• Title/Summary/Keyword: high strain rate

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The friction effects at high strain rates of materials under dynamic compression loads (동압축 하중을 받는 재료의 고변형도율에서의 마찰영향)

  • 김문생
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.11 no.3
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    • pp.454-464
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    • 1987
  • The objective of this research is to analyze and evaluate the dynamic flow curve of metals under impact loading at both high strain rate (.epsilon.=1/h dh/dt > 10$\^$3/m/s/m) and large strain (.epsilon.=In h/h$\_$0/ > 1.0). A test method for dynamic compression of metal disc is described. The velocity of the striker face and the force on the anvil are measured during the impact period. From these primitive data the axial stress, strain, and strain rate of the disc are obtained. The Strain rate is determined by the striker velocity divided by the specimen height. This gives a slightly increasing strain rate over most of the deformation period. Strain rates of 100 to 10,000 per second are achieved. Attainable final strains are 150%. A discussion of several problem areas is presented. The friction on the specimen surfaces, the determination of the frictional coefficient, the influence of the specimen geometry (h$\_$0//d$\_$0/ ratio) on the friction effect, the lock-up condition for a given configuration, the friction correction factor, and the evaluation of several lubricants are given. The flow function(stress verus strain) is dependent on the material condition(e.g., prior cold work), specimen geometry, strain rate, and temperature.

High Strain Rate Superplasticity of Whisker Reinforced Aluminum Alloy Matrix Composites Fabricated by Squeeze Casting (용탕단조법에 의한 휘스커강화 Al합금기 복합재료의 고속초소성)

  • Lim, Suk-Won;Nishida, Yoshinori
    • Journal of Korea Foundry Society
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    • v.21 no.6
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    • pp.359-365
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    • 2001
  • The superplastic behavior of whisker reinforced aluminum alloy matrix composites fabricated by squeeze casting as one of high pressure routes was investigated. The preforms of ${\alpha}-Si_3N_4$ and ${\beta}-SiC$ whiskers without any binder as a reinforcement were used. The matrix materials were 2024 and 7075 aluminum alloys. For the purpose of optimum superplastic condition, respectively, the whiskers volume fraction, extrusion temperature, tensile test temperature and initial strain rate were changed. Fracture surface of tested specimens were observed by SEM. By the results, it became possible to produce superplastic composites by applying only a hot extrusion process to composites obtained by the squeeze casting. The superplastic composites developed are ${\alpha}-Si_3N_4w/7075$, ${\alpha}-Si_3N_4w/2024$ and ${\beta}-SiCw/2024$ systems at high strain rate.

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Evaluation of Dynamic Deformation Behaviors in Metallic Materials under High Strain-Rates Using Taylor Bar Impact Test (Taylor 봉 충격시험을 통한 고 변형률속도하 금속재료의 동적변형거동 평가)

  • Bae, Kyung Oh;Shin, Hyung Seop
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.40 no.9
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    • pp.791-799
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    • 2016
  • To ensure the reliability and safety of various mechanical systems in accordance with their high-speed usage, it is necessary to evaluate the dynamic deformation behavior of structural materials under impact load. However, it is not easy to understand the dynamic deformation behavior of the structural materials using experimental methods in the high strain-rate range exceeding $10^4\;s^{-1}$. In this study, the Taylor bar impact test was conducted to investigate the dynamic deformation behavior of metallic materials in the high strain-rate region, using a high-speed photography system. Numerical analysis of the Taylor bar impact test was performed using AUTODYN S/W. The results of the analysis were compared with the experimental results, and the material behavior in the high strain-rate region was discussed.

A Comparative Study of Computer Simulation using High-Speed Tensile Test Results with Actual Crash Test Results of DP Steels (복합조직강의 고속인장 결과를 이용한 컴퓨터 전산모사와 실제 충돌시험 결과와의 비교 연구)

  • Bang, Hyung Jin;Choi, Il Dong;Kang, Seong Geu;Moon, Man Been
    • Korean Journal of Metals and Materials
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    • v.50 no.12
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    • pp.873-882
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    • 2012
  • Dual Phase (DP) steel which has a soft ferrite phase and a hard martensite phase reveals both high strength and high ductility and has received increased attention for use in automotive applications. To conduct structural analysis to verify vehicle safety, highly credible experimental results are required. In this study, tensile tests were performed in a strain rate range from $10^{-4}/s$ to 300/s for Sink Roll-Less (SRL) hot-dip metal coated sheets. Collision properties were estimated through simulation by LS-DYNA using the stress-strain curve obtained from the tensile test. The simulation results were compared with the actual crash test results to confirm the credibility of the simulation. In addition, a tensile test and a crash test with 2% prestrain and a baking (PB) specimen were evaluated identically because automotive steel is used after forming and painting. The mechanical behaviors were improved with an increasing strain rate regardless of the PB treatment. Thus, plastic deformation with an appropriate strain rate is expected to result in better formability and crash characteristics than plastic deformation with a static strain rate. The ultimate tensile strength (UTS) and absorbed energy up to 10% strain were improved even though the total elongation decreased after PB treatment, The results of the experimental crash test and computer simulation were slightly different but generally, a similar propensity was seen.

The effect of temperature in high temperature SHPB test (고온 SHPB실험에서 온도의 영향)

  • Park, Kyoung-Joon;Yang, Hyun-Mo;Min, Oak-Key
    • Proceedings of the KSME Conference
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    • 2001.11a
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    • pp.349-354
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    • 2001
  • The split Hopkinson pressure bar has been used for a high strain rate impact test. Also it has been developed and modified for compression, shear, tension, elevated temperature and subzero tests. In this paper, SHPB compression tests have been performed with pure titanium at elevated temperatures. The range of temperature is from room temperature to $1000^{\circ}C$ with interval of $200^{\circ}C$. To raise temperature of the specimen, a radiant heater which is composed of a pair of ellipsoidal cavities and halogen lamps is developed at high temperature SHPB test. There are some difficulties in a high temperature test such as temperature gradient, lubrication and prevention of oxidation of specimen. The temperature gradient of specimen is affected by the variation of temperature. Barreling occurred at not properly lubricated specimen. Stress-strain relations of pure titanium have been obtained in the range of strain rate at $1900/sec{\sim}2000/sec$ and temperature at $25^{\circ}C{\sim}1000^{\circ}C$.

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Development of a New LCF Life Prediction Model of 316L Stainless Steel at Elevated Temperature (316L 스테인리스 강의 고온 저주기 피로 수명식 개발)

  • Hong, Seong-Gu;Lee, Soon-Bok
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.26 no.3
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    • pp.521-527
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    • 2002
  • In this paper, tensile behavior and low cycle fatigue behavior of 316L stainless steel which is currently favored structural material for several high temperature components such as the liquid metal cooled fast breeder reactor (LMFBR) were investigated. Research was performed at 55$0^{\circ}C$, $600^{\circ}C$ and $650^{\circ}C$ since working temperature of 316L stainless steel in a real field is from 40$0^{\circ}C$ to $650^{\circ}C$. From tensile tests performed by strain controls with $1{\times}10^{-3}/s,\; l{\times}10^{ -4}/s \;and\; 1{\times}10/^{ -5}/ s $ strain rates at each temperature, negative strain rate response (that is, strain hardening decreases as strain rate increases) and negative temperature response were observed. Strain rate effect was relatively small compared with temperature effect. LCF tests with a constant total strain amplitude were performed by strain control with a high temperature extensometer at R.T, 55$0^{\circ}C$, $600^{\circ}C$, $650^{\circ}C$ and total strain amplitudes of 0.3%~0.8% were used and test strain rates were $1{times}10^{-2} /s,\; 1{times}10^{-3} /s\; and\; 1{times}10^{-4} /s$. A new energy based LCF life prediction model which can explain the effects of temperature, strain amplitude and strain rate on fatigue life was proposed and its excellency was verified by comparing with currently used models.

A Study on the Microstructures and High Temperature Tensile Properties of Ni-base Superalloy Melt-Spun Ribbons (Ni 기 초합금 급냉응고 리본의 미세구조와 고온 인장특성에 관한 연구)

  • Han, Chang-Suk
    • Journal of the Korean Society for Heat Treatment
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    • v.27 no.4
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    • pp.180-184
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    • 2014
  • In order to make clear relationship between high temperature tensile properties and fine microstructure of rapidly solidified cast-type Ni-base superalloys without heat treatment required for consolidation process, tensile test was carried out by changing strain rate from $5{\times}10^{-5}s^{-1}$ to $2{\times}10^{-2}s^{-1}$ and test temperature from $900^{\circ}C$ to $1050^{\circ}C$ using IN738LC and Rene'80 melt-spinning ribbons by twin roll process which were superior to ribbons by single roll process from the viewpoint of structure homogeneity. The dependence of tensile strength on strain rate and test temperature was studied and strain rate sensitivity, m, were estimated from tensile test results. From this study, it was found that tensile strength was influenced by ${\gamma}^{\prime}$ particle diameter, test temperature and strain rate, and m of ribbons exhibited above 0.3 over $950^{\circ}C$.

An Criterion to Minimize FE Mesh-Dependency in Concrete Plate under Impact Loading (충격하중을 받는 판형콘크리트 구조물의 요소의존성 최소화 기준식)

  • Kwak, Hyo-Gyoung;Gang, Han-Gul;Park, Lee-Ju
    • Journal of the Computational Structural Engineering Institute of Korea
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    • v.27 no.3
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    • pp.147-154
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    • 2014
  • In the context of an increasing need for safety in concrete structures under blast and impact loading condition, the behavior of concrete under high strain rate condition has been an important issue. Since concrete subjected to impact loading associated with high strain rate shows quite different material behavior from that in the static state, several material models are proposed and used to describe the high strain rate behavior under blast and impact loading. In the process of modelling high strain rate conditions with these material models, mesh dependency in the used finite element(FE) is the key problem because simulation results under high strain-rate condition are quite sensitive to applied FE mesh size. This paper introduces an criterion which can minimize the mesh-dependency of simulation results on the basis of the fracture energy concept, and HJC(Holmquist Johnson Cook) model is examined to trace sensitivity to the used FE mesh size. To coincide with the purpose of the perforation simulation with a concrete plate under a projectile(bullet), the residual velocities of projectile after perforation are compared. The analytical results show that the variation of residual velocity with the used FE mesh size is quite reduced and accuracy of simulation results are improved by applying a unique failure strain value determined according to the proposed criterion.

Precipitation and Recrystallization of V-Microalloyed Steel during Hot Deformation (V 첨가강의 고온변형시 석출 및 재결정에 관한 연구)

  • 조상현;김성일;유연철
    • Proceedings of the Korean Society for Technology of Plasticity Conference
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    • 1996.03b
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    • pp.48-54
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    • 1996
  • The continuous deformation , multistage deformation and stress relaxation were carried out to investigate the strain induced procipitation by torsion tests in the range of 1000∼800$^{\circ}C$, 0.05/sec∼5/sec for V-microalloyed steel. The starting temperature and time for the initiation of precipitation were determined by stress relaxation tests and the distribution of percipitates increased at higher strain rate and the mean size of precipitates was found to be about 50nm. The precipitation starting time decreased with increasing strain rate from 0.05/sec to 5 /sec and pre-strain. The effect of deformation conditions on the no-recrystallization temperature(Tnr) was determined in the multistage deformation with declining temerature. The Tnr decreased with increasing strain and strain rae. In the controlled rolling, grain refinement and precpitation 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.

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A Study of New Technique Development for Creep Evaluation of Heat Resistant Steel Weldment(I) (내열강 용접부의 크리프 평가 신기술 개발에 관한 연구)

  • 유효선;백승세;권일현;이송인
    • Journal of Welding and Joining
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    • v.20 no.6
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    • pp.30-30
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    • 2002
  • It has been reported that the creep characteristics on weldment which is composed of weld metal(W.M), fusion line(F.L), heat-affected zone(HAZ), and base meta(B.M) could be unpredictably changed in severe service conditions such as high temperature and high pressure. However, the studies done on creep damage in power plant components have been mostly conducted on B.M and not the creep properties of the localized microstructures in weldment have been thoroughly investigated yet. In this paper, it is investigated the creep characteristics for three microstructures like coarse-grain HAZ(CGHAZ), W.M, and B.M in X20CrMoV121 steel weldment by the small punch-creep-(SP-Creep) test using miniaturized specimen(l0×10×0.5mm). The W.M microstructure possesses the higher creep resistance and shows lower creep strain rate than the B.M and CGHAZ. In the lower creep load the highest creep strain rate is exhibited in CGHAZ, whereas in the higher creep load the B.M represents the high creep strain rate. The power law correlation for all microstructures exists between creep rate and creep load at 600℃. The values of creep load index (n) based on creep strain rate for B.M, CGHAZ, and W.M are 7.54, 4.23, and 5.06, respectively and CGHAZ which shows coarse grains owing to high welding heat has the lowest creep loade index. In all creep loads, the creep life for W.M shows the highest value.