3.1 Shear stress responses
Cyclic shear stress was calculated to investigate the plastic
accumulation behavior of the simulation model. The cyclic loads as shown
in Fig. 5d were applied to the boundary layers along the y -axis
and the x -axis, respectively. Fig. 6a and d show the curves of
the shear stress as functions of cycle when d1 is applied along they -axis and the x -axis, respectively. In the first three
cycles, the maximum stress amplitudes remain unchanged in both curves,
indicating that the model is in the elastic stage. Then the stress
amplitudes decrease to some extent from the fourth cycle. The decrease
is called cyclic softening,41 which also stands for
the beginning of plastic accumulation. Although the model is in the
elastic stage when the load is d1 (shown in Fig. 5b), dislocations
nucleate and grow on the interface between cementite and bcc-Fe matrix,
thus resulting in plastic accumulation. The details will be discussed in
the following sections. Fig. 6b and e show the curves of the shear
stress as functions of cycle when d2 is applied along
the y -axis and thex -axis, respectively. Cyclic softening occurs after the first
cycle or in the first cycle due to the diverse loading directions. Fig
.6c and f show that when the load rises to d3, shear stress amplitudes
decrease in the first cycle and then reach a steady level. The
conclusion is that larger load leads to earlier cyclic softening. In
addition, the red boxes in Fig .6c
and f show that the shear stress yields obviously when the load is large
enough, and large residual shear stress exists within the model at the
end of 10 cycles. Fig. 6 also shows that the variational tendencies of
shear stress \(\tau_{\text{yz}}\) and \(\tau_{\text{xz}}\) are basically
similar under the same loading amplitude.