• Title/Summary/Keyword: tensile cracks

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Determination of Optimum Heating Regions for Thermal Prestressing Method Using Artificial Neural Network (인공신경망을 이용한 온도프리스트레싱 공법의 적정 가열구간 설정에 관한 연구)

  • Kim, Jun Hwan;Ahn, Jin-Hee;Kim, Kang Mi;Kim, Sang Hyo
    • Journal of Korean Society of Steel Construction
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    • v.19 no.6
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    • pp.695-702
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    • 2007
  • The Thermal Prestressing Method for continuous composite girder bridges is a new design and construction method developed to induce initial composite stresses in the concrete slab at negative bending regions. Due to the induced initial stresses, prevention of tensile cracks at the concrete slab, reduction of steel girder section, and reduction of reinforcing bars are possible. Thus, the construction efficiency can be improved and the construction can be made more economical. The method for determining the optimum heating region of the thermal prestressing method has not been established although such method is essential for improving the efficiency of the design process. The trial-and-error method used in previous studies is far from efficient, and a more rational method for computing optimal heating region is required. In this study, an efficient method for determining the optimum heating region in using the thermal prestressing method was developed based on the neural network algorithm, which is widely adopted to pattern recognition, optimization, diagnosis, and estimation problems in various fields. Back-propagation algorithm, commonly used as a learning algorithm in neural network problems, was used for the training of the neural network. Through case studies of two-span and three-span continuous composite girder bridges using the developed procedure, the optimal heating regions were obtained.

A Study on the Development of Diagnosing System of Defects on Surface of Inner Overlay Welding of Long Pipes using Liquid Penetrant Test (PT를 이용한 파이프내면 육성용접부 표면결함 진단시스템 개발에 관한 연구)

  • Lho, Tae-Jung
    • Journal of the Korea Academia-Industrial cooperation Society
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    • v.19 no.10
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    • pp.121-127
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    • 2018
  • A system for diagnosing surface defects of long and large pipe inner overlay welds, 1m in diameter and 6m in length, was developed using a Liquid Penetrant Test (PT). First, CATIA was used to model all major units and PT machines in 3-dimensions. They were used for structural strength analysis and strain analysis, and to check the motion interference phenomenon of each unit to produce two-dimensional production drawings. Structural strength analysis and deformation analysis using the ANSYS results in a maximum equivalent stress of 44.901 MPa, which is less than the yield tensile strength of SS400 (200 MPa), a material of the PT Machine. An examination of the performance of the developed equipment revealed a maximum travel speed of 7.2 m/min., maximum rotational speed of 9 rpm, repeatable position accuracy of 1.2 mm, and inspection speed of $1.65m^2/min$. The results of the automatic PT-inspection system developed to check for surface defects, such as cracks, porosity, and undercut, were in accordance with the method of ASME SEC. V&VIII. In addition, the results of corrosion testing of the overlay weld layer in accordance with the ferric chloride fitting test by the method of ASME G48-11 indicated that the weight loss was $0.3g/m^2$, and met the specifications. Furthermore, the chemical composition of the overlay welds was analyzed according to the method described in ASTM A375-14, and all components met the specifications.

A Study on the Electrical Resistivity of Graphene Added Carbon Black Composite Electrode with Tensile Strain (인장변형에 따른 그래핀복합 카본블랙전극의 저항변화연구)

  • Lee, T.W.;Lee, H.S.;Park, H.H.
    • Journal of the Microelectronics and Packaging Society
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    • v.22 no.1
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    • pp.55-61
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    • 2015
  • Stretchable electrode materials are focused to apply to flexible device such as e-skin and wearable computer. Used as a flexible electrode, increase in electrical resistance should be minimalized under physical strain as bend, stretch and twist. Carbon black is one of candidates, for it has many advantages of low cost, simple processing, and especially reduction in resistivity with stretching. However electrical conductivity of carbon black is relatively low to be used for electrodes. Instead graphene is one of the promising electronic materials which have great electrical conductivity and flexibility. So it is expected that graphene added carbon black may be proper to be used for stretchable electrode. In this study, under stretching electrical property of graphene added carbon black composite electrode was investigated. Mechanical stretching induced cracks in electrode which means breakage of conductive path. However stretching induced aligned graphene enhanced connectivity of carbon fillers and maintained conductive network. Above all, electronic structure of carbon electrode was changed to conduct electrons effectively under stretching by adding graphene. In conclusion, an addition of graphene gives potential of carbon black composite as a stretchable electrode.

Development of DHLT Joint for Vertical Cutoff Walls in Offshore Waste Landfill Site (해상처분장 연직차수공을 위한 DHLT 이음부의 개발)

  • Hong, Young-Ho;Lee, Jong-Sub;Lee, Dongsoo;Chae, Kwang-Seok;Yu, Jung-Doung
    • Journal of the Korean Geotechnical Society
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    • v.34 no.3
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    • pp.43-56
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    • 2018
  • Vertical cutoff walls such as steel pipe sheet piles (SPSPs) have been commonly applied for the construction of the offshore waste landfill site. Because the SPSPs are sequentially installed by connecting their joints to those of adjacent piles, their mechanical stability should be ensured against the inherent external forces on the sea. The objective of this study is to evaluate the structural performances of the newly developed types of SPSP joint compared with those of other joint types. The problems of the traditional SPSP joints are investigated, and an advanced joint shape of SPSP, which is named double H with L-T (DHLT) joint, are designed for improving the constructability and maintenance. Full-scale models of the DHLT joint are manufactured, and then its joint areas are filled with grout material. After 28 days of curing time, compressive and tensile strength tests were performed on the joint models and the test results were compared with those of the traditional joints. Experimental results show that the structural capacities of the DHLT joint models are lower than those of traditional joints due to the influence of grout and steel members. In the cases of the compressive strength test, especially, bending occurs on steel H-beam with no distinct cracks in grout due to the asymmetrical structure of joint which has no reaction force. This study shows that the performance of the SPSP joint can be improved by considering the influence factors on the structural capacities estimated by the experimental tests.

Cracking Behavior of Reinforced Concrete Structures due th Reinforcing Steel Corrosion (철근부식에 의한 철근콘크리트 구조물의 균열거동)

  • 오병환;김기현;장승엽;강의영;장봉석
    • Journal of the Korea Concrete Institute
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    • v.14 no.6
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    • pp.851-863
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    • 2002
  • Corrosion products of reinforcement in concrete induce pressure to the adjacent concrete due to the expansion of steel. This expansion causes tensile stresses around the reinforcing bar and eventually induces cracking through the concrete cover The cracking of concrete cover will adversely affect the safety as well as the service life of concrete structures. The purpose of the this study is to examine the critical corrosion amount which causes the cracking of concrete cover. To this end, a comprehensive experimental and theoretical study has been conducted. Major test variables include concrete strength and cover thickness. The strains at the surface of concrete cover have been measured according to the amount of steel corrosion. The corrosion products which penetrate into the pores and cracks around the steel bar have been considered in the calculation of expansive pressure due to steel corrosion. The present study indicates that the critical amount of corrosion, which causes the initiation of cracking, increases with an increase of compressive strength. A realistic relation between the expansive pressure and average strain of corrosion product layer in the corrosion region has been derived and the representative stiffness of corrosion layer was determined. A concept of pressure-free strain of corrosion product layer was introduced to explain the relation between the expansive pressure and corrosion strain. The proposed theory agrees well with experimental data and may be a good base for the realistic durability design of concrete structures.

An Experimental Study on Flexural Behavior of Steel Fiber Reinforced Ultra High Performance Concrete Prestressed Girders (강섬유 보강 초고성능 콘크리트 프리스트레스트 거더의 휨거동 실험 연구)

  • Yang, In-Hwan;Joh, Chang-Bin;Kim, Byung-Suk
    • Journal of the Korea Concrete Institute
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    • v.22 no.6
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    • pp.777-786
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    • 2010
  • This paper examines the flexural behavior of full-scale prestressed concrete girders that were constructed of steel fiber reinforced ultra high performance concrete (UHPC). This study is designed to provide more information about the bending characteristics of UHPC girders in order to establish a reasonable prediction model for flexural resistance and deflection for future structural design codes. Short steel fibers have been introduced into prestressed concrete T-girders in order to study their effects under flexural loads. Round straight high strength steel fibers were used at volume fraction of 2%. The girders were cast using 150~190 MPa steel fiber reinforced UHPC and were designed to assess the ability of steel fiber reinforced UHPC to carry flexural loads in prestressed girders. The experimental results show that steel fiber reinforced UHPC enhances the cracking behavior and ductility of beams. Moreover, when ultimate failure did occur, the failure of girders composed of steel fiber reinforced UHPC was observed to be precipitated by the pullout of steel fibers that were bridging tension cracks in the concrete. Flexural failure of girders occurred when the UHPC at a particular cross section began to lose tensile capacity due to steel fiber pullout. In addition, it was determined that the level of prestressing force influenced the ultimate load capacity.

Residual Stress and Elastic Modulus of Y2O3 Coating Deposited by EB-PVD and its Effects on Surface Crack Formation

  • Kim, Dae-Min;Han, Yoon-Soo;Kim, Seongwon;Oh, Yoon-Suk;Lim, Dae-Soon;Kim, Hyung-Tae;Lee, Sung-Min
    • Journal of the Korean Ceramic Society
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    • v.52 no.6
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    • pp.410-416
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    • 2015
  • Recently, a new $Y_2O_3$ coating deposited using the EB-PVD method has been developed for erosion resistant applications in fluorocarbon plasma environments. In this study, surface crack formation in the $Y_2O_3$ coating has been analyzed in terms of residual stress and elastic modulus. The coating, deposited on silicon substrate at temperatures higher than $600^{\circ}C$, showed itself to be sound, without surface cracks. When the residual stress of the coating was measured using the Stoney formula, it was found to be considerably lower than the value calculated using the elastic modulus and thermal expansion coefficient of bulk $Y_2O_3$. In addition, amorphous $SiO_2$ and crystalline $Al_2O_3$ coatings were similarly prepared and their residual stresses were compared to the calculated values. From nano-indentation measurement, the elastic modulus of the $Y_2O_3$ coating in the direction parallel to the coating surface was found to be lower than that in the normal direction. The lower modulus in the parallel direction was confirmed independently using the load-deflection curves of a micro-cantilever made of $Y_2O_3$ coating and from the average residual stress-temperature curve of the coated sample. The elastic modulus in these experiments was around 33 ~ 35 GPa, which is much lower than that of a sintered bulk sample. Thus, this low elastic modulus, which may come from the columnar feather-like structure of the coating, contributed to decreasing the average residual tensile stress. Finally, in terms of toughness and thermal cycling stability, the implications of the lowered elastic modulus are discussed.

Tension Stiffening Effect of RC Tension Members Reinforced with Amorphous Steel Fibers (비정질 강섬유로 보강된 철근콘크리트 인장부재의 인장강화효과)

  • Park, Kyoung-Woo;Lee, Jun-Seok;Kim, Woo;Kim, Dae-Joong;Lee, Gi-Yeol
    • Journal of the Korea Concrete Institute
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    • v.26 no.5
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    • pp.581-589
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    • 2014
  • This paper presents the tension stiffening behavior from experimental results of each 6 amorphous steel fibers and normal steel fibers reinforced direct tensile specimens with the main variables such as cover thickness to bar diameter ratio. A tension stiffening effect for steel fiber reinforced RC tension members improve on the increase in cover thickness, and also amorphous steel fiber is usually superior to normal steel fiber. The reinforcement of steel fibers controlled the splitting cracks and led to significant increase in the tension stiffening effect. In particular, if cover thickness is more than twice the bar diameter, the amorphous steel fiber reinforced specimen is controlled the splitting crack and increased the tension stiffening effect. And, the tension stiffening effect of amorphous steel fiber reinforced concrete tension members is different to current structural design code provision.

Life Time Prediction and Physical Properties of Chloroprene Rubber Aged by Seawater (클로로프렌 고무의 해수에 의한 물성 변화 및 노화 수명 예측)

  • Lee, Chan Koo;Yun, Ju Ho;Kim, Il;Shim, Sang Eun
    • Elastomers and Composites
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    • v.47 no.1
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    • pp.9-17
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    • 2012
  • Herein, life time prediction based on the deterioration of physical properties of chloroprene rubber (CR)aged by heat and seawater was performed. CR samples were experienced an accelerated test at $80^{\circ}C$, $100^{\circ}C$, $120^{\circ}C$ for heat aging, and $40^{\circ}C$, $60^{\circ}C$, $80^{\circ}C$ for seawater aging for 20,000 hrs. The change in tensile strength, maximum elongation,hardness was measured. As a result, the decrease in elongation was a major factor causing failure. The life time estimated using an Arrhenius model was 125 years at $23^{\circ}C$ for thermal aging and 9 years at $23^{\circ}C$ for seawater aging. SEM and elemental analysis reveal that cracks were generated and the content of oxygen was increased for CR agined by seawater. FT-IR spectrum shows the new C-O and C = O bonds were generated by the chemical reaction with seawater. Also, the glass transtion temperature was increased and the thermal decomposition was decreased by seawater aging.

Material Performance Evaluation of Ceramic Fiber Reinforced Concrete using Energetically Modified Industrial By-products (산업부산물의 활성분체 및 세라믹섬유 혼입 콘크리트의 재료성능 평가)

  • Choi, Seung Jai;Yang, Dal Hun;Lee, Tae Hee;Kim, Jang Ho Jay
    • Ecology and Resilient Infrastructure
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    • v.5 no.3
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    • pp.118-124
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    • 2018
  • Social infrastructures and industrial complexes have been actively constructed in South Korea since the 1960 s as part of the economic development plan, resulting in rapid industrialization. However, side-effects due to the industrialization have occurred. An increase in industrial by-products or wastes is a typical problem. Although some industrial by-products are recycled in Korea as well as worldwide, some wastes are landfilled or dumped in the sea. Although many researchers have executed various technologies for the disposal of industrial wastes, economic and environmental technologies have not been developed. Thus, this study aims to activate paper and fly ashes during the crush process to overcome the drawback of simple concrete mixed with paper and fly ashes, which cause a reduction in workability and strength, derive an optimal content and replacement ratio of concretes mixed with Energetically Modified Material (EMM), and evaluate the material performance. In addition, the basalt fiber is mixed simultaneously to achieve the reduction of cracks and improve the tensile strength.