• 한국해양공학회지
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• 제25권3호
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• pp.66-72
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• 2011

This is the fourth of a series of companion papers dealing with the mechanical property reductions of various marine structural steels. Even though a reduction of the elastic modulus according to temperature increases has not been obtained from experiments, high temperature experiments from room temperature to $900^{\circ}C$ revealed that initial the yield strength and tensile strength are both seriously degraded. The mechanical properties obtained from high temperature experiments are compared with those from EC3 (Eurocode 3). It is found that the high temperature test results generally comply with the prediction values by EC3. Based on the prediction of EC3, time domain nonlinear finite element analyses were carried out for a blast wall installed on a real FPSO. After applying the reduced mechanical properties, corresponding to $600^{\circ}C$ to the FE model of the blast wall, more than three times the deflections were observed and it was observed that most structural parts experience plastic deformations exceeding the reduced yield strength at the high temperature. It is noted that a protection facility such as PFP (passive fire protection) should be required for structures likely to be directly exposed to fire and explosion accident.

• Journal of the Korean Wood Science and Technology
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• 제44권1호
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• pp.67-74
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• 2016

유리섬유 보강집성재의 볼트접합부 항복전단내력을 예측하기 위해 인장형 전단시험을 통해 측정된 실측값과 설계기준에 의한 예측값을 비교하였다. 볼트접합부의 항복전단내력 예측식을 위해 층재의 방향별 탄성계수, 포아송비, 전단계수를 측정하였다. 설계기준의 예측식 보정을 위해 유리섬유 보강집성재의 파괴거동을 반영한 파괴인성계수($K_{ft}$)를 적용하여 보정항복전단내력을 비교하였다. 층재의 탄성계수는 섬유방향에 따라 연륜폭, 연륜각과 높은 상관관계를 보였다. 유리섬유 보강집성재의 보정항복전단내력은 직경과 보강재에 따라 실측치와 비슷한 경향을 보였으며, 직물형 유리섬유보강집성재의 볼트접합부가 가장 높은 내력성능을 보였다. 볼트직경과 보강재에 따른 유리섬유 보강집성재 볼트접합부의 항복전단내력은 건축구조설계기준의 제안식을 보정한 예측치가 가장 잘 일치하였다.

• 대한기계학회논문집A
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• 제25권4호
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• pp.756-763
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• 2001

In the process of integrity evaluation for nuclear power plant components, a series of fracture mechanics evaluation on surface cracks in reactor pressure vessel(RPV) must be conducted. These fracture mechanics evaluation are based on stress intensity factor, K. However, under pressurized thermal shock(PTS) conditions, the combination of thermal and mechanical stress by steep temperature gradient and internal pressure causes considerably high tensile stress at the inside of RPV wall. Besides, the internal pressure during the normal operation produces high tensile stress at the RPV wall. As a result, cracks on inner surface of RPVs may experience elastic-plastic behavior which can be explained with J-integral. In such a case, however, J-integral may possibly lose its validity due to constraint effect. In this paper, in order to verify the suitability of J-integral, tow dimensional finite element analyses were applied for various surface cracks. A total of 18 crack geometries were analyzed, and $\Omega$ stresses were obtained by comparing resulting HRR stress distribution with corresponding actual stress distributions. In conclusion, HRR stress fields were found to overestimate the actual crack-tip stress field due to constraint effect.

• 대한기계학회논문집A
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• 제27권11호
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• pp.1986-1996
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• 2003

Methodology based on the elastic-plastic fracture mechanics has been widely accepted in predicting the critical crack length(CCL) of pressure tubes of CANDU nuclear plants. A conservative estimate of CCL is obtained by employing the J-resistance curves measured with the specimens satisfying plane strain condition as suggested in the ASTM standard. Due to limited thickness of the pressure tubes the curved compact tension(CT) specimens taken out from tile pressure tube have been used in obtaining J-resistance curves. The curved CT specimen inevitably introduce slant fatigue crack during precracking. Hence, effect of specimen geometry and slant crack on J-resistance curve should be explored. In this study, the difference of J integral values between the standard CT specimens satisfying plane strain condition and the nonstandard curved CT with limited thickness (4.2mm) is estimated using finite element analysis. The fracture resistance curves of Zr-2.5Nb obtained previously by other authors are critically discussed. Various finite element analysis were conducted such as 2D analysis under plane stress and plane strain conditions and 3D analysis for flat CT, curved CT with straight crack and curved CT with slant crack front. J-integral values were determined by local contour integration near the crack tip, which was considered as accurate J-values. J value was also determined from the load versus load line displacement curve and the J estimation equation in the ASTM standard. Discrepancies between the two values were shown and suggestion was made for obtaining accurate J values from the load line displacement curves obtained by the curved CT specimens.

• Geomechanics and Engineering
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• 제17권4호
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• pp.355-365
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• 2019

It is normal to observe the presence of numerous cracks in coal body. And it has significantly effective on the mechanical characteristics and realistic failure models of coal mass. Therefore, this paper is to investigate the influence of crack parameters on coal body by comprehensive using theoretical analysis, laboratory experiments and numerical simulation through prepared briquette specimens. Different from intact coal body possessing single peak in stress-strain curve, other specimens with crack angle can be illustrated to own double peaks. Moreover, the unconfined compressive strength (UCS) of specimens decreases and follow by increasing with the increase of crack angle. It seems to like a parabolic shape with an upward opening. And it can be demonstrated that the minimum UCS is obtained in crack angle $45^{\circ}$. In terms of failure types, it is interesting to note that there is a changing trend from tensile failure to tensile-shear mixing failure with tension dominant follow by shear dominant with the increase of crack angle. However, the changing characteristics of UCS and failure forms can be explained by elastic-plastic and fracture mechanics. Lastly, the results of numerical simulations are good consistent with the experimental results. It provides experimental and theoretical foundations to reveal fracture mechanism of coal body with non-penetrating single crack further.

• 한국정밀공학회지
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• 제25권8호
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• pp.80-87
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• 2008

This paper investigated into the impact characteristics of the stainless sheet with thickness of 0.7 mm on the stretching boundary condition through three-dimensional finite element analysis. High speed tensile tests were carried out to obtain strain-stress relationships with the effects of the strain rate. The FE analysis was performed by the ABAQUS explicit code. In order to improve an accuracy of the FE analysis, the hyper-elastic model and the damping factor were introduced. Through the comparison of the results of the FE analyses and those of the impact tests, a proper FE model was obtained. The results of the FE analyses showed that the absorption rate of energy maintains almost 82.5-83.5% irrespective of the impact energy level and the diameter of the impact head. From the results of FE analyses, variations of stress, strain, dissipation energy, strain energy density, and local deformation characteristics in the stainless sheet during the collision and the rebound of the impact head were quantitatively examined. In addition, it was shown that the fracture of the specimen occurs when the plastic strain is 0.42 and the maximum value of the plastic dissipation energy of the specimen is nearly 1.83 J.

• 대한기계학회논문집
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• 제14권1호
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• pp.87-95
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• 1990

본 연구에서는 고온기기의 구조용 부재로 널리 사용되고 있는 304스테인리스 강을 선택하여 상온, $538^{\circ}C$($1000^{\circ}$ F) 및 $593^{\circ}C$($1100^{\circ}C$ F) 고온하의 작은 표면노치 시험편 에서 발생한 표면 피로 균열의 성장거동을 레플리카법에 의하여 상세히 관찰측정하여 파괴역학적으로 해석 연구하고자 한다. 즉 피로균열의 성장특성을 ....$\DELTA\sigma^{n}$a의 매 개변수를 도입하여 정량적으로 평가하고, 수명예측을 실시하여 그 유용성을 비교검토 하였다. 또한 피로균열의 성장에 관한 실험결과 자료들을 PC를 이용하여 전산하고 도식화함으로써 파괴역학적 설계에 유용한 기초적 자료를 제시하고자 하였다.

• 한국재료학회지
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• 제19권1호
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• pp.1-6
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• 2009

In this study, the stress-strain response and the cracking behaviors of ITO film on a PET substrate are investigated. The cracking behaviors of ITO thin films deposited on a thermoplastic semi-crystalline polymer developed for flexible display applications was investigated by means of tensile experiments equipped with an electrical measurement apparatus and an in-situ optical microscope. Electrical resistance increased gradually in the elastic-to-plastic transition region of the stress strain curves and cracks formed. Numerous cracks were found in this region, and the increase of the resistance was linked to the cracking of ITO thin films. Upon loading, the initial cracks perpendicular to the tensile axis were observed at about 1% of the total strain. They propagated to the entire sample width as the strain increased. The spacing between the horizontal cracks is thought to be determined by the fracture strength and the thickness of the ITO film as well as by the interfacial strength between the ITO and PET. The effect of the strain rate on the cracking behavior was also investigated. The crack density increased as the strain increased. The spacing between the horizontal cracks (perpendicular to the stress axis) increased as the strain rate decreased. The increase of the crack density as the strain rate decreased can be attributed to the higher fraction of the plastic strain to the total strain at a given total strain. The higher critical strain for the onset of the increase in the resistance and the crack initiation of the ITO/PET with a thinner ITO film (300 ohms/sq.) suggests a higher strength of the thinner ITO film.

• 대한토목학회논문집
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• 제2권3호
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• pp.1-13
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• 1982

원구멍과 타원구멍을 갖는 두 개의 강판(鋼板)에 완전교번하중(完全交番荷重)(completely reversed load, completely alternating load ;같은 크기의 인장(引張) 압축(壓縮)의 반복)을 가할 때 유한요소법(有限要素法)을 써서 강복요소(降伏要素)가 발생하는 단계마다 각 절점(節點)의 변위(變位), 각 요소(要素)의 응력(應力) 및 변형률(變形率), 하중(荷重)의 크기 등을 계산하여 파괴력학적(破壞力學的)인 검토를 행하였다. 이로부터, 강판(鋼板)의 파괴기구(破壞機構)를 밝히는 데에 핵심이 된다고 생각되는 응력확대계수(應力擴大係數)를 계산할 수 있는 토대가 마련되었으며, 흠선단(先端)의 응력집중(應力集中)현상과 소성역(塑性域)의 변화과정이 밝혀졌다. 또, 재하(載荷) 중에 강복(降伏)을 경험한 부분에서는 강하(降荷)때에 영구변형(永久變形)(잔류변형(殘留變形))이 남게 되고 이것이 나머지의 제하(除荷)를 구속(拘束)하여 반대방향의 재하(載荷)의 효과를 일으킴으로서 흠선단(先端)에 가까운 부분에는 인장(引張) 후의 제하(除荷) 때에 심지어 압축재강복(壓縮再降伏)까지, 압축(壓縮) 후의 제하(除荷) 때에는 심지어 인장재강복(引張再降伏)까지 일으키며 이들이 인장(引張) 및 압축(壓縮)의 재하(載荷) 중의 강복(降伏)과 교번(交番)으로 반복됨으로써 흠선단(先端)에 파로(波勞)현상을 초래하게 된다는 사실을 예견할 수 있었다. 아울러 흠이 원구멍일 때와 타원구멍일 때의 계산결과를 비교하여 홈이 예리한 균열에 가까워질수록 빨리 파괴에 달하게 된다는 사실을 확인할 수 있었다.

• 대한토목학회논문집
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• 제30권3C호
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• pp.149-158
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• 2010

본 연구에서는 말뚝재료의 항복거동 및 기하학적 비선형 거동인 P-${\Delta}$ 효과를 고려한 변단면 단일 현장타설말뚝의 거동특성을 분석하였다. 이를 위하여 실제 시공된 현장타설말뚝에 대한 말뚝재료의 균열 휨모멘트 및 균열 수평하중을 산정하였고, 국내 외에서 적용하고 있는 대표적인 수평변위 기준과의 비교분석을 수행하였다. 또한 구조적 특성을 파악하기 위하여 설계자료를 토대로 재료 및 지반조건을 반영한 현장타설말뚝의 거동을 예측하여 변단면 설계의 타당성을 검토하였다. 분석 결과, 재료의 항복거동과 P-${\Delta}$ 효과 고려시, 이를 고려하지 않는 경우와 비교하여 최대 모멘트가 지표면 부근에서 발생하여 소성힌지 위치에 영향을 주는 것을 나타났으며, 말뚝의 재료파괴는 주로 기둥부의 단면적이 작은 말뚝-기둥 접합부에서 발생하는 것을 알 수 있었다. 또한 주로 말뚝이 수평변위 기준에 도달하기 전에 재료의 파괴가 먼저 발생하기 때문에 말뚝의 항복효과를 고려해야 함을 알 수 있었다. 본 연구 결과, 변단면 단일 현장타설말뚝의 설계 시 불확실성을 고려하여 지지력을 과소평가하고 있는 것으로 나타났으며, 말뚝재료의 항복거동 및 기하학적 비선형 거동인 P-${\Delta}$ 효과를 고려한 해석기법을 통하여 거동특성을 정확히 예측한다면 경제적인 설계가 가능할 것으로 판단되었다.