Abstract
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 KJc or Jc. 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 DPEEQ around the crack fronts at different constraint levels was calculated using the finite element method, which introduces a new non-dimensional constraint parameter Dp, to uniformly characterize the in-plane and out-of-plane constraint effects. Compared with constraint parameters APEEQ or Ap, the process of obtaining parameters DPEEQ or Dp is much simpler and easier. In a wide range, a single correlation curve was drawn between parameter Dp and normalized fracture toughness values KJc/Kref or Jc/Jref 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 Dp 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.
Highlights
Stress corrosion cracking (SCC), which often occurs in nuclear powers‘ structural materials in high-temperature and high-pressure water environments, is a failure mode caused by a variety of factors, including tensile stress, susceptible materials and environmental parameters [1,2]
The existing, popular quantitative prediction model of SCC crack growth rate is mainly influenced by fracture toughness values KJc or Jc
Constraint parameter Ap [14,15] is put forward based on the areas that are surrounded by the equivalent plastic strain isolines ahead of crack tips
Summary
Stress corrosion cracking (SCC), which often occurs in nuclear powers‘ structural materials in high-temperature and high-pressure water environments, is a failure mode caused by a variety of factors, including tensile stress, susceptible materials and environmental parameters [1,2]. Constraint parameter Ap [14,15] is put forward based on the areas that are surrounded by the equivalent plastic strain isolines ahead of crack tips This parameter is able to characterize both of the two types of constraints properly. Since the plastic strain gradient is a characterization parameter of the crack growth rate prediction model, it may be a characterization parameter of composite constraint. By means of the reference average gradient of equivalent plastic strain (Dref) along the crack front of a three-dimensional tensile specimen, the dimensionless constraint parameter Dp (Dp = DPEEQ/Dref) is supposedly an effective parameter to measure both in-plane and out-of-plane constraint effects. The new characterization parameters DPEEQ and Dp were obtained by a three-dimensional FEM, and the results show that plastic strain gradient is a combination of composite constraints in a wide range. It is helpful to build an evaluation method that has structural integrity and safety while containing the composite constraint effect
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