Hiroshi Tachiya

DYNAMIC TENSILE AND COMPRESSIVE STRESS-STRAIN CHARACTERISTICS OF MAGNESIUM ALLOYS AT ELEVATED TEMPERATURES

The dynamic stress-strain characteristics of magnesium alloys have not been clarified sufficiently. Thus, the study investigated both the compressive and tensile dynamic stress-strain characteristics of representative magnesium alloys: AZ61A-F, ZK60A-T5 and AZ31B-F at wide strain rate and temperature ranges. About the strain rate dependency, the dynamic stresses are higher than the static ones under both compressive and tensile loads at elevated temperatures; however the dynamic stress-strain relations change slightly in the dynamic strain rate range. Thus, the magnesium alloys has little strain rate dependence. However, the elongation of the dynamic stress-strain relations under tensile load tends to be larger than that of static one. About the temperature dependency, the yield and flow stresses of the investigated magnesium alloys under compressive load decrease abruptly at temperatures higher than about 600 K in the wide strain rate range. Meanwhile, the ones under tensile load decrease with the temperature more gently. Totally, the magnesium alloys exhibit low temperature dependence. Furthermore, as well known, the yield stresses caused under the tensile load exhibit about twice as high as those under compressive load. This study verified that such a characteristic can be observed over a wide strain rate and temperature ranges.

Approximation of Thermal Deformation Behaviour of a Machine Tool to Improve its Process Precision

This paper presents a stochastic method to approximate the thermal deformation of a machine tool, by using the temperature changes of the body. The method examines the relationship between the thermal deformations and temperature changes of plural positions on a machine tool, and determines the appropriate temperature measuring points. In addition, the approximate equation of the thermal deformation is drawn by t-test, which uses those influential temperature changes as parameters. This study shows that the approximation equation of the thermal deformations can be expressed in a simple form with few parameters, and remarkably improve cutting processing.

Improvement of Energy Absorption of Circular Tubes Subjected to High Velocity Impact

The dynamic behavior of circular straight and stepped tubes made of aluminum alloy under high-velocity impacts was investigated by performing finite element analyses (FEA) and an experiment. The FEA and experiment on the straight tubes suggested that while an increase in the impact velocity enhanced the absorbed energy through compressive deformation just after impact, the peak load at the fixed end was not affected by the velocity. A stepped tube that was thicker near the impacted end was designed on the basis of the results for the straight tubes, and its dynamic behavior was investigated through FEA. The stepped tube absorbed a large amount of impact energy through compressive deformation at the thicker portion during the higher-velocity impact, without increasing the maximum fixed-end load from that of the straight tube.

THE STRENGTH OF HARD STEELS AT HIGH STRAIN RATES

ABSTRACT In order to know the dynamic strength of hard steels, a dynamic three-point bending method based on Hopkinson pressure bar technique using long bars was proposed. The stress applied to a specimen was determined by means of the relation between bending moment and strain. The stress-strain relation of hard steels with the Vickers hardness 3~7.9 GPa at various strain rates up to the order of 102 1/s was obtained. As a result, strain rate dependence of the yield strength of hard steels having martensitic structure was found.

1305 Burnishing Process by a Hybrid Mechanism with Force Control

1. 緒言 パラレルメカニズムは,高剛性,高出力,多自由度とい った利点を持つことから,加工機の機構として注目されて いる.同機構において力制御を可能とすれば,調整された 大きな力を多方向へ発生させられることから,その応用範 囲が拡大される.そこで本研究では,先に開発した空間 5 自由度ハイブリッド式パラレルメカニズム型加工機に関 して力制御を可能とし,同加工機をバニシング加工へ応用 する.バニシング加工は金属材料表面の微小な塑性変形を 利用した仕上げ加工である.同加工法は,高速な鏡面仕上 げが可能で,表面改質による金属的性質の改善,加工スラ ッジが発生しないなど多くの利点を有する. パラレルメカニズムを用いれば,多様な曲面を対象とし たバニシング加工が可能となる.しかし,バニシングに関 する多くの研究は,平面など単純な形状を対象としており, パラレルメカニズムを用いた場合に対象となる自由曲面で の加工条件と表面改質の関係に関する検討は十分でない. そこで本研究では,様々な加工条件でバニシング加工を 行い,表面改質の効果を評価し,パラレルメカニズムによ る適切なバニシング加工方法を検討する.

Determination method for power-saved driving motions of manipulators by heuristic algorithms (in case of PTP control)

Abstract This study proposed a method for suppressing torques and consumed energy of manipulators considering an effect of inertial forces in order to achieve power-saved drive. The present paper describes the case that the start and end points of a work path of a manipulator are specified and the path between the points can be determined arbitrarily, which is so-called point-to-point (PTP) control. The proposed method determines both the initial pose of a manipulator and input motions of its joints so that the torque or consumed energy is suppressed. The input motion of the joint is expressed by a motion curve that indicates the time profiles of input displacements, velocities and accelerations. The proposed method determines the motion curve as a polynomial with appropriate coefficients. Those coefficients and their values are decided by a heuristic algorithm in order to suppress the input torques or consumed energy of a manipulator. The paper actually determines the power-saved drive motion of “PUMA560” by the proposed method and reveals its effectiveness.

MEASUREMENT OF PRESSURE RESPONSES IN A PHYSICAL MODEL OF A HUMAN HEAD WITH HIGH SHAPE FIDELITY BASED ON CT/MRI DATA

This study discusses a head injury mechanism in case of a human head subjected to impact, from results of impact experiments by using a physical model of a human head with high-shape fidelity. The physical model was constructed by using rapid prototyping technology from the three-dimensional CAD data, which obtained from CT/MRI images of a subjects head. As results of the experiments, positive pressure responses occurred at the impacted site, whereas negative pressure responses occurred at opposite the impacted site. Moreover, the absolute maximum value of pressure occurring at the frontal region of the intracranial space of the head model resulted in same or higher than that at the occipital site in each case that the impact force was imposed on frontal or occipital region. This result has not been showed in other study using simple shape physical models. And, the result corresponds with clinical evidences that brain contusion mainly occurs at the frontal part in each impact direction. Thus, physical model with accurate skull shape is needed to clarify the mechanism of brain contusion.

Basic Study of Wearable Robot Hand using a Parallel Mechanism to Rehabilitate Fingers

This study discusses a robot hand which can be mounted on the human hand to rehabilitate damaged fingers or to assist finger motion. Ordinary robot hands tend to be heavy and large, because they require to mount an actuator on each joint and to use many actuators. Thus, the robot hands are not appropriate to be mounted on the hand. This study notes that human hand grasping any objects composes parallel mechanism. As the parallel mechanisms can arrange the actuators on the base, their moving parts are simple and lightweight. Thus, we propose the wearable robot hand using parallel mechanism. The chains of the proposed mechanism are fixed to the tips of the fingers and the mechanism uses the grasped object by fingers as the output link. When human hand equipped with the proposed hand grasps an object, the proposed robot hand composes a parallel mechanism. Then, human fingers can be operated in a variety of motions. Furthermore, the restraint of the fingers by the wearable robot hand can be easily taken off by releasing the grasped object. Thus, human hand will be safe with respect to unexpected motion of the wearable robot hand.

Dynamic Constitutive Equation for Carbon Steels with Consideration of Blue Brittleness

Blue brittleness is exhibited by some steels after being heated to an elevated temperature. Although at high temperature a given strain is associated with a lower stress level, at the temperature at which blue brittleness is observed there is a pronounced work-hardening effect. Furthermore, the temperature at which blue brittleness occurs is a function of the strain rate. Therefore, to improve the accuracy of numerical simulations of hot or warm working which utilizes the thermal softening of steels, a dynamic constitutive equation which can model the blue brittleness phenomenon is needed. In this paper we present such a constitutive equation, which has two parts : one models the ordinary constitutive relation and the other the phenomenon of blue brittleness. Unknown material constants in the equation are determined by a non-linear least-squares method from uniaxial impact and static compressive test results. The equation is then evaluated by comparing it to experiments over a wide temperature range.

A Testing Machine for Two Stage Impact Loading and Some Experiments.

A new impact testing machine was proposed, which was possible for various two stage impact loading tests of tension and torsion. Especially, the testing machine can realize the impact loading and inverse loading in order to examine the Bauschinnger effect at high strain rates. This machine is based on SHPB method, and has two clamp devices that are released in the time lag of 100 μs by connecting the two clamps to a rod. Using the testing machine, some efficiency tests were done. The results showed that the rise time of the impact load is about 50 μs and the present machine is valid as an impact testing machine. Moreover, some impact experiments were conducted with short aluminum specimens. It was confirmed that the Bauschinnger effect appeared under the impact loading as same as the static loading.