• 대한기계학회논문집A
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• 제25권7호
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• pp.1073-1081
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• 2001

Many researches have published numerous papers about the high-strain-rate obtained from Split Hopkinson Pressure Bar(SHPB) tests. And 6.5wt%Si steel is widely known as an excellent magnetic material because its magnetostriction is nearly zero. Single crystals are prepared by the Floating Zone(FZ) method, which melts the alloy by the use of a high temperature electron beam in a pure argon gas condition. In this paper, the fracture behavior of the poly crystals and single crystals (DO$_3$phase) of Fe-6.5wt%Si alloy by SHPB test is observed. The comparison of high-strain-rate results with static results was done. Obtained main results are as follows: (1) Fe-6.5wt%Si alloy has higher strength at high-strain-rate tensile. SHPB results of polycrystal are twice as high as static results. (2) From the fractography, the cleavage steps are remarkably reduced in the SHPB test compared with the static test.

• 한국자동차공학회논문집
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• 제18권1호
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• pp.127-130
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• 2010

This paper deals with the formability of DP590 steel considering the strain rate. The strain hardening coefficient, elongation and r-value were obtained from the static and dynamic tensile test. As strain rate increases from static to 100/s, the strain hardening coefficient and the uniform elongation decrease and the elongation at fracture and r-value decrease to 0.1/s and increase again to 100/s. The high speed forming limit tests with hemi-spherical punch were carried out using the high speed crash testing machine and high speed forming jig. The high speed forming limit of DP590(order of $10^2$/s) decreases compared to the static forming limit(order of $10^{-3}$/s) and the forming limit band in high speed forming test is narrower than that in the static forming test. This tendency may be due to the development of brittleness with increase of stain rate.

• 소성∙가공
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• 제6권2호
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• pp.102-109
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• 1997

The multistage deformation and stress relaxation were carried out to investigate the strain induced precipitation by torsion tests in the range of 1000~80$0^{\circ}C$, 0.05~5/sec for V-microalloyed steel. The starting temperature and time for the initiation of precipitation were determined by stress relaxation tests. The distribution of precipitates increased, as the strain rate increased and the mean size of precipitates was found to be about 10~30nm. The precipitation starting time$(P_s)$ decreased with increasing strain rate and the amount of pre-strain. The effect of deformation conditions on the no-recrystallization temperature$(T_nr)$ was also determined in the multistage deformation. $T_nr$ Tnr decreased with increasing the strain and strain rate. In the controlled rolling simulation, grain refinement and precipitation hardening effects could be achieved by the alternative large pass strain at the latter half pass stage under the condition of low temperature and high strain rate.

• 대한기계학회논문집A
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• 제31권2호
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• pp.174-181
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• 2007

This paper is concerned with the thermo-mechanical behavior of steel sheet for an auto-body including temperature dependent strain rate sensitivity. In order to identify the temperature-dependent strain rate sensitivity of SPRC35R, SPRC45E and TRIP60, uniaxial tensile tests are performed with the variation of the strain rates from 0.001/sec to 200/sec and the variation of environmental temperatures from $-40^{\circ}C$ to $200^{\circ}C$. The thermo-mechanical response at the quasi-static state is obtained from the static tensile test and that at the intermediate strain rate is obtained from the high speed tensile test. Experimental results show that the variation of the flow stress and fracture elongation becomes sensitive to the temperature as the strain rate increases. It is observed that the dynamic strain aging occurs with TRIP60 at the temperature above $150^{\circ}C$. Results also indicate that the flow stress and tincture elongation of SPRC35R are more dependent on the changes of strain rates and temperature than those of SPRC45E and TRIP60.

• 한국자동차공학회논문집
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• 제14권1호
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• pp.48-53
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• 2006

A methodology is described for predicting the fatigue life of the resistance spot weldment including strain rate effect. Because it is difficult to perform a physical failure test with high strain rate, an analytical method is necessary to get the mechanical properties of various strain rate, To this end, quasi-static tensile-shear tests at several strain rate were performed on spot weldments of SPC. These test provided the empirical data with the strain rate. With these results, we formulated the function of fatigue life prediction using the lethargy coefficient which is the global material property from tensile test. And, we predicted the fatigue life of spot weldment at dynamic strain rate. To confirm this method for fatigue life prediction, analytical results were compared with the experimental fatigue data.

• Mycobiology
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• 제38권3호
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• pp.210-214
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• 2010

Selection of fungal strains with high virulence against the developmental stages of Bemisia tabaci was performed using internal transcribed spacer regions. The growth rate of hyphae was measured and bioassay of each developmental stage of B. tabaci was conducted for seven days. All of the fungal strains tested were identified as Lecanicillium spp., with strain 4078 showing the fastest mycelium growth rate (colony diameter, 16.3 $\pm$ 0.9 mm) among the strains. Compared to strain 4075, which showed the slowest growth rate, the growth rate of strain 4078 was increased almost 2-fold after seven days. Strains 4078 and Btab01 were most virulent against the egg and larva stages, respectively. The virulence of fungal strains against the adult stage was high, except for strains 41185 and 3387. Based on the growth rate of mycelium and level of virulence, strains 4078 and Btab01 were selected as the best fungal strains for application to B. tabaci, regardless of developmental stage.

• Composites Research
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• 제35권1호
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• pp.38-41
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• 2022

고 변형률 속도에서 폴리프로필렌-유리 장섬유 복합재료(PP-LGF)와 열가소성 올레핀(TPO) 소재의 동적 압축 특성을 얻기 위해 홉킨슨바(Split-Hopkinson Pressure Bar (SHPB))를 이용하여 실험을 진행하였다. SHPB는 변형률 속도 100 s^-1~10000 s^-1 범위에서 재료의 동적 기계적 물성을 확인할 수 있는 장치이다. SHPB 시험은 입력봉과 전달봉에서 측정된 탄성파를 이용하여 시편의 응력, 변형률 및 변형률 속도를 얻을 수 있는 탄성파 전달 이론을 기반으로 한다. 또한 SHPB에서 얻은 변형률 데이터의 검증을 위해 시편을 초고속카메라로 촬영하여 DIC 기법을 통해 얻은 변형률 데이터와 비교 진행하였다.

• 한국재료학회지
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• 제27권12호
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• pp.643-651
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• 2017

Four types of high Mn TWIP(Twinning Induced Plasticity) steels were fabricated by varying the Mn and Al content, and the tensile properties were measured at various strain rates and temperatures. An examination of the tensile properties at room temperature revealed an increase in strength with increasing strain rate because mobile dislocations interacted rapidly with the dislocations in localized regions, whereas elongation and the number of serrations decreased. The strength decreased with increasing temperature, whereas the elongation increased. A martensitic transformation occurred in the 18Mn, 22Mn and 18Mn1.6Al steels tested at $-196^{\circ}C$ due to a decrease in the stacking fault energies with decreasing temperature. An examination of the tensile properties at $-196^{\circ}C$ showed that the strength of the non-Al added high Mn TWIP steels was high, whereas the elongation was low because of the martensitic transformation and brittle fracture mode. Although a martensitic transformation did not occur in the 18Mn1.9Al steel, the strength increased with decreasing temperature because many twins formed in the early stages of the tensile test and interacted rapidly with the dislocations.

• 소성∙가공
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• 제20권4호
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• pp.309-315
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• 2011

The behavior of metallic materials at high strain rates shows different characteristics from those in quasi-static deformation. Therefore, the strain rate should be considered when simulating crash events. The objective of this paper is to evaluate the dynamic tensile characteristics of SPRC440 as a function of the volume fraction of phases. As-received SPRC440 is composed of ferrite and pearlite phases. However, ferrite and martensite phases were observed after heat treatment at $730^{\circ}C$ and $780^{\circ}C$ for 5 minutes, as expected by calculations based on the curves from dilatometry tests. High cross-head speed tensile tests were performed to acquire strain-stress curves at various strain rates ranging from 0.001 to $300\;s^{-1}$, which are typical in real vehicle crashes. It was observed that the flow stress increases with the strain rate and this trend was more pronounced in the as-received specimens consisting of ferrite and pearlite phases. It is speculated that the dislocation density in each phase has an influence on the strain rate sensitivity.

• 한국구조물진단유지관리공학회 논문집
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• 제19권4호
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• pp.83-91
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• 2015

충돌 또는 폭발 하중 하의 콘크리트는 정적 하중에서 가지는 재료물성과 다른 거동을 보이게 된다. 즉, 고변형율 하의 콘크리트의 재료물성은 크게 변화하게 되며, 이를 시험평가하기 위한 방법 중 압축강도와 관련된 시험법으로는 SHPB(Split Hopkinson Pressure Bar) Test가 있다. 그러나, SHPB Test는 금속과 같은 인성재료를 위해 개발된 시험방법으로서 취성재료인 콘크리트에 적용이 가능한지에 대한 검토가 추가적으로 필요하며, UHPC와 같은 섬유보강 초고성능 콘크리트에 대한 연구는 미미한 실정이다. 이에 본 연구에서는 콘크리트 시험체를 위해 제작된 SHPB Test 장비를 사용하여 일반 콘크리트 및 UHPC 시험체에 대한 시험평가를 수행하였으며, SHPB Test의 적용 적정성을 파악하기 위하여 초고속카메라를 활용한 변위영상 분석을 통한 검증을 수행하였다.