Temperature can significantly affect fatigue delamination growth (FDG) behavior in composites, while fiber bridging has been frequently reported during FDG. The focus of this study was therefore on investigating temperature effects on FDG behavior with fiber bridging. Mode I fatigue delamination experiments were conducted on a thermoset composite laminates M30SC/DT120 at different temperatures. The Paris relation and fatigue resistance curve (i.e. fatigue R-curve) were used to interpret bridging effects on FDG behavior and to explore temperature effects on fiber bridging development. A modified Paris relation was employed to determine the effects of temperature on the intrinsic FDG behavior at the crack front excluding fiber bridging. The Paris interpretations clearly demonstrate that fiber bridging can significantly retard FDG behavior at different temperatures. Temperature can have different effects on fiber bridging development and the intrinsic FDG behavior. Particularly, elevated temperature can promote more bridging fibers, whereas decreased temperature has negligible influence on fiber bridging. When looking at the intrinsic delamination resistance, mode I FDG can accelerate at elevated temperature but decrease at freezing temperature. Fractographic examinations indicate that fiber/matrix interface debonding is the dominant damage mechanism in mode I FDG at different temperatures. Elevated temperature can lead to the weakening of interface adhesion, contributing to faster intrinsic mode I FDG behavior and more fiber bridging development. And a semi-empirical fatigue model based on normalization was finally proposed to determine mode I intrinsic FDG behavior at different temperatures for engineering applications.

In this study, an aerospace thermosetting composite was co-curing joined by Polyether-ether-ketone (PEEK) and Polyethylenimine (PEI) films, with an aim of developing advanced composite joints. The semi-crystalline PEEK films were surface activated upon a UV-irradiation technique to obtain a strong film–composite interface, while the amorphous PEI films could be directly used. The fracture behaviour of the composite joints was evaluated and compared with benchmark aerospace adhesive joints. The experimental results proved remarkable mode-I and mode-II fracture resistance of the PEEK co-cured joints at 22 °C and 130 °C, while the PEI co-cured joints exhibited excellent mode-I fracture resistance at 22 °C and mode-II fracture resistance in both testing temperature cases. Extensive elongation, tearing and fracture of the PEEK/PEI plastics were proved to be the main mechanisms for toughness enhancement. Overall, this work had successfully demonstrated the effectiveness of developing advanced composite joints via a co-curing process using high-performance thermoplastic films.

The exceptional mechanical properties of Polyether-ether-ketone(PEEK) polymers make them ideal candidates for interlayer toughening of carbon fibre/epoxy composites. Herein, ultra-thin PEEK films with a thickness of 8μm, 18μm and 25μm were used for interlayer toughening of an aerospace-grade carbon fibre/epoxy composite. The mode-I and mode-II fracture behaviour of the interleaved laminates were investigated, with the fracture mechanisms being investigated. The surfaces of the PEEK films were treated by a UV-irradiation technique to enhance their intrinsically low surface activities. This significantly increased the adhesion at the interface between the PEEK interlayers and the composite matrix. A topography analysis on the fracture surfaces revealed extensive damage of the PEEK interlayers during the fracture process of the laminates. Owing to the exceptional properties of the PEEK films, significant enhancements in the mode-I and mode-II fracture properties of the laminates were obtained, i.e. the mode-I and mode-II fracture energies were significantly increased by 227% and 441%, respectively. Overall, the UV-treated PEEK films proved superior effectivenesses for laminate toughening when compared to the other state-of-the-art interlayer materials.

The production of advanced composites from recycled carbon fibres (rCFs) is critical for the sustainable development of carbon fibre industry. Herein, non-woven mats consisting of commingled rCFs and Polyphenylene-sulfide (PPS) fibres were compression moulded to manufacture rCF/PPS composites, with the fibre/matrix adhesion being tailored by UV-irradiating the non-woven mats. The intralaminar and interlaminar fracture resistance and mechanical performance of the rCF/PPS composites were characterised. The experimental results had demonstrated that improving the PPS/rCF adhesion of the composites significantly increased the intralaminar fracture energies and mechanical properties under tensile and shear loading conditions. However, it also negatively affected the interlaminar fracture resistance. The main fracture mechanism was observed to be fibre evulsion for the intralaminar fracture mode, while crack bridging by the rCFs was the primary fracture mechanism for the interlaminar fracture condition. That led to the contrary influences of the improved fibre/matrix adhesion on the intralaminar and interlaminar fracture resistance of the rCF/PPS composites. In summary, this study had shedded lights on tailoring the crack resistance and mechanical performance of rCFRPs by adjusting the fibre/matrix adhesion using the UV-treatment technique.

The development of effective methods for the bonding of Poly-etherether-ketone (PEEK) and Polyphenylene-sulphide (PPS) composites to thermoset composites is appealing to expand their applications in aerospace industry. Herein, the surfaces of PEEK and PPS composites were treated by a high-power UV-irradiation technique for 6 s, that proved to significantly improve their intrinsically low surface activities. Carbon fibre reinforced epoxy composites were then directly cured onto the PEEK and PPS composites with or without an aerospace film adhesive at the joining interfaces. The mode-II and mix mode-I/II fracture behaviour of the hybrid joints were studied using an end notched flexural test and a fixed-ratio mixed-mode test, respectively. It was observed that the failure of the hybrid joints without adhesives mainly took place at the joining interfaces. In this case, the lack of resins at the fracture plane resulted in relatively low fracture toughness. Encouragingly, a cohesive failure was observed for the hybrid joints with adhesives in all the cases, owing to the enhanced adhesion between the adhesive and the PEEK/PPS composites upon the UV-treatment. This phenomenon indicated that optimal fracture resistance of the hybrid adhesive joints was obtained for the given material systems.

A novel co-curing process was proposed for the bonding of carbon fibre/epoxy composites by replacing traditional epoxy adhesives with carbon fibre/PEEK (CF/PEEK) tapes, with an attempt to improve the structure integrity. The lap-shear strengths, fatigue resistance and mode-I and mode-II fracture behaviour of the co-cured joints at 22 °C and 130 °C were investigated, and the failure mechanisms were also studied. The experimental results demonstrated that, by replacing an aerospace structural adhesive with surface-treated CF/PEEK tapes for the co-curing bonding of composite joints, the lap-shear strength of the joints had been increased by 47% and 68% at 22 °C and 130 °C, respectively; the fatigue life had been extended by 3.39 times; the mode-I fracture energy had been increased by 70% and 182% at 22 °C and 130 °C, respectively; and the mode-II fracture energy had been increased by 59% and 54% at 22 °C and 130 °C, respectively. An analysis on the failure surfaces of the tested specimens proved significant plastic deformation and breakage of the PEEK resin and extensive carbon fibre delamination being the main failure mechanisms of the CF/PEEK bonded joints. Overall, this study demonstrated a huge potential of replacing traditional film adhesives with CF/PEEK tapes for the co-curing bonding of aerospace composite joints with significantly enhanced structure integrity and thermal stability.

The inherently low surface energy of carbon fibre reinforced Polyether-ether-ketone (CF/PEEK) composites results in an extremely low compatibility with adhesives. This subsequently causes significant challenges in the adhesive joining of them to other dissimilar materials. Herein, the bonding surfaces of the CF/PEEK composites were treated by a high-power UV-irradiation technique prior to the adhesive bonding, with an attempt to develop hybrid composite-to-aluminium joints with excellent fracture resistance. The mode-I, mode-II and mix-mode fracture behaviour of CF/PEEK-to-aluminium joints bonded by two commercial aerospace adhesives was evaluated. Cohesive failure within the adhesive layers or substrate damage to the CF/PEEK composites were observed in all the cases. This indicated that the adhesion between the CF/PEEK composites and the adhesives was sufficient to prevent an adhesive failure at the composite/adhesive interfaces under different fracture modes. This study explored an effective route to develop strong and tough CF/PEEK-to-aluminium joints for aerospace applications. Additionally, it revealed that the form of the adhesive supporting carrier was a key factor affecting the fracture behaviour and fracture energies of the adhesive joints.

This work studied the mix mode-I/II fracture behaviour of an aerospace-grade carbon fibre/epoxy composite that was interlayer-toughened by Polyamide-12 (PA), Polyphenylene-sulphide (PPS), Polyimide (PI), Polyethersulfone (PES) and Polyethylenimine (PEI) fibres. During the laminate curing process, the PA fibres melted, the PPS and PI fibres kept in their original form and the PES and PEI fibres dissolved in the epoxy matrix. This resulted in different toughening mechanisms of the veils for the mix mode-I/II fracture of the laminates, which was studied using a cracked lap-shear test. The main toughening mechanisms were observed to be plastic deformation and failure of the thermoplastic resin for the meltable PA veils, thermoplastic fibre debonding and bridging for the intact PPS and PI veils, and thermoplastic particle debonding and plastic void growth for the dissolvable PES and PEI veils. The experimental results revealed that the fibre debonding and bridging mechanism was superior for toughness enhancement, followed by the thermoplastic particle debonding and plastic void growth mechanism. For instance, interleaving the PPS and PEI veils increased the mix-mode fracture propagation energy of the laminates by 345% and 171%, respectively. However, the toughening performance of the PA and PI veils was poor, since the crack mainly propagated at the vicinity around the interlayer/laminate interface.

Exploring routes for the effective use of recycled carbon fibres (rCFs) is critical to close the loop in the life cycle of carbon fibres. This work demonstrated a potential of using rCFs for interlayer toughening of carbon fibre/epoxy composites. Nonwoven mats based on rCFs and commingled rCFs/Polyphenylene-sulfid (PPS) fibres were used to interlay a laminate, aiming to improve the mode-I and mode-II fracture toughness. The experimental results proved significant enhancements in the interlaminar fracture properties upon interleaving, with the rCF/PPS mats exhibiting a more prominent toughening effectiveness than the rCF mats. For example, the maximum increase in mode-I and mode-II fracture initiation energies of the laminates was 51% and 66%, respectively upon interleaving the rCF mats, and 220% and 105%, respectively by adding the rCFs/PPS mats. The fractography analysis proved that the main toughening mechanisms were fibre debonding and pulling-out for the rCF mats and fibre bridging for the commingled rCFs/PPS mats. The differences in the toughening mechanisms resulted in opposite effects of the interlayer/epoxy adhesion to the fracture toughness, i.e. an improved interlayer/epoxy adhesion increased the toughening effectiveness of the rCF mats, but negatively affected the toughening performance of the rCF/PPS mats.

Carbon fibre/epoxy composite joints were assembled with Poly-etherether-ketone (PEEK) and Poly-ethylenimine (PEI) films using a co-curing process to prepare single-lap joint specimens. The joints were tested under quasi-static loading conditions at 22 °C and 130 °C and a fatigue loading condition. The experimental results demonstrated better or comparable structure integrity of the composite joints co-cured by PEEK and PEI films than the reference joints bonded by aerospace FM300 adhesives. In particular, the PEEK co-cured joints exhibited extraordinary mechanical performance at 130 °C and excellent fatigue resistance. For instance, the lap-shear strength at 130 °C and the fatigue life of the composite joints co-cured by 200μm PEEK films was 2.1 times and 2.7 times higher than that of the aerospace adhesive joints, respectively. Overall, the results of this work proved that advanced thermoplastic films are promising alternatives to epoxy adhesives for the co-cure joining of thermoset composites with significantly enhanced structural integrity and thermal stability.

The present work is concerned with adhesive bonding of thermoplastic composites used in general aerospace applications, including polyphenylene sulfide (PPS), polyetherimide (PEI) and polyetheretherketone (PEEK) carbon fibre composites. Three different surface treatments have been applied to the PEEK, PPS and PEI-based composites in order to enhance the adhesion: atmospheric plasma, ultraviolet radiation (UV) and isopropanol wiping as a control. Water contact angles and free surface energies were measured following the standard experimental procedure based on the employment of three different liquid droplets. Infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) were subsequently performed to characterize the surface chemistry of the samples after treatment. The single lap joints were manufactured and bonded by an Aerospace grade epoxy-based film adhesive originally developed for use on metals but with the ability to bond treated thermoplastics to good strength (supplied by Henkel Ireland). Quasi-static (QS) tests were conducted. The lap shear strength was evaluated, and the failure mechanisms of the different joints were examined for the range of surface treatments considered. It was found that the performances of the PEEK and PPS joints were considerably improved by the plasma and UV treatments resulting in cohesive and delamination failures, while PEI was unaffected by the plasma and UV treatments and performed very well throughout.

This work studied the effects of adding short basalt fibers (BFs) and multi-walled carbon nanotubes (MWCNTs), both separately and in combination, on the mechanical properties, fracture toughness, and electrical conductivity of an epoxy polymer. The surfaces of the short BFs were either treated using a silane coupling agent or further functionalized by atmospheric plasma to enhance the adhesion between the BFs and the epoxy. The results of a single fiber fragmentation test demonstrated a significantly improved BF/epoxy adhesion upon applying the plasma treatment to the BFs. This resulted in better mechanical properties and fracture toughness of the composites containing the plasma-activated BFs. The improved BF/epoxy adhesion also affected the hybrid toughening performance of the BFs and MWCNTs. In particular, synergistic toughening effects were observed when the plasma-activated BFs/MWCNTs hybrid modifiers were used, while only additive toughening effects occurred for the silane-sized BFs/MWCNTs hybrid modifiers. This work demonstrated a potential to develop strong, tough, and electrically conductive epoxy composites by adding hybrid BF/MWCNT modifiers.

The compatibility between the majority of thermoplastic veils (TPVs) and epoxies is typically poor, owing to the inherently low surface energies of thermoplastics. This tends to largely affect the toughening performance of TPVs as interlayer materials of carbon fibre/epoxy composites. The traditional methods for surface activation of thermoplastics, such as corona discharge, plasma treatment and acid etches, are not applicable to TPVs as they could cause significant damage to the thermoplastic fibres with nano-/micro-scale diameters. Herein, a UV-irradiation technique was proposed to active the surfaces of polyphenylene-sulfide (PPS) veils, that effectively improved their adhesion with epoxies. Consequently, the effects of an improved veil/epoxy adhesion on the mode-I and mode-II fracture behaviour and corresponding fracture mechanisms of the interleaved laminates were investigated. It was found that an improved veil/epoxy adhesion significantly enhanced the toughening performance of the PPS veils for the laminates manufactured by resin transfer moulding of non-crimp fabrics, by introducing additional carbon fibre delamination and significant PPS fibre damage during the fracture process. In contrast, the increased level of veil/epoxy adhesion inhibited PPS fibre bridging during the fracture process of the laminates produced from unidirectional prepregs, and caused considerable adverse effects on the fracture performance.

Poly-etherether-ketone (PEEK) fibres (average diameter 30μm) were surface-activated by a UV-irradiation technique, and then used as interlayers of carbon fibre/epoxy composites. The results of a flatwise tensile test demonstrated a significant improvement in the PEEK fibre/epoxy adhesion upon the UV-treatment, i.e. the ultimate strength increased from 0.6–0.7MPa to 7.6MPa. Accordingly, interlaying UV-irradiated PEEK fibres resulted in considerable increases in the maximum values of open-hole tensile strength, Charpy impact strength and mode-I fracture energy, i.e. of 12%, 131% and 293%, respectively. However, it also decreased the flexural strength by 29%, owing to the thickness increase caused by adding interlayers. Fortunately, the load carrying capacity (the maximum failure load under flexural bending) was largely unaffected, and moreover, an average residual strength of 475 ± 23MPa still remained after the damage at the maximum load. The results demonstrated significant benefits of using longitudinal UV-irradiated PEEK fibres as interlayers of CFRPs.

Thermoplastic veils based on Polyethylene-terephthalate (PET), Polyphenylene-sulfide (PPS) and Polyamide-12 (PA) fibres (∼10μm in diameter) were used to interlay unidirectional (UD), non-crimp fabric (NCF) and 5-Harness satin weave (5H) carbon fibre laminates. The PET and PPS veils remained in a fibrous form and the PA veils melted during the laminate curing process. The results of an end-loaded split test demonstrated significant improvements in the mode-II fracture performance in all cases. In general, interlaying thermoplastic veils was most efficient for toughening the UD laminates, with reduced improvements observed for the 5H and NCF laminates, respectively. The main toughening mechanism of the intact PET and PPS veils was thermoplastic fibre bridging. The melted PA veils mainly improved the fracture toughness of the epoxy at the mid-plane. The different toughening mechanisms of the veils, combined with different fracture mechanisms between the UD, NCF and 5H laminates, resulted in significantly different toughening levels.

Polyphenylene-sulfide (PPS) veils doped with MWCNTs and graphene nanoplatelets (GNPs) were used as interleaves of a carbon fibre/epoxy composite, aiming to study its effects on the fracture performance. Interlaying original PPS veils significantly improved the mode-I and mode-II fracture toughness of the laminates due to a PPS fibre bridging mechanism. The addition of MWCNTs on the veils improved the PPS fibre/epoxy adhesion by introducing additional interactions, i.e. MWCNT pull-out and breakage, between the PPS fibres and the epoxy during the fracture process. This further improved the fracture toughness of the laminates at a relatively low content of MWCNTs. In contrast, the incorporation of GNPs on the veils decreased the PPS fibre/epoxy adhesion, resulting in detrimental effects on the fracture performance.

Interlaying thermoplastic veils into carbon fibre/epoxy composites has proved to significantly increase the interlaminar fracture toughness. The main toughening mechanism is thermoplastic fibre bridging for the non-meltable veils and matrix toughening for the meltable veils. Herein, to take advantage of different toughening mechanisms, hybrid meltable/non-meltable thermoplastic veils were used to interlay two types of aerospace-grade composites produced from unidirectional (UD) prepregs and resin transfer moulding of non-crimp carbon fibre fabrics (NCF). The mode-I and mode-II fracture behaviour of the interleaved laminates were investigated. The experimental results demonstrated outstanding toughening performance of the hybrid veils for the mode-I fracture behaviour of the UD laminates and for both of the mode-I and mode-II fracture behaviour of the NCF laminates, resulting from the combination of different toughening mechanisms. For example, the maximum increases in the mode-I and mode-II fracture energies of the NCF laminates were observed to be 273% and 206%, respectively.

Efficient joining of hybrid thermoset/thermoplastic composite joints is critical to produce high performance lightweight structures while keeping the cost low. Herein, a high-power UV-irradiation technique was proposed to rapidly active the surfaces of PEEK and PPS composites for the following co-cure joining with epoxy composites. A single lap-shear joint test and a double cantilever beam test were used to evaluate the mechanical and fracture performance of the hybrid joints. The experimental results revealed that high structural integrity of the hybrid joints was achieved upon applying a 6 s UV-treatment to the thermoplastic composites. For example, the lap-shear strength and fracture energy of the adhesive bonded hybrid joints were above 25 MPa and 800 J/m², respectively. Overall, high-power UV-irradiation proved a highly efficient, rapid and low-cost method to treat thermoplastic composites for the co-cure joining with epoxy composites, and hence it demonstrated significant promise in industrial mass production.

A high-power UV-irradiation technique was proposed for the surface treatment of PPS and PEEK composites, aiming to achieve good adhesion with epoxy adhesives. The composite substrates were rapidly UV-irradiated for a duration of between 2–30s, and then bonded using an aerospace film adhesive to produce joints. Tensile lap-shear strength and mode-I and mode-II fracture energies of the adhesive joints were investigated. It was observed that the application of a short-time UV-irradiation to the substrates transformed the failure mode of the specimens from adhesion failure to substrate damage in all cases. This consequently resulted in remarkable improvements in the mechanical and fracture performance of the adhesive joints. For example, the lap-shear strength increased from 11.8MPa to 31.7MPa upon UV-irradiating the PPS composites for 3s, and from 8.3MPa to 37.3MPa by applying a 5s UV-irradiation to the PEEK composites. Moreover, the mode-I and mode-II fracture energies significantly increased from ∼50J/m² to ∼1500J/m² and from <300J/m² to ∼7000J/m², respectively for both of the adhesively bonded PEEK and PPS composite joints.

Carbon fibre reinforced poly-etherether-ketone (PEEK) and poly-phenylene-sulfide (PPS) composites were rapidly surface-treated by high-power UV light, and then adhesively bonded to aluminium 2024-T3 and carbon fibre/epoxy composites. The results of a single lap-shear joint test demonstrated that a UV-treatment lasting for 5 s was sufficient to prevent joint failure occurring at the composite/adhesive interfaces in all cases, e.g. it increased the failure strength of the PPS composite/aluminium joints from 11.1 MPa to 37.5 MPa. Moreover, the composite/adhesive interfaces performed well upon an exposure of the joints to an environment of high humidity and temperature for 8 weeks. Additionally, an investigation lasting for 6 months showed no degradation of the surface functionalisation from UV-irradiation. Overall, this work highlights high-power UV-irradiation a very promising method for surface preparation of thermoplastic composites (TPCs) for adhesive joining, i.e. TPC adhesive joints with excellent structural integrity can be obtained by using this rapid, eco-friendly and low-cost surface-treatment method.