Quality Evalution And Fatigue Life Analysis Of 6xxx Series Aluminum Alloy Welded Joint

With the widespread use of aluminum alloy welded joints in the marine manufacturing industry,fatigue performance prediction of welded joints is particularly important.At present,the classification societies mainly u se the traditional fatigue assessment methods such as the nominal stress method/hot spot stress method to design and calibrate the fatigue performance of welded hull components,but the life prediction process is tedious and the data dispersion is high,especially for welded joints with special structural forms,which cannot be directly borrowed and can only approximate or simplify the joint form to analyze.This paper is based on the structural stress method of nodal force calculation,combined with finite element simulation and fatigue testing of the fatigue resistance of welded joints and welding quality,in order to propose fatigue life prediction to assist in the specific design of ship structures.For two different forms of extruded aluminum alloy hollow section stir friction welding structure,on the basis of retaining the structural characteristics,with the application of ABAQUS simulation software to design the fatigue loading fixture for extruded sections,simulate the effect of different bolt conn ection positions,bolt connection methods,bolt preload force and friction force on the structural stress at the calculated position,to achieve the investigation of the fatigue performance of the weak position(weld)of the member,and at the same time to achieve Quantitative characterization.Fatigue test and fracture microscopy are used to analyze the fatigue resistance of the profile and to evaluate the welding quality.For the aluminum alloy fusion welded cruciform commonly used in load-bearing components,fatigue performance analysis of aluminum alloy cross-joints with two load-bearing modes(non-load-bearing cruciform /load-bearing cruciform)was carried out separately.With the usage of finite element analysis,the influence of the weld size of fillet weld,fusion depth and other factors on the load bearing cross joint failure mode conversion is carried out and supplemented by experimental verification,the results show that when the fusion depth is 0,the load bearing cross joint weld foot size is greater than 1.02 T or the relative weld foot size is 1 and the fusion depth is greater than 0.13 mm,Mode A failure mode can be achieved,i.e.,the best fatigue resistance design is met,the failure path crosses the base material.Further study on the effect of common galling defects and joint deviation on the fatigue life of cross joints revealed that 0.2～0.3mm galling can greatly reduce the fatigue life of the joint;while the presence of joint deviation can generate additional bending moment at the location of the load bearing weld,which in turn reduces the fatigue life.With the usage of the equivalent structural stress method,the fatigue data of lap section welded structures/joints,cruciform of 8mm/16 mm plate thickness are plotted in the main S-N curve,and the results show that the main S-N curve can be directly used for the fatigue design of welded structures/joints of aluminum alloy sections and aluminum alloy fusion welded cross joints.In comparison with the nominal stress method,the master S-N curve can unify the fatigue data for different plate thicknesses,different joint forms and different welding methods,which will certainly greatly reduce the cost and cycle time of fatigue resistance design of marine structures.