Equal Load-carrying Capacity Design Of Under-matched Overlap Joints

For high strength steels welding, there are existing problems such as weldingcold cracks(which is the most serious), embrittlement and soften of heat affectedzone. In recent years, under-matched joints are widely used in high strength steelswelding structure for their good welding cold cracks resistance and plasticity, butthe load-carrying capacity shortage becomes the new problem. Overlap joints arewidely used in welding structures for their advantage in pre-welding preparationand assembly. The stress and strain concentration of overlap joints are extremelyserious, load-carrying capacity shortage of under-matched overlap joints becomesmore serious, so it’s particularly important to improve the load-carrying capacity ofunder-matched overlap joints.In order to improve the load-carrying capacity of under-matched overlap joints,the equal load-carrying capacity(ELCC) design idea is proposed based on safetyconsideration. The geometric shape design can improve the load-carrying capacityof welded joints for its stress and strain distribution optimization, load-carryingcapacity of the perfect designed joints can be equal with the base metal. Inconsideration of static load capacity and fatigue resistance, the ELCC principle andrealization conditions are presented based on fracture mode analysis.Stress concentration factors at weld root and toe of bilateral weldedunder-matched overlap joints are calculated based on finite element method, theinfluence of overlap length, weld width and weld toe smooth transition radius onstress concentration factors is studied and the stress concentration equation isfounded based on regression analysis method. Design guideline of under-matchedoverlap joints ELCC for high strength steels is given based on stress concentrationequation and ELCC realization conditions.Follow the ELCC design guideline, geometric shape parameters ofunder-matched overlap joints corresponding to given mis-match ratio whichrequired to meet ELCC realization conditions are obtained and ELCC test joints aremachined. The test result shows that the fracture of ELCC under-matched jointsoccurs in the base metal zone under static load and the fracture mode is generalyield fracture mode, the fatigue life of ELCC under-matched joints is significantly higher than ordinary joints. The ELCC designed under-matched overlap joints finelymeet the ELCC design requirements.Further optimization of geometric shape is studied based on the obtainedELCC joints. Two optimizations are proposed:platform smoothly transited and basemetal grooved welded. The result shows that, for the optimized ELCC joints, thestatic load-carrying capacity is equal with base metal, the fatigue life is stillsignificantly higher than ordinary joints.The load-carrying capacity of under-matched overlap joints of high strengthsteels obviously improved by ELCC design. The overlap joints can be designedaccording to strength of base metal, need not regard the influence of mis-matchratio.