Study of Composite Impact Dent Visual Detectability and Damage Relaxation Phenomena

The studies focused on visual detectability of surface damages in aircraft composite skins were performed. The goals were to understand how inspection conditions and service factors affect the detectability during standard field control procedures, to establish reasonable barely visible impact damage threshold and to optimize maintenance program in the frameworks of Irkut MC-21 aircraft certification. The probability of surface damage detection as a function of damage size was experimentally evaluated on empennage-type stringer panels in relation to qualification of experts, paint color, viewing distance and surface contamination. The significance of those factors was estimated by non-parametric methods of mathematical statistics, the bootstrap technique was applied for empirical data reduction. Based on the criteria, accepted in industry, the thresholds of detectability were established for general and detailed visual inspection procedures. The special attention was paid to the investigation of impact dent relaxation phenomena, i.e. to the reduction of damage size in time. The relaxation tests were conducted under normal and under hot-wet conditions and it was determined that combination of moisture and elevated temperature leads to the maximum damage size reduction. The performed studies resulted in recommendations on actual external damage size values which should be used as input data in damage tolerance analysis and as detectability criteria in aircraft maintenance manual.