January 16th 2024 - Jean Sébastien Lecomte
Date/Location: Wednesday 6 March 2024 at 2 pm in building L2 (room 322-324).
Speaker: Thomas Merlette, Solid Mechanics Laboratory (LMS), Ecole Polytechnique
Title: Amorphous glassy polymers and carbon fibre-reinforced polymers: two examples of heterogeneous materials
Abstract: The mechanical properties of heterogeneous materials often result from the behaviour and spatial distribution of heterogeneities in a complex way. Two examples of microstructure-property relationships in polymeric materials are presented here. The first example is a composite material consisting of long carbon fibres embedded in a thermoplastic polymer matrix. The fibres have a diameter of approximately 7 μm and exhibit temperature-independent elastic behavior, whereas the viscoelastic behavior of the matrix varies drastically between the glassy (low-temperature) and rubbery (high-temperature) states. Since the transverse isotropic linear viscoelasticity of the composite is difficult to characterize experimentally, the use of micromechanics is useful for predicting its homogenized behavior. FFT-based homogenization calculations are used to study the impact of microstructure and temperature on effective viscoelastic properties in the frequency domain. The study shows, in particular, that when phase contrast and fiber fraction are significant enough, the spatial distribution of fibers has an important impact on effective properties. The second example is that of amorphous glassy polymers, which are intrinsically heterogeneous materials. The characteristic length scale of heterogeneities is of the order of 5 nm and local viscoelastic behavior varies drastically from one heterogeneity to another, the relaxation times distribution spanning between 4 and 8 decades. Physically, the plasticity of those materials involves local monomeric rearrangements with associated energy barriers. The understanding of these mechanisms, and in particular the physical origin of strain hardening, is still the subject of numerous studies. In this work, a theory for strain hardening in glassy polymers is proposed, according to which monomer orientation increases the energy barriers to be crossed. The predictions of the model are studied using a numerical method analogous to Dissipative Particle Dynamics. Strain hardening then appears as an emergent property: it results from the slowing down of the dynamics of a small fraction of heterogeneous zones that can store higher elastic energy levels during deformation. We also show that this new theory is able to reproduce, at least qualitatively, specific memory effects such as the Bauschinger effect.
07 février 24 - Léonard Turpin
Date/Location: Wednesday 07/02/24 at 3 pm / building L2 room 324-322.
Speaker: Leonard Turpin (Diamond Light Source, Didcot, Oxfordshire, UK)
Title: Integrated image correlation and X-rays
Abstract: Integrated image correlation (IDIC in 2D, typically based on photographs, and IDVC in 3D, typically based on tomographies) is a valuable tool that can be used to solve identification problems based on full-field measurements. In its broadest definition, it consists of solving the problem of image correlation not on a finite element or thumbnail basis, but on a reduced basis incorporating a model or a priori knowledge of the behaviour being studied. Provided that the model is accurate, IDIC/IDVC minimises uncertainty about the parameters identified and is very robust to image noise. The classic application of IDIC/IDVC is the identification of material properties. To illustrate this, we will present the identification of all the thermoelastic parameters of the mesoscopic model of a 3D woven ceramic matrix composite from an in situ thermomechanical test using a tomograph. IDVC can also be used to determine geometric parameters, for example the position of the interface resulting from zone homogenisation of a material.
06 février 2024 - Joseph Paux
Date/Location: Tuesday 06/02/24 at 11am – Bâtiment L2 room 324-322
Speaker: Joseph Paux (EiSINE – ITheMM – University of Reims)
Title: Plasticity of porous single crystals and slip band fractals
Résumé: The growth of cavities in polycrystals is central to the understanding of ductile fracture in metals. At the single-crystal scale, this growth is strongly influenced by the surrounding crystalline plasticity. This tends to produce complex deformation fields structured into slip bands. In the presence of porosity, fractal structures of slip bands form around the cavities. We can take advantage of this recursive structure to estimate the strength of the porous single crystal and anticipate the evolution of the shape of the cavities.
23 January 2024 - Cuong Ha-Minh
Speaker: Cuong Ha-Minh from the Mechanical Engineering Department and member of the Laboratoire Mécanique Paris-Saclay (LMPS)
Date/Location: Tuesday 23 January 2024 at 2 pm in building L1
Title: “Modelling and Identification of the Mechanical Behaviour of Dry Fabrics and Laminates up to Final Failure: Challenges and Solutions for the Future”
Key words: “Woven and Laminated Materials and Structures, Mechanical Behaviour, Simulation, Modelling, Experimental/Digital Dialogue, Advanced Numerical Approaches”
Summary:
A scientific research strategy is presented to study the mechanical behaviour of dry fabrics and laminates in collaboration with academic and industrial partners. The originality of this approach lies in the predominant use of numerical analyses in dialogue with selective experimental data in order to characterise, quantify and correctly predict the physical mechanisms at different scales: micro-meso-macroscopic, etc. Scale transitions, local/global approaches and specific numerical techniques have been studied and developed to optimise prediction.
Experimental data, obtained directly from the literature or through specific tests under our supervision, were used to identify models and validate the numerical results. Based on the numerical and experimental results, theoretical models can be proposed and developed to enable real-time prediction using universal calculation tools such as Matlab or Scilab.
Faced with the major challenges associated with various complexities, an alternative is to propose and develop intelligent numerical tools focused on the search for quantities of interest. These tools are based on advanced approaches such as Isogeometric Analysis, Constitutive Relation Error (CRE), Model Reduction with PGD (Proper Generalized Decomposition), Limited Damage Rate model, as well as identification approaches based on Digital Image Correlation (DIC) and Artificial Intelligence.
January 16th 2024 - Jean Sébastien Lecomte
Speaker: Jean Sébastien Lecomte from the Laboratory for the Study of Microstructures and Mechanics of Materials (LEM3, Metz)
Date/Location: 16 January at 11am at Salles de Réunions L322 bâtiment L2.
Title: “Effect of H and O on the local mechanical properties of hexagonal metals”
Abstract: “The subject of my presentation will be the determination of the local mechanical properties of metals with hexagonal structure (HCP) and the effects of light elements on these properties. Through two examples, I will show the effect of light elements (O and H) on the activation of dislocations and maclage in titanium alloys. The first example will focus on the titanium (α β) alloy, TA6V, which is the best known titanium alloy. Macroscopic temperature tests show the presence of an athermal plateau which can be explained by oxygen atom-dislocation interaction.
The second example will look at the effect of hydrogen on the properties of a commercially pure titanium alloy (T40). The results suggest that, during cold rolling, hydrogen facilitates the formation of {1012} stress macles and the formation of geometrically necessary dislocations.”