Adhesive Bulk Research Articles

Fatigue of structural adhesives under stress concentrations: Notch effect on fatigue strength, crack initiation and damage evolution

The effect of stress concentrations on the fatigue behaviour of two structural adhesives, relevant materials in lightweight applications, was investigated regarding fatigue strength, crack initiation lifetime, damage evolution and fracture behaviour. Internally and externally notched adhesive bulk specimens were tested under constant amplitude loading with a stress ratio of 0.1. Majority of fatigue lifetime (60–95%) was spent on crack initiation, which was shortened by an increasing stress concentration factor. Crack initiation causes a sudden damage increase near to failure. A model was proposed correlating the fatigue strength of notched specimens with highly stressed region and stress concentration factor.

Measuring moisture absorption in structural adhesive joints with fibre Bragg grating sensors

The increased joint efficiency, distribution of loads and decrease in stress concentrations have led to the increased use of adhesives for structural bonding. However, there are a limited number of techniques for verifying and monitoring the integrity and durability of adhesive bonds. This article studies the potential of estimating the curing and ageing of adhesive bulk samples with embedded fibre Bragg grating sensors through measuring the strain associated with hygroscopic expansion. This is achieved by relating the output of a fibre Bragg grating sensor to the deformation of the structure in which it is embedded. This work considers the possibility of mapping the changing structural resistance to mechanical loading (stiffness) of adhesive bonds as a function of time, under the influence of temperature and moisture as environmental factors. The goal is to map the influence of these environmental factors separately on the one hand, and their combined effect on adhesive bonds, on the other hand. This study subjects several bulk specimens to various environmental ageing loads. The swelling, associated to moisture absorption and that results in mechanical strain, is measured with fibre Bragg grating sensors. The moisture absorption behaviour at different temperatures and environmental relative humidity conditions determined in this way is verified using classical test methods (e.g. differential scanning calorimetry, gravimetric) on multiple fibreless specimens.

Dynamic mechanical analysis of carbon black filled, elastomer-toughened ethyl cyanoacrylate adhesive bulk films

This paper reports on a dynamic mechanical analysis (DMA) study of carbon black filled, elastomer-toughened ethyl cyanoacrylate (ETECA) adhesive bulk films. The bulk films were cured at room temperature, both with and without premixed initiators, between poly (ethylene) (PE) substrates and in poly (tetrafluoroethylene) (PTFE) moulds. Adhesive batches with different strong and weak acid concentrations were used to study the influence of the curing speed on the phase separation behaviour of the elastomer. The mechanical properties of the bulk adhesive films and electron micrographs of the fracture surface of the tensile tested bulk specimens are reported in the previous publication by Ștefanov et al. [1; International Journal of Adhesion & Adhesives, 68: 142–155, 2016]. For all tested bulk adhesive films similar peaks were observed in the DMA curves, i.e., a peak at low temperature, before the glass transition region of the polymer, and a peak corresponding to the glass transition temperature of the poly (cyanoacrylate) adhesive. The results of this investigation show that the amount of the phase-separated elastomer increases with the increase in weak acid concentration for the samples cured without the use of initiators. An opposite trend was observed for the samples cured using initiators. Another important finding is that for a complete phase-separation of the elastomer from the poly CA a secondary step, following curing at room temperature, is necessary which should consist of heating the sample to 110 °C.

Shear Strength of an Aluminum Alloy Bonded with a DP-460 Adhesive: Single Lap-shear Joints

Single lap-shear joints (SLJ) specimens with and without partial round fillets were fabricated to measure the average shear strength of adhesives. The effects of the length of the adherend on the SLJ specimens were also investigated. An epoxy adhesive was used to bond aluminum alloy. Tensile tests were performed on the adhesive bulk specimens to measure the mechanical properties. The finite element analysis (FEA) method was used to measure the adhesive stress distributions, i.e., the peel and shear stresses, on the bonded part. The experimental results revealed that the specimen consisting short length of adherend and without the partial round fillets exhibited the smallest average shear strength of adhesive among the investigated specimens. FEA revealed that the low average shear strength for the specimen with a short adherend length was caused by high stress concentrations on the adhesive at the edge of the bonded part.

Recent progress of glycopolymer synthesis for biomedical applications.

Glycopolymers are an important class of biomaterials which include carbohydrate moieties in their polymer structure. In addition to biological research on the interactions of glycopolymers with lectin-carbonate, glycopolymers have recently been used as a new synthetic biomaterial for direct therapeutic methods, medical adhesives, and biosensors. Thus, a comprehensive understanding of new advances in glycopolymer research is essential for the next level of biomaterial studies. This review article highlights commonly used glycopolymer synthesis methods and biomedical applications thereof. Glycopolymers can be synthesized by modern polymerization methods that can control the molecular weight, molecular weight distribution, chemical functionality, and polymer architecture. The polymerizations include free radical polymerization, atom transfer radical polymerization, reversible addition-fragmentation chain-transfer polymerization, and nitroxide-mediated polymerization. Because the carbohydrate-lectin interactions with glycopolymers are involved in many biological processes, carbohydrates containing glycopolymers are used in (1) fundamental studies to understand the specificity and strength of biological binding, (2) controllable interactions to prevent microorganism adhesion to human cells, (3) large scale bulk adhesives for medical applications, (4) biocompatible therapeutic nanoparticles, (5) direct drug delivery vehicles, and (6) precise quantitative measurement of biosensor materials that can detect physiological signals.

Polymer Interphases in Adhesively Bonded Joints – Origin, Properties and Methods for Characterization

Chemically curing adhesives are formulations requiring reactions to convert from liquid to solid. Once cured, these adhesives carry the potential to create strong load bearing joints, resisting even severe detrimental service conditions. In adhesively bonded joints with chemically curing adhesives the term "interphase" relates to the adhesive volume adjacent to the surface of the adherent (interface), which generally will exhibit properties different from those of the adhesive bulk polymer. The properties of these interphases play an important role concerning the performance and durability of structural adhesive joints. Therefore localized strain analysis in the cross-section of shear-loaded adhesive joints was performed by combining a high-precision mechanical testing device with digital microscopy and by developing a method for preparing, marking, and digitally tracking the local deformations in micro shear specimen. Non-uniform shear profiles developing in the cross-section of the adhesive joints after exceeding the yield point serve as a sensitive indication for mechanical surface-affected interphase properties and it could be observed, that deranged crosslinking promotes strain softening of the polymer in the interphase. Infrared analysis of the cross-sectional interphase region in adhesively bonded joints was performed with a Bruker Tensor II Fourier Transform Infrared (FTIR) spectrometer equipped with a Hyperion 3000 microscope with a 20x ATR germanium crystal objective and a MCT-Focal-Plane-Array-Detector (FPA), allowing to conduct high resolution chemical imaging and localized chemical analysis.

Double cantilever-beam test comparisons of Mode I fracture toughness of adherends bonded using DP8010 and DP8005 acrylic-based adhesives

The aim of this study is to not only measure the fracture toughness (GIC) of adherends bonded with a DP8010 acrylic-based adhesive using double cantilever beam (DCB) specimens under Mode I loading, but also to compare them with those bonded with DP8005. Aluminum alloy (5052-H34), glass fiber-reinforced polypropylene matrix composite (GF/PP), and carbon fiber-reinforced epoxy matrix composite (CF/EP) are used as adherends. Tensile tests of the adhesive bulk specimens are performed to measure the mechanical properties. R−curves measured from DP8010 and DP8005 are evaluated by relating them with failure modes observed from the fracture surfaces. Traction−normal separation relationships are considered to compare the fracture behaviors between DP8010 and DP8005. The experimental results show that the modulus of toughness (ur) for the adhesive bulk specimen of DP8010 exhibits similar values with that of DP8005. To maximize the values of GIC using acrylic-based adhesives, the following conditions need to be considered: ① strong bonding between adhesive and adherend, ② defect-free adherends, and ③ high ductile behavior of the adhesive bulk specimen.

Adhesive layer analysis for scarf bonded joint in CFRE composites modified with MWCNTs under tensile and fatigue loads

The effects of bondline thicknesses (0.17 & 0.25-mm) and scarf angles (5° & 10°) on the tensile and fatigue properties are investigated, for the first time, for carbon-fiber/epoxy composites scarf adhesive joints (SAJs) modified with optimum weight percentages of multi-walled carbon nanotubes (CNTs). New experimental/analytical model was developed to measure the adhesive displacement parallel to the adherend surface (Δux) using instrumented specimens with extensometer and strain-gauge. The measured values of Δux were used to determine the deformations, normal stresses, in-plane shear stresses and Young’s moduli of the adhesive layers in the SAJs under tensile loads. Results from tensile tests showed remarkable increase in apparent and local stiffnesses for the SAJs with CNT-adhesive, 0.17-mm bondline thickness and 10°-scarf angle. The Young’s moduli of the adhesive layers of the 5°-SAJs is about 2–3.2 times higher than that of bulk adhesives. The tensile strength of CNT-SAJs is 15.9–33% higher than that of the neat-SAJs with different bondline thicknesses and scarf angles. On the other hand, CNTs have marginal effect on the fatigue strength at lower lives and insignificant effect at fatigue limits (107-cycles). The fatigue strength/life of the SAJs was improved with decreasing scarf angle from 10° to 5° and bondline thickness from 0.25-mm to 0.17-mm.

In situ Investigations of Near Phase Boundary Water Diffusion of Ageing Adhesive Bonds by Scanning Kelvin Probe

Many industries nowadays rely on the adhesive bonding technology for their success, most notably aerospace, automotive, construction, and the electronics industry. One of the main advantages of the adhesives is the ability to bond dissimilar materials and the more efficient load transfer in comparison to other joining methods. Several industrial applications request the adhesive joint to withstand loads under different environmental conditions such as high moisture, and extreme temperatures. Thus, predicting the lifetime of adhesively bonded joints under moist environments is crucial to support tailoring the properties for the respective application demand. Relevant effects of water in adhesively bonded components may be strength degradation and introduction of stresses. Knowing the behaviour of the water in adhesive joints may contribute to directing the design of bonded components, lead to greater confidence and increase the use of adhesives in the industry. A significant cause of joint weakening is the effect of water in the interface between adhesive and substrates. There is evidence that the transport kinetics of water in the adhesive bulk do not correspond to the kinetics in the adhesive joint, studies show that water can be absorbed much faster by the interface than by the bulk. [1, 2] The study of the kinetics of water absorption in the interface is a particular challenge, and is also gravimetrically hardly detectable. A powerful and established method which can detect directly the corrosion induced delamination of polymer coatings is the scanning Kelvin probe (SKP). [3] In the present work the SKP is used to study the water uptake of adhesive bonds. To determine the water uptake processes of an adhesive joint by means of SKP, an adhesive is applied to a metal surface, and as second substrate a very thin non-conductive barrier layer is used. Several scanning Kelvin probe experiments are carried out, under different environmental conditions, to define which are the most suitable adhesives, substrates and barrier layers to investigate the water diffusion in situ and near the interface of an adhesive joint. The potential difference distribution is measured time-dependent in order to obtain information about the water absorption kinetics. The tests are accompanied by analytical and electrochemical methods, e.g., dynamic vapour sorption and electrochemical impedance spectroscopy. [1] Zanni-Deffarges, M. P.; Shanahan, M.E.R. (1995): Diffusion of water into an epoxy adhesive. Comparison between bulk behaviour and adhesive joints. In International Journal of Adhesion and Adhesives 15 (3), pp. 137–142. DOI: 10.1016/0143-7496(95)91624-F. [2] Wapner, K.; Grundmeier, G. (2004): Spatially resolved measurements of the diffusion of water in a model adhesive/silicon lap joint using FTIR-transmission-microscopy. In International Journal of Adhesion and Adhesives 24 (3), pp. 193–200. DOI: 10.1016/j.ijadhadh.2003.09.008. [3] Hausbrand, R.; Stratmann, M.; Rohwerder, M. (2009): Corrosion of zinc–magnesium coatings. Mechanism of paint delamination. In Corrosion Science 51 (9), pp. 2107–2114. DOI: 10.1016/j.corsci.2009.05.042.

Microscale groove effect on shear strength of epoxy-bonded dissimilar metal plate

In this study, the shear strengths of Al 7075–HSS adhesive bonded, grooved and smooth plates were investigated. The proven toughness and durability of adhesives have drawn the attention of researchers who want to take advantage of the technology to benefit the development of ballistic resistance sandwich panels. However, the strength of the panel depends on the design of surface topography. Therefore, it is essential to understand the fracture upon loading parallel to the plane of the adhesive bonded metal plates. In this experiment, toughened epoxy was used to bond dissimilar metal plates at 1 mm thickness. The shear tests were performed with a universal-testing machine to identify the maximum fracture loads. The results showed that a shear lap joint specimen with a grooved surface yields a higher strength than a smooth specimen. From the fracture behaviour of all specimens, interfacial failure with some degree of cohesive failure was observed. This indicates that the strength of the adhesive-bonded metal plate driven by a mechanical interlocking effect and mode of failure for thick bondline was the result of interfacial strength rather than adhesive bulk strength. Shear value results and fractography for 1 mm bond thickness provide insights towards steel fibre application in epoxy.

Finite element analysis of timber-glass walls

The paper presents a finite element analysis (FEA) of experimentally investigated timber-glass composite walls. So far a series of mechanical tests with in-plane monotonous static and quasi-static cyclic loading were carried out on timber-glass walls with the goal to analyse their general behaviour and to experience failure mechanisms that occur. Timber-glass walls consisted of glass panes, adhesively bonded to the timber frame with different types of adhesives, i.e. silicone, polyurethane and epoxy. FEA was limited to timber-glass walls subjected only to monotonous static loading. A commercial finite element code Ansys was used to perform the simulation of experiments. As the types of adhesive are decisive for the behaviour of timber-glass walls, additional experiments on bulk adhesives are briefly discussed. The main aim of the performed FEA was to build as accurate as possible mathematical model of timber-glass walls to achieve the correct general behaviour, failure mechanisms and to confirm the composite action in case of using flexible adhesives, e.g. silicone and polyurethane. Geometrically nonlinear finite element method was used including solid hexahedral finite elements. Some variations of material models and mesh size were performed to calibrate the numerical model with the results of experimental investigations in order to reach the desired response. The above described experimental and FE investigations present a starting point for a comprehensive parametric study.

The mechanical performance of adhesive joints containing active disbonding agents

A controllable adhesive disbonding mechanism can be achieved by incorporating, and then activating, functional additives located within the matrix of an adhesively bonded joint. This action facilitates the disassembly and material recovery of structurally bonded assemblies.The engineering capabilities of bonded joints containing a range of chemical or physical foaming agents were investigated. Joint disassembly was measured as a function of additive chemistries, physical properties and concentration. The short- and long-term mechanical properties of bonded joints and bulk adhesives containing functional additives were subsequently evaluated.The disbonding mechanism was found to be due to a ‘solid-state foaming’ of the adhesive, followed by a proliferation of micro-cracking throughout the matrix. The effect of the physical foaming agents on joint disassembly was mostly attributable to the volumetric expansion efficiency of the additive whilst constrained within an adhesive matrix. For chemical foaming agents, the effectiveness of disassembly was dependent on the chemical compatibility of the adhesive and additive, the concentration and the efficiency to which the additive decomposed to a gas.

Moulds design for adhesive bulk and joint specimens manufacturing

PurposeThe purpose of this paper is to provide an insight into the techniques which are used and developed for adhesive bulk and joint specimens manufacturing.Design/methodology/approachAfter a short introduction, the paper discusses various techniques for adhesive bulk and joint specimens manufacturing and highlights their advantages and limitations. A number of examples in the form of different bulk and joint specimens of different types of adhesives are used to show the methods for determining the adhesive's mechanical properties needed for design in adhesive technology. In order to predict the adhesive joint strength, the stress distribution and a suitable failure criterion are essential. If a continuum mechanics approach is used, the availability of the stress‐strain curve of the adhesive is sufficient (the bulk tensile test or the TAST test is used). For fracture mechanics‐based design, mode I and mode II toughness is needed (DCB and ENF tests are used). Finally, single lap joints (SLJs) are used to assess the adhesive's performance in a joint.FindingsBefore an adhesive can be specified for an application, screening tests should be conducted in order to compare and evaluate the various adhesion parameters. Properties of adhesives can vary greatly and an appropriate selection is essential for a proper joint design. Thus, to determine the stresses and strains in adhesive joints in a variety of configurations, it is necessary to characterize the adhesive behaviour in order to know its mechanical properties. A great variety of test geometries and specimens are used to obtain adhesive properties. However, for manufacturing of adhesive bulk specimens and joints necessary for use in these tests, properly, moulds should be designed.Originality/valueThe paper summarises the main methods of preparing adhesive bulk and joint specimens and the test methods for determining the mechanical properties needed for design in adhesive technology. Emphasis is given to the preparation of specimens of suitable quality for mechanical property determination and the moulds designed for this purpose.

Significance of the conformation of building blocks in curing of barnacle underwater adhesive

Barnacles are a unique sessile crustacean that attach irreversibly and firmly to foreign underwater surfaces. Its biological underwater adhesive is a peculiar extracellular multi-protein complex. Here we characterize one of the two major proteins, a 52 kDa protein found in the barnacle cement complex. Cloning of the cDNA revealed that the protein has no homolog in the nonredundant database. The primary structure consists of four long sequence repeats. The process of dissolving the protein at the adhesive joint of the animal by various treatments was monitored in order to obtain insight into the molecular mechanism involved in curing of the adhesive bulk. Treatments with protein denaturant, reducing agents and/or chemical-specific proteolysis in combination with 2D diagonal PAGE indicated no involvement of the protein in intermolecular cross-linkage/polymerization, including formation of intermolecular disulfide bonds. As solubilization of the proteins required high concentrations of denaturing agents, it appears that both the conformation of the protein as building blocks and non-covalent molecular interactions between the building blocks, possibly hydrophobic interactions and hydrogen bonds, are crucial for curing of the cement. It was also suggested that the protein contributes to surface coupling by an anchoring effect to micro- to nanoscopic roughness of surfaces.

Study on the Effect of Strain Rate of Adhesive Material on the Stress State in Adhesive Joints

The effect of the strain rate sensitivity on the behavior of bulk materials of epoxy resin adhesives is examined experimentally using an INSTRON®-type material testing machine. In the present study, the strain rate is chosen in the range 10−4∼10−1/s. The effects of the strain rate sensitivity of adhesive joints in which solid cylinders and hollow cylinders are bonded (tubular lap adhesive joints) are also examined using an INSTRON-type material testing machine and a split Hopkinson pressure bar apparatus (SHPB). In addition, the stress distributions at the adhesive interface in the tubular lap adhesive joints under static and impact loadings are examined using two-dimentional axi-symmetrical finite element stress analysis. The results of compressive and tensile tests for the adhesive bulk specimens show that the yield stress increases as the strain rate increases. The results of quasi-static and impact shear tests for the tubular lap adhesive joints show that the shear strength increases as the strain rate increases. The numerical results show that the maximum value of the shear stress, τrz, decreases as the adhesive thickness decreases under static loadings, while the maximum value of the shear stress, τrz, decreases as the adhesive thickness is about 0.2 mm under impact loadings. These results under impact loadings show different characteristics from results under static loadings. A fairly good agreement is found between the numerical and the experimental results.

The selection and performance of adhesives for a steel–glass connection

Frame-supported glass structures such as curtain walls consist of glass infill plates mounted onto a framework of metallic elements. The longitudinal shear transfer at the interface of the glass and the metal is generally insufficient to mobilise composite action between the two materials, thereby making the units structurally inefficient. In this paper we investigate five candidate adhesives for load bearing steel–glass connections by means of mechanical testing and numerical modelling. The mechanical tests on representative steel–glass connections provide useful data for the selection of a suitable adhesive. The systematic characterisation of the time-dependent constitutive models of the bulk adhesives provides data essential for analytical and numerical models. Good agreement between the experimental results and the numerical models provides a basis for improving the structural efficiency of frame-supported glass structures.

Rehabilitation of timber structures – Preparation and environmental service condition effects on the bulk performance of epoxy adhesives

Epoxy adhesives have been used for many years in the rehabilitation of timber structures and are currently the most appropriate adhesive type for on-site operations. However, because they exhibit excellent initial joint strength when tested in standard climate conditions, there has not been a major concern about their service durability. In order to contribute to the ongoing discussion on the reliability of these adhesives, a study was conducted to evaluate the effect of environmental service conditions on the durability of four commercial two-component structural epoxy adhesives, as well as the effect that the type of mixing, curing and post-curing conditions, as well as the presence of moisture have on the adhesives viscoelastic properties. It was found that the preparation conditions, cure schedule and moisture have a significant effect on the performance of the bulk adhesives. Moreover, it was found that under ambient conditions there is potential for under-cure or slow progression of cure for the epoxy adhesive products typically employed in these applications, which in turn can compromise the durability of a bonded joint.

The role of alumina nanoparticles in epoxy adhesives

Both untreated and calcined fumed alumina nanoparticles were dispersed into an epoxy-based adhesive at various percentages. The glass transition temperature of the nanofilled adhesives increased up to an optimal filler loading and then decreased, probably due to concurrent and contrasting effects of chain blocking and reduction of the crosslinking degree. Tensile modulus, stress at break, and fracture toughness of bulk adhesive were positively affected by the presence of untreated alumina nanoparticles at an optimal filler content. Mechanical tests on single-lap aluminum bonded joints indicated that untreated alumina nanoparticles markedly improved both the shear strength and fatigue life of the bonded joints. In particular, the shear strength increased by about 60% for an optimal filler content of 1 vol.%, and an adhesive failure mechanism was evidenced for all the tested specimens. Concurrently, a relevant decrease of the equilibrium contact angle with water was observed for nanofilled bulk adhesives. In summary, alumina nanoparticles can effectively improve the mechanical performances of epoxy structural adhesives, both by increasing their mechanical properties and by enhancing the interfacial wettability with an aluminum substrate.

Fracture toughness of bulk adhesives in mode I and mode III and curing effect

Failure load predictions of adhesively bonded lap joints are either done based on a stress/strain limit criterion or using concepts of fracture mechanics. For the former case, the stress-strain curves of the adhesive must be determined accurately and for the latter case the toughness of the adhesive is needed. The present study gives for two adhesives, one brittle and one ductile, the strength and fracture properties in the bulk form. Stress-strain curves are given in tension and shear. The toughness was measured in mode I using a three point bending specimen and in mode III with a circular specimen with a notch under torsion. Mixed-mode criteria are discussed and it is shown that the experimental results are in good agreement with the strain energy theory. Finally, the effect of shrinkage stresses on the fracture toughness was studied and it is shown that they have a substantial effect on the fracture toughness.

In situ ATR-IR spectroscopic and electron microscopic analyses of settlement secretions of Undaria pinnatifida kelp spores.

Knowledge about the settlement of marine organisms on substrates is important for the development of environmentally benign new methods for control of marine biofouling. The adhesion to substrates by spores of Undaria pinnatifida, a kelp species that is invasive to several countries, was studied by scanning electron and transmission electron microscopies (SEM/TEM) as well as by in situ attenuated total reflection infrared (ATR-IR) spectroscopy. The IR spectra showed that adhesive secretion began approximately 15 min after initial settlement and that the adhesive bulk material contained protein and anionic polysaccharides. Energy dispersive X-ray microanalysis of the adhesive identified sulphur and phosphorus as well as calcium and magnesium ions, which facilitate the gelation of the anionic polysaccharides in the sea water. The adhesive may be secreted from Golgi bodies in the spore, which were imaged by TEM of spore thin sections. Additionally, an in situ settlement study on TiO(2) particle film by ATR-IR spectroscopy revealed the presence of phosphorylated moieties directly binding the substrate. The presence of anionic groups dominating the adhesive suggests that inhibition of spore adhesion will be favoured by negatively charged surfaces.