Laurent Sohier

Characterization and modelling of the 3D elastic–plastic behaviour of an adhesively bonded joint under monotonic tension/compression-shear loads: influence of three cure cycles

Characterization and modelling of the 3D elastic–plastic behaviour of ductile adhesive materials are all but straightforward. Advanced models and significant experimental work are required in order to achieve good accuracy over a wide range of different loading conditions combining tension or compression with shear. Indeed, advanced constitutive laws taking into account hydrostatic stress dependency and non-associated formalism have to be defined. As a consequence, the experimental characterization of an adhesive within a bonded assembly has to include different load combinations. In this study, a modified Arcan specimen is used to obtain a large experimental data base covering tension, tension-shear, shear and compression-shear results for a bi-component epoxy-based adhesive (Huntsman™ Araldite 420 A/B). Three different bonding conditions are considered; the curing temperature (50 or 110 °C) and aged time at the test (within a week or after a 6 months storage at room temperature) being the two parameters investigated. A simplified inverse identification method based on these results is proposed for the identification of a 3D elastic–plastic model proposed by Mahnken and Schlimmer. In particular, the number of identification steps involving coupled finite element analysis and optimization software is considerably reduced. The comparison between the predicted and experimental results demonstrates the good capabilities of the model and underlines its limitations considering the flow rule definition in a specific case. Results show that both the curing temperature and the aged time have a substantial influence on the yield surface and final strength of the adhesive whereas the hardening curve seems less affected. Some possible improvements in the modelling of the adhesive under monotonic proportional loads are proposed as a conclusion.

Comparative Study of the Results of Various Experimental Tests Used for the Analysis of the Mechanical Behaviour of an Adhesive in a Bonded Joint

The use of adhesively bonded joints is often limited by a lack of reliable models able to accurately predict their behaviour in industrial applications, in which the stress distribution is often complex. The mechanical behaviour of an adhesive in a bonded joint is often heavily dependent on its stress state (i.e., the tensile–shear combinations). Thus, a large experimental database is required to accurately represent the complex behaviour of an adhesive in a bonded joint. On the one hand, the initial yield surface (initial elastic limit) often has to be described taking into account the two stress invariants, hydrostatic stress and von Mises equivalent stress, and on the other hand the non-linear behaviour of the adhesive is also quite complex to model. However, the mechanical response of adhesively bonded joints often presents quite large stress concentrations; thus, the analysis of experimental tests is made particularly difficult. Obtaining reliable experimental results makes it possible to contribute to optimization of an adhesive in a bonded joint. This paper presents comparisons between results of different experimental tests (with bulk and bonded joints), some of them are designed to greatly limit the edge effects. Results are presented for two adhesives under proportional monotonic loadings. The two adhesives have very different behaviours (a ductile adhesive and a brittle adhesive) and two different surface preparations of aluminium substrates (a mechanical preparation and a chemical preparation recommended by the adhesive manufacturer) were studied.The use of adhesively bonded joints is often limited by a lack of reliable models able to accurately predict their behaviour in industrial applications, in which the stress distribution is often complex. The mechanical behaviour of an adhesive in a bonded joint is often heavily dependent on its stress state (i.e., the tensile–shear combinations). Thus, a large experimental database is required to accurately represent the complex behaviour of an adhesive in a bonded joint. On the one hand, the initial yield surface (initial elastic limit) often has to be described taking into account the two stress invariants, hydrostatic stress and von Mises equivalent stress, and on the other hand the non-linear behaviour of the adhesive is also quite complex to model. However, the mechanical response of adhesively bonded joints often presents quite large stress concentrations; thus, the analysis of experimental tests is made particularly difficult. Obtaining reliable experimental results makes it possible to contribute ...

Mechanical behavior of syntactic foams for deep sea thermally insulated pipeline

Ultra Deep offshore oil exploitation (down to 3000 meters depth) presents new challenges to offshore engineering and operating companies. Flow assurance and particularly the selection of insulation materials to be applied to pipe lines are of primary importance, and are the focus of much industry interest for deepwater applications. Polymeric and composite materials, particularly syntactic foams, are now widely used for this application, so the understanding of their behavior under extreme conditions is essential. These materials, applied as a thick coating (up to 10-15 cm), are subjected in service to: - high hydrostatic compression (up to 30 MPa) - severe thermal gradients (from 4°C at the outer surface to 150°C at the inner wall), and to high bending and shear stresses during installation. Damageable behavior of syntactic foam under service conditions has been observed previously [1] and may strongly affect the long term reliability of the system (loss of thermal properties).This study is a part of a larger project aiming to model the in-service behavior of these structures. For this purpose it is important to identify the constituent mechanical properties correctly [2, 3]. A series of tests has been developed to address this point, which includes: - hydrostatic compression - shear loading using a modified Arcan fixture This paper will describe the different test methods and present results obtained for different types of syntactic foams.

Advances in Testing Adhesively Bonded Composites

Abstract: This chapter will briefly present the current state of the art with respect to tests for composite assemblies, before describing an improved testing approach. The lap shear test is first examined in detail, through a series of results from tests on glass/epoxy and carbon/epoxy specimens. Numerical modelling is used to show the difficulties in analysing results for this specimen configuration. An original approach is then presented, based on modification of the Arcan test method. This allows the determination of adhesive failure envelopes from tests on metal substrates. The approach is then extended to composite assemblies and test results are given. The chapter concludes with a discussion of future testing requirements.

Influence of the geometry of coaxial adhesively bonded joints on the transmitted load under tensile and compression loads

For the assembling of dissimilar material or of composite materials, the use of adhesive for the design of assemblies can reduce the cost and the weight of structures. Adhesive joining techniques do not require holes, such as for riveted or bolted joints, which can lead to stress concentrations, but adhesively bonded joints are often characterized with large edge effects associated with geometrical and material parameters. In the case of single lap type joints, peel and cleavage forces strongly limit the transmitted load of the assembly despite various techniques proposed to limit the influence of edge effects. Cylindrical joints are associated with large strength of the substrates in the radial direction; thus, peel and cleavage forces have different effect with respect to simple lap joints. But, for such assemblies, edge effects also exist. The objective is to analyse the effect of various geometries of the different parts of the assembly in order to optimize the maximal transmitted load of such joints. In the case of axial loads, the stress distributions are analysed using axisymmetric theory of elasticity. A pressure-dependent elastic limit of the adhesive is used, in order to accurately represent the difference between tensile-shear and compression-shear loads in the mechanical response of the adhesive. Designing adhesively bonded assemblies which strongly limit stress concentrations can significantly increase the load transmitted by the assembly. The first 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.

Strategies for the analysis of the behavior of an adhesive in bonded assemblies

Experimental and numerical analyses of the mechanical behavior of bonded joints can be made particularly difficult by the influence of edge effects. Therefore, understanding the stress distribution in an adhesive joint can lead to improvements in adhesively-bonded assemblies. Such an analysis is proposed in the case of usual single lap shear specimens. Stress singularities can contribute to the initiation and propagation of cracks in the adhesive. Thus, in order to obtain reliable experimental data to analyze the nonlinear behavior of an adhesive in an assembly, tests which strongly limit the influence of stress singularities must be proposed. The design and the abilities of such a device for shear tests are presented. Moreover, some experimental results obtained using a modified Arcan fixture, which has been designed to strongly limit edge effects, are presented in the case of monotonic and complex history loadings. Furthermore, a 2D non associated elasto-visco-plastic model is proposed to accurately describe the experimental behavior under tensile-shear monotonic loadings. An extension of this model is also proposed to represent relaxation type effects under shear loadings.

Influence of edge effects on the load transmitted by adhesively-bonded assemblies

This paper presents a contribution to the influence of edge effects on the load transmitted by adhesively-bonded assemblies. Bonded assemblies are mostly characterized by the presence of stress concentrations at the bottom of the adhesive and in its thickness near the interface. This phenomenon, called edge effects, plays a very important role in the macroscopic observed behaviour of an adhesive in an assembly. Thus, to describe its behaviour it is necessary to propose solutions to reduce the edge effects for the bonded test specimens used to characterize the adhesive and for industrial applications. Mastering edge effects improves the efficiency of bonded parts and reduces the dispersions of the loads at failure. In the first part a modified Arcan test fixture is described to analyze the behaviour of an adhesive in an assembly under traction-shear loadings. The test specimens use a specific geometry to reduce the edge effects (beaks). In the second part, edge effects are analyzed by the finite element method under elastic hypothesis. In the third part, experimental results for traction-shear Arcan tests are presented. And finally, this method to reduce edge effects is applied to the bonding of a rail on a composite mast for high competitive sailing boats.