Material strain hardening in pressure vessel design by analysis

The effect of including material strain hardening on the gross plastic deformation (GPD) load of four sample problems from the EPERC Design by Analysis manual is investigated. The models considered are two axisymmetric vessels with flat heads under pressure, a nozzle/knuckle intersection under pressure, and a cylinder-cylinder intersection under combined pressure and moment loading. The results of limit analysis, large deformation elastic-perfectly plastic analysis, and small and large deformation bilinear hardening analysis are presented. Plastic loads are calculated using the ASME twice elastic slope (TES) criterion and the plastic work curvature (PWC) criterion. It is found that the plastic pressures given by the ASME TES criterion are inconsistent with respect to the limit load. The PWC criterion is found to represent strain hardening more consistently and lead to enhanced plastic loads. Von Mises equivalent plastic strain plots show that the form and extent of plastic deformation at the limit load in a perfectly plastic analysis and at the plastic load in a strain hardening analysis are similar, indicating a common definition of GPD