• Proceedings of the Korean Magnestics Society Conference
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• 2000.09a
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• pp.397-406
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• 2000

Hopkinson effect in the HDDR-treated Nd$\sub$15/Fe$\sub$77/B$\sub$8/ alloy was examined in detail by means of a thermomagnetic analysis with low magnetic field (600 Oe). The emergence and magnitude of maximum in magnetisation in the thermomagnetic curve due to the Hopkinson effect was correlated to the grain structure and coercivity of the HDDR-treated material. the HDDR-treated materials showed clear Hopkinson effect (maximum in magnetisation just below the Curie temperature of the Nd$\sub$2/Fe$\sub$14/B phase) on heating in the thermomagnetic curve. Magnitude of the magnetisation rise due to the Hopkinson effect became smaller as the recombination time increased. The magnetisation recovery at room temperature on cooling from above the Curie temperature became smaller as the recombination time increased. The HDDR-treated materials with shorter recombination time, finer grain size and higher coercivity showed larger magnetisation maximum due to Hopkinson effect in the thermomagnetic curve.

• Journal of Magnetics
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• v.6 no.2
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• pp.61-65
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• 2001

The Hopkinson effect in the HDDR-treated $Nd_{15}Fe_{77}B_8$ allay was examined in detail by means of a thermo-magnetic analysis with low magnetic field (600 Oe). The emergence and magnitude of maximum in magnetisation in the thermomagnetic curve due to the Hopkinson effect was correlated with the grain structure and coercivity of the HDDR-treated material. The HDDR-treated materials showed a clear Hopkinson effect (maximum in magnetisation just below the Curie temperature of the $Nd_2Fe_{14}B\;$ phase) on heating. The magnitude of the magnetisation rise due to the Hopkinson effect became smaller as the recombination time increased. The magnetisation recovery at room temperature on cooling from above the Curie temperature became smaller as the recombination time increased. The HDDR-treated materials with shorter recombination time, finer grain size and higher coercivity showed larger magnetisation maximum due to the Hopkinson effect in the thermo-magnetic curve.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.204-209
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• 2008

The interest in the mechanical behavior of materials at high strain rates has increased in recent years, and by now it is well known that mechanical properties can be strongly influenced by the speed of applied load. The split Hopkinson pressure bar (SHPB) has been widely used to determine mechanical properties of materials at high loading rates. However, to ensure test reliability, measurement error source must be accounted for and eliminated. During experiment, the specimens were located between the incident and the transmit bar. The presence of contact frictions between the test bars and specimen may cause errors. In this work, numerical experiments were carried out to investigate the effect of friction on test results. In SHPB test, the measured stress by the transmitted bar is assumed to be flow stress of the test specimen. Through the numerical experiments, however, it is shown that the measured stress by the transmit bar is axial stress components. When, the contact surface is frictionless, the flow stress and the axial stress of the specimen are about the same. When the contact surface is not frictionless, however, the flow stress and the axial stress are not the same anymore. Therefore, the measured stress by the transmitted bar is not flow stress. The effect of friction on the difference between flow stress and axial stress is investigated.

• Transactions of the Korean Society of Automotive Engineers
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• v.6 no.2
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• pp.80-91
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• 1998

In this paper, the estimation of dynamic interlaminar fracture toughness on fracture mode II in CFRP(carbon fiber reinforced plastics) laminates in made. Dynamic ENF(End Notched Flexure) apparatus used in this paper is manufactured by suing Split Hopkinson Pressure Bar. The static and impact load history in the CFRP specimen is measured by using manufactured dynamic ENF tester and 3-point bending test is carried out to find the load history. Also dynamic interlaminar fracture toughness can be found by using the J integral obrained from dynamic analysis in consideration of intertia-force effect.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2004.10a
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• pp.634-637
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• 2004

This paper presents a Split Hopkinson Pressure Bar(SHPB) technique to obtain compressive stress-strain data for rubber materials. An experimental technique that modifies the conventional Split Hopkinson Pressure Bar(SHPB) has been developed for measuring the compressive stress-strain responses of materials with low mechanical impedance and low compressive strengths such as rubber. This paper introduces an all-polymeric pressure bar which achieves a closer impedance match between the pressure bar and the specimen materials. In addition, we are a pulse shaper to lengthen the rising time of the incident wave to ensure stress equilibrium and homogeneous deformation of a rubber materials. It is found that the modified technique can be determine the dynamic deformation behavior of an NBR rubber more accurately.

• Structural Engineering and Mechanics
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• v.19 no.6
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• pp.603-618
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• 2005

The split Hopkinson pressure bar (SHPB) technique is widely used to characterize the dynamic mechanical response of engineering materials at high strain rates. In this paper, attendant problems associated with testing 70 mm diameter concrete specimens are considered, analysed and resolved. An adaptation of a conventional solid circular striker bar, as a means of achieving reliable and repeatable SHPB tests, is then proposed. In the analysis, a pseudo one-dimensional model is used to analyse wave propagation in a non-uniform striker bar. The stress history of the incident wave is then obtained by using the finite difference method. Comparison was made between incident waves determined from the simplified model, finite element solution and experimental data. The results show that the simplified method is adequate for designing striker bar shapes to overcome difficulties commonly encountered in SHPB tests. Using two specifically designed striker bars, tests were conducted on 70 mm diameter steel fibre reinforced concrete specimens. The results are presented in the paper.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.10a
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• pp.795-798
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• 1997

This study is about the dynamic behavior of steel(SM45C). Dynamic tests were performed using SHPB(Split Hopkinson Pressure Bar) which is designed and modified to be used in both tensile and compressive modes. Quasi-static compression tests were also carried out for the comparison to the dynamic results. Not only the dynamic mechanical properties but also the effect of the notch of the specimen on stress-strain curve were investigated. The dynamic test results reveal that strain and stress are sensitively affected by the notch. The depth and the number of notch increase the stress and decrease the strain.

• International Journal of Automotive Technology
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• v.1 no.1
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• pp.35-41
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• 2000

The crashworthiness of vehicles with finite element methods depends on the geometry modeling and the material properties. The vehicle body structures are generally composed of various members such as frames, stamped panels and deep-drawn parts from sheet metals. In order to ensure the impact characteristics of auto-body structures, the dynamic behavior of sheet metals must be examined to provide the appropriate constitutive relation. In this paper, high strain-rate tensile tests have been carried out with a tension type split Hopkinson bar apparatus specially designed for sheet metals. Experimental results from both static and dynamic tests with the tension split Hopkinson bar apparatus are interpolated to construct the Johnson-Cook and a modified Johnson-Cook equation as the constitutive relation, that should be applied to simulation of the dynamic behavior of auto-body structures. Simulation of auto-body structures has been carried out with an elasto-plastic finite element method with explicit time integration. The stress integration scheme with the plastic predictor-elastic corrector method is adopted in order to accurately keep track of the stress-strain relation for the rate-dependent model accurately. The crashworthiness of the structure with quasi-static constitutive relation is compared to the one with the rate-dependent constitutive model. Numerical simulation has been carried out for frontal frames and a hood of an automobile. Deformed shapes and the Impact energy absorption of the structure are investigated with the variation of the strain rate.

• Explosives and Blasting
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• v.23 no.3
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• pp.27-42
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• 2005

An explosion modeling technique was developed by using the spherical discrete element code, $PFC^{3D}$, which can be used to model the dynamic stress wave propagation phenomenon. The modeling technique is simply based on an idea that the explosion pressure should be applied to a $PFC^{3D}$ particle assembly not in the form of an external force (body force), but in the form of a contact force (surface force). The stress wave propagation modeling was conducted by simulating the experimental approach based on the Hopkinson's effect combined with the spatting phenomenon that had previously been developed to determine the dynamic tensile strength of Inada granite. As a result, the stress wave velocity obtained by the proposed modeling technique was 4167 m/s, which is merely $3\%$ lower than the actual wave velocity of 4300 m/s for an Inada granite.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.5
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• pp.511-518
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• 2010

The split Hopkinson pressure bar (SHPB) has been widely used to determine the mechanical properties of materials at high loading rates. However, to ensure test reliability, the source of measurement error must be identified and eliminated. During the experiment, specimens were placed between the incident and the transmit bar. Contact friction between the test bars and specimen may cause errors. In this study, numerical experiments were carried out to investigate the effect of friction on the test results. In the SHPB test, the stress measured by the transmitted bar is assumed to be the flow stress of the test specimen. However, performing numerical experiments, it was shown that the stress measured by the transmit bar is axial stress components. When the contact surface is frictionless, the flow stress and axial stress of the specimen are approximately equal. On the other hand, when the contact surface is not frictionless, the flow stress and axial stress are no longer equal. The effect of friction on the difference between the flow stress and axial stress was investigated.