Lucas F. M. da Silva

Handbook of adhesion technology

Preface Introduction to adhesive bonding technology Part A - Theory of Adhesion Forces involved in adhesion Wetting Work of adhesion Spreading Theories of adhesion Part B - Surface treatments Surfaces General principles Surface treatments Surface assessment Primers and adhesion promoters Surface treatments of selected materials Part C - Adhesive and sealant materials Classification Composition Adhesive families Sealant families Selection Part D - Testing of adhesive properties Physical properties Thermal properties Failure strength tests Fracture tests Impact tests Special tests Part E - Joint design Constitutive adhesive and sealant models Analytical approach Numerical approach Special numerical techniques Design rules and methods to improve joint strength Design with sealants Design for impact loads Vibration damping Part F - Durability High and low temperature effects Humidity, water and chemicals Radiation and vacuum Fatigue load conditions Creep load conditions Combined temperature-moisture-mechanical stress effects Part G - Manufacture Storage Preparation Application Bonding equipment Environment and safety Part H - Quality control Quality control of raw materials Processing quality control Non-destructive testing Techniques for post-fracture analysis Part I - Applications Aeronautical industry Aerospace industry Automotive industry Rail industry Boats and marine Civil construction Electrical Shoe industry Part J - Emerging areas Molecular dynamics simulation and molecular orbital method Bioadhesion Bioadhesives Adhesives with nanoparticles Adhesion in medicine Adhesion in energy applications Recycling and environmental aspects Epilogue

Effect of Adhesive Type and Thickness on the Lap Shear Strength

The effect of the adhesive thickness on the bond strength of single-lap adhesive joints is still not perfectly understood. The classical elastic analyses predict that the strength increases with the adhesive thickness, whereas experimental results show the opposite. Various theories have been proposed to explain this discrepancy, but more experimental tests are necessary to understand all the variables. The objective of the present study was to assess the effect of the adhesive thickness on the strength of single-lap joints for different kinds of adhesives. Three different adhesives were selected and tested in bulk. The strain to failure in tension ranged from 1.3% for the most brittle adhesive to 44% for the most ductile adhesive. The adherend selected was a high-strength steel to keep the adherends in the elastic range and simplify the analysis. Three thicknesses were studied for each adhesive: 0.2, 0.5, and 1 mm. A statistical analysis of the experimental results shows that the lap shear strength increases as the bondline gets thinner and the adhesive gets tougher.

Manufacture of adhesive joints and bulk specimens with high-temperature adhesives

Abstract For high-temperature usage (200°C and above) such as in certain supersonic aircraft structures, the adhesives used are either bismaleimides or polyimides, generally supplied as films, with or without a carrier. Other adhesives, such as modified epoxies, can also be used up to 200°C. For simulation purposes (such as finite element analysis), the adhesive mechanical properties are needed. These are generally obtained by testing a bulk specimen in tension or by testing adhesive joints under pure shear. However, a problem associated with adhesive films is void formation during manufacture. In order to achieve the full potential of the adhesive, it is necessary to understand the origin of the voids and to develop techniques to eliminate or minimise their formation. In this investigation, a bismaleimide adhesive, Redux 326 (Hexcel Composites), was studied in two forms: supported (woven glass) film and paste. Different manufacturing methods such as the vacuum release technique and sheet moulding were tested in order to produce adhesive joints and bulk specimens free of defects. The merits of each technique are discussed in terms of efficiency and practicability. The most appropriate method was used to manufacture single lap joints with and without voids in the supported film and paste form. These were tested at low temperatures where the adhesive is brittle and at high temperatures where the adhesive is ductile, to check the influence of voids and the carrier in the joint strength.

Modeling of adhesively bonded joints

Analytical Modeling.- Simple Lap Joint Geometry.- Analysis of Cracked Lap Shear (CLS) Joints.- Analytical Models with Stress Functions.- Numerical Modeling.- Complex Constitutive Adhesive Models.- Complex Joint Geometry.- Progressive Damage Modelling.- Modelling Fatigue in Adhesively Bonded Joints.- Environmental Degradation.- Non-Linear Thermal Stresses in Adhesive Joints.- Impact.- Stress Analysis of Bonded Joints by Boundary Element Method.

Mechanical Characterization of Flexible Adhesives

In this paper, the performances of two different adhesive types—a polyurethane and a high temperature thixotropic adhesive sealant, room temperature vulcanizing (RTV) silicone rubber—were studied through adhesive joint tests. The standard Thick Adherend Shear Test (TAST) was performed in order to measure the shear properties of the adhesives. Single lap joints (SLJs) were fabricated and tested to assess the adhesive performance in a joint. The influence of temperature on the lap shear strength of the adhesives was investigated. It is shown that the lap shear strength of both adhesives is affected by variation of temperature. The effect of bondline thickness and overlap length on the lap shear strength of the adhesives was studied. The reduction of failure load with increasing the bondline thickness is a very common situation when dealing with structural adhesives. For the low strength flexible adhesive Sikaflex® 552 the failure load as well as the overall stiffness of the SLJs decreases as the bondline gets thicker, whereas for AS1805 RTV adhesive the failure loads increase as the bondline gets thicker. Also, in contrast to joints with brittle adhesives, the failure loads of joints with flexible adhesives increase almost proportionally with increasing overlap length. Fatigue tests were also performed and show a low variability in the results.

Joint Strength Optimization of Adhesively Bonded Patches

Aircraft face damage from impact with objects or birds or due to ageing that leads to fatigue cracks. The conventional methods of repairing aircraft metallic structures generally include the use of a plate joined by screws or rivets. Although these methods are efficient in the short term, they introduce stress concentrations leading to the initiation of new cracks that are difficult or impossible to detect by non-destructive methods. For these reasons, it is necessary to develop new methods to improve the behaviour of the structure (especially for long term) and its manufacture cost. One of the solutions that have been studied by the aeronautical industry is the use of patches bonded with structural adhesives. However, adhesively bonded patches have problems of stress concentration at the edges where crack initiation is prone to occur. This problem can be reduced by the use of a taper and a spew fillet at the end of the patch and by the use of a mixed adhesive technique where a ductile adhesive is placed at the edges of the patch. Double strap specimens from 3 mm thick 6063-T6 aluminium alloy sheet were analysed. Aluminium and straps (or patches) with an internal taper, an adhesive spew fillet, and dual adhesives were experimentally tested. The results obtained were explained by a finite element analysis. A taper angle is beneficial only for the brittle adhesive. The use of two adhesives is advantageous for the taperless configuration.

Advances in Numerical Modelling of Adhesive Joints

The analysis of adhesively bonded joints started in 1938 with the closed-form model of Volkersen. The equilibrium equation of a single lap joint led to a simple governing differential equation with a simple algebraic equation. However, if there is yielding of the adhesive and/or the adherends and substantial peeling is present, a more complex model is necessary. The more complete is an analysis, the more complicated it becomes and the more difficult it is to obtain a simple and effective solution. The finite element (FE) method, the boundary element (BE) method and the finite difference (FD) method are the three major numerical methods for solving differential equations in science and engineering. These methods have also been applied to adhesive joints, especially the FE method. This book deals with the most recent numerical modelling of adhesive joints. Advances in damage mechanics and extended finite element method are described in the context of the FE method with examples of application. The classical continuum mechanics and fracture mechanics approach are also introduced. The BE method and the FD method are also discussed with indication of the cases they are most adapted to. There is not at the moment a numerical technique that can solve any problem and the analyst needs to be aware of the limitations involved in each case.

Multiple-site damage in riveted lap-joints: experimental simulation and finite element prediction

Abstract The multiple-site damage (MSD) phenomenon is discussed, and exemplified by the behaviour of riveted lap-joint specimens of aluminium alloy 2024-T3 alclad. The tests performed, on which the paper is based, are part of the contribution of IDMEC to a project on the fatigue behaviour of ageing aeronautical structures—the BRITE-EURAM project ‘SMAAC’, partially funded by the European Union. The study involves fatigue testing under constant amplitude loading of 1.6-mm-thick riveted lap-joints, and includes examination of the specimens during and subsequent to testing (post-mortem analysis of the fracture surface in a scanning electron microscope) in order to determine the time of occurrence, location and extent of fatigue damage. Crack growth rates are determined from periodic crack length measurements with a travelling microscope. Stress measurements are made using extensometry and the SPATE infrared technique to determine loading distribution of the lap-joints and redistribution due to cracking of fastener holes. Data on the initiation and growth of cracks and on residual static strength are used to assess the predictive model based on the finite element method.

Comparison of the Mechanical Behaviour Between Stiff and Flexible Adhesive Joints for the Automotive Industry

Adhesive bonding is increasingly being used in structural applications such as in automotive joints. The theoretical analyses and experimental data are generally for rigid and strong epoxy adhesives. Elastomeric adhesives such as polyurethanes are used in structural applications such as windshield bonding because they present important advantages in terms of damping, impact, fatigue, and safety which are critical factors in the automotive industry. However, there are other structural applications in the main body where polyurethanes may also be used. The main objective of the present project is to compare the behaviour of structural joints used in the automotive industry, such as single lap joints and T-joints made of rigid adhesives, and those made of elastic adhesives in terms of stiffness, strength, impact, damping, and fatigue. The elastomeric adhesive selected was a polyurethane from Sika (Sikaflex® 256) and the structural adhesive selected was an epoxy from Huntsman (Araldite® AV138/HV998). The shear strength of the polyurethane is approximately four times lower than that of the epoxy. However, the polyurethane shear failure strain is 330%, whereas that of the epoxy is only 6%. The benefits of using elastomeric adhesives in structural adhesive joints used in the automotive industry are described, especially in terms of ductility, impact, and fatigue.

Parametric Study of Adhesively Bonded Single Lap Joints by the Taguchi Method

The single lap joint is the most studied joint in the literature in terms of both theory and practice. It is easy to manufacture and the lap shear strength is a useful parameter for strength assessment and quality control. Simple design rules exist such as the one described in ASTM 1002 standard or in a recent paper by Adams and Davies. A previous study by the present authors gave a simple predictive equation considering the type of adhesive, i.e. ductile or brittle, the adherend yield strength and the overlap length. However, other factors also affect the joint strength such as the adherend and adhesive thicknesses and the surface preparation which have not previously been addressed in the literature. In order to quantify the influence of the adhesive (toughness and thickness), the adherend (yield strength and thickness), the overlap and the surface preparation on the lap shear strength, the experimental design technique of Taguchi was used in the present study. An experimental matrix of eighteen tests was designed and each test was repeated three times. The influence of the six previously-mentioned variables could be assessed using the statistical software Statview®. In this paper a simple predictive equation is proposed for the design of single lap joints.