ON CRACK INTERACTION EFFECTS OF IN-PLANE SURFACE CRACKS USING ELASTIC AND ELASTIC-PLASTIC FINITE ELEMENT ANALYSES

Kim, Jong-Min,Huh, Nam-Su Korean Nuclear Society 2010 Nuclear Engineering and Technology Vol.42 No.6

The crack-tip stress fields and fracture mechanics assessment parameters for a surface crack, such as the elastic stress intensity factor or the elastic-plastic J-integral, can be affected significantly by the adjacent cracks. Such a crack interaction effect due to multiple cracks can alter the fracture mechanics assessment parameters significantly. There are many factors to be considered, for instance the relative distance between adjacent cracks, the crack shape, and the loading condition, to quantify the crack interaction effect on the fracture mechanics assessment parameters. Thus, the current assessment codes on crack interaction effects (crack combination rules), including ASME Sec. XI, BS7910, British Energy R6 and API 579-1/ASME FFS-1, provide different rules for combining multiple surface cracks into a single surface crack. The present paper investigates crack interaction effects by evaluating the elastic stress intensity factor and the elastic-plastic J-integral of adjacent in-plane surface cracks in a plate through detailed 3-dimensional elastic and elastic-plastic finite element analyses. The effects on the fracture mechanics assessment parameters of the geometric parameters, the relative distance between two cracks, and the crack shape are investigated systematically. As for the loading condition, an axial tension is considered. Based on the finite element results, the acceptability of the crack combination rules provided in the existing guidance was investigated, and the relevant recommendations on a crack interaction for in-plane surface cracks are discussed. The present results can be used to develop more concrete guidance on crack interaction effects for crack shape characterization to evaluate the integrity of defective components.

다층세라믹 콘덴서에서 생성된 크랙의 관찰과 분석

이철승,강병성,허강헌,박진우 한국세라믹학회 2009 한국세라믹학회지 Vol.46 No.2

For the Y5V characteristic MLCC which is very prone to crack, it is important to to find out the basic cause of the crack. After finding out the crack origin, the materials and processes should be developed to remove the crack. The microstructures of the cracks were investigated using the fractographic method for the various types of cracks such as an exterior crack, a cyclic thermal shock crack, and an piezo-electric crack. It was found out that the crack origin was the pore at the end of the Ni inner electrode after bake-out. Even though the three dimensional crack shapes were different, the crack origins were seemed to be similar. The exterior crack could grow from the origin with the aids of residual and applied stress. FEM (finite element method) analysis was used to calculate the stress distribution of residual and applied stress. And the concept of fracture mechanics was applied for the explanation of the crack initiation and propagation from the stresses concentration. For the Y5V characteristic MLCC which is very prone to crack, it is important to to find out the basic cause of the crack. After finding out the crack origin, the materials and processes should be developed to remove the crack. The microstructures of the cracks were investigated using the fractographic method for the various types of cracks such as an exterior crack, a cyclic thermal shock crack, and an piezo-electric crack. It was found out that the crack origin was the pore at the end of the Ni inner electrode after bake-out. Even though the three dimensional crack shapes were different, the crack origins were seemed to be similar. The exterior crack could grow from the origin with the aids of residual and applied stress. FEM (finite element method) analysis was used to calculate the stress distribution of residual and applied stress. And the concept of fracture mechanics was applied for the explanation of the crack initiation and propagation from the stresses concentration.

The effect of micro pore on the characteristics of crack tip plastic zone in concrete

Hadi Haeri,V. Sarfarazi 사단법인 한국계산역학회 2016 Computers and Concrete, An International Journal Vol.17 No.1

Concrete is a heterogeneous material containing many weaknesses such as micro-cracks, pores and grain boundaries. The crack growth mechanism and failure behavior of concrete structures depend on the plastic deformation created by these weaknesses. In this article the non-linear finite element method is used to analyze the effect of presence of micro pore near a crack tip on both of the characteristics of crack tip plastic zone (its shape and size) and crack growth properties (such as crack growth length and crack initiation angle) under pure shear loading. The FE Code Franc2D/L is used to carry out these objectives. The effects of the crack-pore configurations and the spacing between micro pore and pre-excising crack tip on the characteristics of crack tip plastic zone and crack growth properties is highlighted. Based on the obtained results, the relative distance between the crack tip and the micro pore affects in very significant way the shape and the size of the crack tip plastic zone. Furthermore, crack growth length and crack initiation angle are mostly influenced by size and shape of plastic zone ahead of crack tip. Also the effects of pore decrease on the crack tip by variation of pore situation from linear to perpendicular configuration. The critical position for a micro pore is in front of the crack tip.

단조증가 및 반복하중 하에서 모사 암석 시료의 균열 성장에 관한 실험적 연구

고태영(Tae Young Ko),이승철(Seung Cheol Lee),김동근(Dong Keun Kim),최영태(Young Tae Choi) 한국암반공학회 2011 터널과지하공간 Vol.21 No.4

교통, 굴착, 발파 등에 의한 반복하중은 오랜 시간에 걸쳐서 암석의 미세균열 성장을 일으키며, 암석의 강도 등에 영향을 미치기 때문에 반복하중에 의한 균열의 성장, 결합은 장시간 안정성 평가에 중요한 영향을 미친다. 본 연구에서는 두 개의 초기 균열을 가지는 모사 암석 시험편에 단조증가 및 반복하중을 가하여 하중조건에 따른 균열의 성장과 결합유형을 조사하였다. 단조증가하중, 반복하중 시험 모두에서 서로 유사한 날개균열 시작 위치, 날개균열 각도, 균열 성장 순서, 균열 결합 형태가 관측되었다. 본 연구에서 관찰된 균열 결합은 크게 3종류로 전단에 의한 결합, 1개의 날개 혹은 인장 균열에 의한 결합 그리고 2개의 날개 혹은 인장 균열에 의한 결합으로 요약될 수 있다. 피로균열은 반복하중 시험에서만 발생하였으며 성장 방향은 이차균열과 유사하게 초기균열과 같은 방향 혹은 하중방향과 직교인 수평방향으로 관찰되었다. Cyclic loading due to traffic, excavation and blasting causes microcrack growth in rocks over long period of time, and this type of loading often causes rock to fail at a lower stress than its monotonically determined strength. Thus, the crack growth and coalescence under cyclic loading are important for the long-term stability problems. In this research, experiments using gypsum as a model material for rock are carried out to investigate crack propagation and coalescence under monotonic and cyclic loading. Both monotonic and cyclic tests have a similar wing crack initiation position, wing crack initiation angle, cracking sequence and coalescence type. Three types of crack coalescence were observed; Type Ⅰ, Ⅱ and Ⅲ. Type Ⅰ coalescence occurs due to a shear crack and Type Ⅱ coalescence occurs through one wing or tension crack. For Type Ⅲ, coalescence occurs through two wing or tension cracks. Fatigue cracks appear in cyclic tests. Two types of fatigue crack initiation directions, coplanar and horizontal directions, are observed.

Investigating the effects of non-persistent cracks’ parameters on the rock fragmentation mechanism underneath the U shape cutters using experimental tests and numerical simulations with PFC2D

Jinwei Fu,Hadi Haeri,Vahab Sarfarazi,Sh. Mohamadi Bolban Abad,Mohammad Fatehi Marji,Gholamreza Saeedi,Yibing Yu 국제구조공학회 2022 Structural Engineering and Mechanics, An Int'l Jou Vol.83 No.4

This paper aims to study the fracture mechanism of rocks under the ‘u’ shape cutters considering the effects of crack (pre-existing crack) distances, crack spacing and crack inclination angles. The effects of loading rates on the rock fragmentation underneath these cutters have been also studied. For this purpose, nine experimental samples with dimensions of 5 cm×10 cm×10 cm consisting of the non-persistent cracks were prepared. The first three specimens’ sets had one non-persistent crack (pre-existing crack) with a length of 2 cm and angularity of 0°, 45°, and 90°. The spacing between the crack and the “u” shape cutter was 2 cm. The second three specimens” set had one non-persistent crack with a length of 2 cm and angularity of 0°, 45°, and 90° but the spacing between pre-existing crack and the “u” shape cutter was 4 cm. The third three specimens’ set has two non-persistent cracks with lengths of 2 cm and angularity of 0°, 45° and 90°. The spacing between the upper crack and the “u” shape cutter was 2 cm and the spacing between the lower crack and the upper crack was 2 cm. The samples were tested under a loading rate of 0.005 mm/s. concurrent with the experimental investigation. The numerical simulations were performed on the modeled samples with non-persistent cracks using PFC2D. These models were tested under three different loading rates of 0.005 mm/s, 0.01 mm/sec and 0.02 mm/sec. These results show that the crack number, crack spacing, crack angularity, and loading rate has important effects on the crack growth mechanism in the rocks underneath the “u” shape cutters. In addition, the failure modes and the fracture patterns in the experimental tests and numerical simulations are similar to one another showing the validity and accuracy of the current study.

Cracked-healing and the bending strength of Si3N4 composite ceramics with SiO2 additions

남기우,S. H. Park,J. S. Kim 한양대학교 세라믹연구소 2009 Journal of Ceramic Processing Research Vol.10 No.4

This study analyzed the crack-healing behavior of Si3N4 composite ceramics based on variations in the heat-treatment temperature and the amount of colloidal SiO2 added. Semi-elliptical cracks about 100 μm length were obtained from a Vickers indenter using a load of 24.5 N. The results showed that the use of an optimum amount of added colloidal SiO2 and the coating of colloidal SiO2 on the cracks could significantly increase the bending strength. Meanwhile, the heat-treatment temperature has a profound influence on the extent of crack healing and the degree of strength recovery. The optimum heat-treatment temperature is dependent on the amount of added colloidal SiO2 employed. The crack healing strength was far greater in the case of the cracked specimen when colloidal SiO2 had been coated on the crack surfaces. When scanned with a microscope probe, the cracks were found to have almost entirely disappeared when heat treated at a temperature of 1,273 K. The bending strength of the SSTS-1 cracked specimen not coated with the colloidal SiO2 recovered to the same extent as the smooth specimen at the optimum healing temperature of 1,273 K. However, the bending strength of the cracked specimen with the colloidal SiO2 coating increased by up to 140%. The optimum amount of the added colloidal SiO2 was found to be 1.3wt.%. Moreover, the bending strength of the crack-healed specimen with the colloidal SiO2 coating was up to 160% greater than that of the smooth specimen with 0.0 wt.% of the added colloidal SiO2 coating. Crack closure and the rebonding of the cracks caused by the oxidation of cracked surfaces were thus identified as the dominant healing mechanism of Si3N4 composite ceramics.

Experimental study on water exchange between crack and clay matrix

Song, Lei,Li, Jinhui,Garg, Ankit,Mei, Guoxiong Techno-Press 2018 Geomechanics & engineering Vol.14 No.3

Cracks in soil provide significant preferential pathways for contaminant transport and rainfall infiltration. Water exchange between the soil matrix and crack is crucial to characterize the preferential flow, which is often quantitatively described by a water exchange ratio. The water exchange ratio is defined as the amount of water flowing from the crack into the clay matrix per unit time. Most of the previous studies on the water exchange ratio mainly focused on cracked sandy soils. The water exchange between cracks and clay matrix were rarely studied mainly due to two reasons: (1) Cracks open upon drying and close upon wetting. The deformable cracks lead to a dynamic change in the water exchange ratio. (2) The aperture of desiccation crack in clay is narrow (generally 0.5 mm to 5 mm) which is difficult to model in experiments. This study will investigate the water exchange between a deformable crack and the clay matrix using a newly developed experimental apparatus. An artificial crack with small aperture was first fabricated in clay without disturbing the clay matrix. Water content sensors and suction sensors were instrumented at different places of the cracked clay to monitor the water content and suction changes. Results showed that the water exchange ratio was relatively large at the initial stage and decreased with the increasing water content in clay matrix. The water exchange ratio increased with increasing crack apertures and approached the largest value when the clay was compacted at the water content to the optimal water content. The effective hydraulic conductivity of the crack-clay matrix interface was about one order of magnitude larger than that of saturated soil matrix.

Crack growth rate of inclined and deflected surface-cracks in round-bar specimens of copper processed by equal channel angular pressing under cyclic loading

Goto, M.,Yamamoto, T.,Han, S.Z.,Kim, S.,Ahn, J-H.,Kitamura, J.,Iwamura, T.,Lee, J. Elsevier 2017 Engineering fracture mechanics Vol.182 No.-

<P>For low-cycle fatigued, ultrafine grained copper, processed by equal channel angular pressing, a unique crack growth direction, either inclined at 45 degrees or perpendicular to the loading direction, was observed around the circumference of round bar specimens. In addition, the perpendicular cracks showed a 45 degrees deflected crack face toward the center of the specimens. To clarify the growth behavior of such inclined and deflected cracks, the role of the deformation mode at the crack-tip areas was discussed in terms of the surface damage caused by cyclic stressing and the mixed-mode deformation at the crack tips. The preexistent shear-bands/shear-cracks around the crack tips and the in-plane shear-mode deformation at the crack tips assisted the formation of shallow inclined- and deep deflected-cracks. To quantitatively estimate the severity of mechanical damage at the crack tips, the comparative stress intensity factor range under combined mode I, II and III conditions were calculated. The crack growth rate along the growth direction at the surface and bottom for inclined and deflected surface-cracks was discussed in terms of the comparative stress intensity factor range. (c) 2017 Elsevier Ltd. All rights reserved.</P>

탄성 유한요소해석을 이용한 동일 평면상에 존재하는 다중 표면균열의 균열상호작용 평가

허남수(Nam-Su Huh),김종민(Jong-Min Kim),최순(Suhn Choi),박근배(Keun-Bae Park),최재붕(Jae-Boong Choi),김영진(Young-Jin Kim) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5

The crack-tip stress fields and fracture mechanics assessment parameters, such as the elastic stress intensity factor and the elastic-plastic J-integral, for a surface crack can be significantly affected by adjacent cracks. Such a crack interaction effect due to multiple cracks can magnify the fracture mechanics assessment parameters. There are many factors to be considered, for instance the relative distance between adjacent cracks, crack shape and loading condition, to quantify a crack interaction effect on the fracture mechanics assessment parameters. Thus, the current guidance on a crack interaction effect (crack combination rule), including ASME Sec. XI, BS7910, British Energy R6 and API RP579, provide different rules for combining multiple surface cracks into a single surface crack. The present paper investigates a crack interaction effect by evaluating the elastic stress intensity factor of adjacent surface cracks in a plate along the crack front through detailed 3-dimensional elastic finite element analyses. The effects of the geometric parameters, the relative distance between cracks and the crack shape, on the stress intensity factor are systematically investigated. As for the loading condition, only axial tension is considered. Based on the elastic finite element results, the acceptability of the crack combination rules provided in the existing guidance was investigated, and the relevant recommendations on a crack interaction for in-plane surface cracks in a plate were discussed.

Crack-tip field characterization of crack-tip opening angle-based crack growth – plane strain single-edge cracked specimen subject to pure extension

정성환,이형일 대한기계학회 2011 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.25 No.5

The objective of this work is to study the crack-tip field of plane strain fully plastic crack growth of a single-edge cracked specimen subject to pure extension. The nodal release technique [1] was implemented into an FEM model so that the nodes arranged in the crack path are released. The fracture criterion for the crack growth in the model adopts the loading geometry-based crack-tip opening angle-(CTOA) proposed in the work of Ref. [2]. The crack-tip field from the CTOA-based crack growth was investigated and characterized with a slip line field. It was observed that the backface configuration of the specimen, which evolves with the crack growth, is engaged in the formation of the shear band and, as a result, significantly elevates the triaxiality at the crack-tip during the crack growth.