• The Journal of Engineering Geology
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• v.11 no.2
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• pp.191-203
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• 2001

We investigate mechanical anisotropy dependent of rock fabric and its relationship with microcracks in the Pocheon Granite. Uniaxial compressive strengths range from 177MPa to 212MPa and the elastic constants are 48GPa-62GPa. The tensile strengths are 6.9MPa~8.5MPa and ultrasonic wave velocities range between 3,200m/sec and 3,700m/sec, indicating that mechanical anisotropy is strongly dependent of rock fabric. The minimum anisotropy ratio is 14% and the maximum is 24%, depend on the mechanical properties. The preferred orientations of microcracks are closely related with the directions of rock fabric. The preferred orientations of microcracks in feldspar are governed by the direction of mineralogical axis and are different from the directions of rock fabric. However, microcracks in quartz grains are very long and parallel to the directions of rock fabric, indicating that directions of rock fabric may be governed by the preferred orientations of microcracks in quartz grains. The preferred orientations of microcracks measured by differential strain analysis and microscopic observation are slightly different. That may be caused by different methodology. Lengths and numbers of microcrack are measured by microscopic observation. However, differential strain analysis measures the widths of microcracks.

• Journal of Periodontal and Implant Science
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• v.45 no.2
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• pp.62-68
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• 2015

Purpose: The goal of this study was to investigate the histomorphometric characteristics of the healing process of microcracks in the cortical bone after the installation of mini-implants (MIs). Methods: Self-drilling MIs were inserted into the tibial diaphysis of twelve adult male New Zealand rabbits. Four MIs per rabbit were placed randomly. The animals were divided into four groups according to the length of the healing period: group A was sacrificed immediately, group B was sacrificed after one week, group C was sacrificed after two weeks, and group D was sacrificed after four weeks. Cortical bone thickness was measured using micro-computed tomography, and histomorphometric analyses of the cumulative length of the microcracks (CLCr) and the total number of microcracks (NCr) were performed using hematoxylin and eosin staining. Results: The microcracks were radially and concentrically aligned in the peri-MI bone. The CLCr decreased significantly one week after the surgery, mainly due to healing of the concentrically aligned microcracks. The CLCr showed another significant decrease from two weeks after the surgery to four weeks after the surgery, mainly reflecting healing of the radially aligned microcracks. A statistically significant decrease in the NCr occurred as the microcracks healed from zero weeks to two weeks. However, no significant difference in the NCr was found between groups C and D. Conclusions: In order to improve the primary stability of MIs, delayed loading and a healing period of a certain length are recommended to ensure the optimal healing of microcracks and bone remodeling.

• Economic and Environmental Geology
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• v.30 no.4
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• pp.353-362
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• 1997

P wave velocities of core samples from the Pocheon granite were measured before and after applications of cyclic loading. Then. distribution of the pre-existing microcracks and microcracks developed due to the cyclic loading was investigated by analyzing P wave velocity anisotropies and microscopic observations from thin sections. Anisotropy constants were calculated with three different ways: (1) $C_A$ between the maximum and the minimum velocities, (2) $C_AI$ between velocities measured along the axial direction and the average of six velocities measured in the planes perpendicular to the loading axis (rift plane) and (3) $C_AII$ between the maximum and the minimum velocities measured in the plane perpendicular to the loading axis. Among anisotropy constants. $C_AI$ was the most effective anisotropy constant to identify the rift plane whose orientation is parallel to the pre-existing microcracks as well as the distribution of stress induced microcracks. $C_AI$ decreased after cyclic loading and the relationship between $C_AI$ and number of cycles shows comparatively coherent negative trends. indicating that stress induced microcracks are aligned perpendicular to the orientation of pre-existing microcracks and that the amounts are proportional to the number of loading cycles. The difference of anisotropy constants before and after cyclic loading was effective in delineating the level of cracks and we called it Induced Crack Index. Velocity measurements and microscopic observations show that anisotropy was caused mainly due to microcracks aligned to a particular direction.

• Journal of the Korea Concrete Institute
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• v.24 no.6
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• pp.651-658
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• 2012

This paper concentrates on the evaluation of microcracks in thermal damaged concrete on the basis of the nonlinear ultrasonic modulation technique. Since concrete structure exposed to high temperature accompanies the development of microcracks due to the physical and chemical changes from temperature and exposed time, the adoption of nonlinear approach is required. Instead of using the conventional ultrasonic nondestructive methods which have the limitation in evaluating excessive microcracks, accordingly, a nonlinear ultrasonic modulation method which shows better sensitivity in quantifying microcracks is introduced. Upon the analysis for the modulation of ultrasonic wave and low frequency impact to measure the nonlinearity parameter, which can be used as an indicator of thermal damage, the verification processes for the introduced technique are followed: SEM investigation and permeable pore space test are performed to characterize thermally induced microcracks in concrete, and ultrasonic pulse velocity tests are performed to confirm the outstanding sensitivity of nonlinear ultrasonic modulation technique. In advance, compressive strength of thermal damaged concrete is measured to represent the effect of microcracks on performance degradation. Correlation studies between experimental data and measured data show that nonlinear ultrasonic modulation technique can effectively be used to quantify thermally induced microcracks, and to estimate the compressive strength of thermally damaged concrete.

• 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.

• Tunnel and Underground Space
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• v.5 no.3
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• pp.240-250
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• 1995

Microcrack development in the macheon gabbro, the Sangju sandstone and the Jungsun marble due to fatigue stresses was investigated using differential strain analysis and microscopic observations from fluorescent-dyed thin sections. In the Macheon gabbro, various types of cracks, including gran boundary cracks, cleavage cracks, intragranular cracks adn intergranular cracks were developed. However, only grain boundary cracks were produced in the Sangju sandstone and the Jungsun marble due to fatigue stress. Many microcracks were produced due to fatigue stresses in the macheon gabbro. However, few microcracks were produced in the Sangju sandstone and the Jungsun marble. Fatigue stresses produced new microcracks in the Junngsun marble and the Macheon gabbro, but only pre-existing grain boundary cracks were lengthened in the Sangju snadstone. Most microcracks were produced within a few tens of numbers of cyclic loading, indicating that rocks under fatigue stresses will be damaged at the early stage of cyclic loading.

• The Journal of the Petrological Society of Korea
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• v.17 no.4
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• pp.206-221
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• 2008

We have studied general orientational characteristics of microcracks distributed in Bulgugsa Granites of southwestern Gyeongsang Basin. Microcracks of 131 sets, which were developed on horizontal surfaces of II rock samples collected from Sacheon-Gosung, Geoje-si and Namhae-gun areas, were distinguished by image processing. Then, 45 sets with a distinct linear array on image were sorted out. These microcracks can be comparable with vertical grain planes. Orientations of these microcracks were compared with those of vertical rift and grain planes developed in Cretaceous and Jurassic granites of Korea. In the distribution chart, the agreement of the distribution pattern between microcracks of 45 sets and above vertical planes suggests that microcrack systems developed all over the study area also occur regionally in Cretaceous and Jurassic granites of Korea. Whole domain of the directional angle-frequency chart can be divided into 20 domains in terms of the phases of the distribution of microcracks. Meanwhile, 18 domains from 45 sets of microcracks were compared with the maximum principal stress orientations suggested from previous studies. The majority of maximum principal stress orientations pertain to domain $1{\sim}2$, $5{\sim}6$, $11{\sim}15$, $17{\sim}18$ and $19{\sim}20$, and these domains are coincident with the orientation of the 1st and 2nd-frequency orders represented in a rose diagram for 45 sets of microcracks. Representative orientations of open microcrack reflect the maximum principal stress orientations suggested in previous studies.

• The Journal of the Petrological Society of Korea
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• v.16 no.3
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• pp.129-143
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• 2007

We have studied orientational characteristics of microcracks in Mesozoic granites and granitic dyke rocks from Seokmo-do, Ganghwa-gun. Microcracks on horizontal surfaces of rock samples from 14 sites were investigated by image processing. Orientations of these microcracks compared with those of 18 sets of joints in Mesozoic granites from Seokmo-do. From the related chart, microcrack sets show strong preferred orientations which obviously are coincident with the direction of vertical common joints. It follows that the formation of macroscopic joints may be the results of further growth and step-wise jointing of pre-existing microcracks. Orientations of microcracks from this result also compared with those of vertical rift and grain planes for Jurassic and Cretaceous granite quarries in Korea. As shown in the distribution chart, the congruence of distribution pattern among microcracks and rift and grain planes suggests that similar microcrack systems probably occur regionally in Jurassic and Cretaceous granites from Korea. In particular, whole domain of the distribution chart was divided into 16 groups in terms of the phases of distribution of microcracks and planes. These microcrack sets in each domains construct complex composite microcrack systems which have formed progressively by different geologic processes and under varying conditions.

• The Journal of the Petrological Society of Korea
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• v.13 no.3
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• pp.133-141
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• 2004

We have studied the characteristics of rock cleavage for the Pocheon granite with Jurassic emplacement age. Photomicrographs were used to observe and analyze microcracks from the granite. Three sets of microcrack planes are recognized; (1) the rift plane developed parallel to the principal sets of microcracks, (2) the grain plane parallel to the secondary sets of microcracks, (3) the hardway plane perpendicular to both rift and grain planes. The microcracks developed in the granite shows higher polution, mean length and density in the order of rift plane, grain plane and hardway plane. The fracturing characteristics of the granite are closely related to the development of these three planes.

• Tunnel and Underground Space
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• v.8 no.3
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• pp.226-233
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• 1998

Microcracks in rock materials, whether natural or induced, provide useful information on the engineering performance of in situ rockmasses. A population of preferentially oriented microcracks has observable effects on the physical properties of a rockmass, but their effects may not be evident if the rock material is highly anisotropic due to other causes. An experimental program was undertaken to investigate the effect of rock fabrics on the physical properties of rock materials. In this study, anisotropy in the circumferential wave velocity and the direction of induced fractures under axial point loading were measured. Rock specimens (NX-size) of the leucocractic Pocheon granite were cored from rock blocks, retaining the relative directions of each specimen. Another set of specimens was prepared from the rock cores of the same meterial, obtained in the field. The master orientation line (MOL) was set to be the representative direction of the microcracks in the specimen. Variation of the circumferential wave velocity of each specimen was then measured along the core, keeping the MOL as reference. The direction of the minimum wave velocity was nearly perpendicular to the direction of the MOL. Coring of smaller-sized (EX-size), concentric specimens from the NX specimens were then followed, and axial point loading was applied. The direction of induced fractures due to axial point loading was closely related to the MOL direction, confirming the prior test result.