do Lee C.,Inha Technical College
Metals and Materials International | Year: 2014
The aim of the present study was to investigate the contribution of the strain rate to the transition of elastic-plastic deformation behavior and the difference between the fatigue strength coefficient and monotonic tensile strength in terms of the modified Basquin’s relation, which considers microporosity variation. The transition phenomena of elastic-plastic deformation were evaluated through comparison of the overall contour of the hysteresis loops measured in high cycle fatigue tests of a low-pressure die-cast A356 alloy. The increase of the alternating stress amplitude at a given excitation frequency causes a variation in the strain rate per unit time interval that depends fundamentally on the excitation frequency. In addition, the transition of elastic-plastic deformation behavior is induced by variation of strain rate in high cycle fatigue test, i.e., typically, a variation of the elastic modulus and extension of the elastic deformation region, compared with monotonic deformation under a very slow strain rate. The dependence of the elastic modulus on the strain rate due to the variation of the stress amplitude can be described in an exponential form of strain rate. The modified Basquin’s equation which includes the contribution of the strain rate and microporosity to the fatigue strength coefficient and fatigue life was re-established, including the dependence of the elastic modulus and transition of elastic-plastic deformation on the variation of the strain rate. © 2014, The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht.
Lee C.D.,Inha Technical College
Materials Science and Engineering A | Year: 2010
The variability in the tensile elongation of squeeze-cast Al-10%Si-2%Cu-0.4%Mg alloy was investigated in terms of the defect susceptibility to the effective void area fraction, which is the sum of the additional void area caused by damage evolution of eutectic Si particles and the void area of pre-existing microvoids. Additional theoretical verification was performed by constitutive prediction. The tensile elongation of as-cast and T6-treated alloys is described as a power law relationship to the variation of the effective void area fraction. The anisotropic fracture damage of eutectic Si particles has a significant contribution to the total effective void area fraction when compared with the variation of load-bearing capacity by microporosity. The constitutive model can precisely predict the defect susceptibility of tensile elongation to the variation of effective void area fraction. Additionally, the model suggests that the tensile deformation of Al-10%Si-2%Cu-0.4%Mg alloy is affected by the maximum effective void area, which corresponds with extremes of the distribution frequency, rather than the average value of the microstructural characteristics, especially over a wide-range distribution of aspect ratio of Si particles. © 2010.
Lee C.D.,Inha Technical College
Metals and Materials International | Year: 2010
The variability in the tensile strength of as-cast AM60 and AZ91 alloys was investigated in terms of the defect susceptibility to the variation in grain size and microporosity. The microporosity was measured from the quantitative fractography analysis through scanning electron microscopy (SEM) observation on fractured surface after tensile test. The ultimate tensile strength (UTS) of both alloys can be characterized as a power law relationship to microporosity variation in terms of the defect susceptibility and maximum strength achievable in the defect-free condition. The defect susceptibility of tensile strength to microporosity variation is decreased remarkably with grain refinement. The defect susceptibility of AZ91 alloy to microporosity variation exhibits more sensitive dependence on the variation in grain size than AM60 alloy. Also, the dependence of UTS on the variation in grain size is described as a power law relationship for various levels of microporosity. The variation on effective void area fraction by the damage evolution of Mg17Al12 phase may introduce a practically significant decrease of load bearing capacity, less than by microporosity variation. The Hall-Petch relation of both alloys in the defect-free condition could be suggested as maximum values of friction stress and locking parameter. ©KIM and Springer.
Lee J.-I.,Inha Technical College
International Journal of Precision Engineering and Manufacturing | Year: 2012
This paper addresses the prediction of the distributed material properties of a microstructure. Predictions were made by measuring the dynamic response of fabricated materials used in the microstructures. When these distributed material properties are used to estimate the mechanical performance of microstructures, differences between computer simulations and the experimental results can be reduced, and reliable design can be developed. The distributed material properties of a micro-electromechanical system(MEMS)-based gyroscope were obtained numerically using the proposed method and experimentally using dynamic testing. The natural frequency of the numerical analysis was slightly higher (first mode: - 0.43 Hz, second mode: 0.33 Hz) than the result from the dynamic testing because some properties such as the microstructure porosity were neglected. © KSPE and Springer 2012.
Lee S.,Inha Technical College |
Lee B.-J.,Namseoul University
Surface and Coatings Technology | Year: 2012
In the present work, we report the formation of residual oxide layer during chemical-mechanical-planarization (CMP) process in the carbon nanotube (CNT) via interconnects and some feasible solutions for its removal. Residual oxide layer makes electrically poor contact between CNTs and metal resulting in high contact resistance in CNT via interconnects. We adopt post-CMP processes such as hydrofluoric acid (HF) or Ar plasma treatment to remove the residual oxide layer. X-ray photoelectron spectroscopy (XPS) was used to confirm the chemical state of samples before and after the post-CMP process. Silicon and oxygen peaks from silicon-based oxide layer observed after the CMP process were disappeared and reduced in its intensity by the post-CMP process, respectively. Furthermore, via resistance decreased more than 1 order of magnitude after the post-CMP process. It is found that the post-CMP process provides good electrical contact between CNTs and metal by removing the residual oxide layer. © 2011 Elsevier B.V.