The Significance of In-Service Factors for the Visual Detectability of Impact Damage in Composite Airframe

Abstract

The inspection conditions may significantly affect the visual detectability of impact damage in aircraft composite skins. The goal of the present study was to identify the most affecting factors related to standard field control procedures and thus establish reasonable barely visible impact damage threshold in the frameworks of Irkut MC-21 aircraft certification. The probability of surface damage detection as a function of damage size was evaluated in relation to qualification of experts, paint color, viewing distance and surface contamination. The experiments with representative panels were carried out, the empirical detectability data was analyzed by methods of mathematical statistics. Using non-parametric tests the statistical significance of mentioned in-service factors was estimated. The bootstrap method was applied to make empirical data sufficiently representative. The minimum reliably detectable sizes of dents caused by 25 mm indenter for considered inspection conditions were identified. For detailed inspection in case of the clean surface the size of 0.3 mm can be accepted as a relevant criterion, in case of contamination this value can grow up to three times. For general inspection the values of 1 mm for clean and 1.7 mm for contaminated surface can be accepted conservatively. The obtained results were shown to be consistent with the literature and were recommended for use on the stages of design and maintenance.