Study on the mechanical state of inner surface crack tips at SA508-52M-316L dissimilar metal welded joint interfaces

• A novel 3D full-scale FE model with continuous mechanical property gradients is developed for DMWJs, surpassing the simplistic sandwich model. • An integrated multi-scale methodology enables high-fidelity analysis from global structure to local crack-tip fields. • Mechanical heterogeneity critically affects crack-tip states: peak stress/triaxiality near SA508 HAZ, max plastic strain in 316L HAZ. • Interface cracks deflect towards the higher-strength base metal’s HAZ, driven by asymmetric mechanical fields. • The conventional sandwich model underestimates crack-tip stress and overestimates strain/triaxiality, underscoring the need for gradient-aware models. Low alloy steel SA508 welded to type 316L austenitic stainless steel using nickel-based filler materials, such as alloy 52 M, represents one of the most frequently employed configurations of dissimilar metal welded joints (DMWJs) within the primary circuit of a pressurized water reactor (PWR). In response to the issue of structural integrity assessment of DMWJs at the primary circuit safe-end, a three-dimensional full-size finite element model considering the material property gradient was established. The mechanical response characteristics of inner surface crack tips under the coupling effects of complex loading and mechanical heterogeneity were systematically analyzed. By integrating sub-modeling techniques with the predefined field method, a multiscale correlation analysis between the global mechanical behavior of the full-size structure and the local stress–strain fields at crack tips was conducted. The influences of crack location on stress triaxiality and plastic strain distribution at crack tips were specifically investigated. A comparative analysis reveals that the conventional sandwich model underestimates crack-tip peak opening stress by approximately 15% at the SA508-52 Mb interface and by about 11% at the 52Mw-316L interface within their respective heat-affected zones (HAZs); corresponding crack-tip peak plastic strain (PEEQ) is overestimated by roughly 13% and 46%. Analysis shows that crack-tip opening stress and plastic strain exhibit higher values in the SA508 and 316L HAZs. Furthermore, cracks near structural discontinuities demonstrate more pronounced mechanical heterogeneity than those in flat regions. The proposed continuous transition model effectively addresses this limitation, providing a more accurate mechanical assessment for engineering applications.