• Title/Summary/Keyword: Cleavage fracture

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Effect of Internal Stress due to Plastic Costraint On Fracture Ductility of Dual Phase Steel (複合組織鋼 의 破斷延性 에 미치는 塑性拘束 에 의한 內部應力 의 영향)

  • 김정규
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.7 no.2
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    • pp.123-129
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    • 1983
  • The effect of the micro-internal stress which is induced in the ferrite grain by plastic constraint, on fracture behavior was investigated. The specimen used has combined microstructure with matrix of ferrite encapsulated by second phase of martensite. The micro-internal stress in the ferrite grain was estimated using a simple mechanical model, and its effect on micro and macro fracture behaviors was discussed. The results obtained are summarized as follows; The micro-internal stress promotes the formation of cleavage cracks in the ferrite during deformation. Consequently, it was concluded that the internal stress is one of the significant factors which cause the fracture ductility to decrease.

The Effects of cathodic protection on fracture toughness of buried gas pipeline (매설가스배관의 음극방식이 배관의 파괴인성에 미치는 영향)

  • Kim, Cheol-Man;Kim, Woo-Sik
    • Proceedings of the KSME Conference
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    • 2001.06a
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    • pp.573-578
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    • 2001
  • For the corrosion protect ion of the natural gas transmission pipelines, two methods are used, cathodic protection and coating technique. In the case of cathodic protection, defects are embrittled by occurring hydrogen at the crack tip or material surface. It is however very important to evaluate whether cracks in the embrittled area can grow or not, especially in weld metal. In this work, on the basis of elastic plastic fracture mechanics, we performed CTOD test ing with varying test conditions, such as the potential and current density. The CTOD of the base steel and weld metal showed a strong dependence of the test conditions. The CTOD decreased with increasing cathodic potential and current density. The morphology of the fracture surface showed quasi-cleavage. Hydrogen introduced fractures, caused by cathodic overprotection.

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Fracture Toughness Evaluation of Natural Gas Pipeline under the Cathodic Protection

  • Kim, Cheol-Man;Baek, Jong-Hyun;Kim, Young-Pyo;Kim, Woo-Sik
    • Corrosion Science and Technology
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    • v.8 no.4
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    • pp.133-138
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    • 2009
  • For the corrosion protection of the natural gas transmission pipelines, two methods are used, cathodic protection and coating technique. In the case of cathodic protection, defects are embrittled by occurring hydrogen at the crack tip or material surface. It is however very important to evaluate whether cracks in the embrittled area can grow or not, especially in weld metal. In this work, on the basis of elastic plastic fracture mechanics, we performed the CTOD testing with various test conditions, such as testing rate and potential. The CTOD of the base metal and the weld metal showed a strong dependence of the test conditions. The CTOD decreased with decreasing testing rate and with increasing cathodic potential. The morphology of the fracture surface showed the quasi-cleavage at low testing rate and cathodic overprotection. The low CTOD was caused by hydrogen embrittlement at crack tip.

The Notch Effects on the Fatigue fracture Behaviour of Ferrite-Martensite Dual Phase Steel (페라이트-마르텐사이트 이상조직강의 피로파괴거동에 미치는 노치효과)

  • 도영민
    • Journal of the Korean Society of Safety
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    • v.18 no.3
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    • pp.46-53
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    • 2003
  • For the tensile tests of the F.E.M., microvoids are created by the boundary separation process at the martensite boundary or neighborhood and at inclusions within the fracture. to grow to the ductile dimple fracture. For the case of the M.E.F., microvoids created at the discontinuities of the martensite phase which exists at the grain boundary of the primary ferrite are grown to coalescence with the cleavage cracks induced at the interior of the ferrite, which as a result show the discontinuous brittle fracture behavior. In spite of their similar tensile strengths, the fatigue limit and the notch sensitivity of the M. E.F. is superior to those of the F.E.M., The M.E.F. is much more insensitive to notch than F.E.M. from the stress concentration factor($\alpha$).

Crack propagation behavior of in-situ structural gradient Ni/Ni-aluminide//Ti/Ti-aluminide laminate materials (Ni/Ni-aluminide//Ti/Ti-aluminide 구조경사형 층상재료의 균열 전파 거동)

  • Chung, D.S.;Kim, J.K.;Cho, H.
    • Journal of the Korean Crystal Growth and Crystal Technology
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    • v.15 no.6
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    • pp.269-275
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    • 2005
  • Ni/Ni-aluminide/Ti/Ti-aluminide laminate composite, considered as a functionally gradient material, was manufactured by thin foil hot press technique. Thick intermetallic layers of NiAl and $TiAl_3$ were formed by a self-propagating high-temperature synthesis (SHS) reaction, and thin continuous taters of $Ni_3Al$ and TiAl were formed by a solid-state diffusion. Fracture resistance with loading along the crack arrester direction is higher than crack divider direction due to the interruption of crack growth in metal layers. The $Ni_3Al$ and NiAl intermetallic layer showed cleavage and intergranular fracture behavior, respectively, while the fracture mode of $TiAl_3$ layer was found to be an intragranular cleavage. The debonding between metal and intermetallic layer and the pores were observed in the Ni/Ni-aluminide layers, resulting in the lower fracture resistance. With the results of acoustic emission (AE) source characterization the real time of failure and the effect of AE to crack growth could be monitored.

Effects of TiN Coating on the Fatigue Fracture of Dental Implant System with Various Cyclic Loads

  • Jung, Da-Un;Chung, Chae-Heon;Son, Mee-Kyoung;Choe, Han-Cheol
    • Journal of the Korean institute of surface engineering
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    • v.48 no.6
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    • pp.283-291
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    • 2015
  • The purpose of this study was to investigate effects of TiN coating on the fatigue fracture of dental implant system with various cyclic loads. TiN coated abutment screw, the fixture, and abutment of internal hex type were prepared for fatigue test. The fatigue test was carried out according to ISO 14801:2003(E) using tensile and compression tester with repeated load from 30% to 80% of static fracture force. Morphology and fractured surface was observed by field emission scanning electron microscope(FE-SEM) and energy dispersive X-ray spectroscope(EDS). The fracture cycle drastically decreased as repeated load increased. Especially, in the case of TiN-coated abutment screw, fracture cycle increased compared to non-coated abutment screw. The fatigue crack was propagated fast as repeated load increased. The plastic deformation region decreased, whereas, cleavage fracture region increased as repeated load increased.

Evaluation of Fracture Toughness by J-A$_2$ Method Considering Size Effect (시편크기의 영향을 고려한 J-A$_2$ 방법에 의한 파괴인성 평가)

  • 이정윤;김영종;김용환;김재훈
    • Journal of the Korean Society for Precision Engineering
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    • v.17 no.1
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    • pp.153-163
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    • 2000
  • The size effect on fracture toughness was investigated by introducing $J-A_2$ theory. For this application,small size specimens were chosen to establish $J-A_2$ assessment curve with FEM analysis. Two-dimensional FEM analysis was conducted with plane strain model using ABAQUS by domain integral method to calculate both crack tip stress and fracture toughness which were used to establish $J-A_2$ curve. The assessment curve predicted the fracture toughness of large specimens very well when compared to the test values. The results showed good prediction for deep crack specimen, though there were acceptable deviations in shallow cracked specimens, presumably caused by constraint effect. When the curve applied to reactor vessel in order to predict end of life fracture toughness with assumption of on-power pressure test condition, it provided the reasonable pressure compared to the existing design value. Better predictions would be possible if more test data were available.

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Fracture Analysis of Implant Components using Scanning Electron Microscope : Part II - Implant Retaining Screw (임플란트 구성요소의 파절면에 관한 주사전자현미경적 연구 : Part II - 임플란트 유지나사)

  • Lim, Kwang-Gil;Kim, Dae-Gon;Cho, Lee-Ra;Park, Chan-Jin
    • Journal of Dental Rehabilitation and Applied Science
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    • v.26 no.4
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    • pp.373-388
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    • 2010
  • Fracture causes serious problems in many instance of prosthetic failures. But it is hard to find the definite causes when fractures occur. Fractography encompasses the examination of fracture surfaces that contain features resulting from the interaction of the advancing crack with the microstructure of the material and the stress fields. All fractured specimens(implant retaining screw) retrieved from Gangneung-Wonju national university dental hospital for 3 years(from 2007 to 2009). After pretreatment of samples, the scanning electon microscope were used for surface examination and fracture analysis. In case of most of the fractured specimens, fracture took place by fatigue fracture and fractured surface represents fatigue striation. Fatigue striation indicate the progression of the crack front under cyclic loading, are characteristic of stage 2 crack growth. The site of crack initiation and stage 1 crack growth were not easily identified in any of the failure, presumably because of the complex microstructural features of the polycrystalline sample. In case of fractured by overload, dimpled or cleavage surface were observed. Using the interpretation of characteristic markings(ratchet mark, fatigue striation, dimple, cleavage et al) in fracture surfaces, failure events containing the crack origin, crack propagation, material deficiency could be understand. Using the interpretation of characteristic markings in fracture surfaces, cause and mechanism of fractures could be analyzed.

Parametric Studies on Hydrogen Embrittlement in Liquified Hydrogen Tank using Molecular Dynamics Simulation (분자동역학을 이용한 액화수소 연료탱크의 수소취성화 파라메터 연구)

  • Song-Hyun, Cha;Hyun-Seok, Kim;Seonho, Cho
    • Journal of the Computational Structural Engineering Institute of Korea
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    • v.35 no.6
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    • pp.325-331
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    • 2022
  • Hydrogen embrittlement in metals has been a serious issue with regard to structural safety. In this study, molecular dynamics simulations revealed that the aggregation of hydrogen atoms at the crack tips suppresses the dislocation emission and thus results in cleavage fracture. A series of molecular dynamics simulations were performed considering factors such as the concentration of hydrogen atoms, loading rate, and diffusion coefficient. We investigated the conditions that minimize hydrogen embrittlement. The simulation results were consistent with the experimental results and used to quantify hydrogen embrittlement.