Jun Hyub Park

Bio: Jun Hyub Park is an academic researcher. The author has contributed to research in topics: Beam (structure) & Tensile testing. The author has an hindex of 1, co-authored 3 publications receiving 6 citations.

Determination of tensile properties and residual stresses of Ni-Co thin films

Abstract: This paper reports tensile properties and residual stresses of Ni-Co thin films. To measure elastic (and plastic) properties, direct tensile tests using dog-bone type specimens are performed first. Assuming that residual stresses vary linearly through the film thickness, bending and membrane residual stress components are measured using cantilever beam and T-structure beam specimens, respectively. Averaged values of Young’s modulus, yield strength and tensile strength are found to be about 163GPa, 1,700MPa and 2,000MPa, respectively. The membrane and bending residual stress components are found to be about 825MPa and 47MPa, respectively.

Characterization of Mechanical Properties and Residual Stresses for Ni-Co Thin Films

Abstract: The present work characterizes mechanical (elastic and plastic) properties and residual stresses of a Ni-Co thin film having 10 m thickness. Mechanical properties are obtained from direct-tension testing together with elastic finite element analysis. Membrane and bending components of residual stresses are measured using micro-cantilever and T-structure tests, respectively. Introduction Measurement of residual stresses in thin films is quite important, as their effect on mechanical integrity of micro-electrical-mechanical components could be significant. A number of measurement methods are available [1-7], but a majority of methods deal with measuring averaged residual stresses, rather than thickness-variation of residual stresses in thin films. Although such thickness variation could be often insignificant due to the fact that the thickness is very small, residual stresses can vary along the thickness of thin films. Measuring thickness variations of residual stresses, however, are not straightforward. A sectioning method can be applied (see for instance Ref. [7]), but can be quite tedious. In this respect, a simple method to measure thickness variations is desirable. In designing (macroscopic) structures, a stresses categorization process from stresses in the thickness direction is popularly employed [8-10]. In the stresses categorization process, stresses in the thickness direction are decomposed into the primary, secondary and peak stresses. Primary stresses are ones resulting mechanical loads and dead weights. Secondary stresses include thermal and residual stresses. Both primary and secondary stresses are typically idealized as a sum of (constant) membrane and (linear) bending stress components. Remaining stresses are classified as the peak stress, which relaxes quickly with plastic deformation by definition. A message that can be learnt from conventional stresses categorization process is that determination of membrane and bending components of residual stresses would be sufficient from mechanical integrity point of view. This paper presents measurement of residual stresses in Ni-Co thin films. A linearly-varying residual stress across the film thickness is assumed. To measure its bending and membrane components, cantilever beam and T-structure beam specimens are used, respectively. Experiments Material and Specimen Fabrication A NiCo thin film is recently widely used for MEMS devices using LIGA processes. Fabrication processes of NiCo thin film specimens are schematically shown in Fig. 1. For tensile test, conventional “dogbone” tensile test specimens were fabricated. Furthermore, for residual stress analysis, specimens of T-type structure and cantilever beams (which will be discussed later in this paper) with various dimensions were also fabricated. A single-side

Characterization of mechanical properties and residual stresses for Ni- Co thin films

Abstract: The present work characterizes mechanical (elastic and plastic) properties and residual stresses of a Ni-Co thin film having 10 m thickness. Mechanical properties are obtained from direct-tension testing together with elastic finite element analysis. Membrane and bending components of residual stresses are measured using micro-cantilever and T-structure tests, respectively. Introduction Measurement of residual stresses in thin films is quite important, as their effect on mechanical integrity of micro-electrical-mechanical components could be significant. A number of measurement methods are available [1-7], but a majority of methods deal with measuring averaged residual stresses, rather than thickness-variation of residual stresses in thin films. Although such thickness variation could be often insignificant due to the fact that the thickness is very small, residual stresses can vary along the thickness of thin films. Measuring thickness variations of residual stresses, however, are not straightforward. A sectioning method can be applied (see for instance Ref. [7]), but can be quite tedious. In this respect, a simple method to measure thickness variations is desirable. In designing (macroscopic) structures, a stresses categorization process from stresses in the thickness direction is popularly employed [8-10]. In the stresses categorization process, stresses in the thickness direction are decomposed into the primary, secondary and peak stresses. Primary stresses are ones resulting mechanical loads and dead weights. Secondary stresses include thermal and residual stresses. Both primary and secondary stresses are typically idealized as a sum of (constant) membrane and (linear) bending stress components. Remaining stresses are classified as the peak stress, which relaxes quickly with plastic deformation by definition. A message that can be learnt from conventional stresses categorization process is that determination of membrane and bending components of residual stresses would be sufficient from mechanical integrity point of view. This paper presents measurement of residual stresses in Ni-Co thin films. A linearly-varying residual stress across the film thickness is assumed. To measure its bending and membrane components, cantilever beam and T-structure beam specimens are used, respectively. Experiments Material and Specimen Fabrication A NiCo thin film is recently widely used for MEMS devices using LIGA processes. Fabrication processes of NiCo thin film specimens are schematically shown in Fig. 1. For tensile test, conventional “dogbone” tensile test specimens were fabricated. Furthermore, for residual stress analysis, specimens of T-type structure and cantilever beams (which will be discussed later in this paper) with various dimensions were also fabricated. A single-side

Analysis of a laser assisted milling process with inclination angles

Abstract: LAM (Laser Assisted Machining) is an effective machining method that can achieve a machining process for materials, which are difficult to cut, and a significant reduction in machining costs. Recently many researches on micromachining of plates or cylinders in connection with LAM have been conducted. However, in laser assisted milling processes the heat sources of laser are irregularly varied according to shapes of workpieces. Also, there are few researches of laser assisted milling processes with inclination angles. In this study, a thermal analysis of the laser assisted milling process for inclination planes is performed as a basic research in milling researches. Regarding the workpiece with various inclination angles, a temperature profile analysis is performed using a moving heat source. In addition, the effects of preheating are verified through the thermal analysis and experiment performed in this study. By analyzing the obtained temperature profile, a proper laser power control method is proposed according to inclination angles. The results of this analysis can be used to estimate thermal deformations on workpieces and tools and to apply as a heat source analysis method in laser assisted milling processes.

Determination of Poisson’s ratio of a beam by time-average ESPI and Euler-Bernoulli equation

Abstract: In this research, a laser holographic technique is used to characterize the Poisson’s ratio of a cantilever beam. We used the principle that one can determine the Poisson’s of a material by means of its vibration characteristics which use the Euler-Bernoulli Equation to link with the specimen’s natural frequency of vibration. The Poisson’s ratio of the beam can be easily determined after the resonance frequencies of the bending and torsional vibration modes are measured by time-averaged electronic speckle pattern interferometry (TA-ESPI) with the advantages of non- destructive, non-contact and real-time measurement technique. The proposed technique is able to give high accurate Poisson’s ratio of thin materials through a simple experimental set-up and analysis.

Estimation of deformed laser heat sources and thermal analysis on laser assisted turning of square member

Abstract: Laser assisted machining (LAM) has difficulties in estimating temperature after applying a LAM process due to its very small heat input area, large energy and movement. In particular, in the case of laser assisted turning (LAT) process, it is more difficult to estimate the temperature after preheating because it has a shape of ellipse when a laser heat source is rotated. A prediction method and thermal analysis method for heat source shapes were proposed as a square shaped member was preheated. The temperature distribution was calculated according to the rotation of the member. Compared with the results of the former study, the maximum temperature of the calculation results, 1 407.1 °C, is 8.5 °C higher than that of the square member, which is 1 398.6 °C. In a LAT process for a square member, the maximum temperature is 1 850.8 °C. It is recognized that a laser power control process is required because square members show a maximum temperature that exceeds a melting temperature at around a vertex of the member according to the rotation.

Evaluation of Cutting Force and Surface Temperature for Round and Square Member in Laser Assisted Turn-Mill

Abstract: LAT (Laser Assisted Turning) is an effective machining method for difficult to cut materials. Especially, because of the characteristics of a significant reduction in machining costs and flexibility in its machining, the applications of LAT have been largely extended to various machining fields. However, Studies on LAT are still staying at the beginning of research and represent a limitation in which a LAT process was applied to round members only. According to increases in customized production of special purpose parts, the researches on LAT for clover or polygon section members are necessary. In this paper, a LATM (laser assisted turn-mill) process was proposed to complement LAT.

New non-contact measurement method of deformation at tensile test of thin film via digital image correlation technique

Abstract: Digital image correlation technique has been widely and effectively applied in the measurement of material properties. This paper presents a new method named Stride-Crawl Approaching Method by combining the steepest descent method and the Levenberg-Marquardt method. It is no longer necessary to provide initial guess of deformation by applying this method. And, the result can always converge to the absolute optimum. Moreover, regardless of the deformations are large or small, this method can always be applied directly. Therefore, this method is more suitable and has great advantages compared to other conventional methods. The proposed method was tested two different simple deformation forms including the rigid body translation, and the uniform tensile elongation by comparing the experimental values with the theoretical values. The images for the verification were generated by numerical simulation. And then the errors were analyzed and it turned out that the error of the displacements was about pixels and the error of displacement gradients was usually limited in 3%. Finally, the stress-strain curves of BeCu was drawn using the output data of the proposed method. And then the tensile properties of BeCu including the elastic modulus, 0.2% yield strength, ultimate tensile strength, and fracture strain were obtained from the stress-strain curves.