This study presents two novel methods for in situ characterization of the reaction-diffusion process during the co-curing of a polyetherimide thermoplastic interlayer with an epoxy-amine thermoset. The first method was based on hot stage experiments using a computer vision point tracker algorithm to detect and trace diffusion fronts, and the second method used space- and time-resolved Raman spectroscopy. Both approaches provided essential information, e.g., type of transport phenomena and diffusion rate. They can also be combined and serve to elucidate phenomena occurring during diffusion up to phase separation of the gradient interphase between the epoxy system and the thermoplastic. Accordingly, it was possible to distinguish reaction-diffusion mechanisms, describe the diffusivity of the present system and evaluate the usability of the above-mentioned methods.

The aim of this study was to characterise the microstructural organisation of staple carbon fibre-reinforced polymer composites and to investigate their mechanical properties. Conventionally, fibre-reinforced materials are manufactured using continuous fibres. However, discontinuous fibres are crucial for developing sustainable structural second-life applications. Specifically, aligning staple fibres into yarn or tape-like structures enables similar usage to continuous fibre-based products. Understanding the effects of fibre orientation, fibre length, and compaction on mechanical performance can facilitate the fibres’ use as standard engineering materials. This study employed methods ranging from microscale to macroscale, such as image analysis, X-ray computed tomography, and mechanical testing, to quantify the microstructural organisations resulting from different alignment processing methods. These results were compared with the results of mechanical tests to validate and comprehend the relationship between fibre alignment and strength. The results show a significant influence of alignment on fibre orientation distribution, fibre volume fraction, tortuosity, and mechanical properties. Furthermore, different characteristics of the staple fibre tapes were identified and attributed to kinematic effects during movement of the sliver alignment unit, resulting in varying tape thicknesses and fuzzy surfaces.

Carbon staple fiber composites are materials reinforced with discrete-length carbon fibers processed using traditional textile technologies, offering moderate mechanical properties and flexibility in manufacturing. These composites can be produced from recycled carbon staple fibers, aligned into yarn and tape-like structures, providing a more sustainable alternative while balancing performance, cost-effectiveness, and environmental impact. Aligning staple fibers into tape-like structures enables similar applications to those of continuous-fiber-based products, while allowing control over fiber orientation distribution, fiber volume fraction, and length distribution, which are all critical factors influencing both mechanical and thermo-mechanical properties. This study focuses on the experimental characterization and numerical investigation of Coefficients of Thermal Expansion (CTEs) in aligned carbon staple fiber composites. The effects of fiber orientation and volume fraction on coefficients of thermal expansion under different fiber alignment parameters are analyzed, revealing distinct thermal expansion behavior compared to typical aligned unidirectional continuous carbon fiber composite laminates. Unlike continuous unidirectional laminates, which typically exhibit transversely isotropic behavior without tensile–shear coupling, staple fiber composites demonstrate different in-plane axial, transverse, and out-of-plane CTE characteristics. To explain these deviations, a modeling approach is introduced, incorporating detailed experimental information on fiber distributions and microstructural features rather than averaged fiber orientation values. This involves a multi-scale analysis based on a laminate analogy through which all composite thermo-elastic properties can be predicted, accounting for variations in fiber orientations, volume fractions, and tape thicknesses. It is shown that while the local variation of fiber volume fraction has a small effect on the homogenized value of the coefficients of thermal expansion, fiber misalignment, tape thickness, and asymmetry in fiber orientation distribution will significantly affect the measurements of CTEs. For the case of carbon staple fiber composites, the asymmetry in fiber orientation distribution significantly influences the measurements of axial CTE. Fiber orientation asymmetry causes tensile–shear coupling under mechanical and thermal loading, leading to an unbalanced laminate with in-plane shear–tensile deformation. This coupling disrupts uniform displacement, complicating strain measurements and the determination of composite properties.

Short carbon fiber-reinforced bio-based polyamide 11 (PA11) composites were developed in filament form for Additive Fusion Technology (AFT) 3D printing and benchmarked against injection-molded samples. Composites containing 15 and 25 weight percent (wt%) recycled carbon fibers (rCFs) were successfully extruded into 1.75 mm diameter filaments, whereas higher fiber contents (35 wt%) led to brittle filament failure. AFT printing with subsequent consolidation produced short fiber composites with highly aligned fibers, while injection molding generated more randomly oriented microstructures. Mechanical testing revealed that AFT-printed composites in the fiber direction achieved significantly higher stiffness and comparable tensile strength to injection-molded counterparts. At 25 wt% fiber content, AFT 0° specimens reached an axial tensile modulus of 14.5 GPa, about 32% higher than injection-molded samples (11.0 GPa), with similar axial tensile strength (~123 vs. 126 MPa). However, AFT specimens displayed pronounced anisotropy: transverse (90°) properties dropped to ~2.3 GPa for transverse modulus and ~46–50 MPa transverse tensile strength, near matrix-dominated levels. Impact testing showed orientation-dependent toughness, with AFT 90° samples at 15% fiber content achieving the highest impact energy (76 kJ·m⁻²), while AFT 0° samples were ~30% lower than injection-molded equivalents. Dynamic mechanical analysis confirmed that AFT 0° composites maintained higher stiffness up to ~80 °C. Overall, these results demonstrate that aligned short fiber filaments enable high stiffness and strength performance comparable to injection molding, with the trade-off of anisotropy that must be carefully considered in design. This study is the first to demonstrate the feasibility of combining bio-based PA11 with recycled short carbon fibers in AFT printing, thereby extending additive manufacturing to sustainable and high-stiffness short fiber composites.

The joining of composites mostly relies on traditional joining technologies, such as film or paste adhesives, or mechanical fasteners. This study focuses on the appealing approach of using standard thermoplastic welding processes to join thermosets. To achieve this, a thermoplastic coupling layer is created by curing with a thermoset composite part. This leads to a functional surface that can be utilized with thermoplastic welding methods. The thermoplastic coupling layer is integrated as a thin film, compatible with the thermoset resin in the sense that it can partially diffuse in a controlled way into the thermoset resin during the curing cycle. Recent studies showed the high affinity for the interphase formation of poly hydroxy ether (phenoxy) film as coupling layer, in combination with a fast-curing epoxy system that cures within 1 min at 140 °C. In this study, an investigation based on resistance and ultrasonic welding techniques with different testing conditions of single-lap shear samples (at room temperature, 60 °C, and 80 °C) was performed. The results showed strong mechanical strengths of 28.9 MPa (±0.7%) for resistance welding and 24.5 MPa (±0.1%) for ultrasonic welding, with only a minor reduction in mechanical properties up to the glass transition temperature of phenoxy (90 °C). The combination of a fast-curing composite material with an ultra-fast ultrasonic joining technology clearly demonstrates the high potential of this joining technique for industrial applications, such as automotive, sporting goods, or wind energy. The innovation allowing structural joining performance presents key advantages versus traditional methods: the thermoplastic film positioning in the mold can be automated and localized, joint formation requires only a fraction of a second, and the joining operation does not require surface preparation/cleaning or structure deterioration (drilling).