Equal Load Carrying Capacity Joint Design Based On Fracture Parameter Of Stress Intensity Factor

In order to reduce weight and ensure the load carrying capacity of structure,high-strength steel is commonly used in bridges, building, aerospace fields andso on due to its high strength and good ductility. As an efficient joining process,welding has been widely used in engineering field. Weld defects can not beenavoided because of the welding particularity. It has been found that weld defectscan significantly deteriorate the load carrying capacity of joint. Typical brittlefracture is often observed in the defect-existent joint. In order to increase loadcarrying capacity of defect-existent joint, equal load carrying capacity (ELCC)method based on stress intensity factor is proposed by using fracture mechanicstheory. ELCC design criteria of joint with different defects are given.In order to guarantee the load-carrying capacity of under-matched joint withdefect reach that of the base metal, four different conditions have beenconsidered. The influences of joint shape parameters on shape factor ofcenter-cracked or edge-cracked butt joint are studied. The results showed that itis unreasonable to calculate the shape factor of center-cracked or edge-crackedbutt joint without considering weld reinforcement. A new approach to obtainingshape factor expression for defect-existent complicated structures is introduced.Firstly, the influences of structural shape parameters on shape factor should beinvestigated by using finite element method. Secondly, a coordinated parameterwhich can unify all influences of shape parameters on shape factor must beintroduced. Then a modified coefficient function on the basis of the traditionalshape factor expression can be obtained with regression analysis of all finiteelement results by using the coordinated parameter as the independent variable.Finally, the accurate shape factor expression considering the effect of allstructural shape parameters can be obtained. The accurate shape factorexpressions for center-cracked and edge-cracked butt joint under tensile load aregiven in this paper. The shape factor expressions given in this paper canefficiently guide defect-existent butt joint design. To conduct safety assessment,the corresponding stress intensity factor, critical stress, critical crack length andfatigue remaining life can also be obtained by using the shape factor expressions proposed in this paper.The procedures of obtaining shape factor and stress intensity factor fordefect-existent complicated structures by using finite element method are asfollowing, establishing finite element model, defining material parameters andboundary conditions, submitting job and processing data. The procedure is toocomplicated and error-prone. In order to improve the efficiency and accuracy ofcalculating the shape factor and stress intensity factor for defect-existentcomplicated structures, the expert system of calculating stress intensity factor fordefect-existent joint is developed. The expert system of ELCC joint design whichcan provide the ELCC joint shape parameters is also developed. This expertsystem can also give the stress intensity factor, crack tip opening displacement,J-integral and fatigue remaining life of existing ELCC joint.Tensile experiment and fatigue experiment of joint with center crack or edgecrack both in weld and base metal designed according to ELCC joint shapeparameters were tested. Although the crack length of weld metal is larger thanthat of the base metal, all the samples were fractured at the cracks in base metalzone which can satisfy the ELCC condition. Tensile experiment and fatigueexperiment of joint with center crack or edge crack only in weld metal designedaccording to ELCC joint shape parameters were also tested. All the tensilesamples were fractured at the base metal zone which is near to the weld toe, andtheir breaking stress can reach the tensile strength of base metal. This can satisfythe static ELCC condition. All the fatigue samples were fractured at the cracks inweld metal zone, but their fatigue remaining life were much higher than those ofthe normal joint with same crack.The dangerous section of joint can be transferred from weld zone to basemetal zone after ELCC joint design. ELCC joint with defect would fracture at thebase metal zone and its load carrying capacity can reach that of the base metal.The stress intensity factor calculating system for defect-existent joint can be usedto efficiently calculate the stress intensity factor. The ELCC joint design systemcan easily provide the corresponding ELCC joint shape parameters.