The Relaxation Of Residual Stress Under Cyclic Loading

Welded components are widely used in the marine engineering. Residual stresses introduced during the welding process can have a significant influence on the fatigue strength of welded components. There are many ways to reduce potentially harmful residual stress to prolong service life, such as annealing, pre-stretching, and shakedown. For the purpose of accurate assessment of fatigue lifetime, not only the magnitude of the residual stresses, but also their stability factors and evolution during service are of great importance.The initial residual stress after welding can be achieved by the numerical simulation with FEM based on the thermal elasto-plastic theory. The changes of temperature and stress fields are obtained, which is from the Q235steel plate being welded both sides. X-ray diffraction method is used to measure the surface residual stresses on the welded specimens to verify the numerical simulation results. The relaxation of the residual stresses are studied by comparing the changed residual stresses and initial residual stresses on the welded specimens, which are applied the cyclic loadings of different levels and different numbers.The relaxation of welding residual stresses as a function of cyclic loading was documented on Q235. Welded specimens were subjected to cyclic loadings (R.=0.1) at different applied stresses, and the residual stress relaxation existing near the welds was measured as a function of cycles. A phenomenological model, which treats dislocation motion during cyclic loading as being analogous to creep of dislocations, is proposed for estimation of the residual stress relaxation. A model based on the principle of creep dislocation movement was proposed to estimate the relaxation of welding residual stress. This model shows good agreement with the experimental data.