified measure and characterization parameter of in-plane and out-of-plane constraint based on crack-tip equivalent plastic strain have been investigated. Results: The results show that the area surrounded by ε_p isoline has a good relevance with the material's fracture toughness on different constraint conditions, so it may be a suitable parameter. Based on the area A_PEEQ, a unified constraint characterization parameter √A_p is defined. It was found that there exists a sole linear relation between the normalized fracture toughness J_IC/J_ref and √A_p regardless of the in-plane, out-of-plane constraint and the selection of the _p isolines. The sole J_IC/J_ref-√A_p line exists for a certain material. For different materials, the slope of J_IC/J_ref-√A_p reference line is different. The material whose slope is larger has a higher J_IC/J_ref and is more sensitive to constraint at the same magnitude of normalized unified parameter. Conclusions: The unified J_IC/J_ref -√A_p reference line may be used to assess the safety of a cracked component with any constraint levels regardless of in-plane or out-of-plane constraint or both. (authors)