Fatigue Resistance Design Of Butt Welded Joints With Heterogeneous Mechanical Properties Based On Principle Of Equal Load-bearing Capacity

Fatigue load is the most common load-bearing type in the service process of welded structures.Improving the fatigue bearing capacity of welding structures through welding structure design is of important theoretical and engineering significance.Due to the influence of filler metal selection and welding thermal cycle,strength mismatch is common in welded joints which will have a significant impact on the mechanical properties and bearing capacity of welded joints.Most of the traditional fatigue design methods of welded joints simplify the welded joints into homogeneous materials with the same strength as the base metal,ignoring the influence of strength mismatch on fatigue bearing capacity of welded joints.This simplification may guarantee the safety of equal-matched or over-matched welded joints,but might lead to unsafe design results if directly used in fatigue design of under-matched welded joints.To solve the fatigue design problem of strength-mismatched welded joints,based on the idea of equal load-bearing capacity,a research on fatigue design methods of strength-mismatched welded joints is carried out in this thesis.Based on three-dimensional elastic theory,the effect of non-homogeneous mechanical properties on stress distribution near interface is studied in this thesis,which laid the foundation for the fatigue design of joints.It is found that the stress distribution near the interface of dissimilar joints is controlled by the difference of materials performance on both sides of the interface,which can be characterized by the interfacial stress coefficient.Moreover,it is found that when the ratio of Poisson’s ratio and Young’s modulus of the materials on both sides of the interface are equal,the interfacial stress singularity disappears under uniaxial load perpendicular to the interface.It is found that by adjusting geometrical size of the local bearing section and changing the local stress distribution of welded joints,the weak area of welded joints could have equivalent or even higher fatigue bearing capacity to the base metal.Based on this,an equal fatigue-bearing capacity design method(EFBC)of welded joints is put forward in this thesis.Taking fatigue life as the evaluation parameter of the fatigue bearing capacity,the realization condition of the EFBC design method is that the designed welded joint should have the same fatigue life as that of the base metal under certain fatigue load.Based on this,a critical design curve for EFBC designed joint can be obtained.The idealized geometric optimization method and non-idealized geometric optimization method for the critical design curve are proposed,and the effects of height,width and transition arc radius of the reinforcement on stress concentration coefficient near the welding toe are analyzed using finite element method.Based on the above research content,the criteria for EFBC design of butt joints are established.Based on criteria of EFBC,for smooth butt welded joints,butt welded joints with initial cracks or crack-like defects,and when the fatigue crack initiation life of welded joints needs to be considered,the influence of material heterogeneity and geometric parameters on fatigue strength of welded joints are studied based on nominal stress method,fracture mechanics method and total life method respectively.The quantitative relationship between mechanical and geometric parameters of welded joints and fatigue life of welded joints is established.The fatigue life estimation formula of welded joints considering material heterogeneity is obtained and used in EFBC design of welded joints.Thus,the EFBC design methods based on nominal stress method,fracture mechanics method and total life method are established respectively.Q550 high strength low alloy steel with ER70S-6 as the filler metal are selected for experimental verification.The experimental results show that the EFBC designed welded joint could have the same fatigue bearing capacity with the base metal,which proves that the EFBC design methods established in this thesis is feasible and can be necessary supplements of current standards,and is of great engineering value.