• Journal of the Korean Ceramic Society
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• v.21 no.2
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• pp.149-155
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• 1984

The effect of $ZrO_2$ dispersed phase on the mechanical properties in $Al_2O_3$-$ZrO_2$system has been studied. There are both metastable tetragonal phase and stable monoclinic phase of $ZrO_2$particles diespersed in Al2O3 matrix at room temperature. Metastable tetragonal $ZrO_2$ changes to the stable monoclinic structure within the stress field of the crack. And microcracks are formed by the expansion of $ZrO_2$during the tetragonal-monoclinic transformation on cooling. Therefore stress-induced phase transformation and inclusion-induced microcracking contribute to the mec-hanical properties of $Al_2O_3$-$ZrO_2$system. Sintered composites containing 10m/o $ZrO_2$ yield KiC values of 6.5MN/$m^{3/2}$ much greater than that of pure $Al_2O_3$ This increase results from microcrack extension and stress-induced phase transformation absor-bing energy by crack propagation. Flexural strength of composites is decreased considerably in comparison with pure $Al_2O_3$ This decrease results from microcrack as a crack former and higher porosity than pure $Al_2O_3$.

• Journal of the Korean Geotechnical Society
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• v.33 no.9
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• pp.23-34
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• 2017

In order to develop urban underground spaces, even microcracks should be reinforced. In this paper, the grouting injection performance for microcracks was investigated considering the viscosity and particle size of the grouting materials, injection pressure, and crack width. There are two types of typical grouting materials used for filling micro-cracks. One is a chemical liquid grouting material which is a solution type and the other is a cementitious grouting material which is a suspension type. The injection performance of the grouting materials for microcracks is generally influenced by the viscosity, and the injection performance of the cementitious grouting material is additionally affected by the particle size. From laboratory tests, the viscosity was calculated inversely to provide a suitable viscosity measurement method for each grouting material. The groutability ratio based on the relationship between the crack width and the particle size was evaluated to estimate the grouting feasibility of the cementitous grouting material through microcracks.

• The Journal of the Petrological Society of Korea
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• v.26 no.1
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• pp.45-54
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• 2017

The characteristics of the rock cleavage in Jurassic Geochang granite were analysed using the distribution of microcrack lengths and spacings. The phases of distribution of the above lengths and spacings were derived from the enlarged photomicrographs(${\times}6.7$) of the thin section. First, the length and spacing-cumulative diagrams for the six directions of rock cleavages were arranged in increasing order($H2{\rightarrow}R1$) on the density(${\rho}$) of microcrack length. The various parameters were extracted through the combination of the above two types of diagrams. The discrimination factors representing the three quarrying planes and three rock cleavages were acquired through the mutual contrast between the values of parameters. The analysis results of the research are summarized as follows. The evaluation for the six directions of rock cleavages was performed using the parameters such as (1) intersection angle(${\alpha}-{\beta}$) and (2) exponent difference(${\lambda}_S-{\lambda}_L$) between two exponential straight lines related to spacing(${\alpha}$, ${\lambda}_S$) and length(${\beta}$, ${\lambda}_L$). The values of parameters(1 and 2) are in order of H(hardway, (H1 + H2)/2) < G(grain, (G1 + G2)/2) < R(rift, (R1 + R2)/2). On the contrary, the values of the above two parameters for three planes are in order of R < G < H. Meanwhile, the direction of convergence between two exponential straight lines was derived. The above direction is compliant to arrangement of the line os' centering around the line ol. The above two lines converge in the direction of the Y-axis when the line ol and line os' occupy the upper region on the left and the lower region on the right, respectively(R-type). On the contrary, the above two lines converge in the direction of the X-axis when the order of arrangement between line ol and line os' is reversed(H-type). Especially, the positive(+) or negative(-) value of intersection angle(${\alpha}-{\beta}$) is determined by the arrangement of two vertical lines. This type of correlation analysis is useful for evaluating the relative strength of rock cleavage and discriminating three quarrying planes.

• Korean Journal of Mineralogy and Petrology
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• v.34 no.3
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• pp.193-208
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• 2021

The characteristics of the microcrack lengths(①), microcrack spacings(②) and Brazilian tensile strengths(③) related to the six directions of rock cleavages(H2~R1) in Geochang granite were analyzed. First, the 18 cumulative graphs for the above three major factors representing unique characteristics of the rock cleavages were made. Through the general chart for these graphs classified into three planes and three rock cleavages, the 28 parameters on the length, spacing and Brazilian tensile strength have been determined. The results of correlation analysis among these parameters are summarized as follows. Second, the above parameters were classified into six groups(I~VI) according to the sorting order on the magnitude of parameter values among three rock cleavages and three planes. The values of parameters belonging to group I and II are in order of R(rift) < G(grain) < H(hardway) and H < G < R. The values of the 8 parameters on the length of line(os₂, 𝚫s, 𝚫L and oS^mean), the exponent(λ_L^mean and λ_S^mean), the slope(a^mean) and the anisotropy coefficient (An^mean) are in order of R < G < H and H'(hardway plane) < G'(grain plane) < R'(rift plane). Third, the noticeable differences in distribution patterns among the six types of charts for three planes and three rock cleavages are as follows. From the chart for three planes, the values of 𝚫L, 𝚫s and 𝚫σ_t, corresponding to the distance between two points where the two fitting lines meet on the X-axis, increase in the order of R' < H' < G'. In particular, the two graphs of R2 and G2 related to the length and Brazilian tensile strength are almost parallel to each other and show the distribution characteristics of hardway plane. Among the graphs related to the Brazilian tensile strength, the overall shape for hardway plane is similar to that for grain. From the chart for three rock cleavages, the slopes of the graphs related to the length increase in the order of R < G < H, while those of the graphs related to the spacing and Brazilian tensile strength decrease in the order of R < G < H. Lastly, the characteristics of variation among the six rock cleavages, the three planes and the three rock cleavages were visualized through the correlation chart among the above parameters from this study.

• Korean Journal of Mineralogy and Petrology
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• v.33 no.2
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• pp.99-114
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• 2020

The characteristics of the Brazilian tensile strengths(σ_t) parallel to the rock cleavages in Jurassic granite from Geochang were analysed. The evaluation for the six directions of rock cleavages was performed using the parameter values on microcrack length and the above strength. The strength values of the five test specimens belonging to each direction were classified into five groups. The strength values of these five groups increase in order of group A < B < C < D < E. The close dependence between the above microcrack and strength was derived. The analysis results of this study are summarized as follows. First, the chart showing the variation and characteristics of strength among the three rock cleavages were made. In the above chart, the strength values of six directions belonging to each group were arranged in order of rift(R1 and R2), grain(G1 and G2) and hardway(H1 and H2). The strength distribution lines of the five groups concentrate in the direction of R1. And the widths among the above five lines indicating strength difference(Δσ_t) are the most narrowest in R1 direction. From the related chart, the variation characteristics among the two directions forming each rock cleavage were derived. G2(2)-test specimen shows higher value and lower value of the difference in strength compared to the case of G1(1)-test specimen. These kinds of phenomena are the same as the case between the test specimen H2(2) and H1(1). The strength characteristics of the above test specimens (2) suggest lower microcrack density value and higher degree of uniformity in the distribution of microcracks arrayed parallel to the loading direction compared to those of test specimens (1). The six strength values belonging to each group were arranged in increasing order in the above chart. The strength values of the test specimens belonging to both group D and E appear in order of R1 < R2 < G1 < H1 < G2 < H2. Therefore, the strength values of group D and E can be indicator values for evaluating the six directions of rock cleavages. Second, the correlation chart between slope angle(θ) and strength difference(Δσ_t) were made. The values of the above two parameters were obtained from the five strength distribution lines connecting between the two directions. From the chart related to rift plane(G1-H1, R'), grain plane(R1-H2, G') and hardway plane(R2-G2, H'), the slope values of linear functions increase in order of R'(0.391) < G'(0.470) < H'(0.485). Among three planes, the charts related to hardway plane show the highest distribution density among the five groups. From the related chart for rift(R1-R2, R), grain(G1-G2, G) and hardway(H1-H2, H), the slope values of linear functions increase in order of rift(0.407) < hardway(0.453) < grain(0.460). Among three rock cleavages, the charts related to rift show the highest frequency of groups belonging to the lower region. Taken together, the width of distribution of the slope angle among the three planes and three rock cleavages increase in order of H' < G < R' < R < G' < H. Third, the correlation analysis among the parameters related to microcrack length and the tensile strengths was performed. These parameters may include frequency(N), total length(Lt), mean length(Lm), median length(Lmed) and density(ρ). The correlation charts among individual parameters on the above microcrack(X) and corresponding five levels of tensile strengths for the five groups(Y) were made. From the five kinds of correlation charts, the values of correlation coefficients(R²) increase along with the five levels of strengths. The mean values of the five correlation coefficients from each chart increase in order of 0.22(N) < 0.34(Lt) < 0.38(ρ) < 0.57(Lmed) < 0.58(Lm). Fourth, the correlation chart among the corresponding maximum strength for group E(X) and the above five parameters(Y) were made. From the related chart, the values of correlation coefficient increase in order of 0.61(N) < 0.81(Lt) < 0.87(ρ) < 0.93(Lm) < 0.96(Lmed). The two parameters that have the highest correlations are median length with maximum strength. Through the above correlation analysis between microcrack and strength, the credibility for the results from this study can be enhanced.

• Structural Engineering and Mechanics
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• v.7 no.2
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• pp.225-240
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• 1999

This paper presents a mechanistic approach to uniaxial viscoelastic constitutive modeling of asphalt concrete that accounts for damage evolution under cyclic loading conditions. An elasticviscoelastic correspondence principle in terms of pseudo variables is applied to separately evaluate viscoelasticity and time-dependent damage growth in asphalt concrete. The time-dependent damage growth in asphalt concrete is modeled by using a damage parameter based on a generalization of microcrack growth law. Internal state variables that describe the hysteretic behavior of asphalt concrete are determined. A constitutive equation in terms of stress and pseudo strain is first established for controlled-strain mode and then transformed to a controlled-stress constitutive equation by simply replacing physical stress and pseudo strain with pseudo stress and physical strain. Tensile uniaxial fatigue tests are performed under the controlled-strain mode to determine model parameters. The constitutive equations in terms of pseudo strain and pseudo stress satisfactorily predict the constitutive behavior of asphalt concrete all the way up to failure under controlled-strain and -stress modes, respectively.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.5 no.1
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• pp.33-38
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• 2006

Silicon has been widely used in electronic parts as a semiconductor equipment. It, however, requires much effort to grind without microcrack and chipping because of its high hardness and brittleness. So far, many studies for the grinding of engineering ceramics have been done, but not for the grinding of silicon. In this paper, a theoretical analysis on the grinding forces is introduced. Grinding experiments were performed at various grinding conditions including grinding directions (Up grinding and Down grinding), table speeds and depth of cuts. The grinding forces were measured to compare at various grinding conditions. The experimental values agree well with theoretical ones.

• Computers and Concrete
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• v.25 no.6
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• pp.517-523
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• 2020

An experimental system on three point bending notched beams was established to study the fracture process of concrete. In this system, the acoustic emission (AE) was used to build the cumulative generation order (AGO) and dynamically track the process of microcrack evolution in concrete. A grey-cusp catastrophe model was built based on AE parameters. The results show that the concrete beams have significant catastrophe characteristic. The developed grey-cusp catastrophe model, based on AGO, can well describe the catastrophe characteristic of concrete fracture process. This study also provides a theoretical and technical support for the application of AE in concrete fracture prediction.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2008.04a
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• pp.56-59
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• 2008

This paper presents results of a parametric study for a constitutive model (Lee et ai, 1989) for particle-reinforced composites considering weakened interfaces and crack nucleation. Eshelby's tensors for particles with imperfect interfaces (Ju and Chen, 1994) and microcracks (Sun and Ju, 2004) are incorporated into a micromechanical formulation. A parametric study for the microcrack nucleation parameter ${\phi}_{{\upsilon}0}$ and ${\epsilon}^{th}$ is conducted to investigate the sensitivity of the parameter to the constitutive model.

• Journal of Welding and Joining
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• v.13 no.2
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• pp.82-95
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• 1995

This paper discusses distribution of thermal stress, strain at near the joint and investigates the reliability of solder joints of electronic devices on a printed circuit board. As Electronic devices are composed of different materials, thermal stresses generate at near the interface, such as solder joints and interface between lC device and lead frame pad due to the differences of thermal expansion coefficients, As results of thermal stress, strain, micro crack often occurs thermal fatigue fracture at the interface of different materials, The initiation and propagation of micro crack depend on the environmental conditions, such as storage temperature and thermal cycling. Finally, this paper experimentally shows a way to suppress micro cracks by using Fe-Ni alloy clad lead frame, and investigates crack and thermal fatigue fracture of TSOP(Thin small outline package) type on printed circuit board.