Quantitative Crack Monitoring Using Diffracted Lamb Waves

Abstract

Due to cyclic loading and longtime exposure under extreme environmental conditions, fatigue cracks are often generated in aircraft structures. When the crack grows to a critical size, the structure will not be able to bear the loads, and the structure may rupture suddenly, thus the crack will cause great damages to the aircraft. To evaluate the severity of the crack, one has to know not only the location, but also the size of the crack. Therefore size monitoring of fatigue cracks in aircraft structures is very important to manufacturers as well as maintenance personnel. Some nondestructive evaluation (NDE) methods can be used for this purpose, such as X-ray, Cscan and magnetic particle testing, however, these techniques is limited to ground testing. Many methods have been developed for crack size monitoring and quantification by sensors mounted on the structure in the past few years. However, most of these methods demand dense sensors or sensor arrays around the crack. So crack size monitoring is still a very challenging problem. In this study, Fastest Time of Flight Diffraction (FTOFD) method was developed to monitor the location and size of the crack. FTOFD method uses a sparse sensor network to activate and receive Lamb waves in a structure. Diffraction waves will be generated at the crack tips when Lamb waves pass the crack. Diffraction waves are then received and analyzed by signal processing methods to get the position of the crack tips. Once two crack tips of the crack are positioned, both the location and size of the crack can be given very conveniently. An aluminum plate was used to verify the FTOFD method. A simulated crack was made on the plate by a wire-electrode cutting machine. Experimental results showed that the monitored size of the simulated crack is very close to the real size of the crack.