The strip placement method was used to manufacture nine carbon fibre reinforced epoxy laminates that were tested in bending to study the influence of asymmetric wrinkles on strength. Due to the accuracy of the process and material flow of the material during consolidation, the achieved wrinkles did not match the intended wrinkles in terms of maximum fibre angles or degree of asymmetry, highlighting the difficulty of creating controlled wrinkles in flat coupon specimens. Nevertheless, the experimental data showed decreasing ultimate bending strength with increasing maximum angle and increasing strength with increasing asymmetry. Therefore, for samples with the same maximum angle, the sample with higher wrinkle asymmetry had a higher ultimate bending failure strength, highlighting the importance of considering wrinkle asymmetry alongside the often-used maximum angle. Simulations of the as-designed coupons were also run to design the experiment. In terms of the relevance of asymmetry, both the numerical and experimental data are in line with previous numerical studies on asymmetric wrinkles under pure tension and compression. Contrasting the as-designed wrinkle morphologies to the manufactured wrinkles, it was found that overall, the study highlights the complexity and difficulty of creating controlled wrinkles within coupon specimens and provides evidence for the relevance of wrinkle shape asymmetry when characterising and simulating wrinkles across multiple load cases.
Carbon fibre-reinforced thermoplastics are attractive for lightweight applications due to their high strength and corrosion resistance. Among engineering polymers, polyamide 12 (PA12) is widely used due to its low moisture absorption; however, its relatively high cost and limited availability are drawbacks. Compatibilised blends based on high-density polyethylene (HDPE) and polyamide 6 (PA6) emerge as an alternative. This study comparatively evaluates PA12- and HDPE/PA6(50/50)-based formulations, reinforced and unreinforced with 10 wt.% carbon fibres, focusing on their mechanical, thermal, morphological, and rheological behaviour. The HDPE/PA6/carbon composite showed higher tensile strength (47.3 MPa) and modulus (2010 MPa), whereas PA12 composites exhibited greater ductility (elongation >190%). Carbon fibre incorporation increased stiffness and reduced melt flow in both systems. Thermal analyses indicated distinct crystallisation behaviours, with PA12 showing a more uniform crystalline structure, while HDPE/PA6 blends exhibited multiphase crystallisation due to their immiscible morphology. Overall, compatibilised HDPE/PA6 blends achieved mechanical performance comparable to or exceeding PA12, albeit with reduced ductility.
This study investigates the influence of sodium hydroxide (NaOH) surface treatment on the out-of-plane auxetic behavior and mechanical properties of hemp/polylactic acid (PLA) nonwoven composites. Untreated and alkali-treated hemp fibers (2.5 wt.%, 5 wt.%, and 7.5 wt.% NaOH) were used to fabricate needle-punched nonwovens with 50 wt.% PLA. Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR) were employed to analyze surface morphology, thermal stability, and chemical changes, respectively. Alkali treatment effectively removed hemicellulose and lignin, enhanced surface roughness, and improved fiber matrix adhesion. Mechanical testing revealed that 5 wt.% NaOH treatment yielded the highest tensile strength due to optimal interfacial bonding and mechanical interlocking. However, out-of-plane Poisson's ratio measurements indicated a decrease in auxetic magnitude with increasing NaOH concentration, with the maximum negative Poisson's ratio of approximately -6.5 observed for untreated composites and -2.3 for treated ones. The results demonstrate that improved interfacial bonding enhances strength but restricts fiber rotation and reorientation, leading to reduced auxetic behavior. This work establishes a direct correlation between fiber surface modification and auxetic tunability, providing valuable insight into designing sustainable, functionally adaptive composites for structural and protective applications.
Fused filament fabrication (FFF) is a printing technology that relies on remelting and modelling physical objects from thermoplastic filament materials. Natural fibres have been studied and used for polymeric reinforcement. Yet, there is limited information in the literature about reinforcing filament materials for FFF printing. This study aimed to evaluate some surface properties of FFF 3D printed polyamide 6 reinforced with kenaf fibres. Kenaf fibres were retted, bleached with sodium hypochlorite solution, and silanised with 3-aminopropyltriethoxysilane solution prior to thermally compounding with polyamide beads to produce filaments for experimental groups. A total of 55 specimens were printed by an FFF printer for 5 groups: control, 0.1%, 0.3%, 0.5% and 1% kenaf fibres-reinforced polyamide (11 specimens for each group). Fourier-transform infrared spectroscopy analysis was conducted on the control and 1% kenaf-reinforced sample for chemical characterisation. The study groups were submitted to surface roughness, surface hardness and water contact angle measurement tests. The results showed no significant chemical change in the composite as a result of fibre incorporation. Surface hardness have shown a significant increase in their mean values after fibres incorporation, which were 93.3, 93.5, 92.2, 91.3 for 0.1, 0.3, 0.5 and 1% respectively compared with the control 88.9, while there was no significant difference in both surface roughness and surface hydrophilicity except for 1% kenaf-reinforced group (roughness = 358, contact angle = 55.1) compared to the control group (roughness = 216, contact angle = 46.4). Kenaf fibres reinforcement with polyamide 6 at concentrations not more than 0.5% has improved the surface hardness of the FFF printed material.
Fused Deposition Modeling (FDM) has promised to revolutionize the fabrication of strain sensors, offering design flexibility and cost-effectiveness. However, printing flexible thermoplastic elastomers with conductive nanomaterials presents challenges, such as nozzle blockages and inconsistent feeding due to high melt viscosity and filler agglomeration. In this study, hybrid thermoplastic polyurethane (TPU) nanocomposites filaments containing carbon nanotubes (CNT), graphene nanoplatelets (GNP), and boron nitride (BN), were developed. The hybrid fillers reduced the filament extrusion force and improved the printability compared to CNT-only composites, while maintaining high electrical conductivity and strain-sensing capabilities. A novel extrusion force testing method was implemented to measure the printability of the nanocomposite filaments. The printed sensors, prepared via FDM, demonstrated an enhanced strain-sensing range and mechanical durability, with TPU-CNT-BN composites achieving sensing up to 250% strain. These findings highlight the potential of hybrid nanocomposites for reliable and scalable production of flexible strain sensors using FDM, offering applications in wearable electronics and structural health monitoring.
