PSI - Issue 60

Sreerag M N et al. / Procedia Structural Integrity 60 (2024) 20–35 Sreerag M N/ Structural Integrity Procedia 00 (2023) 000 – 000

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1. Introduction ISRO’s largest diameter solid rocket booster (SRB) is currently fabricated with ultra-high strength Maraging steel (M250). Solid booster cylindrical hardware is in general fabricated from plates, which are rolled and welded (through long seam welding) to form cylindrical shells and these shells are welded together to form a cylinder of required length through cirseam welds. Welding of M250 plates are carried out by GTAW welding process and it is carried out in four passes using proprietary welding procedure specification during the welding process. Weld strength of maraging steel will reduce with each weld/weld repair and weld repairs are unavoidable due to inherent weld defects. The ultimate strength of M250 material reduces by 6.4%, 8.8%, 18% and 21.5% when it is as welded, repaired once, twice and thrice respectively. As the rocket motor cases are highly mass optimized pressure vessels, the effect of weld repair and the corresponding reduction in material strength need to be accounted appropriately. When an unacceptable defect is detected using NDT at any stage of fabrication of a motor case, repair is carried out and this results in reduction in the mechanical property of the material, high cost of repair and associated quality assurance activities using various NDT technique (e.g., proof pressure test), and longer lead time etc. If repair of the defect is not done, safety of the personnel involved in the proof pressure test of motor case and also the test facility itself is at risk. Considering this, sound engineering judgment can be made only by understanding the effect of weld/weld repair properties as well as weld repair length on the failure of the pressure of vessel. In this paper, Significance of no-flaw burst pressure in estimating the failure pressure of a fracture prone material is described by giving an outline of 3-parameter elastic plastic fracture method followed in ISRO is described first. Then, a description about the finite element procedure that is used to estimate the elastic plastic failure pressure is described. Further the effect of parent metal properties on the failure pressure for shells with weld described. For this long seam weld, cirseam weld and shell with both long seam and cirseam weld is studied separately. Then, the effect of weld repair length (axial weld repair length for long seam weld and circumferential weld repair length for cirseam weld for given weld bead width) on the failure pressure is studied. Finally, the failure pressure of 3.2m diameter maraging steel cylindrical shell is studied considering all the above points. A typical application of this study for salvaging 3.2 mm segment hardware having weld repair deviation is also given towards the end of this paper. Nomenclature SRB Solid Rocket Booster LVM3 Launch Vehicle Mk-III NDT Non-Destructive testing k max Stress intensity factor at failure σ f Hoop failure stress in presence of flaw K F , m, p 3-parameter material constants σ u Hoop stress at burst in unflawed case σ m Membrane stress σ b Bending stress H Bending correction factor a Semi Elliptical Crack depth c Semi Elliptical Crack width Q Shape factor for elliptical crack F Stress-intensity boundary-correction factor σ f Hoop stress at burst in presence of defect t Shell thickness M1 Elastic magnification factor 1 M2 Elastic magnification factor 2 M3 Elastic magnification factor 3 S Stress-intensity correction factor P b No flaw burst pressure P bf Burst pressure in presence of flaw