• 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.

• Computers and Concrete
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• v.26 no.3
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• pp.239-255
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• 2020

Experimental and discrete element method were used to investigate the effects of joint number and its angularities on the shear behaviour of joint's bridge area. A new shear test condition was used to model the gypsum cracks under shear loading. Gypsum samples with dimension of 120 mm×100 mm×50 mm were prepared. the length of joints was 2cm. in experimental tests, the joint number is 1, 2 and 3 and its angularities change from 0° to 90° with increment of 45°. Assuming a plane strain condition, special rectangular models are prepared with dimension of 120 mm×100 mm. similar to joints configuration in experimental test, 9 models with different joint number and joint angularities were prepared. This testing show that the failure process is mostly governed by the joint number and joint angularities. The shear strengths of the specimens are related to the fracture pattern and failure mechanism of the discontinuities. The shear behaviour of discontinuities is related to the number of induced tensile cracks which are increased by increasing the rock bridge length. The strength of samples decreases by increasing the joint number and joint angularities. Failure pattern and failure strength are similar in both of the experimental test and numerical simulation.

• Computers and Concrete
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• v.5 no.4
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• pp.343-358
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• 2008

This article focuses on concrete structures submitted to impact loading and is aimed at predicting local damage in the vicinity of an impact zone as well as the global response of the structure. The Discrete Element Method (DEM) seems particularly well suited in this context for modeling fractures. An identification process of DEM material parameters from macroscopic data (Young's modulus, compressive and tensile strength, fracture energy, etc.) will first be presented for the purpose of enhancing reproducibility and reliability of the simulation results with DE samples of various sizes. The modeling of a large structure by means of DEM may lead to prohibitive computation times. A refined discretization becomes required in the vicinity of the impact, while the structure may be modeled using a coarse FE mesh further from the impact area, where the material behaves elastically. A coupled discrete-finite element approach is thus proposed: the impact zone is modeled by means of DE and elastic FE are used on the rest of the structure. The proposed approach is then applied to a rock impact on a concrete slab in order to validate the coupled method and compare computation times.

• Proceedings of the Korean Geotechical Society Conference
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• 2006.03a
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• pp.229-238
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• 2006

암반터널굴착을 위한 발파시 이로 인한 암반의 최종 손상영역을 예측하는 것은 터널의 안전성을 위해 매우 중요하다. 그러나 복잡한 발파거동은 손상영역을 적절히 예측하는데 상당한 어려움이 있다. 이러한 어려움을 효과적으로 해결하기 위해 발파하중을 응력파와 가스압으로 분리한 많은 연구가 진행되었다. 응력파는 발파공 주위에 분쇄한(crushing annulus)과 파쇄균열대(fracture zone)를 형성시키며, 상당시간 지속되는 준정적인 가스는 파쇄균열대의 닫힌 균열내부에 침투하여 균열을 다시 진행시키는 역할을 하게 된다. 즉, 가스압은 최종적으로 암반에 손상을 가하는데 기여를 한다. 따라서 본 논문은 이러한 가스압에 의해 생성되는 균열의 최종 진행 길이를 예측함으로써 발파로 인한 최종 손상영역을 간단하게 예측할 수 있는 방법을 제시하고자 한다. 이를 위해 무한 탄성평면에서 발파공 주위에 대칭으로 형성되는 균열을 모델로 사용하였다. 이 모델에서 균열이 진행할 수 있는 조건과 가스의 질량이 일정하다는 두가지 조건을 사용하였다. 그 결과 응력집중계수는 균열이 진행할수록 감소하여 최종균열의 길이를 예측할 수 있었고, 그와 동시에 발파공에 작용하는 압력도 감소하는 것을 확인할 수 있었다.

• Proceedings of the Korean Geotechical Society Conference
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• 2004.03b
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• pp.904-909
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• 2004

The study area adjoins with Yangsan fault in Sangbuk-myeon, Samsam-ri, Kyongsang-namdo and consist of the natural steep slope. After drawing data layer which have altitude by using digital topography data, it is converted to lattice DEM of $10m{\times}10m$ size. From this, gradient map of unit lattice, slant direction map and shadow relif map are made. Using flow apportioning algorithm, upper slope contributing area and wetness index by established lattice can be calculated. Area that have high wetness index shows lineament structure of northwest-southeast direction, and this agrees with shear fracture system. The result of electricity specific resistance survey in the study area shows that area of high wetness index has low electricity specific resistance anomaly. That is, wetness index conforms with distribution of fractured zone that accompanied chemical weathering of rock. Therefore, wetness index can be used as the method of detecting fractured zones and judging the stability of the area.

• Proceedings of the Korean Geotechical Society Conference
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• 2004.03b
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• pp.807-814
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• 2004

Land seismic reflection survey has been increasingly demanded in various civil engineering works because of its own ability to delineate layers, water table, to detect cavities or fracture zones, to estimate seismic velocities of each layer. However, our shallow subsurface structures are very complex. The relatively thin layer(mostly soil) to the wavelength directly followed by a basic rock with high impedance used to generate complicated surface waves, kind of channel waves with high amplitude that is dominate in entire seismograms and hence the useful reflection events will be almost hopelessly immersed in the undesired surface waves. Thus, it would seem that the use of traditional seismic survey could not be likely to provide in itself a satisfactory information about our exploration targets. This paper hence introduces an efficient measuring strategy illustrating a properly controlled arrangement of the vertical single force sources commonly used, yielding a very sharply elongated form of P-energy with a minimum of S radiation energy, what we call, P-beam source. Abundant experiments of physical modeling showed that in that way the surface waves could be enormously reduced and the reflection events would be additive and thus reinforced. Examples of field data are also illustrated. The contribution of P-beam source will be great in civil engineering area as well as in general geological exploration area.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2002.10a
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• pp.265-268
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• 2002

In this work, the blend system of epoxy and PMR-15 polyimide is investigated in terms of the cure behaviors and thermal stabilities. The cure behaviors are studied in DSC measurements and thermal stabilities are also carried out by TGA analysis. DDM (4, 4'-diamino diphenyl methane) is used as curing agent for EP and the content of PMR-15 is varied within 0, 5, 10, 35, and 20 phr to neat EP. As a result, the cure activation energy ($E_a$) is increased at 10 phr of PMR-15, compared with that of neat EP. From the TGA results of EP/PMR-15 blend system, the thermal stabilities based in the initial decomposed temperature (IDT) and integral procedural decomposition temperature (IPDT) are increased with increasing the PMR-15 content. The fracture toughness, measured in the context of critical stress intensity factor ($K_{IC}$) and critical strain energy release rate ($G_{IC}$), shows a similar behavior with $E_a$. This result is probably due to the crosslinking developed by the interactions between intermolecules in the polymer chains.

• 한국지구물리탐사학회:학술대회논문집
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• 2003.11a
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• pp.272-280
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• 2003

A definite shut-in pressure in hydraulic fracturing techniques is needed for obtaining the correct information on the in-situ stress regimes in rock masses. The relation between the behaviour of hydraulically induced fractures and the condition of remote stress is considered to be major reasons of an ambiguous shut-in pressure in hydraulic fracturing pressure-time history curves. This paper describes the results of a series of numerical analyses carried out using UDEC(Universal Distinct Element Code, Itasca), which is based on the discrete element method, to compare several methods for determining the shut-in pressure during hydraulic fracturing. The fully coupling of hydraulic and mechanical analysis was applied, and the effects of four different discontinuity geometries in numerical modelling have been investigated for this purpose. The effects of different remote stress regimes and different physical properties on hydraulic fracture propagation have been also analyzed. Several methods for obtaining shut-in pressure from the ambiguous shut-in curves have been applied to all the numerical models. The graphical intersection methods, such as (P vs. t) method, (P vs. log(t)) method, (log(P) vs. log(t)) method, give smaller values of the shut-in pressure than the statistical method, (dP/dt vs. P). Care should be taken in selecting a method for shut-in pressure, because there can be existed a stress anomaly around the wellbore and fracturing from the wellbore by a constant flow rate may have a more complicate mechanism.

• Geophysics and Geophysical Exploration
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• v.9 no.4
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• pp.255-260
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• 2006

Measurements in two adjacent (about 1.5 m separation) boreholes reveal that there is a significant degree of variations in the width and property (permeability) of shear zones in the granitic rock. A high frequency (>10 kHz) cross-well seismic tomography was conducted to characterize the features of permeability distribution at the shear zones in the inter-well region. At the shear zones, the correlation between the permeability at the well location and the velocity pattern shown in the cross-well velocity tomogram suggests that a high resolution velocity tomogram may provide useful information for the shear zone characteristics, such as permeability, fracture density, width, and length.

• Economic and Environmental Geology
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• v.28 no.5
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• pp.445-454
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• 1995

The petrochemical data of the porphyritic granites of Cretaceous age in the Yonghwa-Seolcheon area show the trend of subalkaline magma, calc-alkaline magma, I-type granitoid and magnetite series. This granite is the relevant igneous rock of gold-silver mineralization in this mining district Fluid inclusions have been studied in phenocryst quartz from the Cretaceous porphyritic granite. Three main types of fluid inclusion were found : liquid-rich inclusion(I type), gas-rich inclusion(II type) and solid-bearing inclusions(III-A, III-B). The solid-bearing inclusions(III-A,B) represent the earliest trapped fluids. They have salinities between 41.0 and 67.5 wt% equivalent to NaCl. These are high saline inclusions containing NaCl and KCl daughter crystals. Homogenization temperature inferred from the fluid inclusion study ranges from 650 to $75^{\circ}C$ Type I and II inclusions were observed within the same fracture. This cause for these differences in degree of filling is evidence of boiling. Salinities of type I and II inclusions range from 9.87 wt% to 15.29 wt%, from 8.40 wt% to 14.64 wt% NaCl equivalent, respectively.