Stress indices for elbows using finite element analysis

Full text: Design under plastic deformation in Class I nuclear power plant piping system is governed by equation (9) in N8-3652 ASME boiler and pressure vessel code 1998. In this equation, nominal stresses obtained by strength of material theories are modified by scalar multipliers called stress indices. This facilitates its applicability to components such as elbows and tees in addition to straight pipes. One of the indices is termed B2, which relates the collapse load of the elbow to that of a straight pipe with identical cross section. Whereas B2 is constant for straight pipes, it varies with curvature for elbows. Nonlinear finite element analyses have been performed for a series of stainless steel elbows of different sizes and schedules with different levels of internal pressure. In total 16 elbows with the piping factor h varying from 0.1 to 1.0, Do/t varying from 7 to 60 and R/Do from 1.3 to 2.2 have been considered for analysis. The dimensionless pressures of 0.0, 0.2, 0.4 and 0.5 have been considered to take into account the effect of pressure. Finite element package COSMOS/M has been used for various analyses. Collapse limit loads and collapse moments were determined from the load deflection curves obtained from the above analysis. The limit load and moments were further used to calculate the B2 index. Revised relationship for index with reduced conservatism is proposed