• Title/Summary/Keyword: loading and unloading effect

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Characteristics of Cracks under Vickers Indentation in Glass Using Acoustic Emission (음향방출을 이용한 유리의 비커스 압입 균열 특성)

  • Park, H.Y.;Lee, J.K.;Park, H.I.;Lee, B.W.
    • Journal of Power System Engineering
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    • v.16 no.1
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    • pp.72-77
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    • 2012
  • Acoustic emission (AE) is known to be sensitive to fracture process and so it was expected that AE data may propose as a means of monitoring the fracture information. The aim of this study is to analyze the characteristic of AE signal emitted from glass during Vickers indentation. To observe AE characteristics by surface effect, both glass and coating glass were studied. During Vickers indentation loading, AE signal resulted from penny-like crack is detected. During Vickers indentation unloading, AE signal resulted from both radial/median crack and lateral crack is detected. In case of indentation on glass, the emission energy(${\epsilon}$) is found to be approximately proportional to the fourth power of the crack length. In case of indentation on coating glass, the emission energy(${\epsilon}$) is approximately proportional to the crack length.

A Methodology for Monitoring Prestressed Force of Bridges Using OFS-embedded Strand (광섬유센서가 내장된 강연선을 이용한 교량의 장력 모니터링 방법)

  • Kim, Hyoun-Wo;Kim, Jae-Min;Kim, Jin-Won;Kim, Young-Sang;Yun, Chung-Bang
    • Proceedings of the Computational Structural Engineering Institute Conference
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    • 2008.04a
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    • pp.133-138
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    • 2008
  • This study proposes a novel method for in-service evaluation of force in an external prestressing 7-wire tendon which is employed for retrofitting bridge superstructure. For this propose, a smart strand 7.0m long whose king wire is replaced by a steel tube and the FBG sensor, is developed. Performance of the strand is demonstrated through loading-unloading tests for a RC T-shaped beam 6.4m long. Finally, a couple of test results are presented to discuss effect of temperature change in the FBG sensor.

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Modeling the Effect of Water, Excavation Sequence and Reinforcement on the Response of Tunnels

  • Kim, Yong-Il
    • Journal of the Korean Geotechnical Society
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    • v.15 no.3
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    • pp.161-176
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    • 1999
  • A powerful numerical method that can be used for modeling rock-structure interaction is the Discontinuous Deformation Analysis (D D A) method developed by Shi in 1988. In this method, rock masses are treated as systems of finite and deformable blocks. Large rock mass deformations and block movements are allowed. Although various extensions of the D D A method have been proposed in the literature, the method is not capable of modeling water-block interaction, sequential loading or unloading and rock reinforcement; three features that are needed when modeling surface or underground excavation in fractured rock. This paper presents three new extensions to the D D A method. The extensions consist of hydro-mechanical coupling between rock blocks and steady water flow in fractures, sequential loading or unloading, and rock reinforcement by rockbolts, shotcrete or concrete lining. Examples of application of the D D A method with the new extensions are presented. Simulations of the underground excavation of the \ulcornerUnju Tunnel\ulcorner in Korea were carried out to evaluate the influence of fracture flow, excavation sequence and reinforcement on the tunnel stability. The results of the present study indicate that fracture flow and improper selection of excavation sequence could have a destabilizing effect on the tunnel stability. On the other hand, reinforcement by rockbolts and shotcrete can stabilize the tunnel. It is found that, in general, the D D A program with the three new extensions can now be used as a practical tool in the design of underground structures. In particular, phases of construction (excavation, reinforcement) can now be simulated more realistically.

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Acoustic Emission Signal Analysis for Damage Assessment of the Reinforced Concrete Slab Structures (철근 콘크리트 슬래브 구조 손상 평가를 위한 음향방출 신호분석)

  • Kim, Jeong-Hee;Han, Byeong-Hee;Seo, Dae-Cheol;Yoon, Dong-Jin
    • Journal of the Korean Society for Nondestructive Testing
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    • v.29 no.4
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    • pp.360-367
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    • 2009
  • The acoustic emission(AE) behavior of reinforced concrete slab under flexural loading was investigated to assess the integrity. This study was aimed at identifying the characteristics of AE response associated with damage development. By applying cyclic loading in various load steps, it was able to differentiate each AE source such as distributed micro crack initiation, friction, flexural crack and localized diagonal tension crack. The secondary peak and the change of AE hit rate gave valuable criteria fur assessment. From the analysis of the felicity ratio, furthermore, it was shown that this values can be used for evaluating the degree of concrete damage. Based on the experimental results, this approach for practical AE application may provide a promising method for estimating the level of damage and distress in concrete structures.

Comparison of Rock Young's Moduli Determined from Various Measurement Methods (다양한 시험법으로 규명된 암반 탄성계수 비교)

  • Ryu Kuen-Hwan;Chang Chan-Dong
    • The Journal of Engineering Geology
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    • v.16 no.1 s.47
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    • pp.1-14
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    • 2006
  • Various measurements were carried out to estimate the modulus of deformation in two dominant rock types in Korea: granite and gneiss. Four most commonly used methods were utilized: Goodman jack tests, PS well logging, laboratory ultrasonic tests and laboratory uniaxial loading tests. Laboratory static and dynamic Young's moduli depend on the magnitude of the applied axial stress, range of Sequency used for measurement and the loading/unloading condition. As the laboratory measurement condition approaches to that in situ, the resultant moduli also appear to be comparable to that in situ. This suggests that the simulation of in situ stress condition is important when the modulus of rock is determined in the laboratory Dynamic Young's modulus is generally higher than static Young's modulus because of (micro)crack behavior in response to the stress, different range of frequency used for measurements, and the effect of the amplitude of deformation. Understanding of the relations in moduli from different measurement methods will help estimate appropriate in situ values.

Effect of Cold Cyclic Compaction on Densification of $Al_2O_3$ Powder/SiC Whisker Composite ($Al_2O_3$분말과 SiC 휘스커 복합체의 치밀화에 미치는 상온 반복 압축의 영향)

  • 최승완;김기태
    • Journal of the Korean Ceramic Society
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    • v.34 no.3
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    • pp.296-302
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    • 1997
  • The effect of cold cyclic compaction on densification of SiC whisker/Al2O3 composite was investigated. Re-lative density of the compact increased as the number of cycle and the compaction pressure increased and the bias pressure decreased. The rate of loading and unloading and the frequency of cold cyclic compaction did not affect much on sliding and rearrangement of the particles. Fracture of SiC whisker was hardly ob-served during cold cyclic compaction and the direction of whisker was randomly oriented throughout the compact regardless of the direction of compaction. Thus, cold cyclic compaction may be an efficient method to densify SiC whisker/Al2O3 composite.

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Ultimate Fracture Strength Analysis of Initially Cracked Plate (초기균열을 가진 판의 최종파괴 강도해석)

  • 백점기;서흥원
    • Proceedings of the Computational Structural Engineering Institute Conference
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    • 1991.10a
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    • pp.133-138
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    • 1991
  • The aim of the present paper is to develop a computer program predicting ultimate fracture strength of initially cracked structure under monotonically increasing external loads. For this purpose, two kinds of 3-D isoparametric solid elements, one 6-node wedge element and another 8-node brick element are formulated along the small deformation theory. Plasticity in the element is checked using von Mises' yield criterion. Elasto-plastic stiffness matrix of the element is calculated taking account of strain hardening effect. If the principal strain at crack tip which is one nodal point exceeds the critical strain dependin on the material property, crack tip is supposed to be opened and the crack tip node which was previously constrained in the direction perpendicular to the crack line is released. After that, the crack lay be propagated to the adjacent node. Once a crack tip node is fractured, the energy of the newly fractured node should be released which is to be absorbed by the remaining part. The accumulated reaction force which was carried by the newly fractured node so far is then applied in the opposite direction. During the action of crack tip relief force, since unloading may be occured in the plastic element, unloading check should be made. If a plastic element unloads, elastic stress-strain equation is used in the calculation of the stiffness matrix of the element, while for a loading element, elasto-plastic stress-strain equation is continuously used. Verification of the computer program is made comparing with the experimental results for center cracked panel subjected to uniform tensile load. Also some factors affecting ultimate fracture strength of initially cracked plate are investigated. It is concluded that the computer program developed here gives an accurate solution and becomes useful tool for predicting ultimate fracture load of initially cracked structural system under monotonically increasing external loads.

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Dynamic Analysis of Floating Multi-Bodies Considering Crane Impact Loads (크레인 충격하중을 고려한 다중 부유체 운동해석)

  • Kim, Young-Bok;Kim, Yong-Yook
    • Journal of the Society of Naval Architects of Korea
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    • v.49 no.3
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    • pp.273-279
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    • 2012
  • The concept of the Mobile Harbor had been made recently as a kind of feeder vehicle to transfer a certain amount of container boxes (i.e. 250 TEU at a time) from main ocean container vessels over 5,000 TEU capacity to the container terminal on land. In a harbor a short distance apart from the land, the container loading/unloading operation has to be performed on the main deck of the Mobile Harbor using the container cranes in the state of side-by-side mooring with protection of fenders and robot arms in the gap. Even under the ocean condition of the sea state class 2 or 3, the operation has to be confirmed to be safely performed. In this situation, the floating bodies considering the multiple-body interaction effect also has to be examined whether they might behave safely or not. Especially, this study focuses on the dynamic behavior of the Mobile harbor when a container box is hanged on the crane and the impact load due to the slewing motion is imposed in a certain sea state. The motion response should be controlled within the motion level to assure the safe operation.

Application of Cardiac Electromechanical FE Model for Predicting Pumping Efficacy of LVAD According to Heart Failure Severity (심부전 정도에 따른 좌심실보조장치의 박동효율예측을 위한 심장의 전기역학적 유한요소 모델의 응용)

  • Jung, Dae Hyun;Lim, Ki Moo
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.38 no.8
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    • pp.715-720
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    • 2014
  • In order to maximize the effect of left ventricular assist device (LVAD) on ventricular unloading, the therapy should be begun at appropriate level of heart failure severity. We predicted pumping efficacy of LVAD according to the severity of heart failure theoretically. We used 3 dimensional finite element model of ventricle coupled with 6 Wind-kessel compartmental model of vascular system. Using the computational model, we predicted cardiac responses such as contractile ATP consumption of ventricle, left ventricular pressure, cardiac output, ejection fraction, and stroke work according to the severity of ventricular systolic dysfunction under the treatments of continuous LVAD. Contractile ATP consumption, which indicates the ventricular energetic loading condition decreased maximally at the $5^{th}$ level heart-failure under LVAD therapy. We conclude that optimal timing for LVAD treatment is $5^{th}$ level heart-failure when considering LVAD treatment as "bridge to recovery".

Temperature-dependent Mullins Effect in Anti-vibration Rubber for Railway Vehicles (철도 차량용 방진고무의 온도에 따른 뮬린스 효과)

  • Oh, Sunghun;Lee, Su-Yeong;You, Jihye;Kim, Hong Seok;Cheong, Seong-Kyun;Shin, Ki-Hoon
    • Journal of the Korean Society of Manufacturing Technology Engineers
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    • v.26 no.2
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    • pp.193-198
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    • 2017
  • Rubber materials are widely used for anti-vibration in various industries such as railways, automobile, and aviation. However, various factors hinder the accurate prediction of mechanical properties and lifetime of these materials. Particularly, a stress softening phenomenon Mullins effect greatly affects the accuracy of test results by reducing the initial peak stress. Although the Mullins effect has been studied previously, research on its temperature dependence is lacking. In this study, we performed experiments to estimate the temperature dependence of the Mullins effect. Dumbbell specimens made of natural rubber (NR65) was mounted on a stress softening tester and placed in a heat chamber, where they were tested at temperature of 25, 50, and $80^{\circ}C$. Further, five test sets, each consisting of 10 loading/unloading cycles were sequentially performed at predetermined time intervals. Based on the test results, we assessed the effect of temperature and time interval on stress softening and recovery.