• Smart Structures and Systems
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• 제13권6호
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• pp.943-957
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• 2014

When subjected to fatigue loading, the main failure mode of partially prestressed concrete (PPC) structure is the fatigue fracture of tensile reinforcement. Therefore, monitoring and evaluation of the steel stresses/strains in the structure are essential issues for structural design and healthy assessment. The current study experimentally investigates the possibility of using fiber Bragg grating (FBG) sensors to measure the steel strains in PPC beams in the process of fatigue loading. Six full-scale post-tensioned PPC beams were exposed to fatigue loading. Within the beams, the FBG and resistance strain gauge (RSG) sensors were independently bonded onto the surface of tensile reinforcements. A good agreement was found between the recorded results from the two different sensors. Moreover, FBG sensors show relatively good resistance to fatigue loading compared with RSG sensors, indicating that FBG sensors possess the capability for long-term health monitoring of the tensile reinforcement in PPC structures. Apart from the above findings, it can also be found that during the fatigue loading, there is stress redistribution between prestressed and non-prestressed reinforcements, and the residual strain emerges in the non-prestressed reinforcement. This phenomenon can bring about an increase of the steel stress in the non-prestressed reinforcement.

• 한국군사과학기술학회지
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• 제16권3호
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• pp.382-389
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• 2013

In anti-jamming GPS receiver, adaptive signal processing techniques in which the radiation pattern of adaptive array antenna of elements may be adaptively changed used to reject interference, clutter, and jamming signals. In this paper, I describes adaptive signal processing technique using the sample matrix inversion(SMI) algorithm. This adaptive signal processing technique can be applied effectively to wideband/narrowband anti-jamming GPS receiver because it does not consider the satellite signal directions and GPS signal power level exists below the thermal noise. I also analyzed the effects of covariance matrix sample size and diagonal loading technique on the system performance of five-element circular array antenna. To attain near optimum performance, more samples required for calculation covariance matrix. Diagonal loading technique reduces the system nulling capability against low-power jamming signals, but this technique improves robustness of adaptive array antenna.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 1999년도 봄 학술발표회 논문집(I)
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• pp.840-845
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• 1999

In this study, it was experimentally investigated the effectiveness of repair and strengthening methods for RC beams deteriorated under severs enviromental conditions. Polymer cement were employed to restore the sectional loss and aramid fiber-reinforced sheet was used to reinforce the surface subject to tension. Repaired and strengthened reinforced-concrete samples were subjected to loading tests. The tests revealed that the sectional restoration enhanced the loading capability of the sample structures. Additional strengthening with one aramid fiber-reinforced sheet improved 18% of yielding load and 30% of ultimate load of the structure. Reinforcing with two aramid fiber-reinforced sheets brought about an enhancement of 22% of yielding loading and 49% of ultimate load.

• 전자공학회논문지A
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• 제31A권12호
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• pp.44-55
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• 1994

This study is to apply the theory of fatigue fracture to solder joints under thermal cyclic loading and predict life of solder joint to failure. A 62Sn-36Pb-2Ag solder was used in this study. Tensile tests were preformed at temperatures of 15.dec. C, 50.dec. C and 85.dec. C in order to find terms of crack length "a". plastic strain range ""${\Delta}{\varepsilon}_p$" and temperature "T". Solder joint under thermal cyclic loading was analyzed by FEM. this FEM analysis together with the crack growth rate will provide the capability of the fatigue life prediction of solder joints and enhance the reliability od solder joint.

• Structural Engineering and Mechanics
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• 제31권1호
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• pp.25-38
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• 2009

Local failure of a primary structural component induced by direct air-blast loading may be itself a critical damage and lead to the partial or full collapse of the building. As an extensive research to mitigate blast-induced hazards in steel frame structure, a state-of-art analytical approach or high-fidelity computational nonlinear continuum modeling using computational fluid dynamics was described in this paper. The capability of the approach to produce reasonable blast pressures on a steel wide-flange section column was first evaluated. Parametric studies were conducted to observe the effects of section sizes and boundary conditions on behavior and failure of columns in steel frame structures. This study shows that the analytical approach is reasonable and effective to understand the nature of blast wave and complex interaction between blast loading and steel column behavior.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2011년도 추계학술대회 논문집
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• pp.244-249
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• 2011

The potential hazards resulting from a low-velocity impact (bird-strike, tool drop, runway debris, etc.) on aircraft structures, such as engine nacelle or a leading edges, has been a long-term concern to the aircraft industry. Certification authorities require that exposed aircraft components must be tested to prove their capability to withstand low-velocity impact without suffering critical damage. In most of the past research studies unloaded specimens have been used for impact tests, however, in reality it is much more likely that a composite structure is exposed to a certain stress state when it is being impacted, which can have a significant effect on the impact performance. And the radiated impact sound induced by impact is analyzed for the damage detection evaluation. In this study, an investigation was undertaken to evaluate the effect in-plane loading on the impact force and sound of composite laminates numerically.

• 대한기계학회논문집A
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• 제26권6호
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• pp.993-1000
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• 2002

The objective of this work is to develop the capability to analyze accurately the mixed-mode propagation of a crack in composite structures with elastic orthotropic material stiffness properties and anisotropic material strength characteristics. In order to develop the capability to fully analyze fracture growth and failure in anisotropic structures, we examined the fundamental problem of mixed mode fracture by carrying out the analysis on orthotropic materials with an inclined crack subject to biaxial loading. Our goal here is to include an additional term in the asymptotic expansion of the crack tip stress field and to show that the direction of crack initiation can be significantly affected by that term. We employ the normal stress ratio theory to predict the direction of crack extension. It is shown that the angle of crack extension can be altered by horizontal loads and the use of second order term in the series expansion is important f3r the accurate determination of crack growth direction.

• Structural Engineering and Mechanics
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• 제67권6호
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• pp.659-669
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• 2018

The Dynamic equations in the polar coordinates are drawn out using the MLPG method for the non-symmetric FG cylindrical shell. To simulate the mechanical properties of FGM, the nonlinear volume fractions for radial direction are used. The shape function applied in this paper is a form of the radial basis functions, by using this function all the requirements for an effective and suitable shape function are established. Hence in this study, the multiquadrics (MQ) radial basis functions are exploited as the shape function governing the problem. The MLPG method is combined with the the Newmark time approximation scheme to solve dynamic equations in the time domain. The obtained results by the MLPG method to be verified are compared with the analytical solution and the FEM. The obtained results through the MLPG method show a good agreement in comparison to other results and the MLPG method has high accuracy for dynamic analysis of the non-symmetric FG cylindrical shell. To demonstrate the capability of the present method to dynamic analysis of the non-symmetric FG cylindrical shell, it is analyzed dynamically with different volume fraction exponents under harmonic and rectangular shock loading. The present method shows high accuracy, efficiency and capability to dynamic analysis of the non-symmetric FG cylindrical shell with nonlinear grading patterns.

• 한국국방경영분석학회지
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• 제32권1호
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• pp.36-50
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• 2006

한국군은 전시 상당량의 전략물자를 미국으로부터 항공수송을 통해 도입하고자 한다. 이 연구는 항공수송 모델을 형성하고 전시 동원될 민간 항공기를 포함한 동원자원의 항공수송 능력을 평가하는 것이다. 전략 항공수송계획은 국군수송사령부에 의해 매년 조정 수립되고 있으나 항공수송 과정의 불확실성으로 인해 시뮬레이션을 통해 실현가능성을 평가할 필요가 있다. 불확실한 요인은 미국 공항에서의 전략물자 도착에 대한 불확실성, 적하역 시간, 동원령 발령시 항공기 최초 위치 지점 등이다. 시뮬레이션은 두 개의 시나리오 하에서 실행되었으며 결과에 대한 민감도 분석을 수행하였다. 시뮬레이션 결과, 항공수송 능력에 가장 큰 영향을 미치는 요인들은 미국 공항에서의 물자 도착 불규칙성과 항공기 동원 대수이었다.

• Computers and Concrete
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• 제33권4호
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• pp.399-407
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• 2024

This study nondestructively examined the evolution of crack density in ultra-high performance concrete (UHPC) upon cyclic loading. Uniaxial compression was repeatedly applied to the cylindrical specimens at levels corresponding to 32% and 53% of the maximum load-bearing capacity, each at a steady strain rate. At each stage, both P-wave and S-wave velocities were measured in the absence of the applied load. In particular, the continuous monitoring of P-wave velocity from the first loading prior to the second loading allowed real-time observation of the strengthening effect during loading and the recovery effect afterwards. Increasing the number of cycles resulted in the reduction of both elastic wave velocities and Young's modulus, along with a slight rise in Poisson's ratio in both tested cases. The computed crack density showed a monotonically increasing trend with repeated loading, more significant at 53% than at 32% loading. Furthermore, the spatial distribution of the crack density along the height was achieved, validating the directional dependency of microcracking development. This study demonstrated the capability of the crack density to capture the evolution of microcracks in UHPC under cyclic loading condition, as an early-stage damage indicator.