In-Service Health Monitoring of Composite Structures Using Lamb Waves

摘要

The main aim of the present work was to study the propagation of Lamb waves in composite plates and investigate their interaction with defects. Thin piezoceramic patches that operate in longitudinal mode (d_(13)) were selected as transmitters and receivers. The fundamental anti-symmetric A_0 Lamb mode was generated at low frequencies. Operation at low frequencies reduces the resolution of the inspection but less attenuation is present and cheaper equipment for data acquisition is needed. It has been demonstrated that the use of a linear array of transducers for the inspection of large composite plates can identify typical delamination defects. The use of a linear array of transmitters at one edge of composite plates was simulated using a Finite Element analysis. Delaminations were modelled by reducing the elastic properties of the damaged area. The position of delamination through the thickness of the laminate will cause different reductions in the elastic properties allowing through thickness location to be modelled as well. It was found that by increasing the number of transmitters, detectable changes in the wave propagation are produced. The location of damage in the longitudinal x-axis was successfully determined but the extent of damage across the width was not accurately predicted. It was observed that even though the aspect ratio of the length to the width of the transducer was high and the longitudinal mode was dominant, significant wave propagation was generated across the width of the plate producing unwanted signal between the excitation and the reflection of the back edge of the plate. The spacing of the transmitters is being investigated as this may resolve the problem.