• Proceedings of the Korean Nuclear Society Conference
• /
• 1996.05c
• /
• pp.359-364
• /
• 1996

'93.4월에 고리원자력 4호기 운전중(원자로 출력 100%, 발전기 출력 975MWe) 주증기 차단밸브 (MSIV)의 닫힘으로 인해 발전소가 정지되었다. 밸브분해 점검결과 밸브스템이 Back Seat Ring 조립부위 Notch 부위에서 Steam Flow와 평행한 방향으로 절단되어, 밸브스템의 손상원인을 규명하기 위한 본 연구에서는 피로해석절차도에 따라 S-N 곡선에 근거하여 피로해석을 수행하였다. 피로해석결과 밸브스템의 초기균열 생성원인은 Stem Notch 부위의 제작결함과 발전소 정지시 밸브를 급속히 닫을 때 작용하는 충격하중등에 의해 발생된 것으로 추정되며 인장평균응력과 관내 유체의 진동하중의 변동응력이 조합하여 피로균열을 가속시켜 파손을 일으킨 것으로 사교된다.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.29 no.4A
• /
• pp.299-305
• /
• 2009

In this study, in order to research the causes lead to fatigue crack in the coped stringer of a steel railway bridge, we take the steel railway bridge which actually occurs fatigue crack as a research object and manufacture the full size of crossbeam-stringer and floor system model to perform the experimental test. The results indicates that, the fatigue crack in the top of coped area of stringers is caused by the reciprocal action of the in plane stress in the tip of coped area of web by the negative moment occurred in the end of the stringers. While the fatigue crack in the bottom of coped area of stringers is due to the plane stress caused by the out-plane deformation relative to the bottom of coped area of web of the fixed end in the stringers.

• Proceedings of the Materials Research Society of Korea Conference
• /
• 2009.11a
• /
• pp.35.2-35.2
• /
• 2009

최근 전세계적으로 저탄소, 녹색성장으로 인하여 원자력발전이 주목받고 있다. 또한 에너지의 고효율로 인한 발전소의 설비가 대형화가 됨에 따라 발전소의 수명평가와 건전성평가가 중요해지고 있다. 일반적으로 구조물 내에 존재하는 균열의 크기와 형상을 파악하여 피로균열전파속도를 평가함으로써 건전성평가를 확인하고 있다. 그리고 고온, 고압에서의 피로균열전파속도는직류전위차 (Direct Current Potential Drop : DCPD)법을 사용하고 있다. DCPD법은 균열의 정밀한 측정방법으로써 측정시 오차가 발생하기 때문에 ASTM에서 제시된 incremental polynomial 법을 권고하고 있다. 따라서 본 연구에서는 피로균열전파전파속도의통계적처리를 통해서 합리적인 곡선을 구하여 건전성평가에 활용하고자 한다. 실험에 사용된 시편은 두께 5mm, 폭 25.4mm CT시편을 사용하였으며, 1mm의 예비균열을 주었다. 그리고 실험온도는 상온에서 실시 하였으며, 주파수는 10Hz를 주었다. 그리고 DCPD 측정을 위해 5A의 전류를 주었으며, 이때 측정된 전압값을 ASTM에 제시된 관계식에넣어 균열길이로 환산하였으며, 데이터처리는 ASTM에 제시된 incremental polynomial법을 기본적으로 사용하였다. 또한 ASTM에 제시된 2n+1을 이용하여 데이터의 수 n을 1~7 까지 변화를 주어 3~15 point 까지 데이터를 처리하여곡선을 제시하였다. 분석결과 $R^2$값이 1을 기준으로 했을 때 3~7 point 까지는큰 차이를 보이지 않았지만 9-point 이후부터는 $R^2$ 감소함을 알 수 있었다. 또한 적용된 데이터의수에 따라 피로군열전파속도 곡선에서 측정된 Paris law의 n값과 C 값은 큰차이를 보이지 않았다.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.35 no.10
• /
• pp.1229-1235
• /
• 2011

Fatigue tests were conducted on the aluminum alloy, A7075-T6 to determine the most reliable fretting fatigue damage parameter. Specimens with grooves were used, so that either fretting fatigue crack at the pad/specimen interface or plain fatigue crack at the groove could be nucleated, depending on the pad pressure. Both the crack nucleation location and initial crack orientation were examined using optical microscopy, and the results were used to assess the reliability of the various fretting fatigue damage parameters that have been most commonly used in the literature. Finite element analysis was employed to obtain the stress and strain data of the specimen, which were needed to estimate the parameter values and the orientation of the critical plane. It was revealed that both the Fatemi.Socie and McDiarmid parameters, which assume shear-mode fatigue cracking, are the most reliable.

• Proceedings of the Korean Society of Precision Engineering Conference
• /
• 2004.05a
• /
• pp.76-76
• /
• 2004

하이브리드 복합재료 중에서 적충형태의 Al/GFRP는 단일재 알루미늄에 비해 피로특성, 비강도, 비강성 등이 매우 우수하여 Fig. 1과 같이 항공기 주익 구조에 주로 적용된다. 그러나 이러한 Al/GFRP 적층재 역시 장시간에 걸쳐 비행하중을 받게 되면 다양한 형태의 파손이 발생할 수 있다. 이 중 알루미늄층과 섬유층 사이에서 발생하는 층간분리는 Al/GFRP 적층재의 대표적인 피로파손 형태이며, 현재 이러한 파손은 다 방면으로 연구되고 있다.(중략)

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.28 no.1A
• /
• pp.1-13
• /
• 2008

The web of a horizontally curved plate girder bridge is, in general, subject to not only longitudinal flexural in-plane stress but also out-of-plane bending stress. Therefore, the induced stresses in the fillet welded joints at the intersection of the web and flange plates in the curved plate girder bridge can be considerably high, and the welded joints of gusset plates connecting the main girder to the floor beams or sway bracings can be subject to much more severe situation than those in the ordinary straight plate girder bridge. In order to investigate the cause of fatigue crack occurred in a curved girder bridge that has been served in about 23 years, in this study, field load tests have been performed to obtain the stress characteristics at the welded joint under the real traffic flow. Using the test results, we have investigated the causes of the occurrence of various fatigue cracks and have estimated the fatigue lives for the cracks. In addition, the characteristics of structural behavior at welded joint of the curved girder bridge have been examined by comparing the FE analysis and the field test result.

• Journal of Aerospace System Engineering
• /
• v.12 no.3
• /
• pp.38-44
• /
• 2018

Military aircraft may have fatigue cracks in structurally weak areas due to multiple factors such as the accumulation of flight time while perform various missions and unpredictable air conditions. As a fatigue crack progresses, there is a risk that the structure will be destroyed in extreme cases, which can have a significant impact on flight safety. In this study, a cracking phenomenon was observed during the periodic inspection the inner support of the fairing, which is installed to protect the connection between the wing and the body of the aircraft. Therefore, a study on a series of quality improvement processes for reformation was described. In order to identify the causes of cracks, pre-load generation occurrence during the wing assembly process was investigated and a fracture analysis was performed. Also, the design of the support structure was suggested in terms of preventing recurrence of cracks. The structural integrity was verified using a stress and fatigue life analysis.

• Journal of the Korean Society for Nondestructive Testing
• /
• v.28 no.4
• /
• pp.352-357
• /
• 2008

The detection of micro cracks in materials at the early stage of fracture is important in many structural safety assurance problems. The nonlinear ultrasonic technique (NUT) has been considered as a positive method for this, since it is more sensitive to micro crack than conventional linear ultrasonic methods. The basic principle is that the waveform is distorted by nonlinear stress-displacement relationship on the crack interface when the ultrasonic wave transmits through, and resultantly higher order harmonics are generated. This phenomenon is called the contact acoustic nonlinearity (CAN). The purpose of this paper is to prove the applicability of CAN experimentally by detection of micro fatigue crack artificailly initiated in Aluminum specimen. For this, we prepared fatigue specimens of Al6061 material with V-notch to initiate the crack, and the amplitude of second order harmonic was measured by scanning along the crack direction. From the results, we could see that the harmonic amplitude had good correlation with COD and it can be used to detect the crack depth in more accurately than the common 6 dB drop echo method.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.28 no.6D
• /
• pp.819-829
• /
• 2008

Fatigue cracking occurs over long time period because dynamic strength of slab continuously decreases by vehicle loading repetitively applied to the concrete pavement. To more accurately predict the fatigue life of the concrete pavement, the stress due to environmental loading should be considered prior to calculating the stress due to the vehicle loading because the stress due to temperature and moisture distribution always exists within the slab. Accordingly, a new fatigue model considering the environmental loading was developed in this research by evaluating factors of existing fatigue models most widely used and by making data points from the models. The applicability of the new model was evaluated by performing a fatigue analysis on the general concrete pavement structure using local climatic and traffic conditions in Korea. It was concluded that the top-down cracking due to the tensile stress at top of the slab is dominant cause of the fatigue failure than the bottom-up cracking occurred at bottom of the slab. More advanced fatigue analysis considering vehicle speed is expected by developing this study.

• Journal of the KSME
• /
• v.37 no.10
• /
• pp.41-46
• /
• 1997

철강구조물 부재 내에 노치나 균열이 존재할 수 있고, 외부의 피로하중에 의하여 취약부에서 발생한 균열이 진전하여 전구조물의 최종파손을 야기시킬 수 있다. 부재를 보다 안전하게 사용하고 또한 신뢰성을 확보하기 위해서는 이미 손상된 부재에서 균열의 진전상태를 계측할 수 있는 방법이 확립되어져야 하고, 파괴역학적 파라미터를 이용한 사용재의 균열진전거동특성이 평가되어야 한다. 균열길이의 측정방법은 지금까지 많은 연구자들에 의하여 개발되어져 왔는데 크게 광학현미경을 이용하여 육안으로 직접 균열길이를 측정하는 방법과 컴플라이언스, 초음파, AE 또는 전기적 신호를 통하여 얻어진 결괄부터 균열길이로 환산하는 간접적인 방법으로 대별된다. 대부분의 균열길이의 측정방법은 많은 수작업이 요구되고, 특히 하한계응력확대계수영역의 미세한 균열진전량을 측정하기에는 어려움이 따르고 있다. 이에 대하여 전도체 시험편에 일정전류를 흐르게 하고 균열길이의 증가에 따라 변화하는 전위차로 이를 균열길이로 평가하는 전기적인 측정방법이 있다. 이 방법은 실험장치가 비교적 간단하고 미세한 균열길이의 측정이 용이하여 균열길이의 직접적인 측정이 곤란한 고온역 그리고 충격하중하에서의 균열길이 측정에 이용이 확대되고 있다. 이 글에서는 여러 균열길이 측정방법의 장.단점에 대하여 고찰하고, 그 중 많은 장점을 갖고 있는 직류전위차법의 실험방법을 소개한다.