Experiments have consistently shown that metallic materials display strong size effects at the micron scale, where the peculiar phenomenon “smaller is stronger” appears. Considering the exponentially increasing trend towards miniaturization, correct modeling of such effects has become inescapable in various high-technology fields, including microbotics, microelectronics, micromedicine, etc. As a result, a strong scientific effort has been devoted to the subject and numerous size-dependent theories have been proposed in recent years. Among them, strain gradient plasticity (SGP) theories, which can be seen as an extension of conventional plasticity to small scales, can particularly be cited. Including internal length scale(s), these theories are capable of predicting plastic deformation gradients, which correlate with size effects as experimentally observed and numerically predicted using dislocation mechanics. Thanks to their capabilities in capturing size effects, SGP theories have become increasingly used for the prediction of the size-dependent behavior of materials at the micron scale. However, despite the significant progress made on these theories, they still suffer from some fundamental issues, so far preventing their application to real engineering problems. The present project is proposed to address one of the most challenging of these issues: the physical nature of elastic gaps which are numerically observed using major of such theories. Almost all existing thermodynamically-consistent SGP theories including higher-order dissipation predict elastic gaps (delay in plastic flow) under certain non-proportional loading conditions. Nevertheless, to date, there is no experimental evidence nor is there small-scale numerical confirmation proving the existence of such gaps in reality. This represents a major source of confusion and uncertainty, preventing the development of robust SGP models that can be applied in a real industrial context. In the absence of works studying these gaps from a physical point of view, the scientific investment on SGP theories has reached a bifurcation point making the scientific community divided into those who consider elastic gaps as “unknown” size effects (then possibly physical) and those who see no physical reasons for their occurrence in reality, at least within a single-crystal, as they reflect an instantaneous finite change in the higher-order stresses. This project aims at clearing-up this ambiguity and at providing the compelling answer to the question "are elastic gaps physical?". To achieve its goals, the project will create a multi-disciplinary synergy between original small-scale experiments, implying, for the first time, non-proportional loading conditions, and extensive discrete-dislocation-based simulations. Results of the experimental and numerical investigations will then be considered to review major existing SGP theories, with the aim of developing the first single- and poly-crystal SGP models that are robust enough for real engineering applications. As an application, the proposed models will finally be used to investigate one of the most challenging small-scale problems: impact of size effects on the formability of ultra-thin sheet metals, which show an ever-growing use in various engineering fields.

The current trend for increased capabilities and miniaturization leads to manufactured goods composed of materials showing very heterogeneous mechanical properties. The exposition of these assemblies to thermal loadings during manufacturing or during use induce reliability issues. In this project, printed circuit boards (PCBs) are considered, for they are the ideal multi-material product for the fundamental research in mechanics of materials. Delamination between the constitutive layers is a source of reliability issues in PCBs, and constitutes the topic of this project. After having characterized the mechanical behavior of the different layers involved, the response of the interfaces will be studied at different loading rates and temperatures. For this purpose, new measurement methods will be developed. Furthermore, the measurements and modeling steps achieved will allow to offer predictive simulations to the PCB industry, therefore improving the reliability of the products.

The objective of this project is to design an integrated power module embedded in PCB under electrical and thermal constraints, as well as "0-electromagnetic emission" constraint, reliability and lifetime. The developed electro-thermo-mechanical models and characterizations could be exported, adapted and used in the generic design of integrated converters. The project will bring major advancements in the understanding of physical phenomena (characterizations, failure modes) as well as in the exploration of innovative technologies (new materials for contact recovery) which will have spinoffs in the academic and industrial PCB fields. Significant improvements can then be made to increase the reliability and lifetime of these devices. Finally, the sustainability studies carried out will provide new answers on the economic and environmental level with regard to integration technologies.

In the aim of improving the industrial forging routes for titanium components, the STAMP project is dedicated to the study of the microstructural evolution during thermomechanical treatments of Ti64 alloys. The present project is focused on the ? phase globularization mechanisms: i.e. the fragmentation of the initial ? lamellae into isotropic nodules. This transformation is required, among various reasons, to respect the titanium alloys end-users specifications. Based on the industrial Knowledge of the forging process (including numerical simulations), the microstructure evolution will be experimentally simulated thanks to controlled laboratory devices to reproduce the “incremental” loading of the process: an opportunity stands, to combine the high temperature compression device for monotonic, isothermal plane strain tests, (ARMINES-SMS) and the new Gleeble MaxStrainTM modulus in MATEIS INSA allowing the reproduction of forging sequences with strain path’s changes and inter-pass annealing. A specific attention will be paid to the thermomechanical conditions temperature and strain rate homogeneity in the second device, thanks to Finite element analysis of the tools and the samples. The microstructure morphology and the texture will be studied, as a function of the complex forging routes; Microstructural analyses will account for the transformation from alpha to beta during the cooling stage after forge, thanks to specific tools developed at the LEM3 (Université de Lorraine) in charge of the task in the project. A finite element model will be developed by INSA MATEIS, to capture the effect on the mechanical properties of the globularized microstructure. This model will be fed and validated respectively by the microstructure analyses the forged samples and the exploitation of the room temperature tensile tests performed on the specimen collected in semi-industrial samples pressed by the company Aubert et Duval. As a prospect of the project this model should be also applied to capture the globularization mechanisms during the forge operation of a former lamellar microstructure. These results will highlight the microstructure evolution mechanisms allowing (or preventing) the ? phase globularization to proceed in industrial conditions. This transformation is mandatory for the static and cyclic specifications of many aeronautic components. The industrial scale allowing the problematic definition and the evaluation of the investigation results relevancy will be strongly coupled with the laboratory context easing access to microstructure analyses and versatile use of the thermo-mechanical devices.

Despite many attempts to model effective behavior, there is currently no recognized approach that can capture the most important aspects of deformation of 3D textile reinforcements during their processing, and predict efficiently both the macroscopic response of the textile structure in the dry state or as pre-impregnated from the behavior of the fibers or yarns at the smaller scales. Recent developments in multi-scale simulation and 3D imaging techniques in materials science, particularly X-ray microtomography combined with appropriate image analysis techniques, make it possible to finely analyze the micro-mechanisms of deformation at the level of interactions between fibers and to enrich the interpretation of micro or meso-mechanical tests, which opens up new ways for the exploration and the understanding of the phenomena occurring at this level, in particular for elaborating and identifying models at intermediate scales, essential for an in-depth prediction of macroscopic behavior. The general objective of the project is the development of constitutive laws with an enriched kinematics for dry and pre-impregnated 3D technical textile reinforcements at different scales, which integrate the geometry of the constituents identified by X microtomography, the rheology of fibers and yarns, geometric and structural nonlinearities, singular and dissipative phenomena related to the presence of defects, irregularities in behavior (contact, friction, microcracking), as well as a statistical variability of the geometry and mechanical properties of the yarns or fibers within the armor. These aspects are scientific locks that define the innovative nature of the project compared to the literature works. The project is multidisciplinary since it concerns the science of fibrous materials, the mechanics of discrete and continuous media, multiscale higher order homogenization methods, rheology, stochastic methods, microtomography image analysis techniques, and numerical methods.