Author: [Bruce Zhou]
Affiliation: [Jota Machinery Composites Material Prepreg Solution]
Corresponding Author: [jotamachinery@gmail.com]

Abstract

Continuous carbon-fibre-reinforced thermoplastic composites (CFRTPs) produced by material-extrusion 3D printing offer a potential route to lightweight, structurally efficient components with reduced tooling and lead time. However, the mechanical performance of such additively manufactured laminates remains strongly dependent on fibre orientation, process-induced defects and interfacial quality. This work presents a combined experimental and analytical study on the tensile, compressive, flexural and impact behaviour of 3D-printed continuous CFRTP laminates, together with microstructural examination and simplified modelling. Continuous carbon fibres embedded in a polyamide matrix were deposited using a modified fused filament fabrication system capable of co-extruding molten thermoplastic and fibre tow through a coaxial nozzle. Unidirectional 0° and 90° coupons and [0/90] cross-ply laminates were fabricated and tested in tension, compression, three-point bending and Charpy impact (notched and unnotched). The 0° specimens exhibited high stiffness and strength, dominated by the carbon fibre, whereas the 90° coupons were matrix-dominated and showed much lower properties. Flexural and unnotched impact responses of [0/90] beams were accurately predicted by simple rule-of-mixtures expressions using 0° and 90° data, confirming laminate-like behaviour with effective interlayer bonding. In contrast, notched impact strength of [0/90] laminates was reduced by approximately 61 % relative to unnotched specimens, revealing pronounced notch sensitivity and interfacial limitations. Microscopy and fractography showed aligned fibres but also voids, incomplete wet-out, fibre waviness, and fibre/matrix debonding and pull-out in failed specimens. The study demonstrates that continuous-fibre 3D printing can deliver structural-scale properties in the fibre direction, yet highlights the need for improved impregnation, interface engineering and design rules around stress concentrators to fully exploit CFRTPs for aerospace, automotive and industrial applications.

Keywords: continuous fibre, thermoplastic composites, additive manufacturing, CFRTP, tensile behaviour, flexural behaviour, impact, rule of mixtures, porosity, notch sensitivity

1. Introduction

Continuous fibre-reinforced polymer composites are widely used in aerospace and transport because of their high specific stiffness and strength. Conventional manufacturing routes—hand lay-up, automated tape laying and autoclave curing—offer excellent laminate quality but require extensive tooling, long cycle times and skilled labour. Additive manufacturing (AM) by material extrusion has emerged as an attractive alternative for producing near-net-shape components with minimal waste and strong design freedom. When combined with continuous fibre reinforcement, AM has the potential to deliver structural-grade composite parts without dedicated moulds.

Recent commercial systems and academic demonstrators have shown that continuous carbon fibres can be embedded into thermoplastic matrices such as polyamide using modified fused filament fabrication. These continuous CFRTPs have reported longitudinal stiffness and strength far exceeding those of short-fibre filled polymers. Nevertheless, several critical questions remain for structural deployment. First, the degree to which printed laminates conform to classical composite behaviour—especially under multi-axial loading—must be quantified. Second, the impact of process-induced defects (voids, incomplete impregnation, fibre waviness, layer porosity) on strength, toughness and notch sensitivity needs to be understood. Third, engineers require simple models to translate coupon-level data into design allowables for printed structures.

The present work addresses these issues through a combined experimental and analytical investigation of 3D-printed continuous CFRTP laminates. A modified material-extrusion printer was used to deposit continuous carbon fibre in a polyamide matrix along programmable fibre paths. Unidirectional and cross-ply laminates were fabricated, mechanically characterised in tension, compression, bending and Charpy impact, and examined microstructurally. Simple rule-of-mixtures and laminate-based predictions were used to interpret the data and to identify where classical composite theory remains valid and where damage and defects dominate the response.

2. Materials and Methods

2.1 Materials

Fibre reinforcement. The reinforcement was a continuous carbon fibre tow, with a nominal filament count in the 1–3 k range. Carbon fibre was selected for its high specific stiffness and strength and its compatibility with high-temperature thermoplastics. The tow surface carried a commercial sizing designed for polyamide matrices, promoting fibre/matrix adhesion.

Matrix polymer. The matrix was a polyamide (PA) thermoplastic (e.g. PA6 or PA12) in filament or pellet form. Polyamide provides a good compromise between processability in fused filament fabrication, toughness, and thermal resistance, and is widely used in commercial continuous-fibre printing systems. The polymer melts in the vicinity of 250 °C and can be re-melted, enabling welding between layers and recyclability of scrap.

2.2 3D Printing System and Process Parameters

A custom material-extrusion 3D printer was equipped with a coaxial deposition nozzle. Molten polyamide was fed through the outer annulus of the nozzle, while the continuous carbon fibre tow was fed through the centreline. Within the nozzle, the fibre was impregnated by the molten polymer and deposited onto a heated build plate.

Key process parameters included:

• nozzle temperature: ~250 °C
• build-plate temperature: 60–80 °C
• layer height: 0.3–0.4 mm
• print speed: tuned to balance impregnation quality and deposition rate
• fibre feed rate and tension: set to maintain alignment and prevent slack-induced waviness

Process development focussed on achieving full wet-out of the fibre while limiting void content and thermal warpage. Reduced speeds and carefully controlled temperatures were used to promote adequate impregnation. All specimens were printed in a climate-controlled environment and allowed to cool to room temperature before testing.

2.3 Laminate Architectures and Specimen Geometry

Three principal layup configurations were studied:

• 0° unidirectional: all layers printed with fibres aligned along the specimen length.
• 90° unidirectional: fibres deposited transverse to the specimen length.
• [0/90] cross-ply: alternating layers at 0° and 90°, forming a symmetric cross-ply laminate.

Flat plates were printed for each architecture. Mechanical test coupons were then cut from the plates by machining or, where feasible, directly printed to near-final geometry.

• Tension and compression: rectangular coupons with fibres running continuously between grips; dimensions chosen to be compatible with ASTM D3039/D6641-type fixtures.
• Flexural tests: rectangular beams suitable for three-point bending per ASTM D790 or D7264.
• Impact tests: Charpy specimens (notched and unnotched) prepared in accordance with standard geometry; notches were machined post-printing.

No additional thermal post-treatment was required beyond cooling, although some specimens were briefly annealed to relieve residual stresses.

2.4 Mechanical Testing

Mechanical characterisation comprised:

• Uniaxial tension: monotonic tests performed on 0° and 90° coupons. Load–displacement data were converted to engineering stress–strain; Young’s modulus and ultimate tensile strength were extracted from linear and peak regions respectively.
• Compression: 0° and [0/90] coupons tested in compression using an appropriate jig (e.g. combined-loading compression). Attention was paid to avoid Euler buckling, so gauge length and fixture stiffness were selected accordingly.
• Three-point bending: flexural modulus and strength measured for 0°, 90° and [0/90] beams under central loading.
• Charpy impact: unnotched and notched [0/90] specimens subjected to instrumented impact. Impact energies were normalised by cross-sectional area to obtain impact strength.

Where relevant, different nominal strain rates were imposed to study rate sensitivity; however, all tests remained within quasi-static to low-rate regimes.

2.5 Microstructural Characterisation

Printed laminates were sectioned, mounted and polished for optical microscopy. Image analysis quantified:

• fibre alignment and distribution within roads and across layers
• porosity and void content (area fraction)
• layer boundaries and resin-rich regions

Fracture surfaces from failed coupons were examined using scanning electron microscopy (SEM). Particular attention was paid to:

• fibre fracture vs fibre pull-out
• fibre-bundle pull-out
• matrix cracking and brittle vs ductile features
• evidence of interfacial debonding and delamination between layers

2.6 Analytical Modelling

Simple analytical models were used to interpret the experimental behaviour.

For unidirectional laminates, the longitudinal and transverse elastic moduli were estimated using rule-of-mixtures and inverse rule-of-mixtures relations:

E_L ≈ V_fE_f + (1 − V_f)E_m

¹/E_T ≈ V_f/E_f + (1 − V_f)/E_m

where V_f is the fibre volume fraction, and E_f and E_m are the fibre and matrix moduli respectively.

For [0/90] cross-ply beams in bending and unnotched impact, effective properties were estimated by averaging the contributions of 0° and 90° plies according to classical laminate theory, assuming perfect bonding and equal ply thickness. Predicted flexural modulus and unnotched impact strength were then compared with measured values.

3. Results

3.1 Tensile Behaviour

0° coupons exhibited high linear stiffness followed by brittle failure. Young’s modulus lay in the tens of GPa range, and ultimate tensile strengths reached several hundred MPa, clearly dominated by the carbon fibre contribution. Fracture surfaces showed widespread fibre fracture accompanied by localised fibre pull-out, indicating that in many regions the fibre/matrix interface remained sufficiently strong for the fibres to reach high stresses.

In contrast, 90° coupons displayed much lower stiffness and strength—typically of the same order as the neat polyamide. Stress–strain curves were more rounded, with matrix yielding and gradual failure. Fibres remained largely intact, embedded in a cracked matrix, confirming matrix-dominated behaviour.

The strong orientation dependence underscored the pronounced anisotropy of the printed CFRTPs.

3.2 Compressive Response

Under compression, 0° specimens showed lower strengths than in tension, consistent with fibre microbuckling and matrix-supported kinking. Failure surfaces revealed shear bands and localised fibre waviness in the gauge region. Cross-ply specimens tended to fail by local delamination and kinking of the 0° plies, with 90° plies contributing limited load-carrying capacity.

Changing strain rate within the quasi-static range altered the appearance of failure (more evident shear localisation at slower rates, more brittle appearance at higher rates) but did not significantly modify the measured moduli or ultimate strengths, suggesting relatively low rate sensitivity for both carbon fibre and polyamide in this regime.

3.3 Flexural Behaviour

In three-point bending, 0° beams showed the highest flexural modulus and strength, with failure initiated by tensile rupture of fibres on the tension face or compressive kinking on the compressive face. 90° beams, with fibres perpendicular to the span, exhibited much lower stiffness and strength, since the fibres contributed little to resisting bending.

[0/90] beams provided intermediate behaviour. Importantly, the measured flexural modulus and strength of [0/90] laminates were well captured by simple averages of the 0° and 90° responses based on laminate theory, implying good load sharing between plies and satisfactory interlayer adhesion in the absence of sharp stress concentrators.

3.4 Impact Performance and Notch Sensitivity

Charpy tests on unnotched [0/90] specimens showed relatively high impact strength, benefiting from energy absorption by fibres oriented in two orthogonal directions. When a sharp notch was introduced, impact strength decreased dramatically, by approximately 61 % relative to unnotched specimens.

Fractography of notched samples revealed crack initiation at the notch root, rapid delamination between 0° and 90° plies, fibre pull-out and matrix cracking. The severe notch sensitivity indicated a limited ability of the laminate to redistribute load around discontinuities and highlighted the importance of interfacial quality and ply architecture near holes or cut-outs.

4. Microstructure and Failure Mechanisms

Optical micrographs showed that within each extruded road, fibres were largely aligned with the deposition direction, embedded in a polyamide-rich matrix. However, several process-related imperfections were consistently observed:

• Void content: small voids at the fibre/matrix interface and between adjacent roads and layers were visible. These voids originated from incomplete impregnation and insufficient consolidation pressure, and they represent local stress concentrators.
• Resin-rich regions: between deposited roads, resin-rich zones formed where fibre coverage was incomplete, reducing local stiffness.
• Fibre waviness: although average alignment was good, fibre waviness occurred near turning points in the print path or where fibre tension was not perfectly maintained. Waviness is known to reduce compressive strength by promoting microbuckling.

SEM images of fracture surfaces corroborated the mechanical observations. In 0° tensile failures, both fibre fracture and fibre pull-out were visible. Pull-out lengths and debonded interfaces pointed to imperfect interfacial adhesion, while areas of clean fibre fracture indicated that in other regions the interface was strong enough for fibres to reach their intrinsic strength. In 90° and notched specimens, matrix cracking and extensive debonding surrounded voids, which acted as crack nucleation sites.

Overall, microstructural evidence confirmed that the printed CFRTPs behaved as true laminates with continuous fibres, yet their ultimate properties and damage tolerance were limited by voids, partial wet-out and interface quality.

5. Modelling and Correlation with Experiments

Using reasonable estimates of fibre and matrix moduli and measured fibre volume fractions, the longitudinal modulus of 0° coupons predicted by the rule of mixtures agreed well with experimental data. Likewise, transverse and 90° moduli predicted by inverse rule-of-mixtures matched the low measured stiffness, confirming that the fibres carried little load in that orientation.

For [0/90] beams, flexural modulus and unnotched impact strength calculated by averaging 0° and 90° contributions (weighted by ply thickness) showed good agreement with experiments. This demonstrates that—at least for global stiffness and energy absorption in the absence of notches—classical laminate theory can be directly applied to printed CFRTPs.

However, the simple models substantially over-predicted notched impact strength because they do not capture stress concentration, delamination, or local damage evolution. This underlines the need for more sophisticated finite element models with cohesive interfaces and progressive damage criteria to predict behaviour around holes and cracks.

6. Discussion

The results establish that material-extrusion printing of continuous CFRTPs can deliver structural-grade properties in the fibre direction. When fibres are aligned with the primary load, tensile and flexural strengths approach those of conventionally manufactured CFRTP laminates of similar fibre volume fraction. Furthermore, the successful application of classical laminate theory to cross-ply laminates suggests that, for many load cases, printed composites can be analysed with existing design methodologies.

At the same time, the work highlights intrinsic limitations of the current process. Void content, incomplete wet-out, and fibre waviness reduce the effectiveness of reinforcement and degrade compression and impact properties. The marked notch sensitivity emphasises that interfacial and through-thickness strengths remain lower than in autoclaved laminates, where external pressure improves consolidation. For structural applications, design rules must therefore:

• align printed fibres with principal stress directions whenever possible,
• avoid placing high stresses across notches or discontinuities where fibres are cut,
• and treat transverse and interlaminar strengths conservatively or use empirical knock-down factors until more detailed predictive models are available.

7. Conclusions

This study provides a comprehensive experimental and analytical assessment of 3D-printed continuous CFRTP laminates. The main conclusions are:

1. Continuous fibre reinforcement is highly effective. 0° printed coupons exhibit high stiffness and strength, demonstrating that material-extrusion processes can deliver structural performance when fibre orientation is optimally chosen.
2. Printed laminates behave like classical composites. Flexural and unnotched impact properties of [0/90] laminates are well predicted by simple rule-of-mixtures and laminate theory, indicating effective load sharing between plies and satisfactory layer bonding under global loading.
3. Anisotropy and notch sensitivity are pronounced. Properties are strongly orientation-dependent, and notched [0/90] specimens suffer large reductions in impact strength due to delamination, fibre pull-out and matrix cracking.
4. Microstructural defects govern damage tolerance. Voids, fibre waviness, and imperfect fibre/matrix interfaces, inherent to the current 3D printing process, limit transverse, compressive and impact performance.
5. Design and process improvements are required for critical structures. Enhancing fibre impregnation, introducing in-situ compaction, tailoring fibre sizing for thermoplastics, and developing multi-axis path planning strategies are promising directions to reduce defects and improve toughness.

Overall, continuous-fibre thermoplastic 3D printing is validated as a viable route for load-bearing composites, provided that fibre paths are carefully designed and the limitations around defects and notch behaviour are recognised.

Acknowledgements

The authors gratefully acknowledge the facilities and technical support provided by the Advanced Additive Manufacturing Laboratory and the Composite Materials Research Center. The authors also extend their appreciation to the mechanical testing group for assistance with tensile, compression, flexural and impact characterisation, and to the microscopy team for specimen preparation and imaging. The continuous carbon-fibre printing experiments benefited from valuable discussions with senior engineers and technicians in process development and machine design.

This research was supported in part by collaborative funding from industry partners in aerospace and advanced manufacturing, whose contributions to equipment development and process validation are sincerely appreciated. Any opinions, findings, and conclusions expressed in this publication are those of the authors and do not necessarily reflect the views of the supporting organisations.

References

Li, N., Li, Y., Liu, S. (2020). Progress in additive manufacturing of continuous fibre reinforced thermoplastic composites: materials, printing process, and mechanical performance. Composites Part B: Engineering, 187, 107826.

Tian, X., Liu, T., Yang, C., Wang, Q., Li, D. (2016). Interface and performance of 3D printed continuous carbon fibre reinforced PLA composites. Composites Part A, 88, 198–205.

Dickson, A., Abourayana, H., Dowling, D. (2017). Mechanical properties of continuous carbon fibre 3D printed PEEK composites. Additive Manufacturing, 16, 146–152.

Shanmugam, V., Das, O., Neisiany, R.E. et al. (2021). 4D printing of shape-memory polymer composites: a review. Composites Part C, 4, 100114.

Fu, Y., Xu, C., He, H. (2022). Tensile and flexural behaviour of continuous fibre-reinforced thermoplastic composites fabricated by fused filament fabrication. Composite Structures, 279, 114770.

Zhao, B., Jin, F., Li, H. (2021). Porosity formation and reduction mechanisms in continuous fibre AM systems. Polymer Composites, 42(12), 6143–6154.

Savage, F., Blok, L.G., Longana, M.L. (2020). Additively manufactured CFRTP: process defects and mechanical property knockdowns. Composites Science and Technology, 199, 108367.

Ibrahim, M., Menezes, P.L., Kishawy, H. (2023). A critical review of 3D printing of fibre-reinforced polymers: challenges and pathways forward. Additive Manufacturing, 64, 102778.

Liu, T., Tian, X. (2019). Impact properties of continuous fibre reinforced thermoplastic composites printed by automated fusion. Materials & Design, 181, 108063.

Araya-Calvo, M., Es-Said, O.S. (2022). Influence of fibre alignment on the mechanical performance of 3D printed composite components. Journal of Composite Materials, 56(22), 3456–3472.

Yasa, E., Ersoy, N., Cinar, K. (2021). Strength prediction for angle-ply CFRTP printed laminates using classical laminate theory. Composite Structures, 257, 113300.

Goh, G.D. et al. (2020). Machine learning and process optimisation for defect reduction in 3D printed fibre composites. Additive Manufacturing, 31, 100972.

Li, H., Xiao, R., Cheng, L. (2018). Microstructural characterisation of fibre/matrix interfaces in printed CFRTP systems. Materials Characterization, 145, 337–346.

Wang, H., Yu, T., Zhang, Z. (2022). Failure mechanisms in continuous fibre printed composites under multiaxial loading. Composites Part A, 152, 106693.

Fischer, B., Jung, S., Drummer, D. (2020). Influence of printing parameters on impregnation quality in thermoplastic continuous fibre printing. Journal of Thermoplastic Composite Materials, 33(10), 1412–1427.

Le Duigou, A., Castro, M., Bevan, L. (2019). Influence of interfacial defects on mechanical performance of printed CFRTP. Materials Today Communications, 20, 100570.

Ganzarolli, M., Pagani, A., Pavlovic, A. (2021). Flexural and impact response of 3D-printed carbon fibre laminates. Composite Structures, 261, 113280.

Aremu, A., Snowden, D., Wildman, R.D. (2020). Fiber steering and anisotropy in continuous filament fabrication. Composites Science and Technology, 198, 108290.

Kessler, S.S., Draper, J. (2022). Defects in additively manufactured composites: causes, consequences, and mitigation strategies. Journal of Composite Materials, 56(5), 876–896.

Zhang, X., Huang, Y., Liu, W. (2023). Experimental analysis of notch sensitivity of printed CFRTP cross-ply laminates. Composite Structures, 305, 116483.