• Composites Research
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• v.24 no.3
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• pp.12-16
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• 2011

High strain rate tensile tests were performed to measure the strain rate sensitivity of fiber reinforced composite material. The composite material was developed for the light weight design of an automotive FEM(front end module) carrier. Standard specimens for quasi-static tests of fiber reinforced composites can be found in ASTM D3039. However, in case of high strain rate tests, it was hard to find standard specimen shapes. In this study, three kinds of tensile specimens designed based on ASTM D638 were investigated to determined the adequate gauge width of tensile specimen for fiber reinforced composite. A drop tower type of high speed tensile apparatus was developed for strain rates of about 15/s and 100/s. Gauge width of 6mm, 8mm and 10mm were investigated. Test results showed the specimen of 8mm width was adequate for the high strain rate tensile tests of fiber reinforced composite. It was found the strength of the composite material increased as the strain rate increased.

• Journal of the Korea institute for structural maintenance and inspection
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• v.24 no.5
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• pp.95-102
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• 2020

For the purpose of developing a PE fiber-reinforced highly ductile cementitious composite having high tensile strain capacity more than 2% under the condition of containing aggregates with large particle size, this study investigated the tensile behavior of composites according to the particle size and distribution of aggregates in the composite. Compared with the mixture containing silica sand of which particle size is less than 0.6 mm, mixtures containing river sand and/or gravel with the maximum particle size of 2.36 mm, 4.75 mm, 5.6 mm, 6.7 mm were considered in the experimental design. The particle size distributions of aggregates were adjusted for the optimized distribution curves obtained from modified A&A model by blending different sizes of aggregates. All the mixtures presented clear strain-hardening behavior in the direct tensile tests. The mixtures with the blended aggregates to meet the optimum curves of aggregate size distributions showed higher tensile strain capacity than the mixture with silica sand. It was also found that the tensile strain capacity was improved as the maximum size of aggregate increased which resulted in wider particle size distribution. The mixtures with the maximum size of 5.6 mm and 6.7 mm presented very high tensile strain capacities of 4.83% and 5.89%, respectively. This study demonstrated that it was possible to use coarse aggregates in manufacturing highly ductile fiber-reinforced cementitous composite by adjusting the particle size distribution.

• Journal of the Korea Concrete Institute
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• v.15 no.1
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• pp.69-77
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• 2003

One directional and biaxial tension tests of 13 reinforced concrete panels were conducted to derive a constitutive law of concrete. Based on the test results, a model equation is derived for the stress-strain relationship of concrete in tension. Main test variables are reinforcement ratio and the load ratio applied in two directions. In addition a failure envelope of concrete in tension-tension region is suggested based on the initial crack occurrence. Test results show that the concrete carries substantial tensile stress even after cracking occurrence. However, the application of this proposed stress-strain relationship for concrete is limited to the case where the direction of reinforcement coincides with the direction of the applied principal stresses.

• Magazine of the Korea Concrete Institute
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• v.9 no.5
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• pp.115-125
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• 1997

콘크리트의 변형률국소화는 콘크리트의연화거동에 수반되어 변형이 국부적으로 집중되는 현상이다. 본 연구의 목적은 인장과 압축하중상태에서 콘크리트 부재에 발생하는 콘크리트 변형률 국소화 거동을 해석적으로 재현할 수 있는 통일된 모형을 제안하는 것이다. 본 논문에서는 인장과 압축에 대하여 변형률국소화가 일어나는 콘크리트 부재를 변형률 연화가 일어나는 국소화영역과 탄성제하가 발생하는 비국소화영역으로 구분하여 모델링하는 통일된 모형을 제안하였다. 또한 제안된 모형에서 미시역학적 평균화기법을 이용해 평균등가탄성계수와 수정된 평균등가탄성계수를 구하여 시편의 크기와 국소화영역의 크기에 따는 해석을 수행하였으며 기존의 실험값과 비교하였다. 연구결과, 본 연구에서의 변형률국소화모형이 크기효과를 포함한 콘크리트의 변형률국소화거동 해석에 타당하게 적용될 수 있음을 보여주었다.

• Journal of the Korean Recycled Construction Resources Institute
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• v.7 no.3
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• pp.279-286
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• 2019

In this study, an experimental study on the tensile properties of steel fiber-reinforced ultra high strength concrete(UHSC) with a standard compressive strength of 180MPa was performed. Steel fibers with a volume ratio of 1% were mixed to prepare direct tensile strength specimens and prism specimens for the three-point bending test. The fabricated specimens were set up in the middle section of the specimen to induce cracks, and the test was carried out according to each evaluation method. First, the stress-strain curves were analyzed by performing direct tensile strength tests to investigate the behavior characteristics of concrete after cracking. In addition, the load-CMOD curve was obtained through the three-point bending test, and the inverse analysis was performed to evaluate the stress-strain curve. Tensile behavior characteristics of the direct tensile test and the three-point bending test of the indirect test were similar. In addition, the tensile stress-strain curve modeling presented in the SC structural design guidelines was performed, and the comparative analysis of the measured and predicted values was performed. When the material reduction factor of 1.0 was applied, the predicted value was similar to the measured value up to the strain of 0.02, but when the material reduction factor of 0.8 was applied, the predicted value was close to the lower limit of the measured value. In addition, when the strain was greater than 0.02, the predicted value by SC structural design guideline to underestimated the measured value.

• Journal of the Korea institute for structural maintenance and inspection
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• v.6 no.3
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• pp.193-199
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• 2002

Structral crack of RC structure generally occurs when the tension stress by applied load is larger than tension resistance of concrete, and it means deterioration of structure and the decrease of load resistance. Because structural crack of structure can occur critical damage to structure occasionally, the research on crack detection algorithm of RC structure is needed for assurance of structural safety and effective maintenance of structure. In this paper, we executed the laboratory test on measuring strain of RC beam's tension and compression zone, using strain gauge which is widely used on strain measurement of civil structure. By using measured strain, we analyzed strain change, elastic modulus change, and neutral axis change to detect crack of RC beam. As a result, we proposed the simple and effective crack detection algorithm using trends of neutral axis position change.

• Journal of the Korea institute for structural maintenance and inspection
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• v.24 no.6
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• pp.25-33
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• 2020

To investigate the relationship between strain distribution and tensile properties of brittle material, five types of tensile coupon of carbon fiber reinforced polymer (CFRP) modified the tab portion in order to have a strain distribution including S0, SD1, SD2, SV1, SV2 were tested. The ultimate stress and strain of SD2 and SV2 which was intended to have larger strain distribution were smaller than those of SD1 and SV1, that was more clearly shown in the test results of the symmetric coupons (SV series) than the asymmetric coupons (SD series). In addition, the ultimate stress and strain of most coupons with strain distribution in this study were decreased when compared to the control group with uniform strain. These results were analyzed in various ways through 1) the average of the strain values directly measured by the strain gages, 2) the converted strain calculated by dividing the total deformation by the effective length, and 3) the ultimate effective strain derived from both the elastic modulus and the ultimate load. The values measured by strain gage indicates response of the local region precisely, but it does not represent the response from whole section. However, the converted strain and effective strain can supplement disadvantage of gage because they represent the average response of whole section. In particular, the effective strain can provide rupture strain conservatively, which can be utilized in practice, when the value obtained by strain gage was not effective due to gage damage or abnormal gage readings near ultimate load. This value provides a value that can be used even when partial rupture has occurred and is reasonably useful for specimens with strain distribution.

• Proceedings of the KWS Conference
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• 2010.05a
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• pp.79-79
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• 2010

저온용 무연 솔더의 대표 조성으로 고려되고 있는 Sn-58Bi(융점: $138^{\circ}C$) 공정(eutectic) 조성은 우수한 강도에도 불구하고 연성(ductility) 측면에서의 문제점이 지속적으로 보고되고 있다. 따라서 이 합금계의 연성을 최대로 개선시킬 수 있으면서도 실제 상용화가 가능한 합금 조성의 개발 연구가 요청된다. 본 연구에서는 Sn-Bi 2원계 조성에서 최대의 연성을 나타내는 것으로 보고된 Sn-40Bi 조성에 미량의 합금원소를 첨가함으로써 최대의 연성을 확보하는 한편, 그 연성 특성이 변형속도에 어느 정도 민감한지를 인장 실험을 통해 결정하고자 하였다. 합금원소로는 0.1~0.5 wt%의 Ag, Mn, In, Cu를 선택하였으며, 인장 시편을 제조하여 $10^{-2}$, $10^{-3}$, $10^{-4}\;s^{-1}$의 3종류로 변형속도를 변형시켜가며 응력-변형 곡선(stress-strain curve)을 측정하였고, 조성별, 변형속도별로 최대인장강도(ultimate tensile stress, UTS) 및 연신율 결과들을 정리하였다. 합금원소를 첨가한 조성의 경우는 모든 시험 조건에서 Sn-40Bi보다 우수한 연신률을 나타내는 것으로 측정되었으나, $10^{-2}\;s^{-1}$의 빠른 변형속도에서는 그 향상 정도가 상대적으로 감소하는 경향이 관찰되었다. 특히 Sn-40Bi-0.5Ag 조성의 경우 느린 변형속도에서 특히 눈에 띄는 연신률 값을 나타내며, 모든 변형속도 조건에서 가장 우수한 연성을 나타내었다. 한편 Sn-40Bi-0.1Cu 조성의 경우 변형속도에 따른 연신률의 변화 정도, 즉, 변형속도에 따른 연신률의 민감도가 매우 커 $10^{-4}\;s^{-1}$ 속도에서는 Sn-40Bi-0.5Ag에 버금가는 연신률 값이 측정되었으나, $10^{-2}\;s^{-1}$ 속도에서는 가장 나쁜 연신률 특성을 보여주었다. Sn-40Bi-0.2Mn 조성은 최고의 연신률 향상 특성을 나타내지는 않았으나, In을 첨가한 경우보다는 대체적으로 우수한 연성을 나타내었다. 이상의 각 합금별 연성 특성은 인장시험 전의 미세조직 관찰 결과와 인장시험 후 파면부의 조직변화 관찰 결과로부터 해석되었다. 그 결과 석출상의 형성 여부, 인장 시험 중 재결정 조직의 형성 여부, 라멜라(lamellar) 조직의 분율과 라멜라 간격(lamellar spacing)의 정도 또는 $\beta$-Sn과 라멜라 조직 사이의 결정립계와 라멜라 조직 내 결정립계에서의 슬라이딩 모드(sliding mode) 변형 정도, 석출상의 크기와 분포 정도 등이 연신률 및 변형속도 민감도와 같은 연성 특성에 가장 큰 영향을 미치는 인자인 것으로 분석되었다.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.15 no.1
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• pp.69-84
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• 2002

In this papers, an analytical model which can simulate the post-cracking behavior and tension stiffening effect in a reinforced concrete(RC) tension member is proposed. Unlike the classical approaches using the bond stress-slip relationship or the assumed bond stress distribution, the tension stiffening effect at post-cracking stage is quantified on the basis of polynomial strain distribution functions of steel and concrete, and its contribution is implemented into the reinforcing steel. The introduced model can be effectively used in constructing the stress-strain curve of concrete at post-cracking stage, and the loads carried by concrete and reinforcing steel along the member axis can be directly evaluated on the basis of the introduced model. In advance, the prediction of cracking loads and elongations of reinforced steel using the introduced model shows good agreement with results from the previous analytical studies and experimental data.

• Journal of the Korea institute for structural maintenance and inspection
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• v.22 no.1
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• pp.90-98
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• 2018

Recently, various reinforced concrete joints have been used in reinforced concrete structures. Therefore, it is important to grasp the tensile properties of the spliced rebar. In this study, uniaxial tensile tests were conducted on Grade 60 D22(#7), D29(#9), and two kinds of couplers manufactured according to ASTM A615 standard for evaluating ductility of coupler joints. The strain was measured using an image processing method more accurate and capable of measuring at various points freely. As the result of uniaxial tensile test, it was possible to calculate the stress-strain relationship and the longitudinal strain distribution according to the stress stages and it was founded that the average strain becomes lower as more occupying the coupler joint portions in the same gauge length. In addition, the empirical equations are proposed to account for the effect of the length of the coupler on the ultimate strain and the rupture strain.