• Journal of the Korean Society of Safety
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• v.17 no.3
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• pp.7-12
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• 2002

The object of this paper is to investigate collapse characteristics of CF/Epoxy(Carbon Fiber/Epoxy resin) composite tubes on the change of interlaminar number and fiber orientation angle of outer and to evaluate reappearance of collapse characteristics on the change of tension strength of fibers under static and impact axial compression loads. When a CF/Epoxy composite tube is mushed, static/impact energy is consumed by friction between the loading plate and the splayed fiends of the tube, by fracture of the fibers, matrix and their interface. In general, CF/Epoxy tube with 6 interlaminar number(C-type) absorbed more energy than other tubes(A, B, D-types). The maximum collapse load seemed to increase as the interlaminar number of such tubes increases. The collapse mode depended upon orientation angle of outer of CF/Epoxy tubes and loading status(static/impact). Typical collapse modes of CF/Epoxy tubes are wedge collapse mode, splaying collapse mode and fragmentation collapse mode. The wedge collapse mode was shorn in case of CF/Epoxy tubes with 0$^{\circ}$ orientation angle of outer under static and impact loadings. The splaying collapse mode was shown in only case of CF/Epoxy tubes with 90$^{\circ}$ orientation angie or outer under static loadings, however in impact tests those were collapsed in fragmentation mode. So that CF/Epoxy tube with 6 interlaminar number and 90$^{\circ}$ outer orientation angle presented to the optimal collapse characteristics.

• Journal of the Korean Society for Heat Treatment
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• v.28 no.2
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• pp.75-81
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• 2015

Effects of B and Cu additions on the microstructure and mechanical properties of high-strength bainitic steels were investigated in this study. Six kinds of high-strength bainitic steels with different B and Cu contents were fabricated by thermo-mechanical control process composed of controlled rolling and accelerated cooling. The microstructures of the steels were analyzed using optical and transmission microscopy, and the tensile and impact tests were conducted on them in order to investigate the correlation of microstructure with mechanical properties. Depending on the addition of B and Cu, various low-temperature transformation products such as GB (granular bainite), DUB (degenerated upper bainite), LB (lower bainite), and LM (lath martensite) were formed in the steels. The addition of B and Cu increased the yield and tensile strengths because of improved hardenability and solid solution strengthening, but decreased the ductility and low-temperature toughness. The steels containing both B and Cu had a very high strength above 1.0 GPa, but showed a worse low-temperature toughness of higher DBTT (ductile-to-brittle transition temperature) and lower absorbed energy. On the other hand, the steels having GB and DUB showed a good combination of tensile and impact properties in terms of strength, ductility, yield ratio, absorbed energy, and DBTT.

• Korean Journal of Materials Research
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• v.30 no.3
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• pp.123-130
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• 2020

In this study, two types of thick steel plates are prepared by controlling carbon equivalent and nickel content, and their microstructures are analyzed. Tensile tests, Vickers hardness tests, and Charpy impact tests are conducted to investigate the correlation between microstructure and mechanical properties of the steels. The H steel, which has high carbon equivalent and nickel content, has lower volume fraction of granular bainite (GB) and smaller GB packet size than those of L steel, which has low carbon equivalent and nickel content. However, the volume fraction of secondary phases is higher in the H steel than in the L steel. As a result, the strength of the L steel is higher than that of the H steel, while the Charpy absorbed energy at -40 ℃ is higher than that of the L steel. The heat affected zone (HAZ) simulated H-H specimen has higher volume fraction of acicular ferrite (AF) and lower volume fraction of GB than the HAZ simulated L-H specimen. In addition, the grain size of AF and the packet sizes of GB and BF are smaller in the H-H specimen than in the L-H specimen. For this reason, the Charpy absorbed energy at -20 ℃ is higher for the H-H specimen than for the L-H specimen.

• Korean Journal of Materials Research
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• v.31 no.3
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• pp.139-149
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• 2021

In this study, three kinds of bainitic steels are fabricated by controlling the contents of vanadium and boron. High vanadium steel has a lot of carbides and nitrides, and so, during the cooling process, acicular ferrite is well formed. Carbides and nitrides develop fine grains by inhibiting grain growth. As a result, the low temperature Charpy absorbed energy of high vanadium steel is higher than that of low vanadium steel. In boron added steel, boron segregates at the prior austenite grain boundary, so that acicular ferrite formation occurs well during the cooling process. However, the granular bainite packet size of the boron added steel is larger than that of high vanadium steel because boron cannot effectively suppress grain growth. Therefore, the low temperature Charpy absorbed energy of the boron added steel is lower than that of the low vanadium steel. HAZ (heat affected zone) microstructure formation affects not only vanadium and boron but also the prior austenite grain size. In the HAZ specimen having large prior austenite grain size, acicular ferrite is formed inside the austenite, and granular bainite, bainitic ferrite, and martensite are also formed in a complex, resulting in a mixed acicular ferrite region with a high volume fraction. On the other hand, in the HAZ specimen having small prior austenite grain size, the volume fraction of the mixed acicular ferrite region is low because granular bainite and bainitic ferrite are coarse due to the large number of prior austenite grain boundaries.

• Journal of the Korean Wood Science and Technology
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• v.30 no.2
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• pp.95-101
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• 2002

Physical and mechanical properties of major Korean ash species were examined. For Fraxinus rhynchophylla and Fraxinus sieboldiana, green moisture content of sapwood was almost the same value as heartwood. Heartwood of Fraxinus mandshurica, however, had slightly higher moisture content than sapwood. Green and oven dry densities of F. mandshurica were lower than those of F. rhynchophylla and F. sieboldiana. Swelling and shrinkage of sapwood in F. sieboldiana showed somewhat higher value than those of F. rhynchophylla. Longitudinal compressive strength and modulus of elasticity in F. sieboldiana were lower values than those in the other species. Shearing strength in radial section was higher than that in tangential section of all samples. It could be noted that shearing strength of F. sieboldiana demonstrated higher value than that of the other species. Three species had excellent bending properties in MOR and MOE. Impact bending absorbed energy for F. rhynchophylla and F. mandshurica did not show any significant differences.

• Journal of the Korean Wood Science and Technology
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• v.31 no.4
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• pp.57-62
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• 2003

The Mechanical and drying characteristics of Pinus densiflora wood, which was damaged by forest fire, and sound wood were examined. There were no significant differences in axial compressive properties, shearing strength and impact bending absorbed energy between damaged and sound woods. But the bending properties of damaged wood were slightly higher than those of sound wood. The green moisture content of sapwood in damaged wood was lower than that in sound wood, but oven-drying rate was similar in both woods. Number of initial check in damaged wood was smaller than that in sound wood, and inner check in damaged and sound woods was not observed. Consequently, it is considered that mostly damaged wood by forest fire can be used for wood material after removing the bark.

• Journal of the Korean Society for Precision Engineering
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• v.18 no.7
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• pp.120-126
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• 2001

The widely used spot welded section members of vehicles are structures which absorb most of the energy in a front-end collision. In front-end collision, sufficiently absorbed in the front parts, the impact energy does not reach the passengers. Simultaneously, the frame gets less damaged. This structures have to be very stiff, but collapse progressively to absorb the kinetic energy as expected. In the view of stiffness, the double-hat shaped section member is stiffer than the hat shaped section member. In progress of collapse, the hat shaped section member is collapsing progressively, but the double-hat shaped section member does not due to stiffness. An analysis on the hat shaped section member was previously completed. This paper concerns the collapse characteristic of the double-hat shaped section member. In the program system presented in this study, an explicit finite element code, LS-DYNA3D is adopted for simulating complicate collapse behavior of double hat shaped section members with respect to spot weld pitches. And comparing with the results from the quasi-static and impact experiment, the simulation has been verified.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.10
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• pp.1686-1694
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• 2003

The foam-filled tube beams can be used for the front rail and firewall structures to absorb impact energy during frontal or side collision of vehicles. In the case of side collision where bending is involved in the crushing mechanism, the foam filler would be effective in maintaining progressive crushing of the thin-walled structures so that much impact energy could be absorbed. In this study, bending behaviors of the closed-cell-aluminum-alloy-foam-filled stainless steel tube were investigated. The various foam-filled specimens including piecewise fillers were prepared and tested. The aluminum-alloy-foam filling offered the significant increase of bending resistance. Their suppression of the inward fold formation at the compression flange as well as the multiple propagating folds led to the increase of load carrying capacity of specimens. Moreover, the piecewise foams would provide the easier way to fill the thin-walled shell structures without the drawback of strength.

• Transactions of the Korean Society of Automotive Engineers
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• v.21 no.6
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• pp.24-30
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• 2013

This paper studies the comparative analysis on two different internal cushion devices (the types of needle and relief valve) used to absorb the energy which is generated when the pneumatic cylinder moves with the load at meter-out speed control system. The effect at varying the piston velocity under same driving condition is mainly investigated. The simulation results on pressure in the cushion chamber and the dynamic behavior of the relief valve type cushion device are compared with the needle valve type. Design and performance are improved with the cushion configuration of better quality at high-speed pneumatic cylinder. Based on the relation between absorbed energy and impact energy at cushion process, cushion performance at pneumatic cylinder is evaluated.

• Proceedings of the KSR Conference
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• 2004.10a
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• pp.634-639
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• 2004

Under light collision accidents, the energy absorption strategy for the coupler and expansion tube of the TTX(Tilting Train Express) initial design is established in the paper. Also, 1st shearing bolts are designed. When the absorbed energy of the coupler reaches its maximum, the connecting bolts between the coupler and the car body are sheared off not to transmit the impact force to the car body structure. To absorb more energy after the lst shearing bolts work, a expansion tube is designed conceptually and installed at the rear part of the coupler. Using Hyper-Mesh and LS-DYNA, pre/post processing and light collision analyses are preformed, respectively.