Role of Fragility of the Glass Formers in the Yielding Transition under Oscillatory Shear
Amorphous materials, especially metallic glasses, are known for their exceptional mechanical properties, such as high yield strength and large yield strain. Understanding the microscopic mechanisms behind their failure, particularly the yielding transition, remains an active area of research. Previous studies have shown that yielding behavior depends on the initial age of the sample. Through extensive computer simulations, we demonstrate that this age dependence varies across different materials and is influenced by the specific characteristics of the initial glass former, particularly its fragility. Both strong and fragile glass formers exhibit similar yielding behaviour in poorly annealed conditions with a critical yield strain, $γ_c$ that does not depend on the initial conditions. However, below a critical degree of annealing, the yield point increases significantly with further annealing for fragile glasses, while it remains relatively constant for strong glasses. The results are found to be universal across a wide variety of model glassy systems with varying fragility, including metallic glasses, molecular glasses, model granular glasses, and network-forming glasses like Silica. We rationalise these findings by introducing a modified mean-field elastoplastic model that explicitly incorporates the crucial role of changing energy barrier with increasing annealing in the yielding process. This simple model reproduces all the simulation results and provides critical insights into how energy barriers influence the physics of the yielding transition including the critical yield strain under oscillatory shear deformation.