SAFETY AND RELIABILITY ASSESSMENT OF BIMETALLIC PIPE ' WELDS OF AUSTENITIC AND FERRITIC STAINLESS STEEL AND INCONEL

Abstract

The safety and reliability assessment of iris-matched joints in a nuclear industry is an important issue. Recent international surveys of such welded joints have shown that there are several cracking problems, due to fabrication, ageing or corrosion [41] Thus, there exists a strong need to develop simple methods to assess the fracture assessment of such mis-matched joints. The accurate and realistic assessment of strength mis-matched components is directly related to the accurate estimation of the yield load. The current Leak-Before-Break (LBB) methodology is based on evaluating the integrity of cracked nuclear piping, having mis-match in material properties, by considering the. strength properties of the base material and fracture properties of weld material. This method generally yields safe results but it is highly conservative. At present FEM is considered as the best method to evaluate yield load and crack driving force for these mis-matched structures. However, it is always useful to have some simplified estimation methods for engineering calculations. In this regard most of the available works are concerned with specimens only and no such work exists on yield load solution for cracked welded pipe, which from safety consideration is an important component in a nuclear industry. This dissertation covers both mis-match and bi-metallic effects and provides accurate closed form yield load solutions of cracked welded pipe loaded in threepoint bending. Wide variation in crack sizes and weld widths have been considered to cover almost all practical cases. The weld considerations are idealized as a simple strip model, and the circumferential through-wall crack is assumed- to be located at the center of the weld metal. 3-D elastic-plastic finite element analyses of 424 cracked welded pipes have been performed. For the over-match cracked welded pipes analytical solutions for yield loads have been obtained. A comparison with the finite element results shows good agreement, for almost all values of strength mis-match levels, weld widths and crack sizes. Since due to time constraint analytical equations could not be obtained for under-match and bi-metallic cases, this may be a future work