Modal testing to detect and locate damages in large composite structures

Large complex structures such as aircrafts, ships, locomotive train cabinets, and satellites after some time of operation are subjected to structural damages such as cracks, heavy corrosion and loose joints. These structural damages will change -to some extent-their dynamic behavior which sometimes may not be detected by well-known NDT techniques. In this research paper, Normal Mode Test procedure was implemented as a Ground Vibration Test (GVT) on a CESSNA 172N small aircraft of comparatively large and composite structure to detect structural damages. Since real structural damage was not available in the aircraft by the time of the test, it was decided to simulate these damages by fuel addition in the wing structure. Two tests were carried out on the aircraft, one with empty fuel tanks, and the other with full tanks. In each test, the aircraft was excited with two shakers placed at the wing tips. In order to get a good energy distribution within aircraft structure, four shakers were placed at wing tips and horizontal tail to excite Changes in frequency and amplitudes and phase were measured in each case. Analysis of the predicted modes show the severity of the simulated structural change and its location. This paper strongly recommends further studies of the NMT method applied to real structural damage of large and composite structure.