报告题目：Full-field kinematic, thermal and calorimetric measurements to
characterize the thermomechanical behavior of solid materials
报告人：法国蒙彼利埃大学 Bertrand WATRRISSE
During a mechanical test, the material transforms the received mechanical energy into different forms of energy (elastic energy, stored energy, heat …). The conversion of mechanical energy into heat has been investigated in a wide range of materials by numerous authors. Naturally, the way the mechanical energy is partitioned characterizes the material behavior as a whole. The construction of the energy balance associated with a mechanical loading requires the determination of several “measurable” quantities (overall deformation, temperature) but also of non-measurable ones (stresses). These latter can be “easily” obtained during simple (uniaxial) tests on homogeneous specimens but their estimation is quite more difficult in heterogeneous situations that are far more common in practice (localization, complex loading …).
The combined use of imaging techniques such as Digital Image Correlation and Infrared Thermography allows reaching fields of deformation and temperature measurements even in the presence of highly localized material responses. Apart from the technical difficulties inherent to the simultaneous use of cameras working in different wavelength ranges – which requires temporal (synchronous) and spatial matching, and ad hoc lighting –, it is necessary to develop adapted data processing techniques to extract the relevant thermo-mechanical quantities from the recorded images. These data-processing are often related to inverse problem solving (such as heat equation inversion or mechanical inversion). This presentation will focus on the experimental strategy proposed to reach the local fields necessary to characterize the thermo-mechanical behavior of the material during a mechanical test: displacement and strain fields, temperature, heat sources…
报告题目：Assessment of strain energy on mechanically-loaded materials and
structures using a Constitutive Equation Gap inverse method
报告人：法国蒙彼利埃大学 Bertrand WATRRISSE博士
Energy balances constitute a very efficient tool to characterize the thermo-mechanical response of materials. They require the estimation of the different energies involved during the loading. These energies have to be locally assessed to take into account the heterogeneous response of the specimen in a real (imperfect) situation. The estimation of deformation energy is particularly challenging since it requires the estimation of the stress field that is not directly measurable in most situations.
This presentation will present a strategy proposed for identifying the shape and the parameters of constitutive equations for heterogeneous materials in addition to the stress field locally developed by the material. The method is based on the constitutive equation gap principles and it relies on the knowledge of mechanical kinematic fields (displacement or strain fields obtained by DIC for example) and of the loading (stress-free boundary location, overall applied load …). Combing the stress and strain fields then gives access to the strain energy locally developed by the material during the transformation.
The general framework of this method will be presented and we will focus on its implementation for elastoplastic behaviors. The procedure is then based on the iterative minimization of an energy norm involving the secant elastoplastic tensor.
We will insist on the performances of the procedure for locally identifying heterogeneous property fields and on its robustness and its stability with respect to experimental noise.
Finally, results obtained on various experimental tests with different specimen geometries will be presented.
报告题目：Thermomechanical characterization of polymeric materials
报告人：法国蒙彼利埃大学 Jean-Michel MURACCIOLE博士
Material thermal responses to mechanical loading processes are closely linked to their energetic response. So to perform an analysis of the thermomechanical behavior of polymers, load and unload tensile tests are performed using imaging techniques. Quantitative Infrared Thermography is used to estimate, via the heat equation, the amounts of heat involved in the material transformation.
The local heat equation is established using the Thermodynamics of Irreversible Processes where the equilibrium state is characterised by n+1 state variables including temperature T, stain, while the other (1, …,n-1) variables describe the material microstructure:
Where is the Gibbs Free energy, ρ the material density, C a specific heat and k the thermal conductivity.
We can notice that heat sources involve the intrinsic dissipation, a thermo-elastic coupling and other kinds of thermomechanical couplings.
This equation shows that only a part of the stress induces intrinsic dissipation which allows introducing the stress part associated with reversible processes:
Which can be written, for an isothermal process:
Where stands for the specific internal energy and s the specific entropy.
This last equation distinguishes the well-known classical energetic elasticity, from the entropic one - so-called rubber elasticity. The first one, identified by Lord Kelvin describes the material thermal expansion. It is inphase shift with respect to strain, while the second one is in phase.
In polymers, these two phenomena are linked to the chain flexibility. Above Glass Transition Temperature, main chain motions are easy and changes in shape can occur with small internal energy change: this is the rubbery state.
On the other hand, below, the only motions are to be much localized and may lead to plasticity. Then below a given temperature or over a given strain rate, polymers have been shown to possess a linear, strain-rate independent behavior. Nevertheless, the measured elastic moduli are lower than the theoretical ones.
Working on different polymers, we evidence the existence of the two specified coupling effects - in and out of phase. We propose a thermomechanical model introducing three state variables: the temperature T, the strain and a supplementary variable accounting for chain entanglement. This simple, reversible, model can represent these two phenomena. In this framework, the so-called thermoplastic inversion is then shown to be associated with the competition between these two coupling mechanisms.
报告题目：Mechanical and microstructural investigation of SMAT-induced nanomaterials
报告人：法国蒙彼利埃大学 Laurent WALTZ博士
In the present work, a method is presented combining surface nanocrystalline treatment (SMAT) and the co-rolling process. The aim of this duplex treatment is the development of a 316L stainless steel semi-massive multilayered bulk structure with improved yield and ultimate tensile strengths, while conserving an acceptable elongation to failure by optimizing the volume fraction and distribution of the nano-grains in the laminate. To characterize this composite structure, tensile and bending tests as well as sharp nanoindentation tests were carried out to follow the local hardness evolution through the cross-section of the laminate. Furthermore, electron microscope observations were carried out to determine the correlation between the microstructure, the local hardness and the mechanical response of the structure.