Aluminium Adhesive Joints Research Articles

A comparative study of the effect of various surface treatments on the fracture behavior of aluminum adhesive joints under mode I loading

This study investigates the effect of different surface treatments on the fracture behavior of epoxy-aluminum joints under mode-I loading. Six surface treatments including degreasing, abrasion with varying grit sizes, alkaline etching, acid etching, a combination of alkaline and acid etching, and a combination of abrasion and acid etching were applied to aluminum surfaces before bonding. Surface morphology, roughness parameters, total surface free energy, contact angle, and elemental composition were analyzed. Results showed that the ultimate fracture load and fracture energy initially increased and then decreased with increasing surface roughness. The most effective treatments were found to be acidic etching in combination with alkaline etching and abrasion due to their synergetic effects. These treatments removed the natural oxide layer and created a porous oxide layer, enhancing surface roughness, increasing adhesive-substrate contact areas, and providing more sites for mechanical interlocking. Compared to the degreased specimens, significant improvements were observed, including a 40% and 31% increase in maximum fracture load, a 74% and 53% increase in initiation fracture energy, and a 65% and 47% increase in propagation fracture energy, respectively. Etching treatments demonstrated superior effects on the fracture behavior of aluminum adhesive joints compared to abrasion methods, leading to cohesive failure after etching treatments.

Ultrasonic Features for Evaluation of Adhesive Joints: A Comparative Study of Interface Defects.

Ultrasonic non-destructive evaluation in pulse-echo mode is used for the inspection of single-lap aluminum adhesive joints, which contain interface defects in bonding area. The aim of the research is to increase the probability of defect detection in addition to ensuring that the defect sizes are accurately estimated. To achieve this, this study explores additional ultrasonic features (not only amplitude) that could provide more accurate information about the quality of the structure and the presence of interface defects. In this work, two types of interface defects, namely inclusions and delaminations, were studied based on the extracted ultrasonic features in order to evaluate the expected feasibility of defect detection and the evaluation of its performance. In addition, an analysis of multiple interface reflections, which have been proved to improve detection in our previous works, was applied along with the extraction of various ultrasonic features, since it can increase the probability of defect detection. The ultrasonic features with the best performance for each defect type were identified and a comparative analysis was carried out, showing that it is more challenging to size inclusion-type defects compared to delaminations. The best performance is observed for the features such as peak-to-peak amplitude, ratio coefficients, absolute energy, absolute time of flight, mean value of the amplitude, standard deviation value, and variation coefficient for both types of defects. The maximum relative error of the defect size compared to the real one for these features is 16.9% for inclusions and 3.6% for delaminations, with minimum errors of 11.4% and 2.2%, respectively. In addition, it was determined that analysis of the data from repetitive reflections from the sample interface, namely, the aluminum-adhesive second and third reflections, that these contribute to an increase in the probability of defect detection.

Effects of Double and Triple Hybrid Nanomaterial Reinforcements on Mechanical Performance of Aluminium Adhesive Joints

Effects of Double and Triple Hybrid Nanomaterial Reinforcements on Mechanical Performance of Aluminium Adhesive Joints

The effect of hygrothermal aging on the adhesion performance of nanomaterial reinforced aluminium adhesive joints

Adhesive joints are subjected to various environmental factors such as humidity and temperature, depending on their use in industrial applications. It is important to examine and improve the performance of adhesive joints formed with polymeric adhesives, which are known to be sensitive to humidity and temperature parameters, under these conditions. For this reason, in this study, FPL-etched AA7075-T6 aluminium adherends bonded with nonreinforced and BNNP (boron nitride nanopowder), ANP (alumina nanopowder) and MWCNT (multiwalled carbon nanotubes) reinforced epoxy adhesive as a single lap adhesive joint. Thereafter, performance of the adhesive joints that exposed hygrothermal aging conditions of 1, 2, 4 and 6 weeks at 100% relative humidity (RH) and 60 °C were investigated. Glass transition temperature (Tg) values of nonreinforced and reinforced epoxy adhesives before and after hygrothermal aging were determined by DSC (Differential Scanning Calorimeter) analyses. Depending on the nanomaterial types, the tendency of the adhesive joints to water uptake was determined and compared with nonreinforced adhesive joints. When the shear strength findings were investigated, it was defined that the adhesive joints reached almost saturation after 2 weeks of hygrothermal aging. In addition, the most effective nanomaterial type against hygrothermal aging was determined as MWCNT. After 2 weeks hygrothermal aged, the shear strength decreases of MWCNT reinforced adhesive joints were 21.77% and 16.97% less than BNNP and ANP reinforced adhesive joints, respectively. Although the adhesive deteriorates chemically after hygrothermal aging, its adhesion performance improved with the physical benefits of nanomaterials. However, ANP, which is cheaper in terms of price/shear strength performance in long-term aging times, showed similar results with MWCNT. In addition, fracture mechanisms were examined in more detail with SEM (Scanning Electron Microscope) analyses before and after aging, and the effects of hygrothermal aging on the adhesive joints were determined.

Assessment of Composite Aluminum Adhesive Joints Using Digital Image Correlation

Traditional nondestructive evaluation (NDE) methods present significant challenges to detecting and characterizing kissing or weak bonds in adhesively bonded structures. These kissing or weak bonds also cannot transmit shear stresses or handle complex loading modes and, if not detected, can present a significant threat to the structural integrity of the components or systems. This paper demonstrates the digital image correlation (DIC) technique for evaluating adhesively bonded dissimilar materials joints subjected to kissing or weak bonds. The study employed four adhesively bonded carbon fiber reinforced plastics and aluminum (CFRP-Al) lap-shear test coupons with varied bond quality (i.e., with no contamination and three simulated kissing bond defects). The novelty of the approach presented in this paper was that this technique could detect and demonstrate changes in the normal strain (εyy) contour map of the contaminated specimens at relatively lower load levels. This load level corresponds to 15% of the failure load for the silicone and hydraulic oil contaminated sample and around 30% for the polyvinyl alcohol (PVA) contaminated sample. In addition, higher compressive strains along the overlap edges were observed in the strain map for the single lap joints due to the higher peeling stresses of the adherend and the stress concentration at the edges of an adhesively bonded joint.

Mechanical Performance Enhancement of Aluminum Single-Lap Adhesive Joints Due to Organized Alumina Nanotubes Layer Formation on the Aluminum Adherends

The present investigation aims to take a step forward for the transfer of a simple laboratory electrochemical method of surface nano-treatment of aluminum to industrial applications. The electrochemical method has been applied to process 1050A aluminum. Surface nano-structuring has been achieved and resulted in the formation of an organized alumina nanotubes layer on commercial aluminum plates used as adherends for the manufacturing of aluminum single-lap adhesive joints. The mechanical properties of single-lap aluminum adhesive joints constructed with both non-anodized and anodized adherends were investigated and compared. Two types of epoxy resins were used to prove that the anodization of the adherends is equally effective, independently of the adhesives’ type. Furthermore, three overlap lengths were used (7, 10, and 25 mm) to study the effect of the overlap length on the overall joint mechanical response. Results of both three-point bending and tensile–shear testing showed that there is a considerable improvement of the joints’ mechanical performance with the addition of the nanostructures, for all the overlap lengths. It was found that the anodization method greatly contributes to the strengthening of the joints, leading to a strength increase of up to 176% and 148% for the shear and three-point bending strength, respectively.

Effect of Temperature and Moisture on the Impact Behaviour of Adhesive Joints for the Automotive Industry

This study focuses on evaluating the impact of aluminum adhesive joints as a function of temperature and moisture, in an effort to understand how these conditions affect their mechanical properties and behavior. After preparing the required specimens (using two different adhesives and adherend thicknesses), several tests have been made in order to determine these properties and compare their values to the predictions made using analytical methods. These tests were repeated with several distinct combinations of temperatures and moisture levels so that the effect of these properties can be properly interpreted. It was observed that higher temperatures strongly increase the ductility of the adhesive but mixed with moisture this can degrade them. Moisture can increase the energy absorbed through increased plastic deformation of the adhesive and improve behaviour at low temperatures.

Fracture energy assessment of adhesives – Part I: IsGICan adhesive property? A neural network analysis

Different joint types and geometries have been considered to measure the fracture toughness ( Gc) of adhesives. However, results show that, sometimes, the obtained Gcvalues for the same adhesives are not similar in different reports. Several factors including the joints geometry, material properties of the substrate, and also the test conditions influence the obtained results. This study is conducted to find a logical connection between these factors and to find their influences on the obtained mode I fracture toughness ( GIc) to understand which parameters are the most influential and which ones are less significant. To this aim, 115 values of different GIcalready reported in the literature were collected and examined regarding the geometrical and material parameters. To find the rational relationship between the effective parameters and the reported GIcvalues, a method based on the artificial neural network technique was employed. The results revealed that whilst the fracture energy of aluminum adhesive joints is more influenced by geometrical parameters including the joint type and substrate thickness, the steel adhesive joints are more sensitive to the adhesive properties and bondline thickness. With this study, it is possible to design an optimum test by minimizing the effects of variables that cause errors in obtaining fracture toughness and also to estimate GIcof adhesives by using the developed model. On the other hand, it is also possible to design real structures in which the fracture toughness of the adhesive reaches its maximum value.

Structural health monitoring of adhesive joints under pure mode I loading using the electrical impedance measurement

The addition of Multi-wall carbon nanotubes (MWCNTs) or functionalized-graphene nanoplatelets (f-GNPs) into the epoxy resin create an electrical conductive networks which can be employed for structural health monitoring (SHM) in sensitive adhesive joints. The opening fracture mode (Mode I) loading condition, as one of the important fracture modes in adhesive joints was precisely evaluated in this study. The study aims to investigate the crack extension in the adhesive joint (Fracture mechanics) under mode I loading by both smart adhesives and proper measurement technique. In order to provide high sensing capability in the adhesive joint, several aluminum to aluminum adhesive joints were prepared by dispersion of 2.5 wt% MWCNTs or 12 wt% f-GNPs into the epoxy adhesive, respectively. Considering the electrical impedance changes simultaneously with the mechanical response of the adhesive joints, novel outcomes were obtained. Almost linear trend has been observed in the impedance curve for the strain value of 0.57. During crack growth, the conductive network in epoxy matrix near the crack tip would be break and thus the electrical impedance would be increased. In addition, the electrical impedance curve has exhibited several sharp increases, which are attributed to the rapid crack growth.

Static strength improvement of tubular aluminium adhesive joints by the outer chamfering technique

Tubular adhesive joints find applications in the piping industry, vehicle frames or thin-walled tubes, for instance, but they are seldom studied in the literature. This work numerically assesses the maximum load (Pm) of aluminium tubular joints, by using an outer chamfer in the tubes in the overlap region, after validation of the numerical tool with experiments. The numerical analysis consisted of using the Finite Element (FE) method to analyse peel (σy) and shear stresses (τxy) in the adhesive layer and cohesive zone models (CZM) to predict Pm. The CZM technique was positively validated for the strength analysis of tubular joints. It was also shown that the chamfer highly affects the joints’ behaviour, and that an optimal configuration exists that enables maximum Pm results.

Experimental Investigation of the Mechanical Behavior of Aluminum Adhesive Joints under Mixed-Mode Loading Conditions

Experimental Investigation of the Mechanical Behavior of Aluminum Adhesive Joints under Mixed-Mode Loading Conditions

Experimental and numerical analysis of scarf aluminum adhesive joints

This work consists of a validation of Cohesive Zone Model (CZM) laws in traction and shear, estimated by the application of the direct method, in the strength prediction of joints under a mixed-mode loading. In this context, scarf joints with different scarf angles (α) and adhesives of different ductility were tested. Pure-mode cohesive laws served as the basis for the creation of simplified triangular, trapezoidal and exponential laws that were tested for each of the adhesives. Their validation was accomplished by comparing the numerical predictions with the experimental results. The use of the direct method allowed obtaining very precise maximum load (Pm) predictions, indicating the cohesive law shape more suitable for each adhesive /geometry set. For the geometric and material conditions considered, this study has led to the conclusion that no significant Pm errors are incurred by the choice of a less appropriate law.

Effect of surface treatment on the shear strength of aluminium adhesive single-lap joints for automotive applications

Adhesive joints exhibit very high toughness and good fatigue resistance. This technique is a serious candidate to replace rivets or welding in primary structural components. Nevertheless, there is hesitation on the part of the industry to replace traditional fasteners in primary structural applications, mainly due to the limited understanding of joint performance over the life of structures. In the present research, we focus on the static strength of adhesive bonded aluminium alloys for the automotive industry. So, the aim of this work is to carry out and quantify the various variables affecting the strength of single lap joints, especially the effect of the surface preparation. Aluminium single lap joints (SLJs) were fabricated and tested to assess the adhesive (structural one-component polyurethane adhesive) performance in a joint. We found that the decrease in surface roughness was found to increase the shear strength of single lap joints. Furthermore, it has been possible, qualitatively, to identify the relative sensitivity of the effects of various surface roughnesses on the behaviour of spreading kinetics. Experimental results show that rougher surfaces have less wettability which is in coherent with shear strength tests.

A method for predicting the curing residual stress for CFRP/Al adhesive single-lap joints

Abstract The purpose of this paper is to present a prediction model of the curing residual stress for carbon fiber reinforced plastic (CFRP) and aluminum (Al) adhesive single-lap joints. Based on the lamination theory and formation mechanism of the curing residual stress of composite structures, we have found that high curing temperature in joints will lead to a significant increase of the residual stress. In the CFRP/Al stacks, residual tensile stresses are found in the Al and glue layer, while compressive stresses in CFRP. Meanwhile, the residual stress in glue layer is smaller than that in the Al and CFRP. More comparisons are made among the prediction, finite element analysis (FEA) and experimental tests to prove the efficiently and validity of the predicting model.

Fatigue crack initiation and damage characterization in Brazilian test specimens for adhesive joints

The present study is focused on the fatigue failure initiation at bimaterial corners by means of a configuration based on the Brazilian disc specimens. These specimens were previously used for the generalized fracture toughness determination and prediction of failure in adhesive joints, carried out under static compressive loading. Under static loading, local yielding effects might affect the asymptotic two-dimensional linear elastic stress representation under consideration. Fatigue loading avoids this fact due to the lower load levels used. The present tests were performed using load control; video microscopy and still cameras were used for monitoring initiation and crack growth. The fatigue tests were halted periodically and images of the corner were taken where fatigue damage was anticipated. Damage initiation and subsequent crack growth were observed in some specimens, especially in those which presented brittle failure under static and fatigue tests. These analyses allowed the characterization of damage initiation for a typical bimaterial corner that can be found in composite to aluminium adhesive lap joints.

The effects of test temperature and humidity on the mixed-mode fatigue behavior of a toughened adhesive aluminum joint

The effects of test environment humidity and temperature on the fatigue threshold and crack growth behavior of P2-etched and commercial coil-coated (CC) aluminum adhesive joints were studied under mixed-mode loading using aluminum asymmetric double cantilever beam (ADCB) specimens. Under dry conditions, increasing the temperature to 80 °C had an insignificant effect on the fatigue threshold, but caused an increase in the crack growth rates. At 40 °C, the fatigue behavior was insensitive to moisture at higher crack growth rates, but became sensitive to moisture level in the test environment as crack growth rates slowed to the threshold. The effect of moisture and temperature were explained by the observed changes in the crack path, which in general moved progressively closer to the more highly-strained adherend as the applied strain energy release rate, and consequently the crack growth rate, decreased. Furthermore, the residual adhesive thickness on the more highly-strained adherend, t r , increased with increasing temperature, and the crack path shifted to the hydrated aluminum oxide interface when the test environment was saturated with moisture. The degrading effect of a hot-wet environment was similar for both P2-etch and CC pretreatments. At higher crack growth rates, the joint fatigue performance was degraded solely due to the effect of the increased temperature, whereas at low crack growth rates, the fatigue performance was degraded predominantly because of elevated moisture.

Effect of adhesive thickness on fatigue and fracture of toughened epoxy joints – Part II: Analysis and finite element modeling

The effect of bondline thickness on the fatigue and fracture of aluminum adhesive joints bonded using a rubber-toughened epoxy adhesive was studied using finite element analysis. The fatigue data of Part I examined the dependence of the fatigue threshold and cyclic crack growth rate on the adhesive thickness under both mode-I and mixed-mode loading. The fracture data of Part I illustrated the relation between the adhesive thickness and the quasi-static crack initiation and steady-state critical strain energy release rates. These experimental trends are explained in terms of the effects of the adhesive thickness and the applied strain energy release rate on the stress distribution in the bondline, the stress triaxiality at the crack tip, and the plastic zone size in the adhesive layer.

Improvement of the Adhesive Fracture Toughness of Bonded Aluminum Joints Using E-Glass Fibers at Cryogenic Temperature

Fracture toughness and crack resistance of aluminum adhesive joints were measured at the cryogenic temperature of −150°C, with respect to the orientation and volume fraction of the E-glass fibers in the epoxy adhesive. Cleavage tests on the DCB (Double Cantilever Beam) adhesive joints were performed using two different test rates of 1.67 × 10−2 and 8.33 × 10−4 mm/s to observe the crack propagation trends. From the experiments, it was found that the DCB joints bonded with the epoxy adhesive reinforced with E-glass fibers not only showed a stable crack propagation with a low crack propagation speed, but also higher fracture toughness and crack resistance than those of the DCB joints bonded with the unreinforced epoxy adhesive at a cryogenic temperature of −150°C.

Analysis of manufacturing parameters on the shear strength of aluminium adhesive single-lap joints

An experimental and numerical investigation into the shear strength behaviour of aluminium alloy adhesive lap joints was carried out in order to understand the effect of geometrical and manufacturing parameters on the strength of adhesive bonding joints, with the aim of optimizing shear strength. The adherend material used for the experimental tests was an aluminium alloy in the form of thin sheets, and the adhesive used was a high strength epoxy. Five surface treatments were studied. The surface treatments process using sodium dichromate–sulphuric acid etch (CSA) and abrasive polishing (AP) resulted in improved joint shear strength when compared to acetone cleaning (SW), caustic etch (CE), and Tucker's reagent etch (TR). The decrease in surface roughness was found to increase the shear strength of single-lap joints. An increase in adherend thickness and overlap length was found to increase shear strength which means that an increase in joint rigidity increases its strength. A numerical analysis was developed to explain the effect of the geometrical parameters on rotation angle, stress and strain fields, and failure load. An increase in adherend thickness and overlap length decreases the joint rotation angle, reducing the plastic strain peak and therefore increasing the failure load.

Durability of Aluminium Adhesive Joints Bonded with a Homopolymerised Epoxy Resin

The durability of epoxy–aluminium joints that use a homopolymerised epoxy resin was studied, and the effects of relative humidity, temperature, and salt concentration were analysed. The adhesive properties were measured by lap–shear tests, and the water uptake of the epoxy resin was determined by gravimetric measurements. Ageing and degradation effects on the epoxy resin and on the aluminium substrates were also analysed. The homopolymerised epoxy resin absorbs little water (1.5 wt%) because of its nonpolar network structure. The water uptake is enhanced by increasing relative humidity and temperature; however, the joint strength remains constant because of epoxy plasticization. A saline environment is damaging to the adhesive joints, because of metal corrosion, but was not significantly harmful to the epoxy resin, because of a lower diffusion coefficient of salt water. The Tg decrease of the epoxy adhesive due to water absorption depends only on the amount of absorbed water and is independent of the hydrothermal ageing conditions.