Romain Créac'Hcadec

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.

Experimental Analysis of the Influence of Hydrostatic Stress on the Behaviour of an Adhesive Using a Pressure Vessel

The modelling of the non-linear behaviour of ductile adhesives requires a large experimental database in order to represent accurately the strains which are strongly dependent on the tensile-shear loading combination. Various pressure-dependent constitutive models can be found in the literature, but only a few experimental results are available, for instance, to represent accurately the initial yield surface taking into account the two stress invariants, hydrostatic stress, and von Mises equivalent stress. This paper presents the possibility of combining the use of tests on bulk specimens and tests on bonded assemblies, which strongly limit 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. For a given strain rate of the adhesive, experimental results using various stress paths are presented in order to analyse the influence of the hydrostatic stress on the mechanical behaviour of an adhesive. The analysis of the results focuses herein on the modelling of the initial yield surface, but such results are also useful for the development of the flow rules in the case of 3D pressure-dependent models.

2-D Modeling of the Behavior of an Adhesive in an Assembly Using a Non-Associated Elasto-Visco-Plastic Model

This paper presents a contribution to the development of a numerical model for an adhesive in an assembly, starting from a large data base of experimental results in the case of radial monotonic loadings. The experimental results were obtained with a modified Arcan-type fixture using specific geometries which strongly limit the influence of edge effects in order to obtain reliable information about the non-linear behavior of the adhesive. These results underline that deformations in the adhesive are much larger in shear than in peel. Thus, a non-associated 2D model, with a specific yield function, was proposed to represent accurately the experimental observations. As the stress state is not uniform in the adhesive joint for the proposed Arcan-type fixture, inverse identification techniques using non-linear finite element simulations were used. Firstly, for a given strain rate, an elasto-plastic model was proposed and its behavior was analyzed through different numerical examples. Secondly, an extension to elasto-visco-plastic models was proposed for a wide range of deformation rates under tensile-shear loading tests. Results of numerical examples and comparisons with experimental data are presented using joint-type elements (or interface elements) which allow one to limit the numerical cost in the case of bonded structures with low edge effects.

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 ...

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.

Analysis of the moisture effect on the mechanical behaviour of an adhesively bonded joint under proportional multi-axial loads

The objective of the study is to identify the 3D behaviour of an adhesive in an assembly, and to take into account the effect of ageing in a marine environment. To that end, three different tests were employed. Gravimetric analyses were used to determine the water diffusion kinetics in the adhesive. Bulk tensile tests were performed to highlight the effects of humid ageing on the adhesive behaviour. Modified Arcan tests were performed for several ageing times to obtain the experimental database which was necessary to identify constitutive models. A Mahnken–Schlimmer type model was determined for the unaged state according to a procedure developed in a previous study. This identification used inverse techniques. It was based on the unaged modified Arcan results and on a coupling between an optimisation routine and finite-element analysis. Then, a global inverse identification procedure was developed. Its aim was to relate the unaged parameters to the moisture concentration and overcome the difficulties usually associated with ageing of bonded assemblies in a humid environment: a non-uniformity of the stress state and a gradient of mechanical properties in the adhesive. This procedure was similar to the one used in the first part but needed modified Arcan results for several ageing times. It also required an initial assumption for the evolution of the Mahnken–Schlimmer parameters with the moisture concentration.

Identification of the Elastic–Plastic Behavior of Adhesively Bonded Structures Under Multi-Axial Loadings: Comparison of a Modified Arcan Test and a Tension/Compression–Torsion Test

Structural bonding is nowadays widespread in the industry. However, characterisation methods and 3D modelling of the adhesives need to be improved. The characterisation requires an experimental procedure to obtain a large experimental database under various loading cases, which represents a significant amount of data. The 3D modelling requires advanced models with several parameters to identify and generally uses inverse identification procedures, which can be time expensive. For a good accuracy, the constitutive models need to take into account the dependency on the hydrostatic stress and be written under the non-associated formalism. In this study, the experimental database is obtained via a modified Arcan test that can cover a wide range of loadings between tension, shear, mixed tension–shear, and mixed compression–shear. A second experimental campaign is realized with a tension/compression–torsion (TCT) test that can cover a greater range of loadings: from tension to compression and mixed tension/compression–shear, with an infinite possibility of mixed loadings. The modified Arcan database is used to identify a 3D elastic–plastic Mahnken–Schlimmer type model, according to an inverse identification procedure developed in a previous study. This model identification is validated on the experimental database coming from the TCT test: a numerical/experimental comparison is realized. This allows the validation of the model and emphasizes the benefits of the TCT test. Indeed, it proves that this test is well suited to characterize adhesive joints and presents several capacities that will be really useful for further studies, like an infinite range of non-proportional loadings available.

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.