Mechanical Strength Degradation of Graphite Fiber Reinforced Thermoset Composites Due to Porosity

Abstract

<p>The mechanical strength of composite laminates is sensitive to the presence of porosity. Porosity in laminates is generally considered to be a random distribution of voids incurred during the manufacture process. Larger, interlaminar voids typically result from trapped air or moisture; smaller, intralaminar voids may occur between fibers due to improper wetting or the release of volatile gases during the cure cycle. Porosity has its greatest effects on matrix-dominated mechanical properties such as compressive strength, transverse tensile strength and interlaminar shear strength (ILSS). Judd and Wright [1] have surveyed the existing data and made an appraisal of the effects of voids on the mechanical properties of composites. In a study of porosity in filament wound/CVD carbon-carbon composite [2], the transverse tensile strength was found to decrease exponentially with increasing porosity and followed an empirical equation often attributed to Ryshkewitch [3] and Duckworth [4]: σ = σmaxeBP (1) where σmax is the strength at zero porosity, P is the volume fraction of porosity, and B (a negative number) is an empirical constant that depends on pore size, shape, and orientation. More recently Yoshida et. al.</p>