Real-Time Evaluation of Impact Induced Delamination in UAV Composite Structures Using Fiber Optic Sensors

This study focuses on detection and evaluation of low-velocity impact induced delamination in the thick CFRP laminates used in main structures of UAVs. When the low-velocity impact occurs on the structure during operation or maintenance, acoustic emissions are generated, which are important clues for detecting and evaluating damages caused by the impact. In particular, the real-time detection of delamination using on board sensors is essential for safe operations of structures because the delamination is invisible and hard to be detected by conventional nondestructive methods. In this study, fiber Bragg grating (FBG) sensors were used for measuring the acoustic emission signals generated from the impact. Test articles are CFRP laminates with the thickness of 5 mm, and the FBG sensor was surface attached on the bottom surface of each specimen. The low-velocity impact was given to each specimen with different energies by controlling the drop height. The acoustic emission signal in each case of different impact energy was captured by the FBG sensor in the sampling frequency of 100 kHz. This sampling speed is fast for FBG sensors, but is generally considered to be a limited speed for damage evaluations. By C-scan inspections after low-velocity impact experiments, delaminations were found in the impacted specimens over the impact energy of 15 J. The measured signals have a form in which the main waveform generated by the structural deformation and the acoustic emission signals are mixed. The signal processing method is proposed for detecting and evaluating the delamination using such measured signals from FBG sensors. This method consists of two steps: delamination detection and delamination area prediction. Strong acoustic emission signals from the delamination occurrence affect the waveform of the measured signal, so the delamination could be detected by analyzing the symmetry of the main waveform of the measured signal. Through this first process, signals that are determined to have been damaged are selected. As the next step, the delamination areas were estimated using the induced peaks of acoustic emission signals during generating the delamination in the laminates. In this study, the delamination areas were measured by a linear regression using the counted numbers of peaks related to delamination. As a result, the delamination area could be reasonably evaluated using the method proposed in this study. Through this study, a useful method for a real-time delamination diagnosis with a simple optical fiber sensing system was proposed, which is considered to be a practical impact damage monitoring system applicable to real composite structures.