Low cycle fatigue(LCF) properties of Al-Zn-Mg-Mn alloy have been examined. There were observed the differences in LCF properties and deformation characteristics according to the applied plastic strain range. It is considered that this differences were also reflected on the break point in Coffin-Manson plot at bout $\triangle \varepsilon_p=3 \times 10^{-3}$ for Al-Zn-Mg alloy used in the present study. In addition, the difference of cyclic hardening exponent between high plastic strain range and low plastic strain range was resulted. These differences in the LCF properties with the plastic strain ranges are summerized as follows.
It is well known that the slip on the planes can be possible whenever the shear stress to the slip direction on the slip plane reaches a critical value. At high plastic strain range, the critical resolved shear stress is simultaneously reached on many slip planes, resulting in multiple slip activation. And most of shearable precipitates can be cut by the slip. These result in the homogeneous distribution of the deformation at high plastic strain range. But, due to the multiple slip, many dislocation interaction and especially, the interactions between dislocation and nonshearable Mn-dispersoids which have been uniformly distributed in the matrix, it is considered that the cyclic hardening exponent at high plastic strain range have a higher value than one at low plastic strain range. On the other hand, at a low plastic strain range, fewer planes are oriented to satisfy the requirements for the slip and fewer ppts can be cut. These effects promote inhomogeneous deformation and result in the local and inhomogeneous distribution of the fatigue damage. This idea is supported by the observations with TEM micrographs at high and low plastic strain ranges. Therefore, it is believed that the difference in cyclic hardening exponent with the plastic strain range is originated from the difference in degree of the deformation homogeneity.
Meanwhile, It can be considered that the break in Coffin-Manson plot is also attributed to the difference in degree of deformation homogeneity. That is, in low plastic strain range, the deformation inhomogeneity developes intense slip band. This results in relatively earlier initiation of fatigue crack in low plastic strain range as compared with high plastic strain range in which deformation is homogeneous. This causes the decrease of fatigue life in low plastic strain range, resulting in the break in Coffin-Manson plot.
There are difference in a feature of tensile peak stress reduction between high plastic strain range and low plastic strain range. This result is related with the difference in degree of deformation homogeneity. Due to the local and inhomogeneous distribution of fatigue damage resulted from the inhomogeneous deformation in low plastic strain range, fatigue crack propagation rate is relatively slow as compared with high plastic strain range which the distribution of fatigue damage is homogeneous. In low plastic strain range, load drop is therefore relative smooth. On the other hand, load drop is abrupt in high plastic strain range because of the homogeneous distribution of accumulated fatigue damage. This opinion is confirmed with the observation on fatigue fractured surface. When applied strain is high, dimple structure, the feature of over load fracture(abrupt load drop), is observed in the most area of fracture surface, when low strain is applied, small step is observed on fracture surface as if fatigue crack propagates along with a characteristic crystallographic plane(relatively smooth load drop).
