The uncertainty inherent in most aircraft structural maintenance methodologiesnecessitates inspection intervals, which are established based on either assumed levelsof damage or the results of fatigue tests. Maintenance approaches that rely on onlyscheduled inspections have an intrinsic amount of uncertainty and risk becauseintervals do not reflect the loading and damage history of each specific aircraft. Thisrisk is more pronounced in composite aircraft, because damage is often not visuallyapparent. Human factors in labor-intensive inspections make this damage easy to misswhen performing tedious inspections of the entire aircraft [2].This work presents two case studies of structural health monitoring (SHM)methods that are designed to reduce the risk in aircraft maintenance, as well as the costof frequent, lengthy inspections. The first is an impact identification system which iscapable of locating impacts to a full-scale fuselage using only three sensors. Thisimpact identification method is able to quantify the severity of impacts as well,allowing maintenance personnel to focus inspections on areas that have sustainedfrequent and/or high amplitude impacts. Using this method, over 97% of impacts to aheavy lift helicopter fuselage are located within 9 inches of the true impact location.The second case study details the development of a non-contact wide area inspectionmethod which has the potential to reduce inspection times and uncertainty ascompared to labor-intensive inspection methods such as coin tap testing. Thisinspection method exploits the nonlinear forced vibration characteristics of damagedareas 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 inspectionmethod is shown to be effective in locating subsurface damage in composite helicopterpanels, and has the potential to reduce both labor costs and damage detectionuncertainty.
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