In this paper, penetration strengths of the natural and syntactic rubber sheets with thicknesses of 3.0 and 2.3 mm subjected to transverse pressure were determined when the small impactor having a weight of 24.8 g collided with the sheets by using the measurement system of impact load and sheet deflection based on electromagnetic induction phenomena. The rubber sheet set at the end of the pressure chamber was deformed initially by vacuuming or pressurizing the inside of the chamber before the penetration tests of the sheets. The histories of the impact force and sheet deflection were analyzed from the electromotive force generated in five coils near the deformed sheet when the impactor containing a neodymium magnet collided with the sheets according to Faraday’s law of induction. Every sheet was penetrated when the impactor collided at approximate 60 m/s with the sheet under the initial transverse pressure of 40 or −40 kPa. Determining the absolute deflections from the position before applying the transverse pressure, the impact load–absolute deflection curves of the impact tests under the different transverse pressure conditions were mutually compared to evaluate the penetration strengths of the sheets. As a result, the critical condition for penetrating the rubber sheets was clarified not to be the maximum impact load but the absolute deflections at the maximum impact loads independently of the transverse pressure.
Glass plates are used extensively as windows in structures, cars, and airplanes. However, because it is a brittle material with low practical strength, improving its resistance to puncture impact loading is important for safety and security. Although it is known that fitting shatterproof window film to the glass plate is an easy and effective way to reinforce it against impact loads, the effect of the film fitting has not enough been evaluated quantitatively. In this study, a puncture impact test was conducted with a glass plate fitted with a shatterproof window film. When the general puncture impact testing method is applied to a glass plate, the measured load profile would be intensely oscillated by resonance. Therefore, in this study, the modified testing method utilizing the steel incident bar was adopted. By dropping the steel weight, impact loads were applied to the glass plate with the film through the steel incident bar. The applied impact load was estimated from the strain gauge outputs at the incident bar. The displacement of the incident bar was measured using the high-speed video camera. The crack propagation in the thickness direction of the specimen was observed using the shadowgraph method with another high-speed video camera. Then the fracture mode of the glass plate and failure mode of the film were clarified. With the proposed test method in this study, load-displacement curves without oscillations because of resonance were obtained. It was proven that by the method used in this study, it is possible to quantitatively evaluate the effect of film-fitting on the glass plate. The numerical analysis was conducted to verify the obtained load and displacement in the experiment. It was revealed that further improvement is needed in the measuring method for the load and displacement of the incident bar.
Mode I crack resistance curves of CFRP (Carbon Fiber Reinforced Plastics) laminates were investigated using the W-DCB (Wedge-loaded Double Cantilever Beams) specimen and SHPB (Split Hopkinson Pressure Bars) system under impact loading, where the SWC (Stress Wave Control) technique was applied to reduce the effects of inertia force induced by the impact loading in the specimen. LSB (Local Strain Based) formula, where the dynamic energy release rate could be theoretically calculated from the surface strain of the specimen, was proposed to evaluate the mode I fracture toughness under impact loading. SST (Surface Strain Transition) analysis, where the dynamic crack growth behavior could be was discretely determined from the strain distribution of the specimen, was proposed to evaluate the mode I crack extension under impact loading. Finite element analyses considering the dynamic loading and crack extension demonstrated the validity of the evaluation method of crack resistance curves based on the LSB formula and SST analysis. Rate dependence of the mode I crack resistance curves of unidirectional CFRP laminates (T700S/2500, Toray) was experimentally investigated using the proposed method, where the mode I fracture toughness was not sensitive to the loading rate and crack extension.
Magnetorheological (MR) fluids are categorized as smart fluids, which are made of small iron particles suspended in carrier fluids such as silicone oil. The presence of a magnetic field will instantaneously increase the viscosity of the MR fluid, which is known as the MR effect. To expand the practical application of the MR fluid, it is very useful to study the pressure wave propagation under high-loading conditions in detail. Therefore, in this study, the behavior of the pressure wave propagating in MR fluid that was generated by the collision between an ultra-high speed projectile and MR fluid was investigated. The impact test was carried out using a vertical gas gun. Three underwater impact sensors were used to measure the pressure wave, and the history of pressure at each point was acquired. From the experimental result, it was found that the compressibility of MR fluid affected the magnitude of peak pressure generated by the entry of an ultra-high speed projectile into the MR fluid. Furthermore, it was confirmed that the yield shear stress and the increase in shear stress due to the magnetic field greatly affect the damping of the impact-induced pressure.
“Magnet-Coil” method using electromagnetic induction was developed to determine the position of a moving object during penetration into sand. The electromotive force generated in a coil was formulated when a magnet in the object passed linearly thought the coil with the three conditions such as “tilt”, “orbit eccentricity” and “tilt and orbit eccentricity” to measure the position and attitude of the object. The experiments were conducted to validate the theoretical formulas. The results of the theoretical formulas under the conditions agreed well with the experimental results. The tilt and orbit eccentricity of the magnet were found to affect independently the electromotive force generated in the coil. The distance between the positions when the electromotive forces became peak and zero was clarified to be dependent on only the orbit eccentricity, independently of the tilt and velocity of the magnet. Therefore, the eccentric position of the magnet can be identified from the distance even if the magnet passes through the coil with the tilt.
In recent years, casualties due to the impacts of falling lapilli and ballistic ejecta resulting from volcanic eruptions with unclear precursors have increased. However, while research and development efforts aimed at protecting people from such scattered ejecta via improved volcanic shelters are progressing, there has been little research on volcanic disaster prevention measures aimed at directly protecting the human body. Accordingly, this study reports on the development of an impact test apparatus and a human head model that can be used to evaluate the impacts of volcanic lapilli as well as the results of our impact resistance evaluation of a commercially available mountaineering helmet (ravina FLUQUE made by Tanizawa Seisakusho, Ltd.). In our experiments, vitrified grindstone with a standard density of approximately 2400 kg/m3 were used as simulated lapilli and launched at the helmet at velocities of approximately 40-80 m/s. Impact safety was evaluated from the damage sustained by the helmet and the head injury criterion (HIC) value, which was calculated via a triaxial accelerometer installed on the head model. Experimental results show that the damage to mountaineering helmet used in this study was small and that the HIC value was low even when it was impacted by lapilli equivalents with diameters up to 30 mm travelling at velocities of up to approximately 60 m/s, thereby suggesting that the mountaineering helmet provided sufficient protection up to that impact velocity. Additionally, summarizing the relationship between the interior/exterior damage of the helmet and the HIC value, it was found that the HIC values can vary significantly due to deformation and fracture of the protective foam material mounted inside the helmet.
In order to improve both the crash safety and fuel efficiency of transport machinery, thin-walled structures of various shapes have been utilized as energy absorbers based on the progressive buckling mechanism. The honeycomb structure is used as a structural member of aircraft due to its high specific strength while it also shows excellent performance as an energy absorbing member. In this study, the dynamic crushing behavior of metal honeycombs due to oblique impact loading was studied with laying emphasis on the effects of the crush angle and the direction of impact loading on their energy absorption characteristics. Finite element models of some metal honeycombs were made by considering the strain rate dependence of material, the plastic deformation and fracture of adhesive layer and the initial imperfection. Some calculated results were compared with the corresponding experimental results, and the validly of those numerical models was verified. Then, the effect of crush angle was discussed through the parametric study with varying the crush angle and the direction of impact loading. The oblique impact loading causes the transition from axial collapse to bending one, so the mean buckling load at the crush angle of 45° was 30–50 % smaller than that for the vertical impact loading, while the structure in the direction parallel to the adhesive layer is slightly stronger.
Rotating bending fatigue test was performed to confirm the risk of fatigue failure due to stealth defects which are not visible on X-ray computed tomography. The fatigue test was performed on aluminum die-cast alloy ADC12 specimen that had been previously observed for defects inside specimen by the X-ray CT. After the fatigue fracture, the fracture surface was observed with a scanning electron microscope to identify the defect that became the fracture origin. When the stress intensity factor of the defect that became the fracture origin was predicted from the defect shape of the X-ray CT observation results, all the defects satisfied the crack initiation condition. When a large number of defects exist inside the material, it was found that it is difficult to identify the defect that is the starting point of fracture from CT observation results. On the other hand, it was confirmed that stealth defects cause fatigue failure. In this paper, the risk to the observation result of defects by X-ray CT was mentioned.
It is expected that tissue regeneration can be promoted by using a scaffold, in which a nanofiber layer with good cell adhesion and a microfiber layer that enables three-dimensional cell migration and proliferation are laminated alternately. In this study, conditions was researched to fabricate a nanofiber layer with ϕ400-500 nm and a microfiber layer with around ϕ10 µm by electrospinning, and then the nano/micro fiber laminated nonwoven fabric was developed using a conductive spray that enables the production of thick nonwoven fabric. The fiber diameter tended to increase with the increase of polymer concentration and flow rate, and tended to decrease with the increase of the hexafluoroisopropanol fraction which has a higher dielectric constant than dioxolane. The fabricated nano/micro fiber laminated nonwoven fabric had a thickness of 629 µm. The nano/micro fiber laminated nonwoven fabric with suitable fiber distance and porosity to enhance cell migration and proliferation can be fabricated by the electrospinning method with a conductive spray.
Thermal barrier coating (TBC) system consists of top coat (TC), bond coat, and substrate. High temperature nonlinear stress-strain constitutive equation of TC is one of the most important mechanical properties for stress analysis of TBC system. Using a coating system specimen, the substrate prevents the specimen from rapid fracture, hence constitutive equation up to high strain is obtainable. In this study, new evaluation method for high temperature nonlinear constitutive equation of TC using the thermal stress of TC in TBC system was proposed. Changing the substrate thickness, several thermal stresses of TC under constant temperature occurred, so that we were able to obtain the stress dependent Young’s modulus which gave the nonlinear constitutive equation under constant temperature. Simultaneously, the initial residual stress at room temperature (RT) was obtainable by the proposed method. Flexural resonance method was adopted for the evaluation of Young’s modulus. Experimental data showed significant thermal stress dependent Young’s modulus for both as-sprayed TC and heat treated TC. The residual stress at RT and high temperature constitutive equation were successfully obtained by the proposed method. It was confirmed that the obtained nonlinear constitutive equation was reasonable based on the deformation mechanism of porous thermal sprayed ceramics coating.