Monitoring of the moisture content in carbon-epoxy plates using Lamb waves

Non-destructive material health monitoring is of great importance for aerospace industries, in particular for tanks made of composite materials, the mechanical properties of which can be altered by hydrothermal environment. In this context, the sensitivity of Lamb waves to various levels of moisture in a carbon-epoxy plate has been investigated. The plate has been manufactured and submitted to different cycles of hydrothermal aging and drying. For various steps of these cycles, both changes in weight and ultrasonic data are measured. Low-order A0, S0, S1 Lamb modes are generated and detected using air-coupled transducers, and signal processing allows both real and imaginary parts of the wave-numbers to be measured. The real parts of the wave-numbers of all the generated modes are shown to be not sensitive to the moisture level of the material. However the imaginary part of the wave-number, i.e. the attenuation, of the A0 mode is very sensitive to the moisture content, and its changes follow those of the plate weight, thus making it a promising mode for NDT of composite materials used in humid environments.