A study on the effect of weld repair on the structural margins of a solid rocket booster hardware

Abstract

Cylindrical shells in one of ISRO’s solid rocket booster is fabricated from ultra-high strength steel (M250 Maraging steel) through rolled and welded construction. This steel being a fracture prone material, elastic-plastic fracture mechanics-based design is followed for the design of cylindrical shell. The no-flaw failure pressure i.e. elastic plastic failure pressure (Pb) of the cylinder is an important parameter in design. For a given diameter of cylindrical shell, Pb is a function of shell thickness and mechanical property. If any unacceptable flaw is found on welds during NDT, repairs are carried out in the weld region. Mechanical properties of weld/weld repairs are lower when compared to parent material. It is seen that, at any location, if it is repaired twice, the strength reduces by 21% when compared to the parent metal property. Thus, the mechanical properties of the weld region dictate the overall design thickness of the cylindrical shell. A study is carried out to find the effect of length of weld repair zone on the Pb. A parametric 3D FE model simulating the repair in long seam weld, cirseam weld and combination of long seam & cirseam weld in cylindrical shell is used for this purpose. It is found that the effect of parent metal property needs to be considered while estimating Pb. Further while considering the repair in long seam, the length of repair also needs to be considered. Pb is found to increase considering its effect. This study was helpful in establishing more realistic structural margins in a solid rocket booster case which had weld repair.