Keiko Watanabe

Impact compressive and bending behaviour of rocks accompanied by electromagnetic phenomena

It is well known that electromagnetic phenomena are often observed preceding earthquakes. However, the mechanism by which these electromagnetic waves are generated during the fracture and deformation of rocks has not been fully identified. Therefore, in order to examine the relationship between the electromagnetic phenomena and the mechanical properties of rocks, uniaxial compression and three-point bending tests for two kinds of rocks with different quartz content, granite and gabbro, have been carried out at quasi-static and dynamic rates. Especially, in the bending tests, pre-cracked specimens of granite were also tested. Using a split Hopkinson pressure bar and a ferrite-core antenna in close proximity to the specimens, both the stress–strain (load–displacement) curve and simultaneous electromagnetic wave magnitude were measured. It was found that the dynamic compressive and bending strengths and the stress increase slope of both rocks were higher than those observed in static tests; therefore, there is a strain-rate dependence in their strength and stress increase rate. It was found from the tests using the pre-cracked bending specimens that the intensity of electromagnetic waves measured during crack extension increased almost proportionally to the increase of the maximum stress intensity factor of specimens. This tendency was observed in both the dynamic and quasi-static three-point bending tests for granite.

Measuring Behavior of Impactor Penetrating through Polymer Sheet Based on Electromagnetic Induction

In the paper, the behavior of an impactor penetrating through a polymer sheet was measured using electromagnetic induction phenomena. First, electromotive forces generated in a coil were measured to decide the relation between the impactor velocity and the electromotive force when the impactor with an embedded neodymium magnet passed through a coil at several constant velocities. The intensity of the electromotive force was found to be proportional to the impactor velocity at each impactor position. The relation was used as the calibration data to calculate the velocity and position of the impactor. Next, penetration tests of polyvinyl chloride sheets were conducted with the coil set at the front of the sheet. The electromotive force generated in the coil was measured when the impactor penetrated through the sheet. The impactor velocity and position were calculated from the electromotive force with the calibration data. The validity of the measuring method was confirmed because the calculated results from the measured electromotive force agreed with the observed results by using a high speed video camera.

Dynamic and Quasi-Static Compressive Deformation Behaviour of Polyimide Foam at Various Elevated Temperature

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.

Penetration Velocity Measurement in Sands Using Magnet-Coil Gages

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.

Plasma measurement by optical visualization and triple probe method under high-speed impact

High-speed impact on spacecraft by space debris poses a threat. When a high-speed projectile collides with target, it is conceivable that the heat created by impact causes severe damage at impact point. Investigation of the temperature is necessary for elucidation of high-speed impact phenomena. However, it is very difficult to measure the temperature with standard methods for two main reasons. One reason is that a thermometer placed on the target is instantaneously destroyed upon impact. The other reason is that there is not enough time resolution to measure the transient temperature changes. In this study, the measurement of plasma induced by high-speed impact was investigated to estimate temperature changes near the impact point. High-speed impact experiments were performed with a vertical gas gun. The projectile speed was approximately 700 m/s, and the target material was A5052. The experimental data to calculate the plasma parameters of electron temperature and electron density were measured by triple probe method. In addition, the diffusion behavior of plasma was observed by optical visualization technique using high-speed camera. The frame rate and the exposure time were 260 kfps and 1.0 μs, respectively. These images are considered to be one proof to show the validity of plasma measurement. The experimental results showed that plasma signals were detected for around 70 μs, and the rising phase of the wave form was in good agreement with timing of optical visualization image when the plasma arrived at the tip of triple probe.

Numerical Simulation of Behavior of Sand Particles during High-Speed Penetration with Particle Method

The phenomena that occur during high-speed penetration of a projectile into sand particles are interesting subjects in engineering. The macro-scale research themes are the behavior of the ejected sand particles and the progress of the high-speed projectile, while the micro-scale research themes are the deformation and fragmentation of a single sand particle. Studies of these unique phenomena were conducted using both experiments and numerical simulation. Although accurate simulation of the behavior of sand particles during high-speed penetration is difficult because sand particles have characteristics of both fluids and solids, the reproducibility of the actual phenomena has improved in recent years with the development of particle methods. In our research, we conducted simulations of the phenomena using Smoothed Particle Hydrodynamics (SPH), which is a mesh-free, particle-based method. The results showed the possibility of accurate reproduction during high-speed projectile penetration into sand particles at the macro-scale.

Analysis of Plasma Created by High-Speed Impact with Triple Probe

The mechanism of destruction of a material by high-speed impact is known to be complex, and it is hard to analyze the inner state of the material during the destruction process. In particular, it is difficult to measure the temperature changes within a material during a high-speed impact. In this study, we propose a new method for estimating this temperature change by measuring the plasma produced at the impact point using a triple probe. A plasma produced by laser ablation was measured to ascertain that the triple probe actually worked. Further, some of the parameters related to the triple probe were varied, and the obtained results were compared, in order to determine the optimal parameters for measuring plasmas. A pulsed Nd-YAG laser with a fundamental wavelength of 1064 nm was employed to produce laser ablation. The laser was irradiated on a thin A2024 plate coated with a black paint. The expanding plasma plume was recorded with a high-speed camera, and the signal from the plasma was measured with the triple probe.

Velocity Control of Diaphragmless Vertical Gas Gun for Low Pressure Ranges

In the field of impact engineering, high-speed impact phenomena are simulated using a projectile accelerator. At the Impact Engineering Laboratory in Ritsumeikan University, a single-stage gas gun was designed to investigate the high-speed penetration phenomena of impacts in sand, which is known to show fluid-like behavior. The gas gun consists of a 2 m launch tube that can achieve projectile muzzle velocities of up to around 500 m/s. The theoretical muzzle velocity of the projectile can be calculated by considering the speed of sound and the specific heat ratio of propellant gases. A performance evaluation for high-pressure ranges of 1 MPa and higher in a high-pressure vessel has been conducted. When fitting parameters are introduced to the theoretical formula, good agreement is obtained with the experimental results. In this study, experiments for low pressure ranges were conducted to predict the projectile velocity and to investigate the minimum velocity limit of the projectile. By introducing fitting parameters to the theoretical formula, the projectile velocity could be predicted accurately for pressure ranges less than 1 MPa. Furthermore, the minimum velocity limit of this equipment was found to be around 30 m/s.

Improvement of Flexural Strength of RC Beams by Using the Multi-Layer Multi-Tensioning Method with CF Sheets

A novel retrofitting method using extremely prestressed carbon fiber sheets, MLML (Multi-Layer Multi-Tensioning) method, was proposed for improving the flexural strength of reinforced concrete structures. The experimental results suggested that the crack initiation strength and the energy absorbing capacity of RC beams could be largely improved by the MLMT method. However, the advantage of the MLMT method was not so clear on the rebars yielding strength and the ultimate flexural strength of RC beams, compared with the conventional methods.