| Περίληψη: | Corrosion damage assessment of aircraft aluminum alloys is nowadays mostly made by accounting for the metallographic features of corrosion damage, such as pit shape, density and depth, the onset of exfoliation, etc. Corrosion damage equivalence, therefore, is considered as these features present the same characteristics when induced by different corrosive environments. For the case of static loading, corrosion damage is usually accounted through reducing the metal thickness by the depth of corrosion attack and then calculating the corresponding stress increase. For the case of fatigue, corrosion pits are considered as possible onsets for fatigue cracks. Despite of the evidence provided on the impact of other effects in 2024-T3 aluminum alloy’s mechanical properties, such as hydrogen embrittlement, the approach remains to some extent controversial.
The aim of the present PhD thesis is to contribute to establish a link between the metallographic features of corrosion damage caused by environments of different aggressiveness and the degradation of the mechanical properties of a corroded material, suggesting the need to expand current corrosion damage interpretation, to account not only for geometrical metallographic features but also for mechanical properties of the affected material. Towards this objective, a methodology is developed which allows the numerical simulation of the tensile behavior of the corroded material based on the combined effects of the metallographic features of the corrosion damage and the presence of hydrogen embrittlement. The present work is divided in two parts: a) the experimental investigation and b) the numerical analysis.
The experimental investigation includes an extensive metallographic investigation of the occurring corrosion damage in low-aggressiveness corrosive solution. Moreover, tensile tests were performed on the pre-corroded material which was exposed to the low-aggressiveness corrosive solution for several exposure periods. The results are compared to the ones obtained in a previous work utilizing a high-aggressiveness solution, aiming to trace a correlation between the damages caused by both environments. The results allow the formulation of correlation functions between corrosion damage’s geometrical metallographic features from low- to high-aggressiveness environment exposures. On the other hand, the degradation of tensile properties, and particularly of tensile ductility, is more pronounced in specimens exposed to higher corrosion rate environments, even when damage in both environments leads to equivalent metallographic features. This suggests significant differences in the underlying physical mechanisms of the damage accumulation process when the same material is exposed to different corrosive solutions. A major responsible for such differences is the occurrence of hydrogen embrittlement in high-aggressiveness corrosive solutions. Further evidence of this effect in 2024-T3 aluminum alloy induced by different aggressiveness corrosion environments is presented by means of nanohardness and hydrogen desorption tests. Nanohardness tests were carried out aiming to quantify the depth and local mechanical properties of the affected zone, while hydrogen desorption tests had the objective to verify the correlation between damages of environments of different aggressiveness beyond their metallographic features. The results show that nanohardness tests offer a suitable base to the analysis of the local mechanical properties and that the underlying physics of the corrosion process impact greatly on the degradation of mechanical properties. This implies that the exploitation of current accelerated corrosion tests for assessing the corrosion susceptibility of an alloy at the expected in-service conditions can be considered to be reliable as long as the underlying corrosion damage physical processes activated during the accelerated corrosion test remain the same with the respective mechanisms activated in-service.
The simulation procedure involves the development of an updated multi-scale modeling approach for simulating the tensile behavior of the corroded aluminum alloy 2024 T3, accounting for both the geometrical features of corrosion damage and the effect of corrosion-induced hydrogen embrittlement. The approach combines two FE models: a model of a three-dimensional Representative Unit Cell (RUC) representing an exfoliated area and its correspondent Hydrogen Embrittled Zone (HEZ), and a model of the tensile specimen. The models lie at the micro- and macro-scales, respectively. The characteristics of the HEZ are determined from measurements of nanohardness conducted on pre-corroded specimens. Using the model of the RUC, the local homogenized mechanical behavior of the corroded material is simulated. Then, the behavior of the exfoliated areas is assigned into different areas (elements) of the tensile specimen and final analyses are performed to simulate the tensile behavior of the corroded material. For the validation of the approach, tensile tests have been used. The numerical results show that this approach is suitable for accurately simulating the tensile behavior of pre-corroded experimental specimens, accounting for both geometrical features of corrosion damage and corrosion-induced hydrogen embrittlement.
The developed methodology represents a step towards the establishment of a link between the metallographic features of the corrosion damage and the residual mechanical properties of the material, accounting for a more holistic interpretation of corrosion damage. This leads to a more reliable estimation of the residual strength of the corroded aircraft structures and the possibility of more accurate prediction of corrosion-induced properties’ degradation.
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