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.