Detection of damage in metal lap joint by combined passive and active methods

Abstract

Fatigue cracks and corrosion damage are critical issues for aircraft, especially for the aging fleet still in use today. The most frequent area of damage is on the aircraft skin, more specifically the joint regions. A structural health monitoring system is being sought after as a system of sensors to detect and localize damage during flight, reducing the amount of time spent in ground inspections and amount of ground inspections overall. Currently, studies from companies and universities are being done, using a variety of different sensing methods, including acoustic emission (AE) testing, ultrasonic testing (UT), and by optical fiber with fiber Bragg gratings (FBG) as strain gages. AE and FBG sensors are passive systems by ‘listening’ to cracks growth or measuring stiffness change around the crack, respectively. These two methods can be combined to form an active network, checking the other methods in real-time by using guided waves of UT. This study looks at analyzing the abilities of AE and FBG sensors to work as both passive and active systems, comparing results to one another. Due to temperature problems in skewing responses of guided waves, a network of sensors is formed as well to use a correlation in baseline approach, negating this effect, and is tested for fatigue damage on a metal lap joint configuration over cyclic loading. FBG sensors are found to be more directional based and can work in replacing strain gages, while AE sensors can be used well in conjunction with active UT.