Simulation Research On Fatigue Crack Propagation Behavior In Bulb Stiffeners In Ship Structural Details

Fatigue failure is regarded as one of the most serious problems faced by marine structures during their service life.Bulb stiffeners are basic structural elements and widely used in ships and offshore platforms.Therefore,the accurate information of fatigue crack propagation in the bulb section is important during the fatigue life prediction.Presently,classification societies propose that if fatigue cracks propagated and penetrated through plate thickness,such structure members can be regarded as fatigue failure.As for the bulb stiffeners,the classification rules usually recommend an equivalent angle bar to substitute bulb stiffener in simplified fatigue assessment procedures.Since the crack propagation behavior in bulb section should be different from the one in plate thickness.It is of great important significance to study on fatigue crack propagation behavior in bulb stiffeners in ship structural details.This paper aims at simulation research on the fatigue propagation behavior of a bulb stiffener from a ship bottom longitudinal-go-through-bulkhead in a ship structure.The research work are as follows:1)The simulation of crack propagation based on the finite element methodThe application of FEM in the fatigue crack propagation and the main variables were discussed.Three kinds of crack: CCP,SEN and through-wall crack with curve front were studied based on finite element program FRANC 3D separately to verify the accuracy of the FEM simulation.Calculation results of the stress intensity factor were investigated by comparing to the Stress intensity factor handbook and Newman-Raju results.2)The simulation of crack propagation based on the extended finite element methodTwo kinds of shape function and the discrete equations of XFEM were discussed.The application of XFEM in the fatigue crack propagation was analyzed from the view of failure criterion and the density of mesh.The SIF of SEN and CCP specimen were calculated by the XFEM through ABAQUS.3)The test of fatigue crack propagation in bulb stiffenersThe crack shape and crack propagation rate were investigated both experimentallyand numerically.The fatigue crack propagation were simulated based on FEM and XFEM separately.Based on the simulation and test,failure criterion of the bulb stiffeners was discussed.The following conclusions can be drawn:1)When the crack appears only in the upper surface of a bulb stiffener,the crack front shape follows a half ellipse.While it gradually changes into an approximate quarter ellipse when the crack propagates to the side surface of the bulb.2)From the comparisons between the test results and the simulation,the simulation by program FRANC 3D and ABAQUS based on FEM and XFEM was proved to be reliable and can be used to simulate the fatigue crack in complicated ship details.3)The widely applied fatigue failure criterion of the bulb stiffeners is conservative.The failure criterion of a bulb stiffener is that the fatigue crack penetrated the whole bulb head,in which the crack length is always longer than the flange thickness of proposed equivalent angle steel.It is reasonable to take the length of that the crack penetrates the oblique surface of the bulb as a failure criterion.