Fig. 7 Evolution of dislocation density under different cyclic
loads. (a), (b) and (c) show the evolutions of dislocation density under
d1, d2 and d3, respectively. Shear stress tensor-yz and shear stress
tensor-xz represent the results of loading along the y -axis and
the x -axis, respectively. Average density-yz and average
density-xz represent the average values of dislocation density when
loads are applied along the y -axis and the x -axis,
respectively. (d) Periodic variation of dislocation density. Red dotted
lines indicate the two peaks of dislocation density in each load cycle.
The elasticity of the model promotes dislocation annihilation, while the
plasticity of the model promotes the overgrowth of dislocation.
Therefore, the evolution of dislocation density is another manifestation
of the elastic-plastic properties of the model. In addition to that, the
variation of dislocation density is closely related to the cyclicity of
applied loads. Fig. 7d shows a detailed description of the upper part of
Fig. 7c. The blue curve at the bottom of Fig. 7d shows the variation of
cyclic load with cyclic cycles.
The red dotted lines indicate the
two peaks of dislocation density in each cycle. The variation of
dislocation density is highly periodic and corresponds to the leading
edges of the load peaks in the blue curve below. When the load goes to
zero, the dislocation density appears to be a minimum. And the minimal
values within each cycle are greater than zero, which indicates the
plastic accumulation in the model. Large amounts of line defects and
concomitant point defects in the model will affect the macroscopic
plasticity of the material. When the microscopic defects develop into
macroscopic cracks or holes with the motion of shear slip bands in the
model, the fatigue life of the material will be adversely affected.