An Investigation of a New Parameter Based on the Plastic Strain Gradient to Characterize Composite Constraint around the Crack Front at a Low Temperature

Stress corrosion cracking (SCC) is an important destruction form of materials such as stainless steel, nickel-based alloy and their welded components in nuclear reactor pressure vessels and pipes. The existing popular quantitative prediction models of SCC crack growth rate are mainly influenced by fracture toughness values K(Jc) or J(c). In particular, the composite constraint, containing the in-plane constraints and out-of-plane constraints around the crack front, has a significant influence on the fracture toughness of structures in nuclear power plants. Since the plastic strain gradient is a characterization parameter of the quantitative prediction model for crack growth rate, it may be a characterization parameter of composite constraint. On the basis of the experimental data at a low temperature of alloy steel 22NiMoCr3-7 used in nuclear pressure vessels, the gradient of equivalent plastic strain D(PEEQ) around the crack fronts at different constraint levels was calculated using the finite element method, which introduces a new non-dimensional constraint parameter D(p), to uniformly characterize the in-plane and out-of-plane constraint effects. Compared with constraint parameters A(PEEQ) or A(p), the process of obtaining parameters D(PEEQ) or D(p) is much simpler and easier. In a wide range, a single correlation curve was drawn between parameter D(p) and normalized fracture toughness values K(Jc)/K(ref) or J(c)/J(ref) of specimens at a low or high constraint level. Therefore, regardless of whether the constraint levels of the structures or standard specimens are low or high, constraint parameter D(p) can be used to measure their fracture toughness. To build an evaluation method that has structural integrity and safety while containing the composite constraint effects, in addition to accurate theoretical interpretation, further verification experiments, numerical simulations and detailed discussions are still needed.