Fracture assessment diagram (FAD) based fracture assessment procedures are universally adopted by standards/documents including BS7910, R6, API579-1/ASME FFS-1 and FITNET. In the use of a FAD for structural integrity assessment, one important consideration is to determine the load ratio (Lr) which is defined by two equivalent definitions: Lr is either defined as the ratio of reference stress (σref) to yield strength (σY) as in BS7910, or as the ratio of applied load to plastic limit load as in R6. The solutions of reference stress or limit load are given in the assessment procedures for commonly encountered flawed structures such as a plate containing a surface crack and a cylinder containing an external surface crack. Although the solutions given in the various standards are not all the same, they were invariably derived on the basis of analysis of the force and moment equilibrium with regard to a flawed section and few of them has taken into account the effects of bi-axial stressing on a flawed section, thus remaining a question whether these solutions are still valid in situations involving bi-axial loading such as the presence of pressure in a cylinder in addition to axial tension and bending.

In this work, finite element analysis (FEA) of plastic collapse was systematically performed on circumferential internal surface cracks in a cylinder subjected to various combined loads, including combined tension and pressure, combined bending moment and pressure, and combined tension, bending moment and pressure. The focus was on understanding the effects of bi-axial stressing due to pressure on plastic limit load. The investigation of these cases has demonstrated a significant effect in plastic limit load arising from the application of pressure introducing a state of bi-axial stressing. Comparison of the results of plastic limit load obtained from FEA with those derived from BS 7910 reference stress solutions was carried out to assess the applicability when the standard solutions of plastic collapse are used in the applications containing bi-axial stresses.

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