ABSTRACTThere are currently little widely used structural health monitoring (SHM) techniques for structural adhesive joints. In this study, integrated fibre Bragg grating (FBG) sensors are used to examine the structural integrity and reliability of functional adhesive joints. There is an even greater demand for structural monitoring during operation or quality control of adhesive joints during production as more industries embrace high performance structural adhesives in fail-safe designs. Smartroof® photovoltaic (PV) roof tiles are used in this study to validate a real-time health monitoring of the adhesive connection between the PV glass plate and the polymer frame. A structural epoxy and a modified siloxane (MS) polymer are assessed. To assure measurement accuracy and to provide a better understanding of the strain sensing mechanism, the data is qualitatively compared with a finite element model. It is demonstrated that using embedded FBG sensors is accurate and adequate for in situ real-time structural health monitoring. Nevertheless, it remains difficult to ensure and manage the fibre’s position in the adhesive layer.

ABSTRACTThe double cantilever beam (DCB) specimen is widely used to characterise the mode I fracture of adhesive joints. This paper analyses some particular characteristics of adhesively bonded composite DCB specimens which could affect test results. Three-dimensional (3D) and two-dimensional (2D) finite element analyses (FEA) were conducted in order to evaluate the effects bondline constraint and adherend through-thickness flexibility on the specimen response. Since beam theory-based data reduction schemes are widespread, beam models were also employed to analyze the effects of adherend through-thickness flexibility and fracture process zone relative size. It is shown that, although composite adherends are usually thinner and have much lower transverse moduli than metal adherends, the level of bondline constraint is similarly high. This may: limit the level of adhesive plastic deformations in the fracture process zone; generate high bondline tractions that increase the likelihood of interface failure and interlaminar damage in the composite adherends. The present analyses also show relevant effects of adherend through-thickness flexibility in the adhesive elastic loading stage. Finally, smaller fracture process zones relative to metal adherend DCB specimens were predicted by a beam cohesive zone model. This may explain lower fracture energy values reported with composite adherends in some studies.

ABSTRACTThis study improved laser shock-wave adhesion test (LaSAT) to evaluate the bonding strength of an aluminum alloy/epoxy resin adhesive, since the bonding strength is dependent on loading rate. In this method, strong shock wave due to laser ablation induces interfacial fracture, and the critical stress is computed using numerical simulations of the FEM. To accurately evaluate the bonding strength, the input waveform caused by laser ablation was identified for FEM computation. Such ablation impact (pressure vs. time) and its spatial distribution of ablation impact were experimentally investigated, and this study computed stress-wave propagation in the specimen. It is found that numerical simulation with FEM reproduced well the wave propagation. A wide range of loading rates was achieved by the quasi-static tensile test, split Hopkinson pressure bar (SHPB) test, and LaSAT. As a result, bonding strength exhibited a significant dependence on the strain rate, similar to that of bulk resin materials in general.

ABSTRACTThe effect of different amounts of epoxy functional silane (EFS) as a crosslinking agent in the water-based acrylate adhesives was evaluated for the production of eucalyptus wood panels at different curing temperatures (120°C and 140°C). The thermal stability and dynamic analyses (loss and storage moduli) of the acrylate adhesives with EFS were evaluated. The addition of EFS in the acrylate adhesive increased the thermal degradation, storage, and loss moduli. The thermal degradation, bending test, specific mass, and morphology were evaluated in the wood panels. Significant increases in the storage modulus and bending strength of the wood panels with EFS and acrylate adhesive at 140°C were found. These results demonstrate a potential application of EFS in the production of wood panels aiming towards the manufacture of thicker composites.

ABSTRACTSeveral lignin-based adhesives were developed during the last decades. More recently, lignin isolation methods were improved, leading to technical grade lignins with constant properties. This recent scenario created the required conditions for the industrial use of lignin in adhesive applications, at large scale, with reliable properties. In this work, technical grade kraft lignin was epoxidized and incorporated in industrial epoxy resin, resulting into a partly biobased epoxy resin. Notwithstanding, the mechanical properties of cured partly biobased epoxy were investigated using Arcan device, aiming to reproduce realistic load conditions for the adhesively bonded joints. At last, failure envelopes were obtained from Drucker–Prager and von Mises models, revealing the most reliable model to calculate the failure prediction. Results pointed out to the development of a partly biobased epoxy adhesive with slightly superior mechanical properties, in comparison with industrial epoxy adhesive. Indeed, an important contribution was provided for the lignin revalorization and the mechanical characterization and failure prediction of epoxy adhesive based on the modified lignin.

ABSTRACTSurface tensions of solid materials have been studied over 200 years and widely used for industrial or engineering applications. The surface tensions and surface tension components can be calculated using measured contact angles, for example, by the model studied by Owens and Wendt. The model is often represented in an asymmetric linear form, called the Owens-Wendt-Rabel-Kaelble method, with the use of the linear least squares method. However, due to the practical preference not to use many types of test liquids, the existing statistical analysis is unsuitable, especially when the data measured are scattered. The present work proposes symmetric linear and circular expressions of the model of the two surface tension components. The symmetric linear expression can be used for obtaining the polar and dispersion components of surface tension of a solid; it enables appropriate choices for test liquids, physically meaningful screening of measured values, and clear validation of deduced surface tension components of solids. The symmetric circular expressions can be applied to deduce polar and dispersion components of liquids by using test solids. In conjunction with this, appropriate choices of test solids can be determined.

ABSTRACTIn this study, the enhancement in strength and toughness of multi-piece glass fiber reinforced plastic (GFRP)/GFRP joints was demonstrated. These joints are bonded with repurposed short glass fiber reinforced adhesive and find their application in rehabilitation of sewage pipelines. These short fibers were produced by grinding and milling the fiberglass wastes, collected during the lamination process of pipes. To study the effect of glass fiber content on the mechanical properties of the composite adhesive, short fiber composite (SFC) specimens with varied milled fiber content were tested under flexural loads. A finite element-based numerical homogenization study was carried out on a micro-mechanical periodic unit-cell representing a non-homogeneous SFC to predict the mechanical properties of the microstructure. The dispersion state of fibers and fracture surface characteristics of SFC were analyzed via scanning electron microscopy. Further, the shear strength of the adhesive with and without reinforcement was experimentally evaluated using a lap shear test. Increasing the fiber content in the adhesive transforms it from a compliant state with poor strength into a much stiffer and stronger composite with reduced strain tolerance. The optimized SFC was selected for joining the two segments of the GFRP sewage pipe using a unidirectional composite as a tongue. Subsequently, the tongue-in-groove joints (TGJs) were tested under tension.

ABSTRACTThe development of adhesives with excellent reversibility is necessary for the removal, recycling and reusing of precision components under specific conditions in electronics manufacturing. The integration of photosensitive cycloaddition reactions into common adhesive systems is a viable alternative for reversible modification. In this contribution, a novel reversible acrylate adhesive was successfully prepared by doping thiol-functionalized anthracene derivatives in situ, and its characteristic structure, photophysical properties and mechanical performance were systematically evaluated. As expected, the reversible dimers structure of anthracene provides remarkable bonding and debonding properties for adhesives. Sample #3 achieved an excellent shear strength of 1.67 ± 0.07 MPa and 91% debonding rate, with 283.03% and 89.58% improvement over the control sample #0 without anthracene modification, respectively. Furthermore, the shear strength and debonding rate for secondary bonding were up to 2.37 ± 0.07 MPa and 82%. We believe that our paradigm can provide a feasible approach for reversible adhesive design.

ABSTRACTFatigue cracking is a significant cause of failure in flexible pavements at moderate temperatures. Neat bitumen cannot properly perform at all temperatures and environmental conditions due to the increasing traffic volume. Consequently, this study examined the simultaneous usage of carbon nanotubes (CNTs) and the styrene-ethylene/propylene-styrene (SEPS) polymer as bitumen modifiers. The linear amplitude sweep (LAS) test and surface free energy (SFE) theory were used to determine the rheological characteristics and thermodynamic parameters of neat and modified bitumens, respectively. Using the SEPS nanocomposite up to 6% increased the fatigue life and moisture damage resistance of the asphalt mixtures by improving thermodynamic parameters such as adhesive free energy in dry and wet conditions. According to the LAS results, the modified bitumen outperformed the neat bitumen in terms of fatigue life under different strain levels. The fatigue life of the asphalt mixtures also decreased as the temperature increased from 10 to 20°C. However, for the mixtures containing the SEPS nanocomposite, the reduction in fatigue life was less noticeable due to the lower temperature sensitivity of the modified bitumen. The mixtures containing 6% SEPS nanocomposite demonstrated the highest performance.

ABSTRACTDiethoxy(3-glycidyloxypropyl) methylsilane (GPMS), a surface modification agent, is used to give halloysite nanoparticles a hydrophobic behaviour and improve their ability to disperse in epoxy polymers (EP). Surface roughness and hydrophobicity (WCA: 144°) are both quite high in EP-GPMS/Clay coatings. The corrosion behavior of coated steel surfaces was investigated using electrochemical methods. The EP-GPMS/clay nanocomposite coating was shown to have about 63 times higher coating resistance than the EP coating. The EP-GPMS/Clay displayed better coating resistance (6687.71 kΩ.cm2) and a lower corrosion current density (4.25 μA/cm2) than plain epoxy (1.01 kΩ.cm2; 287.21 μA/cm2) even after prolonged exposure to the electrolyte, according to electrochemical studies. Compared to the pure EP (0.0599 mm/year), the EP-GPMS/clay coated sample had the lowest corrosion rate (0.0017 mm/year). According to SECM data, at 15 d of immersion, the Fe dissipation at the surface of the EP-GPMS/Clay coating (2.6 nA) is much smaller than the plain EP (14.7 nA). Additionally, the EP-GPMS/Clay showed enhanced adhesive properties. The EP with silanized clay offers an outstanding oxygen and water repellent, hydrophobic, and barrier properties. Due to the environmental safety of clay/silane, this kind of coating might be employed as a workable coating substance for industrial applications.

ABSTRACTA large number of structures in various fields are repeatedly subjected to dynamic loading during their lifetime. To avoid or reduce vibration amplitudes possibly occuring, the use of vibration damping measures is necessary. Due to their inherent material damping, viscoelastic adhesives show a potential to reduce the dynamic effects of structures. This has not yet been comprehensively investigated for adhesively bonded joints. This paper reports on experimental and numerical investigations of damping properties of adhesively bonded tubular steel joints in a scale relevant for construction industry and thus represents a fundamental data set. Different adhesive joint geometries and test parameters were investigated experimentally and their influence on the damping properties of adhesively bonded specimens is discussed and evaluated. The test results show that the damping properties of adhesively bonded joints can be enhanced by increasing the adhesive layer thickness and reducing the overlap length. In addition, the damping properties of the test specimens increase with a rise in the stress on the adhesive layer. The influence of the test frequency on the loss factor is of secondary importance. Furthermore, material models of the adhesives used were developed and validated successfully on the basis of the experimental test result presented.

ABSTRACTThis overview aims at gathering the various existing works on interphases within epoxy/metal bonded assemblies. Indeed, this particular area plays an important role on the adhesion but also on the behavior in wet environment. Polymeric materials being hydrophilic, water diffusion occurs when they are in a humid environment. The water molecules then have an important impact on the mechanical and physical properties. The effect of water on the interfacial properties of adhesive joints is large discussed in literature. In this article, a complete interphase formation model is be proposed. This two-scenario model explain all the experimental results observed in the literature on the interfaces of a bonded joint. Moreover, hypotheses is made on the water diffusion mechanisms and the diffusive properties of the interface. This is allow explaining the change of the fracture surface observed during the wet aging of a bonded joint. A large number of studies have observed the transition from cohesive to adhesive failure.

ABSTRACTThis work aims to characterize the water uptake mechanisms of a two-component epoxy adhesive joint immersed in deionized water. The pore-type defects in the bulk adhesive after the cure cycle are highlighted and characterized using X-ray µ-tomography. Two population patterns of defects are generated and analyzed, for two different thicknesses. The waterfront is not detectable by µ-tomography for this adhesive because the densities of the water and the adhesive remain relatively close to each other. Instead, the volume variation and kinetics of pore water filling have been accurately identified. This analysis was completed by optical observations and gravimetric measurements.

ABSTRACTThis study looks at the mechanical and free vibration properties of glass laminated with functionalized aluminium reinforced epoxy. NaOH, K2Cr2O7, Fe2(SO4)3. XH2O, and C8H18O3Si were employed to functionalize aluminum. The flexural, impact, and shear strengths, as well as their failures, were investigated using electron microscope images. The impulsive hammer approach was applied to investigate laminate vibration behaviour. The results showed that the flexural strength (623.83 MPa), impact strength (76.41 J/mm2), and shear strength (55.31 MPa) of C8H18O3Si functionalized laminates outperformed those of other functionalized laminates. The improvement of mechanical properties was due to the change in interfacial adhesion. This is dependent on the functionalization of aluminium used in stacked laminates. Moreover, C8H18O3Si functionalized laminates exhibit a higher natural frequency and higher damping than other functionalized laminates. This natural frequency and damping are enhanced due to the strong mechanical interlocking between polymer and metal adhesion. It was noted that the same laminates have higher toughness than other laminates because of the high impact energy obtained.

ABSTRACTTo investigate the enhancement mechanism and effect of laser treatment on the performance of an interface between a carbon-fiber-reinforced polymer and magnesium (CFRP/Mg), finite element simulations and experiments were conducted to demonstrate the macroscopic and microscopic failure processes of the laser-treated interface. Based on the high controllability of the groove parameters in laser machining, the quantitative relationship between the texture parameters and the shear properties of the CFRP/Mg interface was analyzed. In addition, shear tests and simulations of the laser-treated interfaces with different groove parameters were performed. Increasing the groove depth and decreasing the groove spacing effectively improved the shear strength of the CFRP/Mg interface. In addition, the reliability of the interface cohesion parameters obtained using the representative volume element model was verified by experiments and simulations, which is important for the application of laser technology to composite laminates.

ABSTRACTThe goal of this paper is to determine whether laser-induced surface melting can generate adhesive debonding. Commercial cyanoacrylate and acrylic adhesives are used to attach an aluminum (Al) cylinder to a transparent polymethyl(methacrylate) (PMMA) plate, and then a variable force is applied to create an axially loaded butt joint. High energy nanosecond laser pulses at 1064 and 532 nm are directed through the transparent PMMA to be absorbed at the Al surface, causing transient localized heating that leads to joint failure. The dependence of this debonding on both laser fluence (energy per area) and applied force are investigated. Single shot debonding occurs at fluences on the order of 0.47 J/cm2 for 1064 nm and 0.29 J/cm2 for 532 nm pulses with an applied pressure of 0.22 MPa. Characterization of the Al surface before and after laser impact confirms that the debonding arises from surface melting and causes only slight changes to the Al surface. A simple model of the debonding process is developed to explain the dependence of the debonding on the applied load. Single laser pulses can generate instantaneous, relatively clean separation of bonded joints, suggesting that laser debonding may be a promising strategy to initiate deadhesion.

ABSTRACTIn this paper, the effect of a novel fibre reinforcement architecture in the adhesively bonded joint efficiency of natural fibre reinforced composites (NFRC) was investigated. Two different reinforcement techniques were used: intralaminar reinforcement (2D) and orthogonal-through-the-thickness reinforcement (3D). The aim of the novel architecture is to enhance the transverse properties of the adherend (transverse strength and fracture toughness) in order to delay or avoid delamination failures. A jute bidirectional fabric was used as a base primary reinforcement phase and curauá, sisal, ramie, hemp and glass fibres were used as secondary reinforcement phases for the 2D and 3D fibre reinforcement architectures. Single lap joints (SLJs) bonded with an epoxy adhesive used in the automotive industry were fabricated with these adherends and the efficiency of the joints was investigated by comparing them to glass (GFRP) and carbon (CFRP) pure synthetic fibre reinforced composite joints. It was found that the novel architecture was successful in reaching the failure load of the synthetic composite joints for SISAL 3D, CURAUÁ 2D and CURAUÁ 3D SLJs. Therefore, NFRC bonded joints can be a viable replacement for synthetic fibre composite joints at no load-bearing loss.

ABSTRACTThe presence of surface tension in liquids, which induces a strong capillary force between wetting particles in micro/nano scale, has aroused extensive attention. In present study, a fundamental investigation on capillary forces and rupture behaviors of capillary bridges between a three-finger microgripper and a plate is conducted in quasi-static state. Theoretical analysis is performed for solutions of the capillary force. The capillary bridges between a three-finger microgripper and a plate are established based on the principle of energy minimization. An analytical approach for computing the capillary force for the three-finger/plate geometry is proposed by means of variables obtained from the simulation models. The comparison of the single-finger capillary bridge and three-finger capillary bridge is investigated based on the developed models. The effects of separation distance, capillary bridge volume, radial distance and contact angle on the capillary force of three-finger capillary bridges are analyzed in detail. The results demonstrate that the variation of capillary force with separation distance and volume changing is not monotonic, which is caused by the edge effect of the three-finger microgripper. Capillary force measurements were experimentally characterized to demonstrate the reliability of the simulation models and the capillary force solution method based on an established experimental platform.

ABSTRACTLow surface energy materials are used as foul-release coatings (FRCs) to reduce the forces required to remove biofouling. Previous experiments observed the release behavior of epoxy studs (pseudobarnacles) from silicone coatings with a thickness gradient. The studs were loaded transversely in thick-to-thin and thin-to-thick directions, and the final decohesion always proceeded from the thin to the thick side of the coating. However, trends in the critical transverse forces required for removal were not apparent. In this study, finite element models (FEMs) were created to determine the peak shear stress at the interface of an epoxy stud bonded to a silicone coating with a thickness gradient in response to a transverse load. The effects of the average coating thickness, the thickness gradient of the coating, and the transverse loading direction were determined. At a given average coating thickness, increasing the thickness gradient produced higher peak shear stresses at the interface, which would reduce the critical force required to remove the epoxy stud (i.e., would improve the performance of the FRC). The influence of the increased thickness gradient waned as the average coating thickness increased. Therefore, moderately thin coatings with large thickness gradients would be optimal for FRC performance under transverse loading.

ABSTRACTIn the present work, adhesion has been incorporated in a model for receding contact between an elastic layer and a half plane under flat and parabolic punches. Based on the JKR model for adhesion and with the help of integral transformation, solution of the problem may be reduced to a system of singular integral equations. A numerical scheme has been proposed and verified. The effect of adhesion on contact size and the stresses has been demonstrated.