Nowadays, smart composite materials embed miniaturized sensors for structural health monitoring (SHM) in order to mitigate the risk of failure due to an overload or to unwanted inhomogeneity resulting from the fabrication process. Optical fiber sensors, and more particularly fiber Bragg grating (FBG) sensors, outperform traditional sensor technologies, as they are lightweight, small in size and offer convenient multiplexing capabilities with remote operation. They have thus been extensively associated to composite materials to study their behavior for further SHM purposes. This paper reviews the main challenges arising from the use of FBGs in composite materials. The focus will be made on issues related to temperature-strain discrimination, demodulation of the amplitude spectrum during and after the curing process as well as connection between the embedded optical fibers and the surroundings. The main strategies developed in each of these three topics will be summarized and compared, demonstrating the large progress that has been made in this field in the past few years.

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.

Carbon Fiber Composites (CFCs) recycling has received increasing interest by the composites industry, but it is still in its early stages as an industry. There are two primary challenges that need to be addressed in order to achieve full property retention during CFC recycling: (1) the ability to recover the fiber content without property loss; and (2) conversion of the recycled, short fiber material back into high-performance CFC structures. The ability to manufacture high fiber volume fraction CFCs with end-of-life products would provide an opportunity to reduce material cost and lifetime-embodied energy. In this paper, recycled, short carbon fibers are processed via solvolysis and converted into high-performance CFCs with fiber volume fraction of ~50% and excellent composite property retention. This is enabled through alignment of the discontinuous, recycled fiber feedstock using the Tailorable universal Feedstock for Forming (TuFF) process. The paper introduces the necessary steps to process the fibers in the wet-laid process and explores the resulting mechanical and microstructural properties. The importance of incoming fiber surface quality and the effect of surface contamination from residue left by the recycling process on both the TuFF process and final composite properties is discussed in detail. A pyrolytic process has been adopted to remove the residue that is a by-product of the recycling process from the incoming recycled fiber surface. The approach presents a promising pathway for the recycling of high-performance CFCs.

Recycling of carbon fibers enables a sustainable feedstock for industrial applications of high-performance composite materials. This allows light weighting with recycled carbon fibers due to their superior mechanical properties while reducing the high embodied energy and cost of virgin carbon fiber composites. This study optimizes a pyrolysis cycle for fiber recovery of an aerospace-grade thermoset prepreg and a cleaning (oxidation) step to minimize fiber degradation and left-over resin residue, enabling dispersion and alignment of the recycled, discontinuous fibers in the Tailorable Universal Feedstock for Forming alignment process. The study balances the influence of the optimized thermal cycle (pyrolysis + oxidation step) on recycled carbon fiber strength retention with the ability to disperse at the filament level to create aligned, recycled carbon fiber composite samples with high fiber volume fraction. The optimized thermal cycle for efficient fiber recovery applied a pyrolysis step at 500 °C for 4 h in an inert gas environment and an additional oxidation step at the same temperature for 100 min. This resulted in ~20% strength degradation of the fiber compared to the virgin fiber. The processed recycled composite achieved 44% fiber volume fraction with full modulus translation (~128 GPa) compared to the virgin continuous composite with strength translation (~870 MPa), reaching ~50%.