Damage Mechanics Study Of Environment Corrosion Combining With Stress And Prediction Of Structure Property

The research work in this thesis is based on the theory of damage mechanics and focused on the damage of environmental corrosion and stress coupled corrosion to metallic materials of airframe, which has a great theoretical significance and practical engineering application not only for finding the degradation law of structural mechanical behavior during service, but also for predicting calendar life of aircraft structure. Pits, stress corrosion and corrosive fatigue are three typical damage forms of aircraft structures in service environment and these are studied and analyzed by means of damage mechanics in this thesis. On the base of in-depth study of the environmental damage mechanism, analytical models of damage mechanics are presented to predict quantitatively the mechanical behavior degradation brought by environment or environmental-stress combination. With help of the data of the accelerated corrosion test of a typical aluminum alloy, the reasonableness and predictability of these models are validated. The major research work and innovations are as follows:1. The thermo-dynamical principle of corrosive damage and the thermo-dynamical relationship containing electro-chemical energy are studied and presented. In terms of the damage dissipation potential function, the explicit energy dissipation relationship of mechanical and environmental combination is described. Such a research work completes the theoretical framework of damage mechanics taking account of the environmental corrosion and stress coupled corrosion.2. Acid salt spray experiment to accelerate the corrosion of aluminum alloy samples of 7075-T3 is conducted with and without the sample uni-loadings. Also mechanical properties degenerated with corrosion time are tested. With application of the test data, the accelerating and coupled effect of the corrosive environment and load on the degradation of the aluminum alloy property is studied. As results, the phenomenological quantitative relations of the effects are obtained.3. In view stand of meso-damage mechanics, corrosive pits are taken account of continuum damage. The evolution law of such a meso-damage is researched in accordance with above mentioned experiment and test data, and a new void growth rule of the damage under the Gurson's model is presented in this thesis. In terms of this model, the elastic moduli, yield stresses and ultimate strengths of the material degenerated with time are calculated numerically. A good agreement of the numerical results with test data is shown. 4. The critical criterion of corrosive pits transforming into crack is studied and proposed as a primary step in the prediction of the evolution of stress corrosion damage for this aluminum alloy. In this context, two initial life predicting models for stress corrosion are established and their reasonableness are validated with obtained test data. As far concerned with the growth of stress corrosion crack, two view-points and relevant models of anodic dissolution and fracture mechanics are analyzed and compared. In consequence of the researches, the view stand that different models for different damage growth stage should be applied is set forth.5. A corrosion fatigue life predicting model based on the in-depth study of mechanical damage mechanics is also suggested, which is more coincidence with non-linear accumulative law of corrosion fatigue damage. Also this thesis has discussed the crack growth model of corrosion fatigue, and its applicability in engineering practice is analyzed. Furthermore, some engineering methods and advances are studied for expressing crack growth rate of corrosion fatigue, and some study cases are exemplified to estimate the reasonableness and applicability of these methods for crack growth rate.The total calendar life prediction for airframe structure has been explored in terms of different damage phases, and so chemical short crack in the damage growth exposed on corrosive environment has been discussed. Finally, in accordance with practical damage process of airframe exposed to complicated and alternant loading and corrosion environment, the development of application for the research work has been pointed out.