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권동일(Dongil Kwon),이경우(Kyung Woo Lee),김성훈(Sung-Hoon Kim),김주영(Ju-Young Kim),이윤희(Yun-Hee Lee) 한국정밀공학회 2005 한국정밀공학회 학술발표대회 논문집 Vol.2005 No.10월
The nanoindentation technique is widely used to investigate the mechanical properties of nano-microscale materials. The nanoindentation method for assessing mechanical properties at low loads and shallow depths is already well established for the characterization of thin films as well as bulk materials. In this study, we evaluated residual stress in DLC and Au thin films usign nanoindentation technique with a new stress-relaxation model. Moreover, We suggest a composite hardness equation and quantify the magnitude of hardness increase by using an equation based on the interface hardness and the interface thickness, derived by comparing results derived from this equation and those determined in nanoindentation tests. Finally, We present an indentation size effect (ISE) model that extends the available contact depth for ISE application down to several tens of nanometers by considering the tip bluntness effect.
Oh Min Kwon,Jiyeon Kim,Jinwoo Lee,Jong‑hyoung Kim,Hee‑Jun Ahn,Ju‑Young Kim,Young‑Cheon Kim,Dongil Kwon 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.10
We investigated the analytic relation between hardness and compressive yield stress using an expanding cavity model (ECM)for nanoporous gold (np-Au). We prepared three np-Au samples with ligament sizes 30.61, 59.36 and 116.33 nm by freecorrosiondealloying and post heat treatment. The indentation contact morphology was examined to estimate the hardnessaccurately from the nanoindentation load-depth curve. Unlike conventional dense metals, the deformation was confined to theprojected contact area, and the center of the residual impression was dominated by densification. The projected contact areaestimated by the Oliver–Pharr method was overestimated, so that a new contact area function was proposed that consideredthe indentation contact morphology of np-Au. It was confirmed that a hardness value taking into account the indentationcontact morphology of np-Au matches well with the hardness derived by direct measurement of the residual impression. We modeled the ratio of hardness to compressive yield stress for np-Au using an ECM. The scaling factors, which representthe extra strain-hardening in the core in the ECM, were analyzed for np-Au and dense metals. An ECM that better matchesnp-Au is suggested based on the scaling factor resulting from densification beneath the indenter.
Oh Min Kwon,Jongho Won,Jong‑hyoung Kim,Changhyun Cho,Eun‑chae Jeon,Dongil Kwon 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.6
We suggest a method for accurately estimating the uncertainty of indentation yield strength determined from the modifiedMeyer relation as a mathematical function of the measurement, taking into account Type A and Type B uncertainty. Usingthis method, we quantitatively compared the expanded uncertainty level of the yield strength as measured by instrumentedindentation testing (IIT) and uniaxial tensile testing, and propose a dominant measurand that affects the final expanded uncertaintyof the indentation yield strength. To interpret the difference in uncertainty between IIT and uniaxial tensile testing, weinvestigated the effect of the major sources of uncertainty in the IIT system: sample surface roughness and angular misalignmentbetween the surface normal of the sample and the symmetric axis of the indenter. The surface roughness was controlledusing 400-, 1000- and 2000-grit paper and the misalignment angle ranged over 0°, 1° and 2°. Acceptable surface roughnessand standard uncertainty of misalignment angle are proposed that give the IIT similar uncertainty to uniaxial tensile testing.
강구조물에 대한 현장에서의 비파괴적 인장물성 평가 기술 개발
손동일 ( Son Dongil ),최열 ( Choi Yeol ),장재일 ( Jang Jae-il ),권동일 ( Kwon Dongil ) 한국구조물진단유지관리공학회 2001 한국구조물진단유지관리공학회 학술발표대회 논문집 Vol.5 No.1
A newly developed Advanced Indentation System (AIS), which is a portable and nondestructive system for evaluating tensile properties, was used to measure stress-strain behavior of steel structure. For all test materials and conditions, the AIS-derived results were in good agreement with those from conventional standard test method. Furthermore, the nondestructive AIS test results rigorously indicated the various levels of degradation of materials. Examples of the test results are given and potential applications of the AIS to assess the integrity of aging structures are briefly discussed.
권오준,이성학,권동일,이영국 대한금속재료학회(대한금속학회) 1993 대한금속·재료학회지 Vol.31 No.5
The present study is concered with the processing of ultra-fine steels by thermo-mechanical processes and the grain refinement mechanism. Three steels were deformed at high strain rates using a pilot plant rolling mill, and exhibited good tensile properties and low temperature impact toughness, although they contained only about 0.01% carbon. Especially in the Ni added steel, room temperature tensile strength reached to 850 MPa, and Charpy impact energy at -120℃ showed 78J, indicating the potent effect of Ni addition. Although the microstructure was identified to be mainly acicular ferrite by optical microscopy, TEM microstructures of ultra-fine steels were composed of very fine and highly dislocated grains whose shapes were changed from elongated to equiaxed ones as alloying elements were added. The formation of this microstructure might be explained by a possible mechanism, i.e., a repeated dynamic strain-induced transformation mechanism. These findings suggest that ultra-low carbon steels with high strength and toughness can be processed by grain refining combined with microalloying.