Investigation On Strength Properties Of Aluminium Alloy Weld Joint In Spacecraft Tank

Aluminum alloy welded structure is an important structure form of aerospace tank, withadvantages in light-weight, adaptability of cryogenic/corrosion environment, high efficiency andflexibility in manufacturing process. Due to the particularity of welding process, the welded jointstrength affected by the coupling effect of welded specific factors such as uneven local properties,residual stress and stress concentration caused by weld reinforcement, which makes it difficult toanalyze the strength problem of welded structure.In allusion to these problems, the effects of welded specific factors on welded joint strengthunder static load, pre-corrosion and fatigue load, as well as the methods of strength evaluation, arestudied in this thesis following the local mechanical properties in the welded joint. The main contentsin this thesis are shown as follow:(1) Experimental research of welded joint’s local mechanical properties. The locations of thelocal mechanical properties’ key transitions in the joint are identified by observing and measuring thechanges in the microstructure in the welded area. By measuring the stress and strain of these keypoints during a local tensile test, the accurate but discrete data are acquired. The changing rules oflocal mechanical properties are summarized. The results show the plastic properties of the joint willundergo a substantial, continuous and non-linear change in the local area near the weld. The elasticmodulus is almost the same in the same area, but the elastic ranges are different. The welded jointstrength and fracture position are affected by the coupling effect of uneven local mechanicalproperties and weld reinforcement(2) Numerical analysis method research of welded joint strength. The correlation and differencebetween random positions in the welded joint local area are summarized. The mathematical model ofwelded joint mechanical properties is built. A finite element analysis modeling approach, which givesconsideration to both accuracy and efficiency, is put forward. The results of finite element analysis areverified by tests. The results show any position in the local area of welded joint has the same slope ofthe stress-plastic strain double logarithmic curve. The vertical coordinate of the curve and coordinateaxis’ intersection point is in a certain proportion to yield strength. The mathematical model of weldedjoint mechanical properties can not only provide material property data for finite element analysis, butalso can give a estimation method of local mechanical property, which only needs a few simple tests.The coupling effect of local mechanical properties and other factors is the key of joint’s strength. The computational accuracy and efficiency can be balanced by thinning mesh and material property of thecoupling effect position and simplified discretizing the other areas.(3) Research the relationships between release strain and residual stress of welded joint. Theeffect of welded joint uneven local mechanical property on residual stress computation is analyzedand summarized. The relationship between release strain and residual stress is calculated by numericalanalysis method, and the effect of local mechanical property is considered in the calculation. Theresults show the differences of mechanical property between welded joint’s local area and base metallead to the different relationships between release strain and stress, and finally cause the error of theresidual stress results, the maximum residual stress in particular. The maximum residual stress inaluminum alloys welded joint is normally less than the yield strength of base metal but most likelymeets or excess the actual yield strength of the exact position.(4) Pre-corrosion strength property research of tank’s welded joint. To the special condition ofwelded tank which has to bear high load after long-term storage of corrosive solutions, corrosion testsof welded joint specimens and base metal specimens are conducted to obtain the corrosion propertydata of the welded joints and base metal. The differences of corrosion properties between the weldedjoint and base metal are summarized. Based on the data of corrosion tests and welded joint localmechanical tests, a finite element method is used to predict the pre-corrosion strength, the results ofwhich are verified by pre-corrosion tensile tests. The stress of key positions in the joint which ischanging by time is analyzed by numerical method. The results show the major characteristic of awelded joint’s corrosion resistance is the existence of obvious differences of corrosive resistanceamong different local areas. The corrosion resistance is an important factor for the pre-corrosionstrength of welded joints, and its influence increases with time. The fracture position is transferredwith time by the coupling effect of corrosion and local material property.(5) Fatigue and fracture properties research of tank’s welded joint. To the special condition ofwelded tank which has to bear alternating load, the fatigue property parameters of fracture areestimated based on the local mechanical properties data of welded joint’s fracture position. Thefatigue life is predicted and verified by fatigue tests. The effect of welding and thickness to weldedjoint critical stress intensity factor is analyzed. A method for estimation of the critical stress intensityfactor based on critical crack size is put forward, which estimate the critical stress intensity factor bythe size and type of the crack at the fatigue fracture location. The results show the local mechanicalproperty of the fatigue fracture is the key factor of prediction. The prediction results based on localmechanical property can meet the engineering calculation requires. The estimation method that is applied to measure the factor based on the fracture crack size excludes influences of welding andthickness effects in a convenient way of measurement and calculation. The method can be adopted forwelded structures subjected to vibration loads for fatigue failure analysis and reference of fracturetoughness in engineering practice.