• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2006년도 춘계학술발표회 논문집(I)
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• pp.74-77
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• 2006

The purpose of this study is to investigate the relation between color change and residual compressive strength in concrete exposed to high temperature. In order to study the color image analysis, the specimens have been tested with variables of concrete strengths(20Mpa, 40Mpa, 60Mpa) in transient heating conditions($800^{\circ}C$ heating and 2 hour preservation). The results show that the residual strength of specimens are coincident with the full development of the pink/red color and the method may be used to define the distance from a heated surface where strength degradation has occurred.

• Steel and Composite Structures
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• 제7권1호
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• pp.19-34
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• 2007

This paper presents results of a non-linear finite element analysis of axially loaded slender hollow structural section (HSS) columns, strengthened using high modulus carbon-fiber reinforced polymer (CFRP) longitudinal sheets. The model was developed and verified against both experimental and other analytical models. Both geometric and material nonlinearities, which are attributed to the column's initial imperfection and plasticity of steel, respectively, are accounted for. Residual stresses have also been modeled. The axial strength in the experimental study was found to be highly dependent on the column's imperfection. Consequently, no specific correlation was established experimentally between strength gain and amount of CFRP. The model predicted the ultimate loads and failure modes quite reasonably and was used to isolate the effects of CFRP strengthening from the columns' imperfections. It was then used in a parametric study to examine columns of different slenderness ratios, imperfections, number of CFRP layers, and level of residual stresses. The study demonstrated the effectiveness of high modulus CFRP in increasing stiffness and strength of slender columns. While the columns' imperfections affect their actual strengths before and after strengthening,the percentage gain in strength is highly dependent on slenderness ratio and CFRP reinforcement ratio, rather than the value of imperfection.

• International Journal of Naval Architecture and Ocean Engineering
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• 제9권5호
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• pp.473-483
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• 2017

Comprehensive consideration regarding influence mechanisms of initial imperfections on ultimate strength of spherical shells is taken to satisfy requirement of deep-sea structural design. The feasibility of innovative numerical procedure that combines welding simulation and non-linear buckling analysis is verified by a good agreement to experimental and theoretical results. Spherical shells with a series of wall thicknesses to radius ratios are studied. Residual stress and deformations from welding process are investigated separately. Variant influence mechanisms are discovered. Residual stress is demonstrated to be influential to stress field and buckling behavior but not to the ultimate strength. Deformations are proved to have a significant impact on ultimate strength. When central angles are less than critical value, concave magnitudes reduce ultimate strengths linearly. However, deformations with central angles above critical value are of much greater harm. Less imperfection susceptibility is found in spherical shells with larger wall thicknesses to radius ratios.

• 한국지반신소재학회논문집
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• 제16권1호
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• pp.73-83
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• 2017

모래 및 실트 함유량이 우세한 서해안 저소성 지반(인천, 화성, 군산)에서 실시된 실내 및 현장시험으로부터 얻어진 비배수전단 강도를 이용하여 피에조콘계수(Nkt)를 분석한 후, 이에 대한 적용성을 평가하였다. 인천, 화성 및 군산지역에서 얻어진 일축압축 강도에 의한 콘계수(Nkt)는 19~23, 간이 CU 강도에 의한 값은 13~13.8, 현장베인강도에 의한 값은 11.6~13.1로 평가되었다. 이는 저소성 실트 지반조건에서 일축압축강도가 과소평가되는 원인에 의한 것으로 간이 CU 강도 적용조건과 비교했을 때 콘계수(Nkt)가 약 1.8배 전후로 커지며 분산되는 경향을 나타냈다. 저소성 실트 지반에서 수행된 CPTU 데이터를 이용하여 콘계수(Nkt)를 평가할 때에는 지반의 입도분포, 액소성한계 등의 물리적 특성, 지층 내의 sandseam 분포 등으로 인한 콘선단저항(qt) 및 주면마찰력(fs)의 불규칙한 분포, 간극수압계수(Bq)를 종합적으로 분석하여 수행하는 것이 바람직할 것으로 판단된다.

• 콘크리트학회논문집
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• 제18권1호
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• pp.3-12
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• 2006

화해를 입은 콘크리트 구조물의 거동은 많은 변수에 따라 영향을 받는 복잡한 현상을 나타낸다. 연구에서 고려되는 변수들은 콘크리트 및 철근이 받는 온도크기 및 가열시간, 콘크리트와 철근의 기계적 물성, 함수율, 피복두께, 편심유무, 기둥단면형상 등을 들 수 있다. 이러한 주요변수들이 화해를 입은 기둥의 잔존내력에 미치는 영향을 추적할 수 있는 수치해석 모델이 존재하나 실무에서 용이하게, 그러나 어느 정도 정확성을 갖고 화해 입은 기둥의 잔존내력을 예측할 수 있는 실용예측식이 필요하다. 본 연구에서는 이 목적을 위하여 화해를 입은 보통강도 철근콘크리트 정방형 기둥에 대하여 수치해석적 모델에 의한 잔존내력 예측치를 근사적으로 산정할 수 있는 실용예측식을 개발하였다. 개발된 식은 화해시간, 콘크리트강도, 철근비, 기둥단면크기의 영향을 고려한다. 개발된 실용예측식에 의한 잔존내력값은 수치해석 모델에 의한 잔존내력값을 비교적 근접하게 예측하였다. 화해를 입은 보통강도 철근콘크리트 기둥에 대한 잔존내력, 안정성 및 추가 필요내력의 산정 등에 대한 예제를 수록하였다.

• 한국해안·해양공학회논문집
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• 제24권3호
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• pp.149-158
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• 2012

본 연구는 강화도 서측에 위치한 조력 발전 유력 후보지인 석모수로의 북단과 남단의 2개 정선에서 13시간 연속으로 단면 유속과 염분 관측을 대,소조기시 각각 수행하여 단면 내 잔차류의 변화와 성층간의 관계를 분석 하였다. 석모수로의 염분 분포는 대,소조기에 따라 서로 다른 염분 구조 형태를 보이며, 염분의 침입 범위 또한 다르게 나타난다. 잔차류 성분은 13시간 유속 성분에서 최소 자승법을 이용하여 추출한 $M_2$분조와 $M_4$분조를 제거하여 구하였고, 석모수로 북단과 남단에서 표층의 잔차류 성분은 모두 남쪽 방향의 잔차류 성분을 보인다. 그러나 두 정선의 단면 저층에서는 표층과 저층간의 2층 흐름 구조를 만드는 북쪽 방향의 잔차류가 관측되는데 그 위치는 대,소조기에 따라 다르게 나타나 시,공간적으로 다른 결과를 보인다. 북쪽 방향의 잔차류와 성층의 관계를 분석하기 위해서 Richardson number와 Simpson-hunter index를 분석하였다. 이러한 결과를 종합할 때, 석모수로의 단면 잔차류 패턴은 Richardson number가 상대적으로 급격하게 커지고 Simpson-hunter index가 4 이상 나타나는 지역, 즉 성층이 강하게 나타나는 지역에서 2층 흐름 구조를 보인다.

• Advances in materials Research
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• 제5권1호
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• pp.21-34
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• 2016

Fire is one of the most destructive powers to which a building structure can be subjected, often exposing concrete elements to elevated temperatures. The relative properties of concrete after such an exposure are of significant importance in terms of the serviceability of buildings. Unraveling the heating history of concrete and different cooling regimes is important for forensic research or to determine whether a fire-exposed concrete structure and its components are still structurally sound or not. Assessment of fire-damaged concrete structures usually starts with visual observation of colour change, cracking and spalling. Thus, it is important to know the effect of elevated temperatures on strength retention properties of concrete. This study reports the effect of elevated temperature on the mechanical properties of the concrete specimen with polypropylene fibres and cooled differently under various regimes. In the heating cycle, the specimen were subjected to elevated temperatures ranging from $200^{\circ}C$ to $800^{\circ}C$, in steps of $200^{\circ}C$ with a retention period of 1 hour. Then they were cooled to room temperature differently. The cooling regimes studied include, furnace cooling, air cooling and sudden cooling. After exposure to elevated temperatures and cooled differently, the weight loss, residual compressive and split tensile strengths retention characteristics were studied. Test results indicated that weight and both compressive and tensile strengths significantly reduce, with an increase in temperature and are strongly dependent on cooling regimes adopted.

• 비파괴검사학회지
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• 제32권2호
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• pp.122-130
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• 2012

Adhesive bend and shear tests of micro-sized bonded component have been performed to clarify the relationship between effects of heat-treatment on the adhesive strength and the bonded specimen shape using Weibull analysis. Multiple micro-sized SU-8 columns with four different diameters were fabricated on a Si substrate under the same fabrication condition. Heat-treatment can improve both of the adhesive bend and shear strength. The improvement rate of the adhesive shear strength is much larger than that of the adhesive bend strength, because the residual stress, which must change by heat-treatment, should effect more strongly on the shear loading. In case of bend type test, the adhesive bend strength in the smaller diameters (50 and $75\;{\mu}m$) widely vary, because the critical size of the natural defect (micro-crack) should vary more widely in the smaller diameters. In contrast, in case of shear type test, the adhesive shear strengths in each diameter of the columns little vary. This suggests that the size of the natural defects may not strongly influence on the adhesive shear strength. All the result suggests that both of the adhesive bend and shear strengths should be complicatedly affected by heat-treatment and the bonded columnar diameter.

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2003년도 춘계학술발표대회 논문집
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• pp.129-134
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• 2003

The importance of understanding the response of structural composites to impact and CAI cannot be overstated to develop analytical models for impact damage and CAI strength predictions. This paper presents experimental findings observed from quasi-static lateral load tests, low velocity impact tests, CAI strength and open hole compressive strength tests using 3mm thick composite plates ($[45/-45/0/90]_{3s}$ - IM7/8552). The conclusion is drawn that damage areas for both quasi-static lateral load and impact tests are similar and the curves of several drop weight impacts with varying energy levels (between 5.4 J and 18.7 J) fallow the static curve well. In addition, at a given energy the peak force is in good agreement between the static and impact cases. From the CAI strength and open hole compressive strength tests, it is identified that the failure behaviour of the specimens was very similar to that observed in laminated plates with open holes under compression loading. The residual strengths are in good agreement with the measured open hole compressive strengths, considering the impact damage site as an equivalent hole. The experimental findings suggest that simple analytical models for the prediction of impact damage area and CAI strength can be developed on the basis of the failure mechanism observed from the experimental tests.

• The Journal of Korean Physical Therapy
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• 제25권5호
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• pp.273-281
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• 2013

Purpose: The purpose of this study is to determine the effect of motor imagery training on residual upper extremity strength and activities of daily living of chronic cervical spinal cord injury patients. Methods: Twelve ASIA A B patients, who had more than a 12-month duration of illness and C5 or 6 motor nerve injury level, were randomly divided into experimental group (n=6) and control group (n=6). Patients in the experimental group performed motor imagery training for five minutes prior to general muscle strengthening training, while those in the control group performed general muscle strengthening training only. The training was performed five times per week, 30 minutes per day, for a period of four weeks. General muscle strengthening training consisted of a progressive resistive exercise for residual upper extremity. Motor imagery training consisted of imagining this task performance. Before and after the training, EMG activity using BTS Pocket Electromyography and Spinal Cord Independent Measure III(SCIM III) were compared and analyzed. Results: The residual upper extremity muscle strengths showed improvement in both groups after training. Comparison of muscle strength improvement between the two groups showed a statistically significant improvement in the experimental group compared to the control group (p<0.05). SCIM III measurements showed significant improvement in the scores for Self-care and Transfer items in the experimental group. Conclusion: Motor imagery training was more effective than general muscle strengthening training in improving the residual upper extremity muscle strength and activities of daily living of patients with chronic cervical spinal cord injury.