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For the improvement of car safety in collision, the development of high-speed processing technology, the refinement of practical constitutive equations for numerical simulation, and so forth, it is important to obtain accurate stress-strain relation of various materials in wide ranges of strain and strain rate. Through a series of numerical simulations of the specimen model of practical geometry by using the computer code LS-DYNA, some useful results in the determination of the geometry and dimensions of specimen suitable for dynamic tension test under the specified strain rate in the range 102-103 s-1, such as the effects of the rising time of input velocity and the lengths of grip and fillet on transient vibration, the relationship between the limits of gauge length and strain rate to give an adequate accuracy of dynamic stress-strain curves, and so forth, are obtained.

It is desired to obtain the material properties in a wide range of strain rates and strain to be used for numerical simulations of car crushing, high speed forming and others. To conduct dynamic material testing, adapting suitable specimen size and shape for each testing strain rate is important to obtain the results with good accuracy and interchangeability for a wide range of strain rates. Any standard for the specimen size and shape, however, is not yet established except quasistatic testing. In this study, the upper limit of the gauge length of the test piece, to keep the testing error under a few percent, for the dynamic tensile testing in the strain-rate range from 102 to 104 s-1 were obtained through a series of numerical simulations.

In recent years, an increasing interest has been paid to the accurate measurement of stress-strain relations of materials for wide range of strain rates which are needed to formulate the constitutive equations of rate-sensitive materials precisely. The Hopkinson Pressure Bar system is widely used for this purpose. However, the device should be long enough to prevent the immediate reflection of stress waves from the ends of the input and output bars. The newly developed Sensing Block (SB) system, which consists of a small sensing projection and a relatively large mass block, is expected to be used widely for many materials. In this paper, the bounds of application of the SB method for measuring dynamic load was examined numerically. The relation between the size of sensing projection and the rising time of applied load was cleared to satisfy some criterions on the accuracy for each model. The results ensure the sufficient accuracy for measuring dynamic load under the obtained conditions.

Initial transient behavior of a tall-building-model due to a strong near-source earthquake has been studied based on the numerical simulation using the code MSC. Dytran. It was found that stresses occurred in the initial transient stage exceeded the yield stress of material in many columns, in the case of elastic-plastic building model, and that those exceeded the tensile strength of material in many columns, in the case of elastic building model. Through discussing on the dynamic properties of the structural material with the rate dependency of strain hardening rate of the material, the possibility to be occurred the brittle fracture of column even at strain rate in the range of 0.1∼1 s-1 was pointed out. These results show very possibility that a high stress sufficient to lead the initiation of damage such as local fracture and crack can be reached in some parts of the tall-building-model due to a strong near-source earthquake.

In 1995, the great Hanshin-Awaji earthquake caused a large amount of destruction and structural failures. One example, whose mechanism is not fully clear, is the fracture of a bridge bearing of a Nielsen type bridge that does not occur under the ordinary static or dynamic loading conditions. The fracture probably resulted from very high stress due to an unexpected dynamic mechanism. In this paper, the 3-dimensional dynamic behavior of a Nielsen type bridge was analyzed by assuming a collision between the upper and the lower parts of the bearing, which might have occurred in the great Hanshin-Awaji earthquake. The numerical results show that an impact due to a relative velocity of 5∼6m/s between the upper and the lower parts of the bearing generates a stress sufficient to cause a fracture in the upper bearing. The observed features of the actual fracture surface was also simulated fairly closely.

The main aim of this paper is to give important information for understanding the mechanism of the axial crushing of thin-walled structures under dynamic loading, from the safety point of view in car crushing. For this, the axial crushing experiments of circular tube specimens were conducted by using the dynamic-loading device with the load sensing block to evaluate the properties of dynamic progressive buckling of two types of materials. The thin-walled circular tube specimens of steel and aluminum alloy were chosen, stressing mainly on the behavior of steel specimens. The effect of impact velocity on the magnitude of axial stress corresponding to the first buckling of the dynamic progressive buckling, and the effect of the differences between those material properties on the behavior of the dynamic progressive buckling were clarified. The absorbed energy per unit volume of the steel specimen was also examined. It was found that the ratio of the thickness to the diameter of the specimen affects greatly the absorbed energy per unit volume and the deformation mode of dynamic progressive buckling.

To examine axial crushing properties of aluminum alloys under high strain rate deformations, the axial crushing tests of thin-walled circular tube specimens were conducted. Particularly, we introduced a new apparatus, load sensing block, with a special technique based on the theory of stress wave propagation for the dynamically-loaded axial crushing test. It enables us to get stable outputs to illustrate stress-strain curves for a wide strain range even in the case of the specimens with long length in the axial direction or low impedance. In accordance with the split-Hopkinson bar method, the size of device must be extended considerably in those cases. In order to evaluate the energy absorption abilities of those specimens, we paid attention to the absorbed energy density per unit length. The comparison among the modes of dynamic progressive buckling shows that the concertina buckling mode has the largest absorption ability among those modes. The conditions governing those buckling modes are also discussed.

Elaboration of the preceding one-dimensional analysis of ductile spall damage based on damage mechanics was discussed by performing axisymmetric two-dimensional analyses. The effects of surface energy due to cavity nucleation and elastic-viscoplastic-damage coupling were incorporated into the energy conservation law. Then, the application of the commercial finite-difference program MANJUSRI-3D for nonlinear dynamic analysis to the axisymmetric two-dimensional problem was discussed. Finally, a plate impact of OFHC copper discs was analyzed by the present method, and the effects of the radial stress wave and surface nucleation energy on the spall damage process were elucidated. The results of analysis were compared with those of one-dimensional analysis and experiments.

A systematic method of spall damage analysis was developed on the basis of damage mechanics. By use of a scalar damage variable, the damage evolution equation of Lemaitre et al. and the viscoplastic constitutive equation of Perzyna modified for the damaged materials were incorporated into the commercial finite-difference program MANJUSRI-3 D for nonlinear dynamic analysis. The process of spall damage and histories of stress wave propagation and temperature were analyzed for plate impact of OFHC copper disc targets. The results of analysis of the stress history and the particle velocity at the rear surface of the target plate were compared with those of the corresponding experiments.