Issue 29

L. Zhao et alii, Frattura ed Integrità Strutturale, 29 (2014) 410-418; DOI: 10.3221/IGF-ESIS.29.36 413 Material Elastic modulus, E (MPa) Poisson ratio,  Yield stress, σ 0 (MPa) Hardening exponent, n Constant,  A508 193000 0.288 440 5.333 1.0 Alloy 182 Buttering 193000 0.288 480 5.769 1.0 Alloy 182 Weld 193000 0.288 385 4.779 1.0 304SS 193000 0.288 254 4.402 1.0 Table 1 : Material mechanical parameters for the FEM simulation. Residual stress and operating loads Only the hoop stress is used in the current evaluation based on some assumptions because the pipe flaw is an axial crack. The distribution of residual stress [12] is shown in Fig.4. Note that only the hoop stress for an axial crack was applied in the weld region. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -50 0 50 100 150 200 250 300 350 400 450 Welding residual stress (MPa) Norm alized distance from pipe inside to outside Hoop Stress Figure 4 : Welding residual stress profiles. Normal operating loads for RPV nozzles are listed in Tab. 2. In addition to that, both axial and hoop stresses varying through-thickness induced by internal fluid pressure are considered. The welding residual stress applied as field and remote bending moment are also taken into account. Service temperature [13] (°C) Inner pressure (MPa) Axial pressure (MPa) Bending moment (kNm) Welding residual stress (MPa) 345 15.59 44 2 492 Shown in Fig. 4 Table 2 : Normal operating loads for RPV nozzles. FEM model A commercial FEM code, ABAQUS, was used in this simulation analysis. Based on symmetry condition, half of the model was investigated. The pipe symmetry surface is symmetric restrained in X-Y plane. Appropriate extensions of the left and right ends in the model have been made to reduce the influence of edge effect on the analysis results. Considering the nozzle and pressure vessel are rigidly connected, the fixed constraint on the left is set. The combined operating loads such as internal pressure, weight and moment loads are applied on the safe end DMW joint. Sub-model technique was adopted to investigate the crack local stress-strain field in detail. With different sampling location, three models are calculated based on the sub-model technique. Boundaries of the sub-model are driven by the stress-strain obtained from the global model analysis [14]. Fig.5 gives the mesh of a DMW joint specimen (global model and sub-mode), where X-Y datum is the crack surface and Z-axis is the crack growth direction. Mesh at the vicinity of crack front is observably refined in both global model and