Laser Induced Lamb Wave Generation for Structural Health Monitoring of Carbon Fiber Reinforced Polymers

Abstract

In this paper we present an innovative concept for the excitation of guided acoustic waves (lamb waves) in carbon fiber reinforced polymers (CFRP). The idea is to add this external signal generation to a passive structural health monitoring system (SHM), using the now active system for nondestructive testing (NDT). The whole system consists of piezoelectric sensors, embedded in the polymer matrix of the monitored component, the external laser in combination with a scanning device for spatial resolved generation of acoustic waves and a signal processing unit for data analysis. Using laser excitation for lamb wave generation helps to overcome several dis-advantages compared to the use of piezoelectric transducers only: The flexibility in repositioning of the excitation area allows for easy compensation of the strong signal attenuation of CFRP with a minimum number of piezoelectric transducers. The variation of laser wavelength in the range of 1024 to 3500 nm in combination with variation in intensity allows for a selective coupling of the acoustic waves either into the matrix or in the C fibers. Using piezoelectric transducers for detection only, omits the need for a large number of high-voltage amplifiers for signal generation. In this contribution we present first results of a systematic investigation of the effective generation of lamb waves in CFRP. In addition to the variation of the wavelength of the laser, the intensity was varied too. A potentially damaging influence of the laser radiation on the CFRP material was investigated.