An Impact Event Detection System for Composite Box Structures

A major element of the certification of composite aircraft structures is the evaluation of their tolerance for impact damage. The concept of Barely Visible Impact Damage (BVID) is used to set a damage size threshold below which the structure shall be able to carry Ultimate Load. This is similar to the detectability threshold for fatigue cracks in metal structures. Appearance of BVID, however, cannot be directly associated with the applied load spectrum and the presence of stress concentrations, contrary to fatigue damage. The goal of the research is to replace inspection for BVID by detection of impact events. A system based on acoustic emission was developed to detect and localize impact events on a thermoplastic composite structure (an aircraft elevator). A piezoelectric sensor was attached to the spar of the elevator. A software module based on an artificial neural network was designed to identify the location and energy level of impact events on the elevator skins. The module was able to determine the zonal source locations of impacts using only a single sensor.