THRESHOLD VALUES FOR THE OPERATION OF AVIATION PANEL STRUCTURES WITH FATIGUE CRACKS

Abstract

The article considers the problem of determining the threshold values for the service life of aircraft panel structures with fatigue cracks in accordance with the damage-tolerance concept. The relevance of the study stems from the need to improve the safety and efficiency of aircraft operation by reasonably predicting the residual life of defective structures and optimizing technical inspection intervals. Existing regulatory approaches to determining the safe-life and periodicity of control are based mainly on the use of dispersion coefficients, which often leads either to an excessive reduction in the structural life or to an unjustified increase in maintenance costs. The aim of the work is to develop an approach to justifying safe inspection intervals for aircraft panel structures considering the random growth of fatigue cracks and determining the threshold values for residual life. To achieve this goal, methods of fracture mechanics and statistical modeling were used. The Paris law was applied to describe the growth rate of a fatigue crack. In this case, the random nature of the parameter m of this law was considered, the distribution of which for structural aluminum alloys was assumed to be logarithmically normal. The modeling was performed on a panel structure of an aircraft fuselage skin with a central crack under different levels of cyclic stress. For each scenario of the numerical experiment, random values of the parameter m were generated, after which the residual resource of the structure was determined until the critical crack size was reached. As a result of the studies conducted, it was established that the residual resource of panel structures with fatigue cracks is satisfactorily described by the Pareto distribution. The existence of a threshold value of operation was revealed, below which the destruction of the structure does not occur regardless of the random values of the crack growth parameter. The dependence of the residual resource threshold on the magnitude of the acting stresses was determined; in semi-logarithmic coordinates, it is linear, similar to classical fatigue curves. It is shown that the use of the threshold value of the residual resource as a criterion for assigning inspection intervals allows to increase safe intervals of technical control in comparison with traditional methods based on dispersion coefficients. The results obtained can be used to develop maintenance programs and assess the residual durability of fail-safe aircraft structures. The proposed approach contributes to increasing the operational efficiency of aircraft by optimizing inspection intervals without reducing flight safety