The present work aims at identifying an elastic-viscoplastic material behavior over a wide plastic strain and plastic strain-rate range (up to 0.1 and 1000 s−1 respectively), using the Virtual Fields Method. Image-Based Inertial Impact tests have been performed on the Ti6Al4V titanium alloy. The strain-rate dependency of the material has been identified with the results provided by these tests and compared to references.

In the present work Image-Based Inertial Impact (IBII) tests are performed on Ti6Al4V material. The IBII test uses an impact on the edge of the specimen to generate a short pulse that loads the specimen. Three specimen geometries have been tested: a classic rectangular specimen, and two specimen geometries with stress concentrating geometries (i.e. a hole and notches) to enhance high levels of plastic strain. Full-field measurement of the acceleration and strain are successfully used in combination with the Virtual Fields Method (VFM) to identify the strain rate sensitivity parameter of the Johnson-Cook model. The strain/strain rate spectra covered by each specimen are analysed. Finally, the influence of the virtual field used in the identification process is discussed as well as the simultaneous identification of the Johnson-Cook model strain rate sensitivity parameter and the strain rate threshold parameter.

Les travaux de la these visent a mettre en place une methodologie innovante de caracterisation du comportement viscoplastique des materiaux metalliques sous chargement purement inertiel. Sous chargements mecaniques extremes (e.g., crash, impact ou explosions), leur comportement mecanique presente en effet pour nombre d’entre eux une sensibilite a la vitesse de deformation. Des approches dites statiquement determinees sont majoritairement utilisees pour caracteriser leur comportement, mais elles requierent de nombreux essais dont les conditions experimentales sont souvent contraintes comme par exemple l’homogeneite de la vitesse de deformation qui doit etre maintenue constante en temps par exemple. En revanche, des approches dites statiquement indeterminees permettent l’exploitation d’essais mecaniques avec peu d’hypotheses (voire sans) sur les conditions d’essai. Une methodologie fondee sur un essai d’impact purement intertiel est mise en oeuvre ici pour identifier le comportement viscoplastique de ces materiaux. Avec la Methode des Champs Virtuels, la methodologie permet l’identification des parametres materiaux en exploitant uniquement la mesure des champs de deformation et d’acceleration, potentiellement heterogenes en temps et en espace. Ainsi, celui-ci peut etre caracterise sur une large gamme de deformations et de vitesses de deformation plastiques en procedant a un nombre limite d’experiences. La methode repose sur le developpement d’un simulateur d’images avance permettant de definir au prealable l’ensemble du dispositif experimental (geometrie de l’eprouvette et conditions experimentales). Optimisees numeriquement pour prescrire les parametres d’essai critiques, les realisations experimentales menees sur un alliage de Titane utilise dans l’industrie aeronautique ont permis d’identifier les parametres d’un modele de Johnson-Cook sur un spectre de deformations et de vitesses de deformation plastiques pre-determine. Les incertitudes de la mesure sont egalement integrees et analysees dans ce travail.

Rate-dependent behaviour characterization of metals at high strain rate remains challenging mainly because of the strong hypotheses when tests are processed with statically determinate approaches. As a non-standard methodology, Image-Based Inertial Impact (IBII) test has been proposed to take advantage of the dynamic Virtual Fields Method (VFM) which enables the identification of constitutive parameters with strain and acceleration fields. However, most of the test parameters (e.g. projectile velocity, specimen geometry) are not constrained. Therefore, an FE-based approach is addressed to optimize the identification over a wide range of strain and strain-rate, according to two design criteria: (1) the characterized viscoplastic spectra, (2) the identifiability of the parameters. Whereas the first criterion is assessed by processing the FEA simulations, the second is rated extracting material parameters using synthetic images to input the VFM. Finally, uncertainties regarding the identification of material constants are quantified for each IBII test configuration and different camera performances.

The present work aims at identifying an elastic-viscoplastic material constitutive model over a wide strain and strain-rate range (up to 0.1 and 1000 s−1 respectively), using the so-called Virtual Fields Method. To define the experimental campaign, a design process has been set. It relies on the numerical optimization of the setup – notably the specimen shape, the impact conditions and the measurement resolution (time and space) – with respects to user-defined criteria. Finally, the selected configuration ensures an accurate and robust identification.

The present work aims at identifying an elastic-viscoplastic material behaviour over a wide strain and strain-rate range (up to 0.1 and 1000 s(-1) respectively), using the so-called Virtual Fields Method. To define the experimental campaign, a design process has been set. This relies on the numerical optimization of the setup - notably the specimen shape ? with respects to user-defined criteria. Finally, the selected configuration ensures an accurate and robust identification of material parameters.