Helical Locomotion in Yield Stress Fluids.

We report three stages for locomotion of a helical swimmer in yield stress fluids. In the first stage, the swimmer must overcome the material's yield strain to generate rotational motion. However, exceeding the first threshold is not sufficient for locomotion. Only when the viscous forces are sufficiently strong to plastically deform the material to a finite distance away from the swimmer will net locomotion occur. Once locomotion is underway in the third stage, the yield stress retards swimming at small pitch angles. Conversely, at large pitch angles, yield stress dominates the flow by enhancing swimming speed. Flow visualizations reveal a highly localized flow near the swimmer in yield stress fluids.

The effect of microstructure parameters on the residual stresses in the ultrafine-grained sheets.

In this research, the influence of microstructure parameters on the residual stresses of ultrafine-grained sheets was investigated. For this purpose, the constrained groove pressing (CGP) process was carried out on the copper sheets with 3 mm thickness, and residual stresses of the CGPed sheets was measured using the contour method. Microstructure of the CGPed specimens was evaluated by the optical microscopy, micro x-ray diffraction (micro-XRD), and transmission electron microscopy (TEM) experiments. Microstructure parameters including crystallites size, dislocations density, and lattice strain were calculated using Williamson-Hall and Williamson-Smallman equations, and the calculated results were validated by the TEM images. The influence of these parameters on the residual stresses was investigated by analysis of variance (ANOVA) method, and two approaches were considered in this way. According to the results, the CGP process can create nanostructures in the CGPed sheets, and with increasing number of CGP passes, grains size, crystallites size, lattice strain, and residual stresses decrease, and density of dislocations increases. Microstructure parameters have a significant effect on the macro-residual stresses, and strain is the most effective parameter. Also, in the ultrafine-grained sheets, micro-parameters have an undeniable contribution, which is the same as that of macro-parameters on the macro-residual stresses.