• Composites Research
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• v.23 no.2
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• pp.1-9
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• 2010

This study analyzes the apparent fracture toughness of a thick-walled cylinder with a functionally graded material (FGM) coating at the inner surface of the cylinder. The cylinder is assumed to have a single radial edge crack emanating from its inner surface. The crack surfaces and the inner surface of the cylinder are subjected to an internal pressure. The incompatible eigenstrain developed in the cylinder due to nonuniform coefficient of thermal expansion as a result of cooling from sintering temperature is taken into account. Based on a method of evaluating stress intensity factor introduced in our previous study, an approach is developed to calculate apparent fracture toughness. The approach is demonstrated for a cylinder with a TiC/$Al_{2}O_{3}$ FGM coating and some numerical results of apparent fracture toughness are presented graphically. The effects of material distribution profile, cylinder wall thickness, application temperature, and coating thickness on the apparent fracture toughness are investigated in details. It is found that all of these factors play an important role in controlling the apparent fracture toughness of the cylinder.

• The Journal of Engineering Geology
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• v.26 no.1
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• pp.101-115
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• 2016

We carried out laboratory material tests on two cements (KS-1 ordinary Portland and Class G) with changing W/S (Water/Solid) and the content of fly ash in order to evaluate their physical and mechanical properties. The specimens of KS-1 ordinary Portland cement were prepared with varying W/S (Solid=cement) in weight, while those of Class G cement were prepared with changing the content of fly ash in volume but maintaining W/S (Solid=cement+fly ash). The results of the material tests show that as the W/S in KS-1 ordinary Portland cement and the content of fly ash in Class G cement increase, the properties (density, sonic wave velocity, elastic constants, compressive and tensile strengths, thermal conductivity) decrease, but porosity and specific heat increase. In addition, an increase in confining pressure and in the content of fly ash leads to plastic failure behavior of the cements. The laboratory data were then used in a stability analysis of cement sheath for which an analytical solution for computing the stress distribution induced around a cased, cemented well was employed. The analysis was carried out with varying the injection well parameters such as thickness of casing and cement, injection pressure, dip and dip direction of injection well, and depth of injection well. The analysis results show that cement sheath is stable in the cases of relatively lower injection pressures and inclined and horizontal wells. However, in the other cases, it is damaged by mainly tensile failure.

• Composites Research
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• v.29 no.3
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• pp.111-116
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• 2016

The tilting pad journal bearing for the turbo compressor application has a role to support high speed and heavy loading rotor. White metal has been widely used for the bearing material but the conventional bearing is immediately suspended and induces serious serious damage to the rotor under the unexpected oil cut situation or the insufficient oil film formation. The carbon fiber reinforced composite having high specific stiffness, specific strength and excellent tribological characteristics can solve these seizure problems. In this work, the study on the durability of high thermal resistance carbon fiber/epoxy composite tilting pad journal bearing under oil cut situation was conducted. The material properties of the composite materials including tensile, compressive and interlaminar properties were measured at room and high temperature of oil cut situation. To investigate the possibility of failure of composite tilting pad journal bearing under oil cut situation, the stress distribution of the composite bearing was analyzed via finite element analysis and the Tsai-Wu Failure index was calculated. To verify the failure analysis results, the oil cut tests for the composite tilting pad journal bearing were conducted using industrial test bench.

• Composites Research
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• v.28 no.4
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• pp.212-218
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• 2015

A large composite spar was manufactured using an automatic fiber placement (AFP) machine. To verify its structural performance, the weakest part of the structure, which is called 'corner radius', was tested under bending and examined by finite element analysis. Since the application of AFP machine to composite structure fabrication is still in early stage in Korea, this paper presents the summary of whole process for manufacturing composite spar using AFP machine from mandrel design and analysis to verification test. The deflection and stress by mandrel weight and AFP machine force, thermal deformation and natural frequency were all examined for mandrel design. The target structure was composite C-spar and cured in an autoclave. Test results were compared with nonlinear finite element analysis results to show that the structure has the strength close to the theoretical value. It was confirmed that the corner radius of the spar manufactured by AFP process showed deviation less than 20% compared with first ply failure strength. The results indicate that the AFP technology could be used for large scale composite structure production in the near future.

• Journal of Aerospace System Engineering
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• v.13 no.2
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• pp.34-42
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• 2019

The existence of different thermodynamic properties results in various undesirable effects, such as thermal deformation and residual stress, in heat-welding processes. The solid-state junction, by using explosive or electromagnetic forces, i.e., high-velocity impact welding without employing heat is advantageous in joining materials with different thermodynamic properties. In the solid-state junction, the joining is performed within a short time, a high velocity and large deformations are accompanied by interfacial surfaces. The numerical analysis models play an important role in the understanding of the mechanism of high-velocity impact welding. However, in the analysis of high velocity and large deformations, the conventional Lagrangian method has low reliability due to the occurrence of entanglements. In this study, high-velocity impact welding between Cu and CP-Ti with different thermodynamic properties was performed using a un-gridded numerical method, SPH (Smoothed Particle Hydrodynamics), and interfacial morphology occurred. As a result of the analysis, the interfacial morphology was confirmed and the compared degree of shape (straight, vortex), period, length, and so on appeared differently depending on the relationship between the parameters (impact angle and speed).

• International Journal of Highway Engineering
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• v.5 no.4 s.18
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• pp.1-11
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• 2003

Recently, the new pavement design method considering Korean environment and the specification for improving performance of pavement are being developed in Korea. The Jointed Concrete Pavement Program Applying Load Discretization Algorithm (called HEART-JCP) is one of the results of Korea Pavement Research Project in Korea. HEART-JCP program is developed to analyze various loading condition using the load discretization algorithm without mesh refinement. In addition, it can be modified easily into multi-purpose concrete pavement nidyses program because of the modularized structure characteristic of HEART-JCP. The program consists of basic program part and load discretization part. In basic program part, the displacement and stress are computed in the concrete slab, sub-layer, and dowel bar, which are modeled with plate/shell element, spring element and beam element. In load discretization program part, load discretization algorithm that was used for the continuum solid element is modified to analyze the model with plate and shell element. The program can analyze the distributed load, concentrated load, thermal load and body load using load discretization algorithm. From the result of verification and sensitivity study, it was known that the loading position, the magnitude of load, and the thickness of slab were the major factors of concrete pavement behavior as expected. Since the result of the model developed is similar to the results of Westergaard solution and ILLISLAB, the program can be used to estimate the behavior of jointed concrete pavement reasonably.

• Composites Research
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• v.18 no.2
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• pp.20-29
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• 2005

To evaluate the flexural deformation and strength of composite motor case above the glass transition temperature$(T_g),\;170^{\circ}C$, of resin material, a finite element analysis(FEA) model in which material non-linearity and progressive failure mode were considered was proposed. The laminated flexural specimens which have the same lay-up and thickness as the composite motor case were tested by 4-point bending test to verify the validity of FEA model. Also. mechanical properties in high temperature were evaluated to obtain the input values for FEA. Because the material properties related to resin material were highly deteriorated in the temperature range beyond $T_g$, the flexural stiffness and strength of laminated flexural specimen in $200^{\circ}C$ were degraded by also $70\%\;and\;80\%$ in comparison with normal temperature results. Above $T_g$, the failure mode was changed from progressive failure mode initiated by matrix cracking at $90^{\circ}$ ply in bottom side and terminated by delamination at the center line of specimen to fiber compressive breakage mode at top side. From stress analysis, the progressive failure mechanism was well verified and the predicted bending stiffness and strength showed a good agreement with the test results.

• Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography
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• v.33 no.5
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• pp.453-461
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• 2015

In order to manage the deflection of a curved PCT girder bridge during construction, a GPS receiver was installed at the spot predicted to be the weak point during the incremental launching so as to measure the deflection at each construction stage. The deflections obtained in the experiment were compared with those derived from the monitoring of stress, temperature and inclination. The comparative analysis of the GPS measurement and analytical values obtained from finite element modeling with respect to the launching distance showed that the measured values differ by 0.6 to 1.6 times to the analytical results. This difference could be significantly reduced by thermal calibration. From the analysis of the behavioral pattern of the bridge, deflection occurred during construction in the concrete tip due to the deflection at the head of the nose at the 95m and 75m-spots, and compression and tension developed respectively at the compression weak zone and tension weak zone. The application of GPS appeared to enable more efficient management of the deflection during the erection of the curved PCT girder bridge and is expected to be helpful for the prediction and management of the behavior in future ILM construction sites.

• Journal of Korean Orthopaedic Sports Medicine
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• v.7 no.1
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• pp.27-32
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• 2008

Purpose: To evaluate the clinical results of the patients who underwent radiofrequency thermal shrinkage (RFTS) for treatment of anterior cruciate ligament (ACL) laxity after ACL reconstruction. Material and Methods: From October 1999 to March 2006, we performed 133 cases of ACL reconstruction. Among them we experienced 16 patients who had the laxity of reconstructed ACL in second look arthroscopy. Mean follow-up was 20.4 months. Mean age was 33.5 years. 12 cases were male and 4 cases were female. The elongated ACL were treated by bipolar radiofrequency energy with an output of grade II. Subjective and objective parameters were utilized in analyses, such as: the mean range of motion, Lysholm knee score, Tegner activity score, Lachman test, IKDC score. Wilcoxon signed-rank test was used to perform the data analysis. P<0.05 was considered to be statistically significant. Results: Postoperative mean Lysholm knee score (preop: $82.2{\pm}5.2(77{\sim}85))$ (P=0.04), postop: $85.2{\pm}4.8$(82-90)) and anterior displacement by the Telos stress test (preop: $5.4{\pm}4.6(3{\sim}10)mm$, postop: $2.1{\pm}1.9(0{\sim}4)mm)$ (P=0.02), Lachman's test, and IKDC scores (P=0.04) demonstrated significant differences statistically compared to the preoperative. There were no statistical differences in mean range of motion, Tegner activity scale. Conclusions: Arthroscopic shrinkage for the ACL laxity after ACL reconstruction with radiofrequency device showed good clinical results and was applicable operative technique.

• Journal of the Korean Geotechnical Society
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• v.37 no.3
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• pp.19-30
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• 2021

The presence of the hydrate-bearing sediments in Ulleung Basin of South Korea has been confirmed from previous studies. Researches on gas production methods from the hydrate-bearing sediments have been conducted worldwide. As production mechanism is a complex phenomenon in which thermal, hydraulic, and mechanical phenomena occur simultaneously, it is difficult to accurately conduct the productivity and stability analysis of hydrate bearing sediments through lab-scale experiments. Thus, the importance of numerical analysis in evaluating gas productivity and stability of hydrate-bearing sediments has been emphasized. In this study, the numerical parametric analysis was conducted to investigate the effects of the bottom hole pressure and the depressurization rate on the gas productivity and stability of hydrate-bearing sediments during the depressurization method. The numerical analysis results confirmed that as the bottom hole pressure decreases, the productivity increases and the stability of sediments deteriorates. Meanwhile, it was shown that the depressurization rate did not largely affect the productivity and stability of the hydrate-bearing sediments. In addition, sensitivity analysis for gas productivity and stability of the sediments were conducted according to the depressurization rate in order to establish a production strategy that prevents sand production during gas production. As a result of the analysis, it was confirmed that controlling the depressurization rate from a low value to a high value is effective in securing the stability. Moreover, during gas production, the subsidence of sediments occurred near the production well, and ground heave occurred at the bottom of the production well due to the pressure gradient. From these results, it was concluded that both the productivity and stability analyses should be conducted in order to determine the bottom hole pressure when producing gas using the depressurization method. Additionally, the stress analysis of the production well, which is induced by the vertical displacements of sediments, should be evaluated.