On the application of actively and passively excited guided elastic waves for the monitoring of fiber-reinforced plastics

Nowadays fiber-reinforced plastics are found in numerous industrial applications, such as aerospace, automotive engineering, railway and naval engineering. These materials have high tensile and flexural strengths and nevertheless a low density at the same time. The use of fiber-reinforced plastics is particularly relevant in areas where large masses have to be moved and accelerated. However, testing and monitoring these structures is still a challenge caused by the different damage behavior compared to metal structures. Non-visible structural changes, such as delaminations and fiber-fractures, may cause local degradation and finally the failure of the components. In this work, active and passive ultrasonic methods based on guided elastic waves are investigated for their applicability to carbon fiber-reinforced structures. Therefore, tensile tests with cyclically increasing load are carried out on specimens with different fiber orientations until complete failure. The acoustic emissions in the specimen during the load are recorded. As a second technique, actively excited guided waves are transmitted and received during the rest periods between the measuring ramps. Different parameters are extracted from the measured data, which allow the monitoring of the specimen’s degradation. Finally, a comparison of the results of the active and passive method is carried out addressing issues like the informative value and the sensitivity.