Structural health monitoring-based methodologies for managing uncertainty in aircraft structural life assessment

Aircraft maintenance approaches that rely on only scheduled inspections have an intrinsic amount of uncertainty and risk because intervals do not reflect the loading and damage history of individual aircraft. This risk is more pronounced in composite aircraft, because damage is often not visually apparent. This work presents two case studies of complementary structural health monitoring methods that are designed to reduce the risk in aircraft maintenance, as well as the cost of frequent, lengthy inspections. The first is an impact identification system which is capable of locating impacts to a full-scale fuselage using only three sensors. This impact identification method is able to quantify the severity of impacts, allowing maintenance personnel to focus inspections on areas that have sustained frequent and/or high-amplitude impacts. Using this method, over 97% of impacts to a heavy-lift helicopter fuselage are located within 9 in of the true impact location. The second case study details the development of a noncontact wide-area inspection method, which has the potential to reduce inspection times and uncertainty as compared to labor-intensive inspection methods such as coin tap testing. This inspection method exploits the nonlinear forced vibration characteristics of damaged areas through surface velocity measurements acquired by a scanning laser vibrometer. By comparing the structure's response to forcing functions of differing magnitudes, the local nonlinear characteristics of damage are identified. This automated inspection method is shown to be effective in locating subsurface damage in composite helicopter panels and has the potential to reduce both labor costs and damage detection uncertainty.