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 APEEQ

a unified constraint characterization parameter Ap is defined.It was found that there exists a sole linear relation between the normalized fracture toughness JIC/Jref and Ap regardless of the in-plane

out-of-plane constraint and the selection of the ε p isolines.The sole JIC/Jref-Ap line exists for a certain material.For different materials

the slope of JIC/Jref-Ap reference line is different.The material whose slope is larger has a higher JIC/Jref and is more sensitive to constraint at the same magnitude of normalized unified parameter.Conclusions: The unified JIC/Jref-Ap 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.