Showing papers in "Journal of Adhesion in 2015"

The Effect of Adhesive Thickness on the Mechanical Behavior of a Structural Polyurethane Adhesive

Abstract: One parameter that influences the adhesively bonded joints performance is the adhesive layer thickness Hence, its effect has to be investigated experimentally and should be taken into consideration in the design of adhesive joints Most of the results from literature are for typical structural epoxy adhesives which are generally formulated to perform in thin sections However, polyurethane adhesives are designed to perform in thicker sections and might have a different behavior as a function of adhesive thickness In this study, the effect of adhesive thickness on the mechanical behavior of a structural polyurethane adhesive was investigated The mode I fracture toughness of the adhesive was measured using double-cantilever beam (DCB) tests with various thicknesses of the adhesive layer ranging from 02 to 2 mm In addition, single lap joints (SLJs) were fabricated and tested to assess the influence of adhesive thickness on the lap-shear strength of the adhesive An increasing fracture toughness with inc

Adhesive Joints for Low- and High-Temperature Use: An Overview

Abstract: This work presents a review of several investigations on the topic of adhesive bonding at high and low temperatures. Durability and strength at extreme temperatures have always been a major limitation of adhesives that, given their polymeric nature, exhibit substantial degradation at temperatures where other structural materials (such as metals for example) have minute changes in mechanical properties. However, due to the inherent advantages of bonding, there is a large and continued effort aiming to improve the temperature resistance of adhesive joints, and this effort has been spread among the various topics that are discussed in this review. These topics include adhesive shrinkage and thermal expansion, adhesive properties, joint geometry optimization, and design techniques, among others. The findings of these research efforts have all found use in practical applications, helping to solve complex problems in a variety of high-tech industries where there is a constant need to produce light and strong co...

Adhesive Selection for Single Lap Bonded Joints: Experimentation and Advanced Techniques for Strength Prediction

Abstract: The integrity of multi-component structures is usually determined by their unions. Adhesive-bonding is often used over traditional methods because of the reduction of stress concentrations, reduced weight penalty, and easy manufacturing. Commercial adhesives range from strong and brittle (e.g., Araldite® AV138) to less strong and ductile (e.g., Araldite® 2015). A new family of polyurethane adhesives combines high strength and ductility (e.g., Sikaforce® 7888). In this work, the performance of the three above-mentioned adhesives was tested in single lap joints with varying values of overlap length (LO). The experimental work carried out is accompanied by a detailed numerical analysis by finite elements, either based on cohesive zone models (CZM) or the extended finite element method (XFEM). This procedure enabled detailing the performance of these predictive techniques applied to bonded joints. Moreover, it was possible to evaluate which family of adhesives is more suited for each joint geometry. CZM revea...

Effect of Surface Texture on the Adhesion Performance of Laser Treated Ti6Al4V Alloy

Abstract: The growing demand in lighter and safer structures generates the requirement of lighter joining strategies, particularly for lightweight metal alloys, composites, and also joining dissimilar materi...

Composite Repair in Wind Turbine Blades: An Overview

Abstract: Renewable energy sources such as wind energy—together with energy-efficient technologies—are essential to meet global energy demands and address climate change. Fiber-reinforced polymer composites, with their superior structural properties (e.g., high stiffness-to-weight) that allow lightweight and robust designs, play a significant part in the design and manufacture of modern wind turbines, especially turbine blades, for demanding service conditions. However, with the current global growth in onshore/offshore wind farm installations (with total global capacity of ∼282 GW by the end of 2012) and trend in wind turbine design (∼7–8 MW turbine capacity with ∼70–80 m blade length for offshore installations), one of the challenges that the wind energy industry faces with composite turbine blades is the aspect of structural maintenance and repair. Although wind turbines are typically designed for a service life of about 20 years, robust structural maintenance and repair procedures are essential to ensure the st...

On the Certification of Bonded Repairs to Primary Composite Aircraft Components

Abstract: Airworthiness certification is required when bonded repairs are made to primary composite structure in situations where damage has reduced or has the potential to reduce residual strength to below the design ultimate strength. Generally, certification of bonded primary structure poses many difficulties. As most repairs are one-off events meeting these certification requirements is especially challenging since demonstration by testing will generally not be possible or cost-effective. This paper discusses options for addressing the two key issues relating to certification: (a) how to validate initial and enduring bond strength of adhesive bonds, mainly given the inability of conventional non-destructive inspection to provide this assurance and (b) how to develop acceptable generic design allowables for bonded repairs which represent actual failure modes – especially for cyclic loading, since validation by testing of simulated repairs will generally be infeasible. It is concluded that proof testing of bonded...

Stepped Flush Repairs for Primary Composite Structures

Abstract: The drive towards greater use of fiber-reinforced composites in primary structures, such as aircraft structures and wind turbines that are increasingly unitized, calls for advanced repair techniques that can restore the structural integrity and geometry. Two such repair techniques are stepped repairs and scarf repairs. Under certain conditions, stepped repairs are easier to perform than scarf repairs, but the step corners may cause high level of stress concentrations negatively affecting the strength of the repair. This paper presents an investigation of the effect of step corners on the fracture behavior of stepped joints under compressive loading, particularly after subjecting it impact damage. Joints featuring different corner radii were tested under compressive loading, and results showed that the impact damage reduced the compressive strength, to marginally below the compression-after-impact strength of composite laminates. The findings reveal that stepped repairs can be designed to rival the performance of scarf repairs, due to the similarity in the inherent stress concentrations at ply terminations in both repair configurations.

Finite Element Analysis of Functionally Graded Bond-Lines for Metal/Composite Joints

Abstract: Failure in adhesive joints is usually the result of the non-uniform distribution of stresses that generally appears along the bond-lines, with peak values near the ends of the overlaps and inner zones where the adhesive essentially does not work. For joints comprised of dissimilar materials, the stress fields are also affected by the absence of symmetry. The present work is focused on “functionally graded adhesive joints” to avoid this phenomenon and to improve the strength of aluminum/composite joints under shear loads. Looking for the most favorable grading of properties, a search/optimization procedure is implemented based on finite element calculations and considering continuous variations of the material responses within the adhesive layer. After this, a comparative analysis of the continuous distributions obtained against discrete/“banded” approximations is performed, as these configurations are more feasible for manufacturing and therefore more suitable for industrialization. In order to avoid sing...

On the Fabrication and Automation of Reliable Bonded Composite Repairs

Abstract: For structures made of carbon fiber-reinforced plastics (CFRP), fast, robust, and reliable repair technologies are mandatory for economical usage. In this paper, the authors explain their strategy and experiences. An automated process is proposed to achieve the challenging goals. A general overview on the origin, effects, and analysis of contaminants in CFRP structures and the relationship to the achievable strength of adhesive bonds are given. For the repair of composite structures using adhesive bonding, surface pretreatment is a key factor in terms of reliability and strength. Different surface treatment processes such as grinding, grit blasting, plasma and pulsed lasers treatments are discussed. Furthermore, the possibilities and technical implementation of an automated milling process for the repair of composite structures are presented. This change from manual production to automation tremendously improved the quality and duration of the repair and allows the creation of a uniform surface for adhesi...

Structural Adhesives Modified with Thermally Expandable Particles

Abstract: In this study, the behaviour of two structural adhesives modified with thermally expandable particles (TEPs) was investigated as a preliminary study for further investigations on the potential of TEPs in adhesive joints. Tensile bulk tests were performed to get the tensile properties of the adhesives and TEPs-modified adhesives. In order to determine the expansion temperature of the particles while encapsulated in these particular adhesive systems, the variation of the volume of adhesive samples modified with different TEPs concentration as a function of temperature was measured. Further, the possibility of any chemical interactions between TEPs and adhesives matrix in the TEPs-modified specimens was verified by a Fourier transform infrared spectroscopy analysis. Finally, the fracture surfaces of the unmodified and TEPs-modified specimens, as well as the dispersion and the morphology of the particles, were examined by a scanning electron microscopy analysis. It was found that the stiffness of the TEPs-mod...

Application of Cohesive Zone Modeling to Composite Bonded Repairs

Abstract: Cohesive zone models can play an important role in the definition of repair strategies. These models allow the prediction of damage initiation and propagation. They are based on a softening relationship between stresses and relative displacements between crack faces, thus simulating a gradual degradation of material properties. Typically, stress-based and energetic fracture mechanics criteria are used to simulate damage initiation and growth, respectively. Those elements are placed at the planes where damage is prone to occur which, in the case of bonded repairs, is usually easy to identify a priori. Taking this into consideration, cohesive mixed-mode damage models based on interface finite elements were used with the objective of optimizing the repair efficiency. The determination of the cohesive pure mode softening laws is a key aspect of these models, and the direct method is the most accurate process to do it. The models were validated and applied to two different cases involving repairs. Several geom...

The Effect of Moment and Flexural Rigidity of Adherend on the Strength of Adhesively Bonded Single Lap Joints

Abstract: In the present study, mechanical properties of different single lap joint configurations derived from adherends with different thicknesses subjected to tensile loading were investigated experimentally and numerically. For this purpose, experimental studies were conducted on two different types of SLJ samples, the first type with identical upper and lower adherend thicknesses and the second with different upper and lower adherend thicknesses. For the first type, five different thickness values were tested. For the second type, the lower adherend thickness was constant while five different upper adherend thickness values were tested. The adhesive was prepared from a two-part paste. After the experimental studies, stress analyses on the SLJs were performed with three-dimensional finite element analysis by considering the geometrical non-linearity and the material non-linearities of the adhesive (DP460) and adherend (AA2024-T3). It was observed that, in single lap joint geometry, variation in the thickness of...

Steel-Epoxy Composite Joints Bonded with Nano-TiO2 Reinforced Structural Acrylic Adhesive

Abstract: This article reports a study on the effect of TiO2 nanoparticles on the adhesion strength of steel–glass/epoxy composite joints bonded with two-part structural acrylic adhesives. The introduction of nano-TiO2 in the two-part acrylic adhesive led to a remarkable enhancement in the shear and tensile strength of the composite joints. The shear and tensile strengths of the adhesive joints increased with adding the filler content up to 3 wt.%, after which it decreased with adding more filler content. Also, addition of nanoparticles caused a reduction in the peel strength of the joints. Differential scanning calorimeter analysis revealed that glass transition temperature (Tg) values of the adhesives rose with increasing the nano-filler content. The equilibrium water contact angle decreased for adhesives containing nanoparticles. Scanning electron microscope micrographs revealed that addition of nanoparticles altered the fracture morphology from smooth to rough fracture surfaces.

Degradation of Epoxy-Steel Single Lap Joints Immersed in Water

Abstract: Exposure to environmental factors, especially moisture, is recognized as the major cause of degradation of adhesive joints. In this work, complementing a previous study on exposure to moisture, single lap joints were subjected to immersion in water, up to five weeks, at room temperature and 50 °C. The material of the adherends was mild steel, and the adhesive was a bi-component epoxy. The specimens were fabricated using the open-face technique. Mechanical testing at the end of the relevant period of immersion showed an initial loss of ultimate load, after one week at 50 °C or two at room temperature; then, the strength remained practically constant over the remaining time. The loss was more accentuated after immersion at 50 °C, about 70%, than at room temperature, about 30%. Also a reduction in stiffness of the joints was measured, again dramatic (about 70%) after immersion at 50 °C, moderate (about 10%) after room temperature immersion. Optical examination, performed before closing the open-face specimen...

Progressive Failure Analysis of Reinforced-Adhesively Single-Lap Joint

Abstract: The failure behavior of reinforced-adhesively single-lap joints was investigated experimentally and numerically The reinforced adhesive was produced by mixing waste composite particles and an epoxy-based commercial adhesive The single-lap joint was prepared with an adhesive and unidirectional fiber glass/epoxy composite plates with a (0°/90°)3 stacking sequence Three types of adhesive were used: an un-reinforced adhesive (ADH), an adhesive mixed with glass fiber-reinforced epoxy resin composite plate particles (GFRC), and an adhesive mixed with carbon fiber-reinforced epoxy resin composite plate particles (CFRC) The adhesive thickness (ta) and overlap length (lap) were 04, 08, 12, and 16 mm and 10, 20, 30, and 40 mm, respectively Progressive failure analysis was performed with the ANSYS™ 110 finite element program using ANSYS™ parametric design language (APDL) code In the numerical study, the failure loads of the composite and the adhesive were determined with the Hashin failure criteria and th

Swelling Behaviour of Cured Urea–Formaldehyde Resin Adhesives with Different Formaldehyde to Urea Mole Ratios

Abstract: As a part of understanding of the network structure of urea–formaldehyde (UF) resin adhesives, this study examined the swelling behaviour of cured UF resin adhesives with four different formaldehyde–urea (F/U) mole ratios, using Flory–Rehner thermodynamic theory and field emission-scanning electron microscopy (FE-SEM) to relate the swelling behaviour to consequently induce micromorphological changes. Cured UF resin films before and after acetone extraction were exposed to swelling in dimethyl sulphoxide at three different temperatures. For the first time, this study reported the experimentally determined swelling parameters, such as sol fraction (ωsol), polymer volume fraction (φp), polymer–solvent interaction parameter (χ), and the number average molecular weight between cross-links (Mc), for cured UF resin adhesives. Both ωsol and Mc decreased as the F/U mole ratio increased. But these values increased with an increase in the swelling temperature. The extraction resulted in negative ωsol values, suggest...

Effects of Temperature and Hole Drilling on Adhesively Bonded Single-Lap Joints

Abstract: In this study, the load-carrying capacity of a single-lap joint bonded by an adhesive was determined experimentally. Glass fiber-epoxy composite material was chosen as adherends and Loctite® 9466 A&B2 was used as adhesive. The vacuum assisted resin infusion method (VARIM) was used to manufacture composites. In this experimental study, the effects of hole drilling and temperature were investigated. Five hole configurations and three temperatures (room temperature, 50°C, and 80°C) were considered. The results show that hole drilling elevate the failure load and when the temperature increases the load-carrying capacity decreases.

Size Effect of the Adhesive Bonding Strengths of Metal/Polymer Similar and Dissimilar Material Joints

Abstract: The relationship between the adhesive bonding strengths and the bonding areas (sizes) is important for material and structural designs, because adhesive bonding is widely used not only in small scale electronic devices, but also in large-scale composite–metal joints of modern aircraft/ship structures. Therefore, a systematic experimental investigation on adhesive bonding strengths of polymer/polymer, metal/metal, and polymer/metal joints was conducted. Results show that the size effect of the bonding strengths is small for the current bonding areas (from 30 to 300 mm2), which are widely employed in most of the test standards for engineering designs.

Variability of the Pull-off Technique for Adhesion Strength Evaluation on Ceramic Tile Claddings

Abstract: The periodic assessment of claddings adhesion during the facade life cycle becomes an important preventive in-service issue. This paper discusses the pull-off test, an in-situ diagnosis technique, which characterizes the in-service performance of ceramic tile claddings, by measuring adherence resistance to tension. Adherence is one of the main required properties of external coating, not only for aesthetical reasons but also for safety purposes. Throughout this research, an experimental campaign is performed in laboratory and in-situ, analyzing the variability of the pull-off testing technique. In-situ tests have been undertaken in existing construction sites and test walls. The laboratory results were compared to the in-situ ones. Standards requirements were evaluated and different failure modes were verified. In this work the impact of several parameters is analyzed such as the use of metallic discs with different thickness levels, geometries, and dimensions; the location of the testing at different hei...

Rheology Study of Sugar Cane Bagasse Lignin-Added Phenol–Formaldehyde Adhesives

Abstract: The rheological properties of sugar cane bagasse lignin–phenol formaldehyde (PF) (30% lignin – PF) resins were studied using oscillation tests. The bagasse lignin was introduced in the classic adhesive formulation in order to supply a part of PF. Rheological qualities of optimal lignin–PF (30% lignin – PF) resins and commercial PF resin were assessed by using a rotary rheometer (ARES). Dynamic rheological measurements, performed at low strain in the linear range, are useful to characterize the network properties of resins. The results obtained showed that the time sweep indicates excellent structural stability of optimal lignin–PF (30% lignin–PF) resins and commercial PF resin. The elastic modulus is greater than the viscous one showing a remarkable elastic character of the resins, and the performed frequency sweeps show a typical spectrum of a “weak gels” structure. The time dependence at 125°C shows that the optimum cure time is 7.5 min.

Strength Degradation of Adhesively Bonded Single-Lap Joints in a Cyclic-Temperature Environment Using a Cohesive Zone Model

Abstract: Adhesively bonded joints are widely used in automotive industry. Adhesively bonded joints permit to have more uniform stress distributions, join complex shapes, and reduce the weight of the structures. The requirement to reduce the weight of automobiles is also increasing the application of composites. In this article, CFRP (carbon fiber-reinforced plastics) composite was used in experimental tests. In many cases, adverse environments cause non-negligible degradation in joints mechanical performance. So a combined experimental–numerical approach was developed to characterize the effect of cyclic-temperature environment on adhesively bonded joints. Experimental tests were carried out on single-lap joints with CFRP and steel adherend in a cyclic-temperature environment. A cohesive zone model was taken into consideration to predict the results observed during the experimental tests and an environmental degradation model was developed. Scanning electron microscopy was utilized to investigate the fracture surf...

Surfactant-Assisted Dispersion of MWCNTs in Epoxy Resin Used in CFRP Strengthening Systems

Abstract: The epoxy resin used as the bonding agent in carbon fiber-reinforced polymer (CFRP) strengthening systems was modified by the infusion of multiwalled carbon nanotubes (MWCNTs). Two types of surfactants, Triton X-100 and C12E8, were used to disperse the nanotubes in the epoxy resin employing ultrasonic mixing. Dynamic mechanical analysis and tensile tests were conducted to study the effect of the surfactant-assisted dispersion of nanotubes on the thermal and mechanical properties of epoxy composites. The morphology of the epoxy composites was interpreted using scanning electron microscopy (SEM). Moreover, the effect of surfactant treatment on the structure of nanotubes was investigated by Fourier transform infrared (FT-IR). Based on the experimental results, the tensile strength and the storage modulus of the epoxy resin were increased by 32% and 26%, respectively, by the addition of MWCNTs. This was attributed to the homogeneous dispersion of nanotubes in the epoxy resin according to the SEM images. Anoth...

A Study of the Adhesive Foot of the Gecko: Translation of a Publication by Franz Weitlaner

Abstract: Although it seems that gecko adhesion research is a relatively young branch of science, this recently rediscovered work presents some very old studies with quite remarkable findings. The publication of Dr. F. Weitlaner from 1902 is very impressive, as it covers many recently published topics and – even more impressively – often comes to the same conclusions and provides similar results compared with current publications. Weitlaner published his paper in German which was – at that time – very common in science. This makes it almost impossible for today's international community of bioinspired adhesion researchers to enjoy and appreciate this early gem of scientific work. Thus, we have decided to translate his paper in the hope that it finds the attention it deserves and that it inspires us to stay curious and pursue answers to the questions which have been asked for over a century.

Thermal Conductivity Coefficients of Unidirectional Fiber Composites Defined by the Concept of Interphase

Abstract: In this paper, the thermal conductivity coefficients of unidirectional composites have been estimated. In the present analysis, a model which takes into account the influence of the boundary interphase has been proposed. The thermal conductivity coefficients of the composite were calculated by using this concept to determine the role of the interphase which mainly depends on the quality of adhesion between fibers and matrix. The interphase matter is the part of the polymer matrix lying on the close vicinity of the fiber bounding surface. It has been primarily assumed that the interphase is inhomogeneous with thermophysical properties varying from the fiber surface to the matrix. Five laws of variation have been taken into consideration in order to derive closed form expressions for the thermal conductivity coefficients in which the role of the interphase layer occurs, possessing properties different from those of the constituent phases. A thermal analysis method known in the literature has been adopted in...

Improvement in Strength of Adhesively Bonded Single-Lap Joints Using Reinforcements

Abstract: In order to enhance the strength of adhesively bonded single-lap joints (SLJs), the adhesively bonded SLJs with reinforcements were proposed. Adhesively bonded SLJs of different substrates and with different reinforcements were investigated experimentally and numerically. Scanning electron microscopy was performed on the fracture surfaces of the joints to analyze the failure mechanism. Shear stresses and peeling stresses of the adhesive layer were calculated with finite element analyses (FEA). Results showed that the deformation of the joints decreased with an increase in stiffness at the end of the overlap region. The strength increase in adhesively bonded SLJs with reinforcements was validated by the results from experimental tests and FEA.

Effect of Different Application Techniques of All-in-One Adhesives on Microtensile Bond Strength to Sound and Caries-Affected Dentin

Abstract: The aim of this study was to evaluate the microtensile bond strength of two all-in-one self-etch adhesives applied to sound and caries-affected dentin with four different application techniques. Forty extracted third molars with occlusal caries were randomly divided into four groups for G-Aenial Bond and S3 Bond: (1) according to manufacturer's instructions; (2) with acid etching before applying adhesives; (3) doubling adhesive application time; and (4) doubling adhesive coating. Teeth were sectioned to obtain 1-mm- ± 0.2-mm-thick dentin sticks and subjected to a tensile force. For G-Aenial Bond, doubling the time and application of two consecutive coats produced significantly higher strength than that obtained by following manufacturers' instructions and acid etching application to sound dentin. Prior acid etching and application of two consecutive coats to caries-affected dentin generated significantly higher bond strength than that using other techniques. For S3 Bond, there was no significant differenc...

Numerical and Experimental Damage Identification in Metal-Composite Bonded Joint

Abstract: Experimental and numerical analyses were carried out in order to identify damage in metal-composite bonded joints, which were manufactured from carbon-fiber reinforced polymers and titanium plates joined by an epoxy resin. The monitoring was performed by using vibration-based method through changes in frequency response function (FRF). First, free–free vibration tests were performed on four different specimens (with presence or not of damage and with presence or not piezoelectric sensor). Finite element analyses for the conditions without the transducer were also carried out and compared with the experimental data. FRFs were obtained by using the response of the PZT placed over the titanium plate and accelerometers located at other positions of the joint. The damage was reproduced by replacing 50% of the overlap with a layer of Teflon. Lastly, based on damage identification metric, FRFs for the undamaged and damaged structure were compared, evaluating not only the potentialities and limitations of the app...

Influence of Ammonium Pentaborate (APB) on the Performance of Urea Formaldehyde (UF) Adhesives for Plywood

Abstract: Ammonium pentaborate (APB) was used to modify urea formaldehyde (UF) resins, in which the formaldehyde to urea molar ratio was set at 1.80, 1.50, 1.25, and 1.05. Some specific properties, including gel time, free formaldehyde content in UF, bond strength, and formaldehyde emission levels from plywood were evaluated. The result showed that APB increased the gel time length, but also decreased free formaldehyde content and emission levels, which was reduced mostly by 79.0% and 81.4%, respectively. The result of bond strength indicated that APB was proper to modify high F/U molar ratio of UF resin regardless of the loading level, but a recommended loading level should be considered to relevantly lower the F/U molar ratio of UF. The suggested loading level of APB to UF is 8.0% to 6.0%, 6.0%, and 4.0% to UF resin with F/U molar ratio of 1.8, 1.5, and 1.25 separately.

Sensitivity and Optimization of Peel Strength Based on Composition of Adhesives for Footwear Industry

Abstract: In order to contribute to the research and development of adhesives for the footwear industry, this paper aims to develop a model capable to predict and optimize the peel strength from the composition of adhesives. The proposed approach is based on three stages: experimental planning of measurements, global sensitivity analysis for uncertainty propagation, and optimization procedure. The design variables are the weight percentages of the solid raw material constituents such as polyurethane, resins, and additives of the adhesive joint. Considering the experimental results obtained for Taguchi design points as input/output patterns, an artificial neural network (ANN) is developed based on supervised evolutionary learning. Using the developed ANN a global sensitivity analysis procedure is implemented and the variability of the structural response of adhesive joint is studied. The optimal solution for adhesives composition for maximum peel strength is investigated based on ANN model and using a genetic algori...

A Prediction Method of the Behavior of Adhesively Bonded Structures under Cyclic Shear Loading Based on a Characterization of the Viscous Aspects of the Adhesive in an Assembly

Abstract: The capabilities of structural bonding are more and more used. Estimating the abilities of an adhesive to endure repetitive loadings and to keep stable its mechanical properties along service life is an essential point to analyze in order to conduct fatigue assessments. The aim of this study is to develop a predictive tool for describing the fatigue behavior of an adhesive in an assembly under cyclic loadings. The approach developed analyzes the influence of viscosity on the mechanical behavior of an adhesive in an assembly based on monotonic and creep test results. Thanks to the evaluation of viscous phenomena, it is possible to predict the cyclic response of the adhesive. The experimental approach uses a unique bonded joint designed to limit the stress concentrations and with a maximum stress state in the center of the adhesive. In this paper, following the strategy developed under monotonic loading, experimental results under cyclic loading are presented for different types of loading using several loa...