The Direct Analysis Approach To Compute The Life Of Low Cycle Fatigue Of Ship Structures

Ship and ocean engineering structures are under the action of wind, ocean current and sea waves, the location where stresses concentrated are easy subjected to fatigue. We must take low cycle fatigue into consideration, for the local stress is larger than the yield stress among the structures such as submarines, deep sea vehicles and FPSOs. In this article, we calculate the low cycle fatigue life of structures under cyclic loading based on the macro mechanism of fatigue problem.The fatigue problems of structures are essentially caused by the accumulation of irreversible plastic strain. Therefore, we firstly study the stress-strain relationship of materials. In this article, the author discuss the monotonic stress-strain curve and cyclic stress-strain curve, this is the foundation of fatigue problem.. Meanwhile, the author studied the methods to calculate the local stress and strain, and verify the validity of Neuber's rule. We raise the improvement of Neuber's rule, that is the improved Neuber's rule based on the elastic finite element analysis. There are two types of curves to calculate the fatigue life: stress-life curves and strain-life curves. Taking consideration of the imperfection of strain-life curves, the author proposed the analysis approach based on the persuade stress, then provide the design method about the persuade stress. Obviously, the analysis approach which based on the persuade stress can avoid complicated elastic-plastic FEM and compute the fatigue life from elastic FE analysis directly.There are three main curves about computing the fatigue life: The description of the material fatigue performance and, the calculation of stress and strain history at the hotspot of structures and damage accumulation. We put forward two traditional methods to compute the low cycle fatigue life: the elastic-plastic FE method and the elastic FE method. According to the basic theory of fatigue damage, the author put forward the direct analysis approach to compute the fatigue life of ship structures. At last, the author verified the validity of the three methods and analyzes the scope of the application and how to improve the methods.