Effects Of Residual Stress On Surface Crack Growth At Welded Joints

Ships and marine structures are large and complex welded structures. The brittle fracture of most marine structures always happens in the welding position. The reason is that the welding process is to produce high residual stress in the structure, which affects the fatigue crack growth behavior significantly, and ultimately affects the fatigue life of crack. Therefore, it is a very important thing to discuss the influence of residual stress on the surface crack propagation in the welded joint, which is very meaningful for a deeper understanding of the fatigue behavior of welded structures and the calculation of the fatigue life of the ships and marine structures.The paper studies the finite element numerical simulation of residual stress model and fatigue crack model, as well as the mapping calculation of the residual stress to the fatigue model. It is focused on the influence of the welding residual stress on the stress intensity factor (SIF) of surface crack and fatigue crack life. The paper emphasizes on the works as following aspects:(1) The three-dimensional finite element model, through simulating the weak coupling of temperature field and stress field about flat butt welding model, has been established, which is on the basis of the systematic study of the heat conduction. At last, the residual stress field of crack-free status has been obtained.(2) In order to obtain the effects of residual stress field on the fatigue life of the surface crack, the multi-point constraint (MPC) method and the inverse isoparametric mapping method has been used to import the residual stress field into the crack model. The introduction of the MPC method is to simplify the modeling process and the crack model is divided into the refinement regional and non-refinement region, when the surface crack model has been building in welded joint. The use of the inverse isoparametric mapping method is to calculate the local coordinate and the corresponding displacement of all compute nodes in the refinement region using the interpolation method. Then, they have been imported into the surface crack model, together with the nodal displacements of the non-refined region. Residual stress field is applied through the transmission of the displacement field(3) The SIF in the residual stress field and tensile cyclic loading has been calculated using ANSYS, which is based on the linear elastic fracture mechanics analysis method. The fatigue crack life of the structure in the presence or absence of the residual stress field has been calculated using Forman formula, according to the elastic superposition principle. And the fatigue life has been comparative analyzed.The result of numerical analysis shows that:the introduction of the welding residual stress field has aroused large tensile stress in the direction perpendicular to the crack surface, which has accelerated the fatigue crack propagation and reduced the fatigue life. In addition, residual stress releases with the fatigue crack propagation. And the effects of the residual stress on the crack propagation are getting smaller and smaller.