The Design Of Undermatched Fillet Weld Joints Based On Equal Load Carrying Capacity

Developing high strength steel（HSS）is an effective strategy to reduce the weight of the structure.Equalmatched and overmatched HSS joints suffer from cold cracks.Thus the use of undermatched HSS joints is increasing.However,the strength of undermatched weld is insufficient,and current design methods do not recommend increasing the weld size,the only way to ensure safety is increasing the thickness of the base metal,which goes against the original intention of lightweight and severely restricts the application of HSS in welded structures.The equal load carrying capacity（ELCC）design of undermatched joints,can prevent the weld from failing ahead of the base metal and increase the LCC of the joints to that of the base metal,through enlarging the cross-sectional of the weld reasonably and optimizing the shape of the toe.At present,ELCC studies focu on simple situations such as butt joints and tensile loads,and there is a lack of researches on the complex situations such as fillet weld joints,composite loads,variable amplitude fatigue load,The main applications of high strength steel are large structures that precisely contain a large number of fillet welds and are subjected to very complex natural loads.Fillet weld joints are complex in terms of geometry,stress distribution and fracture mechanism,and the ELCC design of all jo ints is a difficult system engineering.Thus the realization condition and the approach of ELCC with universal applicability are urgently needed.The common characteristics of current ELCC studies are summarized,and then the generalized model of ELCC,ie,the realization conditions and the shape design method of ELCC joint with universal applicability,are put forward.Then,according to the characteristics of fillet weld joints,ELCC researches on fillet weld joint under different fracture mechanisms are carried out systematically.The concepts of generalized matching ratio()and generalized stress concentration factor（6）_N)are defined.The ELCC realization condition is k_N≤μ_N,which equals to that the safety factor of the weld is not less than that of the base metal and is consistent with the traditional design method.That means the proposed ELCC realization condition is widely applicable.The specific form ofand k_Nare related to the failure mode of the joints.By characterizing the LCCs of the base metal and the weld,and substituting them to the equal bearing condition,the critical curve of the weld can be obtained.In complex cases,one specific ELCC realization condition is determined for each failure form and each dangerous position,then the shape of the weld is the envelope of all critical curves.This is the general approach of ELCC design.For fillet weld joints such as lap joints,the strength calculation is carried out according to the shear stress on the calculated section.The specific ELCC realization condition is that the ratio of shear stress of the undermatched weld to that of the equalmatched weld is not greater than their shear yield strength matching ratio.The test results show that the LCCs of the designed undermatched lap joint equal to that of the equalmatched joint,which meets the requirements of ELCC.For fillet weld joints such as full penetration T joints,the fracture mechanism under static load is related to external load and the normal stress cannot be ignored.The specific ELCC realization condition is that the ratio of the average（shear）stress of the weld to that of the base metal is not greater than their（shear）yield strength matching ratio.Based on the ideal elastic-plastic hypothesis,the LCCs of the base metal and the weld are characterized,the ELCC designed T joints under simple load and composite load are determined.The test results show that the static tensile ELCC joints braek at the base metal with a LCC close to that of the base metal and a good ductility,the LCC of the static bending ELCC joints is not lower than that of the base metal,which meet the requirements of ELCC.For fillet weld joints such as T joints with a crack,brittle fracture may occur under static load,the ELCC realization condition based on conventional mechanics and that based on fracture mechanics should be met simultaneously.According to the actual situation and the stress distribution under different loads,T joint with a central crack under static tensile in the root is chosen as an example.The specific ELCC realization condition is that the ratio of the stress intensity factor at the crack tip to the stress of the base metal is not greater the ratio of the critical stress intensity factor of the weld metal to the tensile strength of the base metal,and the theoretical stress concentration factor of the weld root is not greater than the tensile strength ratio.A Saint Venant coefficient is defined to quantitatively describ e the effect of load distribution on the near field stress and to modify the elastic mechanical results of a wedge under tensile at the tip,then the maximum principal stress of the T-joint root was obtained.Combining the mechanical equilibrium condition between the crack surface and the crack tip,then the stress intensity factor at the crack tip is obtained.Then the ELCC designed T joints with a crack are determined.The test result shows that the designed T joints braek at the base metal,which meet the requirements of ELCC.For T joints under fatigue load,the ELCC designed T joints are determined based on nominal stress method,local stress（strain）method,fracture mechanics method and total life method respectively.For the ELCC design under variable amplitude fatigue,the specific ELCC realization conditions based on different method are all in form of k_N≤μ_N.The fatigue test results show that the amplitude bending fatigue strength of ELCC joints with no crack and designed based on nominal stress method is about 98%that of the base metal,the ELCC joints with a crack fracture at the base metal under constant and variable amplitude tensile fatigue loads,which meet the requirements of ELCC.The results show that the proposed design method for fillet weld joint is feasible,and the generalized model of ELCC is reasonable,which can promote the application of HSS in large structures.