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.

This study focuses on producing and processing highly aligned tapes from recycled carbon fibers (rCFs). The rCFs are processed with a modified carding machine, oriented through a specialized subsequent process and consolidated into a semi-finished product. These rCF-tapes are placed onto a two-dimensional tool using an adapted automated fiber placement (AFP) technology to demonstrate a novel approach of producing composites from highly oriented recycled materials. The semi-finished stacks are consolidated in a heating press and test coupons are tested according to corresponding standards. The rCF-tapes are evaluated using methods such as tensile and flexural testing and determination of fiber volume content. Mechanical values are assessed by processing various generations of rCF-tapes and comparing them to each other and to virgin fiber tapes (vCF-tapes) made of the same type of carbon fiber and matrix. Microscopic imaging is also performed to analyze the quality of the resulting composites. In this study, a tensile strength of up to 1100 MPa in the fiber direction and stiffness of up to 80 GPa at a fiber volume content (FVC) of approximately 40% were achieved. The results highlight the strong potential and benefits of using highly oriented rCF-tapes and demonstrate the suitability of fiber placement technologies for those recycled materials.

The aim of this study was to explore the compactibility of unidirectional staple carbon fibre laminates in comparison with their uni- and biaxial continuous fibre counterparts. Resin-preimpregnated plies were inserted into a heated compression mould at an elevated mould temperature of 110 °C. By applying stepwise loading, the correlation between consolidation pressure and fibre volume content was derived and related to fibre orientation distribution. The fibre orientation distribution is obtained from photographic image analyses of 2D ply sections of the same samples using the structure tensor approach. For commonly used autoclave prepreg pressure of 6.8 bar results indicate that lower-oriented staple carbon fibre unidirectional laminates with a fibre orientation distribution factor η₀ = 0.74 can potentially reach a maximum of 39% fibre volume fraction, while higher-oriented laminates with η₀ = 0.78 end up at 43%. An exponential extrapolation suggests that a consolidation pressure of ≥90 bar is required to achieve 60% fibre volume content with highly oriented unidirectional staple carbon fibre laminates.

The increasing availability of recycled carbon fibres (rCFs) from manufacturing waste and end-of-life composite structures offers new opportunities for sustainable, high-performance composites. However, the discontinuous nature of recycled fibres requires efficient alignment and consolidation. In this study, highly aligned discontinuous carbon fibre tapes were manufactured from recycled carbon staple fibres and bio-based polyamide 11 (PA11) fibres using a textile-based processing route and compression moulding. Two material systems containing nominal fibre mass fractions of 50 wt.% and 70 wt.% rCF were investigated. The resulting laminates were characterised in terms of fibre volume content (FVC), fibre orientation distribution, and mechanical performance. FVC of up to 56.72 vol.% were achieved for the 70 wt.% rCF material. Tensile testing revealed a significant increase in stiffness and strength with increasing fibre content, reaching values of 48.1 GPa and 866 MPa. The longitudinal compression modulus and the in-plane shear modulus showed similar trends, while the transverse tensile strength and compressive strength remained strongly influenced by local defects and fibre–matrix interactions. A 2D mesoscopic image analysis demonstrated a pronounced preferential fibre orientation, with approximately 69% of the analysed fibre regions aligned within ±10° of the dominant fibre direction. The derived alignment coefficient η₀ correlated well with the tensile modulus, confirming the strong influence of mesoscopic fibre architecture on mechanical performance. The results demonstrate that the combination of bio-based PA11 and oriented rCF, enabled through textile-based alignment technologies, provides a promising pathway towards sustainable lightweight composite structures.

Re-aligned recycled carbon fiber (rCF) yarns provide a complementary architecture to aligned rCF tapes, enabling load-path-oriented reinforcement placement with potential for automated near-net-shape preforming and three-dimensional formability. This study investigates hybrid rCF/polyamide 11 (PA11) wrap yarns produced through roller carding, hollow-spindle wrap spinning, tailored fiber placement (TFP), and consolidation. Two rCF/PA11 yarn formulations with nominal fiber/matrix mass ratios of 72/28 and 80/20 were developed, yielding fiber volume fractions of 54.4 and 58.5 vol.%, respectively. Mechanical characterization focused on UD coupon testing under axial tensile, transverse tensile, and shear loading. For the rCF/PA11 72/28 and 80/20 formulations, longitudinal tensile testing yielded moduli of 57.8 ± 4.2 and 64.3 ± 6.3 GPa and strengths of 920 ± 41 and 815 ± 86 MPa, respectively. The 72/28 material additionally exhibited an in-plane shear modulus of 1.87 ± 0.03 GPa and shear strength of 48.7 ± 1.4 MPa. Relative to the previously reported aligned rCF/PA11 tape laminates at a comparable fiber volume content, albeit with distinct processing machinery, the 72/28 yarn laminate showed higher longitudinal modulus and longitudinal strength but substantially lower transverse and shear properties, suggesting a possible architectural trade-off associated with discrete yarn lanes, helical wrapping, and TFP preforming. A conrod-shaped preliminary structural benchmark manufactured from rCF/PA11 72/28 sustained 11.3 ± 0.5 kN at a mass of 23.0 ± 1.0 g, corresponding to a 71% higher specific load capacity than the aluminum reference. Proof-of-concept thermoforming of three double-curved shell demonstrators confirmed the capability of the yarn-based preforms to form complex three-dimensional geometries, although local fiber alignment defects highlighted the need for improved draping control. A screening life-cycle assessment estimated 0.42 kg CO₂-eq per rCF/PA11 benchmark part, representing reductions of approximately 63% relative to the virgin-CF/PA12 stretch-broken yarn route and 52% relative to virgin-CF/epoxy routes. The results demonstrate the potential of rCF/PA11 wrap yarns as high-fiber-content textile intermediates for load-path-oriented TFP preforming and subsequent three-dimensional thermoforming.