This study is concerned with increasing the performance of adhesively bonded structures. Adhesively bonded joints offer many advantages, but a lack of confidence limits the current use of this technology. The experimental and numerical analyses of the mechanical behaviour of bonded joints are made particularly difficult by edge effects. Stress singularities can contribute to the initiation and the propagation of crack in adhesive joints. Therefore, understanding the stress distribution in adhesive joints can lead to improvements in the performance of adhesively-bonded assemblies. It can also result in the development of more reliable experimental tests to characterise the non-linear behaviour of the adhesive in an assembly. Several analytical expressions have been proposed to evaluate the influence of the geometry on the stress singularities for bi-material joints. However, precise finite element computations are necessary to analyse the stress singularities, in order to optimise the design of bonded assemblies and take into account the influence of the various parameters, such as the geometry of the assembly, the interactions between the two interfaces in a thin joint, and the non linear behaviour of the adhesive. First, starting from the widely-used single lap shear specimen, the influence of different parameters on the stress distribution through the adhesive joint thickness will be presented. The three main parameters are the geometry of the substrate close to the adhesive joint, the local geometry of the joint near the edge and the thickness of the adhesive joint. Moreover the numerical determination of how stresses evolve through the thickness of the adhesive allows us to analyse the differences between two experimental devices: a modified Arcan fixture, designed in order to limit the influence of the edge effects [1], and the standardised TAST fixture. Then, experimental results, obtained from the modified Arcan test, will underline various aspects of the non-linear behaviour of the adhesive for radial loadings [1], these are useful to develop accurate 2D numerical models for the behaviour of the adhesive in an assembly. Finally, the influence of the hydrostatic pressure on the behavior of adhesives will be examined. Various studies have indicated the importance of hydrostatic pressure on yield behaviour [2] but few experimental data are available in order to validate the models proposed. Thus, the experimental results obtained with the Arcan fixture have to be completed. As a first step, tests under shear loadings will be  presented, performed using a modified TAST specimen [3] in a specially designed pressure vessel mounted on a standard test machine. This allows pressures up to 100 MPa to be applied during mechanical testing. These results are being used to develop a 3D model in order to accurately represent the non-linear behaviour of an adhesive in an assembly.  [1]      J.Y. Cognard, P. Davies, L. Sohier, R. Cr�ac'hcadec, �A study of the non-linear   behaviour of adhesively-bonded composite assemblies�, Composite Structures, 76, 34-46, 2006.[2]      R. Mahnken, M. Schlimmer, Simulation of strength difference in elasto-plasticity for adhesive materials, Int. J. for Num. Meth. in Eng., 63:1461-1477, 2005[3]      J.Y. Cognard , R. Creac'hcadec R., �Analysis of the non linear behaviour of an adhesive in bonded assemblies under shear loadings. Proposal of an improved TAST�, Journal of Adhesion Science and Technology, 23, 1333-1355, 2009

Improved approaches to analyse the non-linear behaviour of adhesives in bonded assemblies

240th National Meeting (2010)

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