Finite element based plastic limit loads for cylinders with part-through surface cracks under combined loading

This paper provides detailed plastic limit load solutions for cylinders containing part-through surface cracks under combined axial tension, internal pressure and global bending. Such solutions are developed based on detailed three-dimensional (3D) finite element (FE) limit analyses using elastic-perfectly-plastic material behaviour, together with analytical solutions based on equilibrium stress fields. For the crack location, both circumferential and axial cracks, and for each case, both external and internal cracks are considered. Furthermore, in terms of the crack shape, calculations are presented for both semi-elliptical and constant-depth surface cracks. The resulting limit load solutions are given in a closed form, and thus can be simply used in practical situations and defect assessment documents. In general, differences between analytical solutions based on equilibrium stress fields and the proposed solutions are more significant for axial cracks than for circumferential cracks. Since the calculations are based on detailed 3D FE limit analysis no assumption is needed concerning the state of stress. The present solutions, therefore, are an accurate estimate of the limit loads of the geometries that have been considered and are thus very useful information for assessing the integrity of pressurised piping.