Nondestructive Testing of CFRP Plates by Lamb Waves

Abstract

In order to improve the reliability and safety of composite materials, it is necessary to have non-destractive testing (NDT) available to detect defects in materials. Although conventional ultrasonic methods have, in principle, been used for NDT, they are inefficient and difficult to inspect large structures, such as ship hulls, because of the high attenuation of ultrasonic waves. In these cases, low frequency waves have an advantage over ultrasonic waves. Therefore, by using low frequency Lamb waves generated by tapping as a means of NDT in composite laminates, the following examinations were made; (1) predicting the thickness of plates, (2) detecting the delamination, (3) predicting the fiber contents in the composite laminates, and (4) measuring the wave velocities depending on propagating directions.
(1) and (2) were carried out by using antisymmetric A₀ mode waves. The results showed that it was possible to predict the thickness and to detect the delamination based on the relationship between the phase velocities of Lamb waves, which has dispersion characteristics, and the product of frequency and plate thickness. (3) was examined by the resonant method of symmetric S₀ mode waves, and (4) was done by S₀ waves. The results showed that the fiber content could be predicted from the stationary waves and the wave velocity propagating at an angle to the fiber direction could be calculated from the Young's modulus.