• Journal of Advanced Marine Engineering and Technology
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• 제37권7호
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• pp.718-728
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• 2013

Analysis of scenarios of transportation oil and gas which produced in the Arctic and others cold seas shows that in the near-term there will be a significant increase of tonnage of tankers for oil and gas and number of ships which should be exploited in difficult ice conditions. For the construction of ice-resistant structures (IRS) intended for production of oil and gas and transportation of these products at ice-class vessels, calculating the load from ice to board the ship and on surface of supports of the platforms are the actuality and urgent tasks. These tasks have one basis in both cases: at beginning of the contact occurs fracture of edge of ice, then occurs compressing of rubble shattered of ice, then they extruding from contact area, after this next layer of ice begin to destruct. At calculating the strength of plating and elements construct of vessels, icebreakers and ice-resistant platforms the specific energy of mechanical destruction ice ${\epsilon}_{cr}$ is an important parameter. For the whole period of study of physical and mechanical characteristics of sea ice have been not many experimental studies various researchers to obtain numerical values of this energetic characteristic of the strength of ice by a method called Ball Drop Test. This study shows that the destruction of the ice from dynamic loading in zone of contact occurs in several cycles, and the ice destructed with a minimum numerical values of ${\epsilon}_{cr}$. The author offer this energy characteristic to take as a base value for the calculation of ice load on ships and offshore structures.

• Restorative Dentistry and Endodontics
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• 제38권1호
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• pp.31-35
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• 2013

Objectives: This study compared the cyclic fatigue resistance of nickel-titanium (NiTi) files obtained in a conventional test using a simulated canal with a newly developed method that allows the application of constant fatigue load conditions. Materials and Methods: ProFile and K3 files of #25/.06, #30/.06, and #40/.04 were selected. Two types of testing devices were built to test their fatigue performance. The first (conventional) device prescribed curvature inside a simulated canal (C-test), the second new device exerted a constant load (L-test) whilst allowing any resulting curvature. Ten new instruments of each size and brand were tested with each device. The files were rotated until fracture and the number of cycles to failure (NCF) was determined. The NCF were subjected to one-way ANOVA and Duncan's post-hoc test for each method. Spearman's rank correlation coefficient was computed to examine any association between methods. Results: Spearman's rank correlation coefficient (${\rho}$ = -0.905) showed a significant negative correlation between methods. Groups with significant difference after the L-test divided into 4 clusters, whilst the C-test gave just 2 clusters. From the L-test, considering the negative correlation of NCF, K3 gave a significantly lower fatigue resistance than ProFile as in the C-test. K3 #30/.06 showed a lower fatigue resistance than K3 #25/.06, which was not found by the C-test. Variation in fatigue test methodology resulted in different cyclic fatigue resistance rankings for various NiTi files. Conclusions: The new methodology standardized the load during fatigue testing, allowing determination fatigue behavior under constant load conditions.

• Smart Structures and Systems
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• 제16권1호
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• pp.163-181
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• 2015

This research investigates the structural health monitoring of nonlinear structures after a major seismic event. It considers the identification of flag-shaped or pinched hysteresis behavior in response to structures as a more general case of a normal hysteresis curve without pinching. The method is based on the overall least squares methods and the log likelihood ratio test. In particular, the structural response is divided into different loading and unloading sub-half cycles. The overall least squares analysis is first implemented to obtain the minimum residual mean square estimates of structural parameters for each sub-half cycle with the number of segments assumed. The log likelihood ratio test is used to assess the likelihood of these nonlinear segments being true representations in the presence of noise and model error. The resulting regression coefficients for identified segmented regression models are finally used to obtain stiffness, yielding deformation and energy dissipation parameters. The performance of the method is illustrated using a single degree of freedom system and a suite of 20 earthquake records. RMS noise of 5%, 10%, 15% and 20% is added to the response data to assess the robustness of the identification routine. The proposed method is computationally efficient and accurate in identifying the damage parameters within 10% average of the known values even with 20% added noise. The method requires no user input and could thus be automated and performed in real-time for each sub-half cycle, with results available effectively immediately after an event as well as during an event, if required.

• 대한기계학회논문집A
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• 제26권2호
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• pp.374-379
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• 2002

Corrosion fatigue tests were carried out to clarify the influence of acid fog as environmental factor on the fatigue strength of SM55C using rotary bending fatigue tester. The fatigue strength of acid fog specimen extremely decreased about 80% compared to that of distilled water specimen. In acid fog environment, a number of cracks commenced at corrosion pit and coalesced with the adjacent cracks during they propagate, and they formed a single non-propagating circumferential crack under the endurance stress of N=5$\times$10$\^$7/ cycles. Also, the depth of the crack is smaller than that of normal fatigue crack, so the crack has a veil small aspect ratio. The reason of this peculiar crack growth characteristics is that the crack opening-closure behaviors are hindered by corrosion products on the surface crack faces, and hence it is thinkable that the strong corrosion action like anodic dissolution for crack growth in depth direction is weaker compared with surface, resulting from faint pumping action of crack during loading-shedding processes.

• Structural Engineering and Mechanics
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• 제27권6호
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• pp.713-739
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• 2007

Nowadays computers can perform symbolic computations in addition to mere number crunching operations for which they were originally designed. Symbolic computation opens up exciting possibilities in Structural Mechanics and engineering. Classical areas have been increasingly neglected due to the advent of computers as well as general purpose finite element software. But now, classical analysis has reemerged as an attractive computer option due to the capabilities of symbolic computation. The repetitive cycles of simultaneous - equation sets required by the finite element technique can be eliminated by solving a single set in symbolic form, thus generating a truly closed-form solution. This consequently saves in data preparation, storage and execution time. The power of Symbolic computation is demonstrated by six examples by applying symbolic computation 1) to solve coupled shear wall 2) to generate beam element matrices 3) to find the natural frequency of a shear frame using transfer matrix method 4) to find the stresses of a plate subjected to in-plane loading using Levy's approach 5) to draw the influence surface for deflection of an isotropic plate simply supported on all sides 6) to get dynamic equilibrium equations from Lagrange equation. This paper also presents yet another computationally efficient and accurate numerical method which is based on the concept of derivative of a function expressed as a weighted linear sum of the function values at all the mesh points. Again this method is applied to solve the problems of 1) coupled shear wall 2) lateral buckling of thin-walled beams due to moment gradient 3) buckling of a column and 4) static and buckling analysis of circular plates of uniform or non-uniform thickness. The numerical results obtained are compared with those available in existing literature in order to verify their accuracy.

• 대한의용생체공학회:학술대회논문집
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• 대한의용생체공학회 1998년도 추계학술대회
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• pp.155-156
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• 1998

Screw loosening and subsequent pullout can be attributed to the reduction in bone mineral density in the vertebrae manifested by osteoporosis in which the decrease in fixation strength between the cancellous bone and screw threads are accelerated by repeated loads exerted by patients own weight and activities following the surgery. In this study, the change in pullout strength of the pedicle screws was investigated before and after repeated loads were imparted. For this purpose. Diapason pedicle screws $(6.7\times40mm)$ were inserted onto fresh porcine spine specimens (T1-L5) after bone mineral density was measured using a DEXA. With an MTS, an axial load was applied at a loading rate of 0.33mm/sec until failure to measure the maximum pullout strength. Flexion moment of 7.5N-m was then imparted at 0.5Hz for 2000 cycles. It was found that the maximum pullout strength was exponentially related to BMD regardless of load types ($107.71\;\times\;\exp^{(1.43{\times}BMD)}r^2=0.93$, P<0.0001 without repeated load; ($107.71\;\times\;\exp^{(2.19{\times}BMD)}r^2=0.78$, P<0.0001 with repeated load). The results suggest that the reduction in pullout strength for pedicle screws is far more prominent in osteoporotic spine than in normal spine especially as number of repeated load was increased. More importantly, it was demonstrated that the level of bone mineral density and the activity level of the patient should be evaluated in more detail for successful implementation of pedicle screw systems in spinal surgery.

• Journal of Biosystems Engineering
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• 제42권4호
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• pp.251-257
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• 2017

Purpose: A chisel mounted on working implement, such as agricultural machinery used in irregular farming conditions, is subjected to highly variable fatigue loading during work. To ensure the safety of the chisel on a working implement for the duration of its service life, fatigue testing must be performed with the proper fatigue test load conditions. In this study, working loads for a chisel were developed by reconstructing loads from strain gage data collected during field tests and used to conduct fatigue tests on the chisel component. Methods: FE analysis with nCode software was utilized to select the proper quantity and locations of strain gages for load measurements. A fatigue test was performed to experimentally verify the fatigue strength of the chisel and to evaluate the validity of the load history developed with the load reconstruction technique. Results: A strain history for the chisel was obtained from data collected during field tests. The data was filtered for the 14-16 km/h speed range, connected, and merged. The chisel load history was developed using the load reconstruction technique. The resulting load history was expressed as a load spectrum using the rain-flow counting method. Conclusions: A fatigue test was conducted on a chisel under a constant load condition with an equivalent load amplitude and number of cycles, as calculated by Miner's Rule for linear damage accumulation. During the fatigue test, there were no cracks at any position. It is concluded that the fatigue test method proposed in this study can be utilized successfully as a durability evaluation method for the chisel.

• 한국방재학회 논문집
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• 제9권5호
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• pp.15-21
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• 2009

AASHTO LRFD 설계 기준에 따른 합성형 교량 전단연결재 설계는 주로 강도보다는 피로에 의해 지배되는 것으로 알려져 있다. 이 피로 설계는 1966년 Slutter와 Fisher의 연구에 근거하고 있다. 당시의 시험은 1면으로 수행되었기 때문에 편심이 작용하였고, 이로 인하여 전단연결재에 별도의 인장력이 가해지게 되어 피로 강도가 감소되는 결과를 낳은 것으로 보인다. 또한, 피로 S-N 곡선은 응력변동폭과 하중반복 횟수 각각의 로그 스케일에 대하여 선형 관계를 보이는 것으로 Fisher에 의해 후에 밝혀졌으나, 전단연결재의 경우에 대해서만은 아직도 응력변동폭에 로그를 취하지 않고 있다. 이 연구는 현재 미국, 영국, 유럽, 일본에서 사용 중인 피로 설계 곡선을 비교 검토한 결과 미국 설계 기준이 비교적 보수적인 설계를 하고 있음을 확인하였다. 나아가, 당시의 실험 세팅과 데이터를 재분석하고 최근까지 전 세계에 공개된 피로 실험 데이터를 수집하여 분석 비교하고, 이를 바탕으로 적절한 설계식을 추천하고자 한다.

• 한국구조물진단유지관리공학회 논문집
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• 제25권4호
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• pp.46-55
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• 2021

콘크리트 구조물은 외부 극한기후환경에 노출 될 경우 공용년수가 증가할수록 다양한 문제점들이 발생할 수 있다. 이러한 문제들 중 최근 가장 문제가 되고 있는 폭우, 폭설과 같은 극한 기후요소의 작용으로 동결융해 현상이 발생한다. 본 연구에서는 서울의 경우, 동결융해가 발생하는 기간 동안에 매우 건조한 날씨를 나타내기 때문에 KS F 2456 를 참고하여 콘크리트의 기중 급속 동결 융해 시험법을 제시하였다. 콘크리트 공시체 및 철근콘크리트 휨 부재를 제작하여 0, 100, 200, 300 사이클의 기중 급속 동결 융해를 수행하였으며 성능 평가를 통해 각 실험체의 재료 및 부재 단위에서의 성능 저하를 확인하였다. 300사이클까지 기중 급속 동결 융해를 수행한 설계 강도 24 MPa의 콘크리트 압축 강도는 5.24 MPa(21%) 만큼 감소하며, 기중 급속 동결 융해가 진행될수록 콘크리트의 재료적 강도 감소에 의해 철근콘크리트 휨 부재의 철근의 응력 부담이 증가되어 지진과 같은 외력 발생에 따른 구조물의 에너지 흡수(소산) 능력이 감소한다.

• 한국지반공학회논문집
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• 제21권3호
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• pp.27-42
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• 2005

본 논문에서는 포화 및 배수 조건에 따른 사질토 지반의 변형특성을 개선된 Stokoe식 비틂전단시험기를 이용하여 비교$\cdot$관찰하였다. 기존의 Stokoe식 비틀 전단 시험기를 개선하여 시료의 완전 포화상태를 만들고, 포화 배수 및 비배수 조건에서 비틂 전단 시험이 가능하도록 하였고, 또한, 비배수 시험에서 간극수압을 동시에 측정할 수 있도록 하였다. 국내에서 채취된 금강모래와 일본의 토요라 모래를 사용하여, 건조, 포화 배수 및 포화 비배수 조건, 3가지 간극비, 4가지 구속응력 조건을 달리하여 비틂전단시험을 수행하였다. 시험결과를 바탕으로, 전단탄성계수와 재료 감쇠비를 각 조건에 대하여 반복하중 및 변형률 크기의 영향을 비교$\cdot$분석하고, 배수 조건에 따라 다르게 나타나는 사질토 지반의 저변형률 (c) 및 중간 변형률 (${10}^{-3}\%<\gamma_c<{10}^{-1}\%$)에서의 변형특성을 관찰하였다.