2.3 Loading conditions
Displacement loads were applied to the boundary layers of the atomic
model to form alternating shear stress conditions. Because of the
deformation properties of bearing steels, it is necessary to discuss the
transition between elasticity and plasticity of the atomic model under
shear deformation. Therefore, monotonic displacement loads were
performed to discuss the transition
(shown in Fig. 5), which were useful
for establishing the load levels to be applied under cyclic deformation.
Displacement loads at a constant velocity of 1Å/ps were applied to the
upper and lower boundary layers of the simulation model in opposite
directions (shown in Fig. 5a). The MD timestep was 2 fs, and the
temperature was maintained at 5K. Here d denotes the displacement
difference between the upper and lower boundary layers. γ represents the
shear strain of the model and is calculated by d and\(L_{zz}\). Considering that loading directions will affect the
simulation results, the movement of the boundary layers is set along they -axis and the x -axis, respectively.
Fig. 5b shows the shear stress-d curves under
monotonic loads along two
directions. Note that the shear stress refers to the average values
calculated along the y -axis (\(\tau_{\text{yz}}\)) and thex -axis (\(\tau_{\text{xz}}\)) of the model except for the
boundary layers,35 respectively. The linear shear
stress tensor-yz and linear shear stress tensor-xz shown in Fig. 5
denote the results of loading along the y -axis and thex -axis, respectively. Although the movement of the boundary
layers along the two directions resulted in different stress states for
the model, both cases contain three different stages: elastic,
elastic-plastic and plastic. Note that the initial stresses of the shear
stress-d curves in Fig. 5b are not zero. This is because that the
MD annealing only kept the normal stress tensors in three directions at
zero, but not shear stress tensors. However, due to these initial shear
stresses are far less than the stress of interest, the influence of them
on the subsequent simulation results can be ignored.