• Geotechnical Engineering
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• v.13 no.2
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• pp.29-38
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• 1997

Strain softening is observed for geomaterials such as rocks when they are sheared. The proper computational modelling for strain softening is very important because this behavior is closely related to failure in geotechnical problems. In this paper, we have investigated the proper FEM techniques for modelling strain softening in order to simulate failure behavior numerically. In showing numerical examples, the effects of element shape, mesh pattern and of imperfection and the difference between small and large deformation theories, of displacement control and pressure control after peak have been discussed.

• Journal of Industrial Technology
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• v.19
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• pp.197-208
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• 1999

The process of localization of cracks and movement of the fracture process zone(FPZ) was studied using the acoustic-emission(AE) techniques. The rate of AE events and sources of AE activity were studied for mortar and rock specimens loaded in uniaxial compression. A series of transducers could be used to detect and AE activity. Based on the time differences between detection of the event at different transducers, source of AE activity could be detected. The rate of AE events increased sharply before peak load. The highest rate occurred just after peak load was attained. The effective crack length estimated from the modified linear-elastic fracture mechanics seemed consistent with the optical and AE measurements.

• Journal of the Korean Geotechnical Society
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• v.18 no.1
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• pp.29-39
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• 2002

In order to evaluate the behavior of overall range from small strain to failure, the triaxial compression tests with LVDTs were performed for local displacement measurements. According to the result it was possible to evaluate the total range behavior from 0.001% to 10% and both secant moduli of undisturbed and disturbed weathered soils had a similar result in the small slain level. The normalized shear moduli$(G/G_{max})$ in the undrained triaxial compression tests were similar to those of resonant column tests but the maximum shear moduli$(G/G_{max})$ were strongly affected by the ratio of saturation. As a result of parametric study a constitutive model with anisotropic hardening could predict the behavior of total strain range.

• Economic and Environmental Geology
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• v.27 no.5
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• pp.499-505
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• 1994

Local stress concentrations often cause new micro-damaging induced by a healed pre-existing defects, and the macro-damage is developed by propagation and coalescence of the micro-damage. The micro-damage causes non-linear deformation in rock material. Considerable work has also been applied to describe mathematically the behavior of cracks under stress. Although these mathematical models can usually be made to agree quite well with the measured data, but it is questionable how well the models describe real rock including microcracks in pre-failure state, such as their micro-damage mechanisms. In the present study, micro-damage initiation and propagation in granitic rock under increasing stress were observed directly. Furthermore, a stress analysis considering the bisphere model was carried out using the homogenization theory to analyze the mechanics of the stress-induced micro-damage.

• Journal of the Korean Society for Nondestructive Testing
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• v.11 no.1
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• pp.31-37
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• 1991

Acoustic emission(AE) signals detected during metal cutting were applied as the experimental test to sensing tool wear and chipping on the NC vertical milling machine. The in-process detection of cutting tool wear including chipping, cracking and fracture has been investigated by means of AE in spite of vibration or noise through intermittent metal cutting, then the following results were obtained 1) When the tool wear is increased suddenly, or the amplitude of AE signals changes largely, it indicates chipping or breaking of the insert tip. 2) It was confirmed that AE signal is highly sensitive to the cutting speed and tool wear. 3) At the early period of cutting, the wear were large and RMS value increased highly by the influence of minute chipping and cracking, etc. Therefore, the above situations should be considered for the time when the tool would be changed.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.21 no.6
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• pp.926-931
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• 2012

Rotary bar was cutting tool being in use for deburring scale after welding metal and for eliminating sharp edges. It was necessary to develop exclusive 6-axis machine being possible to process machining continuously for making an cutting edge of rotary bar and to revise easily machining program. This study aimed at the structural analysis of strength according to the configuration of new made 6-axis machine and at the machining accuracy and durability of rotary bar. As a result of test, the runout of new rotary bar manufactured out of 6-axis machine was decreased from 0.385 mm to 0.027mm extensively than old one. And good cutting ability and durability was obtained equal quality compared with imported products.

• Proceedings of the KIEE Conference
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• 2005.07c
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• pp.1896-1897
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• 2005

보이드, 이물, 돌기 둥 고분자 절연체 내의 결함은 이곳에 전계가 가해질 때, 국부적 전계집중을 유발하며, 절연체 내의 미소 방전을 일으킨다. 이 때 발생한 미소 방전에 의해 보이드 표면의 고분자 물질은 기계적, 화학적 열화를 일으키며, 이에 따라 전기적 트리를 형성하게 된다. 이러한 트리는 결국 절연체의 절연파괴로 진전되므로, 고분자 물질을 절연물로 사용하는 전력기기의 경우에는 신뢰성에 큰 문제를 유발한다. 따라서, 이러한 방전(부분방전)이 일어날 때 생기는 전기적 신호를 검출하여, 절연물의 신회성을 평가하는 것은 매우 중요하다. 이러한 부분방전은 IEC둥 국제기술기준에 그 제한 값이 정하여져 있으며, 본 논문에서는 이러한 값을 수학적으로 평가하는 방법에 대해 연구하였다.

• Proceedings of the KIPE Conference
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• 2015.11a
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• pp.205-206
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• 2015

케이블 및 버스바는 전원을 공급하는 부대 장치로 산업 전반에 걸쳐 널리 사용되고 있다. 대부분의 경우 일정 주기로 점검 및 교체 등의 유지 보수를 진행하나 ITER IVC 버스바 및 지중케이블 등의 특수한 경우에서는 접근성 및 작업성의 이유로 유지 보수를 진행 할 수 없다. 이러한 경우, 절연물 내부에 전기 트리가 발생했을 때에는 대책이 없다. 전기 트리는 부분적인 고전계에 의한 진성파괴, 전하의 주입 또는 추출에 의한 파괴 및 전기-기계적 응력(맥스웰 응력)에 의한 파괴, 미소 부분 방전에 의한 파괴의 가장 큰 고장요인이 되는 현상이며, 다양한 사고 사례가 알려져 있다. 본 연구에서는 유한요소해석법을 이용하여 전기 트리를 가지는 사각 버스바에 대한 전계해석을 진행하였고, 전기 트리를 방지 할 수 있는 사각 버스바의 절연 설계에 대한 방안을 제시한다. 이 설계 방안은 향후 전기 트리를 고려한 사각 버스바의 절연 설계 기초 데이터로 활용 될 것이다.

• Tunnel and Underground Space
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• v.17 no.2 s.67
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• pp.128-138
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• 2007

At low in-situ stress, the continuity and distribution of natural fractures in rock mass predominantly control the failure processes. However at high in-situ stress, the failure process are affected and eventually dominated by stress-induced fractures preferentially growing parallel to the excavation boundary. This fracturing is often observed in brittle type of failure such as slabbing or spatting. Recent studies on the stress- or excavation-induced damage of rock revealed its importance especially in a highly stressed regime. In order to evaluate the brittle failure around a deep underground opening, physical model experiments were carried out. For the experiments a new tue triaxial testing system was made. According to visual observation and acoustic emission detection, brittle failure grades were classified under three categories. The test results indicate that where higher horizontal stress, acting perpendicular $(S_{H2})$ and parallel $(S_{H1})$ to the axis of the tunnel respectively, were applied, the failure grade at a constant vertical stress level (Sy) was lowered. The failure initiation stress was also increased with the increasing $S_{H1}\;and\;S_{H2}$. From the multi-variable regression on failure initiation stress and true triaxial stress conditions, $f(S_v,\;S_{H1},\;S_{H2})$ was proposed.

• The Journal of Engineering Geology
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• v.31 no.2
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• pp.149-163
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• 2021

Damage characteristics of granite, marble and sandstone whose properties were different were investigated by uniaxial compression test and cyclic loading-unloading test. Strength, elastic constants and damage threshold stresses were measured by uniaxial compression test and were compared with those measured by cyclic loading-unloading test. Average rock strengths measured by cyclic loading-unloading test were either lower than or similar with those measured by uniaxial compression test. Rocks with high strength and low porosity were more sensitive to fatigue than that with low strength and high porosity. Although permanent strains caused by cyclic loading-unloading were different according to rock types, they could be good indicators representing damage characteristics of rock. Damage threshold stress of granite and marble might be measured from stress-permanent strain curves. Acoustic emissions were measured during both tests and felicity ratios which represented damage characteristics of rocks were calculated. Felicity ratio of sandstone which was weak in strength and highly porous could not be calculated because of very few measurements of acoustic emissions. On the other hand, damage threshold could be predicted from felicity ratios of granite and marble which were brittle and low in porosity. The deformation behaviors and damage characteristics of rock mass could be investigated if additional tests for various rock types were performed.