Continuous fibre-reinforced thermoplastic acrylic-matrix composites prepared by liquid resin infusion – a review

Increasing demand for lightweight materials is a major driving force for the steady growth of the continuous fibre-reinforced polymer composite industry. In recent years, strict global targets demanding greater environmental responsibility have led to a shift in research focus to address the end-of-life challenges posed by the use of thermoset matrices. Thermosets offer lower-cost processibility than thermoplastics, which historically required cost- and energy-intensive production methodologies. Consequently, despite their well-demonstrated recyclability, thermoformability and weldability, thermoplastics are yet to attain the same technological maturity as thermosets. In situ polymerisable thermoplastic resins have been identified as attractive emerging solutions for improving the processibility of thermoplastics. Thus, are essential materials in meeting the demand for fibre-reinforced thermoplastic composites. This review presents a comprehensive summary of recent works on room-temperature-processible liquid thermoplastic acrylic resins and their composites. Moreover, open problems and research opportunities are identified and discussed

‘Resin welding’: A novel route to joining acrylic composite components at room temperature

The solubility of acrylic polymer in its own liquid monomer creates the opportunity to ‘weld’ acrylic-matrix (Elium®) composites without the application of heat. In this method, termed resin welding, acrylic monomeric resin is infused between acrylic-matrix composite parts. The resin dissolves and diffuses into the acrylic matrix and creates a continuous material, and a strong bond, when it polymerises, without the sensitivities of traditional welding methods to adherend or bondline thickness. Single lap shear testing was conducted on resin-welded and adhesively-bonded coupons with varying bondline thicknesses and filling fibres, and the bonding and fracture mechanisms were investigated using SEM and the diffusion of dyed acrylic resin. The highest bond strength of resin-welded coupons reached 27.9 MPa, which is 24 % higher than the strongest weld reported in the literature, indicating that resin welding is a promising alternative to traditional bonding and welding methods for acrylic-matrix composites

3D printing and epoxy-infusion treatment of curved continuous carbon fibre reinforced dual-polymer composites

A manufacturing technique was developed to fabricate curved continuous carbon fibre reinforced composites based on 3D printing and epoxy-infusion treatment. Composite preforms were first manufactured by material-extrusion based 3D printing of continuous carbon fibre reinforced thermoplastic polyamide-6 (PA-6) filaments. Powder thermoset epoxy was added to the preforms to fill up the gaps, remove air voids and enhance the interfacial bonding through a traditional vacuum bagging and oven curing process. Uniaxial tensile tests showed that the stiffness and strength of the printed composites were increased by 29.3% and 22.1%, respectively, compared to the thermoplastic-only composite specimens. The epoxy-infusion treatment technique was also adopted to manufacture composites with curved fibre alignment and investigate the performance of 3D printed notched specimens under uniaxial tension. It was shown that the placement of continuous carbon fibres along the principal stress trajectories increased the failure strength and the fracture toughness of the composites by 81% and 157% respectively, compared to the unidirectional and concentric placement methods

Effects of thermal process conditions on crystallinity and mechanical properties in material extrusion additive manufacturing of discontinuous carbon fibre reinforced polyphenylene sulphide composites

This study investigates the thermal behaviour of discontinuous carbon fibre reinforced polyphenylene sulphide (CF/PPS), additively manufactured by material extrusion, with a focus on the effects of thermal process conditions on the degree of crystallinity, oxidation crosslinking and mechanical properties of CF/PPS from filament fabrication, material extrusion to annealing treatment. The screw extrusion parameters are optimised by performing a thermal analysis of the fabricated filaments. The effect of crosslinking reactions on the crystallinity process in determining the mechanical properties of the printed samples is illustrated by investigating the influence of the printing conditions. Furthermore, the effect of annealing treatment on the semi-crystalline polyphenylene sulphide (PPS) is studied by measuring the degree of crystallinity and viscoelasticity behaviours. Results demonstrate that the flexural properties of the printed CF/PPS composites at elevated processing temperatures are determined by the oxidation crosslinking between PPS chains. These enhance the crystallisation process of semi-crystalline polymers by acting as the nucleating agent first but negatively affect the mechanical properties at higher temperatures because of the detrimental effects of the polymer inter-chain bonding. The maximum flexural strength of printed CF/PPS reached 164.65 MPa when processing at an extrusion temperature of 280°C, a printing temperature of 320°C, and an annealing temperature of 130°C for 6 h. By adjusting the thermal treatment conditions, the degree of the crystallinity and the mechanical properties of the printed CF/PPS composites can be designed, controlled and tailored

Crystallisation behaviour and morphological studies of PEKK and carbon fibre/PEKK composites

The increased interest in carbon fibre/poly(etherketoneketone) (CF/PEKK) as an option for high-performance applications calls for a thorough understanding of the composite's crystallisation behaviour, due to the essential role that crystallinity plays in performance. In this study, differential scanning calorimetry was used with a variety of thermal cycles to evaluate the effect of thermal history on crystallinity development in unreinforced PEKK and CF/PEKK. Different isothermal holding temperatures during cooling affected the ratio between primary and secondary crystallisation, and non-isothermal cooling cycles influenced the extent of crystallisation. The inclusion of carbon fibres increased the proportion of secondary crystallisation in the matrix and slowed down crystallisation kinetics. A Velisaris-Seferis model was used to model crystallisation kinetics for the isothermal data, and adapted Nakamura models were used for the non-isothermal data. Based on this work, optimum isothermal hold temperatures during cooling for CF/PEKK are estimated to lie in the range of 220–260 °C

The fatigue of carbon fibre reinforced plastics - A review

Engineering structures are often subjected to the conditions of cyclic-loading, which onsets material fatigue, detrimentally affecting the service-life and damage tolerance of components and joints. Carbon fibre reinforced plastics (CFRP) are high-strength, low-weight composites that are gaining ubiquity in place of metals and glass fibre reinforced plastics (GFRP) not only due to their outstanding strength-to-weight properties, but also because carbon fibres are relatively inert to environmental degradation and thus show potential as corrosion resistant materials. The effects of cyclic loading on the fatigue of CFRP are detailed in several papers. As such, collating research on CFRP fatigue into a single document is a worthwhile exercise, as it will benefit the engineering-readership interested in designing fatigue resistant structures and components using CFRP. This review article aims to provide the most relevant and up-to-date information on the fatigue of CFRP. The review focuses in particular on defining fatigue and the mechanics of cyclically-loaded composites, elucidating the fatigue response and fatigue properties of CFRP in different forms, discussing the importance of environmental factors on the fatigue performance and service-life, and summarising the different approaches taken to modelling fatigue in CFRP

Numerical and Experimental Investigation of Joule Heating in a Carbon Fibre Powder Epoxy Towpregging Line

Powder epoxy based towpregs offer favourable processing and storage properties, thanks to the low viscosity and thermal stability of the powder epoxy. Low-cost, high-quality towpregs, which are suitable for automated fibre placement or filament winding applications, can be produced at a high production rate with an automated towpregging line. This study focuses on improving the towpregging process by analysing the heating charac-teristics of a towpregging line that employs Joule heating to impregnate carbon fibre tows with powder epoxy. A finite element analysis heat transfer model was developed to identify the relationship between processing pa-rameters and heating of the carbon fibre tows. Model predictions matched well with experimental results. Using the temperature distribution predicted by the model, powder epoxy melting and sintering behaviour was investigated using semi-empirical equations. Results revealed that Joule heating provides efficient heating with very low power consumption. It was found that while it is possible to produce towpregs at high production speeds (15 m/min), slower speeds might yield more consistent quality. Using parametric studies in the model, it was shown that it is possible to increase towpregging line production rate without compromising the towpreg quality, by altering some of the key process parameters (supplied current, electrode distance etc.)

Seawater ageing of thermoplastic acrylic hybrid matrix composites for marine applications

Increasing usage of polymer composite materials necessitates the development of recyclable alternatives to traditional thermoset matrices or new techniques for recycling these materials. One family of promising recyclable matrices are the room temperature infusible acrylic resins, known commercially as Elium®. If these new materials are to be used in the tidal stream energy and shipping sectors, they must be able to withstand long-term immersion in seawater without significant losses in mechanical properties. In this study, accelerated seawater ageing is applied to acrylic/glass fibre and modified acrylic/glass fibre composites along with a traditional epoxy/glass fibre baseline. The mechanical properties (tensile, flexural, and short beam) are compared before and after ageing, and electron microscopy is used to examine fracture surfaces to determine the effects of water ingress on fracture propagation. In addition, the diffusion coefficients of the composites in seawater are compared and the changes in glass transition temperatures are used to determine the effects of plasticisation

Characterising the shear resistance of a unidirectional non‑crimp glass fabric using modified picture frame and uniaxial bias extension test methods

The forming behaviour of a unidirectional non-crimp fabric (UD-NCF) consisting of polyamide stitches with a tricot-chain stitching pattern is explored. Notably, there are no stabilising tows orientated transverse to the main tow direction in this fabric, a common feature in many ‘quasi’ UD-NCFs, this allows extension of the stitch in the transverse direction under certain loading conditions. The lack of stabilising tows introduces a possible low-energy deformation mode to the UD-NCF, which is absent in biaxial fabrics and to a large extent in ‘quasi’ UD-NCFs. The in-plane shear behaviour is initially investigated using both standard ‘tightly-clamped’ picture frame tests and uniaxial bias extension tests. Preliminary results show a dramatic difference in results produced by the two test methods. During the picture frame test, fibres can be subjected to unintended tension due to sample misalignment in the picture frame rig. To mitigate error arising from this effect, the picture frame test procedure is modified in two different ways: by using an intentional pre-displacement of the picture frame rig, and by changing the clamping condition of test specimen. Results show that the modified picture frame test data contain less error than the standard ‘tightly-clamped’ test but also that the shear stiffness of the UD-NCF is notably lower when measured in the bias extension test compared to the picture frame test, mainly due to the difference in loading conditions imposed during the two tests