In the traffic machines such as the automobile, many thin wall closed section members made of steel are used for the improvement in the collision safety. In these members, the local buckling under compressive stress is mentioned as a main problem. In order to improve the mechanical performance of the member, the technique which fills the inside of closed section with the low-density foaming material is used. In this study, axial compression test and 3-point bending test at the various deformation velocities from elastic region to plastic collapse were carried out on the composite material which filled the hat type thin-walled cross section member with 2 kinds of epoxy resin foaming material in which the foaming rate (density) is different. Characteristics of the composite member were estimated from superposition using the test results of thin-walled and foaming material member, and the stress contribution ratio of the constructional member. It is possible to estimate the maximum load of composite member in this method.
In this research, dynamic buckling behavior of plate- and column- type long aluminum rods whose ends were tightly clamped and middle portions were free or supported was investigated experimentally in the transition velocity range : V = 0.001 to 1m/s, where the loading condition was changing from static to dynamic one. In the experiments, a hydraulic loading machine and a free fall drop-weight type impact testing machine were employed. Dynamic buckling loads at various velocities were measured by the load cell set on the push rod of the hydraulic loading machine or the load sensing block set just below the lower end of the specimen. On the other hand, displacements were measured by a high speed magnetic-resistance device. From experimental results, it was found that the dynamic- to static- buckling load ratio α were successfully described by a power low of slenderness ratio λ and the effective velocity Ve (=V- V0), where V0 is the lower critical or threshold velocity at which the effect of the axial and/or transverse inertia on the dynamic buckling load disappear and the dynamic buckling load becomes equal to the static buckling load. Furthermore, it was also shown that ζ, a ratio of the dynamic buckling load in the intermediately supported condition to the dynamic buckling load in the intermediately free condition, decreases from 4, that was the theoretical value of Euler's buckling equation, to 1 with the increasing slenderness ratio and effective velocity, and this ratio ζ was found to be well expressed by the exponential of slenderness ratio λ and the effective velocity Ve.
After accelerated weathering, dynamic and static compressive properties of polypropylene and Degra-Novon blends were measured using a split Hopkinson pressure bar and universal testing machine. Degra-Novon is an additive mixed with traditional plastics that promotes the spontaneous decomposition of the plastics by sunlight, heat, and microorganisms in soil and water. The use of Degra-Novon achieves a low-cost, biodegradable polymer. The change in yield stress and Young's modulus were examined in detail. The relationship between the compressive properties, differential scanning calorimeter results, and Fourier transform infrared spectroscopy data was discussed.
In previous study, the authors reported the experiment for dynamic crack branch which arises under quasi static load. From the experimental results, the dynamic crack branch condition (Φtotal criticality theory) was derived. The Φtotal criticality theory means that crack branch arises when the energy flux to crack tip per unit time reaches a steady value of material. Furthermore, the numerical simulation using the moving finite element method was performed, and it has succeeded in mechanical prediction of the crack propagation path after crack branching. This crack branching is observed under quasi static load, and the validity of the Φtotal criticality theory under impact load is not discussed until now. In this study, the crack branching phenomenon under impact load was photographed using ultra high speed camera, and the moving finite element analysis is based on the experimental result. The Delaunay automatic triangulation is used for updated mesh subdivision with crack propagation. In numerical simulation, dynamic J integral is evaluated from path integration around crack tip. Then, Φtotal history with crack propagation is discussed by comparison of experimental results and numerical results. The result has confirmed that Φtotal criticality theory is also recognized in dynamic crack branch under impact load, and that Φtotal critical value of PMMA is about 11 MN/s.
In order to characterize the dynamic fracture of Al plate caused by impact with a hypervelocity spherical Al projectile, experiments were performed using a mini two-stage light gas gun. Spherical Al-alloy projectiles with a diameter of 2.1 mm were accelerated to 3.5 km/s and allowed to impact with Al targets under normal impact conditions. The dynamic deformation and fragmentation of the target were observed using a high-speed digital framing camera. The digital images revealed local displacement (bulging) of the target surface, and fragmentation resulting from the large displacement was also observed. Additionally, it was found that the motion of the target fragments was strongly correlated with the bulge growth process. The sequence of dynamic processes from bulging to fragmentation is discussed.
TiNi shape memory alloys (SMAs) have received much attention owing to their unique shape memory properties. In this study, the size effects of hydrogen absorption on the shape memory behavior and strength of TiNi alloy were investigated using thin wires with diameters of 1.0mm and 0.1mm. A brittle layer was formed on the surface of the wires owing to hydrogen absorption. The thickness of the brittle layer increased with increasing hydrogen charging time and was found to be independent of wire diameter. The shape recovery rate and tensile strength after the hydrogen charging decreased with increasing hydrogen charging time, and the rate of decrease was larger for a wire with smaller diameter. In the tensile test, a periodic array of circumferential cracks was observed in the brittle surface layer before the tensile fracture. Thus, the fracture condition of thin hydrogen-degraded wires of TiNi alloy could be successfully predicted by combining the fracture mechanics and net stress criteria.
In this study, we aim to clarify the strengthening mechanism of age-hardenable Ni-P alloy where very fine Ni3P precipitates have been believed to harden the nanocrystalline structures by the so-called precipitation hardening. Specimens were synthesized by electrodeposition followed by aging treatment at temperatures ranging from 200 to 600°C. As-deposited structures were amorphous and became crystallized into nanocrystals by the subsequent aging treatment accompanying Ni3P precipitation. Microstructures, and distributions of grain size of Ni matrix and Ni3P particles size were examined in detail by field-emission type transmission electron microscopy. After the aging treatment under 300°C, average grain size of Ni matrix was smaller than 20nm with finer Ni3P precipitates inside the Ni grains. On the other hand, after aging at 350°C and higher temperatures, both the grain size of Ni matrix and Ni3P particle size were grown to be comparable. Maximum hardness was obtained after the aging treatment at 350°C, and it was harder than that predicted by rule-of-mixture between pure Ni and Ni3P. Therefore, the additional strengthening may operate, which is not associated with the precipitate hardening. Less hardening by the precipitate in nanocrystalline structures is associated with a different deformation mechanism such as grain boundary sliding, which may operate in nanocrystalline regime.