• Proceedings of the Korean Society of Precision Engineering Conference
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• 1992.10a
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• pp.153-157
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• 1992

여러 가지 공학 재료들의 기계적인 특성이나 물리적인 특성들은 하중, 온도, 환경과 같은 재료들이 사용되는 여러 가지 조건들에 적합하게 대응되어 결정되어야 한다. 탄화텅스텐계의 초경합금은 내충격성, 내마모성, 내식성, 고항절력등 공구에 필요한 요구사항을 만족시키기 때문에 여러 가지 공구용으로 많이 사용되어 왔다. 본 연구에서는 충격시험기의 텁에스트레인 게이지를 붙여 회로를 구성한 후, 충격실험에서 얻게 되는 최대점의 하중신호를 정적보정 및 동적보정을 사용하여 하중값으로 환산하여 동적 균열개시인성치를 구하였다.

• Composites Research
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• v.16 no.4
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• pp.44-50
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• 2003

The damage zone around a crack tip occurring before the fracture is a significant domain. which affects the toughening mechanism of materials. In this study. the growth process of damage zone in the vicinity of crack tip for rubber-modified epoxy resin is investigated using an acoustic emission(AE) analysis. The weight fractions of rubber(CTBN 1300$\times$B) in rubber-modified epoxy resin are 5 wt% and 15 wt%. The fracture toughness($K_{IC}$) and the fracture energy($G_{IC}$) were measured using 3 point bending single-edge notched specimens. The damage zone and rubber particles distributed around the crack tip were observed by a polarized optical microscope and an atomic force microscope(AFM). The damage zone around crack tip of rubber-modified epoxy resin was formed at 13 % loading and developed until 57 % loading of the fracture load. The crack initiated at 57 % loading grew repeatedly in the stick-slip propagation behavior. Based on time-frequency analysis, it was confirmed that AE signals with frequency bands of 0.15~0.20 MHz and 0.20~0.30 MHz were generated from cavitation and stable/unstable cracking inside the damage zone.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1992.03a
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• pp.47-58
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• 1992

탄소성 재료의 파괴에서 최대하중을 계산할 수 있는 방법이 제시되었다. 사용된 재 료상수는 파괴개시인성, 항복강도, 진행된 균열선단에서 열림 변위 증분에 대한 균열성장의 비이고, 계산을 실험결과에 맞추어 재료상수를 결정한다. 이들 상수로 다른 시편의 최대하 중을 계산하는 간단한 방법을 평면변형하의 A572 강(상온)과 4533(B)강(-10C)에 적용하여 기존의 실험결과와 비교하였다. 또한 균열 선단 열림 변위와 J-적분값에 기초한 다른 방법 과 비교·논의되었다.

• Composites Research
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• v.14 no.5
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• pp.46-53
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• 2001

In this research, damage behavior of singly oriented ply (SOP) fiber metal laminate (FML) subjected to concentrated load was studied. The static indentation tests were conducted to study fiber orientation effect on damage behavior of FML. During the static indentation tests, acoustic emission technique (AE) was adopted to study damage characteristics of FML. AE signals were obtained by using AE sensor with 150kHz resonance frequency and the signals were compared with indentation curves of FML. The damage process of SOP FML was divided by three parts, i.e., crack initiation, crack propagation, and penetration. The AE characteristics during crack initiation show that the micro crack is initiated at lower ply of the plate, then propagate along the thickness of the plate with creating tiber debonding. The crack grow along the fiber direction with occurring 60∼80dB AE signal. During the penetration, the fiber breakage was observed. As fiber orientation increases, talc fiber breakage occurs more frequently. The AE signal behaviors support these results. Cumulative AE counts could well predict crack initiation and crack propagation and AE amplitude were useful for the prediction of damage failure mode.

• Journal of the Korea Concrete Institute
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• v.13 no.2
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• pp.161-167
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• 2001

It is difficult to obtain accurate fracture toughness values using three point bending test(TPB) proposed by RILEM committees because the shape of load-deflection curve is irregular and final crack propagation occurs after some slow stable cracking. However, fracture toughness is easily obtained from crack initiation load in the disk test. In this paper, the fracture properties of high strength concrete disks with center-crack was investigated. For this purpose, the experimental results were compared with the results by finite element analysis(FEA). And the experimental fracture locus was compared with theoretical fracture locus. Also, the results of fracture properties for the degree of concrete strength are presented. It is concluded from this study that results from FEA with maximum stress theory were compared well with the results from experiment. And the degree of concrete strength was contributed to the crack initiation load and fracture toughness, but was not contributed to the failure angle. Also, The discrepancy of fracture locus between the maximum stress theory and the experiment for concrete is considered to depend upon a large energy requirement for inducing the mixed-mode and sliding mode fractures.

• Tunnel and Underground Space
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• v.14 no.5
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• pp.381-390
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• 2004

In this study, scaled model tests were performed to investigate the deformation behaviors around twin tunnels. Eleven types of test models which had respectively different pillar widths, rock types and loading conditions were mode, where the modelling materials were the mixture of sand, plaster and water. The models with shallower pillar width were cracked under lower pressure than the models with thicker pillar width, and they showed the more tunnel convergences and the clear spatting failures. The models of hard rock were cracked under 50% higher pressure than the models of soft rock and they showed the less tunnel convergences. The failure and deformation behaviors of twin tunnels were also dependent on the loading conditions of models. Futhermore, the results of FLAC analysis were qualitatively coincident with the test results.

• Journal of Korean Tunnelling and Underground Space Association
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• v.3 no.2
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• pp.23-32
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• 2001

Rock masses are usually discontinuous in nature, as a result of various geological processes they have underdone and they contain rock joints and bridges. Crack propagation and coalescence processes mainly cause rock failures in tunnels. In this study, we focused on the crack initiation, propagation and coalescence process of rock materials containing two pre-existing open cracks arranged in different geometries. During uniaxial compression, wing crack initiation stress, wing crack propagation angle, and crack coalescence stress of Diastone gypsum and Yeosan Marble specimens were examined. And crack initiation, propagation, and coalescence processes were observed. Shear, tensile and mixed (shear+tensile) types of crack coalescence occurred. To compare the experimental results with Ashby & Hallam model, crack coalescence stress was normalized and it generally agreed with the experimental results.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.1
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• pp.112-118
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• 1990

Catastrophic fracture cannot be avoided after cracks(initiated from pre-existing defects) propagate rapidly with speeds comparable to a sound wave velocity of the materials. Preventing catastropic failure, crack arrest fracture toughness defined from dynamic(or kinetic) fracture mechanics point of view has been introduced in determining accurate and/or proper crack arrest fracture toughness of a material. For the past decades, many studies have been carried out to render proper theoretical and experimental backgrounds on the use of the static plain strain crack arrest fracture toughness, $K_{1a}$ (which seems to be a material property). $K_{1a}$ has been used to predict the performance of thick walled structures and has been considered as a measure of the ability of a material to stop a fast running crack. Determination of such a material property is of prime importance to the nuclear reactor pressure vessel and bridge materials industries. However, standards procedures for measuring toughness associated with fast running cracks are yet to exist. This study intends to give insight on the determination of the crack arrest fracture toughness of materials such as polymethylmethacrylate(PMMA), SM45C-steel, and A1 7075-T6. The effects of crack jump lengths and fast crack initiation stress intensity factor on the determination of $K_{1a}$ have been experimentally observed.erved.

• Journal of the Korean Recycled Construction Resources Institute
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• v.11 no.4
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• pp.476-483
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• 2023

The present study concerns modelling the structural behaviour for concrete structure into the crack initiation at corrosion of steels. The degradation source included the axial load and steel corrosion. A development of the rust formed on the steel surface was considered with the interfacial gap between steel and concrete. As a result, the tensile damage could occur on the surface of concrete into the cracking with no steel corrosion, which could be further developed by the increasing rust formation, while the cracking at the steel-concrete interface was mainly attributed to the compressive deformation, being restricted within the interfacial zone.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.11
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• pp.1257-1264
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• 2014

Efforts are presently underway for developing an optimal design methodology for GEN-IV nuclear reactors based on target failure probabilities. A typical example is the system-based code, in which the results are represented in the form of partial safety factors (PSFs). Thus, a PSF is one of the crucial elements in either component design or integrity assessment based on target failure probabilities during the operation period. In the present study, a procedure for calculating the PSF of a circumferential through-wall cracked pipe based on the elastic-plastic crack initiation criterion is established, in which the importance of each input variable is assessed. Elastic-plastic J-integrals are calculated using the GE/EPRI and reference stress methods, and the PSF values are calculated using both first- and second-order reliability methods. Moreover, the effect of statistical distributions of assessment variables on the PSF is also evaluated.