• 한국강구조학회 논문집
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• 제10권3호통권36호
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• pp.339-353
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• 1998

본 논문에서는 정량적인 지진 손상모델의 유도를 위해 먼저 지진 반복하중하에서 구조물 및 그들 요소에 대한 기존의 손상 모델들을 체계적으로 정리한 후 문제점을 평가하였다. 또한, 강한 지진과 같은 심한 반복가력을 받는 강구조 부재에 대한 파괴기준을 묘사하였으며, 강부재에 대한 새로운 지진 손상도 평가방법을 제안하였다. 이 손상모델은 극저 사이클 하중하에서 강부재에 대한 일련의 실험과 수치해석 연구로부터 얻어진 것이다. 본 연구에서 극저 사이클 하중은 큰 소성변형 영역안에서 5~20 사이클의 반복하중을 의미한다. 제안된 내진 손상평가 방법은 변형이 가장 심하게 집중된 단면에 있어서 국소 변형율의 이력에 초점을 맞추어 유도되었다.

• 한국농공학회지
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• 제41권4호
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• pp.92-103
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• 1999

Recently structural damage has been frequently observed in reinforced concrete brdiges due to repeated loads such as vehicular traffic an due to continual overloads by heavy duty trucks. Therefore, the purpose of this experimental stduy is to investigate the damage mechanism due to fatigue behavior of high-strength reinforced concrete beams under repeated loads. From the test results, the relation of cycle loading to deflection is on the mid-span , the crack growth and the modes of failure according to cycle number, fatigue life and S-N curve were observed through the fatigue test. Based on the fatigue test results , high-strength reinforced concrete beams failed to 57 ∼66 percent of the static ultimate strength . Fatigue strength aobut two million cycles from S-N curves was certified by 60 percent of static ultimate strength.

• Geomechanics and Engineering
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• 제16권2호
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• pp.113-124
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• 2018

Under repeated loading, the residual stresses within the subgrade and subsoil can accelerate the deformation of the road structures. In this paper, a series of laboratory cyclic loading model tests and small-scale model tests were conducted to investigate the dynamic stress response within soils under different loading conditions. The experimental results showed that a dynamic stress accumulation effect occurred if the soil showed cumulative deformation: (1) the residual stress increased and accumulated with an increasing number of loading cycles, and (2) the residual stress was superimposed on the stress response of the subsequent loading cycles, inducing a greater peak stress response. There are two conditions that must be met for the dynamic stress accumulation effect to occur. A threshold state exists only if the external load exceeds the cyclic threshold stress. Then, the stress accumulation effect occurs. A higher loading frequency results in a higher rate of increase for the residual stress. In addition to the superposition of the increasing residual stress, soil densification might contribute to the increasing peak stress during cyclic loading. An increase in soil stiffness and a decrease in dissipative energy induce a greater stress transmission within the material.

• 한국재료학회지
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• 제4권2호
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• pp.219-226
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• 1994

반복회수에 대한 하중-변형률 선도로부터 구산 잔류변형률의 비교 결과에서 피로하중하의 GFRP관의 강성은 GFRP관의 유리섬유의 적층수가 클수록 크게 나타났으며, 이러한 현상은 피로파괴 직전까지 나타났다. 아울러 본 피로실험 결과를 회귀분석하여 구한 S-N선도에 의하면 정적극한강도 백분율에 대한 피로강도는 GFRP관의 유리섬유 적층수가 증가할수록 증가하였으며, 유리섬유의 적층수가 15, 25, 35층인 GFRP관의 반복회수 200만회에 대한 피로강도는 정적극한강도는 각각 약 75.2%, 79.5%, 84.2%로 나타났다.

• Structural Engineering and Mechanics
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• 제70권1호
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• pp.67-80
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• 2019

Strengthening with near surface mounted carbon fibre reinforced polymers (NSM-CFRP) is a strengthening technique that have been used for several decades to increase the load carrying capacity of reinforced concrete members. In Iraq, many concrete buildings and bridges were subjected to a wide range of damage as a result of the last war and many other events. Accordingly, there is a progressive increase in the strengthening of concrete structures, bridges in particular, by using CFRP strengthening techniques. Near-surface mounted carbon fibre polymer has been recently proved as a powerful strengthening technique in which the CFRP strips are sufficiently protected against external environmental conditions especially the high-temperature rates in Iraq. However, this technique has not been examined yet under repeated loading conditions such as traffic loads on bridge girders. The main objective of this research was to investigate the effectiveness of NSM-CFRP strips in reinforced concrete beams under repeated loads. Different parameters such as the number of strips, groove size, and two types of bonding materials (epoxy resin and cement-based adhesive) were considered. Fifteen NSM-CFRP strengthened beams were tested under concentrated monotonic and repeated loadings. Three beams were non-strengthened as reference specimens while the remaining were strengthened with NSM-CFRP strips and divided into three groups. Each group comprises two beams tested under monotonic loads and used as control for those tested under repeated loads in the same group. The experimental results are discussed in terms of load-deflection behavior up to failure, ductility factor, cumulative energy absorption, number of cycles to failure, and the mode of failure. The test results proved that strengthening with NSM-CFRP strips increased both the flexural strength and stiffness of the tested beams. An increase in load carrying capacity was obtained in a range of (1.47 to 4.49) times that for the non-strengthened specimens. Also, the increase in total area of CFRPs showed a slight increase in flexural capacity of (1.02) times the value of the control strengthened one tested under repeated loading. Increasing the total area of CFRP strips resulted in a reduction in ductility factor reached to (0.71) while the cumulative energy absorption increased by (1.22) times the values of the strengthened reference specimens tested under repeated loading. Moreover, the replacement of epoxy resin with cement-based adhesive as a bonding material exhibited higher ductility than specimen with epoxy resin tested under monotonic and repeated loading.

• Earthquakes and Structures
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• 제7권2호
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• pp.179-199
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• 2014

The steel tube-reinforced concrete (ST-RC) composite column is a novel type of composite column, consisting of a steel tube embedded in reinforced concrete. The objective of this paper is to investigate the effect of cumulative damage on the seismic behavior of ST-RC columns through experimental testing. Six large-scale ST-RC column specimens were subjected to high axial forces and cyclic lateral loading. The specimens included two groups, where Group I had a higher amount of transverse reinforcement than Group II. The test results indicate that all specimens failed in a flexural mode, characterized by buckling and yielding of longitudinal rebars, failure of transverse rebars, compressive crushing of concrete, and steel tube buckling at the base of the columns. The number of loading cycles was found to have minimal effect on the strength capacity of the specimens. The number of loading cycles had limited effect on the deformation capacity for the Group I specimens, while an obvious effect on the deformation capacity for the Group II specimens was observed. The Group I specimen showed significantly larger deformation and energy dissipation capacities than the corresponding Group II specimen, for the case where the lateral cyclic loads were repeated ten cycles at each drift level. The ultimate displacement of the Group I specimen was 25% larger than that of the Group II counterpart, and the cumulative energy dissipated by the former was 2.8 times that of the latter. Based on the test results, recommendations are made for the amount of transverse reinforcement required in seismic design of ST-RC columns for ensuring adequate deformation capacity.

• Geomechanics and Engineering
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• 제9권2호
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• pp.219-241
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• 2015

In this paper, a series of repeated load tests were carried out on a 150 mm diameter plate simulative of vehicle passes, to demonstrate the benefits of soil-rubber shred mixture in decreasing the soil surface settlement of road embankment. The results show that the efficiency of rubber reinforcement is significantly a function of the rubber content, thickness of rubber-soil mixture and soil cap thickness over the mixture. Minimum surface settlement is provided by 2.5% of rubber in rubber-soil mixture, the thickness of mixture layer and soil cap of 0.5 times the loading surface diameter, giving values of 0.32-0.68 times those obtained in the unreinforced system for low and high values of amplitude of repeated load. In this installation, in contrast with unreinforced bed that shows unstable response, the rate of enhancement in settlement decreases significantly as the number of loading cycles increase and system behaves resiliently without undergoing plastic deformation. The findings encourage the use of rubber shreds obtained from non-reusable tires as a viable material in road works.

• 콘크리트학회논문집
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• 제22권2호
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• pp.179-186
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• 2010

철근과 콘크리트의 부착은 콘크리트와 철근의 일체거동에 의한 안전성 확보를 위해서도 필요하지만, 적정수준의 연성을 확보하여, 부재의 거동을 조절하기 위해서도 필요하다. 현재까지 반복하중 하에서의 부착응력-슬립관계에 대한 연구는 진행되어 왔으나, 철근 부식이 이루어진 경우의 반복하중 하에서의 부착응력-슬립관계에 대한 연구는 거의 없는 실정이다. 따라서 이 연구에서는 부식된 철근의 부착 실험을 통하여 단조하중과 반복하중 하에서의 부식률에 따른 부착응력-슬립 관계를 살펴보았다. 실험결과로부터 철근 부식은 할열 균열이 발생하기 전까지는 단조하중에서의 부착강도를 증가시키는 것을 확인하였다. 그리고 반복하중 횟수와 반복하중 하에서의 슬립 및 잔류슬립을 이중로그스케일로 도시한 결과, 철근 부식이 있는 경우에도 철근 부식이 없는 경우와 마찬가지로 선형 관계임을 확인하였다. 그리고 부식에 의한 균열이 발생하지 않는 한, 철근 부식이 반복하중 재하 이후의 부착강도 및 부착강도에서의 슬립 등 부착 특성에 영향을 미치지 않는 것도 알 수 있었다. 마지막으로, 철근 부식으로 인한 균열이후에서의 응력수준에 따른 피로수명은 급격히 감소하는 것을 확인하였다. 이 연구에서 제시하는 철근 부식을 고려한 반복하중 하에서의 부착응력-슬립의 관계는 철근콘크리트 구조물의 내구수명 예측 시 유용하게 사용될 수 있을 것으로 사료된다.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2003년도 봄 학술발표회 논문집
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• pp.311-314
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• 2003

Concrete structures such as bridge, pavement, airfield, and offshore structure are normally subjected to repeated load. This paper proposes a failure probability models of concrete subjected to split tension repeated-loads, based on experimental results. The fatigue tests were performed at the stress ratio of 0.1, the loading shape of sine, the frequency of 20Hz, and the stress levels of 90, 80 and 70%. The fatigue test specimen was 150mm in diameter and 75mm in thickness. The fatigue analysis did not include which exceeded 0.9 of statistical coefficient of determination values or did not failure at 2$\times$$10^6$ cycles. The graphical method, the moment method, and maximum likelihood estimation method were used to obtain Weibull distribution parameters. The goodness-of-fit test by Kolmogorov-Smirnov test was acceptable 5% level of significance. As a result, the proposed failure probability model based on the two-parameter($\alpha and \mu$) Weibull distribution was good enough to estimate accurately the fatigue life subjected to tension mode.

• 한국초전도ㆍ저온공학회논문지
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• 제6권4호
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• pp.22-26
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• 2004

For practical applications, the evaluation of reliability or endurance of HTS conductors is necessary. The mechanical properties and the critical current, Ie, of multifilamentary Bi-2223 superconducting tapes, externally reinforced with stainless steel foils, subjected to high cycle fatigue loading in the longitudinal direction were investigated at 77K. The S-N curves were obtained and its transport property was evaluated with the increase of repeated cycles at different stress amplitudes. The effect of the stress ratio, R, on the Ie degradation behavior under fatigue loading was also examined considering the practical application situation of HTS tapes. Microstructure observation was conducted in order to understand the Ie degradation mechanism in fatigued Bi-2223 tapes.