Tsutomu Umeda

Numerical Evaluation of Dynamic Behavior of Axial Crushing Honeycomb Based on Adhesively-Bonded Model

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.

Experimental Evaluation of Dynamic Buckling Load of GFRP Rod for Gas Circuit Breaker

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.

Development of 3-Dimensional Compression Device and its Application to Clarification of Densification Behavior of Urethane-Foam

This paper deals with the results of three dimensional compression tests carried out for high stiffness urethane foams (Penguin-foam, Sunstar Engineering Ltd.), and also deals with the constitutive modelling base on Shima-Oyane’s consolidation condition for the tested foamed urethane. Three kinds of urethane foams, relative densities of which were 0.1, 0.2 and 0.33, were employed in the experiments. Like metallic porous materials, the tested urethane foams show the strong plastic-compressibility. On the other hand, in modelling, unlike metallic porous materials, the identified material constants for different density foams do not take the same (or unified) values but take the different values when Shima-Oyane’s constitutive model is assumed. Furthermore, the experimentally derived stress-relative density curves could not be satisfactorily described by Shima-Oyane’s original constitutive model; the experimental stress-relative density curves show stronger work hardening as compared with the simulated ones especially in the large deformation stage. To avoid those inconvenience, in this paper, a modified Shima-Oyane type constitutive equation was also proposed, and it was shown that the proposed model could well express both the low work hardening area of the stress-relative density curves at the initial deformation stage and the strong work hardening area at the final deformation stage by supposing the stress restriction at initial deformation stage due to the buckling of cell walls of each foam, and the rapid stress increase at the large deformation stage caused by the successive contact and the friction between the bent cellular walls, respectively.

Experimental Study of the Effects of Geometry and Strain Rate on Dynamic Behavior of Axial Crushing Honeycomb

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.

Numerical Study of the Effects of Strain Rate and Cell Geometry on Dynamic Behavior of Axial Crushing Honeycomb

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.

Dynamic Buckling Behavior of Long Column made of Grass Fiber Reinforced Plastics for Gas Circuit Breaker

In a gas circuit breaker of an electric power substation, a long operating rod made of an insulating material such as grass fiber reinforced plastics is employed, and when the breaker is operated, the rod was subjected to a strong impulsive compression load. In designing of the operating rods, therefore, the precise knowledge of their dynamic buckling loads is required. In this study, the dynamic elastic buckling behavior of GFRP rods the ends of which were tightly clamped was experimentally investigated. In the experiments, elastic buckling loads of slender GFRP rods in a wide range of loading velocities were measured by using the three different type testing machines: universal testing machine, hydraulic servo system and drop weight type impact loading machine. From the obtained 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, we may show possibility that the proposed criterion can successfully describe buckling behaviors of long rods of many sorts of engineering materials with the same material parameters.

Improvement of Measurement Accuracy of Non-Coaxial Hopkinson Bar Method

The high measurement accuracy in dynamic tension testing is required for designs and numerical simulations based on the accurate modeling of stress-strain relations at various strain-rates. The non-coaxial Hopkinson bar method (NCHBM) is one of the recently proposed methods for dynamic tension tests. In this study, the accuracy of the stress-strain relations obtained on the basis of NCHBM was investigated by the analysis based on FEM and by experiment. The FE models of NCHBM apparatus and plate-type specimens of various dimensions were made in detail. The target materials employed in this study were mild steel and SUS316 stainless steel. The effects of the rising, the dimensions of specimen, and the strain rate were examined systematically. The results calculated for the specimens of SUS316 were compared with those obtained by experiment. At last, the improving methods of the specimen fixation were proposed.

3-Dimensionality and Effects of Void Volume Fraction on the Plate-Impact Analysis Taking Account of Dynamic Void Development.

Spall damage analyses are conducted in the framework of continuum damage mechanics with special emphasis on the 3-dimensionality and the effect of void volume fraction on the spall process. The evolution equation of a scalar damage variable taking account of the nucleation and growth of spherical voids, and the elastic-viscoplastic constitutive equations extended to damaged materials are incorporated into the commercial hydrocode MANJUSRI-3D. The axisymmetric two-dimensional analyses of the plate-impact tests on OFHC copper plates are performed to examine the effect of the ratio of the target radius to its thickness and that of the volumetric strain related to the void development on the relation between volumetric strain and hydrostatic pressure.