Optimal pultrusion process conditions for improving the dynamic properties of graphite-epoxy composite beams

The manufacturing process variables significantly influence the mechanical properties of pultruded composites. In this study, a statistical central composite design (CCD) test pattern has been used to manufacture unidirectional graphite-epoxy pultruded composite beams under carefully controlled process conditions. The influences of significant pultrusion process variables and their effects/interactions on the dynamic mechanical properties were investigated. The pultruded specimens were subjected to free vibration decay tests to determine nondestructively the dynamic flexural modulus and loss factor (a measure of internal damping). Mathematical models were derived based on the observed values of the dynamic properties using regression analysis procedures. These models were used to determine the optimal pultrusion process conditions for improving the dynamic mechanical properties of the finished product. A theoretical model postulating varying distribution of fiber content through the thickness of the pultruded composite is also presented. Static flexural tests and microscopic evaluation were employed to validate the assumption that a thin distinct layer of matrix material is formed on the outer surface of these pultruded products.