• 연구논문집
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• s.32
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• pp.55-64
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• 2002

Performance demonstration with real flawed specimens has been strongly required for nondestructive evaluation of safety class components in nuclear power plant. Specimen has been designed to produce mechanical fatigue flaw with tension stress and fatigue flaw has been produced to control stress and cycle, for suitable roughness. Notch condition is considered for control of fracture mode. After seal welding for fracture surface, final welding was performed to complete flaw specimen with GTAW(Gas Tungsten Arc welding) and FCAW(Flux Cored Arc Welding). It was demonstrated flaw size of flawed specimen by radiographic. testing and ultrasonic testing.

• Journal of the Korean Society for Nondestructive Testing
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• v.23 no.1
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• pp.38-44
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• 2003

Performance demonstration with real flawed specimens has been strongly required for nondestructive evaluation of safety class components in nuclear power plant. Mechanical or thermal fatigue crack and intergranular stress corrosion cracking could be occured in the in-service nuclear power plant and mechanical fatigue crack was selected to study in this paper. Specimen was designed to produce mechanical fatigue flaw under tensile stress. The number of cycles and the level of stress were controlled to obtain the desired flaw roughness. After the accurate physical measurement of the flaw size and location, fracture surface was seal-welded in place to ensure the designed location and site. The remaining weld groove was then filled by using gas-tungsten are welding(GTAW) and flux-cored arc welding(FCAW). Results of radio graphic and ultrasonic testing showed that fatigue cracks were consistent with the designed size and location in the final specimens.

• Geomechanics and Engineering
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• v.29 no.5
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• pp.567-582
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• 2022

In the present study, a series of physical experiments and numerical simulations were conducted to investigate the effects of mode I and mixed-mode I/II cracks on the fracture modes and stability of roadway tunnel models. The experiments and simulations incorporated different inclination angle flaws under both static and dynamic loads. The quasi-static and dynamic testing were conducted by using an electro-hydraulic servo control device and drop weight impact system (DWIS), and the failure process was simulated by using rock failure process analysis (RFPA) and AUTODYN software. The stress intensity factor was also calculated to evaluate the stability of the flawed roadway tunnel models by using ABAQUS software. According to comparisons between the test and numerical results, it is observed that for flawed roadways with a single radical crack and inclination angle of 45°, the static and dynamic stability are the lowest relative to other angles of fractured rock masses. For mixed-mode I/II cracks in flawed roadway tunnel models under dynamic loading, a wing crack is produced and the pre-existing cracks increase the stress concentration factor in the right part of the specimen, but this factor will not be larger than the maximum principal stress region in the roadway tunnel models. Additionally, damage to the sidewalls will be involved in the flawed roadway tunnel models under static loads.

• Geomechanics and Engineering
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• v.8 no.4
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• pp.541-557
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• 2015

This research aims to analyze the fracture coalescence characteristics of brittle sandstone specimen ($80{\times}160{\times}30mm$ in size) containing various flaws (a single fissure, double squares and combined flaws). Using a rock mechanics servo-controlled testing system, the strength and deformation behaviours of sandstone specimen containing various flaws are experimentally investigated. The results show that the crack initiation stress, uniaxial compressive strength and peak axial strain of specimen containing a single fissure are all higher than those containing double squares, while which are higher than those containing combined flaws. For sandstone specimen containing combined flaws, the uniaxial compressive strength of sandstone increase as fissure angle (${\alpha}$) increases from $30^{\circ}$ to $90^{\circ}$, which indicates that the specimens with steeper fissure angles can support higher axial capacity for ${\alpha}$ greater than $30^{\circ}$. In the entire deformation process of flawed sandstone specimen, crack evolution process is discussed detailed using photographic monitoring technique. For the specimen containing a single fissure, tensile wing cracks are first initiated at the upper and under tips of fissure, and anti-tensile cracks and far-field cracks are also observed in the deformation process; moreover anti-tensile cracks usually accompanies with tensile wing cracks. For the specimen containing double squares, tensile cracks are usually initiated from the top and bottom edge of two squares along the direction of axial stress, and in the process of final unstable failure, more vertical splitting failures are observed in the ligament region. When a single fissure and double squares are formed together into combined flaws, the crack coalescence between the fissure tips and double squares plays a significant role for ultimate failure of the specimen containing combined flaws.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.10 no.1
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• pp.44-50
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• 2014

There were many flaw issues of reactor vessel head penetration in USA fleets. USNRC issued 10CFR50.55a to implement reactor vessel head penetration ultrasonic examination performance demonstration(PD) in US for enhancement of inspection reliability. After September 2009, all US utilities inspected their RVHP with PD qualified system. Korea Hydro and Nuclear Power Company(KHNP) have developed reactor vessel head penetration performance demonstration system for ultrasonic test to apply for pressurized light-water reactor power plants in accordance with 10CFR50.55a since September 2011. RVHP configuration surveying and analysis, code requirement analysis, and performance demonstration specimen design were performed up to this day. Fingerprinting of manufactured specimen, development of test data management program, development of operation procedure, input of flawed data, and development of final report will be performed for the next step. This paper describes the development status of the performance demonstration system for reactor vessel head penetration ultrasonic examination in Korea.

• Journal of Advanced Marine Engineering and Technology
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• v.30 no.4
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• pp.494-504
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• 2006

It is well known that the two-parameter $J-A_2$ solution can well characterize the crack-tip fields and quantify the crack-tip constraint for different flawed geometries in variety of loading conditions. However, this solution fails to do so for bending dominated specimens or geometries at large deformation because of the influence of significant global bending stress on the crack-tip field. To solve this issue, a modified $J-A_2$ solution is developed in this paper by introducing an additional term to address the global bending influence. Using the $J_2$ flow theory of plasticity and within the small-strain framework detailed finite element analyses are carried out for the single edge notched bend (SENB) specimen with a deep crack in A533B steel at different deformation levels ranging from small-scale Yielding to large-scale Yielding conditions. The numerical results of the crack-tip stress field are then compared with those determined from the $J-A_2$ solution and from the modified $J-A_2$ solution at the same level of applied loading Results indicate that the modified $J-A_2$ solution largely improves the $J-A_2$ solution, and match very well with the numerical results in the region of interest at all deformation levels. Therefore, the proposed solution can effectively describe the crack-tip field and the constraint for bending dominated specimens or geometries.

• Geomechanics and Engineering
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• v.12 no.3
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• pp.541-560
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• 2017

In this research, experimental and numerical simulations were adopted to investigate the effects of ligament angle on compressive strength and failure mode of rock-like material specimens containing two non-coplanar filled fissures under uniaxial compression. The experimental results show that with the increase of ligament angle, the compressive strength decreases to a nadir at the ligament angle of $60^{\circ}$, before increasing to the maximum at the ligament angle of $120^{\circ}$, while the elastic modulus is not obviously related to the ligament angle. The shear coalescence type easily occurred when ${\alpha}$ < ${\beta}$, although having the same degree difference between the angle of ligament and fissure. Numerical simulations using $PFC^{2D}$ were performed for flawed specimens under uniaxial compression, and the results are in good consistency with the experimental results. By analyzing the crack evolution process and parallel bond force field of rock-like material specimen containing two non-coplanar filled fissures, we can conclude that the coalescence and propagation of crack are mainly derived from parallel bond force, and the crack initiation and propagation also affect the distribution of parallel bond force. Finally, the displacement vectors in ligament region were used to identify the type of coalescence, and the results coincided with that obtained by analyzing parallel bond force field. These experimental and numerical results are expected to improve the understanding of the mechanism of flawed rock engineering structures.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.4 no.2
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• pp.50-56
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• 2008

The tubes in heat exchanger are typically made of copper alloy, stainless steel, carbon steel, titanium alloy material. Type-439 ferritic stainless steel is ferromagnetic material, and furnish higher heat transfer rates than austenitic stainless steels and higher resistance to corrosion-induced flaws. Ferritic stainless steel can be found in low-pressure(LP) feedwater heaters and moisture separator reheaters(MSRs) in turbine system. LP feedwater heaters generally utilize thin wall Type-439 stainless steel tubing, whereas MSRs typically employ a heavier wall tubing with integral fins. Service-induced damage can occur on the O.D(outside diameter) surface of Type-439 ferritic stainless steel tubing which is employed for MSRs tubing, and the most typical damage mechanism is vibration-induced tube-to-TSP(tube support plate) wear and fatigue cracking. The wear has been reported that occurs mainly on the OD surface. Accordingly, in this study, we have evaluated the flaw sizing capability of magnetic saturation eddy current technique using magnetic saturation probe and flawed specimen.

• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.5
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• pp.407-415
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• 2008

The tubes in heat exchanger are typically made from copper alloy, stainless steel, carbon steel, titanium alloy material. type-439 ferritic stainless steel is ferromagnetic material, and furnish higher heat transfer rates than austenitic stainless steels and higher resistance to corrosion-induced flaws. Ferritic stainless steel can typically be found in low-pressure(LP) feedwater heaters and moisture separator reheaters(MSRs). LP feedwater heaters generally utilize thin wall type-439 stainless steel tubing, whereas MSRs typically employ a heavier wall tubing with integral fins. Service-induced damage can occur on the OD(outside diameter) surface of type-439 ferritic stainless steel tubing which is employed for MSRs tubing, and the most typical damage mechanism is vibration-induced tube-to-TSP(tube support plate) wear and fatigue cracking. The wear has been reported that occurs mainly on the OD surface. Accordingly, in this study, we have evaluated the flaw sizing capability of magnetic saturation eddy current technique using magnetic saturation probe and flawed specimen.

• The Journal of Korean Academy of Prosthodontics
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• v.32 no.1
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• pp.37-46
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• 1994

The purpose of this study was to evaluate the effect of various cements on the retention of casting crown and the cement film thickness. To evaluate the retention of crown, thirty maxillary premolars were used and prepared to largely same dimension. According to the routine method, Non-precious metal crowns were made. The teeth and the metal crowns were divided into three groups and cemented under 5kg static pressure. Group I was composed of 10 teeth and 10 metal crowns and was cemented with zinc phohsphste cement. Group II was composed of 10 teeth and 10 metal crowns and was cemented with Panavia-EX cement. Group III was composed of 10 teeth and 10 metal crowns and was cemented with All-Bond & composite resin cement. After 5 days, the cemented specimens were mounted and the failure loads were measured by an Instron Universal Testing Machine. To evaluate the cement film thickness. 5 metal teeth and 5metal crowns from a prepared maxillary premolar were made. Two marks were flawed on the margin part of each surface at 4-surfaces of each specimen(one mark : crown, the other : metal tooth) and were measured the width with SEM photograph(80 sheets) before and after cementation(Panavia-EX, All-Bond cement, & ZPC) was made. Differences of the widths of marks between before and after were measured, and differences from 4-surfaces of a specimen cemented with a cement were measured and calculated. The results were as follows ; 1. There was a statistically significant difference between the failure loads of group III and the others(p<0.05). 2. There was a statistically significant difference between the cement film thickness of group III and the others(p<0.05).