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The yield behavior of amorphous metals including the metallic glass shows intrinsic dependence on the hydrostatic stress, so that yield criterion models such as Mohr-Coulomb and Drucker-Prager are often used. Both the models can explain the asymmetry in the yield stress under uniaxial compression and tension conditions, while the asymmetry in the angle of fracture surface is not able to be determined based on any of those models. The free volume model is able to provide that foundation. Shibutani et al. proposed a new constitutive model for amorphous metals that was derived from some free volume models and the flow rule using the Drucker-Prager yield function as a plastic potential, and investigated the yield behavior and the formation of localized shear band under some temperature conditions using the implicit static FEM code. The formation of shear bands is an unstable phenomenon that is greatly affected by the initial imperfection. In this model, on the other hand, the temperature or the strain rate also affects the yield behavior considerably. In this study, the impact fracture of metallic glass was investigated by implementing the constitutive model proposed by Shibutani et al. into the explicit dynamic FEM code DYNA3D, with laying emphasis on reproducing asymmetry in the angle of fracture surface and the examination of effects of strain rate and temperature change.

The dynamic axially crushing behavior of metal honeycombs was studied with placing emphasis on the effects of boundary condition and cell structure on its characteristics as an energy absorber. Numerical honeycomb models of some metal foil materials were made by taking the plastic deformation of adhesive layer, the failure of adhesively-bonded joint and the initial imperfection into account. Then, firstly, the foil material and the branch angle φ of cell geometry were varied to examine the effects on the crushing behavior and the energy absorption capacity. Secondly, to investigate the effects of boundary condition on the crushing mode and the energy absorption capacity, some boundary conditions such as the fixed end condition, the contact condition at the upper and lower ends and the periodic boundary condition at the side surfaces were applied to the honeycomb model. Typical calculated results under different strain rates and geometric conditions were compared with the corresponding experimental results, and the effects of material properties on the mean buckling stress were discussed. It was found that for the material with strain rate dependence, the stress increment of crushing honeycomb due to increasing the strain rate was 30-40 % as large as that of the material itself. Small φ causes different deformation mode of wrinkles, and the mean buckling stress σm decreased about 50 % by changing φ from 120° to 30°. Furthermore, small-scale honeycomb with small φ is easy to be affected by free edges considerably at φ = 60°. The oblique loading condition caused the transition from axial collapse to bending collapse so that at the crush angle of 45° σm became 30-50 % smaller than that under the vertical loading condition, while the effect of the direction of oblique load on σm is not so large.

The axial crushing behavior of commercial metal honeycombs was studied with laying emphasis on the effects of strain rate and geometry on its characteristics as an energy absorber. To investigate the effect of strain rate on the energy absorption capacity, the honeycombs of some metal foil materials were numerically modeled by taking the plastic deformation and failure of adhesively-bonded joint between corrugated sheets and the initial imperfection into consideration. The relationship between the enhancement of mean buckling stress and the strain rate was discussed. Furthermore, A3003 honeycomb model was examined by changing its branch angle from 30° to 180° because the geometrical dispersion will also affect the energy absorption capacity. Typical calculated results under different strain rate and geometric conditions were compared with the corresponding experimental results. It was found that the effect of strain rate on the stress – strain relation of the honeycomb structure is greatly relaxed as compared with that of the material itself. The effects of the boundary condition on the crushing behavior of irregular honeycombs were also discussed.

The axial crushing behavior of some metal honeycombs, of which materials show different characteristics including the strain rate sensitivity with each other, was studied with varying the branch angle at the node of honeycomb, and the effects of geometry and strain rate on that as an energy absorber were discussed. Honeycomb specimens of different branch angles were made by the corrugation technique, and axial crushing tests were carried out under low-speed and impact loading conditions. Then, the effects of the characteristics of stress – strain relation of the material itself, the branch angle and the strain rate on the stress – strain relation of honeycomb structure and the mean buckling stress were examined. The tendency of deterioration in the plateau load or the mean buckling stress due to the irregularity of branch angle was evaluated with considering the influence of boundary conditions by the aid with the numerical simulation. It was also found that the strain rate dependence of metal honeycomb is greatly relaxed as compared with that of the material itself.

From the viewpoint of improving both the crash safety and the fuel efficiency, various shaped thin-walled tubes have been utilized as energy absorbers of automobiles such as front side members, crash boxes and so forth. In the axial crushing test of the regular polygonal tube, if the number of angles was small enough, it showed a certain inherent wrinkle mode, and the mean buckling load increased with that number, while it showed the mode of cylindrical tube if that number became larger. In the oblique crushing test, the same tendency was also shown within the range that the transition from axial collapse to bending collapse did not occur. This transition considerably decreases the mean buckling load so that it is important to know the threshold crush angle for the transition. Then, the crushing behavior of regular 4-12 angled tubes were investigated with changing the crush angle mainly by the experiment. The threshold angle is sensitively influenced by the initial imperfection and the boundary condition so that both the threshold angles obtained by the experiment and by the calculation for the square tube are 8-13° smaller than that predicted by the equation proposed by Han and Park. For the carbon steel S25C, the increase of the strain rate in the axial collapse mode raises the mean buckling load, while it shows little strain rate dependence once the transition occurs.

Dynamic elastic buckling behaviors of GFRP and Aluminum rods were experimentally investigated. In a gas circuit breaker of an electric power substation, an operating rod consisting of insulating materials is connected to an interrupter. The rod securely insulate between the interrupter and an operating mechanism. The rod is configured with a slender rod made of Glass Fiber Reinforced Plastics (GFRP). When the gas circuit breaker ends the opening operation, impulsive compression load acts on the GFRP rod. To develop a smaller and lighter GFRP rod, dynamic buckling loads of the rod must be studied. In this study, dynamic elastic buckling loads for slender GFRP and aluminum rods were investigated by an experimental method. The drop weight impact tests of the GFRP rods and aluminum rods were employed. In the testing device, a special load cell called the “Load Sensing Block” was used to measure the dynamic load of long time duration. Slender GFRP rods with various lengths were axially loaded at the impact velocities ranging from 0.75m/s to 4.5m/s. From the experimental results, an empirical criterion for the dynamic buckling load of the GFRP rod was proposed in terms of the impact velocity and the slenderness ratio. Furthermore, results showed the proposed criterion could successfully describe the buckling behavior of not only the GFRP rod but also the aluminum rod.

The axial crushing behavior of metal honeycomb was studied with laying emphasis on the effects of adhesively-bonded joint and strain rate dependence on its characteristics as an energy absorber. To introduce the influences of the plastic deformation of adhesive layer and the fracture of adhesively-bonded joint, the embedded-process-zone (EPZ) model was used. First, the axial crushing behavior of metal honeycombs (A3003 aluminum alloy and SUS430 stainless steel) was studied with changing the strain rate by low-speed and drop-weight experiments, and then the numerical model of honeycomb, which included EPZ model and the initial imperfection, was investigated by the explicit FEM code LS-DYNA. The mean buckling load calculated with EPZ model becomes smaller than that calculated with the combined model and agrees well with the experimental result. The effect of the strain rate dependence of material on the structure was also studied by the numerical simulation as well as by the experiment. A3003 honeycomb shows no strain rate dependence on the load –displacement relation while SUS430 honeycomb shows significant increase in the mean buckling load.