The application of coherent optics to the study of adhesive joints II. Holographic interferometry

The technique of holographic interferometry is complementary to the technique of speckle photography described previously. It allows the out-of-plane motion to be visualized with a sensitivity of about 0.3 μm. This technique has been applied to the study of the behavior of model glass-glass joints bonded by different adhesives under shear stress. Its high sensitivity allowed the presence of stress concentrations at the ends of the joints to be shown in the case of high-modulus adhesives and the existence of additional torsion and/or cleavage stresses to be demonstrated in the case of lower modulus adhesives. Work in progress should lead to more quantitative data.     

Joint strength optimization by the mixed-adhesive technique

An ideal adhesive lap joint is one in which the adhesive flexibility and strength properties vary along the overlap length. Because of greater adhesive shear strains at the edges of the overlap, a ductile and flexible adhesive should be used at the overlap ends, while in the middle a stiff and less-ductile adhesive should be used. This technique has been investigated in the past but only a few studies have reported any experimental evidence. In the present study, single-lap adhesive joints were manufactured and tested maintaining the same brittle adhesive in the middle of the overlap and using three different ductile adhesives of increasing ductility at the ends of the overlap. A simple joint strength prediction is proposed for mixed-adhesive joints. The mixed-adhesive technique gives joint strength improvements in relation to a brittle adhesive alone in all cases. For a mixed adhesive joint to be stronger than the brittle adhesive and the ductile adhesive used individually, the load carried by the brittle adhesive must be higher than that carried by the ductile adhesive.     

Influence of water immersion on the single-lap shear strength of aluminum joints bonded with aluminum-powder-filled epoxy adhesive

Durability of adhesively-bonded aluminum joints was investigated by measuring the joint strength using the single-lap shear test before and after exposure to distilled water and seawater. Fractured specimens were examined by photography and scanning electron microscopy to determine the failure modes. Addition of Al particles as much as 50 wt% did not cause any significant decrease in adhesive joint strength. Moreover, varying the Al filler content in the adhesive did not have a significant effect on adhesive behavior in either of the two environments studied. The unexposed adhesive joints failed almost completely in a cohesive (in the adhesive) failure mode. Some decrease in strength was observed in adhesive joints after exposure to both distilled water and seawater for 6 months. The decrease in adhesive joint strength was more significant for specimens immersed in distilled water than those immersed in seawater, probably due to the higher amount of moisture in the adhesive in distilled water than in seawater, as observed in a related moisture diffusion study. The joints exposed to distilled water or sea water failed in more than one mode. The interior part of the adhesive lap area failed in a cohesive mode while an adhesion failure mode was observed near the edges of the adhesive lap area, which is believed to be a result of moisture diffusion through the edges.     

Adhesive bonding of polyethylene

The treatment of polyethylene film with aqueous ammonium peroxydisulfate solutions prior to adhesive bonding to aluminum has been studied. Such pretreatments resulted in the formation of adhesive joints of high strengths when bonded with a conventional epoxy adhesive. The effect on the tensile shear strength of adhesive joints of variations in the treatment time and temperature and the peroxydisulfate concentration has been examined. The use of certain catalysts for the reaction has also been studied. Tensile shear strengths at least as high as with other pretreatment methods have been obtained.     

Mode I characterization of toughened epoxy adhesive joints under shock-wave loading

With the advances in adhesive technology, the use of structural adhesive joints has extended to the broader engineering field as an alternative to traditional joining methods such as bolting, riveting, and welding. Therefore, characterization the adhesive joints under different loading and environmental conditions has been becoming more significant in designing adhesives joints for an engineering application. Since most of the polymer-based adhesives have non-linear mechanical behavior and loading rate sensitivity caused by their viscoelastic properties, testing adhesive joints under quasi-static loading cannot give adequate information to predict the response of adhesive joints to high loading conditions. It is therefore imperative to characterize the adhesive joints under high loading rates in order to integrate them into the applications that require high impact resistance. This study focused on the bending (mode I) characterization of adhesive joints under shock-wave loading generated by a large-scale shock tube. A specially designed adhesive joint that transfers the shock-wave loading to the bond in the mode-I form was designed, fabricated and tested. A series of shock-wave loading experiments were carried out with two different adhesive joints: aluminum-epoxy and steel-epoxy and their performances were compared. In addition to the experimental work, an FEM parametric study by an inverse problem-solving technique was used to estimate the mechanical properties of adhesive in both adhesive joints under different shock wave loading conditions. This technique also allowed to estimate the energy absorption capabilities of aluminum-epoxy and steel-epoxy joints.     

The effect of adhesive thickness on tensile and shear strength of polyimide adhesive

The effect of adhesive thickness on tensile and shear strength of a polyimide adhesive has been investigated. Tensile and shear tests were carried out using butt and single lap joints. Commercially available polyimide (Skybond 703) was used as adhesive and aluminum alloy (5052-H34) was used as adherends. The tensile strength of the butt joints decreased with increasing adhesive thickness. In contrast, adhesive thickness did not seem to affect the shear strength of single lap joints. The fabricated joints using the polyimide adhesive failed in an interfacial manner regardless of adhesive thickness. The linear elastic stress analysis using a finite element method (FEM) indicates that the normal stress concentrated at the interface between the adherend and the adhesive. The FEM analysis considering the interfacial stress well explains the effect of adhesive thickness on the joint strength.     

An investigation of fatigue failure prediction of adhesively bonded metal/metal joints

A fracture mechanics-based model for fatigue failure prediction of adhesive joints has been applied in this work. The model is based on the integration of the kinetic law of evolution of defects originated at stress concentrations within the joint. Final failure can be either brittle (fracture toughness-driven) or ductile (tensile/shear strength-driven) depending on the adhesive. The model has been validated against experiments conducted on single-lap shear joints bonded with a structural adhesive. Three different kinds of adhesives, namely a modified methacrylate, a one-part epoxy and a two-part epoxy supplied by Henkel, have been considered and three different overlap lengths have been tested. Fracture toughness and fatigue crack growth properties of the adhesives have been determined with mode I tests. The number of cycles to failure has been successfully predicted in several cases. It is interesting to notice that in the case of joints loaded at the same average shear stress, the shorter the joint, the longer the duration. This fact is also captured by the model.     

Analysis of adhesive joints in corrugated board under shear loading

The aim of this study is the development and validation of a methodology for modelling corrugated board adhesive joints subjected to shear, considered to be the main load in most of these joints, and the analysis of the influence of several joint parameters. Testing and simulation of double-lap corrugated cardboard adhesive joints has been carried out. The corrugated board was glued with a hot-melt adhesive, the type of adhesive commonly used in the assembly of corrugated boxes. The corrugated board adhesive joint model reproduced quite well the stiffness obtained in the test samples, as well as the failure load with a deviation of less than 14%. A simplified plane strain model has also been evaluated in order to obtain a significant reduction in generation and computational costs. Finally, the influence of some of the most significant parameters in the corrugated board adhesive joint has been analysed independently using the simplified model.     

Three-parameter,elastic foundation model for analysis of adhesively bonded joints

A novel three-parameter, elastic foundation model is proposed in this study to analyze interface stresses of adhesively bonded joints. The classical two-parameter, elastic foundation model of adhesive joints models the adhesive layer as a layer of normal and a layer of shear springs. This model does not satisfy the zero-shear-stress boundary conditions at the free edges of the adhesive layer due to the inherent flaw of the two-parameter, elastic foundation model, which violates the equilibrium condition of the adhesive layer. To eliminate this flaw, this study models the adhesive layer as two normal spring layers interconnected by a shear layer. This new three-parameter, elastic foundation model allows the peel stresses along the two adherend/adhesive interfaces of the joint to be different, and therefore, satisfies the equilibrium condition of the adhesive layer. This model regains the missing “degree of freedom” in the two-parameter, elastic foundation model of the adhesive layer by introducing the transverse displacement of the adhesive layer as a new independent parameter. Explicit closed-form expressions of interface stresses and beam forces are obtained. The new model not only satisfies all boundary conditions, but also predicts correctly which interface has the strongest stress concentration. The new model is verified by continuum models existing in the literature and finite element analysis. The new three-parameter, elastic foundation model provides an effective and efficient tool for analysis and design of general adhesive joints.     

Water-Resistant Oxide Layers at the Interface of Zinc/Epoxy Bonded Joints

The durability properties of bonded lap shear joints made from an epoxy/dicyandiamide adhesive and zinc metal coupons have been investigated. The metal coupons were anodized in sodium hydroxide solutions before bonding. The influence of the anodizing conditions on the morphology and composition of the oxide layers has been studied using SEM and TEM imaging analyses as well as X-ray photoelectron spectroscopy. The hydrolytic stability of the bonded joints has been assessed by storing the joints in water at 70 °C or 90 °C for periods of time up to 5 weeks. Polypropylene has been used as a model adhesive to study the influence of mechanical interlocking effects on the performance of the bonded joints. Depending on the anodizing conditions, the improved durability properties have been attributed either to „mechanical interlocking effects“ or to the higher hydrolytic stability of the oxide layers generated during the anodizing treatment.

Some of the results gained from the anodization of zinc have been extrapolated to hot-dipped galvanized steel. Bonded joints made from hot-dipped galvanized coupons anodized under smooth conditions (2% NaOH) displayed residual shear strengths of up to 70% higher than specimens simply degreased after immersion test. The generation of stable oxide layers as well as the suppression of intergranular corrosion phenomena at the metal/adhesive interface can explain the improved durability properties.     

Strain Distributions in Adhesive Wood Joints

The influence of overlap length on the strain distribution in a double-lap joint was determined using a computer vision, white light speckle technique. The overlap lengths were 12.7 mm (0.5') and 38.1 mm (1.5'). This range represents a one-tenth scale of overlap lengths in double-lap joints used in adhesive connections in timber trusses. The side members were 50.8 mm (2') long and the center member was 88.9 mm (3.5') long. All joint members were 25.4 mm × 25.4 mm (1' × 1') in cross section. The joint members were yellow-poplar (Liriodendron tulipifera) wood and the adhesive was a resorcinol-formaldehyde.

Even though the connection was loaded in shear, both transverse and shear strains were shown to exist along the gluelines. These strains were significantly intensified at the ends of the overlap. The strain distributions were altered by the degree of overlap and the level of strain concentration was reduced with increased overlap.

The full-field strain distributions as measured with the new computer vision technique revealed progressive failure development, the eventual failure mode and location, and knowledge of the actual distribution of the strains in the entire joint assembly.     

The effects of Al surface treatment,adhesive thickness and microcapsule inclusion on the shear strength of bonded joints

The influences of various Al surface treatments, adhesive thicknesses as well as the incorporation of synthesized microcapsules into epoxy adhesive on the shear strength of adhesive/ Al joints have been investigated using lap-shear tensile tests. First, the influence of adhesive thickness on the shear strength of joints has been presented. Then, the effects of various Al surface treatments on the surface roughness of Al and shear strength of joint have been researched. Atomic force microscopy was used to study the Al surface morphologies and textures. Finally the few micron-sized polymeric microcapsules were synthesized and the shear performances of microcapsule filled epoxy adhesives were inspected. It was observed that the HCl acid based etching increased both micro-roughness and nano-texture of the Al surface and led to the peak shear strength. Moreover, HCl-nitric acid treatment offered the maximum value for the cohesive failure. Capsule inclusions into the adhesive displayed different influences on the joint shear performances depending on the capsule morphology and the surface treatment of Al.     

Shear behaviour of structural silicone adhesively bonded steel-glass orthogonal lap joints

This paper outlines an experimental study on the shear behaviour of structural silicone adhesively bonded steel-glass orthogonal lap joints. In the combination of steel plate and glass panel to form a hybrid structural glazing system, bonded joints with structural silicones can provide certain flexibility which relieves stress peaks at critical points of glass panel. The cohesive failure and its related fracture pattern of test joints with varied geometries of adhesives are examined experimentally. It is shown that the presence of two failure modes as discrete voids and macro cracks is closely related to the adhesive thickness. The effects of geometric parameters of adhesives on the joint shear strength are examined. It is demonstrated that the joint shear strengths are increased with increased individual overlap length, reduced adhesive thickness or increased adhesive width while the shear deformation corresponding to maximum shear force is mostly influenced by adhesive thickness. Mechanical contributions for those effects are analyzed accordingly. Finally, an analytical formula allowing for the equilibrium of strain and force on the adhesive and adherend is proposed for the analysis of shear strength. It is demonstrated that calculated normalized shear force ratios predicted by proposed formula agree well with those from experimental results.     

Strength Improvement of Adhesively-Bonded Joints Using a Reverse-Bent Geometry

Adhesive bonding of components has become more efficient in recent years due to the developments in adhesive technology, which has resulted in higher peel and shear strengths, and also in allowable ductility up to failure. As a result, fastening and riveting methods are being progressively replaced by adhesive bonding, allowing a big step towards stronger and lighter unions. However, single-lap bonded joints still generate substantial peel and shear stress concentrations at the overlap edges that can be harmful to the structure, especially when using brittle adhesives that do not allow plasticization in these regions. In this work, a numerical and experimental study is performed to evaluate the feasibility of bending the adherends at the ends of the overlap for the strength improvement of single-lap aluminium joints bonded with a brittle and a ductile adhesive. Different combinations of joint eccentricity were tested, including absence of eccentricity, allowing the optimization of the joint. A Finite Element stress and failure analysis in ABAQUS^® was also carried out to provide a better understanding of the bent configuration. Results showed a major advantage of using the proposed modification for the brittle adhesive, but the joints with the ductile adhesive were not much affected by the bending technique.     

Accuracy of cohesive laws with different shape for the shear behaviour prediction of bonded joints

The use of adhesive bonding as a joining technique is increasingly being used in many industries because of its convenience and high efficiency. Cohesive Zone Models (CZM) are a powerful tool for the strength prediction of bonded joints, but they require an accurate estimation of the tensile and shear cohesive laws of the adhesive layer. This work evaluated the shear fracture toughness (J_IIC) and CZM laws of bonded joints for three adhesives with distinct ductility. The End-Notched Flexure (ENF) test geometry was used. The experimental work consisted of the shear fracture characterization of the bond by the J-integral. Additionally, by this technique, the precise shape of the cohesive law was defined. For the J-integral, digital image correlation was used for the evaluation of the adhesive layer shear displacement at the crack tip during the test, coupled to a Matlab sub-routine for extraction of this parameter automatically. Finite Element Method (FEM) simulations were carried out in Abaqus® to assess the accuracy of triangular, trapezoidal and linear-exponential CZM laws in predicting the experimental behaviour of the ENF tests. As output of this work, fracture data is provided in shear for the selected adhesives, allowing the subsequent strength prediction of bonded joints.     

Mechanical behaviour of a combined joint composed of mechanical fastening and adhesive bonding

A study has been made of ‘combined’ joints consisting of mechanical fastening and adhesive bonding between rigid adherends. The mechanical characteristics of such ‘hybrid’ butt and lap shear joints have been derived by theoretical analysis. Results from experimental tests in combined joints giver reasonable agreement with the theoretical equations relating joint load to deformation.     

Internal resistance heating for homogeneous curing of adhesively bonded repairs

Carbon-fibre/bismaleimide (BMI) composite joints were manufactured using resistive heating of a stainless steel mesh embedded between two layers of structural adhesive. In comparison with joints cured thermally, electrically cured joints achieved similar shear strengths. The surface treatment of the stainless steel mesh was critical in achieving satisfactory shear strength. The electrical curing process has been shown to be adaptable to field repairs of composite structures through the use of a modified low voltage AC power supply, traditionally used with heater blanket technology. The embedded mesh curing technique might obviate problems associated with achieving even, through thickness heating of composite structure during bonding of scarf and doubler repairs.     

Adhesives and adhesive joints under impact loadings: An overview

The study of the behaviour of adhesive joints under impact loadings is a very active field of research, driven by significant industrial interest. Many industries, such as the automotive industry, are currently employing adhesive joints extensively, making use of the inherent properties of adhesive joints to improve the mechanical behaviour, reduce weight, and simplify manufacturing. Reduced structural weight is achieved by combining multiple lightweight materials, which is made possible by using adhesive joints. Impact strength is also a major factor, as vehicles must be able to provide adequate safety levels for their occupants during collisions. Another example of industrial application is the defence industry, which uses bonded structures to withstand ballistic impacts, with extremely high impact velocities. Understanding the behaviour of adhesive joints under impact is, therefore, crucial for designing stronger and safer structures. This document aims to review the research that has been previously undertaken in this field. Discussed research topics include high strain rate property determination, adhesive joint testing, effects of coupling environmental conditions with impact loads, and sections on numerical and constitutive modelling procedures. The final sections describe some practical applications of adhesive joints under large strain rates and relate them to the fundamental concepts previously discussed.     

Fatigue life prediction of adhesively bonded single lap joints

Better fatigue performance of adhesively bonded joints makes them suitable for most structural applications. However, predicting the service life of bonded joints accurately remains a challenge. In this present study, nonlinear computational simulations have been performed on adhesively bonded single lap ASTM-D1002 shear joint considering both geometrical and material nonlinearities to predict the fatigue life by judiciously applying the modified Coffin-Manson equation for adhesive joints. Elasto-plastic material models have been employed for both the adhesive and the adherends. The predicted life has close agreement in the high cycle fatigue (HCF) regime with empirical observations reported in the literature.     

Bond behavior of epoxy resin–polydicyclopentadiene phase separated interpenetrating networks for adhering carbon fiber reinforced polymer to steel

The performance of bonded joints of carbon fiber reinforced polymer (CFRP) and steel relies on the mechanical properties of the adhesive used. Despite the high strength and modulus of epoxy adhesives, their brittleness limits their application to defect-sensitive structures. The development of interpenetrating polymer networks (IPNs), either homogeneous or phase separated, provides a route to toughen the epoxy while maintaining its high strength and modulus. Microphase separated IPNs consisting of a diglycidyl ether of bisphenol A-based epoxy resin and a thermoset with high toughness, polydicyclopentadiene (PDCPD), has been previously shown to demonstrate superior combinations of strength and toughness. This work investigates the most critical adhesive properties that affect bond strength by characterizing CFRP-steel double-lap shear joints containing the epoxy resin–PDCPD blend as the adhesive, using a wet lay-up manufacturing technique. The epoxy resin–PDCPD blend adhesives realized much higher bond strengths compared to either neat epoxy or neat PDCPD. Correlations between the bond strength and the bulk material properties are presented. Theoretical calculation of the bond strength indicates that the higher bond strength that can be achieved by using the epoxy resin–PDCPD blend adhesive is due to the increased shear toughness of the new formulations. POLYM. ENG. SCI., 60:104–112, 2020. © 2019 Society of Plastics Engineers