Damage Mechanisms And Numerical Model Of Aluminum Alloy Welded Joint Under Load And Space Thermal Cycling Conditions

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.