Development of a Failure Model for the Adhesively Bonded Tubular Single Lap Joint

The accurate calculation of the stresses and torque capacities of adhesively bonded joints is not possible without understanding the failure phenomena of the adhesive joints and the nonlinear behavior of the adhesive.

In this paper, an adhesive failure model of the adhesively bonded tubular single lap joint with steel-steel adherends was proposed to predict the torque capacity accurately.

The model incorporated the nonlinear behavior of the adhesive and the different failure modes in which the adhesive failure mode changed from bulk shear failure, via transient failure, to interfacial failure between the adhesive and the adherend, according to the magnitudes of the residual thermally-induced stresses from fabrication.     

The Effect of High Humidity on the Dynamic Mechanical Properties and Thermal Transitions of an Epoxy-polyamide Adhesive

Exposure of a cured epoxy-polyamide adhesive to high humidity resulted in a substantial decrease in the complex dynamic tensile modulus of the material. The effect could be reversed and the original modulus essentially regained by drying the adhesive. Thermal transitions in the dry adhesive were displaced by approximately 40°C to lower temperatures in the wet material; this effect could be reversed by drying.

Strength losses experienced by aluminium joints bonded with this adhesive on exposure to humid conditions could not be regained by removal of water from the joints. The mode of failure of these joints changed progressively from wholly cohesive to predominantly adhesive on exposure to high humidity.

It is concluded that the primary role of water in joint degradation is to displace adhesive from its metal substrate and not to induce cohesive failure of the adhesive.     

Development of a Fatigue Failure Model for the Adhesively Bonded Tubular Single Lap Joint under Dynamic Torsional Loading

The adhesively bonded tubular single lap joint shows nonlinear torque transmission capability and deformation characteristics under static torsional loading because of nonlinear properties of the adhesive. However, the dynamic or fatigue torque transmission capability can be calculated with linear, analysis because the stress-strain relation under torsional fatigue loading is linear, due to the small dynamic transmission capability compared with the static torque transmission capability.

In this paper, a failure model for the adhesively bonded tubular single lap joint under torsional fatigue loading was developed with respect to the adhesive thickness, which is the critical factor for the static torque transmission capability. Also, a design method for the adhesively bonded tubular single lap joint under torsional fatigue loading was proposed.     

A Study of Chemical Interactions at the Stainless Steel/Polymer Interface by Infrared Spectroscopy. Part 2: Mechanical Properties and Study of the Interphase

The interaction of a thermoplastic ethylene-maleic anhydride copolymer with stainless steel has been studied by infrared spectroscopic techniques (FTIR). The aim was to improve understanding of the reaction processes at the steel/polymer interface in order to optimize the quality of assemblies in terms of adhesion and durability under the conditions which will subsequently be those of normal operation.

Steel/polymer associations have been tested after being submitted to several different conditions of treatment and aging in order to understand the various phenomena which occur at the steel/polymer interphase.

Mechanical behavior improves after heat treatment, and similar conclusions can be transposed to the structure after use, such as in domestic equipment. Modifications in interactions between stainless steel and polymer are caused first by the chemical reactivity of anhydride functions, and second by the mobility of organic chains which reorganize at the interphase.

Analysis of failure surfaces shows several correlations between the mechanical behavior and the chemical nature of residual polymer on the metal substrate. Localization of failure depends on aging conditions and can be explained by minimization of interfaical energy between the polar structure of the metal surface and the organic chains.     

The Adhesively Bonded Aluminium Joint: The Effect of Pretreatment on Durability

The interface in aluminium bonded structures can be revealed by ultramicrotomy and subsequently studied by transmission electron microscopy. By these means, the more usual surface pretreatments encountered, have been characterised in depth.

A similar examination has been effected following exposure of bonded joints (floating roller peel specimens) to 85% relative humidity at 70°C. Although a drop in peel performance is noted over the exposure time, interfacial examination reveals little damage to the adhesive or adherend. Possible mechanisms for bond strength reduction are discussed: subtle undermining of the alumina film and disruption of physico-chemical bonds across the interface. Both are initiated by moisture reaching the alumina film, either passing along the interface itself or travelling through the adhesive matrix. Also considered are the effects of surface pretreatment and “oxide” penetration, by the adhesive, on durability.

The effect of priming the adherend surface prior to bonding, using a heavily strontium chromate filled adhesive primer, is mentioned and its possible influence on durability is briefly discussed.     

The Effect of Moisture Content in Epoxy Film Adhesives on their Performance. III: Bulk Properties

Humidity absorbed by epoxy film adhesives during low temperature storage or exposure to atmosphere may result in reversible changes and irreversible modifications. Vacuum treatment may partially remedy the reversible changes. The consequences of vacuum drying are manifested in enhancement of both the peel and shear properties of bonded joints (Part I and Part II of this series of papers) and the thermal, physical and mechanical properties of the bulk adhesive, characterized in the present study.

Experimental results have shown that the bulk properties of structural epoxy based adhesives are highly correlated with the aging processes caused by water absorption in the prepolymerized adhesive. Applying the vacuum process is harmful to fresh unaged adhesive due to devolatization of low molecular species of the film adhesive.

The characterization of bulk properties for the purpose of following the aging and recovery processes is advantageous, since the bulk is independent of geometrical and interfacial effects which dominate in the case of property evaluation of the adhesive in a bonded joint.     

A Test Method for Accelerated Humidity Conditioning and Estimation of Adhesive Bond Durability

The ability to determine the durability of adhesive bonds remains an elusive task, especially when the service environment involves exposure to diluents such as water. Moisture continues to be of major concern for many adhesive bond systems for a number of reasons including:

1) many adhesives are hydrophilic, picking up significant amounts of moisture over time;

2) most adhesives and some adherends allow moisture permeation, eventually reaching the adhesive/adherend interface;

3) the high surface energies of metallic and certain other substrates result in moisture migrating to the adherend surfaces and displacing the adhesive from the substrates, and possibly oxidizing the adherend, etc., and

4) absorbed moisture induces swelling stresses which can reduce the bond strength.

Recognition of this susceptibility to moisture has led to extensive studies aimed at evaluating the effects of moisture, developing an understanding of the responsible mechanisms, and predicting the performance of adhesive bonds subjected to humid environments. While some studies have focused on the effect of humidity on neat adhesive samples, most studies have recognized the significance of the adhesive/adherend interactions, and have evaluated strength of actual bonded joints. Unfortunately, the time required for typical bonded geometries to reach moisture equilibrium can be quite long. Single lap joints (SLJ) and double cantilever beam (DCB) specimens with a width of 25mm may take several years to equilibrate, depending on the temperature and adhesive. Such lengthy conditioning times hamper the development of improved adhesives, and may delay the acceptance of these adhesives because of the time required to certify them. Methods to accelerate the conditioning of test specimens would be of significant benefit to adhesive formulators and users.     

The Effects of Molecular Weight on the Single Lap Shear Creep and Constant Strain Rate Behavior of Thermoplastic Polyimidesulfone Adhesive

The bonded shear creep and constant strain rate behavior of zero, one, and three percent end capped Thermoplastic Polyimidesulfone adhesive were examined at room and elevated temperatures. End capping was accomplished by the addition of phthalic anhydrides.

The viscoelastic Chase-Goldsmith and elastic nonlinear relations gave a good fit to the experimental stress strain behavior. Ultimate stress levels and the safe levels for creep stresses were found to decrease as molecular weight was reduced.

The primary objective was to determine the effects of molecular weight on the mechanical properties of the adhesive in the bonded form. Viscoelastic and nonlinear elastic constitutive equations were utilized to model the adhesive. Crochet's relation was used to describe the experimental creep failure data. The effects of molecular weight changes on the above mentioned mechanical behavior were assessed.     

Annealing of a Cyanurate Prepolymer Adhesive on Aluminium and Gold Substrates

Two major effects are observed during annealing. First, some cyanate groups hydrolyze into carbamate structures on Al and Au. The reaction is driven by the metal surface and both hydroxyl groups and adsorbed water can be involved. On Al, where the carbamate production is most prominent, a part of the cyanate groups in the prepolymer is expected to react with substrate hydroxyl groups. The resulting carbamate coupling to the oxide provides a new chemical adhesion mechanism.

As a second effect, an unexpected strong loss of monomer molecules from the prepolymer layers is observed on both substrates at elevated temperatures. It is concluded that the monomer molecules are pushed out of the interphase layer with the substrates due to negative adsorption. Hence, the less mobile, bulky oligomers remain on the substrate and thermal curing of the adhesive must result in a polycyanurate network which is much weaker than in the bulk polymer. This could reduce the mechanical strength of an adhesive joint made with pure polycyanurates and could afford additional chemical modification for practical applications.     

Aging Process of Metal/Epoxy Laminates Investigated with X-ray Photoelectron Microscopy and Spectroscopy

In this article we describe the application of X-ray photoelectron spectroscopy to epoxy/dicyandiamide laminates on zinc galvanized steel which were aged under different environmental conditions involving high humidity and temperatures.

X-ray photoelectron microscopy allows us to identify the distribution of chemical elements with a lateral resolution of 10μm. Areas selected in the microscopy mode were then analyzed in the spectroscopy mode in order to get information on the local chemical composition.

We compared the spectroscopic features of the aged but freshly delaminated surfaces of samples stored under ambient conditions at room temperature with samples exposed to the “Kataplasmann” and the “KWT” test, respectively. Furthermore, a comparison was made with a model sample which was prepared in vacuum and on which the curing process was investigated.

Though there is no substantial loss in the lap-shear strength of the samples, we find drastic spectroscopic changes in the Kataplasma and KWT treated samples compared with the sample kept at room temperature. We conclude that the chemical changes induced by these tests cause an internal interphase boundary between the epoxy/metal interface and the bulk adhesive along which delamination occurs. Comparison with the behavior of the water-vapor-treated model sample gives evidence that hydrolysis is the main reaction in these tests.

The results described here complement our former study.¹     

Effect of Peel Load on Stringiness Phenomena and Peel Speed of Pressure-Sensitive Adhesive Tape

The factors governing interfacial separation in lightly cross-linked polymer adhesives at low pulling rates as demonstrated by their stringiness phenomenon are investigated.

Cohesive failure and adhesive/substrate interfacial separation of uncross-linked polymer adhesives have been adequately explained. However, in lightly cross-linked polymer adhesives, where cohesive failure cannot occur because there is no viscous flow, there are two regions of interfacial separation at low rate and this phemonenon cannot be readily explained by present viscoelastic theories.

Investigation of the stringiness phenomenon of peeling pressure-sensitive adhesive tapes at constant loads shows that two peeling speeds exist for any peeling load up to the vicinity of 200 g/25 mm. Also it is clear that stringiness structure differs greatly at each peeling speed. The stringiness phenomenon of each of these two regions is analyzed using Miyagi's observation apparatus. These two measurements are then reversed and a comparison shows that the two peeling speeds correspond to each steady peeling region.

This field of investigation, when added to the present viscoelastic property studies, should lead to a new peeling adhesive theory which, in turn, may lead to the development of new high peel force pressure-sensitive adhesives.     

Optimal Design of the Adhesively-Bonded Tubular Single Lap Joint

In this paper, a method for the optimal design of the adhesively-bonded tubular single lap joint was proposed based on the failure model of the adhesively-bonded tubular single lap joint. The failure model incorporated the nonlinear mechanical behavior of the adhesive as well as the different failure modes in which the adhesive failure mode changed from bulk shear failure, via transient failure, to interfacial failure between the adhesive and the adherend, according to the magnitudes of the residual thermal stresses induced by fabrication.

The effects of the design parameters for the adhesively-bonded tubular single lap joint, such as the thicknesses of adhesive layer and adherends, the bonding length, and the scarfs of adherends, on the torque transmission capability and the efficiency of the adhesive joint were investigated.     

Preadhesion Laser Treatment of Aluminum Surfaces

An excimer laser may be used for preadhesion treatment of aluminum alloys. This method presents an alternative to the use of ecologically unfriendly chemicals involved in conventional anodizing pretreatments.

Experimental results indicate that preadhesion laser surface treatment significantly improved the shear strength of modified-epoxy bonded aluminum specimens compared with untreated and anodized substrates. The best results were obtained with laser energy of about 0.2 J/Pulse/cm² where single lap shear strength was improved by 600-700% compared with that of untreated Al alloy, and by 40% compared with chromic acid anodizing pretreatment.

The mode of failure changed from adhesive to cohesive as the number of laser pulses increased during treatment. The latter phenomenon has been correlated with morphology changes as revealed by electron microscopy, and chemical modification as indicated by Auger and infrared spectroscopy.

It can be concluded that the excimer laser has potential as a precise, clean and simple preadhesion treatment of Al alloys.     

The Effect of Temperature on the Strength of Adhesively-Bonded Composite-Aluminium Joints

Different materials have different coefficients of thermal expansion, which is a measure of the change in length for a given change in temperature. When different materials are combined structurally, as in a bonded joint, a temperature change leads to stresses being set up. These stresses are present even in an unloaded joint which has been cured at say 150°C and cooled to room temperature. Further stresses result from operations at even lower temperatures.

In addition to temperature-induced stresses, account also has to be taken of changes in adhesive properties. Low temperatures cause the adhesive to become more brittle (reduced strain to failure), while high temperatures cause the adhesive to become more ductile, but make it less strong and more liable to creep.

Theoretical predictions are made of the strength of a series of aluminium/CFRP joints using three different adhesives at 20°C and 55°C. Various failure criteria are used to show good correlation with experimental results.     

Strength Analysis of Adhesively-Bonded Tubular Single Lap Steel-Steel Joints Under Axial Loads Considering Residual Thermal Stresses

The static tensile load bearing capability of adhesively-bonded tubular single lap joints calculated using linear mechanical adhesive properties is usually far less than the experimentally-determined one because the majority of the load transfer of adhesively-bonded joints is accomplished by the nonlinear behavior of the rubber-toughened epoxy adhesive

In this paper, both the nonlinear mechanical properties and the residual thermal stresses in the adhesive resulting from joint fabrication were included in the stress calculation of adhesively-bonded joints. The nonlinear tensile properties of the adhesive were approximated by an exponential equation which was represented by the initial tensile modulus and ultimate tensile strength of the adhesive.

From the tensile tests and the stress analyses of adhesively-bonded joints, a failure model for adhesively-bonded tubular single lap joints under axial loads was proposed.     

Non-destructive Inspection of Adhesively-bonded Joints

The objective of any system of non-destructively examining an adhesive joint must be to obtain a direct correlation between the strength of the joint and some mechanical, physical or chemical parameter which can readily be measured without causing damage. Faults or defects are defined as anything which adversely affect the short or long term strength of a joint. There are two basic areas for examination, the cohesive strength of the polymeric adhesive, and the adhesive strength of the bond between polymer and substrate.

Adhesive strength is very difficult to measure since it is an interfacial phenomenon involving a very thin layer of material, thin even in comparison with bond-line dimensions. Effectively, it would be necessary to assess intermolecular forces and this is not readily possible with existing techniques. This aspect of quality control is usually reduced to assessing the nature of the adherend surfaces prior to bonding.

The cohesive strength of the adhesive is really the only parameter which can be estimated with any degree of confidence, and it is this which features most on destructive tests of bonded joints.

In this paper, defects including porosity, surface un-bonds, zero-volume unbonds, poor cure and so on are discussed, together with the various methods currently used (and some new methods) for physical non-destructive testing.     

Modelling Environmental Degradation in EA9321-Bonded Joints using a Progressive Damage Failure Model

A progressive cohesive failure model has been proposed to predict the residual strength of adhesively bonded joints using a moisture-dependent critical equivalent plastic strain for the adhesive. Joints bonded with a ductile adhesive (EA9321) were studied for a range of environmental degradations. A single, moisture-dependent failure parameter, the critical strain, was calibrated using an aged, mixed-mode flexure (MMF) test. The mesh dependence of this parameter was also investigated. The parameter was then used without further modification to model failure in aluminum and composite single-lap joints (SLJ) bonded with the same adhesive. The FEA package ABAQUS was used to implement the coupled mechanical-diffusion analyses required. The elastic-plastic response of the adhesive and the substrates, both obtained from the bulk tensile tests, were incorporated. Both two-dimensional and three-dimensional modelling was undertaken and the results compared. The predicted joint residual strengths agreed well with the corresponding experimental data, and the damage propagation pattern in the adhesive was also predicted correctly. This cohesive failure model provides a simple but reliable method to model environmental degradation in ductile adhesive bonded joints, where failure is predominantly within the adhesive layer.     

Modelling Environmental Degradation in EA9321-Bonded Joints using a Progressive Damage Failure Model

A progressive cohesive failure model has been proposed to predict the residual strength of adhesively bonded joints using a moisture-dependent critical equivalent plastic strain for the adhesive. Joints bonded with a ductile adhesive (EA9321) were studied for a range of environmental degradations. A single, moisture-dependent failure parameter, the critical strain, was calibrated using an aged, mixed-mode flexure (MMF) test. The mesh dependence of this parameter was also investigated. The parameter was then used without further modification to model failure in aluminum and composite single-lap joints (SLJ) bonded with the same adhesive. The FEA package ABAQUS was used to implement the coupled mechanical-diffusion analyses required. The elastic–plastic response of the adhesive and the substrates, both obtained from the bulk tensile tests, were incorporated. Both two-dimensional and three-dimensional modelling was undertaken and the results compared. The predicted joint residual strengths agreed well with the corresponding experimental data, and the damage propagation pattern in the adhesive was also predicted correctly. This cohesive failure model provides a simple but reliable method to model environmental degradation in ductile adhesive bonded joints, where failure is predominantly within the adhesive layer.     

Peel Strength of Aluminium-Aluminium Joints Bonded by Adhesive Based on Carboxylated Nitrile Rubber-Chlorobutyl Rubber Blend

The peel strength of aluminium-aluminium joints bonded by an adhesive based on carboxylated nitrile rubber and chlorobutyl rubber was found to depend on surface topography and use of a silane primer. Anodization causes a marginal increase in bond strength while the silane primer improves the adhesive joint strength remarkably.

The peel strength was also found to be dependent on test conditions (test rate and temperature). The threshold peel strength value obtained by measurements at low peel rate and high test temperature was found to depend on the type of failure during peeling (cohesive or interfacial) which, in turn, is controlled by the presence of silica filler in the adhesive. Two different threshold values of peel strength were obtained: 60 N/m for interfacial failure (in silica-filled adhesive), 140 N/m for cohesive failure (in unfilled adhesive).     

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.