Takashi YOKOYAMA - Academia.edu (original) (raw)

Takashi Yokoyama has been a Professor from Okayama University of Science since 1990. He obtained a Ph.D. in Engineering from Osaka University in 1982. His research interests include dynamic fracture mechanics, impact strength of materials and stress waves in rods and beams. He is much interested in the application of the Split Hopkinson Pressure Bar in impact testing of materials.

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Papers by Takashi YOKOYAMA

Springer eBooks, 1986

A study of the isoparametric Timoshenko beam element with reduced integration is presented which ... more A study of the isoparametric Timoshenko beam element with reduced integration is presented which has four nodes and two degrees of freedom per node. The element stiffness and mass matrices are derived from the same cubic polynomials for the total lateral displacement and the bending slope. The performance of the reduced integration Timoshenko beam element is studied by calculating the natural frequencies of three cantilever beams with different slenderness ratios. It is shown that the rate of convergence of the isoparametric Timoshenko beam element is significantly improved by the use of reduced integration especially for a thin beam.

The split Hopkinson pressure bar (SHPB) technique developed by H. Kolsky has been widely used to ... more The split Hopkinson pressure bar (SHPB) technique developed by H. Kolsky has been widely used to determine stress-strain curves of materials at strain rates from 102 to 104 s'. In the SHPB method, it is usually impossible to predict the maximum strain and strain rate obtainable in the specimen before testing even if an impact velocity of a striker bar, diameters and acoustic impedances (pc) of the Hopkinson bars and the specimen geometry are specified. This is because the strain rate dependence of test materials is unknown' until the SHPB tests are conducted. The objective of the present work is to predict the maximum plastic strain and strain rate in the SHPB specimen based on the energy analysis using rate-independent plasticity. Two different material models are used to express the rate-independent mechanical behavior of the test materials. The present energy analysis enables one to approximately estimate the maximum plastic strain and strain rate in the SHPB specimen in advance from the static properties of the test materials. The prediction accuracy of the energy analysis is checked by comparison with the experimental results for three high-strength aluminum alloys obtained from the SHPB tests. It is demonstrated that the maximum strain and strain rate in the SHPB specimens are slightly overestimated by the energy analysis due to the neglect of the kinetic energy within them.

Journal of Japan Institute of Light Metals, 1999

Materials Science and Technology, 2003

The tensile strength and energy absorption for dissimilar metal friction welds between 6061-T6 Al... more The tensile strength and energy absorption for dissimilar metal friction welds between 6061-T6 Al alloy and type 304 stainless steel at high rates of loading were determined using the split Hopkinson bar. Cylindrical tensile specimens machined from as welded butt joints of 13 mm in diameter were used in both static and impact tests. Friction welding was conducted using a brake type friction welding machine under two different welding conditions. The effects of welding conditions and loading rate on the joint tensile properties were examined. Results show that the joint tensile properties were greatly affected by the welding parameters, and were slightly enhanced with increased loading rate. Scanning electron microscope observations revealed that the tensile fracture modes in the butt joint specimens varied with loading rate and depend on welding conditions. Microhardness pro les across the weld interface were measured to investigate the extent of the heat affected zone. The slight enhancement of the joint tensile properties with increasing loading rate is primarily attributed to the strain rate dependence of the thermally softened 6061-T6 Al alloy base material. MST/5787

The Proceedings of the JSME Materials and Processing Conference (M&P), 2002

The tensile strength and energy absorption of aissimilar metal friction welds'between 6061 Al all... more The tensile strength and energy absorption of aissimilar metal friction welds'between 6061 Al alloy and Type 3P4 stainless steel at high rates of loading are determined using the split Hopkinson bar. Round tensile specimens machined from as-welded butt joints of 12 mm diameter are used in both static and impact tension tests, Frictipn welding is conducted using a. brake type friction welding mqchine under two diffbrent welding gp. nditions. The effects of welding ,parameters and loading rate on the tensile properties of frictioh welded butt jointsi are investigated. It is demonstrated that the tensile properties are greatly affbcted by the welding conditions, and are slightly enhanced with inereasing loading rate. Macroscopic observations-reveal that the tensile fraeture mode of friction welded butt joints varies with ioading rate, depending on the welding conditions. Microhardness measurements are performed to examine the extent of the heat･affected zone (HAZ) across the weld interface, The slight enhancement in the tensile properties of fricti6,n we}ded butt joints with increasing loading rate is due to the strain rate dependence of the thermally-softened 6061 Al alloy base material, ' ' ' '

Transactions of the Japan Society of Mechanical Engineers Series A, 1997

An experimental technique has been developed for investigating behaviour of materials at high str... more An experimental technique has been developed for investigating behaviour of materials at high strain rates and cryogenic temperature using a split Hopkinson bar system and a liquid helium cryostat. Yield stress and fracture toughness for a mild steel (JIS-S25C) and an aluminum alloy (Ai 7075-T651) were measured at strain rates (loading rates) up to 3x103 S-I (3x10 6 MPa/ffi/s) and temperatures down to 4.6K. For the mild steel, the dynamic and static yield stresses increased with decreasing temperature and the dynamic yield stress was fairly larger than the static one. The variation of the dynamic yield stress due to temperature was not seen below 77K. The fracture toughness was dropped with decreasing temperature and increasing loading rate. The brittle-to-ductile transition was shifted to higher temperature range by the increase of loading rate. For the aluminum alloy, the yield stress scarcely depended on the temperature. The influences of temperature and loading rate on the fracture toughness were not clearly seen for Ai 7075-T651.

Springer eBooks, 1986

A study of the isoparametric Timoshenko beam element with reduced integration is presented which ... more A study of the isoparametric Timoshenko beam element with reduced integration is presented which has four nodes and two degrees of freedom per node. The element stiffness and mass matrices are derived from the same cubic polynomials for the total lateral displacement and the bending slope. The performance of the reduced integration Timoshenko beam element is studied by calculating the natural frequencies of three cantilever beams with different slenderness ratios. It is shown that the rate of convergence of the isoparametric Timoshenko beam element is significantly improved by the use of reduced integration especially for a thin beam.

The split Hopkinson pressure bar (SHPB) technique developed by H. Kolsky has been widely used to ... more The split Hopkinson pressure bar (SHPB) technique developed by H. Kolsky has been widely used to determine stress-strain curves of materials at strain rates from 102 to 104 s'. In the SHPB method, it is usually impossible to predict the maximum strain and strain rate obtainable in the specimen before testing even if an impact velocity of a striker bar, diameters and acoustic impedances (pc) of the Hopkinson bars and the specimen geometry are specified. This is because the strain rate dependence of test materials is unknown' until the SHPB tests are conducted. The objective of the present work is to predict the maximum plastic strain and strain rate in the SHPB specimen based on the energy analysis using rate-independent plasticity. Two different material models are used to express the rate-independent mechanical behavior of the test materials. The present energy analysis enables one to approximately estimate the maximum plastic strain and strain rate in the SHPB specimen in advance from the static properties of the test materials. The prediction accuracy of the energy analysis is checked by comparison with the experimental results for three high-strength aluminum alloys obtained from the SHPB tests. It is demonstrated that the maximum strain and strain rate in the SHPB specimens are slightly overestimated by the energy analysis due to the neglect of the kinetic energy within them.

Journal of Japan Institute of Light Metals, 1999

Materials Science and Technology, 2003

The tensile strength and energy absorption for dissimilar metal friction welds between 6061-T6 Al... more The tensile strength and energy absorption for dissimilar metal friction welds between 6061-T6 Al alloy and type 304 stainless steel at high rates of loading were determined using the split Hopkinson bar. Cylindrical tensile specimens machined from as welded butt joints of 13 mm in diameter were used in both static and impact tests. Friction welding was conducted using a brake type friction welding machine under two different welding conditions. The effects of welding conditions and loading rate on the joint tensile properties were examined. Results show that the joint tensile properties were greatly affected by the welding parameters, and were slightly enhanced with increased loading rate. Scanning electron microscope observations revealed that the tensile fracture modes in the butt joint specimens varied with loading rate and depend on welding conditions. Microhardness pro les across the weld interface were measured to investigate the extent of the heat affected zone. The slight enhancement of the joint tensile properties with increasing loading rate is primarily attributed to the strain rate dependence of the thermally softened 6061-T6 Al alloy base material. MST/5787

The Proceedings of the JSME Materials and Processing Conference (M&P), 2002

The tensile strength and energy absorption of aissimilar metal friction welds'between 6061 Al all... more The tensile strength and energy absorption of aissimilar metal friction welds'between 6061 Al alloy and Type 3P4 stainless steel at high rates of loading are determined using the split Hopkinson bar. Round tensile specimens machined from as-welded butt joints of 12 mm diameter are used in both static and impact tension tests, Frictipn welding is conducted using a. brake type friction welding mqchine under two diffbrent welding gp. nditions. The effects of welding ,parameters and loading rate on the tensile properties of frictioh welded butt jointsi are investigated. It is demonstrated that the tensile properties are greatly affbcted by the welding conditions, and are slightly enhanced with inereasing loading rate. Macroscopic observations-reveal that the tensile fraeture mode of friction welded butt joints varies with ioading rate, depending on the welding conditions. Microhardness measurements are performed to examine the extent of the heat･affected zone (HAZ) across the weld interface, The slight enhancement in the tensile properties of fricti6,n we}ded butt joints with increasing loading rate is due to the strain rate dependence of the thermally-softened 6061 Al alloy base material, ' ' ' '

Transactions of the Japan Society of Mechanical Engineers Series A, 1997

An experimental technique has been developed for investigating behaviour of materials at high str... more An experimental technique has been developed for investigating behaviour of materials at high strain rates and cryogenic temperature using a split Hopkinson bar system and a liquid helium cryostat. Yield stress and fracture toughness for a mild steel (JIS-S25C) and an aluminum alloy (Ai 7075-T651) were measured at strain rates (loading rates) up to 3x103 S-I (3x10 6 MPa/ffi/s) and temperatures down to 4.6K. For the mild steel, the dynamic and static yield stresses increased with decreasing temperature and the dynamic yield stress was fairly larger than the static one. The variation of the dynamic yield stress due to temperature was not seen below 77K. The fracture toughness was dropped with decreasing temperature and increasing loading rate. The brittle-to-ductile transition was shifted to higher temperature range by the increase of loading rate. For the aluminum alloy, the yield stress scarcely depended on the temperature. The influences of temperature and loading rate on the fracture toughness were not clearly seen for Ai 7075-T651.

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