The dynamic stress-strain characteristics of magnesium alloys have not been clarified sufficiently. Thus, the study investigated both the compressive and tensile dynamic stress-strain characteristics of representative magnesium alloys: AZ61A-F, ZK60A-T5 and AZ31B-F at wide strain rate and temperature ranges. About the strain rate dependency, the dynamic stresses are higher than the static ones under both compressive and tensile loads at elevated temperatures; however the dynamic stress-strain relations change slightly in the dynamic strain rate range. Thus, the magnesium alloys has little strain rate dependence. However, the elongation of the dynamic stress-strain relations under tensile load tends to be larger than that of static one. About the temperature dependency, the yield and flow stresses of the investigated magnesium alloys under compressive load decrease abruptly at temperatures higher than about 600 K in the wide strain rate range. Meanwhile, the ones under tensile load decrease with the temperature more gently. Totally, the magnesium alloys exhibit low temperature dependence. Furthermore, as well known, the yield stresses caused under the tensile load exhibit about twice as high as those under compressive load. This study verified that such a characteristic can be observed over a wide strain rate and temperature ranges.
This paper proposes a novel diamond tip burnishing process to improve the integrity of various free-curved surfaces using a spherical 5-Degree-Of-Freedom (5-DOF), hybrid parallelmechanism. The developed parallel mechanism, which has high rigidity and a large workspace, is composed of a spherical 3-DOF parallel mechanism and anXYstage, and is equipped with a burnishing tool on its output link. Using a threedimensional force control system, the parallel mechanism can adjust the thrust force in the burnishing process. The surface roughness and profile of the stainless steel (AISI 316) workpiece, burnished by the proposed method, were evaluated. The surface integrity depended on the values of cross-feed and thrust force, which were controlled by the hybrid parallel mechanism. In addition, the surface roughness improved as cross-feed decreased and thrust force increased. The preliminary surface roughness ofRa= 2.5 µm was improved toRa= 0.25 µm in the burnishing process of the free curved surface, and homogeneous surface integrity was obtained. The results thus suggest that the proposed burnishing method can achieve a highquality surface finish, even on a free curved surface.
A Three dimensional transparent physical model of a human head with real shape was constructed to visualize and measure relative motion between skull and brain in order to clarify the mechanism of acute subdural hematoma. The model consists of a transparent skull, brain made of silicone gel, cerebrospinal fluid to reconstruct relative motion between brain and skull, and meninges which constraints the motion. The shape was based on real-shaped three dimensional CAD data constructed from CT/MRI images of a specific individual. The results of experiments in the cases of impactor collision to the occipital skull showed that shape of impact force pulse applied to the head was not affected on peak bridging vein stretch ratios because significant relative motion between skull and brain occurred by existence of cerebro spinal fluid.
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