PhD Thesis Defense of Benjamin SALA from FEMTO – ST

Congratulations to Benjamin SALA for having successfully defended his PhD Thesis in the frame of the SSUCHY Project. 

He is the fourth PhD student to graduate in the frame of the project. The defence took place on Friday 26th of November 2021, in ENSMM (Besançon, France). So far, Benjamin SALA’s work led to the publication of 3 peer-reviewed articles with first authorship and 3 oral presentations in international conferences.  

More information about him: https://www.researchgate.net/profile/Sala-Benjamin  

Title: Creep and recovery behaviour of bio-based composite sandwich structures in hygrothermal environment. 

SUMMARY 

Today, plant fibre reinforced composites represent an alternative to traditional glass fibre composites in the design of high performance and environmentally friendly structures. However, these biobased materials are for the moment mainly limited to non-structural uses due to the lack of knowledge on their long-term behaviour, in particular under various and varying environmental conditions. This PhD thesis is part of the European project SSUCHY, dedicated to the development and optimization of biobased composite materials for structural and multifunctional applications. The objective of the thesis is to characterize and model the creep behaviour of biobased sandwich structures under different hygrothermal conditions. The studied structures are made of two thin biobased composite skins between which a light and environmentally friendly core material is bonded. The composite skins are made of continuous reinforcements of long flax and hemp fibres and a partially biobased epoxy matrix. The core materials selected are balsa wood panels, a cardboard honeycomb and a recycled PET foam.  

The methodology used in this work aims to develop a tool for the reliable prediction of the behaviour of sandwich materials, based on the mechanical characterization at the scale of its constituents, and this, under ambient and severe hygrothermal conditions. The main results of this work show that the increase in temperature and relative humidity of the environment result in an increase in the levels of total, delayed and irreversible strain of sandwiches and their constituents. A behaviour, unique to biosourced composites, which depends on the stress level and hygrothermal conditions, is highlighted. It induces additional and partially irreversible strains under creep loading and a stiffening of the material observable in the recovery phase. The increase of the creep strain of the core materials, in severe environment, is explained by the activation of the viscoelastic properties of the core constituents. At the scale of the sandwich material, this work shows that it is possible to develop a biobased material with instantaneous mechanical performances that can compete with those of a usual petroleum-based solution, while guaranteeing a lower mass and environmental impact. However, one of the limitations of these materials is the exhibition of significant time-delayed strains under creep stress. These strains are all the more important when the temperature and the relative humidity of the surrounding environment increase. The collected results also show that, although the behaviour and the mechanical properties are very scattered at the scale of the plant fibres, the variability at the scale of the sandwich and its components is significantly lower and comparable to that measured for traditional composites. All the results and tools developed in this thesis will contribute to a more precise design and sizing of biobased sandwich structures in the future. 

Jury Members: 

•  Fabienne TOUCHARD DR CNRS, PPRIME, ISAE-ENSMA (Poitiers) – Rapporteure 
• Peter DAVIES IR HDR, IFREMER (Brest) – Rapporteur 
• Patrick IENNY PU, LMGC, IMT Mines Alès (Alès) – Examinateur 
• Damien SOULAT PU, GEMTEX, ENSAIT (Roubaix) – Examinateur 
• Lamine BOUBAKAR PU, FEMTO-ST, Université de Franche-Comté (Besançon) – Examinateur 
• Frédérique TRIVAUDEY MCF, FEMTO-ST, ENSMM (Besançon) – Co-encadrante de thèse 
• Violaine GUICHERET-RETEL MCF, FEMTO-ST, ENSMM (Besançon) – Co-directrice de thèse 
• Vincent PLACET IR HDR, FEMTO-ST, Université de Franche-Comté (Besançon) – Directeur de thèse