Papers by Author: Hidetoshi Kobayashi

Abstract: We have developed a new testing device which is capable of detecting hydrogen gas release during slow strain rate tensile testing (SSRT) under ordinary pressure. The device is composed of an SSRT machine equipped with a closed chamber with an inspection window that is connected to gas chromatography with a semiconductor hydrogen sensor. Local strain distribution in the specimen during the SSRT is monitored dynamically with a digital image correlation (DIC) method. Hydrogen was pre-charged to aluminum alloys by means of friction in water process. Using the device, it was shown that hydrogen was released particularly in the stage of plastic deformation and fracture. In addition, the hydrogen gas release at the moment of fracture was clearly increased when the alloys were hydrogen-charged and tested at a slow strain rate. When we calculated hydrogen gas release from the fracture surface in Al-Zn-Mg base alloys tested at 3.3×10^-6 s^-1, the hydrogen amount was estimated to be 6.24×10^-10 mol /mm² in a hydrogen-uncharged alloy, and 1.30×10^-9 mol / mm² in a hydrogen-charged alloy.

Abstract: In order to clarify the relationship between the mechanical properties of synthetic quartz and the electromagnetic phenomena during its fracture, a series of uniaxial compression tests were carried out at quasi-static and dynamic rates. Not only the stress-strain curves but also the output of ferrite-core antenna located close to the specimens were measured in a shield box made of permalloy plates. Since the synthetic quartz has three characteristic axes, i.e. optical axis, electric axis and machine axis, the effect of loading direction on the mechanical properties and electromagnetic phenomena of quarts was also examined. The dynamic compressive strength was greater than those in static tests and there is strain-rate dependence in their strength of synthetic quartz. It was also found that there are not any remarkable differences due to the loading direction with respect to the intensity of electromagnetic waves measured in the dynamic compression tests, i.e. the electromagnetic phenomenon does not depend on the loading direction.

Abstract: Hexagonal diamond is considered to be a metastable high-pressure phase of carbon. In previous ab-initio studies, elastic constants of hexagonal diamond were suggested to be higher than those of cubic diamond, which is the stiffest known material. However, the elastic constants of hexagonal diamond have not been investigated experimentally because the size of hexagonal diamond single crystal ever synthesized is not more than 0.5 μm. In this study, we synthesized hexagonal diamond with the size of more than 50 μm in order to measure the elastic constants accurately. Kish graphite powder was used as a starting material and a green compact of kish graphite and copper powder was fabricated as a target. The target was placed into a container made of stainless steel and shock-compressed by a copper projectile accelerated using a single stage powder gun with the velocity of approximately 800 m/s. The estimated shock pressure and temperature were 22 GPa and 1200 °C, respectively. After the shock compression, copper component of the target was dissolved in nitric acid for 24 hours and only carbon component was recovered. A clear (100) peak of hexagonal diamond was observed by a XRD result of the recovered carbon. No peak of cubic diamond was observed. Opaque particles including hexagonal diamond with the size of approximately 100 μm were separated from the graphite and SiO₂ contaminations using heavy-liquid separation method with sodium polytungstate aqueous solution.

Abstract: Dynamic elastic Finite Element Method (FEM) and Discrete element method (DEM) simulations are carried out to investigate dynamic penetration of a projectile into a target of granular medium. It was found that the highly densified region of granular medium was generated just ahead of the projectile and began to propagate spherically with much higher velocity than that of projectile which leaves relatively rarefied medium region. This propagation phenomenon was probably the result of a collision and momentum transfer between particles in target granular medium. The propagation velocity of the densified region decreased during penetration as depending not only on the packing ratio of target medium but also on the projectile velocity. The resistance force of projectile was also investigated in the case of penetration of projectiles with various body lengths. The resistance force increased rapidly and reached to the peak. The peak value was expressed in terms of momentum change of target particles. The resistance force decreased periodically after the peak value. The period clearly depended on the length of projectile. It is obvious that this was caused by the stress wave reverberations in the projectiles with various body lengths.

Abstract: The effect of impact compression on age hardening behavior was examined for Meso20 and 6061 aluminum alloys using a single stage gun. The hardness of Meso20 and 6061 aluminum alloy applied with an impact compression (about 5.0GPa) after the solution treatment increased with the aging time. The cluster of point defects like stacking fault tetrahedral (SFT) was observed in the 6061 aluminum alloys with the impact compression (5.3GPa) after the solution treatment. Even after the impact compression, distribution of the aging precipitates was clearly identified.

Abstract: Dynamics of projectile penetration into sand depends greatly on the features of motion and state of the sand material at the interface with the projectile. The goal of this study is to clarify the behavior of projectile during penetration under the impulse loading induced by the plate impact using a vertical powder gun. In this paper, we constructed an accurate and reliable technique for measuring projectile penetration velocity into sand. “Magnet-Coil” gage method was suggested on the basis of electromagnetic induction phenomena due to movement of the projectile having magnet. We confirmed that it has sufficient measurement accuracy.

Abstract: In order to clarify the effect of strain rate and test temperature on the compressive strength and energy absorption of polyimide foam, a series of compression tests for the polyimide foam with two different densities were carried out. By using three testing devices, i.e. universal testing machine, dropping weight machine and sprit Hopkinson pressure bar apparatus, we performed a series of compression tests at various strain rates (10^-3~10³ s^-1) and at several test temperatures in the range of room temperature to 280 ̊C. At over 100 s-1, the remarkable increase of flow stress was observed. The negative temperature dependence of strength was also observed.

Abstract: The compressive properties of foamed polyethylene (PE) film with a closed cell for electronic devices have been investigated. A commercial closed cell foamed PE film with a density of 330 kg/m³ was used. Quasi-static testing was carried out at strain rates of 10⁻³ to 10⁻¹ s⁻¹. The strain rate of the impact test was approximately 10⁵ s⁻¹ by means of split Hopkinson pressure bar method. Within the set of experiments, the compressive stress increased with the strain rate in both the quasi-static and impact test. In particular, the flow stress increased substantially with the increasing strain rate in the impact deformation. At strains of less than 0.4, the trapped air was locally compressed within the cells, which led to the strain rate dependency of strength in the quasi-static test and the impact test.

Abstract: Effects of high-speed deformation on age hardening and microstructural evolution behavior of 6061 aluminum alloys were studied. By affecting the high-speed impact compression (about 5 GPa) to the 6061 aluminum alloy plate in the state of quenching after the solution heat treatment, the maximum hardness became twice as high as the original hardness. Even after the impact compression, age-hardening was clearly identified both at 175 °C and 100 °C. TEM observation revealed that point defect clusters were distributed densely inside grains after the impact compression, possibly due to the effect of high-speed deformation. The point defect clusters observed were assumed to be stacking fault tetrahedra on the basis of high resolution TEM analysis. The point defect clusters and precipitates were both visible even after the peak-aged condition at 175 °C. The 6061 aluminum alloy specimen after the solution heat treatment, followed by the impact compression (8.0 GPa) and the peak-aged condition showed the highest hardness value (154 Hv) among the testing conditions selected in the present study.

Abstract: We have developed a new testing device, which is capable of detecting hydrogen gas evolution from the microstructural changes at the same timing. The device is composed of the tensile testing machine equipped with a high-speed microscope and two types of quadrupole mass spectrometers installed in the ultrahigh vacuum chamber. Sampling rate of microscopic observation is 2000 fps. Hydrogen or deuterium was pre-charged to the 7075 aluminum alloy by means of the slow strain rate deformation, together with the exposure under the humid air atmosphere. The hydrogen amount was measured by using a thermal desorption analysis in advance. As a result, it was revealed that hydrogen gas was evolved when the surface crack was generated around the notch root of the test specimen. SEM observation also showed that the initial crack is related to the propagation of grain boundary fracture around the notch root. When compared to the microstructure and the hydrogen gas evolution near the notch root, the hydrogen amount evolved at the grain boundary was estimated to be about 3.0×10^-7 mol/m².