Application Of Numerical Integration And Spectral Methods To Fatigue Crack Propagation In Marine Structures

It is a hot issue to evaluate the fatigue strength and residual life of ship and marine structure by fatigue crack growth.The calculation cost of fatigue crack growth is expensive due to the high cycle in marine structure,which hinders the widespread application of this theory to some extent.In this paper,numerical integration method is applied to fatigue crack growth life calculation,and the high-order calculation methods are introduced to reduce the calculation steps.In addition,fatigue crack propagation under the spectral analysis was investigated by using the fatigue crack propagation finite element program written byANSYS APDL and Matlab.Specifically,this article includes the following sections.First,a new expression of fatigue crack growth is deduced for a steady random load.The fatigue crack growth rate model used in the equation can consider the stress ratio and the threshold value.Based on this,the application of high-order numerical method is proposed.Applying this method to the fatigue crack propagation of penetrating crack and surface crack found that the step length of higher-order calculation method can be larger than that of Euler method,and the calculation precision can be higher.As the loading sequence will have an influence on fatigue crack growth life,the numerical integration method is further investigated,and it was applied to the fatigue crack growth model considering load sequence.the calculation formula which combine cycle-by-cycle(CBC)and numerical calculation method(Euler method,Trapezoidal method,third order Runge-Kutta method)are proposed.Compared with the conventional CBC calculation results,it is found that with the increase of the number of steps,all the three numerical methods can achieve satisfactory results.Compared with the Euler method,The higher-order calculation method can take larger steps,and the calculation accuracy can be higher.In the actual structure,the boundary conditions are often complicated.In this case,finite element method(FEM)is widely recognized to calculate the fracture parameters to simulate the fatigue crack propagation.The authors developed a crack propagation simulation program based on ANSYS and MATLAB.The code combines the advantages of ANSYS Parametric Design Language(APDL)and MATLAB numerical calculation.The fracture submodel FEM mesh was updated step-by-step to simulate crack growth.The code can be applied to simulate a)arbitrary path propagation of through-thickness crack,b)quasi-3D crack propagation of the elliptical/semi-elliptical crack.A more accurate calculation method of fatigue crack growth is proposed based on spectral analysis.For a certain crack size,the same stress transfer function can correspond to different stress intensity factors,which led to the stress spectrum method can not calculate the fatigue crack growth accurately.In this paper,short-term distribution of SIF is obtained by SIF transfer function and wave spectrum,and the short-term SIF distribution and improved Paris formula are used to calculate fatigue crack growth life.The method presented in this paper provides a new way of combining spectral analysis and fracture mechanics.