Effects And Mechanism Of Material Constraint On Creep Crack Growth Behavior In Welded Joints

In this paper, the finite element model of CT specimen of P92 steel welded joints with multi-materials was established, and effects of material constraint caused by some factors on creep crack growth (CCG) behavior was investigated based on creep ductility exhaustion damage model systematically, which mainly includes the mismatch in creep properties between weldment constituents, the widths of the materials containing the crack, the initial crack positions and load levels in the welded joints containing only single crack. It also studied both the effects of interaction between dual-crack and material constraint for the welded joints with dual-crack through changing the locations of dual-crack in weldments and the relative distance between dual-crack. Based on the calculations and analyses for the distribution of stress-strain field, the creep crack initiation (CCI) time, CCG path, CCG rate, rupture life and the comparison with the experiments in the literature, it investigated the mechanism of the effects of material constraint caused by the above factors on CCG behavior, and discussed the effects of the material constraint on creep life assessment and design of the welded joints at high temperature. The main conclusions are as followings:(1) When the zone containing the crack is softer than at least one of the other two surrounding materials or both, the crack propagates straight along the initial crack plane, and the material constraint effects can decrease the rupture life of the welded joints. Otherwise, it will form a second crack (SC) in the soft material near interface after certain straight extension in original crack plane. The CCG rate of the SC was much higher than that in original crack plane, and the CCI time of the SC determined the rupture life of welded joints.(2) For the welded joints with softer HAZ containing crack, the crack always propagates straight, and rupture life increases with increasing HAZ width due to the decreasing of materials constraint effect. For the welded joints with harder HAZ containing crack, a SC in soft WM near interface generally forms, and the rupture life increases with increasing HAZ width due to the decreasing of the stress triaxiality and the later initiation of the SC.(3) For some cracks with specific positions, the load level causes the change of CCG path due to the stress-regime dependent creep constitutive and the difference of the material constraint. At low load level (low stress regime), the significant mismatch effect in creep properties promotes the initiation and propagation of SC, and the CCG rate was higher; At high load level (high stress regime), all the crack propagates straight along the initial crack plane, and the CCG rate was lower. If the extrapolation of CCG rate from higher load levels is used in creep life assessment and design of a welded joint at lower load levels, the non-conservative (unsafe) results will be produced.(4) Both the interaction between dual-crack and material constraint have much effects on the creep rupture for the welded joints with dual-cracks, and both the different locations of dual-crack and relative distance between dual-crack lead to different CCG behavior and rupture life.(5) The effects of material constraint which was induced by the mismatch in creep properties between weldment constituents, the widths of the materials containing the crack, the initial crack positions and load levels should be fully considered in the creep life assessment and design for the welded joints at high temperature, and also including the effects of the positions of dual-crack and the relative distance between dual-cracks on the CCG and coalescence for the weldments with dual-crack.