Akihiro Hojo

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.

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.

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.

Determination of Dynamic Constitutive Equations Based on an Information Criterion by the Elasto-Plastic Waves Propagation Analysis

Previously, we presented the determination method of a dynamic constitutive equation with strain- rate sensitivity from the results of an uniaxial longitudinal impact compression test. The proposed method could derive an appropriate strain-rate sensitive dynamic constitutive equation as a polynomial without consideration based on underlying material physics. However, the method needs a lot of test results over a wide strain-rate range. This paper presents the determination method of a dynamic constitutive equation from a few impact test results. The proposed method determines unknown coefficients involved in a polynomial that appears dynamic constitutive relations of a material by nonlinear least squares method. Furthermore, the proposed method determines the optimum degree of a polynomial constitutive equation by AIC (Akaike information criterion). This paper, further, presents the split Hopkinson pressure bar method using a stepped striker bar. The proposed method can obtain the relations between the dynamic stresses and strain-rates in a wide range from a single impact test.

A Simple Measurement Method of Dynamic Yield Strength using Strain Data.

Generally, measurements of the stress-strain curve are required in order to determine the yield strength. However, it is difficult to measure dynamic stresses induced in materials due to the complicated propagation of the stress waves. In contrast to this, measuring strain data is relatively easy. When the materials are subjected to impact force, elastic waves propagate first. After the strain becomes larger than yield strain, plastic waves occur. Therefore the strain rate changes discontinuously at this point, although the strain increases smoothly. By measuring the strain waves induced in a test piece rod subjected to longitudinal impact force as a step waveform, the strain rate profiles are obtained based on numerical differentiation. For the previous reason, the dynamic yield point can be estimated by change of the strain rate. The dynamic yield stresses are obtained by multiplying those values by Youngs modulus measured from the static compress or tensile tests. This paper confirmed the possibility of the present method based on numerical simulations and discussed practicality by measuring the dynamic yield strength of materials under various strain rates.