Characterization of interlayer air permeability of thermoplastic prepreg stacks

Abstract

An important mechanism for void air removal during oven vacuum bag processing for thermoplastic composites relies on void air flow to the part perimeters through the permeable pathways created by the rough surfaces of two adjacent prepreg layers. In this paper, the interlayer air permeability of AS4/APC2 carbon poly (ether ether ketone) thermoplastic prepreg was investigated before and after oven vacuum bag processing conditions. The permeability thickness product was measured with an experimental set-up and data reduced based on one-dimensional Darcy’s flow. The interlayer permeability exhibit directional dependency and are uniquely determined by the angle between two adjacent prepreg layers. Before oven vacuum bag processing, the principal permeability thickness products for lay-ups with included angle of 0°, 30°, 60°, and 90° were determined. Good agreement was achieved between the rotation matrix predicted permeability thickness products and the measured results. During processing, when temperature is above resin glass transition and below melting, the permeability of 0°/0° and 90°/90° interlayers reduces dramatically with increasing temperature and dwell time. For the 0°/90° interlayer, only a slight reduction compared to the room temperature baseline was obtained for temperature ramps up to 300℃ and dwell time up to 8 h at 240℃. The changes of the surfaces roughness were correlated to the reduction of measured permeability. The 0°/90° interlayer surface shows the evidence of fiber–fiber contact that limits the contact between layers and prevents significant drop of permeability during processing. Off-axis stacking sequences with fiber–fiber contact are advantageous for oven vacuum bag processing of large thermoplastic composite parts.