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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3.1. FE model procedure to estimate the failure pressure of motor with seamless cylinder A motor case generally fails in the cylindrical shell region. Hence to estimate P b an axisymmetric model simulating the membrane stresses of cylindrical region (i.e. free from end effects) are used for analysis. True stress strain curve of material is used in the analysis. To simulate the axial load, one end of the model is fixed whereas at the other end, equivalent axial load is applied. Uniform internal pressure is applied in the inner surface. Geometric non linearity to be activated in the FE package. Load is applied in small steps. Maximum number of sub steps for FEM solver is kept very high. At the onset of instability, net section yielding occurs due to sudden decrease in the thickness and increase in the internal radius, the FEM solver reports an un-converged solution error message. Previous converged load step is taken as the failure pressure. 3.2. Cases studies (burst test) showing the comparison of estimated FEM failure pressure and measured failure pressure In this section the failure pressure of two burst test are studied one with Ø 204mm retro motor (RM) and another one with Ø 346 mm Special Purpose Motor. Elastic plastic burst pressure estimated with FEM is compared with actual observed failure pressure during burst test for both cases in table 2. In both the cases, motors failed in classical mode of failure i.e., Longitudinal opening of cylinder. The error in prediction from FEM is within 5%.

Table 2: Elastic plastic burst pressure estimated with FEM and actual observed . Motor Failure pressure (ksc) Error (%) FEM (MPa) Observed (MPa) Ø204 RM burst test 22.97 22.32 2.9% Ø346 SPM burst test 19.82 20.45 3.2%

4. Effect of parent metal properties on the failure pressure for shells having inferior weld properties In rolled and welded shells having inferior weld properties, the dilation of the shell is governed by superior parent metal properties whereas the instability criterion is governed by the reduced weld properties. This effect of parent metal property on the failure pressure have not been studied so far as designers generally conservatively assume the reduced weld properties globally and estimate the failure pressure using FEM method as mentioned in previous section. This approach is here forth called conservative model. To study this effect a separate FEM model simulating the weld as well as parent metal properties is used. This approach is called realistic model. In this FE model, weld property and parent material property are applied in the respective regions. The details are described below.

Fig 1: FE model simulating the long seam weld and parent metal properties

4.1. Effect in long seam weld To simulate the long seam weld of shell, 3D FE model is used. FE model used is a shell sector. Symmetric boundary condition is applied to the circumferential ends of the model. Uniform internal pressure is applied at inner surface. To