Gas Permeability Measurements: Effect of Measurement Technique

Abstract

There is a desire in aerospace industry to replace traditional autoclave processing methods with lower cost out-of-autoclave (OOA) methods such as OOA processing of prepregs. However, high porosity is a challenge in this process. One of the main means of void reduction in OOA processing of prepregs is reducing the gas pressure inside voids by gas transport. It is therefore important to understand the gas permeability in a laminate throughout the process. A key problem is that when the resin is soft, sending gas through this soft porous structure alters the microstructure and thereby the gas permeability. This study investigates the effect of the measurement technique on microstructure and permeability of the test sample both in-plane and through thickness. Permeability measurements in continuous and interrupted modes are performed and the void morphology of the test samples is characterized using optical microscopy. It is shown that the porous structure and the permeability are affected by permeability measurements when taken continuously during the low viscosity region as it creates an interconnected network of inter-laminar voids. This effect can be reduced by discrete measurements of permeability, which is more representative of the situation during actual processing conditions