Lamb Wave Techniques for Damage Detection in CFRP-Components - is this really possible?

Abstract

Damage detection based on Structural Health Monitoring (SHM) with Lamb Waves (LWs) enables in principle a testing of components on demand without time consuming scanning. Their excitation and reception is easily possible with piezoelectric transducers (PZTs). LWs can penetrate large areas with a small attenuation and interact with defects. The localization of damage requires a network of PZTs on the component. However, in spite of a narrowband excitation in minimum two modes are generated (S0 and A0). Both modes are dispersive and propagate with different velocities. The interaction between both LW modes and defects is very complex and the received signals are difficult to evaluate. Especially composite components additionally show anisotropic velocities. The DLR part of the European AISHA II project is the detection of impact damages in the sandwich composite structure of an Eurocopter EC135 Tailboom. In order to get reliable information it is useful to visualize the wave propagation and the interaction with defects. At DLR the air-coupled ultrasonic technique has been successfully enhanced for this visualisation. The investigations of long-range wave propagation show mode conversions at all stiffness discontinuities. This paper describes damage detection based on mode conversions which enable clearer signals than techniques with bonded PZTs for excitation and receiving. This new method requires only a few actuators for the S0 mode and a network of air-coupled sensors which are sensitive for the A0 mode.