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.

다중 표면균열의 균열형상 이상화 기준 적용성 평가

김종민(Jong-Min Kim),허남수(Nam-Su Huh),최순(Suhn Choi),박근배(Keun-Bae Park),최재붕(Jae-Boong Choi) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5

The 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. Regarding such an interaction effect, the relative distance between adjacent cracks, crack aspect ratio and loading condition were known to be important factors for multiple cracks, which affects the fracture mechanics assessment parameters. Although several guidance (ASME Sec. XI, BS7910, British Energy R6 and API RP579) on a crack interaction effect (crack combination rule) have been proposed and used for assessing the interaction effect, each guidance provides different rules for combining multiple surface cracks into a single surface crack. Based on the systematic elastic and elatic-plastic finite element analyses, the present study investigated the acceptability of the crack combination rules provided in the existing guidance, and the relevant recommendations on a crack interaction for in-plane surface cracks in a plate were discussed. To quantify the interaction effect, the elastic stress intensity factor and elastic-plastic J-integral along the crack front were used. As for the loading condition, only axial tension was considered. As a result, BS7910 seems to provide the most relevant crack combination rule for in-plane dual surface cracks, whereas API RP579 provides the most conservative results. In particular, ASME Sec. XI still seems to have some room for a revision to shorten the critical distance between two adjacent cracks for a crack combination. The overall tendency of the elastic-plastic analyses results is identical to that of the elastic analyses results.

ON CRACK INTERACTION EFFECTS OF IN-PLANESURFACE CRACKS USING ELASTIC AND ELASTIC-PLASTICFINITE ELEMENT ANALYSES

JONG-MIN KIM,허남수 한국원자력학회 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 stressintensity factor or the elastic-plastic J-integral, can be affected significantly by the adjacent cracks. Such a crack interactioneffect due to multiple cracks can alter the fracture mechanics assessment parameters significantly. There are many factors tobe considered, for instance the relative distance between adjacent cracks, the crack shape, and the loading condition, to quantifythe crack interaction effect on the fracture mechanics assessment parameters. Thus, the current assessment codes on crackinteraction 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 crackinteraction effects by evaluating the elastic stress intensity factor and the elastic-plastic J-integral of adjacent in-plane surfacecracks in a plate through detailed 3-dimensional elastic and elastic-plastic finite element analyses. The effects on the fracturemechanics assessment parameters of the geometric parameters, the relative distance between two cracks, and the crack shapeare investigated systematically. As for the loading condition, an axial tension is considered. Based on the finite elementresults, the acceptability of the crack combination rules provided in the existing guidance was investigated, and the relevantrecommendations on a crack interaction for in-plane surface cracks are discussed. The present results can be used to developmore concrete guidance on crack interaction effects for crack shape characterization to evaluate the integrity of defectivecomponents.

비파괴적 광학 장비를 이용한 치아 균열 탐지

전미경,구혜민,김백일 대한치과의사협회 2022 대한치과의사협회지 Vol.60 No.8

Purpose: The purpose of this study is to quantify the depth of enamel cracks using Quantitative light induced fluorescence-digital (QLF-D) and swept source optical coherence tomography (SS-OCT), and to compare the validity of detection methods. Methods: A total of 80 extracted teeth with cracks in the coronal portion or suspected cracks were selected. First, the nondestructive devices QLF-D and SS-OCT were taken, and then histological evaluation was performed. Cracks was divided into 4 types with depth of the crack: Sound, Initial crack, Moderate crack, and Dentin-enamel junction (DEJ) crack. Results: As a result of the validity of the crack depth, the SS-OCT showed the highest sensitivity for detecting initial cracks at 0.98, and the lowest sensitivity for detecting DEJ cracks at 0.70. When detected with QLF-D, the cut-off value of the maximum fluorescence loss for detecting initial cracks was 5, and showed a sensitivity and a specificity of 1.00. Conclusions: The application of SS-OCT and QLF-D in diagnosing cracked teeth will not only be effective in detecting early stage cracks, but will also help to establish an appropriate treatment plan for crack progression.

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 Sealing vs. Crack Filling

Evan Humphries,Soon-Jae Lee 한국도로학회 2015 한국도로학회 학술발표회 논문집 Vol.2015 No.10

This study is examining the potential benefits of routing in hot mix asphalt pavement prior to installing crack sealant. (1) Definition. Crack sealing and crack filling are two separate activities. While both crack sealing and crack filling involve placing sealants in pavement cracks, they differ in process. Generally, crack sealing is defined as using a router or saw to create a reservoir in a crack which is then filled with a sealant material. Crack filling is defined as minor crack preparation, such as using an air gun to blow debris out of cracks, prior to installation of the sealant. There is no pavement removed with crack filling. Additionally, crack sealing is performed on working cracks, whereas crack filling is generally the term used to refer to the treatment of nonworking cracks. (2) Implementation. Crack sealing should be carried out on structurally sound pavement which has low pavement distress. The pavement selection consideration should be based on pavement age, pavement and geometric design, pavement selection boundaries, traffic, type and extent of previous maintenance treatments and condition rating. The best candidates for crack sealing are newer pavements which are in the range of 1 to 3 years, and the majority of pavement distress can be found in terms of longitudinal or transverse having slight to moderate crack density. (3) Evaluation. The performance life of a treatment mostly depends on the preparation of crack and the type of the material used. One inspection should be made each year to chart the rate of failure and plan for subsequent maintenance. A mid winter evaluation is highly recommended as it will indicate treatment effectiveness when there is maximum pavement contraction and the crack is near the maximum opening. A small representative sample of the pavement, minimum of 150 m length should be selected for the evaluation. The first step in determining a treatment’s effectiveness is establishing how much of the treatment has failed in relation to the total length of treatment applied: Percent failure = (failed length after treatment / total length of treatment) × 100 After that the treatment’s effectiveness can be determined by subtracting the percentage of treatment failure from 100 percent: Effectiveness = 100 - Percent failure After a number of inspections a graph of effectiveness versus time can be developed. (4) Cost. Crack treatments can be considered as effective if it delays pavement deterioration and extends the pavement service life. Generally, the effective treatment extends the pavement life by two to five years. The effectiveness of rout and seal maintenance depends upon three points: (a) Performance of the sealant materials and appropriate rout width and depth; (b) restraining of crack development and delaying the existing pavement distress; and (c) crack treatment implication period. Chip seal treatment cost 3-14 times more than crack sealing and an overlay cost 8-26 times as much as crack sealing. The cost of crack sealing varies depending on state, materials, whether or not routing is required, and unit being priced.

Crack analysis of reinforced concrete members with and without crack queuing algorithm

P.L. Ng,F.J. Ma,A.K.H. Kwan 국제구조공학회 2019 Structural Engineering and Mechanics, An Int'l Jou Vol.70 No.1

Due to various numerical problems, crack analysis of reinforced concrete members using the finite element method is confronting with substantial difficulties, rendering the prediction of crack patterns and crack widths a formidable task. The root cause is that the conventional analysis methods are not capable of tracking the crack sequence and accounting for the stress relief and re-distribution during cracking. To address this deficiency, the crack queuing algorithm has been proposed. Basically, at each load increment, iterations are carried out and within each iteration step, only the most critical concrete element is allowed to crack and the stress re-distribution is captured in subsequent iteration by re-formulating the cracked concrete element and re-analysing the whole concrete structure. To demonstrate the effectiveness of the crack queuing algorithm, crack analysis of concrete members tested in the literature is performed with and without the crack queuing algorithm incorporated.

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>

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.

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.