A study of the static yield stress in a binary Lennard-Jones glass

Abstract

The stress-strain relations and the yield behavior of a model glass (a 80:20 binary Lennard-Jones mixture) is studied by means of molecular dynamics simulations. In a previous paper it was shown that, at temperatures below the glass transition temperature, Tg, the model exhibits shear banding under imposed shear. It was also suggested that this behavior is closely related to the existence of a (static) yield stress (under applied stress, the system does not flow until the stress sigma exceeds a threshold value sigmay). A thorough analysis of the static yield stress is presented via simulations under imposed stress. Furthermore, using steady shear simulations, the effect of physical aging, shear rate and temperature on the stress-strain relation is investigated. In particular, we find that the stress at the yield point (the "peak"-value of the stress-strain curve) exhibits a logarithmic dependence both on the imposed shear rate and on the "age" of the system in qualitative agreement with experiments on amorphous polymers, and on metallic glasses. In addition to the very observation of the yield stress which is an important feature seen in experiments on complex systems like pastes, dense colloidal suspensions and foams, further links between our model and soft glassy materials are found. An example is the existence of hysteresis loops in the system response to a varying imposed stress. Finally, we measure the static yield stress for our model and study its dependence on temperature. We find that for temperatures far below the mode coupling critical temperature of the model (Tc = 0.435 in Lennard-Jones units), sigmay decreases slowly upon heating followed by a stronger decrease as Tc is approached. We discuss the reliability of results on the static yield stress and give a criterion for its validity in terms of the time scales relevant to the problem.

Simulations of aging and plastic deformation in polymer glasses

“…For instance, calculations of particle correlation functions have shown explicitly that the characteristic time scale for particle relaxations increases with wait time [19]. Recent work [13,20] has focused on the effect of aging on the mechanical properties; results showed that the shear yield stress (defined as the overshoot or maximum of the stress-strain curve) in deformation at constant strain rate generally increases logarithmically with t w . Based on a large number of simulations at different strain rates and temperatures, a phenomenological rate-state model was developed that describes the combined effect of rate and age on the shear yield stress for many temperatures below the glass transition [21].…”

From equilibrium to steady state: the transient dynamics of colloidal liquids under shear

“…Here, we are concerned with a suddenly commencing shear flow, imposed upon a quiescent, equilibrated system. While the measurement of macroscopic quantities like the shear stress σ is standard rheology procedure, the detailed study of microscopic dynamics has been a recent contribution to the field from both computer simulation [2,3,4,5,6,7,8,9] and direct-imaging techniques for colloidal suspensions, namely confocal microscopy [10].…”

Heterogeneous Shear in Hard Sphere Glasses

“…The rheological response of these systems is essentially determined by the competition between an inherent slow dynamics and the acceleration caused by the external drive [8,9]. This may lead to a spatially and temporally heterogeneous flow if the system is close to the yielding threshold [10][11][12]. …”