Comparison of methods for the characterization of voids in glass fiber composites

Voids are a concern in composite materials, as they may have a negative effect on the mechanical properties of the laminates. Voids may develop especially in low cost or off-optimum process conditions. In this study, samples of glass reinforced epoxy laminates with void volume fractions in the 0.5-7% range were successfully obtained by varying the vacuum in the hand layup vacuum bagging manufacturing process. Void content was experimentally characterized using four different methods: ultrasonic scanning, epoxy burn off, serial sectioning, and X-ray computed tomography. The goal of this paper was to determine how the methods compared with respect to each other at quantifying void content. The specimens were taken from nearby locations in the same panels, so a true comparison of the methods could be obtained. The results showed, for the specific material and manufacturing conditions used, that the four different techniques can quantify voids content but with a large variation in the accuracy. X-ray computed tomography was the most successful technique to characterize voids, followed by serial sectioning. Ultrasonic scanning and epoxy burn off were not recommended techniques to characterize voids for laminates manufactured with these materials and process conditions. However, epoxy burn off was a successful technique to calculate fiber and resin weight fraction.