• Title/Summary/Keyword: cracking and brittle fracture

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A Study on Thermomechanical Analysis of Laser Ablation on Cr thin film (크롬박막의 레이저 어블레이션에서 열적.기계적 해석에 관한 연구)

  • 윤경구;장원석;이성국;김재구;나석주
    • Proceedings of the Korean Society of Precision Engineering Conference
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    • 2001.04a
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    • pp.914-917
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    • 2001
  • Single-shot laser damage of thin Cr films on glass substrates has been studied to understand the cracking and peeling-off mechanism. A numerical model is developed for the calculation of transient heat transfer and thermal stresses in Cr films during excimer laser irradiation and cooling, the transient temperature, and the stress-strain fields are analyzed by using a three-dimensional finite-element model of heat flow. According to the numerical analysis for the experimentally determined cracking and peeling-off conditions, cracking is found to be the result of the tensile brittle fracture due to the excessive thermal stresses formed during the cooling process, while peeling-off is found to be the combined result of films bulging from the softened glass surface at higher temperature and the tensile brittle fracture during the cooling process.

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Study of Hot Salt Stress Corrosion Crack Initiation of Alloy IMI 834 by using DC Potential Drop Method

  • Pustode, Mangesh D.;Dewangan, Bhupendra;Raja, V.S.;Paulose, Neeta;Babu, Narendra
    • Corrosion Science and Technology
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    • v.15 no.5
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    • pp.203-208
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    • 2016
  • DC potential drop technique was employed during the slow strain rate tests to study the hot salt stress corrosion crack (HSSCC) initiation at 300 and $400^{\circ}C$. Threshold stresses for HSSCC initiation were found to about 88 % of the yield strength at both temperatures, but the time from crack initiation to final failure (${\Delta}t_{scc}$) decreased significantly with temperature, which reflects larger tendency for brittle fracture and secondary cracking. The brittle fracture features consisted of transgranular cracking through the primary ${\alpha}$ grain and discontinuous faceted cracking through the transformed ${\beta}$ grains.

Cracking Susceptibility of Laser Cladding Process with Co-Based Metal Matrix Composite Powders (레이저 클래딩 공정 조건이 코발트 합금-텅스텐 카바이드 혼합 코팅층의 균열 발생에 미치는 영향)

  • Lee, Changmin;Park, Hyungkwon;Lee, Changhee
    • Journal of Welding and Joining
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    • v.32 no.6
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    • pp.41-46
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    • 2014
  • In this study, cracking susceptibility of laser cladding was investigated according to the processing parameters such as laser power, scan speed and feeding rate with blended powders of stellite#6 and technolase40s (WC+NiCr). The solidification microstructure of clad was composed of Co-based dendrite structures with ${\gamma}+Cr7C3$ eutectic phases at the dendritic boundaries. The crack propagation showed transgranular fracture along dendritic boundaries due to brittle chrome carbide at the eutectic phases. From results of fractography experiments, the fracture surface was typical cleavage brittle fracture in the clad and substrate. The number of clad cracks, caused by a tensile stress after the solidification, increased with increase of laser power, scan speed and feeding rate. Increase of the laser power caused large pores by facilitating WC decarburizing reaction. And the pores affected increase of crack susceptibility. High scan speed caused increment of clad cracks due to thermal stress and WC particle fractures. Also, increase of the feeding rate accompanied an amount of WC particles causing crack initiation and decarburizing reaction.

Temperature effects on brittle fracture in cracked asphalt concretes

  • Ayatollahi, Majid-Reza;Pirmohammad, Sadjad
    • Structural Engineering and Mechanics
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    • v.45 no.1
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    • pp.19-32
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    • 2013
  • Cracking at low temperatures is one of the frequently observed modes of failure in asphalt concretes. In this investigation, fracture tests were performed on cracked asphalt concrete subjected to pure mode I and pure mode II loading at different subzero temperatures. An improved semi-circular bend (SCB) specimen containing a vertical crack was used to conduct the experiments. The SCB specimens produced from the gyratory compacted cylindrical samples were compressively loaded, and critical stress intensity factors, $K_{If}$ and $K_{IIf}$, were then calculated using peak loads obtained from the tests. The experimental results showed that with decreasing the temperature, mode I and mode II critical stress intensity factors increased first but below a certain temperature they both decreased. It was also found that at a fixed temperature, the mode II fracture resistance of the asphalt concrete was higher than its mode I fracture resistance.

Natural Element Analysis on Micro-cracking Behavior of Brittle Solids (취성 재료의 마이크로 크랙킹 거동에 관한 자연요소해석)

  • Kang, Sung-Soo
    • Journal of Advanced Marine Engineering and Technology
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    • v.30 no.6
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    • pp.724-730
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    • 2006
  • Fracture behavior of brittle solids is closely related to microcracking. A meso-scale analysis method using the natural element method is proposed for the analysis of brittle microcracking solids. The microcracking is assumed to occur along Voronoi edges in the Voronoi diagram generated using the nodal points as the generators. The mechanical effect of microcracks is considered by controlling the material constants in the neighborhood of the microcracks. The meso-analysis method is applied to the simulation of the microcracking behaviors of brittle solids subjected to tensile macrostress. The method is also applied to the analysis of the propagation of a macrocrack accompanied by the coalescence with microcracks formed near the macrocrack-tip.

Evaluation of R-curve Behavior Analysis and Machinability of $Si_3N_4-hBN$ Machinable Ceramics ($Si_3N_4-hBN$ 머시너블 세라믹의 R-curve 거동분석과 가공성 평가)

  • 장성민;조명우;조원승;이재형
    • Journal of the Korean Society for Precision Engineering
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    • v.21 no.1
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    • pp.61-70
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    • 2004
  • Generally, ceramics are very difficult-to-cut materials because of its high strength and hardness. The machining process of ceramics can be characterized by cracking and brittle fracture. In the machining of ceramics, edge chipping and crack propagation are the principal reasons to cause surface integrity deterioration. Such phenomenon can cause not only poor dimensional and geometric accuracy, but also possible failure of the ceramic parts. Ceramics can be machined with traditional method such as grinding and polishing. However, such processes are generally cost-expensive and have low material removal rate. Thus, in this paper, to overcome these problems. BN powder, which gives good cutting property, is added for the fabrication of machinable ceramics by volume of 5,10,15,20,25 and 30%. And, mechanical properties, R-curve behavior and machining tests are carried out to evaluate the machining properties of the manufactured machinable ceramics.

Local brittle zone of offshore structural steel welds (해양구조용 강재의 국부취화영역에 관한 연구)

  • 김병천;엄정현;이종섭;이성학;이두영
    • Journal of Welding and Joining
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    • v.7 no.2
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    • pp.35-48
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    • 1989
  • This study is concerned with a correlation of microstructure and local brittle zone (LBZ) in offshore structural steel welds. The influence of the LBZ on fracture toughness was investigated by means of simulated heat-affected zone (HAZ) tests as well as welded joint tests. Micromechanical processes involved in void and cleavage microcrack formation were also identified using notched round tensile tests and subsequent SEM observations. The LBZ in the HAZ of a multiphase welded joint is the interstitially reheated coarse grained HAZ, which is influenced by metallurgical factors such as effective grain size, the major matrix structure and the amount of high-carbon martensite-austenite (M-A) constituents. The experimental results indicate that Chirpy energy was found to scale monotonically with the amount of M-A constituents, confirming that the M-A constituent is the major microstructural factor controlling the HAZ toughness. In addition, voids and microcracks are observed to initiate at M-A constituents by the shear cracking process. Thus, the M-A constituent played an important role in initiating the voids and microcracks, and consequently caused brittle fracture.

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Ductile cracking simulation procedure for welded joints under monotonic tension

  • Jia, Liang-Jiu;Ikai, Toyoki;Kang, Lan;Ge, Hanbin;Kato, Tomoya
    • Structural Engineering and Mechanics
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    • v.60 no.1
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    • pp.51-69
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    • 2016
  • A large number of welded steel moment-resisting framed (SMRF) structures failed due to brittle fracture induced by ductile fracture at beam-to-column connections during 1994 Northridge earthquake and 1995 Kobe (Hyogoken-Nanbu) earthquake. Extensive research efforts have been devoted to clarifying the mechanism of the observed failures and corresponding countermeasures to ensure more ductile design of welded SMRF structures, while limited research on the failure analysis of the ductile cracking was conducted due to lack of computational capacity and proper theoretical models. As the first step to solve this complicated problem, this paper aims to establish a straightforward procedure to simulate ductile cracking of welded joints under monotonic tension. There are two difficulties in achieving the aim of this study, including measurement of true stress-true strain data and ductile fracture parameters of different subzones in a welded joint, such as weld deposit, heat affected zone and the boundary between the two. Butt joints are employed in this study for their simple configuration. Both experimental and numerical studies on two types of butt joints are conducted. The validity of the proposed procedure is proved by comparison between the experimental and numerical results.

Surface Properties and Tool Wear of Si3n4-hBN Machinable Ceramics in Endmill Machining using Tungsten Carbide Tool (텅스텐 카바이드 공구를 사용한 앤드밀 가공에서 Si3n4-hBN 머시너블 세라믹스의 표면특성과 공구마멸)

  • Jang, Sung-Min;Cho, Myeong-Woo
    • Journal of the Korean Society of Manufacturing Process Engineers
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    • v.3 no.1
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    • pp.15-21
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    • 2004
  • The machining process of ceramics can be characterized by cracking and brittle fracture. In the machining of ceramics, edge chipping and crack propagation are the principal reasons to cause surface integrity deterioration. Such phenomenon can cause not only poor dimensional and geometric accuracy, but also possible failure of the ceramic parts. Thus, traditional ceramics are very difficult-to-cut materials. To overcome such problems, in this paper, h-BN powder, which gives good cutting property, is added for the fabrication of machinable ceramics by volume of 5, 10, 15, 20, 25 and 30%. The objectives of this paper is to evaluate the fracture phenomenon of the tungsten carbide tool and the variation of surface integrity of the manufactured machinable ceramics under various cutting conditions during end mill machining With CNC machining center.

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The influence of microstructure size on the tensile fracture behavior or dual phase steel (복합조직망의 파괴거동에 미치는 미시조직크기의 영향)

  • ;Kim, Jung Kyu
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.5 no.3
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    • pp.223-229
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    • 1981
  • A study has been made of the fracture strength and ductility of the dual phase microstructure, in which the martensitic phase encapsulated islands of ferritic phase in association with the cleavage cracking of ferrite grains. It was found the final fracture occured in a brittle manner, starting from the Griffith crack which consisted of the cleavage crack in the ferrite grains and the cracks in second phase. Furthermore, the effects of the ferrite grain sizes on the Griffith crack were also discussed.