Thesis Defense – Benedicte ADOGOU – September 19, 2024

10 September 2024
Seminar

Bénédicte ADOGOU will defend her thesis titled « Measurement of Micro-Mechanical Properties of Materials at High Strain Rates by Micropillar Compression Tests » on September 19 at 10 AM in Amphitheater F1, at École des Mines de Saint-Étienne (158 cours Fauriel 42100 Saint-Étienne).

Jury

MERLE Benoît, Professor, University of Kassel, Reviewer
GUIN Jean-Pierre, CNRS Research Director, University of Rennes, Reviewer
AUBIN Véronique,Professor, Centrale Supélec – Université Paris Saclay, Examiner
DEQUIEDT Jean-Lin, HDR Research Engineer, CEA DAM, Examiner
KALACSKA Szilvia, CNRS Research Fellow, École des Mines Saint-Étienne, Examiner
KERMOUCHE Guillaume, Professor, École des Mines de Saint-Étienne, Thesis Supervisor
FIVEL Marc, CNRS Research Director, Université Grenoble-Alpes, Thesis Supervisor
GUILLONNEAU Gaylord, Senior Lecturer HDR, Ecole Centrale de Lyon, Thesis Co-supervisor

Abstract

Understanding the mechanical behavior of materials at high strain rates, not only at the macroscopic but also at the microscopic scale, is essential for several engineering applications. However, micromechanical tests at high strain rates have long been limited due to challenges in developing suitable experimental setups and difficulties encountered during the execution of such tests.

This work was subdivided into two main parts. In the first part, the strain rate sensitivity of copper and iron single crystals was studied following the development of the experimental setup. Micropillar compression tests across several orders of magnitude of strain rate showed that the yield strength of all materials increases with increasing strain rate. Using an analytical model for predicting dislocation velocities, the evolution of yield strength with strain rate was addressed for copper, and the material’s strain rate sensitivity was continuously characterized across the entire explored strain rate range. This work highlights important aspects concerning the influence of the drag phenomenon on the collective mobility of dislocations depending on the scale considered. The crystal orientation-dependent strain rate sensitivity was also studied for copper and iron, considering two different orientations. Furthermore, 3D discrete dislocation dynamics simulation was implemented for copper. The results revealed that, contrary to what is observed in macroscopic samples, at the microscopic scale, the pure drag regime of dislocation mobility occurs earlier, which is explained by the very high velocities that dislocations can reach in microscopic pillars where the mobile dislocation density is often limited.

The main objective of this project was to achieve an advance in the field of high strain rate micromechanics. Thanks to the development of high strain rate micropillar compression tests, an investigation of strain rate sensitivity was carried out on different materials with tests up to 1000 s^{-1}. The investigations revealed that at small scales and high strain rates, the behavior of materials can be very different from what is known at the macroscopic scale.

In the second part of the project, we proposed a new pillar geometry for tests at higher strain rates. Inherited from the macroscopic “Shear compression specimen,” a microshear specimen was developed and validated on silica and amorphous selenium. This specimen was designed to be machinable by FIB (Focused Ion Beam) and easily testable on standard micromechanical devices. Through compression tests and finite element simulations, the final geometry, after improvement, proved to allow for strain rate multiplication, investigation of shear properties, and large deformation testing of materials.