A Review of Recent Developments in Composites Made of Recycled Carbon Fiber Textiles

Carbon fiber recycling has garnered significant attention in recent years due to the large volume of manufacturing waste and upcoming end-of-life products that will enter the waste stream as the current generation of aircraft is retired from service. Recycled carbon fibers have been shown to retain most of their virgin mechanical properties, but their length is generally reduced such that continuous fiber laminates cannot be remade. As such, these fibers are typically used in low-performance applications including injection molding, extrusion/compression molding, and 3D printing that further degrade the fiber length and resulting composite properties. However, recent advances in the processing of long discontinuous fiber textiles have led to medium- to high-performance composites using recycled carbon fibers. This review paper describes the recent advances in recycled carbon fiber textile processing that have made these improvements possible. The techniques used to manufacture high-value polymer composites reinforced with discontinuous recycled carbon fiber are described. The resulting mechanical and multifunctional properties are also discussed to illustrate the advantages of these new textile-based recycled fiber composites over the prior art.

EXPERIMENTAL AND NUMERICAL INVESTIGATIONS OF STOCHASTIC THICKNESS EFFECTS IN DISCONTINUOUS FIBER COMPOSITES

Discontinuous fiber composites (DFCs) made of chopped prepreg tapes have recently drawn the attention of the aerospace and automobile industries thanks to their flexible manufacturing capability. Because of the discontinuity in a form of prepreg tapes, the fibers easily follow the mold contours while maintaining comparable stiffness and strength to continuous quasi-isotropic laminates. Furthermore, the high production rates and part complexities enabled by DFCs make them a competitive alternative to the use of metals in several applications. However, one of the current roadblocks for the use of DFCs is the lack of reliable analysis methods to predict their mechanical behavior, which depends on different parameters such as platelet sizes, aspect ratios, and spatial distribution. In this paper, we first experimentally investigated tensile elastic modulus and strength of unnotched coupons made of two different platelet aspect ratios (square and narrow) for varying coupon thicknesses. From the experiments, the square platelets showed significant thickness effects on both elastic modulus and strength. The narrow platelets also had significant thickness effects on strength but relatively constant modulus with varying thicknesses. In both modulus and strength, the narrow platelets had higher average values with larger deviations. Next, we computationally examined the relationship between the platelet distributions and the corresponding thickness effects. To get a thorough understanding of the effects of the platelet distribution on the mechanical behavior, the analysis was performed in two steps. In the first step, computational models were generated utilizing a uniform platelet distribution. In the second step, the models were generated leveraging platelet orientation tensors obtained from X-ray micro-computed tomography characterization. It was found that the assumption of a uniform orientation distribution condition was sufficient to capture the average modulus and strength with varying thicknesses for both platelet sizes. However, the associated Coefficient of Variation (CoV) of the results were significantly underpredicted, especially in the case of narrow platelets. On the other hand, numerical results using the orientation tensor obtained via micro-CT provided significantly improved predictions of the CoVs with varying thicknesses. These numerical investigations suggest that, for parts manufactured in conditions of limited platelet flow, the average mechanical performance can be accurately predicted by stochastic Finite Element models featuring a uniform platelet orientation distribution. On the other hand, the prediction of the CoV of moduli and strengths urges the use of an accurate representation of the real platelet morphology.

Editorial for the Special Issue on Discontinuous Fiber Composites, Volume II

This Special Issue on discontinuous fiber composites and its published papers, like its predecessor, give the polymer engineer and scientist an insight into challenges and research topics in the field of discontinuous fiber-reinforced composites [...]