Strength Of Adhesive Joints Research Articles

Quantification of mixed mode loading and bond line thickness on adhesive joint strength using novel test specimen geometry

This study quantifies the effect of mixed mode loading and bond line thickness on adhesive joint strength for automotive structural applications. This research is motivated by the need to address the complex loading that occurs during automotive crash events, as well as the variation in bond line thickness that may occur due to gap variability when joining mass-produced structural components. A newly developed specimen geometry for Mode II and Mixed Mode loading is presented, while a recently published test methodology was used to characterize the Mode I response. Three nominal bond line thicknesses (0.18, 0.30 and 0.64 mm), were investigated for a toughened structural adhesive and steel adherends. The traction-separation response, required for cohesive zone modeling (CZM) of adhesive joints, was determined for each combination of bond line thickness and mode of loading. Mode I loading resulted in higher peak traction and lower critical energy release rates compared to Mode II loading, with the Mixed Mode responses typically falling between Mode I and II, in relation to the loading angle tested. Increasing bond line thickness resulted in a reduction in initial stiffness and peak traction, as well as an increase in critical energy release rate for all modes of loading. Two existing CZM mixed mode failure criteria were assessed and demonstrated a good fit to the tested mixed mode responses, despite the limited ability of the CZM implementation to predict the end of the plateau region of the traction-separation response. The experimental approach described in this study was shown to provide repeatable results that could be directly used to fully define an adhesive CZM, ready for use in finite element modeling without the need for inverse modeling.

Epoxy Adhesives and Their Application

Characteristics of epoxy film adhesives modified with various rubbers and polysulfone, epoxy adhesive pastes of cold hardening, foaming adhesives, and functional adhesives including conductive adhesives are considered. Advantages of epoxy film adhesives over phenolic-elastomer adhesives are given. Features of adhesion technology with epoxy adhesive and their thermosetting ability providing high strength of adhesive joints of honeycomb structure are given. The fields of application of epoxy adhesives of various types are described.

Determination of the shear traction-separation law of adhesive layers using the end-notched flexure specimen

Cohesive zone modelling is increasingly applied to predict the strength of structural adhesive joints. A method is here proposed to determine the bondline shear traction-separation law needed for cohesive zone models. The method combines an effective crack length approach derived from a beam model and an existing J-integral analysis of the end-notched flexure specimen. The resulting work done by the tractions versus separation curves can be easily processed with direct or mixed direct-inverse approaches. Application to some published experimental results demonstrates the potential of the method while highlighting the importance of adherend material and specimen geometry selection.

Variation of intensity of singular stress field (ISSF) along the interface outer edge of prismatic butt joint and debonding condition expressed by the ISSF

In our previous study, the adhesive strength of butt joints having three-dimensional (3D) geometries was investigated by using the intensity of singular stress field (ISSF) in two-dimensional modelling. In this paper, by considering the 3D geometry, the ISSF variation along the butt-joint interface side is discussed to explain the experimental results. The results show that the critical ISSF distributions when debonding occur are almost the same and independent of the adhesive bondline thickness. The validity of the 2D modelling is investigated experimentally for two kinds of brittle and ductile adhesives considering the location of the maximum ISSF. It is found that the adhesive strength can be expressed as a constant value of the ISSF at the center side and also at the corner of the adhesive interface.

Influence of carbon fibre stiffness and adhesive ductility on CFRP-steel adhesive joints with short bond lengths

The use of adhesively-bonded CFRP laminates is a promising technique to strengthen steel structures that have been deteriorated due to corrosion, ageing or increasing loads, as in the case of old metallic riveted bridges. But the relatively short available space between rivets requires the use of adhesively-bonded CFRP laminates with short bond lengths, which needs to be deeply studied as most previous research works have focused on large bond lengths. To study the bond behaviour between CFRP laminates and steel plates in such strengthened structures, a series of tests has been carried out in double-strap joints under tensile loading, evaluating the influence of CFRP stiffness and adhesive ductility on the strength and failure mode of short bond length adhesive joints. Based on the experimental results of the present work, together with a large database collected from literature, a fracture-mechanics model based on interfacial fracture energy in shear GII is calibrated, and a simple expression is developed to be used in design for the strength prediction of such adhesive joints. Finally, double-strap joint specimens are simulated using cohesive zone models (CZM) for the adhesive layers, and the results are compared to the analytical model and experimental tests.

Experimental study on the influences of different surface treatment processes and adhesive type on the aluminum adhesive-bonded joint strength

Surface roughness obtained from different preparation processes such as sanding, shot blasting, and sand blasting were considered in this study. The effects of surface roughness on shear strength of single-lap adhesive joint were examined. 2024-T3 aluminum sheets were cut into standard pieces and then sanded with seven sandpapers with different mesh, shot blasted for four different duration times and sandblasted under four different pressures. Pairs of prepared surfaces were attached using Araldite 2015 adhesive with high viscosity and HPL1012/HPH112 epoxy adhesive with low viscosity. Results show that the ultimate lap shear strength of the sanded samples initially increase and then decrease as surface roughness is increased. The lap joint strength continuously increases as the surface roughness of shot blasted and sand blasted samples are increased. In sanding, the optimum surface roughness is different for both high and low viscosity adhesives. However, for shot blasting and sand blasting, the optimum surface roughness is the same for both adhesive types. The maximum ultimate lap shear strength of joints for both types of adhesives obtained for sand blasting under 0.6 MPa pressure with 0.6 μm surface roughness.

Material and adhesive effect in adhesively-bonded composite stepped-lap joints

Adhesive bonding is a predominant bonding technique in the aeronautical and automotive industries. Cohesive zone models, used together with the finite element method, are a viable tool to predict the strength of adhesive joints. The main objective of this study is to evaluate experimentally and numerically (by cohesive zone model) the mechanical performance of carbon-fiber reinforced polymer stepped-lap bonded joints submitted to tensile loads, for different overlap lengths ( LO) and adhesives. The failure mode analysis showed a predominant failure type for all adhesives and good correspondence with the numerical predictions. Normalized peel ( σy) and shear ( τxy) stresses in the adhesive highly increased with LO, which then reflected on different maximum load ( Pm) evolution with LO, depending on the adhesive's ductility. The damage variable SDEG (stiffness degradation) was also evaluated and emphasized on the smaller damage zone at Pm for the brittle adhesive. A significant geometry and material effect were found on Pm of the stepped-lap joints, with benefit for large LO. In this regard, cohesive zone model revealed to be a suitable tool in determining the behavior of different joints. Comparison with joints with aluminum showed that, provided that no carbon-fiber reinforced polymer delamination occurs, stepped-lap joints between carbon-fiber reinforced polymer adherends give better results due to the higher composite stiffness.

Experimental study of the CTBN effect on mechanical properties and mode I and II fracture toughness of a new epoxy resin

Due to the high strength and adhesiveness of epoxies, these materials are widely used in various industries such as aerospace. But because of their inherent brittleness and poor fracture toughness, they do not have enough resistance against crack propagation. In adhesive joints, maximum stresses near the ends of the overlaps resulted in premature failure of the joints. In this study, for toughening the epoxy, carboxyl-terminated butadiene acrylonitrile (CTBN) as a liquid rubber was used. Blends of a new epoxy resin (R510) and the CTBN in several weight fractions were produced and characterized. In this article, four tests have been done to find the mechanical properties and mode I and II fracture toughness of the adhesives. The tensile and shear tests were performed to achieve Young’s modulus (E), maximum tensile stress (σf), shear modulus (G), maximum shear stress (τf) and Poisson’s ratio (ν) of the adhesives. Also, the double-cantilever beam (DCB) and end-notched flexure (ENF) tests were used to obtain the mode I fracture toughness (GIc) and mode II fracture toughness (GIIc), respectively. The results from each of the DCB and the ENF tests were analyzed by three different methods and compared. Results showed that by adding CTBN to epoxy, E, G, ν, σf and τf are reduced, but toughness, the resistance to crack growth and as a result, the GIc and GIIc increased efficiently. These data can be used to predict the adhesive joint strength and simulating the damage initiation and its evolution until failure with cohesive zone modeling (CZM).

Strength and failure analysis of adhesive single-lap joints under shear loading: Effects of surface morphologies and overlap zone parameters

ABSTRACT The adhesive bonding process is widely used in the automobile industry because of sealing, insulation and good mechanical properties. In this study, the effects of surface treatments and joint parameters on shear strength and failure mode were investigated. Four different sandpaper treatments, overlap length and adhesive thickness were selected to carry out quasi-static shear test. The failure behaviour of aluminium alloy single-lap adhesive joints was analysed and a dimensionless parameters δ was introduced to quantitatively evaluate the effect of overlap length on joint strength. Results showed that the joint strength was enhanced by sandpaper grinding due to the change of surface roughness. The specimens with 90°/0° grinding direction using P360 sandpaper had the best performance rather than an excessively rough surface. The joint shear strength decreased with the increase of adhesive thickness but the effect of overlap length on joint strength was limited. The adhesive thickness had little impact on joint shear strength when the overlap length over 40 mm. In addition, the joint strength could be improved by increasing overlap length until δ below 0.95. However, with the decrease of δ, there was almost no effect on joint strength because of high peeling stress at the both ends of the long joints.

Influence of microcork particles on the lap shear strength of an epoxy adhesive subjected to fatigue loading and different environmental conditions

The present study deals with the effects of temperature and cyclic loading on the strength of single lap adhesive joints enhanced with different amounts of microcork particles. To achieve this, different joints with various overlap lengths were manufactured and tested. The joints were tested at two different temperatures (−20 ℃ and 75 ℃) in quasi-static conditions. Results of high temperature (75 ℃) show that the joints tensile strength decreases with the amount of cork particles. For low temperature (−20 ℃), the strength of the joint with 1 vol.% of cork increased; however, higher amounts of cork led to a reduction of the joint strength. To investigate the influence of cork particles on the fatigue endurance, joints were tested at room temperature under sinusoidal cyclic loading. Based on the experimental data it was found that the cork particles can significantly improve the fatigue life of adhesive joints (up to 4.6 times). Microfailure analysis of the tested joints was also performed using the scanning electron microscope technique.

The Influence of Technological Factors on the Strength of Adhesive Joints of Steel Sheets

The Influence of Technological Factors on the Strength of Adhesive Joints of Steel Sheets

On increasing wet-web strength with adhesive polymers

Fiber-fiber adhesion, called “bonding” in the old paper physics literature, is a critical component of the overall strength of dry paper. With freshly formed very wet pulp fiber webs, all evidence suggests there are no fiber-fiber crossings with significant adhesive joint strength. With water removal, a point will be reached where fiber-fiber adhesion starts to contribute to the overall wet-web strength. The literature reveals very few examples of polymers that increase fiber-fiber joint strength in freshly formed webs. Here, we summarize the literature and explain why it is so difficult to promote fiber-fiber wet adhesion with polymers. Nevertheless, ongoing research in areas as diverse as tissue engineering scaffolds and biomimetic adhe-sives gives clues to future developments. Advances in paper machine engineering have lessened the importance of wet-web strength. By contrast, a critical issue in many of the evolving nanocellulose technologies is the strength of objects first formed by aqueous processing, the green strength—the strength of wet bodies before drying. For exam-ple, 3-D printed nanocellulose objects and ultralow density cellulosic aerogels can be destroyed by capillary forces during drying. There is a need for adhesives that strengthen freshly formed, wet lignocellulosic joints.

Effect of residual strains on the static strength of dissimilar single lap adhesive joints

ABSTRACT Adhesive joints usually experience different thermal conditions not only during their service life but even also when the joints are stored, and no external force is applied. One of the main concerns for such a condition is the residual strains/stresses induced into the adhesive layer due to the cyclic ambient temperature. In this research, the effects of residual strains on the static strength of dissimilar single lap adhesive joints (SLJ) are experimentally investigated. The thermal cycles were applied in an aluminum/carbon fiber-reinforced polymer (Al/CFRP) adhesively bonded SLJ with different adhesive thicknesses. Residual strains due to the differential thermal contraction of the two materials were measured inside the adhesive layer using the digital image correlation (DIC) technique. It was shown that for the studied conditions, the residual strains increase by thermal cycles, leading to a decrease in residual static strength of the joints. A 2D finite element thermal analysis was also performed and the numerical data were compared with the experimental results.

Strength improvement of adhesively bonded single lap joints with date palm fibers: Effect of type, size, treatment method and density of fibers

In this paper, natural fibers and particles provided from different parts of date palm trees are considered to improve the static strength of adhesively bonded joints. Some influencing factors such as type, size, alkaline treatment and the weight ratio of the natural fibers/particles are the parameters that have been experimentally studied. The date palm fibers are provided from four parts of the tree including Mesh, Rachis, Petiole and Bunch. In order to investigate the effect of treatment of the fiber surface, the raw fibers and treated fibers with two different concentrations of aqueous NaOH solution were considered. To study the effect of fiber size, fibers with different sizes including short fibers (0.5–2 mm) and long fibers (30 mm) were separately added to the adhesive. To find the best weight ratio of the fibers corresponding to maximum joint strength, single lap joints with three different weight ratios of date palm fibers (2, 5 and 10%) were experimentally tested. The results show that date palm fibers significantly improve the strength of adhesively bonded joints. For example, the strength of adhesive single-lap joints reinforced by 2 wt% of Rachis fiber treated with 6 wt% NaOH solution, increased by 140%. To investigate the micromechanisms of failure, the fractured surfaces were analyzed. The results show that the date palm fibers can improve the strength of adhesive joints by slowing down the crack propagation, absorbing energy and load endurance.

Theoretical Assessment Of The Bearing Capacity Of A Steel Beam Reinforced With Carbon Fiber

To reinforce steel beams, modern carbon fiber materials that are attached to the beam from the stretched side using a special two-component adhesive can be effectively used. Compared to traditional methods of cross-section increase by means of additional steel elements attached by welding or on bolts, reinforcing with carbon fiber plastic has a number of advantages. Carbon plastic has a much smaller weight than a steel element that has the same load-bearing capacity; the adhesive layer prevents corrosion of the reinforced element; work on strengthening is carried out in a short time and with less labor expenditures etc. The article presents the results of theoretical studies of beams reinforced with carbon fiber lamellas. For the theoretical assessment of the strength of a reinforced steel beam, deformations and stresses in the section at the elastic, elastic-plastic and plastic stages of steel work are considered. It is established that a significant effect of carbon fiber reinforcement is observed even when the stresses in it are determined by the strength of the adhesive joint. Formulas, taking into account the strength of the adhesive layer determined experimentally, for determining the strength of a reinforced beam are obtained. A theoretical model of the operation of bent steel elements is constructed and theoretical dependencies are obtained that can be used for calculations of steel beams reinforced with carbon plastic.

Single lap joint strength prediction using the radial point interpolation method and the critical longitudinal strain criterion

Adhesive joining is currently a very popular joining method. This increases the necessity for better strength prediction tools to aid in the design of these joints. Currently, Cohesive Zone Modeling (CZM) is the most popular method to study joint strength. However, CZM requires traction-separation laws, which define the adhesive behavior, and these are dependent on the adhesive thickness (tA). This means that, when using CZM, the traction separation law parameters have to be measured multiple times to predict the strength of joints with different tA. The recently proposed Critical Longitudinal Strain (CLS) criterion is a criterion based on continuum mechanics, which was previously used with the Finite Element Method (FEM) to predict the strength of Single Lap Joints (SLJ). The use of meshless methods to predict the strength of adhesive joints is scarce and the CLS criterion has never been used with the Radial Point Interpolation Method (RPIM). In this work, the CLS criterion was used with the RPIM to determine the strength of SLJ bonded with three different adhesives. The strength predictions with this approach were accurate for the three adhesives, which ranged from brittle to highly ductile.

The Role of Fiberglass Surface Preparation for Adhesive Bonding

The influence of the surface preparation method of fiberglass VPS–53K on the adhesive strength is researched. To prepare the surface, both standard methods (degreasing, sanding, sandblasting, sacrificial layers) and ultraviolet and normal-pressure plasma treatment are used. The strength of adhesive joints is evaluated by measuring peel resistance according to GOST (State Standard) R 57832–2017 (ASTM D3167). It is shown that the most effective surface preparation method for bonding is normal-pressure plasma treatment. Through this process the fiberglass surface becomes activated, the fiberglass roughness changes conditionally warpwise and fillingwise, which, together with the ongoing physicochemical processes on the fiberglass surface, leads to an increase in the strength of adhesive joints. The application of normal-pressure plasma improves the wettability of the treated surface with cold-setting liquid adhesives.

Influence of surface texturing on adhesive joint strength

Influence of surface texturing on adhesive joint strength

Rheological Properties of Epoxy Urethane Oligomers and Сuring Kinetics of Polymer Composites on Their Basis

The rheological properties of epoxy urethane oligomers (EUOs) synthesized at different stoichiometric ratios of reaction groups NCO : OH have been examined over a wide range of temperatures and shear rates using rotational viscometry. The effect of the NCO : OH ratio on the kinetic parameters (dynamic viscosity increase constant and gelation time) of epoxy polyurethane compositions curing at a temperature of 25 ± 1°C have been revealed. Evaluation of the adhesion tensile strength of cold-cured adhesive EUOs joints is presented.

Strength prediction and stress analysis of adhesively bonded composite joints using meshless methods

The applications of adhesive joints have grown in recent years in various industries, ranging from automotive to maritime. Compared to traditional joining techniques, such as bolts, welding or riveting, adhesive bonding has some advantages, namely, they are low weight, have high strength and have a more uniform stress distribution. However, they also have some disadvantages, many times they are impossible to disassemble, they have low peel strength and some adhesive require high curing temperatures. A particular advantage of using adhesives in composite joints is that they do not require holes in the composite to join the joint, which are associated with delamination failure. The present work aims at using radial point interpolation meshless methods to study the stress distributions and predict the strength of composite adhesive single lap joints (SLJ). To this effect, a brittle adhesive was tested in composite joints with varying overlap lengths (LO). The stress distribution obtained using meshless methods was compared to one obtained using the well-established Finite Element Method (FEM), resulting in a similar behaviour. The strength predictions, using the critical longitudinal strain (CLS) criterion, were similar to the experimental joint strength.