PSI - Issue 14

V. Viswanath et al. / Procedia Structural Integrity 14 (2019) 442–448

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V Viswanath/ StructuralIntegrity Procedia 00 (2018) 000 – 000

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1. Introduction

Liquid propellant tanks, which form a major structural component of any launch vehicle system, are designed for internal pressure loading. The upper stage of a new generation launch vehicle of ISRO is a liquid stage carrying Cryogenic propellants, Liquid Hydrogen (LH2) and Liquid Oxygen (LOX) stored at 20K and 80K respectively in propellant tanks. An Aluminum Alloy compatible with these propellants at low temperatures and possessing high specific strength is employed. However owing to its poor weld efficiency, of the order of only 40%, additional thickness has to be provided at the weld locations. The transition from the higher thickness at the weld regions to lower thickness is achieved by means of steps through chemical milling process. These tanks are of cylindrical configuration with domes welded at its ends as shown in fig. 1.

Fig. 1. Configuration of the propellant tank.

These domes are fabricated by welding together formed petals and then welding to end rings of cylindrical shells to make a complete tank. To cater to functional requirements, openings, called nozzles are provided on these domes that are reinforced by thick rings to compensate for stress concentration effects. These openings are provided for pressurisation, venting, measurement cable lines, thermal conditioning, human access (manhole) etc. These nozzles are attached to the dome petals of the propellant tank by welding after shrink fitting. Whenever there are unacceptable weld defects, it will result in manual weld rework at those local zones. Welding on the domes is done by TIG welding due to its high strength. Aluminum alloys as compared to steel has high thermal conductivity, large coefficient of thermal expansion and lower modulus of elasticity. Therefore, welds in Aluminum alloys have more shrinkage. This shrinkage gives rise to residual stresses in welds. Also, Aluminum alloys are more prone to weld porosities. These, if above acceptable limits, are removed by carrying out a local weld repair which results in additional residual stress. In one such propellant tank shown in fig 1; high residual strains of more than 8000  were observed at two nozzle welds in a direction perpendicular to the weld after a successful proof pressure test. The structural integrity of the tank is evaluated in presence of these residual stresses from fracture mechanics point of view, which is the subject matter of this paper.

Nomenclature a

Depth for a surface flaw Half-length for a surface flaw

c

K K I

Stress Intensity Factor

Applied tensile stress intensity factor Stress intensity factor due to primary stresses Stress intensity factor due to secondary stresses Plane strain fracture toughness of the material Material toughness measured by stress intensity factor

p I K I K K IC K ma t s

K r Crack driving force in terms of stress intensity factor normalised by fracture toughness of material L r Ligament yield parameter L r(max) Permitted limit of L r MOS Margin of safety t Wall thickness V Plasticity correction factor  flow Flow stress, average of yield and ultimate strength of the material  ref Reference stress