Lamb Wave Propagation in Sandwich Composite Structures for the Detection of Impact Damage

Abstract

Lightweight composite sandwich structures are particularly susceptible to impact damage. Barely Visible Impact Damage (BVID) is known to affect the short and long term load bearing capacities of composite structures. Active acoustic Structural Health Monitoring (SHM) systems have the potential to detect such damages before they lead to structural failure, and could thus be applied in a condition based maintenance approach for the DragonFly fuselage. This study was aimed at increasing the level of understanding on the propagation of ultrasonic (Lamb) waves through sandwich composite media and its response to the DragonFly fuselage structural integrity. These two aspects were combined to arrive at a theoretical prediction on the influence of BVID on the through-transmitted signal in an active acoustic SHM system with a pitch-catch setup. The propagation mode and impact response were verified experimentally. A Finite Element (FE) model was created to verify the predictive power of the FE modelling technique for the specific application of active acoustic SHM. Two Lamb wave propagation modes were identified both theoretically and experimentally: the Global Lamb Wave mode and the Leaky Lamb Wave mode. Occurrence of these modes was dependent on the central frequency of the transmitted acoustic signal and the bulk wave velocities in the sandwich structure’s core compared to the Lamb wave mode propagation velocity in the structure’s skins. Other modes (True modes and Rayleigh modes) were identified theoretically, but could not be experimentally confirmed. It was expected and shown experimentally that the presence of impact damage on the propagation path of a signal with 120kHz central frequency reduced the time travelled by this signal between two points, while increasing the amount of energy lost. The latter effect was shown to be reversed at 160kHz, which was explained by the propagation of the signal at this frequency as Leaky Lamb Wave. The FE model could predict the overall trend at 120kHz, but not the exact magnitude of the influence of impact damage on the through-transmitted signal.