Microstructure Evolution in Forming Aligned Discontinuous Fiber Composites

• Erin Mcknight, Chemical Engineering, University of Delaware

• Thomas Cender, Center of Composite Materials, University of Delaware

This research studies the effect that stretch forming has on the Aligned Discontinuous Fiber composite microstructure. Aligned discontinuous fiber composites (ADFC’s) are comprised of a fiber preform, that is impregnated and can be consolidated to produce composite blanks. The advantage they hold over continuous fiber composites is their increased ability to stretch form biaxially into more complex shapes. The blanks are constructed from ADF and a thermoplastic matrix to form a [0/90]2s laminate and formed using a bladder molding process to create a sample formed to 60% plane strain. When the mold has cooled, samples are cut from the center gauge section and potted in clear resin. After the samples are sanded and polished, microscopic pictures are taken for microstructure analysis. Based on the equation of constant volume we can take the actual thickness (tE) compared to the expected thickness (tE) and make a prediction of the local porosity (ɸ). The Results show a clear distinction between the degree of porosity in the 90 and 0 degree plies. Plies which were stretch 90 degrees relative to the fiber direction retained 30% porosity after consolidation, whereas the 0 degree plies were able to consolidate to 5% porosity. This findings indicate that microstructural change is much larger when stretching the ADF plies in the 90 degree direction. This is potentially due to a loss of fiber alignment. Future work will investigate the effect of consolidation pressure which may result in higher squeeze flow dynamics to close residual porosity.