Joining techniques for multi-material structures are critical for increased use of lightweight materials in the automotive industry. This paper investigates the combined effect of spew geometry and load eccentricity angle on the mechanical performance of bonded single-lap joints (SLJ), employing carbon-fibre thermoplastic composite and aluminium adherends. Spew geometries - half-rounded and flat, and eccentricity angles - 0.48 degrees, 0.62 degrees, 0.78 degrees, and 0.91 degrees, considered here, are relevant to typical joints in thin-walled automotive structures. Of the geometries investigated, in the first the excess adhesive was allowed to take its "natural" half-rounded shape, while the second involved a simple additional step, suitable for high-volume production. Moreover, considering the use of dissimilar adherends and crash-durable epoxy adhesives, and limited accessibility to both adherend free ends in practical joints, the flat-spew geometry was formed at the free end of the composite adherend, resulting in SLJs with asymmetrical spew geometries. The SLJs were tested at dynamic crash loading rates (0.5 m/s and 3 m/s) as well as in-service quasi-static loading rates. The load eccentricity was found to significantly affect the half-rounded spew joint performance, particularly at the dynamic loading rates. The asymmetric spew shape significantly minimised the detrimental eccentricity effects, at all loading rates. The SLJs with flat-spew shows 92%-120% higher energy absorption under dynamic loading, relative to half-rounded spew joints. The joint deformation mechanisms and post-failure surfaces are examined. Further, finite element analysis was performed to understand the influence of the investigated spew geometries on the adhesive stress distribution. The proposed flat spew geometry achieved the lowest peel stresses and the highest shear to peel stress ratio, in addition to having low sensitivity to changes in geometry.

Joining techniques for multi-material structures are critical for increased use of lightweight materials such as aluminium alloys and thermoplastic composites in the automotive industry. Interlocking adhesive joints (IAJs) can provide increased mechanical performance over standard adhesive joints, but manufacturing methods suitable for industrial applications must be developed. Here, three methods are examined for fabricating composite adherends with recessed macroscale features. The methods differ in the way the fabric material is draped over a mould and are referred to as "simple-stacking", "moulding-in", and "fibre-cutting". The IAJs are tested under quasi-static, 0.5 m/s and 3 m/s loading rates and the fibre-cutting method achieves the best mechanical performance. One reason is that it gives a homogenous fibre distribution across the overlap width, providing good flexural properties at the recessed features. It also results in resin-rich regions along the overlap length, which lead to beneficial "snubbing" for improved interlocking, and progressive, energy-absorbing failure. The fibre-cutting method is simple to automate and well-suited for scale-up to industrial manufacturing.

This paper shows modifying binder float-length, an easily adjustable parameter, there is significant influence on impact energy absorption, impact resistance and damage tolerance in 3D-woven layer-to-layer carbon/epoxy composites. Binder float-length was changed by modifying textile design without changing loom set-up. Three float lengths (1/2, 2/2 and 3/2) in consistent architecture were woven using constant warp density. Out-of-plane drop-weight impact was performed at 32 J&42 J energy and showed increases in float-length decreased energy absorption by 49% and 32% respectively in warp direction with no significant changes in weft. Conversely, in axial impact tests, higher float length showed higher crush force efficiency and specific energy absorption. This study has also concluded, in both out-of-plane and axial impact scenarios, higher float lengths increase damage tolerance. This work has expanded how minor changes in preform parameters can significantly change both out-of-plane and in-plane impact performance of 3D-woven composites without increased manufacturing cost, time or complexity.

This study presents a novel hybrid technique for joining composites to metal, employing an array of macroscale interlocking features on the faying surfaces of adhesively bonded adherends. Single-lap, interlocking adhesive joints (IAJs) and baseline adhesive joints (BAJs), are tested at quasi-static and transient dynamic (0.5 m/s and 3 m/s) loading rates. The joint deformation mechanisms are examined and fractography analysis is performed at the macro and micro scales. Results indicate a 10% increase in lap-shear strength, and 75-120% increase in work to failure for the IAJs compared to the BAJs, at all loading rates. In addition, IAJs exhibit improved damage tolerance compared to adhesive joints, due to reduced joint rotation, more stable adhesive fracture growth, and the ability to sustain load even after cracks have propagated through the adhesive at the ends of the overlap region. The high energy absorption capacity (23-38 J) of IAJs indicates they could be used to significantly improve the crashworthiness performance of multi-material transportation structures.

"Tension-absorber" joints are bolted joints designed to absorb energy in a survivable crash landing, through an extended version of bearing failure. They have been proposed for use in future transport aircraft narrow-body composite fuselages. Herein, the influence of layup (percentage of each ply orientation), stacking sequence (exact location of each ply) and loading rate, on energy absorption is examined. Quasi-static and dynamic (3 m/s) tests are performed on pin-loaded IM7/8552 carbon-fibre/epoxy laminates. Seven layups and 11 stacking sequences are tested, with key variables being the percentage of 0 degrees plies (from 12.5% to 62.5%), the position of the 0 degrees plies, and the changes in orientation at ply interfaces. Performance measures include ultimate bearing strength (UBS), mass-specific energy absorption (SEA) and crush load efficiency (CLE). Computed tomography is used to examine damage progression in the quasi-static tests. It is found that the most important factor in maximising SEA is having small changes in orientation at ply interfaces. This is even more important than 0 degrees content. A laminate with only 12.5% 0 degrees plies, performed remarkably well due to its low changes in ply orientation. Laminates with a high SEA tend to have a low UBS. Highest UBS was for quasi-isotropic laminates. Increased loading rate results in increased UBS but decreased SEA. The results allow selection of a stacking sequence with a desired combination of UBS and SEA, and provide a valuable database for validation of composites damage models.

Innovative crashworthiness strategies are needed for future narrow-body composite fuselage aircraft due to limited crash energy absorption capability below the cargo floor. A recently-proposed approach is to use specially-designed “tension absorber” joints which absorb energy through an extended bearing failure process. To explore the design space, experimental tests are performed on pin-loaded joints in a widely-used carbon fibre/epoxy composite, with varying stacking sequence, pin diameter and laminate thickness. A bespoke rig is used to pull the pin completely through the laminate. Performance parameters include ultimate bearing strength, mean crushing stress and mass-specific energy absorption. Three-dimensional computed tomography (3D CT) and scanning electron microscopy are used to examine failure and damage. Diameter-to-thickness ratio is found to be an excellent predictor of energy absorption, with small values giving best results, provided the thickness is sufficient to avoid global bending of the specimen. The use of a well-characterised material and availability of 3D CT data enables the results to be used for validation of analysis tools.

Light-weighting of transportation structures necessitates multi-material design employing composites and aluminium, with thermoplastic composites being of increasing interest to the industry. Adhesive bonding is a viable solution for joining dissimilar materials, but joint performance can be considerably affected by surface preparation. In this paper, alumina grit-blasting is investigated as a surface preparation technique for thermoplastic-matrix composites to be bonded to aluminium alloys. Grit-blasting is performed on composite adherends for varying durations, and the resulting chemical and morphological modifications are analysed using goniometry, profilometry, scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. Adhesively-bonded single-lap joints are tested at quasi-static and dynamic (0.5 m/s) loading rates, and fractography analysis is performed at macro and micro scales. It is found that high lap shear strength and work-to-failure can be achieved through optimisation of the grit-blasting parameters. The optimised process produces a composite surface with plasticised matrix, minimal fibre exposure, and favourable surface chemistry for adhesive bonding. Grit-blasting can thus be a simple, yet effective surface preparation technique for composites to be bonded to aluminium.

This paper presents the development and validation of a mesoscale composites damage model for predicting the energy absorption capability of "tension-absorber" joints. Tension-absorber joints are composite bolted joints specially designed to absorb energy in a crash via "extended bearing failure", which involves the bolt forcing its way through the composite over a long distance. They have been proposed for use in future narrow-body composite fuselages. Here, extended bearing failure tests on a carbon fibre/epoxy laminate, are simulated using explicit three-dimensional finite element analysis. A physically based damage model is implemented in a user-defined subroutine. The model uses in-situ ply strengths, stress-based fibre failure criteria, Puck's criteria for matrix damage, a nonlinear law for in-plane shear, a cohesive zone model for delamination, a crack band model to mitigate mesh sensitivity, and frictional contact between the pin and the laminate, and between adjacent plies once they delaminate. The model is found to accurately predict the global response, in terms of bearing strength, mean crush stress and energy absorption, and comparison with CT scans shows that it also captures the mesoscale damage very well. The model is used to predict the effects of pin diameter, laminate thickness and stacking sequence, and the results show excellent agreement with experimental findings.

Tension-absorbing composite joints are mechanically-fastened joints designed to absorb energy when loaded in tension in a crash scenario, by guiding the bolt to push through and crush the laminate over a long distance. They have been developed by Airbus and DLR [1]. Layup and laminate stacking sequence plays a significant role in the performance of a composite joint [2-6]. The situation is highly complex, due to the numerous possible stacking sequences, and the various possible failure modes. A two-fold optimisation algorithm is proposed, using a three-dimensional, finite element composite damage model. First, for each candidate lay-up (i.e. a given percentage of plies in each direction), the stacking sequence (exact sequence of ply directions) is optimised. Then optimisation is performed on the candidate lay-ups. Two objective criteria are used: offset bearing strength and energy absorption. Figure 1 shows the process. The laminate thickness is kept at 2 mm and only symmetric and balanced layups are considered to reduce the size of the optimisation problem. This work is a first attempt to incorporate a high-fidelity, three-dimensional model, in an optimisation framework for bolted joints.

Carbon Fibre Reinforced Plastics (CFRP) offer excellent specific mechanical properties, making them ideal for lightweight structural applications. However, in practice it is difficult to produce a viable design concept from just one material system, so a variety of different materials are required to produce an optimum design. Joining of dissimilar materials efficiently poses a significant challenge to the implementation of multi-material design concepts. Hybrid joining processes possess the advantages of both the mechanical fastening and adhesive bonding methods. This research considers a novel, interlocked/bonded technique for joining metals to CFRPs. The technique presented herein employs interlocking bond-surface morphology formed on the faying surfaces of male (metal) and female (composite) adherends that mechanically interlock in shear when coupled with a layer of adhesive. In this study, a single-lap joint (SLJ) configuration is employed to examine the effect of the interlocking features on lap shear strength and work-to-failure. The results are compared to data obtained from standard bonded SLJs. Two adhesives were studied for bonding of the standard SLJ adherends: (i) a two-part acrylic-based adhesive and (ii) a one-part crash-toughened epoxy adhesive. The toughened epoxy exhibited preferable performance characteristics, so was chosen to bond the interlocking adhesive joints (IAJs). The baseline SLJ and IAJ were tested at two test velocities – 1 mm/min (quasi-static) and 0.5 m/s (intermediate). The quasi-static and intermediate loading rate tests were performed on an electromechanical universal test machine (UTM) and high-speed servo-hydraulic UTM, respectively. The IAJs displayed an increased performance at intermediate loading rate compared to the standard SLJs but showed a reduced performance at quasi-static loading rates.

Although carbon fibre/polyetheretherketone has been extensively characterised, literature reveals large variations in reported values of melt viscosity. Partly due to this lack of clarity, process models of carbon fibre/polyetheretherketone during automated tape placement tend to be overly simplistic, often assuming Newtonian behaviour, even though this is completely at odds with experimental data. This paper seeks to provide insight into why these wide variations exist, via rheological characterisation, utilising a novel single-ply test method to eliminate inter-ply slip. Several previously unreported and non-intuitive trends are found, e.g. shear viscosity increases with temperature, depends significantly on applied pressure, and increases substantially with time, even in an inert atmosphere. The results here, and in the literature, are explainable if carbon fibre/polyetheretherketone melt is regarded as a yield-stress fluid in which boundary-lubricated, fibre-to-fibre friction determines the viscosity at low strain rates. Additionally, shear banding can occur at low strain rates, if pressure and strain magnitude are low, significantly affecting the results obtained.

A recent rheological study of carbon-fibre-reinforced PEEK (CF/PEEK) demonstrated highly complex behaviour, involving phenomenological differences at low and high strain rates. To explain the behaviour, it was hypothesised that CF/PEEK responds as a yield-stress fluid at low strain rates, with boundary-lubricated, fibre-fibre friction determining the viscosity, and as a viscous fluid at high strain rates, with polymer melt viscosity dominating the response. In this paper, a novel finite-element methodology, incorporating fibre friction and melt viscosity in the same model, is employed to study this hypothesis. Two-fibre models investigate how fibre friction and melt viscosity combine to produce an overall composite viscosity. Representative-volume-element (RVE) models examine multi-fibre/melt response, and demonstrate that inclusion of fibre friction produces the observed yield-stress behaviour at low strain rates, and viscous behaviour at high strain rates. Another phenomenon which affects rheological measurements of such composites is shear banding in the sample, which occurs in the yield-stress regime. This effect is demonstrated in the models, and analysis of load transfer between fibres and melt explains how it arises, and how it leads to diminished values of measured viscosity. The results pave the way for improved process models for high-throughput manufacturing processes such as Automated Tape Placement.

This chapter explores the design, testing and analysis of composite bolted joints and begins with a review of literature relating to joint mechanical behavior. The fundamental influence of joint geometry and stacking sequence on the joint response is discussed along with the significant effects of bolt–hole clearance, lateral constraint and loading velocity. The large number of studies featured in the literature review is an indication of the complexity involved in optimizing composite bolted joint design. In order to handle this complexity, accurate 3D FE modeling is considered an invaluable aid in the design of composite bolted joints and is a key focus of the chapter. The use of 3D FE modeling in the design of composite bolted joints is examined in the form of two case studies. The first case study outlines the prediction of bolt–hole clearance effects in single-bolt, protruding-head and countersunk joints, using linear elastic implicit FE models. The models are shown to accurately capture the delay in load take-up and reduced joint stiffness associated with bolt–hole clearance, while layer-by-layer stress distributions provide a detailed insight into the ply loading at the bolt hole. The nature of the ply loading in countersunk joints is shown to be very different to that of protruding-head joints and in all cases, ply stresses are shown to be highly dependent on the level of clearance. The second case study focuses on the prediction of bearing failure in a single-bolt countersunk joint, using a 3D explicit FE model. A physically based damage model is implemented in a VUMAT in order to predict bearing damage and includes Puck failure criteria, a nonlinear shear law and a crack band model to mitigate mesh sensitivity. The resulting progressive damage analysis is both predictive and robust, affirming the benefits of using explicit finite element analysis which are discussed earlier in the chapter.

Fibres made from carbon nanotubes (CNTs) have not yet achieved strengths approaching that of individual CNTs. The problem is that load is not effectively transferred between the constituent, discontinuous CNTs. High energy irradiation has shown promise on small CNT bundles, in creating covalent crosslinks to enhance load transfer, but cannot sufficiently penetrate real CNT fibres, which typically contain 106 or more CNTs. Here, we suggest that the "draw-twist" process for producing CNT fibres from forests offers an opportunity for CNT bundles to be individually treated with irradiation before being twisted to form a fibre. We use molecular dynamics to examine the effectiveness of low energy (1 eV) carbon ion irradiation (or deposition) in this context. We find that very small amounts of deposition can significantly enhance both intra-bundle and inter-bundle load transfer. Within bundles, deposition atoms mediate covalent links between both the sides and ends of neighbouring CNTs. Inter-bundle load transfer is improved as under-coordinated carbon adatom branches formed during deposition, spontaneously form inter-bundle cross-links as the bundles are forced together by the twisting action. The effects of varying fluence and twisting angles are examined, and the potential to add a prior higher energy irradiation step to penetrate larger bundles is explored. The possibility to produce an amorphous carbon/CNT composite fibre is also discussed. (C) 2015 Elsevier Ltd. All rights reserved.

ABSTRACTThe use of interleaved polyethylene terephthalate (PET) veils to increase the interlaminar fracture toughness of glass fiber‐reinforced, low‐styrene emission, unsaturated polyester resin composites, was investigated. PET, being chemically similar to the unsaturated polyester resin, was expected to exhibit good wetting and strong interaction with the matrix. Composite laminates were manufactured by hand lay‐up, with the veil content varying up to 7%. The effects of PET veils on the interlaminar shear strength, flexural strength, flexural modulus, glass transition temperature, damping parameters, and Mode‐I interlaminar fracture toughness of the composite were studied. The veils were found to enhance most of these properties, with only minor negative effects on flexural stiffness and Tg. The PET/resin bonding did indeed prove to be strong, but the enhancement of fracture toughness was not as much as expected, because of the weaker glass/resin interface providing an alternative crack propagation path. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 42877.

ABSTRACTThe effectiveness of using interleaved nylon veils to increase the interlaminar toughness of glass fiber reinforced, low‐styrene emission unsaturated polyester resin composites has been investigated. Samples were manufactured by a hand lay‐up technique followed by compression moulding. Nylon 66 veils were used, with the veil content varying from 0% to 4% by weight. Double cantilever beam, short beam shear, and three point bend tests were performed. The increasing levels of nylon veil content improved the interlaminar toughness of the composites, which was characterized by critical strain energy release rate (GIC). The maximum GIC for crack propagation of a nylon interleaved composite increased by almost 170% over the baseline glass fiber reinforced composite. Dynamic Mechanical Analysis revealed an increase in the damping parameter of up to 117%. Image analysis via Digital Image Correlation and Scanning Electron Microscopy revealed increased fiber bridging between adjacent plies as a key reason for these improvements. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41462.

This paper introduces the idea of nonlocal normal modes arising in the dynamic analysis of nanoscale structures. A nonlocal finite element approach is developed for the axial vibration of nanorods, bending vibration of nanobeams and transverse vibration of nanoplates. Explicit expressions of the element mass and stiffness matrices are derived in closed-form as functions of a length-scale parameter. In general the mass matrix can be expressed as a sum of the classical local mass matrix and a nonlocal part. The nonlocal part of the mass matrix is scale-dependent and vanishes for systems with larger lengths. Classical modal analysis and perturbation method are used to understand the dynamic behaviour of discrete nonlocal systems in the light of classical local systems. The conditions for the existence of classical normal modes for undamped and damped nonlocal systems are established. Closed-form approximate expressions of nonlocal natural frequencies, modes and frequency response functions are derived. Results derived in the paper are illustrated using examples of axial and bending vibration of nanotubes and transverse vibration of graphene sheets.

A three-dimensional, implicit, finite element composites damage model is developed, and applied to a problem involving highly complex, three-dimensional loading, i.e. a single-lap, multi-bolt, composite joint, having variable clearances. The model extends a previous model, adding Puck’s criteria to allow delamination-like, transverse cracking-like or mixed mode failures to be simulated. Plasticity is also added for non-reversible strains in the undamaged parts of the matrix, and a mesh independence strategy is incorporated. A back-up strategy for when the Newton–Raphson method fails to converge, adds significantly to model robustness. The model is implemented in a commercial, implicit solver and demonstrates excellent robustness, being capable of following damage progression from onset, through bearing failure at the holes, to catastrophic, net-tension failure. The predicted failure modes and loads are in good agreement with experiment. The effect of clearance on secondary bending, and consequently on damage progression, is demonstrated. The current model shows better capability than our previous model for predicting matrix damage near the hole, which will be important for future fatigue modelling studies.

Accurate models of dynamic structural failure are important for crashworthiness studies. To date, catastrophic failure of dynamically-loaded composite bolted joints has been studied using global or stacked shell element models. In this paper, high-fidelity (three-dimensional solid) explicit FE models are used to simulate catastrophic failure of countersunk composite fuselage joints. While current state-of-the-art 3D modelling approaches focus almost exclusively on the prediction of composite damage, this study also investigates the treatment of fastener damage. Fastener fracture is a common catastrophic joint failure mode, particularly in joints designed to initially fail in bearing. A Johnson–Cook material model and cohesive elements were used to predict plasticity, damage and fracture of the titanium (Ti–6Al–4V) fastener. Although a model calibration was required, due to the complex interaction of model parameters, numerical results demonstrate key trends of experiments and provide a starting point for the development of more predictive approaches for simulating fastener failure.

This chapter presents an experimental and numerical study on the design and failure analysis of composite bolted joints. The primary geometrical variable under investigation is bolt-hole clearance, and is chosen as it induces significant three-dimensional stresses into the joint, and significantly alters the bolt-load distribution in multibolt joints, and so provides a rigorous test case. It is shown that the increases in bolt-hole clearance lead to higher stresses at the bolt-hole, and this, in turn, causes both matrix and fiber damage to occur earlier in the loading history. Hence, for limit load design, or design that requires no fiber damage, clearance should be considered an important design parameter. However, it was found that bolt-hole clearance did not have a significant effect on the ultimate failure load of joints, which was due to the extensive bearing damage that takes place in the joints, which masks any initial clearance effect and redistributes loads to other fasteners in the multibolt joints.