An updated review of adhesively bonded joints in composite materials

International Journal of Adhesion & Adhesives 72 (2017) 30–42

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International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh

An updated review of adhesively bonded joints in composite materials a

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S. Budhe , M.D. Banea , S. de Barros , L.F.M. da Silva a b

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Federal Center of Technological Education Celso Suckow da Fonseca - CEFET/RJ, Av. Maracanã, 229, 20271-110 Rio de Janeiro/RJ, Brazil Departamento de Engenharia Mecânica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal

A R T I C L E I N F O

A BS T RAC T

Keywords: Adhesively bonded joints Composite materials Damage modelling Finite element method Environmental factors

Continuing interest and more developments in recent years indicated that it would be useful to update Banea and da Silva paper entitled “Adhesively bonded joints in composite materials: an overview”. This paper presents an updated review of adhesively bonded joints in composite materials, which covers articles published from 2009 to 2016. The main parameters that affect the performance of bonded joints such as surface treatment, joint configuration, geometric and material parameters, failure mode etc. are discussed. The environmental factors such as pre-bond moisture, moisture and temperature are also discussed in detail and how they affect the durability of adhesive joints. Lots of shortcomings were resolved during the last years by developing new materials, new methods and models. However, there is still a potential to evaluate and identify the best possible combination of parameters which would give the best performance of composite bonded joints.

1. Introduction In recent years, there has been an increased use of adhesive bonding in all industries, as it is more suitable in many aspects, such as high strength to weight ratio, design flexibility, damage tolerance, fatigue resistance etc. over conventional joining methods. In fact, the adhesive bonding has found application in various sectors such as aeronautics, electronics, automobile, sports, marine, oil and even construction industries, etc. The application of adhesively bonded joints for the composite repair of a damaged structure has been also increased in all sectors mentioned above. Banea and da Silva [1] presented a comprehensive review on the adhesively bonded joints in composite materials in 2009. Since then, a number of researchers have been studied the performance of adhesively bonded composite joints, with the development of new advanced adhesive materials and increased focus on damage modelling (i.e. Cohesive Zone Modelling), among others. Along with this, the environmental factors have been also studied by many researchers, as it is highly demanded by the industries. In recent years, some new review papers [2–18] and several handbooks [19–22] were published on this subject. The review papers focused on the bonded repair of composite structures [2,3], adhesive materials [4–7], fatigue strength [8,9], environmental durability [10–14], analytical methods [15,16] and finite element method [17,18]. The present review paper summarizes the recent developments concerning the main parameters (i.e. surface preparation, joint configuration, material parameters, geometrical parameters, failure mode, ⁎

Corresponding author. E-mail address: [email protected] (M.D. Banea).

http://dx.doi.org/10.1016/j.ijadhadh.2016.10.010 Accepted 7 October 2016 Available online 15 October 2016 0143-7496/ © 2016 Elsevier Ltd. All rights reserved.

etc.) that affect the performance of composite bonded joints. Also, recently developed new analytical models and finite element methods (FEM) are discussed. Some new topics such as manufacturing methods of composite bonded joints and pre-bond moisture are added in a separate section in accordance to their importance to bonded joint performance. Finally, future trends on adhesively bonded joints are included in the conclusions section. 2. Parameters that affect the performance of the bonded joints The performance of adhesively bonded joints depends on many parameters such as composite bonding methods, surface preparation, material parameters (adhesive and adherend properties), geometrical parameters (adhesive thickness, overlap length, stacking sequence, ply angle, fillet etc.) [23–32]. All these parameters must be taken into account during the design of bonded joints for a good performance of the bonded structure. These parameters will be discussed in the next subsections. 2.1. Manufacturing bonding process The bonding process is one of the aspects to take into account when manufacturing a bonded joint. Most of the parameters such as the failure process, the failure mode and the joint strength are all influenced by the bonding process [33–37]. There are mainly three manufacturing bonding processes, namely co-curing, co-bonding and

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Fig. 1. Schematic of the common manufacturing bonding processes between composite components.

2.2. Surface preparation

secondary bonding method to manufacture the bonded joints between composite substrates. A scheme of each of these manufacturing processes is shown in Fig. 1. A co-bonding process is performed when one adherend is cured with the adhesive, while in the co-curing process both parts are simultaneously cured. The secondary bonding is when the adhesive layer is cured between two pre-cured composite panels (substrates). The multi-material bonding method is same as the secondary bonding except with a combination of metal (e.g steel, Al, Ti etc.) and composite substrates instead of only cured composite substrates. The co-curing or co-bonding is usually preferred over the secondary bonding because the number of parts and curing cycles are reduced. Hence, this method is the most commonly used for the repair of composite structures. However, for large and complex structures, the secondary bonding process is more suitable. Mohan et al. [35,36] investigated the influence of the bonding method between the co-cured joint and the secondary bonded joint method under mode I and mixed-mode loading. They found that the co-cured bonded joints showed lower strength than the secondary bonded joints in both loading conditions. Song et al. [25] also noticed the same trend in strength (co-cured without adhesive > secondary bonded > co-cured with adhesive > co-bonded) of single lap joints. Moisture present in prepreg was released during curing and spread in the adhesive layer which lead to weaken the interface and caused a lower strength of the co-cured joints [35,36,38]. It should be noted that moisture content before bonding is an important parameter and along with other factors such as cure temperature and adhesive material should be taken into account during fabrication of bonded composite joints. Therefore, a proper selection of the manufacturing bonding method is very important, especially in the case of composite repair method, as the parent material (damaged) already contains moisture and suffered other changes during the service period. Nowadays, there is a growing interest to optimize the strength, weight and durability of structures by combining traditional metals with composite materials. For instance, composites are structurally more efficient than metals, but metals have better damage tolerance and failure predictability than composites and are unaffected by solvents and temperature which tend to degrade polymers. Therefore, in order to optimize the benefits provided by both types of materials, multi-material joints between metals and composite materials are increasingly being developed. The effect of multi-material bonding process on joint strength was investigated by several researchers [37,39,40] and a positive influence on the performance of bonded structures was found. Therefore, smart combination of materials might be the key to develop lighter structures.

Surface preparation also plays an important role and it is directly related to the quality of the bonded joint. In order to get a strong and durable joint, a surface treatment of adherends should ensure the following aspects: removal of all contaminants (lubricants, dusts, loose corrosion layers, micro-organisms) from the surfaces, good surface wettability, surface energy, good activation of material surfaces being bonded etc [23,26,41–44]. There are different chemical and physical surface treatments available and a proper selection of surface treatment is very important [42,45–47]. Primary and minimum surface pre-treatment is necessary prior to adhesive bonding to provide a clean and preferably active surface. State of the art in achieving this is by either using peel ply technique or different mechanical treatments [43,48,49]. Peel ply is one of the techniques which can protect the surface from the contamination (dry,dust, moisture) and also create and maintain the specific surface texture [26,50,51]. Fig. 2 shows a SEM image of a typical surface morphology (texture) of composite laminate after peel ply treatment [52]. It was noticed that the composite matrix resin interacts with the peel ply material and that interphase or residues of the peel ply are left on the composite surface. Also, in terms of shear strength or fracture toughness, the strength of a joint after the peel ply treatment is hardly ever as high as for a joint pre-treated by an abrasive mechanical treatment [50]. Because of these issues, a careful selection of peel ply products and peel ply removal techniques are necessary for the successful adhesive bonding. Although peel plies have been widely studied in the existing literature, there are only limited studies concerning the durability of peel ply pre-treated joints. It was shown that surface treatment of the substrate prior to adhesive bonding plays an important role in enhancing the durability of the bonded joints [8,53–56]. For example, a different trend of fracture energy was observed with respect to the different surface treatment over the exposure time [10,57–59]. To ensure an environmentally durable interface, it is important to perform an additional surface preparation in combination with the primary treatments. For example, the durability of bonded CFRP/steel joints increased by using a silane coupling agent in addition of grit blasting treatment [10]. A careful selection of the surface preparation method with respect to the substrate material is needed, as some methods may degrade their properties. Azari et al. [53] investigated the effect of surface roughness on the fatigue and fracture behavior of a toughened epoxy adhesive system. A significant dependency on surface roughness was observed, in case of mixed mode fatigue test, but not for the mode I fatigue loading. Therefore, it is prime importance to focus on this issue in 31

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a) Adherend Surface at 50x

b) Adherend Surface at 200x

Fig. 2. Images of a composite adherent surface after removing a peel ply [52].

order to improve the fatigue life of adhesive joints, as most of the structures are subjected to dynamic loading and fail at very low static strength.

2.3. Geometric parameters 2.3.1. Bondline thickness Many researchers studied the effect of adhesive thickness on bonded joints using analytical models [27,60], finite element methods (FEMs) [61,62] and experimental methods [63–66]. The effect of the adhesive thickness was investigated by studying the adhesive bonded joint fracture under mode-I using double-cantilever beam (DCB) joints [65,67–69], tapered double-cantilever beam (TDCB) joints [65,70] and butt joints [71,72]. Mode II has also been studied using the endnotched flexure joints [65,68,73], while mixed mode loading was assessed through single lap joints (SLJ) [60,72,74,75]. For instance, for SLJs, it was shown that strength decreases as the adhesive thickness increases [76], with the exception of SLJs bonded with elastomeric adhesives [77,78]. The reduction in joint strength was attributed to the fact that thicker bondline contain more defects such as voids, microcracks and higher interface stresses [60,69,72,79]. Liao et al. [27] found that the fracture energy of the joint increases as the adhesive thickness increases for a ductile adhesive but for a brittle adhesive, it's reverse. Also, numerical results supported that the ductility of the adhesive increases as the adhesive thickness is increased [62]. Moreover, there is no any generalized trend between strength and adhesive thickness and these mixed behaviour may be attributed to various factors such as the type of loading (mode I, mode II, or mixed), the adherend behavior (elastic or plastic), type of adhesive (ductile or brittle), geometry of joints etc. which can modify the behavior of bonded joints as their thickness is varied [27,67,72,80]. Consequently, interpreting variations in properties can be quite complex. For example, in case of DCB joints, it appears that the toughness increases with the adhesive thickness and reaches a maximum value at some certain value of thickness (i.e. between 1 and 1.5 mm in [68] or between 1 and 2 mm in [65]) above which the fracture toughness reaches the bulk adhesive fracture energy. On the other hand, for composite bonded joints, it's difficult to maintain the bondline thickness constant. This is often achieved by embedding a textile membrane in the adhesive film (i.e. carrier). Fig. 3 [81] shows a SEM image of failure surface of composite bonded joint, where the carrier fiber and the adhesive layer can be observed. The carrier is used to control the bondline thickness and the adhesive bleeding during the curing phase. However, in co-bonding or co-curing processes involving woven fabrics, an uniform adhesive thickness is difficult to achieve due to the geometry of the fabric itself, resulting in fairly significant differences in thickness as shown in Fig. 4. In summary, it is important to consider the adhesive properties, geometrical parameters and also loading type for optimizing the

Fig. 3. SEM image of the fractured specimen to show the carrier fiber in adhesive film to support the bond line thickness [81].

adhesive thickness. Most of the work was performed under mode I and mode II loading conditions and there are limited research papers dealing with the effect of bondline thickness under mixed-mode loading. Most of the practical joints are loaded under mixed-mode conditions and this represents a significant gap in the literature. 2.3.2. Joint configuration The joint should be designed in such a way that the bonded area equally shares the stress. The joint configuration that produces local stress concentrations, high peel stresses and interfacial stresses should be avoided because it leads to premature failure of joints. A wide variety of joint configurations are used in practices and most of them, have been discussed in the previous review paper [1]. The joint strength is affected by the stress concentration at the end of overlap. To reduce the stress concentration many researchers [82– 86] have modified the joints geometry. For example, Kishore et al. [86] modified a SLJ into a Flat Joggle Flat joint (FJF) (Fig. 5) which overcome the eccentricity by the presence of joggle so that the loads remain in-plane and also avoid the bending effect. A 90% increase in load of FJF bonded joints compared to the flat joints was found. Many researchers [84,85,87–90] proposed several techniques for reducing the peel and interfacial stresses (i.e. spew fillet, adhesive thickness, mixed adhesive, tapered plate with different end, different thickness, adherend width, tapered length and thickness, etc). The combination of taper end plate and bi-adhesive in reinforced structures is more beneficial and not only reduces the interfacial stresses but also eliminate the singularities [87]. Fig. 6, demonstrate the beneficial effect of a taper end plate and bi-adhesive in lowering the stress concentration at the end of overlap. In bi-adhesive, the joint strength can be optimized by appropriate geometric (L1/L2) and material property (E1/E2) selection. The results demonstrate the possibility of using both (E1/E2) and (L1/L2) as variable parameters to modify the maximum stresses in a joint with two adhesives. The authors [84,89] suggested an internal taper and an adhesive fillet to reduce the peel stresses. However, manufacturing is a difficult task. Thus, instead of applying 32

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Fig. 4. Variation of adhesive thickness over the length of composite bonded joints (a co-bonding process involving woven fabrics) [81].

the taper along the entire adhesive layer, applying it to a smaller portion was found to be more efficient. Stacking sequence and fiber orientation of the different plies, has also a significant effect on the performance of the joints. Stacking sequence of the composite substrate affect the failure mode [8], load carrying capacity [91,92], and fatigue strength [24] of the bonded joints. Stress concentration of the composite bonded joints can be reduced by an appropriate stacking sequence of the composite substrate [91]. It is thus important to have a proper stacking sequence of composite substrate, as it directly relates to the thickness of the substrate and both should be optimized with respect to joint performance and economically. Overall, all the geometrical parameters (i.e. adherend thickness, width, adhesive thickness, tapered length, tapered thickness, stacking sequence, ply angle, fillet etc.) have an effect on bonded joints performance. Therefore, it's necessary to optimize these parameters for maximum strength of joints provided its feasibility. 2.3.3. Overlap length Increasing the overlap length increases the joint strength up to a certain limit [63,88,92–94]. However, the increment rate depends on the adhesive material [84,88], adherend material [95] and the type of loading [96]. In a study conducted by Neto et al. [84] an increment in failure load proportionally with the overlap length for the ductile adhesive was found, while for the brittle adhesive studied the failure load increased up to a certain extent and then remained constant. Failure mode is also influenced by the overlap length. Cohesive failure mode was observed when the overlap length was in the range of 10– 20 mm and after that an interlaminar failure mode occurred [84]. Actually, the ideal overlap length depends on the pairs of adhesiveadherends.

Fig. 6. Shear stress distribution in the CFRP tapered plate-strengthened steel beam with mixed adhesive joints (bi-adhesive) under UDL load [87].

process. The functional additive approach such as chemical foaming agents and thermally expandable particles (TEPs) has generally received more attention because they can be introduced easily into existing adhesive products [98,102]. Pressure sensitive adhesives are also one of the options for the bonding (pressure is applied to attach the substrate and the adhesive) and the debonding adhesive can be detached without leaving a trace [103]. A smart adhesive that can bond, re-bond, and/or de-bond open up new exciting opportunities of development in this field and offer a promising potential in the future. Furthermore, there are many recent developments in smart adhesive materials such as self-healing adhesive material, dis-bond adhesive materials, among others, as presented by Banea et al. [5] in a recent review paper. Smart adhesive material with self-healing properties can improve the durability of the structures and also are more economical when compared to repair of damaged structure. The dicyclopentadiene (DCPD) Grubbs catalyst self-healing system was incorporated into the bulk matrices of epoxy, fiber-reinforced epoxy composites, and epoxy vinyl ester [104–106]. It was shown that the addition of the microcapsules increases the fracture toughness of the polymer [105,107]. An alternative self-healing system contains epoxy as the encapsulated healing agent [108,109]. Self-healing can also be achieved with dual microcapsule epoxy amine chemistry in thermoset epoxy [110]. However, the research in the application of self-healing materials to adhesive joints is in the initial stages and still there are many technical challenges to incorporate the self-healing concepts in the adhesive of the bonded joints. Another trend is the development of crash resistant adhesives with

2.4. Material parameters 2.4.1. Adhesive properties Most of the industries are demanding new adhesive materials with advanced properties which should satisfy the required conditions for a specific application. In order to meet the necessary requirements, new adhesive materials are developed with more advanced properties. Although, there are numerous adhesive materials available in the market, selection of adhesive materials for a specific application is not an easy task as it depends on many factors (i.e. adherend type to be bonded, curing temperature, expected environmental condition during service, type of load, cost etc.). Nowadays, the extensive application of adhesive bonding in the industry is a positive step, but reuse, recycling and recovery of bonded parts are the major concerns mainly because of environmental issues. To overcome this, the adhesive bonding should easily disbond without damaging the structure. There are several research papers [97–101] on incorporating certain additives, or agents to trigger the debonding

Fig. 5. Flat joggle flat joint [86].

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high toughness, high impact resistance and more deformation than the conventional epoxies. These adhesives are particularly relevant for the automotive industry [19]. There are some research on enhancing toughness without a major decrease in strength by adding polyurethane and/or rubber particles to epoxy adhesives [111–113]. Several authors [7,114–119] provide studies on the use of reinforcement particles and methods to improve the toughness and other properties of structural adhesives. Currently, there are serious concerns related to the environmental issues and safety, as synthetic adhesives are toxic in nature (pollution), nonrenewable, difficult to debond, expensive etc [120,121]. In order to combat these issues, natural adhesives (bio-adhesives) represent a better alternative which can satisfy different issues such as easy release, reusability, no harm to nature etc [122]. and hence more researchers are inclined towards it. In recent years, there are plenty of natural adhesive materials developed, such as: water based adhesive [123,124], natural rubber [125,126] and modified soybean-flour (MSF) adhesives [127], among others. However, these adhesives are used mainly for the wood composite materials and not for metal and/or CFRP. Therefore, still there is an open question about application of natural adhesive materials to adhesive bonding of fibre composite materials. There are several ASTM and ISO standard test methods available to determine the adhesive material properties. The standard fracture mechanic tests of adhesively bonded joints until 2013 are covered in a recent review paper [9]. However, there are still some shortcomings to be overpassed. For example, in fatigue test standard, monitoring the crack growth is still under developed and represents a challenging issue. Brunner et al. [128] used three techniques (visual, compliance based approach and an effective delamination length) for the crack growth monitoring under fatigue mode I. However, further developments are needed to incorporate automated data acquisition and analysis. Same authors [129] continued this work for the mode II fatigue test. For instance, an automated procedure was proposed for mode II fatigue delamination test based on real time monitoring of the specimens compliance [130]. Recently, Chaves et al. [131] proposed a new apparatus and method for measuring the toughness of adhesive joints under mixed mode loading and this method is independent of crack length measurement (the displacement obtained from the Linear Variable Differential Transformer (LVDT)). Although standard methods are well developed for the static and fatigue tests under different mode loading condition, advanced techniques (process and apparatus) are still needed for fatigue test under mixed mode for example, especially monitoring the crack growth during real fatigue test.

Fig. 7. Materials used in a co-bonded adhesive joint with removal of wet peel-ply.

2.5. Failure mode According to AITM1-0053 [136] standard there are six failure modes. Fig. 7 shows the schema of typical layups of co-bonded joints with a pre-cured wet peel-ply. In this case the resin layer is introduced when the wet peel ply is removed from the pre-cured panel. Therefore, 6 failure modes must be taken into account for the co-bonded adhesive joints; they are detailed in the internal testing protocol AITM1-0053 from Airbus [136]. The failure modes are: i) inside the pre-cured adherent (delamination); ii) at the interface inter adherent-fibre/peel ply resin; iii) at the peel-ply adhesive interface; iv) cohesive inside the adhesive; v) at the adhesive co-bonded panel interface (wet-wet interphase) and; vi) inside the co-bonded panel (delamination). Many researchers [35,80,81,84,137,138] have experimentally and numerically investigated the parameters that have an effect on the failure mode of adhesive bonded joints. It has been observed that the failure mode and the strength of the joints are interlinked and influenced by different parameters such as bonding methods, moisture, temperature, type of adhesive, surface preparation, geometrical parameters etc. Temperature and moisture have a high influence on the failure mode of the bonded joint compared to the other parameters. It is well known, that at lower temperature a brittle fracture occurs [137,139– 141], whereas at high temperatures there is a ductile crack growth [137,139,140]. Several authors [81,138] observed the changes in failure mode which depends on the moisture percentage content in the joints. The specimen failed in a different fashion, even though the material and other parameters were the same, only because the joints were manufactured with different bonding methods such as co-bonded, co-cured and secondary bonding [35,38]. Thus, understanding of the failure mode is needed for an optimum design of adhesively bonded joints, as it depend on a number of parameters (i.e. bonding methods, moisture, temperature, type of adhesive, surface preparation, geometrical parameters etc).

2.4.2. Adherend material Special attention should be taken for the selection of the adherend materials, as different materials behave differently and affect the final performance of the joints. A significant difference in strength was observed, in the case of joints bonded with the different adherend material, under the same conditions [92,132–135]. For example, the freeze-thaw cycle showed a negligible effect on the tensile properties of both Glass Fiber Reinforced Polymer (GFRP) and Boron Fiber Reinforced Polymer (BFRP) but exhibited an adverse effect on Carbon Fiber Reinforced Polymer (CFRP) [135]. This highlights the importance of the compatibility of fibres and resin and their crucial role in the long-term performance of composite materials. In addition, the multi-material adherend bonding method is increasingly being used because of their advantages compared to similar adherend joints. However, there should be a good compatibility between the adherend and adhesive material in both chemical and mechanical aspects. In depth understanding of the fracture behavior of adhesively bonded joints is definitely needed in order to fully achieve the benefits of adhesive bonding of different adherend materials.

3. Environmental factors The main environmental threats are related to the effect of temperature and moisture absorption (humidity) which can affect the strength and durability of the joints. Fig. 8 shows the main factors, such as: moisture, temperature, fire, UV (ultra violet) radiation, etc. that influence the durability of adhesively bonded joints. Only moisture and temperature are discussed in details in the following subsections, as they are considered the most important. 3.1. Pre-bond moisture Pre-bond moisture issue is very important for joints formed between polymeric-composite substrates as it directly influence on the performance of adhesive joints. Pre-bond moisture study investigates the effect of moisture content in the substrate before bonding on the mechanical performance of the joints. There are different ways by 34

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Fig. 8. Diagram of environmental parameters influencing the durability of adhesively bonded joints.

exposure condition the ductility is lost along with the elastic modulus [148]. It was suggested that water extract plasticise the resin and, as a result, adhesive ductility is lost after cyclic moisture exposure [148,149]. These observations emphasize the need for the characterization of the material properties of adhesives and resins based on their moisture uptake history. The stability of interfacial adhesion in the presence of moisture is one of the most important factors in the long-term durability of adhesive joints. A review of the available literature reveals that the degradation of the interface is often found to be significantly larger than that of the adhesive [30,81,148]. Furthermore, it is well accepted that, upon moisture penetration, the failure locus, almost always switches from cohesive within the adhesive to at, or near, the interfaces [81,148]. The transport of moisture to the interface of the adherend may lead to irreversible changes as a result of weakening of intermolecular adhesive forces. In general, moisture can potentially attack fiber reinforced plastic (FRP) composites by one or a combination of the following mechanisms: (i) altering the resin matrix; (ii) damaging the fibre/matrix interface; (iii) fibre-level degradation. The majority of researchers [150–153] found a reduction of mechanical properties of composite material in the presence of moisture. Moisture uptake mainly affects the properties of the matrix which result in the matrix swelling, causing stresses large enough to pull the matrix away from the fiber and damage the fibre matrix interface. Hence, the resin-dominated properties of FRPs, such as interlaminar shear strength, are more susceptible to moisture-induced degradation than the fibre-dominated properties, such as tensile strength [150]. However, a direct correlation between moisture content and the degradation of the properties cannot be identified because of different damaging mechanisms at the fibre/ matrix interface for each FRP composite. In addition, the fibre orientation and type and the resin matrix are other influential factors. Recently, a lot of research has been carried out on the environmental conditions and most of the researchers [38,132,153,154,1376] reported the reduction of bonded joint strength in the presence of moisture or ageing effect. The strength reduction rate depend on the exposure time, exposure environmental conditions and also adherendadhesive material combination of bonded joints (see Fig. 9). The possible reasons for the decrement in strength are adhesive plasticization and weak interfacial failure [38]. Drying is the best suited treatment to recover the strength. However, the full strength recovery was not achieved [81]. The strength recovery was attributed to the reverse effect of plasticization while the irrecoverable strength was attributed to the irreversible disruption at the interface due to the effect of moisture [81,148,149,155]. In such cases, the failure of conditioned joints usually remains cohesive in the adhesive layer, but shifts to an area closer to the interface [81,148]. It should be noted that the proper drying process is necessary along with the careful selection of adhesive and composite. Despite numerous studies reporting on the loss of strength in

which the substrates absorb moisture before bonding such as: during the manufacturing process, (CFRP panel undergoes several treatments procedure like wet abrasion, water break test, transportation of CFRP panel from one place to another when it may be exposed to moisture (atmosphere)), storing the laminate for a long period of time and during manufacturing bonded joints, etc. The pre-bond moisture scenario is also plausible in the implementation of adhesively bonded composite repairs since the repair surface was possibly subjected to moisture conditions during the service period. Despite the structural design procedures take into account the detrimental effect of moisture on the mechanical properties of CFRP, its effect on composite structure is not well documented/developed. Few studies [34,38,81,142–144] were reported on the pre-bond moisture effect on the mechanical properties of the bonded joints and most of them found that the presence of moisture in the composite lead to reduction in joint strength. Budhe et al. and Markatos et al. [34,81,143] attributed the decrease in mode I fracture toughness (GIC) to voiding, plasticization of the adhesive and reduction in interfacial adhesion. Extending the drying time of the substrate with pre-bond moisture cause an improvement in the fracture toughness of the joint although not full recovery [52]. A small percentage of prebond moisture appears to have a positive or no/little influence on the fracture toughness [81,142,143]. Matrix ductility and toughness of the adhesive are the possible reason for the improvement in the fracture toughness of the bonded joints. However, the presence of pre-bond moisture in the substrates produced a remarkable change in the failure mode, as cohesive failure changed to interface failure and also multiple crack failure occurred in the presence of pre-bond moisture [34,81,143]. Nevertheless, it is necessary to have more experimental studies in order to justify or set the proper process parameter which link up the relationship between pre-bond moisture and bonded joint strength. Drying temperature and time, both should be well prescribed keeping in mind the blistering problem and repair time, respectively, which led to the maximum performance of the joint. Study on the effect of prebond moisture on the fatigue behavior is still not well developed. Thus, this is one of the areas that need further attention. 3.2. Post bond moisture The moisture uptake by a composite structure during the service period depends on several parameters, such as: adhesive material, adherend material, bonding method, exposure condition and time, curing temperature etc [30,35,118,131,145–147]. It is well known that the adhesive and the resin matrix are the most affected by moisture in composite bonded structures. In general, moisture can change the adhesive through plasticization, swelling, increase of cracks, hydrolysis and by dropping its glass transition temperature. On the other hand, moisture is known to increase the ductility and reduce the elastic modulus and strength of resin/adhesive, but in cyclic moisture 35

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Fig. 9. Maximum load for different environments and exposure time (DW=distilled water at room temperature, SW=sea water at room temperature) [132]. Fig. 10. Fracture toughness vs. temperature for carbon/epoxy composite bonded joints [140].

adhesively bonded joints in the presence of moisture conditions, some researchers found a negligible effect [132,156,157], while others noticed a positive influence in strength with low moisture content in joints [38,140,158]. The increase in matrix ductility, stress relaxation within the adhesive matrix [140] and adhesive toughening [158] are the possible causes of strength increment. The available studies focusing on the effect of moisture on the mechanical behaviour of adhesively bonded joints still have considerable differences in terms of the adherend-adhesive materials used and the manufacturing bonding methods, ageing conditions and specimen configurations. Thus, it is important to consider these parameters in the presence of different moisture condition for design and repair of a specific adhesively bonded composite structure.

quently, the superior performance of tested CFRP specimen coupons. Fluctuating temperatures can also lead to the progressive debonding and weakening of the materials and the fibre/matrix interface [169]. This phenomenon is mainly due to the discrepancy in the thermal expansion coefficients of fibres and resin. Sub-zero and extremely low temperatures can cause FRP matrix embrittlement, matrix hardening, matrix micro-cracking and fibre/ matrix bond degradation [135,170,171]. This behaviour is caused by changes in the FRP constituents at low temperatures, or the incompatibility of CTE between fibres and resins. This highlights the importance of the compatibility of fibres and resin and its crucial role in the longterm performance of composite materials subjected to cyclic freeze/ thaw loading. Temperature affects the fracture behaviour of adhesive joints and it has generally been found that the mode I toughness, GIc, increases with increasing temperature and slight decrease below room temperature [133,139,140,172,173]. Fig. 10 shows the general trend of fracture toughness of adhesive joints with respect to the test temperature. The most common explanation for the increment is matrix ductility, increase in fiber bridging and fiber breakage [133,140,173]. The brittle behaviour of the matrix at low temperature results in lower GIc [173]. If the test temperature is above the glass transition temperature (Tg), the fracture toughness decreases due to the loss of adhesion between the fibers and the matrix, but below the Tg an increase in GIc was observed [138,140,174]. Fracture toughness under mode II, GIIc has most commonly observed to decrease with increasing temperature [140,175,176]. This behaviour was attributed to a reduced toughness of the fiber/matrix interface with increasing temperature [140]. Few mixed-mode results are available and follow the mode I and mode II trends found in literature [139,177]. However, mixed mode behaviour under the influence of temperature is still not well studied in the literature and need further attention. Nowadays, new adhesive materials are continuously developed, thus it is important to have their long term performance when subjected to various temperatures. On the other hand, the design of the bonded repair also must consider the temperature changes that the component will experience during the service period of the composite structures.

3.3. Temperature In the last years, there has been a growing requirement, particularly in the aerospace industry, for the adhesives to withstand high temperatures and to maintain their structural integrity. The most significant factors that determine the strength of an adhesive joint when used over a wide temperature range are: the cure shrinkage, the coefficients of thermal expansion (CTE), (especially when compared to the CTE of the substrates), and different adhesive mechanical properties with temperature [10,159–161]. However, due to the polymeric nature of adhesives, the most important factor to consider when designing a bonded joint is the variation of adhesive mechanical properties with temperature such as the stress-strain curve and the toughness. Studies that present experimental results of adhesive joints with structural adhesives (especially epoxies) as a function of temperature generally show a decrease in strength with increasing and decreasing temperatures [77,161–164]. At high temperatures, the cause is the low adhesive strength while at low temperatures the high thermal stresses and the brittleness of the adhesive is the origin of such behaviour. It was shown that the temperature exposure below glass transition temperature is in fact advantageous for FRP composites and adhesives as a result of further post-curing [165–167]. At higher temperature the softening of the resin matrix and adhesives take place and cause an increase in the viscoelastic response, but the fibres do not undergo any degradation [166]. The deleterious effects of these exposure conditions are, to a large extent, limited to the resin matrix. As a result, the degradation of matrix-dependent properties such as interlaminar shear strength is generally reduced at orders of magnitude larger than that of fibres. Di Ludovico et al. [168] replaced the conventional resin matrix with an innovative epoxy which resulted in a higher Tg and, subse-

3.4. Combined moisture and temperature effects The combined effect of moisture and temperature (hydrothermal) conditions is more damaging than the adverse effect of each individual 36

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tional method was used for the determination of the interfacial stresses in a single-sided strap joint subjected to mechanical and thermal loads [187]. Jailai et al. [188] proposed a novel three-parameter, elastic foundation model to study the interface stresses of adhesively bonded joints taking into account the zero shear stress boundary conditions at free edge of adhesive layer which was ignored in classical twoparameter elastic foundation model. This model does not satisfy the zero-shear-stress boundary conditions at the free edges of the adhesive layer, which violates the equilibrium condition of the adhesive layer. But, three-parameter elastic foundation model allows the peel stresses along the two adherend/adhesive interfaces of the joint to be different, and therefore, satisfies the equilibrium condition of the adhesive layer. Compared to results based on FEM and other analytical methods presented in the literature, this variational method is capable of predicting highly accurate interfacial stresses. A number of geometrical parameters influence the final performance by reducing the peel stress and interfacial stress, etc. Furthermore, it is more interesting to account these geometrical parameters (adhesive thickness, loading and boundary condition, tapered length, tapered angle, overlap length etc.) in an analytical model. Using the analytical model, one can get fast results and calculated results can be updated very quickly to any changes in dimension, size and material properties.

condition [30,33,138,158,178–181]. However, limited research was carried out on the mechanical behaviour of composite laminates and adhesives of adhesively bonded structures after exposure to humidity and temperature. The moisture absorption sensitivity is increased at high temperature, which lead to more damaged structure. A drastic drop of both elastic modulus and tensile strength was observed when the adhesive was subjected to elevated temperatures, whereas the plasticity characteristic becomes notable [178]. The specimens that were hydro-thermally aged, subsequently dried, and then tested did not exhibit complete recovery of unaged apparent shear strengths, indicating some irreversible changes. Also, the shear strength decreased and the failure mode progressed from cohesive (within the adhesive layer) to fiber tear compared to those of the control specimens [138]. To summarize, the individual effect of moisture and temperature on the mechanical properties of adhesive material and joints is well understood, but there is still a lack of systematic ageing conditions to clearly identify the combined effect of each environmental parameter. Further research is needed in order to conduct durability tests at both material and joint level, with minimum variants each time. 4. Analysis of adhesively bonded joints Analysis of adhesively bonded joints can be carried out by analytical methods and finite element methods (FEM). Analytical methods analyse the joints easily, fast and with high accuracy, but certain assumptions are necessary for complex joints and that might limit the accuracy of the results. On the other hand, the FEM has the capability to analyze complex geometries, complex material model and without introducing any assumption, only the computing time is the constraint. In the following subsections, the recent developments in analytical methods and FEMs are discussed.

4.2. FEMs The mechanical behaviour of adhesively bonded joints is not only influenced by the geometry of the joints but also by material properties and different boundary conditions. The increasing complex joint geometry and its three-dimensional nature combine to increase the difficulty to get an overall system of governing equations for predicting the mechanical properties of the adhesively bonded joints. In addition, the material non-linearity due to plastic behaviour is also difficult to incorporate in models for the reason that the analysis becomes very complex in the mathematical formulation. However, the experiments are often time consuming and costly. Thus, numerical solutions derived from FEMs are preferable. Several methodologies can be found in the literature for the finite element analysis (FEA) of adhesive joints. Recently, da Silva and Campilho et al. [189] covered in a book the advancement in numerical modelling of adhesive bonded joints to date. In recent years, most of the developments took place in damage modelling. Therefore, this paper will focus only on this part.

4.1. Analytical methods An analytical tool is useful for the preliminary design purposes of bonded joints, which reduces the costly tests and analysis time for the joints. Banea and da Silva [1] presented the analytical methods and their improvements and limitations. Also, several researchers [15–18] provided reviews of analytical and numerical investigations concerning the analysis of adhesively bonded joints. However, there are some new analytical solutions proposed in recent years, which will be briefly discussed here. Yousefsani et al. [182] proposed an analytical solution for adhesively bonded composites single-lap joints with thick and thin adhesive layers using the full layerwise theory. Each SLJ is divided into N numerical sub-layers through the thickness. For each region, a set of equilibrium equations can be obtained using the principle of minimum total potential energy and three-dimensional elasticity equations. The results presented were very accurate in comparison with other methods. Same author [183] continue the application of this theory to determine interlaminar stresses through the adhesive thickness and along the bondlines of single and double-lap composite joints. Semilayerwise approach was used to model double-lap joints with good results [184]. Icardi et al. [185] developed a refined three-dimensional zig-zag plate model with hierarchic representation of displacements across the thickness that accurately captured interlaminar stresses directly from constitutive equations with a low computational effort. It is well known that the loading and boundary conditions can significantly affect the interfacial stress distributions in the adhesive layer. Still, there are no available practical experimental methods to determine the interfacial stress distributions. Tahani et al. [186] proposed a new analytical solution for adhesively bonded composite single-lap joints with different boundary and loading conditions using the energy method and Timoshenko's beam theory. Shear stress distribution experience significant changes through the thickness of adhesive layer, particularly near the end points. Stress-function varia-

4.2.1. Damage modelling In recent years, most of the research carried out was on the damage mechanics approach, as it gives the most accurate failure prediction results. This approach permits to consider a complete damage behaviour from crack initiation to failure. Banea and da Silva [1] presented the progress on damage modelling until 2009, but in the last years there were more new models developed. An important feature of cohesive zone modelling (CZM) is that it can be easily incorporated into the conventional finite element softwares to model the fracture behaviour in various materials, including adhesively bonded joints. However, the toughest challenges of employing CZM are to assign the cohesive parameters, undefined crack path, mesh convergence, etc [190–194]. The traditional cohesive-zone model considers the adhesive layer as zero thickness, thus CZM approach cannot predict adhesive failure and mixed failure mode also. Extending the theory of CZM, several authors [195–197] proposed an atomistic-based process zone model to simulate fracture and fragmentation of polycrystalline solids at mesoscale, which considers the material interphase, such as grain boundary or persistent slip bands, as a finite thickness zone. In a subsequent study, Rein et al. [198] proposed an adhesive process zone model (APZM), in which adhesive layer was considered as a 3D solid material. Consequently, adhesive failures, cohesive failure, as well as the mixed 37

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Fig. 11. Modelling approach used to analyze bonded composite joints [199].

tional FE, a much more accurate prediction is achieved, since different shapes can be developed for the cohesive laws, depending on the nature of the material or interface to be simulated. The determination of the cohesive parameters and selection of cohesive laws is very important as it depends on a number of parameters such as the adhesive material, whether ductile or brittle, moisture and temperature etc [174,208,209]. Still, there is space to model practical situations which take into consideration the combined effect of moisture and temperature on the strength of adhesive joints, for example. One of the most important advantages of CZMs is related to their ability to simulate onset and growth of damage without the requirement of an initial flaw. However, the cohesive elements should be placed along the paths where damage is prone to occur. Because of it, the damage growth is restricted to well defined planes, (i.e., at the interfaces between the adhesive and the adherend or cohesively in the adhesive), which restrict to allow the mixed failure mode. Campilho et al. [210] used the extended finite element method (XFEM), which allow the growth of discontinuities within bulk solids along an arbitrary path, thus overcoming the main restriction of CZMs that damage grows only at predefined paths. They concluded that this method can be an alternative method for accurate prediction of bonded structures, but is restricted to mixed mode propagation. Same author [192] compared the XFEM method with the standard one to simulate adhesively bonded single and double lap joints. XFEM method showed satisfactory results in terms of quantity, but it's more mesh dependent which ultimately led to more time and cost of the analysis. Nowadays, most of the structures are made up of different materials, in order to have the best performance. Some authors [39,63,211] conducted a series of tests and CZM simulations of joints between different adherend materials. Anyfantis [211] modelled the joints between CFRP composites and steel bonded with a ductile adhesive layer. The elasto-plastic loading and fracture response were modelled by a recently developed mixed-mode CZM law. A comparison was also performed to a numerical analysis based on the Damage Zone Theory (DZT). The industrial application of CZMs to model large and complex structures has been delayed by the requirement of extremely fine meshes along the crack propagation path [193,194,212]. To sort out this, several researchers [193,212–215] proposed different models and methods. Alvarez et al. [193] proposed a two-dimensional cohesive element formulation to model crack initiation and growth in adhesively-bonded joints by implementing in Abaqus a user (UEL) subroutine. This formulation (model) enables the use of far coarser meshes, resulting in significantly shorter simulation times, which in turn should permit an increase in the industrial applications of the CZM approach. A fine local mesh at the interface between the adhesive and the substrate can be generated by using the hierarchical superimposed finite element method [213]. Another way to solve this issue are the meshless methods, such as moving least squares (MLS), reproducing kernel particle method (RKPM), symmetric smoothed

failure modes in bonded joints with different geometric configurations or under complex loadings can be captured. A combined cohesive-zone and continuum approach was also successfully employed to incorporate both interfacial (composite-adhesive interface) and bulk damage in the bondline [199]. The approach adopted by Mahoney in order to model a bonded composite single-lap joint is shown schematically in Fig. 11 [199]. A combined material model can be used to predict all the possible failure modes. Coupled stress and energy criterion model can predict all the possible failure mode of adhesive bonded joints [200,201]. The bondline thickness is one of the most significant geometrical parameters which affect the overall strength of bonding structure. However, the tough challenge of employing CZM on this topic is how to assign the values of the cohesive parameters for various bondline thicknesses and the placement of the cohesive element. Some authors [76,192] used only a single row of cohesive elements to describe the entire adhesive, while others used solid element to describe the adhesive and the cohesive elements are inserted within the solid elements to describe the failure of the adhesive [202]. To overpass this difficulty, it is possible to introduce continuum damage model in solid elements [203]. On the other hand, using thick flexible adhesive behaviour in the continuum element could lead to mesh dependency and therefore solution convergence difficulties. To overcome this, Hasegawa et al. [204] modelled the elastic adhesive layers separately adjacent to the cohesive element. This is generally good practice, but, due to the high ductility of the adhesive resulting in an extensive damage zone, the same fracture parameters may not be as applicable to structural joints with other bondline thickness. The numerical results perfectly matched with the damage growth behaviour observed in the experimental tests by Hasegawa et al. [205]. Based on these results, it can be stated that the modelling approach presented can be applicable to bonded joints with a rubbery and thick adhesive layer. However, this is an area that needs more research in the future. The prediction of the failure process and strength of adhesive joints with different adhesives depend on the cohesive laws shape (i.e. triangular, trapezoidal, exponential, linear etc.) [85,190,206,207]. Usually, the CZMs gives accurate predictions with brittle adhesives, but not with ductile adhesives. This probably happened because a triangular law was used and the behaviour of the ductile adhesive is closer to a trapezium shape [85]. However, the trapezoidal law is not yet implemented in the available commercial softwares (i.e ABAQUS or ANSYS). Campilho et al. [192,206] studied the influence of CZM shapes (triangular, trapezoidal and exponential) used to model a thin adhesive layer in single lap joints. They concluded that the triangular CZM is most suited for the brittle adhesive and the trapezoidal CZM for the ductile adhesive, while the exponential CZM provided over prediction. Hence, CZM accurately predicts the strength of bonded joints if the cohesive laws are estimated correctly. However, compared to conven38

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Damage modelling is still an innovative field under intense development, regarding more accurate modelling techniques, reliable and simple parameter determination methods, increase of robustness and elimination of convergence issues, and it is also under heavy implementation in commercial FE software packages.

particle hydrodynamic (SSPH) [212,214,215]. A major advantage of a meshless method over the FEM, for crack propagation problems, is that a crack path is independent of the distribution of particles in the domain where as in the FEM it depends upon the mesh design. The first constitutive model was a classical bi-linear traction– separation model and is easy and faster to determine the cohesive parameters. But the wide variety of new adhesive materials and their behaviour during damage (elastic, plastic, visco elastic etc) are difficult to accurate predict by the same model. Jousset et al. [216] compare the pressure-dependent elasto-plastic-damage model and traction separation model. Pressure-dependent elastic-plastic damage model provides accurate results in the case of structural joints where the adhesive is subjected to plastic flow compared to bilinear traction-separation model. But it is difficult to predict in advance if such plastic flows will occur or not. Consequently, it is necessary to define the parameter which accounts the plastic flow property of adhesive during analysis as there are different adhesive behaviours. However, this is still an innovative field under intense development, regarding more accurate modelling techniques, reliable and simple parameter determination methods, increase of robustness and elimination of convergence issues, and it is also under heavy implementation in commercial FE software packages.

References [1] Banea MD, da Silva LFM. Adhesively bonded joints in composite materials: an overview. Proc IME J Mater Des Appl 2009;223:1–18. [2] Katnam KB, da Silva LFM, Young TM. Bonded repair of composite aircraft structures: a review of scientific challenges and opportunities. Prog Aerosp Sci 2013;61:26–42. [3] Katnam KB, Comer AJ, Roy D, da Silva LFM, Young TM. Composite repair in wind turbine blades: an overview. J Adhes 2015;91:113–39. [4] Vallée T, Tanert T, Fecht S. Adhesively bonded connections in the context of timber engineering-a review. J Adhes 2016. http://dx.doi.org/10.1080/ 00218464.2015.1071255. [5] Banea MD, da Silva LFM, Campilho RDSG, Sato C. Smart adhesive joints: an overview of recent developments. J Adhes 2014;90:16–40. [6] Paiva RMM, Marques EAS, da Silva LFM, António CAC, Arán-Ais F. Adhesives in the footwear industry. Proc IME J Mater Des Appl 2016;230:357–74. [7] Barbosa AQ, da Silva LFM, Banea MD, Öchsner A. Methods to increase the toughness of structural adhesives with micro particles: an overview with focus on cork particles. Mat.-wiss. u. Werkstofftech. 2016;47:307–25. [8] Abdel Wahab MM. Fatigue in adhesively bonded joints: a review. ISRN Mater Sci 2012;2012:1–25. [9] Chaves FJP, da Silva LFM, de Moura MFSF , Dillard DA, Esteves VHC. Fracture mechanics tests in adhesively bonded joints: a literature review. J Adhes 2014;90:955–92. [10] Heshmati M, Haghani R, Al-Emrani M. Environmental durability of adhesively bonded FRP/steel joints in civil engineering applications: state of the art. Compos Part B-Eng 2015;81:259–75. [11] Marques EAS, da Silva LFM, Banea MD, Carbas RJC. Adhesive joints for low- and high-temperature use: an overview. J Adhes 2015;91:556–85. [12] Kusano Y. Atmospheric pressure plasma processing for polymer adhesion: a review. J Adhes 2014;90:755–77. [13] Costa M, Viana G, da Silva LFM, Campilho RDSG. Environmental effect on the fatigue degradation of adhesive joints: a review. J Adhes 2016. [14] Pethrick RA. Design and ageing of adhesives for structural adhesive bonding – a review. Proc IME J Mater Des Appl 2015;229:349–79. [15] da Silva LFM, das Neves PJC, Adams RD, Spelt JK. Analytical models of adhesively bonded joints—Part I: literature survey. Int J Adhes Adhes 2009;29:319–30. [16] da Silva LFM, das Neves PJC, Adams RD, Wang A, Spelt JK. Analytical models of adhesively bonded joints—Part II:comparative study. Int J Adhes Adhes 2009;29:331–41. [17] He X. A review of finite element analysis of adhesively bonded joints. Int J Adhes Adhes 2011;31:248–64. [18] Sauer RA. A survey of computational models for adhesion. J Adhes 2016;92:81–120. [19] da Silva LFM, Öchsner A, Adams RD. Handbook of adhesion technology. Springer; 2011. [20] da Silva LFM, Dillard DA, Blackman B, Adams RD. Testing adhesive joints, best practices. Weinheim: Wiley; 2012. [21] da Silva LFM, Pirondi A, Öchsner A. Hybrid adhesive joints. Heidelberg: Springer; 2011. [22] da Silva LFM, Sato C. Design of adhesive joints under humid conditions. Heidelberg: Springer; 2013. [23] da Silva LFM, Carbas RJC, Critchlow GW, Figueiredo MAV, Brown K. Effect of material, geometry, surface treatment and environment on the shear strength of single lap joints. Int J Adhes Adhes 2009;29:621–32. [24] Meneghetti G, Quaresimin M, Ricotta M. Influence of the interface ply orientation on the fatigue behaviour of bonded joints in composite materials. Int J Fatigue 2010;32:82–93. [25] Song MG, Kweon JH, Choi JH, Byun JH, Song MH, Shin SJ, Lee J. Effect of manufacturing methods on the shear strength of composite single-lap bonded joints. Compos Struct 2010;92:2194–202. [26] Kanerva M, Saarela O. The peel ply surface treatment for adhesive bonding of composites: a review. Int J Adhes Adhes 2013;43:60–9. [27] Liao L, Huang C, Sawa T. Effect of adhesive thickness, adhesive type and scarf angle on the mechanical properties of scarf adhesive joints. Int JSolid Struct 2013;50:4333–40. [28] Moradi A, Carrère N, Leguillon D, Martin E, Cognard JY. Strength prediction of bonded assemblies using a coupled criterion under elastic assumptions: effect of material and geometrical parameters. Int J Adhes Adhes 2013;47:73–82. [29] Nguyen TC, Bai Y, Zhao XL, Al-Mahaidi R. Curing effects on steel/CFRP double strap joints under combined mechanical load, temperature and humidity. Constr Build Mater 2013;40:899–907. [30] Stazi F, Giampaoli M, Rossi M, Munafò P. Environmental ageing on GFRP pultruded joints: comparison between different adhesives. Compos Struct 2015;133:404–14. [31] Akpinar S. Effects of laminate carbon/epoxy composite patches on the strength of

5. Conclusions Adhesively bonded joints in composite materials have been reviewed by studying articles published between 2009 to 2016. The parameters which directly or indirectly affect the performance of the bonded joint structures are discussed: geometric parameters (adhesive thickness, overlap length, joint configuration), material parameters (adhesive and adherend material), environmental parameters (prebond moisture, post-bond moisture, temperature, humidity), manufacturing method, surface preparation and failure modes. The selection of the manufacturing bonding method usually depends on the substrate to be bonded, service condition, area of application, etc. However, there is a lack of understanding of bonding methods on the failure behaviour and the relationship between bulk adhesive strength and joint strength. Suitable manufacturing bonded joint method for a particular area of application is important, especially in case of the composite repair method, as the parent material (damaged) already contain moisture and suffered other changes during the service period. There is no generalized relationship between the bonded joint strength with respect to the geometric parameters (overlap length, bondline thickness and joint configuration) as there are other factors such as adhesive material properties (ductile or brittle), type of loading and adherend material etc. involved. Thus, it is important to consider these parameters while optimizing the geometrical parameters for maximum performance of the joints. Debonding adhesives, crash resistant adhesives and natural adhesives are continuously developed. However, there is still a need to improve their applicability according to their characteristics and properties. For instance, self-healing adhesives offer a promising potential in the future. Long-term performance and uncertainty relating to environmental durability represent a critical barrier to the wide application of adhesively composite bonded joints in structural applications. Environmental factors such as pre-bond moisture, post bond moisture and temperature should be well discussed before and after the bonding method as they directly influence the performance of the joints. Bonded joints should be designed keeping in mind all these parameters and also the service conditions. Analytical method would be the best option to optimize the geometrical parameters. This gives fast results and calculated results can be updated very quickly to any changes in dimension, size and material properties. However, they are limited to simple geometries. 39

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