Review of ASME III Code Case for the Application of Finite Element Based Limit Load Analysis

The use of finite element based limit load analysis for the assessment of the primary load capacity of a pressure vessel is well established and numerous papers on the subject, including experimental results, have been published in the last decade. Finite element based limit load analysis is often used in the context of NB-3228.1 Limit Analysis to demonstrate a margin against ductile burst as an alternative to satisfaction of the NB-3200 limits on general, local and primary membrane plus bending stress intensity. However, although NB-3200 permits the use of ‘limit analysis’, no specific guidance on the use of finite element methods for this purpose is provided. Other pressure vessel codes, including ASME VIII Division 2 and EN13445 contain explicit guidance on the use of finite element methods for limit load analysis. To address this, a Code Case is currently under development to provide technical guidance on the use of finite element based limit load analysis within the context of NB-3200 assessments. The Code Case provides a step-by-step procedure which guides the analyst in the application of limit load analysis and ensures that a valid analysis has been undertaken. The topics of geometric weakening, yield surface selection, tentative wall thickness, element selection and selection of S_m are accounted for in the Code Case and discussed. This paper provides a detailed review of the Code Case and shows how it can be used in practice.