• Geomechanics and Engineering
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• v.12 no.1
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• pp.1-8
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• 2017

An experimental technique for determining the spalling strength of rock-like materials under a high strain rate is developed. It is observed that the spalling strength of a specimen can be determined by only knowing the wavelength, loading peak value and length of the first spallation of an incident wave under a specific loading waveform. Using this method in combination with a split-Hopkinson pressure bar (SHPB) and other experimental devices, the spalling strength of granite specimens under a high strain rate is tested. Comparisons with other experimental results show that the new measuring method can accurately calculate the dynamic tensile strength of rock materials under a high strain rate.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1999.03b
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• pp.135-139
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• 1999

The static softening behavior of AISI 4140 could be characterized by the hot torsion test in the temperature ranges of 10$0^{\circ}C$~120$0^{\circ}C$ and strain rate ranges of 0.05/sec~5/sec. Deformation efficiency which was based on dynamic materials model was calculated from flow stress curves obtained continuous deformation. Interrupted deformation was performed with 2 pass deformation in the pass strain ranges of 0.25{{{{ epsilon _p}}}} ~3{{{{ epsilon _p}}}} and interrupted time ranges of 0.5~100sec. The dependences of process variables pass strain ({{{{ epsilon _i}}}}) stain rate ({{{{ {. } atop {$\varepsilon$ } }}}}) temperature (T) and interpass time ({{{{ {t }_{i } }}}}) on static recrystallization (SRX) and metadynamic recrystallization .(MDRX) could be indicidually predicted from the modified Avrami's equations. Comparison of the softening kinetics between MDRX and SRX showed that the rate of MDRX was more rapid than that of SRX for the same deformation variables. Controlled multipass deformations were performed using deformation efficiency static and metadynamic recrystallization of AISI 4140.

• Journal of Mechanical Science and Technology
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• v.14 no.5
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• pp.483-489
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• 2000

The purpose of this work is to investigate fatigue damage of quasi-isotropic laminates under tensile loading in different directions. Low cycle fatigue tests of $[0/-60/60]_s$ laminates and $[30/-30/90]_s$ laminates were carried out. Material systems used are AS4/Epoxy and AS4/PEEK. The fatigue damage of $[30/-30/90]_s$ is very different from that of $[0/-60/60]_s$. The experimental results are compared with the result obtained from the method for determining strain energy release rate components proposed by the authors. The analytical results were in good agreement with the experimental results. It is proved that the failure criterion based on the strain energy release rate is an appropriate approach to predict the initiation and growth of delaminations under cyclic loading.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.28 no.3
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• pp.295-305
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• 1992

The critical strain energy release rate and the failure mechanisms of glass-carbon epoxy resin hybrid composites are investigated in the temperature range of the ambient temperature to 8$0^{\circ}C$. The direction of laminates and the volume fraction are [(+45, -45, 0, 0) sub(2) ] sub(s), 50%, respectively. The major failure mechanisms of these composites are studied using the scanning electron microscope for the fracture surface. Results are summarized as follows: 1) The critical strain energy release rate shows a maximum at ambient temperature and it tends to decrease as temperature goes up. 2) The critical strain energy release rate increases as the content of glass increases, and especially shows dramatic increase for the high glass fiber content specimens. 3) Major failure mechanisms can be classfied such as localized shear yielding, fiber-matrix debonding, matrix micro-cracking, and fiber pull-out and/or delamination.

• Journal of the Korea Institute of Military Science and Technology
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• v.20 no.4
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• pp.520-527
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• 2017

In this study, the experimental methods are compared for obtaining the parameters of the Johnson-Cook constitutive model. The parameters used for numerical simulation are very important in making an accurate estimation of numerical simulation. So, the testing method of obtaining the parameters is also very important. We compared the difference of conventional method, compression method and tensile method of AISI-4340 steel at various strain rate by using MTS, SHPB and SHTB. Taylor impact test and M&S were carried out to compare differences among these three types of JC constitutive parameter.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.4
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• pp.494-502
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• 1997

Experiments on corresponding jet flames with stagnant point diffusion flames have been carried out in initial injection periods. A compensated measurement of maximum flame temperature, which is based on the ion signal, has been employed to inspect flame responses to time-varying strain rates. The flame responses are obtained at two conditions for the slowly time-varying strain rate and the case of flame extinction, and analyzed to confirm similarity between a stagnant point diffusion flame and an evolving jet diffusion flame. Nonsteady effects are addressed via the comparison between several time scales. The time variation with low strain rates, in which illustrates the flame behavior of the upper branch far from extinction in the well-known S-curve, is confirmed to produce a quasi-steady flame response through the nonsteady experiments. The time variation with strain rates in the case of flame extinction indicates an unsteady effect of flame response. It is therefore found that the flame responses near jet tip depend on time histories of characterized strain rates in the developing process.

• Clinical and Experimental Pediatrics
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• v.53 no.1
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• pp.72-79
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• 2010

Purpose: Left ventricular (LV) hypertrophy and impaired diastolic function may occur early in systemic hypertension. Diastolic dysfunction is associated with increased cardiovascular risk. Tissue Doppler imaging (TDI)-derived tissue velocity and strain rate are new parameters for assessing diastolic dysfunction. The aim of this study is to determine whether TDI and strain rate imaging (SRI) would improve the ability to recognize early impaired diastolic and systolic functions compared with conventional echocardiography in hypertensive adolescents. Methods: We included 38 hypertensive patients with systolic blood pressure above 140 mmHg or diastolic blood pressure above 90 mmHg. Ejection fraction and myocardial performance index (MPI) were estimated by conventional echocardiography. Peak systolic myocardial velocity, early diastolic myocardial velocity (Em), and peak late diastolic myocardial velocity (Am) were obtained by using TDI and SRI. Results: In the hypertensive group, interventricular septal thickness was significantly increased on M-mode echocardiography. Em/Am was significantly decreased at the mitral valve annulus. Among hypertensive subjects, the E strain rate at basal, mid, and apex was significantly decreased. Systolic strain was significantly decreased at the septum in the hypertensive group. Conclusion: Strain rate might be a useful new parameter for the quantification of both regional and global LV functions and could be used in long-term follow up in hypertensive patients. Early identification by SRI of subjects at risk for hypertensive and ventricular dysfunction may help to stratify risk and guide therapy. Further studies, including serial assessment of LV structure and function in a larger number of adolescents with hypertension, is necessary.

• Journal of the Korean Ceramic Society
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• v.29 no.4
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• pp.312-318
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• 1992

Two kinds of alumina specimens with different grain size (1 and 51 $\mu\textrm{m}$) but same density were prepared by hot-pressing. Fracture strength and fracture toughness of these specimens at low strain rate, sonic velocity, and elastic property were evaluated. Ballistic performance against Cal. 50 AP projectile was characterized by thick-backing method by using A16061-T6 reference block. Mechanical properties measured at low strain rate showed that the specimen with samll grain (SG) were better than specimen with large grain (LG). Fracture strength and fracture toughness of LG specimen were 131 MPa and 3.01 MPa{{{{ SQRT { m} }}, but those of SG specimen were 349 and 4.23, respectively. Sonic velocity and elastic properties of these specimen were similar, but bulk velocity and bulk modulus were different at amount of 4 and 9%. The tendency of ballistic performance was not consistent with the mechnaical properties at low strain rate. The ballistic performance based on quantitative efficiency revealed that the LG specimen (5.13) was ballistically better than the SG specimen (4.00) in spite of their lower mechanical properties.

• Journal of the Korean Society of Combustion
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• v.16 no.4
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• pp.31-37
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• 2011

This paper presents both experimental and numerical investigation of the combustion characteristics of stretched premixed lift-off flames using synthetic gas($H_2$/CO) in an impinging burner. We used "Spin code" for numerical analysis. An ICCD camera was employed to measure flame location and flame thickness. The impinging surface temperature was affected by local strain rate K, equivalence ratio, and composition ratio of fuel. In spite of the difference of boundary conditions in experimental and numerical results, the tendencies of surface temperatures were agreed. From result of this work, we also found that flame location and flame thickness directly related to surface temperature are greatly affected by local strain rate K.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.12
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• pp.2072-2078
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• 2003

In this paper, the boundary element analysis of viscoelastic strain energy release rate G(t) for the cracked linear viscoelastic solids has been attempted. This study proposes the G(t) equation and the calculating method of G(t) by time-domain boundary element analysis for the viscoelastic solids. The G(t) is defined as the derivative of the viscoelastic potential energy II(t) with respect to crack length a. Two example problems are presented to show the applicability of the proposed method to the analysis of the cracked linear viscoelastic solids. Numerical results of example problems show the accuracy and effectiveness of the proposed method.