Julien Maurice

Analysis of the Influence of Hydrostatic Stress on the Behaviour of an Adhesive in a Bonded Assembly

Generally, adhesives are viscoelastic–plastic materials, for which the development of viscosity and plasticity varies depending on the type of adhesive and the stress state. Various models exist to represent the yield surface, or the so-called elastic limit, taking into account the two stress invariants, hydrostatic stress and von Mises equivalent stress. Moreover, to develop precise pressure-dependent constitutive models, it is necessary to have a large experimental database in order to accurately represent the adhesive strains which are strongly dependent on the tensile-shear loading combination. Under quasi-static loadings, for a given strain rate range viscous effects can be neglected, but only a few experimental results are available to model the behaviour of the adhesive in a bonded assembly accurately under realistic loadings. Moreover, edge effects often have a large influence on the mechanical response. This paper presents the possibility of combining the use of an experimental device, which strongly limits the influence of the edge effects, with a pressure vessel especially designed to study the influence of hydrostatic stress. The latter allows pressures up to 100 MPa to be applied during mechanical testing. Comparisons with results obtained with a modified Arcan device are presented. Such results are useful for the development of 3D pressure-dependent models for the yield function and for the analysis of more complex loading.

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.

3D Modeling of the Elastic-Plastic Behavior of Thin Aeronautical Adhesive Films Suited for a Wide Range of Tensile/Compression-Shear Loads

Adhesive bonding is an interesting structural assembling technique for weight saving in modern commercial aircraft, in which the use of composites materials is increasing. In order to meet both optimization and respect of safety conception constraints, the development of accurate numerical strategies is required. Thus, improvement in the experimental characterization and in the design of reliable numerical tools for bonded assemblies is necessary. This paper presents the characterization of the elastic-plastic behaviour of four aeronautical adhesive films, consisting of two epoxy-based resins supported by two types of carrier. The characterization over a wide range of monotonic proportional tensile-shear loads is performed using a modified Arcan test device designed to strongly limit the influence of edge effects. Moreover, to obtain an accurate definition of the initial elastic limit of the adhesives, further experimental tests have been performed using a pressure vessel especially designed to study the influence of the hydrostatic stress. Inverse identification techniques using finite element analysis have been used to identify the material parameters of an elastic-plastic model based on the experimental results (the load-displacement curves). Results underline the potential of such a model to represent the non-linear behaviour of ductile adhesives under tensile/compression-shear proportional monotonic loads.