|Ph.D Student||Nieto Fuentes Juan Carlos|
|Subject||A Reassessment of the Thermomechanical Coupling in|
Solids Subjected to Dynamic Loading
|Department||Department of Nanoscience and Nanotechnology||Supervisors||Professor Daniel Rittel|
|Dr. Shmuel Osovski|
|Full Thesis text|
Heat dissipation still remains an unsolved problem in dynamic plasticity, where nearly adiabatic conditions prevail during high-rate loading conditions. It is well known that the mechanical energy that is not dissipated as heat during material straining remains stored in the lattice as microstructural defects, and thus a one-to-one relationship can be expected between the stored energy, the material’s microstructure and its mechanical characteristics. This work demonstrates that this is not so straightforward.
High-rate experiments on a Kolsky bar, combined with in situ non-invasive thermal measurements, were performed on three pure well-studied materials: copper, nickel and aluminum. The macroscopic dynamic thermomechanical material behavior was then brought face to face with the microscopic response. Dislocations, as the main lattice defect present in FCC metal plasticity, were considered on a physically-based constitutive model to predict the mechanical and the thermomechanical material behavior.
The applicability of a widely-used expression to calculate the energy stored in the material after deformation was called into question. This expression, where the strain energy of a plastically deformed material is proportional to the density of dislocations on it, and can be used in parallel with the micromechanical model to estimate the temperature rise during dynamic plastic deformation, was thoroughly verified. The analytical thermomechanical conversion (based on dislocation mechanics) was compared with the experimental one, revealing a significant discrepancy between the two. An empirical ad hoc factor was introduced in the analytical expression in order to describe adequately the thermomechanical response of the material under dynamic (impact) loading conditions.
Moreover, a large experimental campaign on nickel and aluminum, revealed that while the observed thermal response was strongly strain rate sensitive, the mechanical flow, and microstructural characteristics (as characterized by transmission electron microscopy at similar strains), were not. Considering that nowadays experimental techniques are not capable of monitoring in real time microstructural evolutions under dynamic loading, this apparent discrepancy between mechanical and microstructural vs. thermal results is discussed, and the concept of thermomechanical conversion is therefore reassessed.