Résumé
Mechanical response of amorphous solids is a fundamental research topic in glass science. Whereas amorphous solids respond elastically to a small strain (or stress), the system eventually yields at a large strain, showing plastic response. Although amorphous solids such as glasses are ubiquitous and relevant for industrial applications, understanding of mechanical response is still poor.Computer simulations play an important role in this field, since they are able to provide atomic resolution of mechanical response of glasses. However, there are huge gaps in the accessible timescales between ordinary computer simulations and experiments in terms of glass preparation (e.g. cooling rate) and timescale of deformation (e.g. strain rate). Timescale of current computer simulations are about 10 orders of magnitudes smaller, behind as compared to that of experiments, which makes possible studies of poorly annealed glasses with very large strain rate. For the strain rate, one can overcome the timescale gap by using the athermal quasi-static shear (zero strain rate limit) method which neglects thermal fluctuations [1]. However, fast cooling rate of ordinary simulations still prevents us from studying glasses in experimental condition.Recently, we have developed very efficient simulation setup using particle swap Monte-Carlo algorithm with size polydisperse particles [2]. By applying this setup for a realistic glass-forming system with a continuous potential in three dimensions, we produce very well-annealed glasses whose cooling rate is even slower than experiments, that is ultrastable glasses. We perform athermal quasi-static shear deformation on these systems. Aim of this work is to establish the first simulation study of mechanical response of glasses in and beyond experimental timescales in both cooling and strain rates [3].Our athermal quasi-static shear simulation of the ultrastable glasses show qualitatively different behaviors from ordinary computer glasses [3]. 1) Sharp yielding transition: The stress-strain curve of the ultrastable glasses show very big stress overshoot and sharp yielding, which corresponds to shear band spanning entire sample. With the help of the finite size scaling, we unambiguously determine the yielding transition point at the thermodynamic limit. 2) Burst of stress drops before yielding: We observe sudden emergence of burst of stress drops slightly before the yielding transition point, which can be regarded as a precursor of the yielding. 3) Strong persistent initial memory: After the yielding transition, the systems do not converge to a steady state until more than 1000% shear strain, which is in stark contrast to ordinary computer glasses. REFERENCES[1].Maloney, C E. and Lemaitre, A., Amorphous systems in athermal, quasistatic shear, Phys. Rev. E, 74, 016118 (2006).[2].Berthier, L., Coslovich, D., Ninarello, A., and Ozawa, M., Equilibrium Sampling of Hard Spheres up to the Jamming Density and Beyond, Phys. Rev. Lett.. 116, 238002 (2016).[3].Ozawa, M., Ninarello, A., and Berthier, L., in preparation.