• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.2
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• pp.221-228
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• 1989

The objective of this paper is to develop a numerical technique for analyzing crack driving forces in multi-phase materials. The analysis was based on finite element method coupled with a virtual crack extension technique which is known as the most efficient tool in computational fracture mechanics analysis. The modified J-integral method, proposed by Miyamoto and Kikuchi for the analysis of dual-phase material was carried out by subtracting the J-values for contours surrounding each phase boundary from the J-values for overall contour. It was shown that the proposed numerical procedure, based on the modified J-integral coupled with a virtual crack extension technique, can be used as an effective numerical tool for determining crack driving forces in multi-phase materials.

• Journal of Welding and Joining
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• v.25 no.3
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• pp.37-44
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• 2007

Mechanical and thermomechanical properties of the bulk metallic glass (BMG) are so unique that the deformation behavior is largely dependent on the temperature and the strain rate. Impacting behavior of NiTiZrSiSn bulk metallic glass powder during kinetic spraying was investigated in this study. Considering the impact behavior of the BMG, the kinetic spraying system was modified and attached the powder preheating system to make the transition from the inhomogeneous deformation to the homogeneous deformation of impacting BMG particle easy BMG splat formation is considered from the viewpoint of the adiabatic shear instability. It is suggested that the impact behavior of bulk metallic glass particle is determined by the competition between fracture and deformation. The bonding of the impacting NiTiZrSiSn bulk amorphous particle was primarily caused by the temperature-dependent deformation and fracture (local liquid formation) behavior.

• Structural Engineering and Mechanics
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• v.58 no.5
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• pp.799-823
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• 2016

A finite element model for the non-linear dynamic analysis of a reinforced concrete (RC) containment shell of a nuclear power plant subjected to extreme loads such as impact and earthquake is presented in this work. The impact is modeled by using an uncoupled approach in which a load function is applied at the impact zone. The earthquake load is modeled by prescribing ground accelerations at the base of the structure. The nuclear containment is discretized spatially by using 20-node brick finite elements. The concrete in compression is modeled by using a modified $Dr{\ddot{u}}cker$-Prager elasto-plastic constitutive law where strain rate effects are considered. Cracking of concrete is modeled by using a smeared cracking approach where the tension-stiffening effect is included via a strain-softening rule. A model based on fracture mechanics, using the concept of constant fracture energy release, is used to relate the strain softening effect to the element size in order to guaranty mesh independency in the numerical prediction. The reinforcing bars are represented by incorporated membrane elements with a von Mises elasto-plastic law. Two benchmarks are used to verify the numerical implementation of the present model. Results are presented graphically in terms of displacement histories and cracking patterns. Finally, the influence of the shear transfer model used for cracked concrete as well as the effect due to a base slab incorporation in the numerical modeling are analyzed.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.20 no.1
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• pp.42-47
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• 2021

Recently, many structures using lightweight materials have been developed. This study was conducted by using Al6061-T6 and carbon fiber reinforced plastic (CFRP), two common lightweight materials. In addition, the failure characteristics of an interface bonded between a single material and a heterogeneous bonding material were analyzed. The specimens bonded with CFRP and Al6061-T6 were utilized by the combination of the heterogeneous bonding material. The specimens had a double cantilevered shape and the bonding between the materials was achieved by applying a structural adhesive. The experiments were conducted in opening mode: the lower part of the samples was fixed, while their upper part was subjected to a forced displacement of 3 mm/min by using a tensile tester. Under the tested amount of strength, energy release rate, and considering the specimens' fracture characteristics in opening mode, the specimen "CFRP-Al" presented the maximum stress, followed by "Al" and "CFRP". We can hence conclude that the inhomogeneous material "CFRP-Al" is useful for the construction of lightweight structures bonded with structural adhesive.

• Journal of Mechanical Science and Technology
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• v.16 no.11
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• pp.1420-1427
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• 2002

The Monkman-Grant (M-G) and its modified parameters were evaluated for type 316LN and modified 9Cr-Mo stainless steels prepared with minor element variations. Several sets of creep data for the two alloy systems were obtained by constant-load creep tests in 550～650$\^{C}$ temperature range. The M-G parameters, m, m', C, and C' were proposed and discussed for the two alloy systems. The m value of the M-C relation was 0.90 in type 316LN steel and 0.84 in modified 9Cr-Mo steel. The m' value of the modified relation was 0.94 in type 316LN steel and 0.89 in 9Cr-Mo steel. Although creep fracture modes and creep properties between type 316LN and modified 9Cr-Mo steels showed a basic difference, the M-G and its modified relations demonstrated linearity quite well. The m' of modified relation almost overlapped regardless of the creep testing conditions and chemical variations in the two alloy systems, and the parameter m' was closer to unity than that of the M-G relation.

• Korean Journal of Metals and Materials
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• v.46 no.5
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• pp.296-303
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• 2008

Hydrogen permeation through dense palladium-based membranes has attracted the attention of many scientists largely due to their unmatched potential as hydrogen-selective membranes for membrane reactor applications. Although it is well known that the permeation mechanism of hydrogen through Pd involves various processes such as dissociative adsorption, transitions to and from the bulk Pd, diffusion within Pd, and recombinative desorption, it is still unclear which process mainly limits hydrogen permeation at a given temperature and hydrogen partial pressure. In this study, we report an all-electron density-functional theory study of hydrogen permeation through Pd membrane (using VASP code). Especially, we focus on the variation of the energy barrier of the penetration process from the surface to the bulk with hydrogen coverage, which means the large reduction of the fracture stress in the brittle crack propagation considering Griffith's criterion. It is also found that the penetration energy barrier from the surface to the bulk largely decreases so that it almost vanishes at the coverage 1.25, which means that the penetration process cannot be the rate determining process.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.24 no.5
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• pp.553-560
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• 2015

Fracture mechanics analysis for cracked pipes is essential for applying the leak-before-break (LBB) concept to nuclear piping design. For LBB assessment, crack instability and leak rate should be predicted accurately for through-wall cracked pipes. In a nuclear piping system, elbows are connected with straight pipes by circumferential welding; this weld region is often considered a critical location. Hence, accurate crack assessment is necessary for cracks in the interface between elbows and straight pipes. In this study, the stress intensity factor (SIF) and elastic crack opening displacement (COD) were estimated through detailed 3D elastic finite element (FE) analyses. Based on the results, closed-form solutions of shape factors for calculating the SIFs and elastic CODs were proposed for circumferential through-wall cracks in the abovementioned interfaces under internal pressure. In addition, the effect of the elbow on shape factors was investigated by comparing the results with the existing solutions for a straight pipe.

• Journal of the Korean Society for Heat Treatment
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• v.13 no.6
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• pp.383-390
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• 2000

The static creep behaviour of AlSl 420F stainless steel was investigated over the temperature range of $540{\sim}585^{\circ}C$ and the stress range of $13{\sim}19kg/mm^2$ (127.4~186.2MPa). Constant stress creep tests were carried out in the experiment. Measured stress exponent, n, for the creep deformation of the alloy under the given conditions was found to vary at the range of 9.59, 9.15, 8.78, and 8.53 for the temperature of 540, 555, 570, and $585^{\circ}C$ respectively. The activation energy, Qc, for the creep deformation was 106.42, 102.58,97.81, and 94.58 kcal/mole for the stress of 13, 15, 17, and $19kg/mm^2$, respectively. Lason-Miller parameter, P, for the crept specimens for AlSl 420F stainless steel was measured as $P=T(log\;t_T+21)$. The empirical static creep rate obtained by the regression analysis was as follows. $${\varepsilon}={\exp}[(3.79{\times}10^{-2}{\sigma}+2.722)T-3.0747{\sigma}+28.109]{\times}{\sigma}^{(-2.367{\times}10^{-2}T+22.33)}{\exp}\left[-\frac{(-2.015{\sigma}+132.580){\times}10^3}{RT}\right]$$ The failure plane were observed, intergranular fracture was dominated by r (round) type crack over the experimental range.

• Journal of Nuclear Fuel Cycle and Waste Technology(JNFCWT)
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• v.3 no.4
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• pp.341-348
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• 2005

Bentonite has been considered as a candidate buffer material in the underground repository for the disposal of high-level radioactive waste because of its low permeability, high sorption capacity, self sealing characteristics, and durability in nature. In this study, the potential for separation of bentonite particles caused by the groundwater erosion was studied experimentally for a Korean Ca-bentonite under the relevant repository conditions. Results showed that bentonite particles can be generated at the bentonite/granite interface and mobilized by the water flow although the intrusion of bentonite into fracture by swelling pressure was observed to be small. Different processes of mobilization of theses colloids from the compacted bentonite block have been identified in this study. The concentration of particles eluted in water was increased as the flow rate increased. Thus the result reveals that the erosion of the bentonite surface due to the groundwater flow together with intrusion processes is the main mechanism that can mobilize bentonite colloids in the fracture of the granite.

• Nuclear Engineering and Technology
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• v.51 no.7
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• pp.1805-1815
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• 2019

While nickel-based alloys have been widely used for power plants due to corrosion resistance and good mechanical properties, during the last couple of decades, failures of nuclear components increased gradually. One of main degradation mechanisms was primary water stress corrosion cracking at dissimilar metal welds of piping and reactor head penetrations. In this context, precise estimation of welding effects became an important issue for ensuring reliability of them. The present study deals with a series of finite element analyses and crack growth rate evaluation of Alloys 690/152. Firstly, variation of residual stresses and equivalent plastic strains was simulated taking into account welding of a cylindrical block. Subsequently, extraction and pre-cracking of compact tension (CT) specimens were considered from different locations of the block. Finally, crack growth curves of the alloys and heat affected zone were developed based on analyses results combined with experimental data in references. Characteristics of crack growth behaviors were also discussed in relation to mechanical and fracture parameters.