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Although highly desirable, the experimental technology of the dynamic mechanical properties of materials under multiaxial impact loading is rarely explored. In this study, a true-biaxial split Hopkinson pressure bar device is developed to achieve the biaxial synchronous impact loading of a specimen. A symmetrical wedge-shaped, dual-wave bar is designed to decompose a single stress wave into two independent and symmetric stress waves that eventually form an orthogonal system and load the specimen synchronously. Furthermore, a combination of ground gaskets and lubricant is employed to eliminate the shear stress wave and separate the coupling of the shear and axial stress waves propagating in bars. Some confirmatory and applied tests are carried out, and the results show not only the feasibility of this modified device but also the dynamic mechanical characteristics of specimens under biaxial impact loading. This novel technique is readily implementable and also has good application potential in material mechanics testing.

During the experimental measurement of solid materials by SHPB, a pulse-shaper technique is used to eliminate high-frequency oscillation components and acquire an incident pulse with a rising time that is greater than the time needed by the test specimen to reach a stress equilibrium state. This paper discusses a modified method of pulse-shaper technique for considering strain rate of pulse-shaper applied in SHPB. We present a numerical process for this method and verify the computational procedure by considering two factors (velocity of the striker bar and density of the material). We also present the results of experimental verification with copper and PC pulse-shapers. The results show that the curves of both fit very well, which could validate the modified method.

As a macroscopically orthotropic material, beech wood has different mechanical properties along the fiber direction and the direction perpendicular to the fiber direction, presenting a complicated strain rate sensitivity under impact or blast loadings. To understand the effect of the strain rate on the mechanical properties of beech wood, dynamic compression tests were conducted for the strain rate range of 800 s&minus, 1&ndash, 2000 s&minus, 1, and quasi-static compression tests for obtaining the static mechanical properties of beech wood were also performed for comparison. The fiber direction effect on the mechanical properties was also analyzed, considering two loading directions: one perpendicular to the beech fiber direction and the other parallel to the beech fiber direction. The results show that beech wood for both loading directions has a significant strain rate sensitivity, and the mechanical properties of beech wood along the fiber direction are superior to those along the direction perpendicular to the fiber direction. An analysis of the macrostructures and microstructures of beech specimens is also presented to illustrate the failure mechanisms. The beech wood, as a natural protective material, has special dynamic mechanical properties in the aspect of transverse isotropy. This research provides a theoretical basis for application in protective structures.

高知工科大学博士(工学) 平成18年9月29日授与 (甲第99号)