Image credits: Julie Diani

At the crossroads between physics, chemistry and mechanics, adhesion was the focus of the annual symposium of the " Design and modelling of innovative materials " Chair organised on November 30th and entitled " Adhesion: Experiments, theory and applications ". This programme is led by Julie Diani, CNRS Research Director at the Solid Mechanics Laboratory (LMS*).

Bruno Charrière, Scientific Director of Arkéma, sponsor of the programme, did not hide his enthusiasm for this symposium: "There are very few events of this type with high-level presentations in this field of research. This symposium responds to the need for industrialists to exchange with experts, especially in very interdisciplinary subjects like this one. A wide range of academic issues, often brought forward by industry, were presented in the eight presentations at the symposium. Four doctoral students were also able to take part in the discussions during a poster session where they presented their work to Arkéma's experts and the speakers.

The day opened with a presentation by Matteo Ciccotti (École supérieure de physique et de chimie industrielles de la ville de Paris), who sought to compare three of the main tests to which adhesives are subjected. To do this, he considers the adhesive as a set of springs subjected to peel tests (tearing off a glued surface, such as tape), tack tests (adhering a cylinder to a surface and then removing it) and lap joint tests (sliding two glued surfaces together). He observes the formation of adhesive fibrils and voids to explain the results of the peel and tack tests. The shear test, which is more difficult to model, remains an objective of his future studies.

> Presentation : Matteo Ciccotti, ESPCI, Linking Peel, Tack and Shear in pressure sensitive adhesives

Nicolas Carrere (École Nationale Supérieure de Techniques Avancées Bretagne) is developing a model to study the failures that occur at the interface of bonded assemblies. When two materials are bonded by an adhesive, edge effects are more likely to cause premature failure. In addition, the size and number of defects present in the joints are of interest. He shows that when porosities and defects are present in the adhesive, they can cause ease of failure, but also help to stop the progression of a crack in the adhesive. In the future, he hopes to be able to deflect a crack out of the adhesive to maintain consistency between the two bonded materials.

> Presentation : Nicolas Carrere, ENSTA Bretagne, Outils numériques et expérimentaux pour l’étude du comportement des assemblages collés

Plasma polymerisation, a system that uses a plasma to dissociate elements in order to cover a surface, is at the heart of the research work presented by Florence Bally-le-Gall (Haute-Alsace University). This highly versatile method makes it possible to process, sculpt on a very small scale, or deposit a thin film of material on a surface. She has thus been able to create adhesive surfaces that respond to external stimuli, for example a surface to which bacteria will not adhere unless it is subjected to a tensile force. Her recent work has led her to create a strong adhesion method, based on a specific reaction called the Diels-Alder reaction, but reversible when the bond is heated.

> Presentation :Florence Bally-le-Gall, Université Haute-Alsace Mulhouse, Polymérisation plasma, un procédé pour la fabrication d’interfaces stimulables

Valérie Nassiet (Ecole Nationale des Ingénieurs de Tarbes) presented her model for studying adhesive joints with a property gradient, i.e. an adhesive composed of two different substances distributed in the adhesive junction. By comparing two unique adhesive mixtures supplied by Arkema, she was able to study the different dynamics of cross-linking, i.e. the formation of adhesive bonds by creating a solid 3D network of molecules. She was able to determine that the more separated the adhesives are in the joint (the steeper the gradient and the more heterogeneous the adhesive), the stronger the joint.

> Presentation :Valérie Nassiet, Ecole Nationale des Ingénieurs de Tarbes, Etude de la modularité de l’adhérence d’assemblages collés : application à un joint à gradient de propriétés obtenu par nanostructuration d’adhésif époxyde 

Taking up the modelling of the adhesive as a series of springs, Eric Paroissien (ISAE Supaéro) studied the transmission of stresses exerted on a structure by modelling it as an assembly of "bricks" called macro-elements. He shows that a bonded assembly can be represented with a single macro-element, which simplifies the analysis and calculation cost of bonded structures.

> Presentation : Eric Paroissien et Frédéric Lachaud, ISAE Toulouse, Modélisation du transfert d’effort au travers d’une interface adhésive

Denouncing the concept of interface as an idealised view at the macroscopic level, Maëlenn Aufray (Carnot Institute Chimie Balard Cirimat) developed the concept of "interphase", with the postulate that in an adhesive junction, the material "extends" and mixes into the adhesive instead of considering the two to be separate as is the case with the concept of interface. This interphase varies according to the adhesive and its viscosity, as well as the time before the adhesive is fixed. Finally, she proposes an innovative mechanical test to better distinguish adhesive rupture.

>Presentation :

Part 1, Maëlenn AUFRAY, CIRIMAT Toulouse, Influence de la formation des interphases sur l’adhérence des systèmes collés 

Part 2, Maëlenn AUFRAY, CIRIMAT Toulouse, Influence de la formation des interphases sur l’adhérence des systèmes collés 

Noelig Dagorn (Safran R&D) detailed the characterisation and validation processes of bonded assemblies in the industrial world. He took the example of titanium aircraft engine blades and detailed the behaviour law of the adhesive that he established in order to analyse the criteria of interest (viscoelasticity, work hardening, flow law, etc.). This characterisation stage is followed by a validation stage, with tests involving high-speed impacts to test resistance. He would later like to adapt this procedure to composite bonded assemblies of titanium with another material.

To close the day, Franck Lauro (Université Polytechnique Hauts-de-France Valenciennes) presented his study of the failure of adhesives subjected to dynamic loading in an industrial context. He used a mechanical arm to detach a flexible plate at different speeds and establish a dynamic rupture model requiring little calculation time. He is now looking to apply his viscoelastic model to other pull-out angles, as well as to different speeds, including simulating higher speeds by reducing the temperature of the adhesive.

> Presentation : Franck LAURO, Université Polytechnique Hauts-de-France Valenciennes, Impact et crash, modélisation d’une colle modèle par éléments finis 

For Julie Diani, "This symposium perfectly illustrated the multidisciplinary nature of the issues addressed in the framework of the " Design and modelling of innovative materials " chair. It also showed the positive dynamic for the advancement of knowledge that partnerships between industry and academic research represent. I would like to thank the speakers for their availability and the high quality of the discussions we had during the day."

*LMS: a joint research unit CNRS, École Polytechnique - Institut Polytechnique de Paris