Broadband Laser-Ultrasonic Spectroscopy for Quantitative Characterization of Porosity Effect on Acoustic Attenuation and Phase Velocity in CFRP Laminates

This work aims at applying the method of broadband laser-ultrasonic spectroscopy for quantitative evaluation of the effect of isolated dispersed voids and additional extended interply delaminations on the acoustic attenuation and on the phase velocity in CFRP laminates. This method is based on the laser thermoelastic generation of broadband reference pulses of longitudinal ultrasonic waves in the specially designed source of ultrasound. The high-sensitivity piezoelectric transducer is used to detect these pulses propagating normal to the fiber plies in composite specimens. The laminate specimens investigated have different total porosity levels up to 10.5 % determined by the X-ray computer tomography. The resonance peak of the attenuation coefficient and the corresponding jump of the phase velocity are observed governed by the periodic layered structure of the specimens. The absolute maximum and the frequency bandwidth of the resonance attenuation peak depend on the total porosity level formed by the predominant type of imperfections, either of isolated spheroidal voids entrapped in epoxy layers or of extended interply delaminations. With an increase of the specimen’s total porosity dispersion of the phase velocity becomes noticeable in the low-frequency band before the resonance jump. The derived empirical relations between the total porosity level and the parameters of the frequency dependencies of the ultrasonic attenuation coefficient and of the phase velocity can be used for rapid quantitative characterization of the structure of CFRP laminates subject to different fabrication conditions.