Keywords: coaxial joint, adhesion, stress concentration, finite element analysis, joint design.

The use of adhesive for the design of assemblies can reduce the cost and the weight of structures, especially in the case of assembling dissimilar material or of composite materials, but a lack of confidence limits the current use of this technology. Thus, the optimisation of the design of adhesively-bonded assemblies requires accurate numerical tools which have to take into account the possible stress concentrations arising from edge effects. Stress concentrations can contribute to the initiation and the propagation of cracks in the adhesive. Various simplified models have been proposed in order to describe the behaviour of some bonded joints using one- or two-dimensional models, but often such models are not able to describe the effect of stress singularities which often have an influence on the maximum load transmitted by the bonded assemblies. Therefore, understanding the stress distribution in an adhesive joint can lead to improvements in adhesively-bonded assemblies; for instance, the design of assemblies which strongly limit the edge effects can be very interesting.

In this paper the influence of various geometries of the bonded area of a coaxial assembly is analysed in order to optimize the maximal transmitted load. In the case of tensile loads, the stress distributions are analysed using the axisymmetric theory of elasticity. A pressure-dependent elastic limit of the adhesive is used to accurately optimize the maximum transmitted load. Such models are often used to represent the behaviour of polymers, and they allow an accurate description of the difference between tensile-shear and compression-shear loads in the mechanical response of the adhesive. In the first part, the differences between tensile and compressive loads are analysed using cylindrical joints. Such model can be seen as a first approach in order to analyse the influence of bending loads in a perpendicular direction of the assembly axis. Moreover, a comparison between the mechanical behaviour of cylindrical joints and single lap joints is proposed starting from two-dimensional refined finite element computations with elastic assumptions. The aim is to analyse the possibility of using experimental results of single lap shear specimens to design cylindrical type joints. Secondly, the influence of the angle of conical geometries of the bonded area, which can be easily used for coaxial assemblies, is analysed with respect to the stress distributions. Examples of assemblies of tubes of the same diameters are analysed. Moreover, the influence of several geometries which strongly limit stress concentrations and which have been designed for other geometries of bonded joints, are proposed.

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