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TRENDS IN COMPUTATIONAL STRUCTURES TECHNOLOGY
Edited by: B.H.V. Topping, M. Papadrakakis
Analysis of the Stress Distribution in Adhesive Joints and Optimisation of the Design of Hybrid Bonded Assemblies
J.Y. Cognard1, R. Créac'hcadec1 and L. Sohier2
1ENSIETA, Brest, France
J.Y. Cognard, R. Créac'hcadec, L. Sohier, "Analysis of the Stress Distribution in Adhesive Joints and Optimisation of the Design of Hybrid Bonded Assemblies", in B.H.V. Topping, M. Papadrakakis, (Editors), "Trends in Computational Structures Technology", Saxe-Coburg Publications, Stirlingshire, UK, Chapter 10, pp 223-246, 2008. doi:10.4203/csets.19.10
Keywords: adhesive testing, joint design, stress analysis, edge effects, non-linear behaviour, hybrid bonded assemblies.
This paper presents contributions of numerical modelling for the optimization of adhesively-bonded assemblies. The applications are particularly related to the marine field which is characterized by the use of relatively thick, 0.5 mm or more, adhesive joints. The experimental and numerical analyses of the mechanical behaviour of bonded joints are made particularly difficult by the stress singularities due to edge effects . The stress singularities can contribute to the initiation of fracture in adhesive joints and thus can lead to an incorrect analysis of the behaviour of the adhesive. Therefore, understanding the stress distribution in an adhesive joint can lead to improvements in adhesively-bonded assemblies. Particularly, as it can be difficult to take the effects of stress singularities into account when analyzing experimental results, it is useful to design experimental fixtures which strongly limit the edge effects in order to obtain reliable data. Several analytical studies have been proposed to analyze the influence of the geometry on the stress singularities for bi-material joints [2, 3]. In the case of elastic behaviour, the two main parameters are the relative elastic properties of the materials and the geometry of the substrates. For bi-material, these studies have shown that the use of beaks can limit the edge effects. However, with those approaches it is difficult to analyze the influence of the various parameters. For instance, the geometry of the bonded assembly, the interaction between the two interfaces of the thin joint, the non linear behaviour of the adhesive and the external loading on the structure can have an influence on the stress distribution. Precise finite element computations are therefore useful to analyze the stress singularities in order to optimize the design of bounded assemblies [4, 5].
A modified Arcan fixture, which allows compression or tension to be combined with shear loads, has been designed enabling the adhesives of interest to be characterized up to failure . It has been numerically shown, on the one hand, that the use of a beak close to the adhesive joint makes it possible to strongly limit the edge effects; and on the other hand, that the local geometry of the joint near the edge is an important parameter. The experimental results allowed us to analyse different aspects of the non-linear behaviour of the thin adhesive film.
The paper presents, in the case of tensile-shear loadings, the influence of the geometry of the substrate close to the free edge of the joint and of the local geometry of the joint near the free edge on the stress distribution within the adhesive. Numerical determination of how stresses evolve through the thickness of the adhesive joint requires refined meshes, especially for large material heterogeneity of the structure. Moreover, in order to obtain precise results, the influence of the fixture must also be taken into account, since the rigidity of the different parts of the bonded assembly and the external loading on the structure can influence the stress distribution within the adhesive joint. The numerical results, obtained under elastic assumption, allow us to propose some rules for manufacturing conditions in order to optimize bonded joints. Those numerical results have been confirmed by experimental tests. Moreover, numerical simulations taking into account some aspects of the non-linear behaviour of the adhesive joint provide more information about the way the adhesive behaves in an assembly. A modification in the geometry of the substrates is proposed in order to nearly cancel out the solicitation of the adhesive near the free edge of the joint; thus, this modification can improve the analysis of the behaviour of the joint before failure.
The optimization of adhesively bonded joints in naval applications requires improved characterization of the adhesive. In particular, it is important to analyze the influence of the temperature, and of ageing in the marine environment (seawater, sun, temperature), on the non-linear behaviour of the adhesive. The proposed Arcan fixture makes it possible to carry out these tests but the standardized fixture TAST (shear test with thick substrates) appears better adapted for the analysis of the ageing of the adhesive. A comparison of the experimental results in shear for these two tests showed differences in the non-linear behaviour. A detailed study of the distribution of the stresses in the adhesive joint, for the TAST fixture, showed that the edge effects are very significant. Tests carried out with various substrates and with various adhesives also show that failure initiates in the same place. These edge effects in the TAST fixture can lead to an incorrect analysis of the behaviour of the adhesive, in particular whenever an adhesive failure type is dominating. Using experience gained during improvement of the design of the Arcan assembly made it possible to propose a modification of the TAST fixture to obtain a more "reliable" analysis of the behaviour of the adhesive. This device is also well adapted to the study of the influence of ageing on adhesive properties; indeed, it uses small samples which can be removed from bonded plates having been exposed to ageing.
Hybrid bounded assemblies involving composite materials can improve productivity by simplifying design constraints. Thus an experimental study of the behaviour of the adhesive in a metal/composite joint has been proposed using the Arcan test developed previously to characterize assemblies of metallic substrates. A composite plate was bonded between two metallic substrates. The results indicate that the test fixture is suitable for obtaining the response of adhesive systems under a large range of loading conditions. Steel and aluminium assemblies appear to show similar behaviour but for the metal/composite assembly a rupture by delamination of the composite is often obtained. More work is under-way to clarify the role of composite failure mechanisms in mixed joints. A detailed analysis of the distribution of the stress in the thickness of the adhesive joint has provided important information on the loadings of the adhesive joint and of the composite. This study makes it possible to optimize the dimensioning of this type of assembly. The objective is to limit the edge effects in order to increase the acceptable maximum loading of the assembly. Moreover this experimental test allows us to characterize the behaviour of thin composites under out of plane loadings: the Arcan fixture allows us to apply traction or compression loadings in the normal direction of the middle plane of the composite combined with tangential loadings.
Those numerical results provide some rules for manufacturing conditions in order to optimize bonded joints. This strategy must be applied in order to optimize industrial adhesively-bonded assemblies, while taking manufacturing constraints into account. A test to characterize the bonding of a rail (used to guide the mainsail) to a composite mast on a racing yacht is being employed to analyse the influence of critical parameters (rail geometry, mast surface characteristics...) and will allow the potential of adhesive bonding to be demonstrated on a real industrial structure.
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