Application of Broadband Laser-Ultrasonic Spectroscopy for Nondestructive Testing of the Porosity in Carbon Fiber Reinforced Plastics with Various Volume Contents of Carbon Fibers

The method of broadband laser-ultrasonic spectroscopy based on laser thermooptical excitation of broadband acoustic pulses is used to obtain the frequency dependences of the phase velocity of longitudinal acoustic waves in composite materials. The frequency dependences of the phase velocity in the 0.8-10 MHz spectral range are studied for samples of the unidirectional carbon fiber reinforced plastics with various volume contents of the matrix and fiber. The occurrence of the phase velocity dispersion in this range is observed. The phase velocity dispersion of longitudinal acoustic waves in a sample is calculated using the phase spectra of the probe ultrasonic pulse and the pulse passed through the sample under study. It is shown that the relative dispersion of the phase velocity characterizes the porosity of the sample, and the higher the porosity, the larger the relative dispersion. The empirical relations obtained for the relationship between the porosity and relative dispersion can be used for rapid nondestructive evaluation of the porosity of composites. The obtained results can be useful for both quality control of a material to improve its manufacturing technologies and to predict the behavior of items and products made of this material under the action of external loadings.