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.

This article was published in the journal, International Journal of Adhesion & Adhesives [© Elsevier] and the definitive version is available at: http://www.sciencedirect.com/science/journal/01437496The single lap joint is the most studied type of adhesive joint in the literature. However, the joint strength prediction of such joints is still a controversial issue as it involves a lot of factors that are difficult to quantify such as the overlap length, the yielding of the adherend, the plasticity of the adhesive and the bondline thickness. The most complicated case is that where the adhesive is brittle and the overlap long. In any case, there is still a problem that is even more difficult to take into account which is the durability. There is a lack of experimental data and design criteria when the joint is subjected to high, low or variable temperature and/or humidity. The objective of this work is to carry out and quantify the various variables affecting the strength of single lap joints in long term, especially the effect of the surface preparation. The Taguchi method is used to decrease the number of experimental tests. The effect of material, geometry, surface treatment and environment is studied and it is shown that the main effect is that of the overlap length. In order to quantify the influence of the adhesive (toughness and thickness), the adherend (yield strength and thickness), the overlap, the test speed, the surface preparation and durability 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 eight 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

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.

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 assess...