Research On Residual Life And Residual Strength Of Metals With Corrosion Damage

Metallic material such as aluminum, steel, is sensitive to corrosive environment. Corrosiondamage and fatigue damage together seriously affect the integrity and security of the metal structure.How to calculate residual life and residual strength of metal structure contained the corrosion damageby surface morphology of metal structure without large number of tests become ankey problem.In this thesis, metal material pre-corrosion fatigue damage was concerned, from both local andglobal perspective, the impact on the residual life of metal material by pre-corrosion damage wasin-depth studied. Based on the fracture mechanic and fractal dimension, theoretical models to predictthe pre-corrosion fatigue residual life which were only need the data of corrosion morphology wereestablished. The rationality and predictive ability of the models were analyzed and experimentalverified. The probability distribution of the pre-corrosion fatigue residual strength was also given byexperimental analysis. This model unified conflicting views of previous studies. The main work andinnovations in this paper are as follows:(1) Pre-corrosion fatigue residual life and residual strength tests (227specimens) were done. Inthe laboratory ambient conditions, pre-corrosion fatigue residual life and residual strength tests onaluminum alloy LC4CS and steel were completed. Provided observational data for the follow-upstudy and experimental verification of predicted results of residual life and residual strength.(2) Proposed new criteria for critical pits-opening angle. For existing CM-EIFS method cannotreasonably determine crack initiation pit, the definition of opening angle was proposed by analyzed alot of corrosion morphology observed by three-dimensional microscopy and topography details ofSEM image which influenced crack initiation. Through filtering crack initiation pits by opening angle,remaining life was calculated by CM-EIFS method. The predicted results showed that the openingangle definition was reasonable. Predicted results were accurate, so the CM-EIFS method wasimproved.(3) Proposed an equivalent area method to calculate the initial crack size and crack propagationmodel. The pit is equivalent to a semi-elliptical surface initial crack by projected area of the pit. Thismethod associated initial crack size with the most critical factors of crack propagation life-stressintensity factorand the morphology of initial crack was consistent with experimental results. Based onfiltering the critical pits by opening angle, the equivalent initial crack size could be gotten by equivalent area method. A new crack growth model was also given in this paper, and constituted anew, complete CM-EIFS calculation system. Compared with the experimental results showed that themethod was accurate enough.(4) Proposed a new method to estimate pre-corrosion fatigue residual life of smooth specimenbased on fractal dimension.Because fractal dimension can describe the complexity of overallmorphology and will not ignore any details, an innovative method based on fractal dimension todescribe corrosion morphology and the relationship between fractal dimension and residual life wasproposed. Proposed a new three-dimensional fractal dimension calculation method to predict residuallife. Predicted results with the experimental results were in good agreement and showed that thismethod could well predict the residual life in the only corrosion morphology and pristine S-N curveconditions, saved test time and expenses.(5) Pre-corrosion fatigue notch factor was defined and its empirical formula with pre-corrosiontime change was given. This formula combined with pre-corrosion residual life of smooth specimencould calculate residual life of notched specimen.(6) Proposed a new method to estimate pre-corrosion fatigue residual life of notched specimenbased on fractal dimension. Defined pre-corrosion stress concentration factor and discussed itsphysical meaning. Given the method to calculate pre-corrosion fatigue notch factor by fractaldimension. Predicted results with the experimental results were in good agreement and showed thatthis method could well predict pre-corrosion fatigue notch factor in the only corrosion morphologyand geometry size of notch conditions, and then calculated residual life of notch specimen.(7) Experimental study of pre-corrosion fatigue residual strength was completed, probabilitydistribution model of residual strength of metallic materials containing corrosion fatigue damage wasproposed. In a laboratory environment, residual strength test of the domestic LC4CS aluminum alloyand steel used more than30years in a real environment was implemented (49specimens). A bimodaldistribution model of residual strength was proposed and it provided a good explanation for theinconsistency of experiment results of residual strength in literatures and this thesis. This modelprovided reference for future residual strength check.