Study On Fatigue Crack Propagation Behavior Of Butt Joints Under Complex Stress Field

One of the main causes of failure and even fracture of engineering welded structures is the initiation and propagation of fatigue cracks near the weld toe.The stress intensity factor K at the crack tip is an important parameter for judging crack propagation and structural fracture.Therefore,accurately describing the stress intensity factor at the crack tip near the butt joint of the butt joint is particularly important in studying the crack growth rate of the welded joint.In this paper,the butt joint is taken as the research object,and the calculation method of stress intensity factor suitable for crack tip under biaxial stress field is discussed.The method is verified by fatigue crack propagation test.The results show that the modified stress intensity factor calculation formula can more accurately predict the fatigue crack growth rate,and provide reference for the fatigue life assessment of the butt joint and its similar engineering structure.Firstly,through the existing stress intensity factor calculation method and fatigue crack propagation theory,the stress field near the weld is analyzed under different stress combinations,and the welding residual stress,biaxial stress of bending and bending,stress concentration of weld toe and other factors are considered.Based on the influence of stress intensity factor,a method for calculating the equivalent stress intensity factor of crack tip under biaxial compression load is proposed.The applicability of the method is demonstrated by the biaxial fatigue test data.The results show that the improved equivalent stress intensity factor calculation method can reasonably and effectively describe the crack growth rate near the weld.Then,the fatigue crack propagation law of the butt joint under the complex stress field of compression and bending is studied.The crack propagation path is delineated by the crack expansion incremental calculation method.It is found that the fatigue crack at the weld toe extends in an elliptical or semi-elliptical form,but different aspect ratios have different effects on the crack growth rate.When the crack depth is the same,the amplitude of the stress intensity factor corresponding to the crack tip will gradually decrease as the crack length to width ratio increases.When the butt joint is subjected to the longitudinal compressive stress and the bending stress,the crack propagation rate will be slower than the bending stress alone.When the vertical compressive stress and the bending stress act together,the vertical compressive stress accelerates the crack propagation and can not be ignored in crack propagation analysis.Finally,the equivalent stress intensity factor calculation method of the crack tip under the biaxial compression load is applied to the actual structure of the engineering-the pressure shell shell and column combined shell.The size of the weld defects in the welded structure is determined by statistical methods and engineering methods,and the defects near the weld of the pressure-resistant shell are regarded as the initial cracks,and the fatigue crack growth life of the cone-column joint shell is calculated.The results show that the method can effectively predict the fatigue crack growth life of the cone-column joint shell when the initial defect size is selected properly.