Study on the fatigue strength of AA 6082-T6 adhesive lap joints

A research study on the fatigue behaviour of aluminium alloy adhesive lap joints was carried out to understand the effect of surface pre-treatment and adherends thickness on the fatigue strength of adhesive joints. The adherend material used for the experimental tests was an aluminium alloy 6082-T6 in the form of thin sheets, and the adhesive used was a high strength epoxy (Araldite 420 A/B). The surface preparation included an abrasive preparation (AP joints) and sodium dichromate–sulphuric acid etch (CSA joints).A maximum fatigue strength was obtained for the CSA surface treatment with a 1.0 mm adherends’ thickness. The fastest fatigue damage was related with a high surface roughness and a high stress perpendicular to adhesive surface, which helps to promote the adhesive failure. A numerical analysis was also performed to understand the effect of the adherends thickness on the stress level. Results showed an increase of the out-of-plane peak stresses with the increase of adherends thickness.     

The effect of cataphoretic and powder coatings on the strength and failure modes of EN AW-5754 aluminium alloy adhesive joints

The aim of this study is to determine the effect of cataphoretic and powder coatings and also the method of application the primer on the adherends surface on the strength and failure modes of EN AW-5754 aluminium alloy adhesive joints. The study is performed on lap joints made of EN AW-5754 aluminium alloy, subjected to three different types of surface treatment; namely a) polyurethane cataphoretic coating, b) powder coating based on black mat RAL 9005 UL polyester resin and c) no coating. The tested adhesive joints were made using a one-component polyurethane adhesive Terostat 8596, which was dedicated for automotive and cured under a constant load of 0.018 MPa at 20 ± 2 °C. In addition, this study investigates the effect of the application of Terostat 8519P adhesion promoter which is a liquid polyurethane-based primer containing solvents and which is corresponding to Terostat 8596 polyurethane adhesive. Terostat 8519P adhesion promoter was applied in two different ways: a) to one substrate and b) to both substrates. The produced adhesive joints were subjected to strength tests using the Zwick/Roell Z150 testing machine. The examination of fracture in the tested adhesive joints was performed in accordance with the EN ISO 10365 standard. The shear strength results have demonstrated that both the method of application of the adhesion promoter (Terostat 8519 P) and the presence of cataphoretic coating had an influence on adhesive joints strength. The use of the adhesion promoter significantly affects the strength of both uncoated EN AW-5754 aluminium alloy adhesive joints and the adhesive joints subjected to powder coating. The use of the adhesion promoter has a less significant effect on the cataphoretic-coated samples.     

Effect of chain length of self-assembled monolayers on adhesion force measurement by AFM

It has been confirmed experimentally that the adhesion force measured between an atomic force microscope (AFM) tip and the self-assembled monolayers (SAMs) has a direct correlation with the chain length of SAMs, and that the adhesion force decreases with the increase of the chain length. In this paper, a theoretical model is put forward to calculate the adhesion force between the AFM tip and the SAMs by integrating the Lennard–Jones potential. The theoretical results are in good agreement with the existing experimental results.     

Electrodeposition and characterization of polypyrrole films on aluminium alloy 6061-T6

Polypyrrole films on aluminium alloy 6061-T6 were prepared by electropolymerization of pyrrole in sulphuric acid using two different processes - cyclic voltammetry and potentiostatic polarization - and assessed through SEM observation and voltammetry. The anticorrosive properties of these films were studied by polarization curves and electrochemical impedance spectroscopy.The polypyrrole films formed by both methods are homogeneous and present a globular structure. However, it was found that the films produced by cyclic voltammetry are thicker than those produced potentiostatically at a potential equal to the upper limit of the cyclic voltammetry (E_λa). It was also found that there is an optimum value for the formation potential (E_appl, in the case of the potentiostatic method or E_λa for cyclic voltammetry). Above this value, overoxidation of the polymer occurs, which is found to be deleterious to the coated system behaviour.From polarization curves no major differences were detected between the PPy-coated alloy and the bare material, indicating that no significant protection is achieved by the polymer coating. On the other hand, Bode diagrams are typical of a system undergoing pitting corrosion and show lower impedance values for the alloy covered with polypyrrole than for the bare metal. This can be attributed to the conductive character of the polypyrrole films.     

Polyaniline coatings on aluminium alloy 6061-T6: Electrosynthesis and characterization

The production of polyaniline films on aluminium alloy 6061-T6 in sulphuric acid was carried out by electrodeposition, using cyclic voltammetry and potentiostatic polarization. The films obtained were characterized by SEM and XPS analysis. Electrochemical techniques were also used to assess the anticorrosive properties of the coatings.The choice of the upper potential limits for potential cycling and of the fixed potential for potentiostatic production should take in consideration the balance between their effect on the minimum number of cycles (or time) needed to produce the film and on the rate of overoxidation.Polarization curves show a slight increase of the corrosion potential for polyaniline coated substrate as compared with bare metal, indicating that no significant protection is achieved by the polymer coating. The Bode diagrams show low impedance values for the alloy covered with polyaniline when compared with the results obtained for the bare metal. This can be attributed to the conductive character of the polyaniline films.     

Temperature dependence of microscale adhesion force between solid surfaces using an AFM

The adhesion force between two solid surfaces is of great interest with the rapid development of micro–nanodevices and instruments. The effect of temperature on the microscale adhesion force has been studied by recording force–displacement curves with an atomic force microscope. A flat tip with a diameter ~1.73 μm was used to prevent wear. The adhesion force measurements were carried out under ambient conditions and in a nitrogen-filled glove box. The substrate temperature was varied between 30 and 200 °C. The results show that when the temperature is <200 °C, the influence of temperature on the normal spring constant of the cantilever can be ignored. In this temperature range, the adhesion force distribution for each temperature exhibits a Gaussian-like distribution under both situations. Under ambient conditions, the mean adhesion force first increases with the increase in temperature and reaches the maximum at ~100 °C. Then the adhesion force begins to decline slightly. At about 150 °C, the adhesion force decreases dramatically, and remains relatively stable at high temperatures. The increase in adhesion force is associated with the capillary force. The elevated temperature leads to larger and more numerous liquid bridges. The capillary nucleation, the diffusion of water molecules, and the flow of thin water film are all enhanced with the elevated temperature. However, in dry nitrogen, the mean adhesion force decreases with the increase in temperature. This trend is attributed to the broken van der Waals bonds.     

Tracking the hydrophobicity recovery of PDMS compounds using the adhesive force determined by AFM force distance measurements

Polydimethylsiloxane (PDMS) materials show the unique phenomenon that when exposed to electrical discharge, such as corona discharge, their hydrophobic surface becomes hydrophilic. However, after a certain relaxation time they gradually regain their hydrophobicity. In this study the adhesive force obtained by AFM force distance measurements using a hydrophilic Si₃N₄ probe is used to track the recovery of the hydrophobicity. The time constant of the recovery can be determined by measuring the adhesive force as a function of the recovery time after corona exposure. It is shown how these time constants can be used to monitor the recovery rate as a function of corona treatment time for both filled and unfilled PDMS compounds.     

Degradation behaviour of 6013-T6, 2024-T3 alloys and pure aluminium in different aqueous media

The electrochemical and corrosion behaviour of 6013-T6, 2024-T3 aluminium alloys and pure aluminium in 0.6m NaCl and 0.3m Na2SO4 aqueous solutions was investigated using d.c. and a.c. electrochemical techniques. Results show that the 6013-T6 alloy exhibits superior corrosion resistance compared with that exhibited by the 2024-T3 alloy in both environments. These findings were interpreted on the basis of a barrier oxide film model.     

Experimental analysis of the influence of surface topography on the adhesion force as measured by an AFM

Force curves have been acquired using an atomic force microscope (AFM) on homogeneous microspheres of three different materials (latex, glass and yttria), in order to study the possible influence of the surface topography/geometry on the adhesion force as measured by an AFM. Forces were measured in regions at the top of the spheres ( ≈ 90°), at half-heights ( ≈ 0°) and in an intermediate region between these two ( ≈ 45°), where the angle is measured from the equatorial plane of the sphere to its polar axis. A very irregular and non-reproducible behaviour was found at ≈ 0°, so only the other two regions were quantitatively analysed. For all the three materials, a much smaller adhesion force was obtained in the region corresponding to ≈ 45° as compared to ≈ 90°. Moreover, a quite similar adhesion decrease ratio of about 1.60 ± 0.5 was obtained for all the three materials, which may suggest that the observed behavior might be due to geometrical factors. This observed influence could, in part, explain the observed heterogeneity in adhesion maps of microbial cells reported in the literature. The influence of the surface roughness is also discussed and it seems to result in a poor reproducibility of force curves.     

Logarithmic decrease of adhesion force with lateral dynamic revealed by an AFM cantilever at different humidities

Adhesion forces between a tipless cantilever and an Au film were determined to investigate the influence of lateral velocity by recording force curves with an atomic force microscope at 20%–90% relative humidities. The sample was moved laterally, forth and back, with a frequency of 0.001–100 Hz and scan distances of 0.8, 8, and 80 μm to achieve a velocity ranging over 7 orders of magnitude. Experimental results show that at low lateral velocities (between 1.6 nm/s and 1–10 μm/s), the adhesion force either increases or decreases or remains stable with the lateral velocity without a certain characteristic trend. However, after a critical velocity, the adhesion force decreases logarithmically with the lateral velocity (between 1–10 and 16,000 μm/s). The decreasing magnitude can be as large as 97.3% of the maximum adhesion force. This decrease is well-explained by the contact time dependence of water bridges formed by capillary condensation.     

Effect of laser beam focusing point on AFM measurements

The optical beam deflection method, which is used in AFM to obtain surface images, may distort the resulting image. The flexible and long cantilever is easily overdamped by the laser radiation pressure, resulting in steady deflection of the cantilever (<1 nm). This deflective force distorts the image and influences the force-distance (F-D) curve. The present study investigated the effect of laser radiation pressure on image distortion. As a proof-of-concept test, two grating samples (with step heights of 150 and 18 nm for TGX01 and TGZ01, respectively) were examined with an NSC36 series cantilever in air and water media.     

Evolution and level behavior of adhesion force by repeated contacts of an AFM colloid probe in dry environment

Adhesion forces between a colloid probe and some samples were consecutively measured at a single location with an atomic force microscope (AFM) in a dry environment. The outcomes show that the adhesion force depends considerably on material, contact history, and number and distribution of asperities within the contact zone. Generally, there are four different stages for the adhesion force with increasing measurement number: random behavior for the first several or tens of contacts, then increasing monotonically, later remaining stable, and finally decreasing. Usually, the measured data points are grouped into several levels. Moreover, the adhesion force jumps frequently between different levels and is more inclined to jump to a neighboring level. The level behavior was attributed to the fact that the real contact region is not exactly the same between successive measurements. The differences in the adhesion force between two neighboring levels are almost the same for one location. The adhesion force in some levels usually increases or decreases discontinuously after jumping to other levels and then jumping back. The magnitude and fluctuations of the adhesion force and the number of levels depend on the number and distribution of asperities in the contact zone.     

Influence of lateral velocity on adhesion force of surfaces with different hydrophilicity revealed by an AFM colloidal probe at humid environments

Adhesion forces between a polystyrene microsphere and some samples with different hydrophilicity were measured to investigate the effect of lateral velocity by using an atomic force microscope (AFM) at 30% ± 1% and 70% ± 1% relative humidities. The sample at test was driven to move laterally (forth and back) with a scan distance 80 μm and lateral velocities between 0.16 and 16,000 μm/s. For hydrophilic samples, the mean adhesion force follows two regimes with lateral velocity: At first it either increases or decreases or remains stable, and then it decreases logarithmically after a critical velocity. However, for hydrophobic samples, the mean adhesion force not only follows these two regimes but also has a third regime, i.e., remains stable at large lateral velocities. The logarithmical decrease was ascribed to the contact time dependence of water bridges. The stable trend at high lateral velocities on hydrophobic samples was attributed to the fact that the capillary force vanishes and the microsphere can pull off a sample surface at a contact region with the least number of adhesive asperity junctions on the scan path. The water contact angle (CA) has some effect on the critical velocity—the smaller the CA, the larger the critical velocity.     

On the fatigue durability of clad 7075-T6 aluminium alloy bonded joints representative of aircraft repair

The fatigue durability of bonded joints representative of repairs to aircraft structure with and without the presence of a clad layer was investigated by testing aluminium alloy 7075-T6 double lap shear joint specimens. This was done by changing the bonding interface of the outer strap. The joint geometry, central adherend material, adhesive and surface preparation method were all kept the same. On two of the specimen types, the strap material was clad 7075-T6, with one type fabricated with the clad layer left on prior to surface preparation, and the other with the clad layer removed. On the last specimen type, the strap material was unclad 7075-T6. The test results showed that the fatigue durability was lowest when the clad layer was left in-situ, followed closely by those with the clad layer removed. The unclad specimens achieved a fatigue life one order of magnitude greater than those with the clad layer physically removed. Under constant amplitude loading, adhesive fatigue cracking was observed at the location of peak load transfer, which progressed to the interface. Analysis showed that the cracking caused a substrate stress concentration which may have caused the clad fracture. Further analysis, supported by test observations, showed that once a small notch had formed at the interface, damage progression through the outer strap was rapid.     

Interaction force measurements using atomic force microscopy for characterization and control of adhesion,dispersion and lubrication in particulate systems

Atomic force microscopy is used as a vital tool in understanding the fundamental mechanisms of particulate processes in dry, humid and aqueous systems. Adhesion forces in both dry and humid systems were studied between surfaces of varying roughness, taking into account the capillary forces at high humidity conditions. Colloidal stability in aqueous systems due to non-DLVO forces and steric effects of surfactant aggregates formed on particle surfaces at varying pH and ionic strength conditions were investigated. The force–distance curves obtained by atomic force microscopy were used to determine the mechanical and thermodynamic properties of the self-assembled surfactant structures formed on the surface. Besides determining the repulsive force barrier provided by the surfactant aggregates in dispersion of slurries, the frictional interactions between surfactant adsorbed surfaces were measured using lateral force microscopy, providing valuable insights into the role of dispersants acting as lubricants. The range of interaction forces that can be explored using the Atomic Force Microscopy (AFM) can be utilized to predict, optimize and design a variety of industrially relevant processes such as chemical mechanical polishing (CMP), powder flow and handling and nano-dispersions, just to name a few.     

Visualization of nanomechanical mapping on polymer nanocomposites by AFM force measurement

Nanocomposite based on an elastomer, natural rubber (NR), and pristine multi-walled carbon nanotubes (MWCNT) was prepared using a two-roll mill mixer. The high shearing stress induced homogeneous dispersion of 5 phr. MWCNTs in NR matrix. A procedure based on combination of Johnson-Kendall-Robert (JKR) contact mechanics and “two-point method” together with AFM force measurements, was successfully used to visualize nanomechanical mapping on the resulting nanocomposites. Topography, elastic modulus, and adhesive energy distribution maps were obtained at the same point and at the same time in a single scan. Such maps were successfully used to identify and characterize CNTs and NR regions in nanocomposites. The intermediate modulus region formed around CNTs was investigated on the quantitative evaluation in real space and demonstrated the existence of interaction between CNTs and NR matrix.     

Some aspects of chemistry in adhesion on anodized aluminium

The chemistry of adsorption of typical compounds of epoxy and phenolic resins on various anodized aluminium surfaces was studied by Fourier Transform infrared spectroscopy. It was found that resols and epoxynovolak form strong complex compounds with the oxide on the aluminium surface and that dicyandiamide is partially hydrolyzed to guanylurea. These results are correlated with the properties of adhesive joints.     

Surface contaminants on annealed aluminium foil and their effects on adhesion

Surface fast atom bombardment mass spectroscopy (FABMS) and X-ray photoelectron spectroscopy (XPS) have been used to study the chemistry of the residues remaining on aluminium foil surfaces following annealing. In addition surface FABMS spectra were obtained of the base oil and additives used in the rolling lubricant. Surface FABMS analysis showed that none of the original additives or components of base oil remained on the surface following annealing. Instead the surface was partially covered with organic acid anions of varying molecular weight. The total concentrations of residues determined by XPS and surface FABMS correlated well with the variation in adhesion properties across the width of coiled, annealed foil.     

Atomic force microscopic (AFM) study on a self-organizing polymer film

Summary The structure of a self-organized polymer film prepared by slow evaporation of organic solvents is studied by atomic force microscopy, and compared with that of polymer film prepared by natural evaporation of organic solvents. AFM studies clearly indicates that there exists a self-organizing process of alkyl chains resulting in the partial interdigitated layer structure under the condition of slow evaporation of organic solvents. Received: 27 July 1998/Revised version: 7 October 1998/Accepted: 2 November 1998     

The effect of residual stresses on adhesion measurements

The adhesion of films and coatings is often measured by determining the load required to separate them from their substrate. If there are residual stresses that are relaxed upon delamination, then an additional contribution to the energy-release rate will affect the measurements. These residual stresses may also cause a shift in the mode-mixedness of the interface crack which, in turn, can affect the interfacial toughness. To ensure an accurate interpretation of adhesion measurements, therefore, the effects of these stresses must be considered. These effects are discussed with particular reference to two commonly used test geometries: the blister test and the peel test.