Although some details of microstructure in strain rate effect have been observed via experiments, a reasonable theory that explains those experimental findings has not yet been built. To obtain a reasonable explanation of strain rate effects at mesoscopic level, more work is needed. 3.2 Analyses Based on Dislocation Dynamics 3.2.1 Kinematics Relationship of Strain Rate Effect Orowan’s relationship links directly strain rate and dislocation parameters in dislocation theory. Orowan’s equation is kinematic and independent of materials. Straight dislocation models are described by [1]: _εP ¼αbρm v (3.1) where αis an orientation factor, b is the length of the Burgers vector, ρmis the density of movable dislocations and v is the average velocity of dislocations. 3.2.2 Kinetic Relationship of Strain Rate Effect Experiments by Gilmand and Johnston (1957) demonstrated the relationship between dislocation velocity and resolved shear stress can be described by: v ¼Aτm (3.2) where A and mare material constants [13]. According to this relationship, the dislocation velocity is proportional to the resolved shear stress, which in turn is related to the applied stress. Although this relationship is empirical and can be presented in various formulas, it does describe the kinetic relationship of strain rate effect at mesoscopic scale. It indicates that dislocation velocity increases with stress. Fig. 3.1 Strain rate dependent of flow stress 22 K. Qin et al.
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