【作者】 张俊苗；

【导师】 聂宏；

【作者基本信息】 南京航空航天大学 ， 机械设计及理论， 2013， 博士

【摘要】 由于铝合金焊接结构在减轻结构重量、适应低温/腐蚀环境和提高结构设计制造的灵活性及效率等方面具备显著优势，因而成为航天运载工具中贮箱的重要结构形式。但由于焊接制造过程的特殊性，焊接接头强度受到局部力学性能不均匀、残余应力和焊缝余高引起的应力集中等焊接特有因素的复杂耦合影响，给焊接结构强度问题的分析计算带来很大困难。针对以上问题，本文以焊接接头局部力学性能特性为主线，分析研究了静态加载、预腐蚀和疲劳加载条件下焊接因素对接头强度的影响以及相应的强度评估方法。本文的主要研究内容包括以下五个方面。（1）焊接接头局部力学性能试验测量方法研究。根据微观组织形貌，确定局部力学性能在接头中变化的关键点；在局部拉伸试验中对关键点应力-应变数据进行了测量，获取了局部力学性能准确的离散性数据；归纳了焊接接头局部力学性能的离散特性。结果表明，焊接接头塑性力学性能在近焊缝的局部区域内发生着连续、非线性且大幅的变化，而该区域内弹性模量则较为一致，但弹性段范围各不相同；局部力学性能的不均匀性与焊缝余高之间的耦合作用对接头强度和断裂位置存在重要影响。（2）焊接接头强度数值分析方法研究。归纳总结了焊接接头局部区域内任意点之间在力学性能上的相关性和差异性；构建了描述该区域内任意点力学性能的数学模型；提出了兼顾计算精度和效率的有限元建模方法；对焊接接头拉伸试样进行了有限元分析和试验验证。结果表明，局部区域内任意点应力-塑性应变双对数曲线具有相同的斜率，曲线与坐标轴交点的纵坐标和屈服强度呈比例关系；所提出的数学模型，不仅提供了有限元分析所需数据，而且给出了一种仅需少量简单试验数据的局部力学性能估算方法；局部力学性能与其它因素的耦合作用是决定接头强度的关键因素，通过对耦合作用位置的细化和对其它区域的简化，可兼顾分析精度和效率。（3）焊接接头释放应变-残余应力关系研究。分析总结了局部力学性能不均匀影响残余应力计算结果的原因；通过数值分析方法，计算了考虑局部力学性能影响后的释放应变-残余应力数值关系；分析总结了局部力学性能对结果的影响。结果表明，接头局部区域力学性能与母材的差异，使得区域内释放应变-应力关系与母材存在区别，导致残余应力计算结果产生误差，其对残余拉应力极大值的影响最大；铝合金焊接接头中残余拉应力极值一般小于母材的屈服强度，但很可能已达到甚至超过所处位置材料的实际屈服强度。（4）贮箱焊接接头预腐蚀强度性能研究。针对长期贮存腐蚀性溶液的贮箱中的焊接接头在腐蚀后承受高强度拉伸载荷的特殊工况，通过铝合金焊接接头和母材试样的腐蚀试验，比较分析了焊接接头与母材之间在腐蚀性能上的差异；提出了结合局部力学试验、腐蚀试验和有限元方法的焊接接头预腐蚀强度预测方法；通过试验进行了验证；对接头中关键位置断裂应力随腐蚀时间的变化进行了数值分析。结果表明，焊接接头不同区域的腐蚀性能存在显著差异；腐蚀性能是影响接头预腐蚀强度的重要因素，随着腐蚀时间的增加腐蚀的影响逐渐增大；在腐蚀和局部力学性能的共同作用下，预腐蚀断裂的位置随着时间而发生变化。（5）贮箱焊接接头疲劳与断裂性能研究。针对重复使用的贮箱中的焊接接头承受交变载荷的特殊工况，根据疲劳危险部位的局部力学性能数据估算了该处疲劳性能参数；预测了疲劳寿命并通过疲劳试验进行了验证；分析了焊接和厚度效应对焊接薄板临界应力强度因子的影响；提出了一种通过断口处临界裂纹类型和尺寸来估算断口处临界应力强度因子的方法。结果表明，疲劳断裂部位的局部力学性能是影响估算结果的关键因素，采用断裂部位的局部拉伸性能数据估算得到的疲劳性能参数能够满足工程计算的要求；利用断口裂纹尺寸对临界应力强度因子的估算，考虑了焊接和厚度效应耦合影响且结果不受试验参数影响，可作为断口处断裂韧性的参考值，与焊接接头断裂韧性分区测量结果互为补充，应用于焊接结构的疲劳失效分析。更多还原

【Abstract】 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.更多还原