Some effects of alloying elements on the performance of bonded aluminium alloy joints

The objectives of the motor industry to achieve vehicle weight reduction has led to a growing interest in the use of aluminium alloys in body construction. Recent trends and developments indicate that adhesive bonding will figure prominently in car body assembly in the future. While there may be some temptation to extrapolate design data from experiences in the aerospace industry, it is likely that the automotive industry will be constrained by different alloys and different production conditions. This pilot study demonstrates some effects of these considerations.     

A new pretreatment for the adhesive bonding of aluminium

A new pretreatment for the adhesive bonding of aluminium has been developed. Hard anodizing in sulphuric acid gave a dense, corrosion-resistant coating which was then textured by controlled dissolution in phosphoric acid. The topography and chemical properties of the surface were then suitable for adhesive bonding. The structure of the anodic film, the bond strengths obtained and the bond durability in humid atmospheres were assessed and compared with those from conventional treatments.     

Durability of aluminium bonded joints in long-term tropical exposure

Aluminium bonded joints were exposed to the jungle weathering conditions near the equator in Suriname, South America, for 12 years. Distinct differences were found between different jungle locations, with open jungle exposures being more severe than more protected, secluded jungle sites. More durable bonds were made to a higher purity surface alloy, such as Alclad 2024-T3, than to a more highly alloyed surface adherend, such as 6061-T6 alloy. Deoxidizing the aluminium surfaces before bonding produced both higher initial strength and more durable joints than only vapour degreasing the aluminium. The highest strength retentions were obtained with a room temperature-curing epoxy as compared with a rubber modified heat-cured epoxy. A silicone sealant which had effectively protected similar joints from a seacoast exposure for eight years was relatively ineffectual in the jungle environment.     

Impact behaviour of bonded mild steel lap joints

A simple drop-weight impact rig has been developed for high strain-rate testing of bonded joints. Initial experiments indicate that single lap shear joints in mild steel give considerably higher ultimate strengths under impact conditions than with quasi-static loading. It is suggested that the improved properties may arise from the strain-rate sensitivity of mild steel.     

Effect of alloy composition and surface pretreatment on the durability of adhesive-bonded aluminium alloy joints

Boeing wedge test data which indicate the influences of six surface pretreatments and four alloy compositions on the durability of adhesively bonded joints are reported. X-ray photoelectron spectroscopy data which indicate the effects of the same surface pretreatments and alloy compositions on the chemical compositions of the adherend surfaces are summarized. Possible correlations between the durability and X PS data are discussed. Three different methods of modifying the chromic acid anodize (Def 151) treatment of clad alloy adherends, in order to improve durability, are described and discussed.     

Long endurance fatigue performance of bonded structural joints

Resin bonded joints are being used for a variety of applications in highway bridges. Such uses involve operation under different combinations of dead and live loading plus weathering. This paper is concerned with the fatigue performance of steel-to-steel lap joints for endurances of up to 10⁸ cycles. The effects on fatigue performance of curing temperature and operating temperature have been examined and work in progress to investigate effects of weathering is described. Special attention is given to performances at long endurances and low stresses relevant to traffic loading during service.     

Strength prediction of bonded single lap joints by non-linear finite element methods

A non-linear finite element technique has been used to predict the mode of failure and failure load of single lap joints made from three aluminium alloys and four epoxy adhesives, and the results compared with those obtained from experiment and closed-form analyses. The finite element program used was able to account for the large displacement rotations that occur in a single lap joint under load, and allowed the effects of elasto-plasticity in both the adhesive and adherends to be modelled. A failure criterion based on the uniaxial tensile properties of the adhesive was used: for two untoughened adhesives a maximum stress criterion was found to be appropriate while for two toughened adhesives a maximum strain criterion was employed.     

Strength, deformation and relaxation of joints bonded with modified cyanoacrylate adhesives

The introduction of high molecular weight poly(methyl methacrylate) or poly(butadiene-co-acrylonitrile) into ethyl 2-cyanoacrylate produced viscous adhesives with a homogeneous or heterogeneous structure after cure. Steel joints bonded with these adhesives are shown to have improved tensile shear strength, deformability and stress relaxation of bonds compared with pure cyanoacrylate adhesive. Poly(methyl methacrylate)-modified adhesive is recommended for static load-bearing joints while poly(butadiene-co-acrylonitrile)-modified adhesive is more suited to cyclic or vibrating loads.     

Surface roughness effects on the stress analysis of adhesive joints

A finite element program was used to perform a parametric analysis of stresses generated by adherend surface roughness in lap and butt joints. Roughness asperities were idealized as having tip radius of curvature R1, height H1, and slopes A1. Failure criteria (maximum stress, yield, and fracture) were related to the geometric parameters by functions of the form: $\text{σ = σ}{}_{\mathrm{o}}\text{+ cR}{}^{\mathrm{r}}\text{H}{}^{\mathrm{h}}\text{A}{}^{\mathrm{a}}$The specific material properties used for the analysis were those of tooth enamel and an orthodontic cement. Qualitatively however, these results are applicable to all structural adhesives.     

Study of aluminium bonded joints by analytical electron microscopy: Microstructural changes with aging

This study concerns the influence of aging on the microstructural characteristics of each component in bonded joint systems consisting of an aluminium alloy substrate (2024) clad with an aluminium sheet, an oxide layer obtained by surface treatment ('optimized' chromic-sulphuric acid etching, chromic acid anodizing, or phosphoric acid anodizing), an epoxy-novolac-based composite primer, and an epoxy-based adhesive film. The samples studied were obtained by ultrathin cross-section microtomy of the bonded joints (before and after aging). The microcharacterization was carried out using a transmission electron microscope (TEM) equipped with an X-ray energy-dispersive spectrometer (EDS), an electron energy-loss spectrometer (EELS), and an energy-filtered imaging system. Detailed analyses revealed that microstructural changes occur during aging in the primer in contact with the oxidized substrate and that the thickness of the modified zone depends on the underlying oxide. In addition, it is shown that a 'true' interphase about 200 nm thick is formed between the primer and adhesive by a diffusional process.     

Novel surface and interfacial analysis techniques as aids to the development of new, high fracture toughness film adhesives

An experimental film adhesive of high fracture toughness had given a promising range of adhesive properties on pickled 2024-T3 clad aluminum alloy, but a wide variation in peel performance resulted when this adhesive was used to bond anodized aluminum adherends. High resolution scanning electron microscopy/ energy dispersive X-ray analysis of the fracture interfaces coupled with transmission electron microscopy of ultramicrotomed sections through the same specimens has shown that the problem was non-wetting of the substrate on a microscopic scale. The rheological properties of the adhesive system have been altered to overcome the problem, yielding a high fracture toughness film adhesive of consistent performance.     

Durability of structural adhesive joints

Single lap joint specimens bonded with two commercial epoxy-based film adhesives, FM-73 and FM-300K, were exposed to combinations of different types of ‘load’, ie mechanical load, temperature and moisture/water. The applied mechanical load was high—90% of the nominal strength of the specimen (as determined in tensile experiments). The initial strength of the joint and the residual strength at various time intervals were then determined. The specimens were not influenced by a single type of ‘load’ or even by a combination of two types of ‘load’. However, combining all three ‘loads’ caused spontaneous failure of the specimen. When applying a constant load of up to 60% of the nominal strength, no spontaneous failures were observed within 30 days (even after exposure to hot-wet environment).     

A method to predict fracture in an adhesively bonded joint

The application of a fracture criterion, formulated in terms of material-induced stress/strain singularities at the terminus of an adhesive joint, to cohesive fracture in a single lap joint is presented. The criterion can be interpreted physically in terms of the elastic strain energy density. The strength of the singularities depends on the elastic properties of the adhesive and adherends and the geometry of the bond terminus, but is independent of loading and global geometry. A finite element method is used to predict the limit load of an adhesively bonded single lap joint from a known value of Q_crit, the critical singular intensity factor. This method may be applicable to general joint geometries.     

A comparison of acrylic adhesives for bonding aluminum alloys after using various surface preparation methods

The effects of four surface preparation methods on bonds produced with three acrylic adhesives have been studied and compared with those for a previously tested acrylic adhesive. The comparisons were made using lap shear strengths, and fracture energies obtained from the Boeing wedge test.     

Analysis of bonded double containment cantilever joints

An important part of adhesive development is the formulation of analyses for predicting the stresses and strains of bonded joints. An analysis of a double containment cantilever joint was made using the finite element method. A double containment joint comprising steel adherends bonded by a two-part epoxy resin adhesive was constructed and the strains in the adhesive layer were measured by a two axes co-ordinate table. The experimental results were found to be in good agreement with those of the theoretical analysis.     

A comparison of different grades of an acrylic adhesive for bonding an aluminium alloy

Six different grades of ‘second-generation’ acrylic adhesives from one manufacturer's range are compared using lap shear and wedge tests.     

An analytical model for interfacial stresses in double-lap bonded joints

Due to their many advantages, adhesively bonded joints are widely used to join components in composite structures. However, premature failure due to debonding and peeling of the joint is the major concern for this technique. Existing analytical models suffer from two major drawbacks: 1) not satisfying zero-shear stress boundary conditions at the adhesive layer’s free edges^[1] and 2) failure to distinguish the peel stress along two adherend/adhesive interfaces^[2]. In this study, we develop a novel three parameter elastic foundation (3PEF) model to analyze a representative adhesively bonded joint, the symmetric double-lap joint, which is believed to have relatively low peel stresses. Explicit closed-form expressions of shear and peel stresses along two adhesive/adherend interfaces are yielded. This new model overcomes the existing model’s major drawbacks by satisfying all boundary conditions and predicting various peeling stresses along two adherend/adhesive interfaces. It not only reaches excellent agreement with existing solutions and numerical results based on finite element analysis but also correctly predicts the failure mode of an experimentally tested double-lap joint. This new model therefore reveals the peel stresses’ significant role in the failure of the double-lap joint, but the classical 2PEF model cannot create it.     

A microcomputer program for stress analysis of adhesive-bonded joints

This paper presents the equations used in a stress analysis program for adhesive-bonded joints which is based on the finite element method and is written in BASIC for HP-9000 series computers. Thermal expansion of both the adherends and the adhesive layer is considered.     

A hybrid bondline concept for bonded composite joints

Based on the experience in the past and the occurrence of in-service damages, the authorities restrict today the application of adhesive bonding of composite structures for aircraft applications. However, certification limitations can be overcome if occurring disbonds within a bond are stopped by implemented design features, so-called disbond stopping features. Consequently, a novel bondline architecture for bonded composite joints is proposed. By implementing a distinct rather ductile thermoplastic phase, a physical barrier for growing disbonds is obtained and thus a fail-safe design, respectively. Moreover, the joint is established by using two different joining technologies, namely adhesive bonding and thermoset composite welding. A sophisticated manufacturing technique is developed for the hybrid bondline concept to achieve a high strength joint. The joint׳s quality is examined by means of several analytical methods like microsections, scanning electron microscopy (SEM), and energy-dispersive X-Ray (EDX) analysis. Additionally, the mechanical performance is evaluated by static Double Cantilever Beam (DCB) and Single Lap Shear (SLS) tests.     

A three-point flexure test configuration for improved sensitivity to metal/ adhesive interfacial phenomena

A three-point flexure test was used to evaluate the effects of chemical surface treatments on the performance of adhesively-bonded structures of Ti6A14V. A single adherend/adhesive structure rather than a double adherend sandwich configuration is shown to be sensitive to pre-bonding adherend surface treatments. Data is presented which relates mechanical properties with chemically treated adherend surfaces. Photoelastic isochromatic fringes were recorded simultaneously with the flexure test as a means to monitor stress distribution, failure initiation, and crack propagation.