Analysis of early damage in a woven carbon fiber reinforced composite by means of Coda Wave Interferometry (CWI) and terahertz imaging.
Résumé
This work is focused on the investigation of damage induced by bending tests at various loading levels on a carbon woven fiber reinforced composite. During the loading micro-cracks will first appear in the matrix and grow increasingly important, large cracks on the surface then occur just before the sample’s failure. The evaluation of the early micro-cracks is often difficult because of its diffuse aspect and its size comparable to that of the fibers. The sensitivity of several nondestructive evaluation (NDE) methods was investigated for this specific case. First, coda-wave interferometry (CWI) was considered because of its known higher sensitivity to defects than the earlier wave packets typically analysis. This technique indeed consists in the examination of the late arrival part of the propagated signal. The results were found to be in agreement with the evolution of the macroscopic damage measured from the elastic bending modulus reduction. Then, the damage sample was investigated using terahertz imaging; a signal deconvolution processing was applied to the measured signal. It was able to increase the capability of subsurface damage detection in the composite. Those results were positively compared with X-ray tomography analysis. Micro-cracks, as well as the delamination linked to the final large cracks, were successfully observed using terahertz and X-ray imagings and are shown to be imaged with higher resolution than ultrasonic C-scan.