Characterization of Fiber Volume Fraction Gradients in Composite Laminates

Experimental methods were assessed for accurately characterizing through-the-thickness fiber volume fraction gradients in fiber-reinforced composite laminates. Two optical microscopy-based image analysis methods were considered for this purpose: the areal method and the fiber counting method. Although the areal method currently appears to be the more commonly used method, the fiber counting method was determined to be the preferred optical microscopy-based method when an accurate measure of fiber cross-sectional area is available. Based on this determination, the fiber counting method was used to perform ply-by-ply fiber volume fraction determinations on four sets of autoclave-cured carbon/ epoxy unidirectional and symmetric cross-ply laminates. Although all four laminates displayed out-of-plane curvature following curing, no statistically significant through-the-thickness fiber volume fraction gradient was found in any of the laminates.