• Title/Summary/Keyword: cracks coalescence

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Fatigue Life Estimation of Cruciform Welded Joint Considering Multiple Collinear Surface Cracks (십자형 필렛용접 이음부의 복수균열 진전수명 평가)

  • Han Seung Ho;Shin Byung Chun;Kim Jae Hoon;Han Jeong Woo
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.28 no.10
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    • pp.1549-1557
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    • 2004
  • Fatigue life of welded joints is governed by the propagation of multiple collinear surface cracks distributed randomly along weld toe. These cracks propagate under the mechanisms of mutual interaction and coalescence of the adjacent two cracks. To estimate the fatigue life, its influences on the above two mechanisms should be taken into account, which appear through the stress intensity factors disturbed mutually. However, it is difficult to calculate the stress intensity factors of the multiple surface cracks located in vicinity of weld toe due to its geometrical complexity. They are calculated normally by using the Μk-factors, but such Mk-factors are very rare in literature. In this study, the Μ$textsc{k}$-factors were obtained from a parametric study on crack length and depth, for which a finite element method is used. A fatigue test for a cruciform welded Joint was conducted and the fatigue life of the tested specimen was estimated using the present method with the informations obtained from the test, such as the number, size, and locations of the cracks. The estimated and measured fatigue life showed a good agreement.

PFC3D simulation of the effect of particle size on the single edge-notched rectangle bar in bending test

  • Haeri, Hadi;Sarfarazi, Vahab;Zhu, Zheming
    • Structural Engineering and Mechanics
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    • v.68 no.4
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    • pp.497-505
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    • 2018
  • Three points bending flexural test was modeled numerically to study the crack propagation in the pre-cracked beams. The pre-existing edge cracks in the beam models were considered to investigate the crack propagation and coalescence paths within the modeled samples. The effects of particle size on the single edge-notched round bar in bending test were considered too. The results show that Failure pattern is constant by increasing the ball diameter. Tensile cracks are dominant mode of failure. These crack initiates from notch tip, propagate parallel to loading axis and coalescence with upper model boundary. Number of cracks increase by decreasing the ball diameter. Also, tensile fracture toughness was decreased with increasing the particle size. In the present study, the influences of particles sizes on the cracks propagations and coalescences in the brittle materials such as rocks and concretes are numerically analyzed by using a three dimensional particle flow code (PFC3D). These analyses improve the understanding of the stability of rocks and concretes structures such as rock slopes, tunnel constructions and underground openings.

Initiation and Growth Behavior of Small Surface Fatigue Cracks on SiC Whisker Reinforced Aluminum Composite (SiC 휘스커 강화 알미늄기 복합재료의 미소 표면 피로균열의 발생 및 진전거동)

  • Choe, Yeong-Geun;Lee, Taek-Sun;Kim, Sang-Tae;Seo, Chang-Min;Lee, Mun-Hwan
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.24 no.6 s.177
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    • pp.1584-1592
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    • 2000
  • Reversed plane bending fatigue tests were conducted on SiC whisker reinforced aluminum composite which were consolidated by squeeze casting process. Initiation and growth of small surface fatigue c racks were investigated by means of a plastic replica technique. The fatigue crack initiated in the vicinity of SiC whisker/matrix interface. It was found that a fatigue crack deflected along SiC whisker and grew in a zig-zag manner microscopically, although the crack propagated along the direction normal to the loading axis macroscopically. The coalescence of micro-cracks was observed in the tests conducted at high stress levels, but were not evident in tests in which lower levels of stress were applied. Due to the coalescence, a higher crack growth rate of small cracks rather than those of long cracks was recognized in da/dn -ΔK realtionship.

Simulation of crack initiation and propagation in three point bending test using PFC2D

  • Haeri, Hadi;Sarfarazi, Vahab;Zhu, Zheming;Hedayat, Ahmadreza;Nezamabadi, Maryam Firoozi;Karbala, Mohammadamin
    • Structural Engineering and Mechanics
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    • v.66 no.4
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    • pp.453-463
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    • 2018
  • Three points bending flexural test was modelled numerically to study the crack propagation in the pre-cracked beams. The pre-existing double internal cracks inside the beam models were considered to investigate the crack propagation and coalescence paths within the modelled samples. Notch configuration effects on the failure stress were considered too. This numerical analysis shown that the propagation of wing cracks emanating from the tips of the pre-existing internal cracks caused the final breaking of beams specimens. It was also shown that when two notches were overlapped, they both mobilized in the failure process and the failure stress was decreased when the notches were located in centre line. However, the failure stress was increased by increasing the bridge area angle. Finally, it was shown that in all cases, there were good agreements between the discrete element method results and, the other numerical and experimental results. In this research, it is tried to improve the understanding of the crack propagation and crack coalescence phenomena in brittle materials which is of paramount importance in the stability analyses of rock and concrete structures, such as the underground openings, rock slopes and tunnel construction.

Cohesive Interface Model on Concrete Materials

  • Rhee In-Kyu;Roh Young-Sook
    • Journal of the Korea Concrete Institute
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    • v.17 no.6 s.90
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    • pp.1053-1064
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    • 2005
  • The mechanical damage of concrete is normally attributed to the formation of microcracks and their propagation and coalescence into macroscopic cracks. This physical degradation is caused from progressive and hierarchical damage of the microstructure due to debonding and slip along bimaterial interfaces at the mesoscale. Their growth and coalescence leads to initiation of hairline discrete cracks at the mesoscale. Eventually, single or multiple major discrete cracks develop at the macroscale. In this paper, from this conceptual model of mechanical damage in concrete, the computational efforts were made in order to characterize physical cracks and how to quantify the damage of concrete materials within the laws of thermodynamics with the aid of interface element in traditional finite element methodology. One dimensional effective traction/jump constitutive interface law is introduced in order to accommodate the normal opening and tangential slips on the interfaces between different materials(adhesion) or similar materials(cohesion) in two and three dimensional problems. Mode I failure and mixed mode failure of various geometries and boundary conditions are discussed in the sense of crack propagation and their spent of fracture energy under monotonic displacement control.

Micro-Surface-Cracks Behavior of 304 Stainless Steel Under Creep-Fatigue Interaction at Elevated Temperature (고온하 304 스테인리스강의 크리프-피로상호작용하의 미소표면균열에 관한 파괴거동)

  • 서창민;이상돈;조일현
    • Journal of Ocean Engineering and Technology
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    • v.2 no.2
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    • pp.104-111
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    • 1988
  • This paper deals with the micro-surface-cracks behavior on the unnotched smooth specimens of Type 304 stainless steel at $593^{\circ}C$ in air under creep and creep-fatigue conditions that have 10 mim and 1 min load holding times respectively. The behaviors of the micro-surface-cracks have been visualized by means of surface replica method and optical micro-photography. The quantitative characteristics of initiation, growth and coalescence of micro-surface-cracks have been investigated by observing and measuring the crack growth behaviors. some of the important results are as follows: Main crack initiates at grain boundary in the early stage(10 to 20%)of its life time and grows through coalescence and finally leads to fracture. The distribution of micro-surface-crack length, 2a, can be plotted against the composite Weibull distribution. The growth rate of the main crack can be plotted against the stress intensity factor, crack tip opering displacement and J integral.

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The effect of particle size on the edge notched disk (END) using particle flow code in three dimension

  • Haeri, Hadi;Sarfarazi, Vahab;Zhu, Zheming;Marji, Mohammad Fatehi
    • Smart Structures and Systems
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    • v.22 no.6
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    • pp.663-673
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    • 2018
  • In this study, the effect of particle size on the cracks propagation and coalescence or cracking pattern of the edge notched disc specimens are investigated. Firstly, calibration of PFC3D was performed using Brazilian experimental test output. Then micro parameters were used to build edge notched disc specimen. The horizontal wall of the assembly is let to move downward with a standard low speed of 0.016 m/s. The numerical results show that the tensile cracks are dominant failure pattern for the modeled discs. These tensile cracks initiate from the pre-existing notch tip and propagate parallel to the loading direction then interact with the upper boundary of the modeled specimen. As the size of the balls (ball diameter) decrease the number of tensile cracks increase. The tensile fracture toughness of the samples also decreases as the particle size increases. Understanding the crack propagation and crack coalescence phenomena in brittle materials such as concretes and rocks is of paramount importance in the stability analyses for engineering structures such as rock slopes, underground structures and tunneling.

Initiation and Growth Behavior of Small Surface Fatigue Crack on SiC Particle Reinforced Aluminum Composite (SiC 입자 강화 알루미늄기 복합재료의 표면미소 피로균열 발생 및 초기진전거동)

  • Lee, Sang-Hyoup;Choi, Young-Geun;Kim, Sang-Tae;Lee, Moon-Hwan
    • Composites Research
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    • v.21 no.6
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    • pp.15-22
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    • 2008
  • Reversed plane bending fatigue tests were conducted on SiC particle aluminum composite. The initiation and growth behaviors of small surface fatigue cracks were continuously monitored by the replica technique and investigated in detail. The fatigue life of MMC is shorter than that of matrix because there exists interface debonding of SiC particles and matrix on the whole face of the notch part in the casting metal matrix composite(MMC). The coalescence of micro-cracks was observed in the tests conducted at high stress levels. Due to the coalescence, a higher crack growth rate of small cracks rather than those of long cracks was recognized in da/dn-$K_{max}$ relationship.

Prediction of the crack aspect change in twin surface cracks (2개의 대칭표면구열의 구열형상변화 예측)

  • 최용식;김재원
    • Journal of the korean Society of Automotive Engineers
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    • v.14 no.2
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    • pp.65-75
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    • 1992
  • An analytical scheme for predicting the crack aspect pattern of materials which contain twin surface cracks was developed. Fatigue tests were performed on twin surface cracked PMMA plate specimens to obtain the interaction factor accounting for the interference effect of adjacent cracks. Here, the interaction factor is defined as the ratio of the stress intensity factor for twin surface cracks to that for a single surface crack. From the analysis of the fatigue test result, the interaction factor was presented as the ninth-order polynomial expression having a function of dimensionless crack spacing ratio. Then the polynomial expression was incorporated into the prediction program of the crack aspect pattern for twin surface cracked materials. And, the interaction effect and the coalescence condition of adjacent cracks were simplified in the newly developed prediction scheme of the crack aspect pattern. The predicted crack growth pattern using the prediction scheme was compared with test data from PMMA specimen. The predicted pattern agreed well with the test data.

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The coalescence and strength of rock-like materials containing two aligned X-type flaws under uniaxial compression

  • Zhang, Bo;Li, Shucai;Yang, Xueying;Xia, Kaiwen;Liu, Jiyang;Guo, Shuai;Wang, Shugang
    • Geomechanics and Engineering
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    • v.17 no.1
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    • pp.47-56
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    • 2019
  • Crossing (X-type) flaws are commonly encountered in rock mass. However, the crack coalescence and failure mechanisms of rock mass with X-type flaws remain unclear. In this study, we investigate the compressive failure process of rock-like specimens containing two X-type flaws aligned in the loading direction. For comparison purposes, compressive failure behavior of specimens containing two aligned single flaws is also studied. By examining the crack coalescence behavior, two characteristics for the aligned X-type flaws under uniaxial compression are revealed. The flaws tend to coalesce by cracks emanating from flaw tips along a potential path that is parallel to the maximum compressive stress direction. The flaws are more likely to coalesce along the coalescence path linked by flaw tips with greater maximum circumferential stress if there are several potential coalescence paths almost parallel to the maximum compressive stress direction. In addition, we find that some of the specimens containing two aligned X-type flaws exhibit higher strengths than that of the specimens containing two single parallel flaws. The two underlying reasons that may influence the strengths of specimens containing two aligned X-type flaws are the values of flaw tips maximum circumferential stresses and maximum shear stresses, as well as the shear crack propagation tendencies of some secondary flaws. The research reported here provides increased understanding of the fundamental nature of rock/rock-like material failure in uniaxial compression.