Crystal Plasticity Analysis on the Fundamental Processes of Strain Localization and Damage Formation in Pure Iron Polycrystals under Cyclic Fatigue

By using finite element method for crystal plasticity, we investigated the accumulation behavior of dislocation and atomic vacancies introduced by non-uniform deformation in pure iron polycrystals. Dislocation density was calculated from the increment of plastic shear strain and spatial gradient of the slip systems for SS and GN dislocation densities. Vacancy density was calculated from the edge component of SS dislocation density and the incremental plastic shear strain by expanding the theory of Essmann and Mughrabi, in which atomic vacancies are released by the annihilation of edge dislocations, for each slip system. The cyclic loading analysis was performed under strain-controlled with 10 cycles between a tensile process up to 0.5% nominal strain and a compressive process down to 0%. For comparison, a monotonic loading analysis was also performed. The macroscopic mechanical responses were significantly different under the two conditions, and the work hardening rate under cyclic loading was less than half that under monotonic loading. The localization of plastic strain was more pronounced in the cyclic loading deformation than in the monotonic one. The low work hardening rate for cyclic loading deformation was attributed to the low accumulation of GN dislocations due to the relaxation of the plastic shear strain gradient caused by the load reversal. The average vacancy density was twice higher for monotonic loading deformation than for cyclic loading deformation. On the other hand, the maximum value of vacancy density was almost the same in both conditions, indicating that the cyclic loading deformation was more localized.