Abstract
The mechanical behavior of materials under extreme conditions can be investigated by using laser driven shocks. Actually, femtosecond (fs) technologies allow to reach strong pressures over a very fast duration. This work is dedicated to characterize metals behavior in this ultra-short mode, (aluminum, tantalum), leading to an extreme dynamic solicitation in the target (>10 7 s −1 ). The study includes the validation of experimental results obtained on the LULI 100TW facility by comparison with numerical model. Three modeling steps are considered. First, we characterize the pressure loading resulting from the fs laser- matter interaction, different from what happens in the classical nanosecond regime. Then, the shock wave propagation is observed through the target and particularly its pressure decay, strong in this regime. The elastic-plastic influence on the shock attenuation is discussed, particularly for tantalum which has a high elastic limit. Dynamic damage appears with spallation. Experimentally, spallation is characterized by VISAR measurements and post-test observations. Shots with different thicknesses have been carried out to determine the damage properties in function of strain rate. We show in this work that a simple instantaneous rupture criterion is not sufficient to reproduce the damage induced in the sample. Only the Kanel model, which includes damage kinetics, is able to reproduce experimental data (VISAR measurements, spall thickness). A generalization of this model to any strain rate can be performed by confronting these results to other shock generators data (ns laser driven shocks, plate impacts). One remarkable result is that every Kanel parameters follows a power law with strain rate in dynamic regime (10 5 to 10 8 s −1 ) for both aluminum and tantalum.
Highlights
2 Experimental setupDynamic behaviour and damage investigation in materials present many interests for research applications and some industry fields
The main advantage of such process is the possibility of recovering the shocked sample to analyze the spalled damaged zones, contrary to more conventional methods as plate impacts or explosions
The latest laser technologies evolutions provide an access to shorter regimes in durations, going below the picosecond [4]. This kind of regime is leading to extreme dynamic solicitations in the target, and spallation at a micrometric scale [5]
Summary
Dynamic behaviour and damage investigation in materials present many interests for research applications and some industry fields (aeronautic, automobile, defense. . . ). Previous studies have shown the generation a LULI = Laboratoire d’Utilisation des Laser Intenses – UMR CNRS 7605 (Ecole Polytechnique, France) Website : http//www.luli-polytechnique.fr The imposed gradient of temperature imposed causes phase transitions at high-speed rates, vaporizing and melting the matter which is ejected, leading to an ablation of matter in the micron range [7] This yields to a pressure profile at the origin of the shock wave. One remarkable feature is that a 300fs laser pulse generates a pressure profile whose characteristic duration is more than two hundred times greater at FWMH This dilatation is due to both ion/electron thermal equilibration and the beginning of decay for the wave
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