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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3.1. Procedure

The proof testing of the tank is carried out at room temperature using R Grade Nitrogen gas at 0.46MPa (absolute During proof pressure testing of the tank, the following were observed:

 High strains of 11372  were observed at a nozzle weld location in a direction perpendicular to the weld line.

 Maximum post test residual strain of 8200  was observed at the location after removal of the pressure.

3.2. Estimation of residual stress

The strain gauges that measured the residual strains are in the weld rework area. Therefore, to evaluate the residual stresses; finite element analysis, using assumed residual stresses at these locations, is carried out. Constraint equations are used to simulate the initial residual stress at the heat affected zone (HAZ). The analysis is carried out using version 14.5 of ANSYS finite element package. Fig 4 depicts the axi-symmetric model of a typical nozzle in the propellant tank dome. While the dome and closure plate are modeled using PLANE182 (4 noded linear solid of revolution) elements, the bolts used for the cover plate are simulated using BEAM4 (2 noded linear beam) elements. The only other load, the tank internal pressure, is also applied as shown in Fig 3.

Fig. 3.Finite Element Model of a typical nozzle location with pressure loads & boundary condition

Strain data at the gauge locations were found to have a good match with the test data with an assumed residual stress of 95N/mm 2 which was arrived at by trial and error. Fig 4 gives the comparison of test and post test prediction with residual stress of 95N/mm 2 .

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Fig. 4.Strain Vs Internal pressure during proof pressure test of the propellant tank.

4. Evaluation of structural integrity of the tank

To carry out fracture mechanics based evaluation of integrity of any structure, three parameters namely stress, crack size and fracture toughness of the material under consideration are to be known apriori. While, fracture toughness is a material property that is evaluated through mechanical testing, the crack sizes will be available through non destructive evaluation techniques like radiography or Ultrasonic testing. The stress in the region under consideration is referred to as reference stress and is estimated through finite element analysis.