Study On Evaluating Fatigue Life For Welded Structure

Fatigue failure is one of the main failure models for welded structure under cycle loading over a long period of time. It is an important research subject in the engineering field. The fatigue failure process of welded steel structure consists of two stages: fatigue crack initiation and fatigue crack propagation. The life of fatigue crack propagation can be relatively accurate calculated in the engineering research, but the life of first stage is often ignored.In this paper, research work and research achievement in this field have been reviewed in brief, which includes research and innovation in theory and several important approaches in experiment. Referring to the experiments done by former researchers, the principle of fatigue failure has been analyzed in detail and the process of fatigue failure has been illustrated, which includes micro-crack's initiation and growth, macro-crack's propagation and final instantaneous fracture.Combining with engineering facts, the fatigue process for welded structure is divided into two stages. A better calculation model predicting welded structure's fatigue life is proposed. The fatigue life of crack initiation can be predicted with the local stress-strain method and the life of crack propagation can be predicted with Paris formula. The total fatigue life for welded structure is the sum.A new effective stress concentration factor(SFC) is defined in this paper, the Neuber's equation can be modified with this effective SFC. The theory calculation models of effective SFC for welded structure are established under the condition of symmetrical and unsymmetrical stress cycle respectively, which take account of the follow factors: mechanical properties of matrix material, geometrical shape of welded joint, ratio of stress, residual stress on the weld toe.The SFCs on weld toe are analyzed by FEA software of ANSYS in butt joint and T-joint respectively. The relative curves of SFC affected by different geometrical parameter on section of welded joint are mastered.Based on experiment data, the fatigue life of a crane box girder is calculated by the method brought forward in the paper. Compared the result with the experiment data, the error is in accepted range. It shows that the method in this paper is feasible and can solve engineering problem.