The Effect Of Nonlinear Distribution Stress On Surface Crack Growth At Welded Joints

Ships and offshore platforms are large and complex welded steel structures. The weld toe leads to high nonlinear stress distributed across plate thickness, which affects the stress field around surface crack tip. Meanwhile, the stress becomes larger at intersection points of crack surface and free surface of the plate because of nonlinear stress peak, which accelerates the growth of the crack along length direction. Furthermore, the changes of the stress field and crack shape during propagation influence the magnitude of crack-tip stress intensity factors. Variations of both the crack shape and the stress intensity factor result in alteration of crack propagation life. So it is important to study the effects of the nonlinear distribution stress on the stress intensity factor, crack shape, and crack propagation life for more accurate fatigue life estimation of welded structures.Finite element method is used to analyze the surface crack growth at butt welded joints. The effects of residual stresses and welded defects such as slag inclusions, gas pores, misalignment, incomplete penetration at weld root are not considered. The thesis emphasizes on the effects of the nonlinear distribution stress on surface crack and the main works included are as follows:1. A correction factor of the stress intensity factor Mk is introduced to describe the effect of the nonlinear distribution stress. And the geometry parameters of weld toe, aspect ratio a/c and combined load conditions are considered. Numerical fitting is adopted on the formulas.2. The effect of the nonlinear distribution stress on the surface crack shape is discussed based on assumption of semi-ellipse proposed by Newman and Raju.3. The effect of the nonlinear distribution stress on crack propagation life is studied considering the variations of stress field around the surface crack and crack shape.It is found that the influence of the nonlinear distribution stress is significant on the stress intensity factor of small surface crack and, to a less degree, on that of relatively lager crack. The fitting formulas of stress intensity factors of nonlinear distribution stress are proved precise according to finite element analysis. The surface crack shape change is influenced by the nonlinear distribution stress and tends to be slim. The crack propagation life of the surface crack at weld toe is much lower than that in plate. The effect of the nonlinear stress on crack propagation life is considered by the variation of both the stress field around the crack and the crack shape. The results show that the change of crack-tip stress is the main reason for the decrease of the fatigue life for the small crack, but the alteration of crack shape is the main account for the big crack.