This review thoroughly investigates the mechanical recycling of carbon fiber-reinforced polymer composites (CFRPCs), a critical area for sustainable material management. With CFRPC widely used in high-performance areas like aerospace, transportation, and energy, developing effective recycling methods is essential for tackling environmental and economic issues. Mechanical recycling stands out for its low energy consumption and minimal environmental impact. This paper reviews current mechanical recycling techniques, highlighting their benefits in terms of energy efficiency and material recovery, but also points out their challenges, such as the degradation of mechanical properties due to fiber damage and difficulties in achieving strong interfacial adhesion in recycled composites. A novel part of this review is the use of finite element analysis (FEA) to predict the behavior of recycled CFRPCs, showing the potential of recycled fibers to preserve structural integrity and performance. This review also emphasizes the need for more research to develop standardized mechanical recycling protocols for CFRPCs that enhance material properties, optimize recycling processes, and assess environmental impacts thoroughly. By combining experimental and numerical studies, this review identifies knowledge gaps and suggests future research directions. It aims to advance the development of sustainable, efficient, and economically viable CFRPC recycling methods. The insights from this review could significantly benefit the circular economy by reducing waste and enabling the reuse of valuable carbon fibers in new composite materials.

The demand for fiber-reinforced polymers (FRPs) has significantly increased in various industries due to their attributes, including low weight, high strength, corrosion resistance, and cost-efficiency. Nevertheless, FRPs, such as glass and Kevlar fiber composites, exhibit anisotropic properties and relatively low interlaminar strength, rendering them susceptible to undetected damage. The integration of real-time damage detection processes can effectively mitigate this issue. This paper introduces a novel method for fabricating embedded capacitive sensors within FRPs using a coating technique. The study encompasses two types of fibers, namely glass and Kevlar fiber/epoxy composites. The physical vapor deposition (PVD) technique is employed to coat bundle fibers with conductive material, thus creating embedded electrodes. The results demonstrate the uniform distribution of nanoparticles of gold (Au) along the fibers using PVD, resulting in a favorable resistance of approximately ≈100 Ω. Two sensor configurations are explored: axial and lateral embedding of the coated yarn (electrodes) to investigate the influence of load direction on the coating yarn. Axial-sensor configuration specimens undergo tensile testing, showcasing a linear response to axial loads with average sensitivities of 1 for glass and 1.5 for Kevlar fiber/epoxy composites. Additionally, onset damage is detected in both types of fiber composites, occurring before final fracture, with average stress at the turning point measuring 208 MPa for glass and 144 MPa for Kevlar. The lateral-sensor configuration for glass fiber-reinforced polymer (GFRP) exhibits good linearity towards strain until failure, with average gauge factors of 0.25 and −2.44 in the x and y axes, respectively.

Combining two types of reinforcement fiber in a common matrix may lead to different failure modes such as micro-cracks between the layers when the structure is subjected to lower stress levels. Real-time damage detection should be integrated into the hybrid composite structure to provide structural integrity and mitigate this problem. This paper outlines the working mechanisms and the fabrication of an integrated capacitive sensor in an intra-ply hybrid composite (2 × 2 twill weave). Uniaxial tensile and flexural tests were conducted to characterize the proposed sensor and provide self-sensing functionality (smart structure). The sensitivity and repeatability of the capacitive sensor were measured to be around 1.3 and 185 µΔC/Co, respectively. The results illustrate that onset of damage between layers can be detected by in situ monitoring. It can be seen that the initial damage was detected at the turning point where the relative change in capacitance begins to reduce while the load increases. Finite element modeling was also constructed to analyze the test results and explain the reasons behind the turning point. It was shown that the carbon yarns experienced high transverse shear stress (τ_xz) in the crimp region, leading to inter-fiber cracks.

Waste tires have emerged as a severe environmental threat worldwide as they create a number of disposal and landfill burden issues. In the present study, environmental pollutant crump rubber derived from waste discarded tires was reinforced with epoxy resin and prepared by means of an open-mold casting method to assess its mechanical properties. The impact of crump rubber content (0, 10, 20 and 30 vol.%) on the mechanical behavior of the composites was assessed using three-point bending tests at a constant strain rate of 0.1 and 0.01 mm/s. The stress–strain profiles of the 0.01 mm/s specimens revealed higher strains to failure compared with the 0.1 mm/s tested specimens and all the specimens showed brittle failure. Irrespective of the strain rates, tests revealed a marginal increase in the strength values of the composites and a significant increase in the modulus of all the composites compared with neat epoxy specimens. The results suggest that crump rubber can be effectively used in utilitarian composites requiring good flexural modulus and strength properties. Crump rubber epoxy composites with 30 vol.% of crump rubber showed higher modulus and strength compared with neat epoxy and other composites owing to the toughening phase induced by the crump rubber particles. The failure and fracture features of the specimens were analyzed using scanning electron microscopy.

This paper discusses the use of Kevlar-carbon hybrid composites in the aerospace industry for structural and non-structural components. By combining the high tensile strength and impact resistance of Kevlar fibers with the high stiffness and dimensional stability of carbon fibers, a material with enhanced mechanical properties can be achieved. However, the combination of two materials with different properties can cause delamination between layers, making real-time monitoring of structural integrity important. This study investigates the feasibility of embedding sensors into Kevlar-carbon hybrid composites and presents the basic characterization using a mechanical tensile test. It been shown the embedded sensor has a linear response toward the axial stress and the damage can be detected at the turning point where the capacitance begins to decrease.