In this paper, the distributions of plastic strain near grain boundaries were investigated by the fiducial grid method for pure Aluminium specimens, which have been previously fatigued for certain cycles. The slip heterogenei ties and grain boundary sliding (GBS) during cyclic deformation, are analysed by micromechanics methods. Crystal is anisotropic, so the elastic stress field is heterogeneous near grain boundary. As a result, the different slip system is active in different area. The heterogeneities of plastic slip make the heterogeneities of stress more strongly and more widely. the stress concentration of triple node of grain boundaries is th strongest. That causes the grain boundary sliding. The test result shows that the homogeneous sliping does not cause the heterogeneous deformation in grains, but the strain field is heterogeneous near grain boundary, so is the GBS, GBS causes the discontinuity of displacement and strain, and a peak value of shear strain along the grain boundary. The affected area is small. GBS is similar to the slip system. It seems that the process of GBS in cyclic loading closely related to the cyclic hardening and softening of the material. It is found that the control factor of GBS is not mainly depend on the Σ value, but on the relative orientation between two grains and macro orientation of grain boundaries. The grain boundary which is along the 45 degree with the load axis slides the most easyly.
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