Abolikhina O. Material-science aspects of formation and evolution of damages which define the resource exploitation of aluminum structures of airplanes.

This thesis is dedicated to the study of evolution of flaws in aluminium alloys of AlZn-Mg-Cu, Al-Cu-Mg-Мn alloying systems and to the determination of their connection with structural factors of the material such as the size and composition of intermetallic phases conditioned by heat treatment tempers, and also to the study of the effect of big number of physical factors on the long-term behaviour of structural elements of aluminium alloys, and to the determination of the rate of formation of corrosion damages in structural elements of the aircraft wing. The work is aimed at securing the long-term safe operation of airplanes. The novelty of the thesis consists in the following. In fact, the thesis is the beginning of a new direction in materials science - operational materials science of deformable aluminum alloys. Structural factors which play a determining role in the development of flaws in high-strength aluminium alloys of Al-Zn-Mg-Cu alloying system were set for the first time. There was noted an important negative role of inclusions of intermetallic phases containing copper at the grain boundaries that does not correspond to the statements of developers of the alloy. There was proposed a summarizing chart of evolution of flaws in aluminium alloys, which included a latent period of formation of corrosion damage, a stage of corrosion cracking and its transition to a stage of fatigue failure. The effect of structural and external factors on the duration of each stage was analyzed for high-strength aluminium alloys used in the aircraft structure. There were for the first time obtained the results of statistical processing of data concerning the damage of wing panels of over 2000 airplanes which were used for optimization of scheduled maintenance intervals, that from one side would allow to ensure an accident-free operation of structural elements, and from the other side to minimize capital expenditures for their maintenance. Using the methods of mathematical statistics, there were for the first time analyzed the factors which have the biggest effect on the development of flaws during operation of airplanes. It was shown, that the time between adjacent inspections and the climatic zone with the content of sulfur dioxide or chlorides in the atmosphere, where the airplane was based, had the most substantial effect on corrosion processes. For the first time the original equations of regression have been obtained using the methods of mathematical statistics, that relate the depth of corrosion damage to the terms between adjacent technical inspections and the intensity of flights depending on climatic conditions. The equations of regression which determined the corrosion damage growth rate allowed to increase the duration of interconnected technical inspections of the caissons of the center wing planes 6 times, to ensure continuity of flights and to obtain a significant economic effect. Basing on the obtained results of the analysis, technical documentation was developed for ultrasonic inspection of the thickness of lower panels of the wing centre section of An-24 and An-26 airplanes and there were issued the recommendations for timely detection of damages of different structural elements. This will make it possible to operate these airplanes without disassembly of panels during not less than 10 years after preliminary inspection and despite the climatic zone where the airplane is based. The obtained results were used to predict the corrosion damage growth rate, to evaluate the reduction of strength of the ageing aircraft structure, and to develop the repair works when determining their service life and time limits. The results of corrosion growth rate analysis for different structural elements of the wing were used to adjust the inspection intervals and to determine the optimum time for renovation of anticorrosion protection . Basing on the obtained values of the maximum corrosion damage per year, there were determined the inspection start points for new aircraft types and their inspection intervals in service considering the appropriate safety margins. General scheme of acquisition and analysis of corrosion damage data based on service experience of the available aircraft fleet allows solving both the problems of its continued airworthiness, and the problems of reliable anticorrosion protection of newly designed aircraft based on available data. Research of the microstructure of industrial aluminium alloys in terms of susceptibility to corrosion cracking is taken into account when evaluating the life of a separate element and that of the entire structure, and it is also used for successful selection of materials with required set of features for existing and future aircraft structures.