PSI - Issue 14

V. Viswanath et al. / Procedia Structural Integrity 14 (2019) 442–448 V Viswanath/ Structural Integrity Procedia 00 (2018) 000 – 000

444

3

Ultimate strength of material Yield strength of material

 u  y

2. Residual stress during welding

Residual stress is that which exists in a body even in the absence of any external load and are generated when a body is subjected to non-uniform temperature change. Residual stress in a welded plate develops during shrinkage of the weld pool from the melting point to ambient temperature. Tensile stresses are developed along the weld direction near the molten zone while compressive stresses are developed away from the weld pool to equilibrate the tensile stresses. Distribution of temperature and stresses in various sections of a butt-welded plate is given in Fig 1(a). Since these stresses exist without any external force, the resultant force and moment are always zero. i.e.    0 dA  and   0 dM on any section (1) Also, the residual stresses are always limited to yield stress of material as beyond yield residual stresses are compensated by plastic deformations. Along section A-A thermal stresses due to welding are zero. At section B-B stresses below the welding torch are zero as the metal is in molten state. Stresses in the region at a short distance from weld are compressive because of restriction in expansion by the surrounding material at lower temperature. At section C-C the metal has started to cool and contract. This gives rise to tensile stresses in weld and compensatory compressive stress in area away from weld. Section D-D shows final stress field in the weld. The metal has completely cooled down and high tensile stresses are present in the weld region and compressive stresses are there in away from weld region. Fig 2(b) gives the schematic of a circular weld patch. Residual stresses in such welds are produced basically due to:

 Shrinkage of weld metal in direction parallel to weld  Shrinkage of weld metal in direction perpendicular to weld.

Combined effect of these two factors is shown in Stress distribution for a circular patch weld in Fig 2(c).

Fig. 2.(a) Mechanism of development of residual stresses in a welded plate (b) Circular weld patch (c) stress distribution across a circular patch weld [Koichi Masubuchi (1980)].

3. Proof pressure testing of propellant tank

All flight hardware are subjected to proof/ acceptance tests prior to integration in a launch vehicle for flight. By subjecting the hardware to a proof load, which is higher than the maximum expected load experienced by the hardware during its life, the evidence of satisfactory workmanship and material quality is ensured. The structural integrity of the hardware is also assured as this process helps in screening any initial flaws if present in the hardware.