The Mechanical Characteristics Of Stress Corrosion Cracking Crack Tip Of Dissimilar Metal Welded Joints In Nuclear Power Plants

It is one of the key factors to study the mechanism of SCC and quantitatively predict the stress corrosion cracking(SCC) crack growth rate that the mechanical characteristic of SCC tip in dissimilar metal welded(DMW) joints of nuclear power plants The influence factors on the local mechanical characteristics at the crack tip on interface crack and sub-interface crack were studied by establishing three-point bend specimens of DMW based on theory research and elastic-plastic finite element analysis(FEA). It was also analyzed that the SCC crack growth driving force on different interface and sub-interface in the safety-end dissimilar metal welded joints of the reactor pressure vessel. The main works performed in this dissertation are as followings:(1) The SCC mechanisms of nuclear structure material in high temperature water environments was studied, which clarified that the stress and strain distribution nearby the crack tip is the main driving force to induce the crack tip oxide film rupture during SCC process in nuclear structure materials. Simultaneously, the study also indicates that the SCC crack growth rates largely depend on the stress and strain nearby the crack tip. And an approach to combine elastic-plastic fracture mechanics and FEM was proposed to calculate the driving force of SCC in the dissimilar metal welded joints.(2) The local mechanical characteristics nearby the SCC crack tip in the safety-end dissimilar metal welded joints materials were studied by homogeneous specimens. And the effects of the crack growth on the local mechanical field nearby the crack tip in various materials used in the safety-end dissimilar metal welded joint were obtained, which provide a benchmark to study the SCC crack driving force in the safety-end dissimilar metal welded joints.(3) An elastic-plastic finite element model of the bimetallic material interface was built, in which the mechanical inhomogeneity was considered in the dissimilar metal welded joints. The effects of the mismatch of the yield strength and strain hardening exponent and the crack growth on the local mechanical field nearby the crack tip of interface crack were studied, and the driving force of SCC on the interface of welded joint was adopted.(4) The crack growth analysis model of DMW joints were built by the extended finite element method(XFEM). The crack growth behavior on the interface in mechanical inhomogeneity materials was analyzed and discussed by the XFEM-based cohesive segment method and phantom node method. And the crack initiation and growth crack trend in DMW joints were also discussed.(5) The effects of the yield strength mismatch of the material, the distance between the crack to interface on the local mechanical field nearby the crack tip was analyzed and discussed by a model with sub-interface crack when the crack near the interface. And the effects of the crack growth on the local stress and strain field nearby the crack tip of the sub-interface crack were also analyzed and obtained.(6) The local stress and strain distribution nearby the crack tip of the interface crack and sub-interface crack was analyzed and discussed by a finite element model of the safety-end dissimilar metal welded joints which consist of low alloy steel, nickel-based alloy and austenitic stainless steel. And an approach to study the driving force of SCC crack growth in the safety-end dissimilar metal welded joints was established.