Ultrasonic Measurements of Porosity in Woven Graphite Polyimide Composites

Abstract

A method for evaluating the porosity volume content in composite laminates using the slope of the ultrasonic attenuation with respect to frequency has been reported before in these proceedings1,2. The method was first applied to laminates laid up from unidirectional prepreg tapes and has now been extended to the case of woven composite laminates. It was found that a linear relationship still holds between the porosity volume fraction and the attenuation slope; however, the constant of proportionality is quite different for the case of woven laminates. This difference is attributed to the differences in pore morphology in the woven and nonwoven laminates. Qualitatively, the voids are flatter and longer (needle-like or strip-crack-like) in the interlaminar region of nonwoven laminates of unidirectional prepregs and are very effective in blocking the sound energy. On the other hand, voids tend to be localized in the resin-rich pockets of woven laminates and remain spherical, as shown in Fig. 1. The attenuation and the slope of the attenuation are therefore lower (by about a factor of 2) in the woven composite for the same amount of porosity content. Quantitatively, the “void content-attenuation slope” relationship is modeled for both woven and nonwoven laminates in terms of the scattering amplitude of P-waves from voids of different shapes.