Nondestructive Porosity Assessment of CFRP Composites with Spectral Analysis of Backscattered Laser-Induced Ultrasonic Pulses

The laser-ultrasonic method for nondestructive quantitative local porosity assessment for CFRP composites is proposed and realized experimentally for only one available flat surface of a specimen or a product. This method combines the laser thermoelastic generation and the high-sensitivity piezoelectric detection of broadband pulses of longitudinal ultrasonic waves and does not require the detection of the backwall echo ultrasonic signal. The generation and the detection of ultrasonic pulses is carried out with the specially designed laser-ultrasonic transducer, which allows one to obtain both the temporal profile and the frequency spectrum of a part of the ultrasonic signal backscattered by gas voids in a composite specimen. The frequency spectrum of backscattered ultrasonic pulses is analyzed for three sets of CFRP specimens with different epoxy matrix fractions and porosity. The empirical relation between porosity of CFRP specimens and the spectral power (structural noise power) of ultrasonic signals backscattered by voids is obtained for porosity values up to 0.15. This relation allows one to evaluate the local porosity from measured structural noise power both for CFRP specimens and products fabricated from the same composite material. The proposed laser-ultrasonic setup demonstrates a basis for a system of CFRP porosity assessment in field conditions. It can be very useful especially for nondestructive detection of structural changes of composite materials that will allow evaluation of products during their life time.