• Structural Engineering and Mechanics
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• 제86권6호
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• pp.779-791
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• 2023

Long-termly used in water supply, an underground concrete pipe is easily subjected to the coupled action of pressure loading and flowing water, which can cause the chemo-mechanical damage of the pipe, resulting in its premature failure and lifetime reduction. Based on the leaching characteristics and damage mechanism of concrete pipe, this paper proposes a coupled chemo-mechanical damage and failure model of underground concrete pipe for water supply, including a calcium leaching model, mechanical damage equation and a failure criterion. By using the model, a numerical simulation is performed to analyze the failure process of underground concrete pipe, such as the time-varying calcium concentration in concrete, the thickness variation of pipe wall, the evolution of chemo-mechanical damage, the distribution of concrete stress on the pipe and the lifetime of the pipe. Results show that, the failure of the pipe is a coupled chemo-mechanical damage process companied with calcium leaching. During its damage and failure, the concentrations of calcium phase in concrete decrease obviously with the time, and it can cause an increase in the chemo-mechanical damage of the pipe, while the leaching and abrasion induced by flowing water can lead to the boundary movement and wall thickness reduction of the pipe, and it results in the stress redistribution on the pipe section, a premature failure and lifetime reduction of the pipe.

• Geomechanics and Engineering
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• 제29권5호
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• pp.567-582
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• 2022

In the present study, a series of physical experiments and numerical simulations were conducted to investigate the effects of mode I and mixed-mode I/II cracks on the fracture modes and stability of roadway tunnel models. The experiments and simulations incorporated different inclination angle flaws under both static and dynamic loads. The quasi-static and dynamic testing were conducted by using an electro-hydraulic servo control device and drop weight impact system (DWIS), and the failure process was simulated by using rock failure process analysis (RFPA) and AUTODYN software. The stress intensity factor was also calculated to evaluate the stability of the flawed roadway tunnel models by using ABAQUS software. According to comparisons between the test and numerical results, it is observed that for flawed roadways with a single radical crack and inclination angle of 45°, the static and dynamic stability are the lowest relative to other angles of fractured rock masses. For mixed-mode I/II cracks in flawed roadway tunnel models under dynamic loading, a wing crack is produced and the pre-existing cracks increase the stress concentration factor in the right part of the specimen, but this factor will not be larger than the maximum principal stress region in the roadway tunnel models. Additionally, damage to the sidewalls will be involved in the flawed roadway tunnel models under static loads.

• 한국자동차공학회논문집
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• 제8권3호
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• pp.77-84
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• 2000

Nonlinear transient analysis is executed to obtain the temperature distribution, and to evaluate the thermal stress of brake drum by using FEA(finite element analysis). The result induces the reason why hair crack and the cause of drum failure occurs and the way how stress of drum decreases. The temperature of drum is in proportion to the drum thickness and it processes nonlinear changes at every points of drum. The higher bulk temperature raises, the more stress difference between inner surface and outer surface makes and the highest bulk temperature is at the corner section. It is necessary for the diminishment of the drum stress to make air flow, between drum and rim, move lively and use the materials of higher conductivity. The hair crack and the cause of drum failure seem to be started at the near corner section.

• 대한산업공학회지
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• 제25권4호
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• pp.517-526
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• 1999

This paper deals with two economic optimal designs of constant-stress accelerated life test(ALT) where failure distribution follows one of location-scale family, i. e., exponential, Weibull, and lognormal distributions which have been ones of the popular choices of failure distributions. Two optimization criteria to develop ALT plans are the statistical efficiency per unit expected cost which consists of the fixed test cost, cost being proportional to the number of test units, and variable test cost depending on test period and stress level, and the expected loss which combines Taguchi's quadratic loss function and expected test cost. Optimum plan determines the low stress level, test units allocated to each stress, and censoring times at two stress levels under Type I censoring. The proposed ALT plans are illustrated with a numerical example and sensitivity analyses are conducted to study effects of pre-estimates of design parameters.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.237-242
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• 2001

Residual stress is a dominant obstacle to efficient production and safe usage of products by reducing the mechanical strength and failure properties. Especially, it causes interfacial failure and substrate deflection in the case of thin film. So, the exact evaluation and optimum control of thin film residual stress is indispensable. However, hole drilling or X-ray diffraction techniques have some limits in application to thin film. And, curvature technique for thin film materials cannot give the information about local stress variation. Therefore, we applied the nanoindentation technique in evaluating the thin film residual stress. In this study, we modeled the change of indentation loading curve for residually stressed and stress-free thin films during stress relaxation. The value of residual stress was directly related to the indentation depth change by relaxation. The residual stress from nanoindentation analysis was consistent with the result from curvature technique.

• 동력기계공학회지
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• 제7권2호
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• pp.66-72
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• 2003

Recently, the steel parts used at the aerospace and automobile industries are required to be used light weight parts. Therefore, used material, steel have to be a high stress, which is an indispensable condition in this field. At the consideration of parts design, high hardness of the lightweight parts have an benefit of saving fuel and material. A high stress of metal has a point of difference according to the shape of design, external cyclic load and condition of vibration. A crack generates on the surface of metal or under yield stress by defect of inner metal defect or surface defect and slowly, this crack grow stable growth. Finally, rapidity failure phenomena is happen. Fatigue failure_phenomena, which happen in metal, bring on danger in human life and property therefor, anti-fatigue failure technology take an important part of current industries Currently, the shot peening is used for removing the defect from the surface of steel and improving the fatigue strength on surface. Therefore, this paper investigated the effect on frcature toughness using shot peening which is improve the resistance of crack growth and crack expansion rate by fatigue that make a compressive residual stress on surface.

• 터널과지하공간
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• 제19권2호
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• pp.86-94
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• 2009

암석파괴역학과 파괴인성(rock fracture toughness) 이하의 응력확대계수(stress intensity factor)에서 균열이 성장하는 현상을 이용하여 암석 절리면의 비선형 강도특성과, 시간의 경과에 따라 파괴가 진행되는 특성을 고려한 수치해석용 3차원 절리면 요소를 개발하였다. 이 절리면 요소를 사용하여 암석 절리면 전단시험을 수치해석으로 모사한 결과, 전단응력이 증가하고 시간이 경과함에 따라 절리면 사이에 연결된 절리면 내 접점(asperity in joint)에서 암석의 파괴인성보다 응력확대계수가 작음에도 불구하고 균열이 발생하였고 시간이 경과하면서 균열이 성장, 절리면 내 접점이 파괴되었다. 이와 같이 각각의 절리면 내 접점의 파괴에 따라 절리면의 강도는 감소하고, 절리면의 전단응력은 응력경화와 응력연화 후 잔류응력에 도달하는 비선형거동을 보이면서 시간의 경과에 따라 점진적으로 파괴되었다.

• Geomechanics and Engineering
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• 제18권1호
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• pp.97-109
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• 2019

Physical conditions play vital role on the mechanical properties of frozen soil, especially for the temperature and moisture content of frozen soil. Subsequently, they influence the subsidence and stress law of permafrost layer. Taking Jiangcang No. 1 Coal Mine as engineering background, combined with laboratory experiment, field measurements and empirical formula to obtain the mechanical parameters of frozen soil, the thick plate mechanical model of permafrost was established to evaluate the safety of permafrost roof. At the same time, $FLAC^{3D}$ was used to study the influence of temperature and moisture content on the deformation and stress law of frozen soil layer. The results show that the failure tensile stress of frozen soil is larger than the maximum tensile stress of permafrost roof occurring in the process of mining. It indicates that the permafrost roof cannot collapse under the conditions of moisture content in the range from 20% to 27% as well as temperature in the range from $-35^{\circ}C$ to $-15^{\circ}C$. Moreover, the maximum subsidence of the upper and lower boundary of the overlying permafrost layer decreases with the increase of moisture content in the range of 15% to 27% or the decrease of temperature in the range of $-35^{\circ}C$ to $-15^{\circ}C$ if the temperature or moisture content keeps consistent with $-25^{\circ}C$ or 20%, respectively.

• 지질공학
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• 제32권4호
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• pp.597-610
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• 2022

중간주응력을 고려한 마찰재료의 파괴거동에 대한 확고한 이해는 대심도 보어홀 안정성 및 단층해석 등과 관련된 현장 적용의 고도화를 위한 필수적인 과정이다. 본 연구는 진삼축압축 조건을 물리적으로 구현하는 장비를 설계·제작하였으며 마찰재료로 제작된 석고 시료에 대한 진삼축압축시험을 통하여 재료의 파괴거동 특성을 논의하고 삼차원파괴함수의 적용성을 검토하였다. 진삼축압축시험을 위한 석고 재료는 52(w) × 52(l) × 104(h) mm의 직육면체 시료로 성형하였으며 다양한 조합의 𝜎₃, 𝜎₂의 조건으로 총 24회의 진삼축압축시험이 수행되었다. 또한, 삼차원 파괴기준식의 파라미터로 사용되는 석고의 강도정수 측정을 위하여 전통적인 일축압축시험 및 삼축압축시험이 수행되었다. 석고 재료의 응력-변형 특성은 중간주응력과 최소주응력의 차이가 클수록 취성거동이 더욱 강하게 나타났으며, 시료의 강도 및 변형은 중간주응력의 변화를 반영하는 것으로 평가되었다. 주응력 좌표계에서 시험 데이터에 대한 비선형 다중회귀분석을 수행한 결과 수정 Wiebols-Cook 파괴기준 및 수정 Lade 파괴기준이 석고 시료에 대한 삼차원 파괴기준으로 가장 적합하였다.

• Transactions on Electrical and Electronic Materials
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• 제2권3호
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• pp.12-17
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• 2001

It is shown in the present study that during the HTS (hot temperature storage) test, the metal contamination by impure elements can be highly susceptible to the void formation, leading to the open failure of the power line in the memory device. Such a functional failure associated with the metal contamination was investigated to be dominant in the early stages of the HTS test while the formation of a stress-driven void is mainly observed in the later stages. In particular, it was found that the void formed in the contaminated metal takes on a slit-like shape which has been known to be characteristic of the stress-related voiding. The impure elements leading to the metal degradation were identified to be carbon and oxygen introduced during the metal sputtering process. The experimental works show that the device reliability was significantly improved by reducing the level of such impure elements within metal. It is shown in the present study that during the HTS (hot temperature storage) test, the metal contamination by impure elements can be highly susceptible to the void formation, leading to the open failure of the power line in the memory device. Such a functional failure associated with the metal contamination was investigated to be dominant in the early stages of the HTS test while the formation of a stress-driven void is mainly observed in the later stages. In particular, it was found that the void formed in the contaminated metal takes on a slit-like shape which has been known to be characteristic of the stress-related voiding. The impure elements leading to the metal degradation were identified to be carbon and oxygen introduced during the metal sputtering process. The experimental works show that the device reliability was significantly improved by reducing the level of such impure elements within metal.