• Transactions of the Korean Society of Mechanical Engineers
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• v.8 no.2
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• pp.97-103
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• 1984

Post weld heat treatment(PWHT) of weldment of the low alloy steel is carried out to remove residual stress existing in weldment and to improve fracture toughness, but it is often observed that there occurs grain boundary failure and that fracture toughness decreases in weld heat affected zone(HAZ)because of PWHT. In this paper, the effect of heating rate and holding time of PWHT on fracture toughness were evaluated by crack opening displacement (CDD)test and micro-hardness test under the constant stress simulated residual stress in HAZ of Cr-Mo steel. The experimental results are as follow; (1)Transition temperature of weld HAZ after PWHT was dependent upon heating rate greater than holding time, and fracture toughness was decreased with an increase of the heating rate. (2)Softening ration of the notch tip was increased with holding time within one hour and saturated after one hour, but under applied stress it was increasing continuously. (3)The average hardness value in weld HAZ was increased with heating rate of PWHT.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.13-18
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• 2003

Reduced activation ferritic steel, JLF-1 steel (Fe-9Cr-2W-V-Ta), is one of the promising candidate materials for fusion reactor applications. Fracture toughness ($J_IC$) and tensile tests were carried out at room temperature and elevated temperature ($400^{\circ}C$). Two types of CT specimen were prepared to examine the effect of rolling direction on the fracture toughness of JLF-1 steel. Four types of tensile specimen were also prepared to investigate the property by the rolling direction and welding. The Micro Vickers hardness was measured at various distances of a cross section of the TIG joints of JLF-1 steel according to the heating history of each position. Finally, the fracture surface was observed by scanning electron microscopy (SEM).

• Nuclear Engineering and Technology
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• v.44 no.1
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• pp.43-52
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• 2012

To compute a permeability coefficient along a rough fracture that takes into account the fracture geometry, this study performed detailed measurements of fracture roughness using a confocal laser scanning microscope, a quantitative analysis of roughness using a spectral analysis, and a homogenization analysis to calculate the permeability coefficient on the microand macro-scale. The homogenization analysis is a type of perturbation theory that characterizes the behavior of microscopically inhomogeneous material with a periodic boundary condition in the microstructure. Therefore, it is possible to analyze accurate permeability characteristics that are represented by the local effect of the facture geometry. The Cpermeability coefficients that are calculated using the homogenization analysis for each rough fracture model exhibit an irregular distribution and do not follow the relationship of the cubic law. This distribution suggests that the permeability characteristics strongly depend on the geometric conditions of the fractures, such as the roughness and the aperture variation. The homogenization analysis may allow us to produce more accurate results than are possible with the preexisting equations for calculating permeability.

• Journal of Korea Foundry Society
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• v.14 no.1
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• pp.52-61
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• 1994

The effect of Ni addition, on the mechanical properties and fracture characteristics of Mo-Cu and Mo-Ni-Cu alloyed ductile iron austenitized at $900^{\circ}C$ and austempering temperatures of $250^{\circ}C$, $300^{\circ}C$ and $350^{\circ}C$. The tensile strength, yield strength and hardness are decreased and elongation and impact value are increased in both Mo-Cu and Mo-Ni-Cu alloyed austempered ductile iron, with increased austempering temperature. According to the austempering temperature are increased, the amount of retained austenite are increased. Maximum value of fracture toughness is obtained at $350^{\circ}C$ austempering temperature at this condition, the amount of retained austenite came to 40% in Mo-Ni-Cu alloyed ADI and 34% in Mo-Cu alloyed ADI. The fracture surface of ADI which had represented high toughness are showed a quasi-cleavage pattern and a dimple pattern with micro void. Comparing the fracture characteristics of Mo-Cu alloyed ADI with that of Mo-Ni-Cu alloyed ADI, the latter was superior to the former.

• Nuclear Engineering and Technology
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• v.56 no.6
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• pp.1975-1988
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• 2024

It is important to maintain cladding integrity in spent nuclear fuel management. This study proposes a numerical analysis method to evaluate the fracture resistance of irradiated zirconium alloy cladding under pinch load known to cause Mode-III failure. The mechanical behavior and fracture of the cladding under pinch loading can be evaluated by a Ring Compression Test (RCT). To simulate the fracture of hydride precipitates, zirconium matrix, and Zr/hydride interfaces under the stress field generated by RCT, a micro-structure crack propagation simulation method based on Continuum Damage Mechanics (CDM) has been proposed. Our RCT simulation model was constructed from microscopic images of irradiated cladding. In this study, we developed an automated process to generate a pixel-based finite element model by separating the hydride precipitates, zirconium matrix, and interfaces using an image segmentation method. The appropriate element size was selected to ensure the efficiency and accuracy of a crack propagation simulation. The load-displacement curves and strain energies from RCT were compared and analyzed with the simulation results of different element sizes. The finalized RCT simulation model can be used to establish the failure criterion of fuel rods under pinch loading. The advantages and limitations of the proposed method are fully discussed here.

• Journal of the Korean Society for Precision Engineering
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• v.20 no.9
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• pp.55-62
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• 2003

This paper investigates the micro-cutting of cemented carbides using PCD (polycrystalline diamond) and PCBN (polycrystalline cubic boron nitride) cutting tools are performed with SEM direct observation method. The purpose of this study is to make clear the cutting mechanism of cemented carbides and the fracture of WC particles at the plastic deformation zone in orthogonal micro-cutting. And also to achieve systematic understanding, the effect of machining parameter on chip formation and machined surface was studied, including cutting speed, depth of cut and various tool rake angle. Summary of the results are shown below. (1) Three type of chip formation process have been proposed by the results of the direct observation in orthogonal micro-cutting of cemented carbide materials. (2) From the whole observation of chip formation, primary WC particles are crushed and/or fine grained in the shearing deformation zone. A part of them are observed to collide directly with a cutting edge of tool by following the micro-cutting. (3) Surface finish, surface morphology and surface integrity is good to obtain by cutting with PCD cutting tool compared with PCBN. (4) The machined surface has the best quality near the low cutting speed of 10${\mu}m$/sec with a cutting depth of 10 ${\mu}m$ using 0$^\circ$ rake angle and 3$^\circ$ flank angle in this condition, but it was found that excessively low speed, for example the extent of 1 ${\mu}m$/sec, is not good enough to select for various reason.

• Interaction and multiscale mechanics
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• v.3 no.1
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• pp.23-37
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• 2010

Concrete is a heterogeneous material consisting of coarse aggregate, mortar matrix and interfacial zones at the meso level. Though studies have been done to interpret the fracture process in concrete using meso level models, not much work has been done for simulating the macroscopic behaviour of reinforced concrete structures using the meso level models. This paper presents a procedure for the mesoscopic analysis of reinforced concrete beams using a modified micro truss model. The micro truss model is derived based on the framework method and uses the lattice meshes for representing the coarse aggregate (CA), mortar matrix, interfacial zones and reinforcement bars. A simple procedure for generating a random aggregate structure is developed using the constitutive model at meso level. The study reveals the potential of the mesoscopic numerical simulation using a modified micro truss model to predict the nonlinear response of reinforced concrete structures. The modified micro truss model correctly predicts the load-deflection behaviour, crack pattern and ultimate load of reinforced concrete beams failing under different failure modes.

• Computers and Concrete
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• v.8 no.5
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• pp.563-581
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• 2011

The signal-based acoustic emission (AE) characterization of concrete fracture process utilizing home-programmed AE monitoring system was performed for three kinds of static loading tests (Cubic-splitting, Direct-shear and Pull-out). Each test was carried out to induce a distinct fracture mode of concrete. Apart from monitoring and recording the corresponding fracture process of concrete, various methods were utilized to distinguish the characteristics of detected AE waveform to interpret the information of fracture behavior of AE sources (i.e. micro-cracks of concrete). Further, more signal-based characters of AE in different stages were analyzed and compared in this study. This research focused on the relationship between AE signal characteristics and fracture processes of concrete. Thereafter, the mode of concrete fracture could be represented in terms of AE signal characteristics. By using cement-based piezoelectric composite sensors, the AE signals could be detected and collected with better sensitivity and minimized waveform distortion, which made the characterization of AE during concrete fracture process feasible. The continuous wavelet analysis technique was employed to analyze the wave-front of AE and figure out the frequency region of the P-wave & S-wave. Defined RA (rising amplitude), AF (average frequency) and P-wave & S-wave importance index were also introduced to study the characters of AE from concrete fracture. It was found that the characters of AE signals detected during monitoring could be used as an indication of the cracking behavior of concrete.

• Restorative Dentistry and Endodontics
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• v.24 no.4
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• pp.604-613
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• 1999

This study was performed to evaluate the effect of resin and filler type on the fracture toughness of light-activated composites. Experimental composites were prepared using urethane tetramethacrylate(UTMA) and bisphenol glycidylmethacrylate(Bis-GMA) monomers and five different types of silica fillers. Fracture toughness was measured by a single edge V-notched beam(SEVNB) method, which was discussed from ASTM E399-78. Rectangular bars of $2.5{\times}5{\times}26mm$ were prepared with experimental composites and a notch about 2.25mm deep was carved at the center of the long axis of the specimen using a dental diamond disk driven by a dental micro engine. The flexural test was carried out at a crosshead speed of 0.05mm/min and fracture surfaces were observed under scanning electron microscope. The results obtained were summarized as follows: 1. The fracture toughness values of UTMA-based composites were relatively higher than those of Bis-GMA-based composites. 2. The highest fracture toughness value was observed in the UTMA-based composite containing the $1.5{\mu}m$-spherical fillers. 3 Aging in the distilled water at $37^{\circ}C$ for 10 days showed the increase of fracture toughness, which was severer in the Bis-GMA-based composites than those of UTMA-based composites. 4. The AE amplitude occurring during the fracture toughness tests was the highest at the point of macroscopic fracture.

• Transactions of the Korean Society of Mechanical Engineers
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• v.9 no.2
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• pp.150-157
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• 1985

In terms of behavior of fatigue cracks propagated after build-up around the artificial drilled miro-hole, this study has been made of the build-up process of slips and micro cracks, behavior of micro-crack propagation and the definition of fatigue limit under the rotating bending stress with low carbon steel. The results of this study are as follows: (1) The fatigue limit is the repropagating critical stress for the nonpropagating cracks which have grown to some limit around the micro-hole in regard of the magnitude of micro-hole. (2) Behavior of the slips and micro-cracks initiation are occurring simultaneously in front and in rear of micro-hole tips in the view of the rotational direction, regardless of the magnitude of micro-hole. (3) Behavior of fatigue crack propagation is different from magnitude of micro-hole, its behavior is propagation of single crack about respectively large hole, but about respectively small hole, fatigue crack propagated joining phenomena of micro-cracks. (4) The behavior of fatigue fracture is affected by the factor of its defects in the view of magnitude of micro-hole when the diameter of the micro-holes are smaller than 50.mu.m, and this is also affected with the size effect of micro-hole diameter.