Process development of pre-impregnated hybrid thin-ply composites

Thin-ply composites, obtained with recently developed fiber spreading techniques, rapidly gained industrial interest because they offer a large composite design freedom, and lead to a composite tensile strength that is close to that of the individual fibers, as well as improved fatigue properties. However, their wider adoption towards critical lightweight composite applications remains limited due to their inherent toughness reduction and high production cost. Fiber hybridization demonstrates a great potential for alleviating some of these drawbacks and promoting the manufacturing of laminates with balanced characteristics. Currently, most studies on thin ply hybrids employ simple interlayer configurations mainly due to difficulties in manufacturing more complex hybrid architec-tures, combining fibers that do not have similar diameters, stiffness, surface conditions and spreading charac-teristics. However, recent model predictions indicate that significant mechanical performance improvements should be attained with carefully designed intralayer (tow-by-tow) and intrayarn (fiber-by-fiber) hybrid ar-chitectures. This research, performed at North Thin Ply Technology Sarl (NTPT) and EPFL in the framework of the Eu-ropean project Hyfisyn, aims to evaluate the relative influence of key process parameters on the quality, in terms of microstructural organization as well as resulting composite properties, of thin-ply hybrid prepregs and composite laminates. To reach this objective, the processing variables for fiber spreading and impregna-tion were analyzed, to adapt and improve the current pilot and industrial prepreg line aiming for the cost effective, yet high quality production of thin-ply composites prepregs at the ply-by-ply, tow-by-tow, and fiber-by-fiber levels. A first analysis was carried out on thin-ply glass fiber prepregs, to evaluate the effect of mechanical pre-spreading and thermal treatment on the resulting single fiber and prepreg mechanical properties. The statisti-cal strength distribution of the fibers and resulting composite strength was indeed negatively affected by the treatments, although fiber and composite modulus remained unchanged, opening some possibilities to modi-fy the current process with limited strength reduction. Then, tow level hybrid prepregs were progressively optimized, first using glass and carbon fibers, then two types of carbon fibers with high and low strain to failure, until reaching high quality prepregs. The next step was to produce intimately commingled hybrid prepregs. However, the manufacturing challenges encoun-tered in the controlled mixing of dissimilar fibers for the production of hybrid thin-ply prepregs at an indus-trial scale made this process too unreliable. As an alternative, a novel calendering method, for fiber-level hybrid prepreg tape manufacturing was developed and showed promising results. Composites were produced with several combinations of tow by tow and fiber by fiber hybrid configura-tions. New tools were developed to analyze the microstructure and quantify the degree of hybridization of these carbon fiber hybrids, highlighting the high quality of the materials obtained. Unnotched tensile tests were carried out, showing that the composite stiffness and strength did not reveal any additional fiber dam-age due to the processes. The study contributed to development of thin ply hybrid composites, with hybridization degrees that reached unprecedented levels, and pointed the