This chapter discusses the mixed-mode loading of adhesive joints. The importance of mixed-mode loading is first introduced and then test methods commonly used to measure the mixed-mode fracture resistance of adhesive joints are presented and briefly discussed. The approaches to determine the fracture resistance are briefly reviewed and then the partitioning of mixed-mode fracture energies is discussed. The limitations of the local singular field and global approaches to mixed-mode partitioning are discussed and the use and application of a semianalytical cohesive zone analysis partitioning scheme is evaluated. The limitations of the global partitioning approach are further discussed in the context of developing a scheme to design and analyze adhesive joints with dissimilar adherends (a bi-material interface). A longitudinal strain criterion is proposed in addition to the matching of flexural rigidities and the approach is validated numerically. Finally, the practical issues of crack stability, failure path selection, and the use of mixed-mode failure envelopes is considered.

Interleaving thermoplastic veils has proved to enhance the interlaminar fracture toughness of carbon fibre/epoxy composites under static loading conditions. However, the fatigue delamination behaviour has yet to be investigated. Herein, meltable Polyamide-12 (PA) veils and non-meltable Polyphenylene-sulphide (PPS) veils were used for interlay toughening of unidirectional (UD) and non-crimp fabric (NCF) laminates that were manufactured using a prepreg process and resin transfer moulding process, respectively. The results of Mode-I fatigue delamination tests demonstrated a significant improvement in the fatigue life of the laminates due to interleaving. Additionally, the fatigue resistance energy has been maximumly increased by 143% and 190% for the UD and NCF laminates, respectively. The microscopy analysis revealed that the toughening mechanisms of thermoplastic veils were affected by the form of the thermoplastic veils in the laminates (melted or non-melted), the fracture mechanisms of the reference laminates and the adhesion/miscibility between the thermoplastic veils and the epoxy.

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.

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.