PhD Defence – Glued multi-material structures

On 21 October 2021 at 9:30 am, Johnatan Leplat will defend his thesis in the premises of ENSTA Bretagne in Brest. This thesis deals with the mechanical behaviour and the failure of the glue/substrate interface in a saturated environment. This work was carried out as part of the INDUSCOL project.

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The durability of ORE structures over periods of up to 15 years remains a major issue because the manufacture of these structures is based on combinations of materials often assembled by structural bonding. This project focused on the study of the long-term behaviour of the glue seal in a marine environment, which is still poorly understood and leads to the use of overestimated safety coefficients, thus limiting the introduction of innovative multi-material structures. Water ageing of glue seals increases the risk of failure at the glue/substrate interface (adhesive failure). Only failure at the core of the glue joint (cohesive failure) is conventionally taken into account by the standards. The objective of this work is to integrate adhesive failure into the prediction of glue seal failure.

In addition to the steps of choosing the glue polymerisation conditions, the choice of ageing conditions, and the identification of a behaviour model depending on the water concentration, particular attention was given to the study of cohesive failure and the study of adhesive failure. The rate of critical energy restitution (Gc) of cohesive failure was evaluated from tests conducted on TDCB specimens, which were modified during this work in order to systematically obtain a cohesive fracture surface (regardless of the ageing time). Numerical models integrating the mechanical behaviour were used to obtain the evolution of the Gc of the adhesive as a function of the water concentration. The initiation of cracks at the glue/substrate interface, obtained by end-to-end tests developed specifically for this purpose, made it possible to define and identify a stress initiation criterion. Also developed during this work, a crack propagation test on a single interface allowed the evolution of the energy restitution rate of the interface to be observed for different water concentrations.

All of these tools have finally allowed the development of a predictive numerical model of the failure of a glued multi-material structureaged in water, taking into account the adhesive failure.

Composition of the jury

  • Maëlenn AUFRAY, Senior Lecturer, CIRIMAT
  • Pierre BIDAUD, Senior Lecturer from ENSTA Bretagne, IRDL
  • Emilien BILLAUDEAU, R&D Project Manager Engineer, NAVAL Group
  • Silvio DE BARROS, Professor, Cefet/RJ
  • Bruno FAVOLLE, Professor, PIMM
  • Frédéric LACHAUD, Professor, ICA
  • Antoine MAISON, Research Engineer, France Energies Marines
  • Georgios STAMOULIS, Senior Lecturer, IRDL
  • David THEVENET, Senior Lecturer from ENSTA Bretagne, IRDL

Photo credit: ENSTA Bretagne

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