Design Of Undermatching Welded Joints For High Strength Steels Based On Principle Of The Equal Load-Carrying Capacity

Welded joints of high strength steels or ultra-high strength steels face such problems as welding cold cracks, insufficient reserve of weld toughness and longest development periodicity of its special welding materials, if these welded joints are still designed by the equalmatching principle. When the welding materials which strength is lower and toughness is higher than base metals are used to join high strength steels, the weldment toughness can be improved, and the sensitivity of cold cracks can be decreased obviously, meanwhile the weld of high strength steels can be proceed under lower preheat temperature or even no preheating. Unfortunately, however load-carrying capacity of undermatching weld joints is lower than the one of equalmatching weld joints. To improve the load-carrying capacity of joints, the concept"equal load-carring capacity (ELCC)", i.e. undermatching weld joints and base metal have equal static load-carring capacity and fatigue resistance, and the corresponding design thinking are presented in this thesis. Based on determination of the realization conditions of ELCC, the design guideline to ELCC of undermatching butt joints and undermatching K groove cruciform joints are also presented. The undermatching weld joints of ELCC are designed, and its load-carrying capacity is studied by experimental research as well.Based on safety consideration, the welded structures must have sufficient strength and ability to stress relaxation. Considering the type of fracture mode reflects the welding joint strength, plastic deformation ability and the degree of non-uniform of plastic deformation between weld bead and base metal, the most safe structure fracture model is the general yield fracture which plastic deformation of the base metal will occur prior to weld bead, so the structural design thinking of ELCC which make the yield load of weld bead for undermatching weld joints not less than the yield load of base metal is presented.According to the structural design thinking of ELCC, and principle that geometric parameters designs of weld joints can regulate the distribution of stress and strain, reduce the stress concentration factors and improve the carrying capacity of low strength weld zone, the stress concentration factor at weld root and weld toe are chosen as the assessment parameters of carrying capacity of joints, and the realization conditions of the ELCC under static load and fatigue load for undermatching weld joints were presented, i.e. make the stress concentration factor at weld root equal to the yield strength mis-match ratio of joints and minimize the stress concentration factors at weld toe.Results of feasibility study show that the realization conditions of the ELCC can be met by design of weld geometric parameters, even for cases of extreme strength undermatch. The weld shape proposals to minimize stress concentration for the butt joint and the K groove cruciform joint were analyzed and determined. Influence of such geometric parameters as the weld toe radius, height of excess weld metal, width of the cover pass, width of excess weld metal and leg length on stress concentration factors at weld toe and weld root were studied, and the relationship equations between the geometric parameters of joints and stress concentration factors at weld toe and weld root were also built by the FEA calculating and regression analysis.Based on the ELCC realization conditions and the relationship equations of stress concentration factors, the geometry parameters of undermatching butt joints and K groove cruciform joints corresponding to given mis-match ratio which required to ELCC were determined. In this research, the design guidelines and its scope of application which describe the ELCC design process of undermatching butt joints and K groove cruciform joints are presented.The carrying characteristics and advantages of ELCC undermatching weld joints were analyzed by FEM under tensile load. Because plastic deformation of the weld bead does not occur before plastic deformation of base metal, the triaxial state of stress of weld zone can be avoided even in the extreme undermatching ELCC joints, and the fracture mode can change into a much safer general yield fracture. The stress and strain at weld toe and weld root of the ELCC joints increase slowly with the load increase , the peak values of equivalent plastic strain firstly locates at weld toe and then transferres to the base metal area outside the weld toe. Therefore the ELCC design joints will be broken in the base metal under tensile load, when the ductility of weld bead material is better.The experimental investigation of carrying capacity of ELCC joints for high strength steels was completed, and the feasibility of the ELCC design was proved. The joint strength which equal to the tensile strength of base mental are obtained. And the critical crack size is greater than the one of base metal, the ELCC design of joints decreases the sensitivity to defects like crack. At the region of high stress, the fatigue strength of ELCC joints can be closed to the equalmatching flush butt joints, at the region of low stress, the fatigue strength slightly decreases with the decrease of mis-match ratio. Compare with equalmatching butt joint as welded, the fatigue strength of ELCC joint which yield strength mis-match ratio is 0.57 increases 115% under to 1.0×107cycles, and achieves 80% of equalmatching flush butt joints.The vulnerable area of undermatching welded joints can be transferred from weld bead to base metal by the ELCC design, so the welded structure can be designed directly in the light of strength of base metal, need not regard the influence of mis-match ratio.