【作者】 金成；

【导师】 何世禹；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2008， 博士

【摘要】 航天器的壳体、舱体通常采用铝合金焊接结构。由于空间环境的复杂性加之焊接接头本身所固有的组织不均匀性、几何不连续性,以及焊接残余应力的存在,使得接头部位往往成为焊接结构中最薄弱的部位。实践经验告诉我们,焊接结构的破坏大多都是从焊接接头中起源的。因此,对处于特定空间环境下承载焊接接头的损伤失效行为研究具有十分重要的意义。在地面常规条件下的有关焊接结构失效行为的研究方法和理论已经趋向成熟,但在空间特殊环境中并非完全适用。航天器焊接结构部件与地面结构相比,不但所处的环境不同,而且其破坏与失效的机理也有其特殊性。作者以航天器壳体用铝合金焊接接头为研究对象,从材料细观损伤入手,分析了近地空间热循环条件下承载焊接接头的特殊的损伤与失效机理;采用对比实验研究与解析计算相结合的方法,研究了热循环条件下接头材料细观损伤规律;并在此基础上,用跨尺度的研究方法揭示了细观损伤演化与宏观本构响应之间的定量关系。空间热循环是造成航天器故障的重要环境因素之一,长期的热循环作用会造成材料宏观性能的劣化。模拟空间热循环试验结果表明,长时间的热循环会导致承载焊接接头内部第二相粒子与基体材料脱开,进而形成微孔洞,微孔洞在应力场的作用下不断演化是造成材料宏观性能劣化的重要因素。作者从焊接冶金学角度对接头中第二相粒子的成因进行了分析,分析表明,TIG焊接热过程会在接头区域、尤其是在焊接热影响区中生成较多第二相粒子。温度的上下波动导致第二相粒子与基体之间产生热错配应力,热错配应力与外加载荷的联合作用会造成材料局部的塑性流。在热循环过程中塑性流的不断累积最终造成第二相粒子与基体脱粘,形成微孔洞。微孔洞的形成与演化会导致接头承载面积的减小,最终对接头宏观性能产生影响。孔洞型细观损伤是韧性材料在受载破坏和长期蠕变条件下的一种重要的损伤现象。迄今,已有几种理论模型可以描述微孔洞在材料内部的形成以及演化规律,但在热循环条件下孔洞损伤规律尚未见报道。作者通过研究细观尺度上的代表性体积单元在热循环作用下的行为响应,得出了热循环下细观尺度上的热应力、应变的解析解。以细观孔洞形核规律为切入点,对Gurson孔洞形核方程进行有效修正,引入热错配应力的影响,着力建立一套热循环辅助诱发孔洞形核机制。借助于Gurson孔洞型细观损伤理论,将细观层次上的“损伤”与宏观本构响应联系在一起,为热循环条件下损伤的定量计算奠定了理论基础。模型参数的选择与确定是影响模型计算精确度的一个重要因素。为将建立的损伤模型应用到热循环条件下焊接接头的损伤计算,还需特别考虑一些因焊接接头本身所固有的特性带来的初始条件和参数测定问题,包括焊接残余应力场、区域机械性能以及区域细观损伤参数的测定。作者采用实验测定与数值计算相结合的方法对构件中焊接残余应力场进行了测定与计算,并分析了其在受载过程的变化及对构件形变的影响。结果表明,即使在外载小于构件屈服极限的情况下,焊接残余应力的存在也会导致焊缝及近缝区产生塑性变形,造成附加损伤。采用物理模拟与数值模拟相结合的方法,对焊接接头各区域力学性能以及细观损伤参数分别进行了测定与计算,为损伤模型的应用提供了前提条件。采用中点积分算法,编写用户子程序对建立的材料本构模型“数值化”,子程序可与有限元软件完美结合,并对焊接接头试样和航天器壳体焊接结构在空间热循环条件下的细观损伤演化及宏观响应进行模拟计算。综上,本论文提供了有关空间热循环条件下承载铝合金焊接接头细观损伤与失效机制的新的实验发现,建立了热循环辅助孔洞形核机制,对热循环条件下材料体的宏观性能演化及尺寸不稳定效应进行了理论解析与定量计算。研究成果有助于丰富和发展孔洞型细观损伤理论,亦对航天器设计有所裨益。更多还原

【Abstract】 Aluminum alloy welded structures have been widely used to construct the shell module of space vehicles. In some conditions, the welded joints often become the weakest part of the whole welded construction due to their inhomogeneties microstructure, discontinuous geometry as well as the welding residual stress. The experience tell us that the failure of welded structures always initiates from the welded joints. So, it is very important to study the damage and failure behaviors of the joints under special space conditions.The theory and methodology are driving to a maturity state concerning the evaluation of performance and failure behaviors in the welded structures under normal conditions. However, it does not always hold true in the aerospace conditions. The differences not only lie in the service conditions but also the particularities in the failure mechanisms. Taking the aluminum alloy welded joints as the object of the study, the author investigated the special damage and failure mechanisms in welded joint under load and thermal cycling conditions. Based on that, the quantitative relations between meso-damage revolution and constitutive behaviors were also investigated by the trans-scale approach.Thermal cycling is one of the important factor which causes the failure of the space vehicles. The materials performance may degrade after longtime thermal cycling. The results of the simulated thermal cycling tests show that the longtime thermal cycling may cause debonding between the second particle and the matrix of the welded joint, therefore nucleating void. The void evolutes under the stresses field and it is the key factor which causes the degradation of macro performance. The formation of the particles in the welded joint was analyzed from the welding metallurgy point of view. Results show that the TIG welding process may introduce many particles in the joint area especially in the HAZ. The thermal mismatch stress may be caused between the particles and the matrix under temperature variation conditions. The local plastic flow may be formed under the combining thermal mismatch stress and external loads. The accumulated plastic deformation causes the interface debonding between the particles and matrix. The nucleation and evolution of the voids decrease the load-carrying ability of the joints.The void-damage is a typical damage phenomenon for the failure of ductile metals and longtime creep. So far, several models have been established to describe the evolution rules of the voids in materials, but the rules under thermal cycling conditions has not been documented. Based on the analysis of constitutive response of a respective volume element (RVE) under combining thermal cycling and external loads, the author deduce the in-closed form results of the local stress and strain. By studying the void nucleation rule, the Gurson model was modified to introduce the effect of thermal cycling and tried to describe the thermal cycling assisted void nucleation mechanism. With the help of Gurson’s void-damage theory, the relationship between the damage in meso-scale and the response of macro behaviors was bridged. That is the theoretical foundation for the quantitative calculation of the damage under thermal cycling conditions.The determination of the model parameters is an important factor which affects the simulation accuracy. In order to apply the damage model established for calculation the damage in welded joints, additional issues should be considered. The issues that correlating closely to the welded joint itself include the determination of the welding residual stress field, local mechanical constants and the damage parameters.The determination of the welding residual stress was carried out by both experimental and numerical calculation methods. Its evolution under service and its effects on the deformation were also investigated. Results show that the plastic deformation and additional damage may occur in the joint area even the external loading no higher than its yield limit.The local mechanical constants and damage parameters of the welded joint were determined using physical and numerical simulation methods. It provides the preconditions for the applications of the damage model. The constitutive model established was implemented into the finite element code by means of mid-integration algorithm through users’subroutines. The subroutines can be used to simulate the meso-damage evolution and the macro constitutive behaviors of the welded joint specimens and the welded structures of the space vehicle. The simulation results fit well with the experimental data. In summary, the new experimental findings were presented regarding the meso-damage and failure mechanisms of aluminum alloy welded joint under combined thermal cycling and external loads conditions. The thermal cycling assisted void nucleating model was established. Theoretical calculation and numerical simulation of meso-damage evolution, macro performance degradation and its dimensional instability were carried out successfully. The achievements of this work are helpful for developing and expanding the theory of meso-damage and also of great benefit to the design of space vehicles.更多还原