Alternating Shear Orientation During Cyclic Loading Facilitates Yielding in Amorphous Materials

Abstract

The influence of alternating shear orientation and strain amplitude of cyclic loading on yielding in amorphous solids is investigated using molecular dynamics simulations. The model glass is represented via a binary mixture that was rapidly cooled well below the glass transition temperature and then subjected to oscillatory shear deformation. It was shown that periodic loading at strain amplitudes above the critical value first induces structural relaxation via irreversible displacements of clusters of atoms during a number of transient cycles, followed by an increase in potential energy due to the formation of a system-spanning shear band. Upon approaching the critical strain amplitude from above, the number of transient cycles required to reach the yielding transition increases. Interestingly, it was found that when the shear orientation is periodically alternated in two or three dimensions, the number of transient cycles is reduced but the critical strain amplitude remains the same as in the case of periodic shear along a single plane. After the yielding transition, the material outside the shear band continues strain-induced relaxation, except when the shear orientation is alternated in three dimensions and the glass is deformed along the shear band with the imposed strain amplitude every third cycle.

Shear band healing in amorphous materials by small-amplitude oscillatory shear deformation

“…2). The strain amplitude γ 0 = 0.080 is slightly larger than the critical strain amplitude γ 0 ≈ 0.067 at T LJ = 1.0 ε/k B and ρ = 1.2 σ − 3 [37], and, therefore, the periodic loading induced the formation of a shear band across the system after about 20 cycles. As shown in Fig.…”

“…2 and 3, the potential energy remains above the energy levels of initially stable glasses (before a shear band is formed) even for loading at the strain amplitude γ 0 = 0.060. The results of a previous MD study on mechanical annealing of rapidly quenched glasses imply that the energy level U ≈ − 8.31 ε can be reached via cyclic loading at T LJ = 0.01 ε/k B but it might take thousands of additional cycles [37].…”

“…2). This observation can be rationalized by realizing that the strain remains localized within the shear band, and the effective strain amplitude within the band is greater than the critical value γ 0 ≈ 0.067 [37].…”

“…Once a shear band was formed at γ 0 = 0.080, the glasses were periodically strained at the strain amplitudes γ 0 = 0.030, 0.040, 0.050, 0.060, and 0.065 during 3000 cycles. It was previously found that in the case of poorly annealed (rapidly cooled) glasses, the critical value of the strain amplitude at the temperature T LJ = 0.01 ε/k B and density ρ = 1.2 σ − 3 is γ 0 ≈ 0.067 [37]. The typical simulation during 3000 cycles takes about 80 days using 40 processors in parallel.…”

“…More recently, it was shown that the critical strain amplitude increases in more stable athermal glasses [35,36], whereas the yielding transition can be significantly delayed in mechanically annealed binary glasses at finite temperature [39]. In general, the formation of a shear band during the yielding transition is accelerated in more rapidly annealed glasses periodically loaded at a higher strain amplitude or when the shear orientation is alternated in two or three spatial dimensions [20,23,28,31,37]. Interestingly, after a shear band is formed during cyclic loading, the glass outside the band remains well annealed, and upon reducing strain amplitude below yield, the initial shear band anneals out, which leads to reversible dynamics in the whole domain [28].…”