• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.7
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• pp.1058-1072
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• 1997

A head-neck complex dummy, for measuring brain pressure and reaction force in the cervical spine was developed for experimental study related in injury mechanism. Dummy comprised aluminium-casted head with water filled cavity for simulating brain and mechanical neck assembled with six motion segments. Several kinds of experiments (compression, bending, cyclic modulus, relaxation and constant velocity profile) for the developed mechanical neck showed that this neck model is biomechanically reliable compared with in-vitro test results. As an application of developed head-neck complex dummy, shock absorbing properties of protective helmet was chosen. The experiments showed that the maximum pressure increment of brain after impact was tolerable compared with the guide line for mild brain injury pressure (25psi). Constrast to this results, the reaction force in the neck was high enough to produce failure in the cervical spine.

• International Journal of Aeronautical and Space Sciences
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• v.13 no.4
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• pp.458-467
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• 2012

In this study, the linear viscoelastic response of a rectangular laminated plate is investigated. The viscoelastic properties, expressed by two basic spring-dashpot models, that is Kelvin and Maxwell models, is assumed in the range to investigate the influence of viscoelastic coefficients to mechanical behavior. In the present study, viscoelastic responses are performed for two popular equivalent single-layered theories, such as the first-order shear deformation theory (FSDT) and third-order shear deformation theory (TSDT). Compliance and relaxation modulus of time-dependent viscoelastic behavior are approximately determined by Prony series. The constitutive equation for linear viscoelastic material as the Boltzmann superposition integral equation is simplified by the convolution theorem of Laplace transformation to avoid direct time integration as well as to improve both accuracy and computational efficiency. The viscoelastic responses of composite laminates in the real time domain are obtained by applying the inverse Laplace transformation. The numerical results of viscoelastic phenomena such as creep, cyclic creep and recovery creep are presented.

• Journal of the Korean Applied Science and Technology
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• v.32 no.3
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• pp.386-393
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• 2015

The rheological properties of complex materials such as colloid dispersion show complicated non-Newtonian flow phenomena when they are subjected to shear flow. These flow properties are controlled by the characteristics of flow units and the interactions among the flow segments. The rheological parameters of relaxation time $({\beta}_2)_0$, structure factor $C_2$ and shear modulus $X_2/{\alpha}_2$ for various thixotropic flow curves was obtained by applying thixotropic equation to flow curves. The variations of rheological parameters are directly related to non-Newtonian flows, viscosities and activation energies of flow segments.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.10 no.2
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• pp.49-58
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• 1990

A numerical procedure is developed to analyze the time-dependent deflections of prestressed concrete bridges constructed by the segmental cantilever method. The developed computer program accounts for the time-dependent properties of prestressed concrete materials due to the varying modulus of elasticity, creep and shrinkage of concrete and the stress relaxation of prestressing steel. It also accounts for the stiffness increase due to the presence of the steel reinforcements and the effects of the shear deformation of the prestressed concrete bridge girders. The program is applied to a multi-span continuous segmental prestressed concrete bridge to demonstrate its capabilities and to explore the behavior characteristics of the segmental bridges.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.354.1-354.1
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• 2014

Hafnium nitride (HfN) 박막은 고온에서의 안정성과 낮은 비저항 그리고 산소확산에 대한 억제력을 가지고 있기 때문에 확산방지막으로 많은 연구가 진행 되고 있다. 현재까지 진행된 대부분의 연구는 HfN 박막의 전기적인 특성과 구조적인 특성에 대한 것이었고 다양한 연구 결과가 보고되었다. 하지만 기존의 연구들은 박막의 nano-electrotribology 특성에 대한 연구가 부족하여 박막 적층 공정시 요구되는 물성에 대한 연구가 절실하다. 따라서 본 연구에서는 HfN 박막의 증착조건 및 열처리조건에 따른 nano-electrotribology 특성 변화를 확인하고자 하였다. HfN박막은 rf magnetron sputter를 이용하여 Si 기판위에 Hf target으로 질소 유량을 변화시키며 증착하였고 가열로에서 $600^{\circ}C$와 $800^{\circ}C$로 20분간 열처리를 실시하였다. 열처리한 박막과 as-deposited 상태의 박막을 nano-indenter를 통하여 나노기계 전기적인 특성을 분석하였다. nano-indenter는 박막에 인가된 stress와 탄성계수(elastic modulus), 표면경도(surface hardness)와 같은 특성을 직접적인 tip 접촉을 통하여 in-situ로 분석할 수 있는 장비이다. 실험결과 HfN박막을 $600^{\circ}C$로 열처리 한 경우 표면경도가 16.20에서 18.59 GPa로 증가하였다. 표면경도의 증가는 열처리 시 박막내에 compressive stress가 생성되었기 때문이라고 생각된다. 그러나 $800^{\circ}C$로 열처리 한 경우 표면경도가 16.93 GPa로 감소하였는데 이는 표면균열 발생으로 인한 stress relaxation 때문인 것으로 생각된다. 증착 시 주입되는 질소의 유량과 열처리 온도는 HfN박막의 기계적 안정성에 영향을 미치는 중요한 요소임을 본 실험을 통해 확인하였다.

• Journal of Korean Association for Spatial Structures
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• v.10 no.3
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• pp.57-64
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• 2010

Uniaxial tensile tests of ETFE membrane are performed in this paper. Three kinds ETFE membrane with different thickness are used in the tests. The tensile strength, the tensile strain at break and the stress-strain curve are obtained from the tests. Futhermore, The cycle loading test of ETFE membrane is carried out through using different values of cycle stress. The residual strain, the relaxation of stress and the change of the elastic modulus of foil are investigated. In the creep test, three kinds of temperature (25, 40 and 60 $^{\circ}C$)and three kinds of stress(3,6and9 MPa) are set respectively and the creep time lasts 24 hours.

• Macromolecular Research
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• v.9 no.3
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• pp.129-142
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• 2001

To efficiently demonstrate the molecular motion, physical properties, and mechanical properties of polycarbonates, we studied the differences between bisphenol-A polycarbonate(BPA-PC) and tetramethyl bisphenol-A-polycarbonate(TMBPA-PC) using molecular modeling techniques. To investigate the conformations of BPA-PC and TMBPA-PC and the effect of the conformation on mechanical properties, we performed conformational energy calculation, molecular dynamics calculation, and stress-strain curves based on molecular mechanics method. From the result obtained from conformational energy calculations of each segment, the molecular motions of the carbonate and the phenylene group in BPA-PC were seen to be more vigorous and have lower restriction to mobility than those in TMBPA-PC, respectively. In addition, from the results of radial distribution function, velocity autocorrelation function, and power spectrum, BPA-PC appeared to have higher diffusion constant than TMBPA-PC and is easier to have various conformations because of the less severe restrictions in molecular motion. The result of stress-strain calculation for TMBPA-PC seemed to be in accordance with the experimental value of strain-to-failure ∼4%. From these results of conformational energy calculations of segments, molecular dynamics, and mechanical properties, it can be concluded that TMBPA-PC has higher modulus and brittleness than BPA-PC because the former has no efficient relaxation mode against the external deformations.

• Journal of the Korea institute for structural maintenance and inspection
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• v.6 no.2
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• pp.183-194
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• 2002

A general procedure to evaluate the sensitivity of design variables to stresses and strains in PSC flexural members is proposed. To accomplish the purpose of this study, long-term losses including creep, shrinkage, and PS steel relaxation are formulated based on the equilibrium states of the deformed sectional geometry. Thereby, the formulation follows the basic steps which consider the fundamental formulas adopted by CEB-FIP, ACI, and KCI rather than the age adjusted effective modulus concept. Twenty-one design variable including the material and geometrical properties of concrete, nonprestressing steel and prestressing steel, and the geometry of the cross section are considered in the sensitivity analysis. The gradients of the stresses and strains needed for the sensitivity assessment are calculated in a closed format. The derived formulation is applied to the T-type section PSC beam with prestressing and nonprestressing steels for the sensitivity analysis. The analytically calculated sensitivity results are compared with those numerically calculated to ensure the validity of the proposed procedure.

• Journal of Theoretical and Applied Mechanics
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• v.1 no.1
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• pp.69-88
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• 1995

The paper attempts to estimate the incubation time of a cavity in the interface between a power law creep particle and an elastic matrix subjected to a uniaxial stress. Since the power law creep particle is time dependent, the stresses in the interface relax. Through previous stress analysis related to the present physical model, the relaxation time is defined by ${\alpha}$2 which satisfies the equation $\Gamma$0 ｜1+${\alpha}$2k｜m=1-${\alpha}$2 [19]. $\Gamma$0=2(1/√3)1+m($\sigma$$\infty$/2${\mu}$)m($\sigma$0/$\sigma$$\infty$tm) where $\sigma$$\infty$ is an applied stress, ${\mu}$ is a shear modulus of a matrix, $\sigma$$\infty$ is a material constant of a power law particle, $\sigma$=$\sigma$0 $\varepsilon$ and t elapsed time. the volume free energy associated with Helmholtz free energy includes strain energies associated with Helmholtz free energy includes strain energies caused by applied stress anddislocations piled up in interface (DPI). The energy due to DPI is found by modifying the results of Dundurs and Mura[20]. The volume free energies caused by both applied stress and DPI are a function of the cavity size(${\gamma}$) and elapsed time(t) and arise from stress relaxation in the interface. Critical radius ${\gamma}$ and incubation time t to maximize Helmholtz free energy is found in present analysis. Also, kinetics of cavity fourmation are investigated using the results obtained by Riede[16]. The incubation time is defied in the analysis as the time required to satisfy both the thermodynamic and kinetic conditions. Through the analysis it is found that [1] strain energy caused by the applied stress does not contribute significantly to the thermodynamic and kinetic conditions of a cavity formation, 2) in order to satisfy both thermodynamic and kinetic conditions, critical radius ${\gamma}$ decreases or holds constant with increase of time until the kinetic condition(eq.40) is satisfied. Therefore the cavity may not grow right after it is formed, as postulated by Harris[11], and Ishida and Mclean[12], 3) the effects of strain rate exponent (m), material constant $\sigma$0, volume fraction of the particle to matrix(f) and particle size on the incubation time are estimated using material constants of the copper as matrix.

• Elastomers and Composites
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• v.36 no.2
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• pp.111-120
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• 2001

Mechanical properties and their adhesion behavior with zinc- and brass-plated steel cords of natural rubber/acrylonitrile-butadiene blend compounds were investigated as a function of blend ratio. The Mooney viscosity and stress relaxation time were found to be lowered with increasing NBR content. Tensile modulus generally increased with increasing NBR content. Tensile stress at break stayed constant up to about 40 phr and showed minimum at $50{\sim}60 phr$, and thereafter increased with increasing NBR content. Strain at break decreased linearly below 50 phr, and above the level it showed nearly constant value. Based on the abrupt drops in elastic modulus and tan ${\delta}$ peak, the glass transition temperature of NR and NBR were found to be -55 and $-10^{\circ}C$, respectively. In the case of NR/NBR blend compounds, two distinct transition points were observed and each transition position was not affected by NBR level indicating an incompatible nature of NR/NBR blend system. The pullout force and rubber coverage decreased to the level of about 40% to that of pure m compound, when the 50 phr of NR was replaced by NBR. However, the pure NBR compound showed the comparable adhesion performance with NR(${\sim}90%$). The sulfur concentration was found to become lower with the increased NBR content at the adhesion interface based on the Auger spectrometer results, representing a lack of adhesion layer formation, and this was explained for a possible cause of low adhesion performance with adding NBR.