EXPERIMENTAL-ANALYSIS OF POROSITY-INDUCED ULTRASONIC-ATTENUATION AND VELOCITY CHANGE IN CARBON COMPOSITES

in this paper, ultrasonic measurement results are presented for analysing the wave propagation in carbon fibre reinforced plastics (CFRP) containing voids. The composite samples studied include laminates fabricated from unidirectional and woven fabric prepregs. The ultrasonic methods stress the utilization of spectral analysis and frequency dependence of the attenuation and phase velocity due to porosity. Morphological parameters of pores are found to play an important role in the proper interpretation of measured data. The measured attenuation shows approximately linear behaviour over the frequency range used. The corresponding phase velocity decreases substantially with increasing void content. In addition, the velocity in porous CFRP is found to be more dispersive than that in void-free composites, The relationship between the ultrasonic attenuation and dispersion is subsequently tested using the local form of the Kramers-Kronig relation. The linear relationship between the void content and the attenuation slope (d alpha/df) is found to hold, but the constant of proportionality is quite different for samples with different pore morphology. This morphology has a large effect on the dispersion of phase velocity. The fractional velocity decrease (Delta V/V-0) is correlated with the void content, and its dependence on the frequency and void shape is discussed. The present analysis will aid in the model development of wave propagation in porous composites for detecting and characterizing the porosity in these materials.