Assessment of the integrity of structures such as reactor pressure vessels is a critical issue in relevant industries. In a full integrity assessment, the presence of initial residual stresses (RS) needs to be taken into account. An initial RS field is introduced into a type 316 stainless steel cylindrical vessel with no defects and to one with a partial circumferential crack on its outer surface. Relaxation of RS following several proof load cycles, in form of internal pressure, applied to the vessel is explored using finite element simulations. It is found that the proof loading process generally relaxes the RS and is proved to be beneficial to both cracked and un-cracked vessels with or without the presence of initial RS. Interaction of residual stresses with warm pre-stressing is further investigated using A533B steel at room and low temperature subjected to axial loading. The results are compared with similar analyses but with no introduction of an initial RS field to explore the interaction effects on fracture resistance, as well as the role of partial crack on the RS distribution / redistribution. The differences are discussed and illustrated.