EVALUATION OF STRESS INTENSITY FACTOR FOR TRANSVERSE CRACKS IN WELD JOINT OF BWR SHROUD SUPPORT AND ITS APPLICATION TO THE EVALUATION OF TRAPEZOIDAL CRACK

摘要

Since the weld joints of BWR shroud support are made of Ni-base weld metal that has susceptibility to SCC in the BWR water, the SCC crack prefers to grow in weld metal rather than base metal of Ni-base alloys. It is important to prepare the evaluation method for SCC crack initiated in these alloys. To develop the flaw evaluation method for shroud support, stress intensity factor of postulated axial cracks in H10 weld of shroud support in BWR were analyzed by FEM. Results were compared with simplified SIF solutions in codes and standards to confirm the applicability of these simplified methods. It was understood that these simplified methods were available to evaluate the SIFs of axial crack in H10 weld although the thickness of H10 weld is not constant and is tapered. In the present study, SIF for trapezoidal crack was also analyzed because the front shape of stress corrosion crack in the weld metal of Ni-base alloy is not always semi-elliptical and is thought to have complex shape. It was found that the trapezoidal crack could be substituted by elliptical crack for conservative evaluation when the crack is deep and the ratio of upper base 2d to lower base 2c is small. Based on this result, the authors analyzed SIF for more number of trapezoidal cracks in a plate and compared with SIF of semi-elliptical crack which has the same depth a and surface length 2c of trapezoidal crack to investigate the effective range of substitution by an elliptical crack. It was found that the SIF of deepest point K_A of semi-elliptical crack becomes larger than that of trapezoidal crack when d/c becomes smaller than about 0.37. These results show the substitution of trapezoidal crack with small d/c by semi-elliptical crack makes conservative SIF evaluation and it is very useful because the SIF solutions for semi-elliptical crack provided in many codes and handbooks can be applied.