• Structural Engineering and Mechanics
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• v.58 no.4
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• pp.641-659
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• 2016

This paper deals with the pure bending of incompressible elastic perfectly plastic two-layer sheets under plane strain conditions at large strains. Each layer is classified by its yield stress, shear modulus of elasticity and its initial percentage thickness in relation to the whole sheet. The solution found is semi-analytic. In particular, a numerical technique is only necessary to solve transcendental equations. The general solution is cumbersome because different analytic expressions for the radial and circumferential stresses should be adopted in different regions of the whole sheet. In particular, there are several alternative ways a plastic region (or plastic regions) can propagate. However, for any given set of material and process parameters the solution to the problem consists of a sequence of rather simple analytic expressions connected by transcendental equations. The general solution is illustrated by a simple example.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.4 s.247
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• pp.420-427
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• 2006

The structural and leakage integrity of steam generator tubes should be sustained against all postulated loads even if a crack is present. During the past three decades, most of the efforts with respect to integrity evaluation of steam generator tubes have been focused on limit load solutions but, recently, the applicability of elastic-plastic fracture mechanics was examined cautiously due to its effectiveness. The purpose of this paper is to introduce a testing method to estimate fracture resistance characteristics of steam generator tubes with a through-wall crack. Due to limited thickness and diameter, inevitably, the steam generator tubes themselves were tested instead of standard specimen or alternative ones. Also, a series of three dimensional elastic-plastic finite element analyses were carried out to derive closed-form estimation equations with respect to J-integral and crack extension for direct current potential drop method. Since the effectiveness of $J_{IC}$ as well as J-R curves was proven through comparison with those of standard specimens taken from pipes, it is believed that the proposed scheme can be utilized as an efficient tool for integrity evaluation of cracked steam generator tubes.

• Proceedings of the KSME Conference
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• 2000.04a
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• pp.908-913
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• 2000

In the process of integrity evaluation for nuclear power plant components, a series of fracture mechanics evaluation on surface cracks in reactor pressure vessel(RPV) must be conducted. These fracture mechanics evaluations are based on stress intensity factor, K. However, under pressurized thermal shock(PTS) conditions, the combination of thermal and mechanical stress by steep temperature gradient and internal pressure causes considerably high tensile stress at the inside of RPV wall. Besides, the internal pressure during the normal operation produces high tensile stress at the RPV wall. As a result cracks on inner surface of RPVs may experience elastic-plastic behavior which can be explained with J-integral. In such a case, however, J-integral may possibly lose its validity due to constraint effect. In this paper, in order to verify the suitability of J-integral, two dimensional finite element analyses were applied for various surface crack. Total of 18 crack geometries were analyzed, and Q stresses were obtained by comparing resulting HRR stress distribution with corresponding actual stress distributions. In conclusion, HRR stress fields were found to overestimate the actual crack-tin stress field due to constraint effect.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.72-77
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• 2004

In a primary reactor cooling system(RCS), a dissimilar weld zone exists between cast stainless steel(CF8M) in a pipe and low-alloy steel(SA508 cl.3) in a nozzle. Thermal aging is observed in CF8M as the RCS is exposed for a long period of time to a reactor operating temperature between 290 and $330^{\circ}C$, while no effect is observed in SA508 cl.3. The specimens are prepared by an artificially accelerated aging technique maintained for 300, 1800 and 3600 hrs at $430^{\circ}C$, respectively. The specimens for elastic-plastic fracture toughness tests are prepared one type, which notch is created in the heat affected zone(HAZ) of CF8M. And, the specimens for fatigue crack growth tests are prepared in three classes, which notches are created at the center of deposited zone, the HAZ of CF8M, and the HAZ of SA508 cl.3. From the experiments, the J-integral values with the increase of aging time decrease, and the differences of the fatigue crack growth behaviors are relatively small in the three classes specimens.

• Bulletin of the Society of Naval Architects of Korea
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• v.7 no.2
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• pp.27-40
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• 1970

The distribution of stress and strain in elastic stages is investigated by the experiments of two dimensional photoelastic coating and Moire fringe method. Center block type and cover plate type of fillet welds are used as specimens in the test. The results are as follows. 1) Center block type gets less uniform stress distribution than cover plate type. And its stress concentration factor, especially at root, is larger than that at toe. 2) When main plate and cover plate closely contact and it cause friction, stress concentration decreases more than that in case of slit. That is because stress can be transmitted on the contact surface. 3) When slit is made, the outside of fillet gets more stress than the inside of it. 4) While the plastic strain distribution of center block type reaches the maximum at root and differs very slightly from that under lower loading, the plastic strain distribution of cover plate type is inclined to get the maximum at the outside of fillet rather than at root. 5) When the plastic strain value of cover plate type is compared with that of center block type at toe and root, the relations between the former and the latter shows root<toe and root>toe. 6) Because stress distribution becomes changed according to loading, fracture angle cannot be estimated by the peaks of elastic stress distribution. 7) The strain distribution just before fracture can be found by Moire fringe method.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.10
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• pp.1009-1016
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• 2007

Finite element alternating method has been suggested and used effectively to obtain the fracture parameters in assessing the integrity of cracked structures. The method obtains the solution from alternating independently between the FEM solution for an uncracked body and the crack solution in an infinite body. In the paper, the finite element alternating method is extended in order to obtain the elastic-plastic stress fields of a three dimensional inner crack. The three dimensional crack solutions for an infinite body were obtained using symmetric Galerkin boundary element method. As an example of a three dimensional inner crack, a penny-shaped crack in a finite body was analyzed and the obtained elastc-plastic stress fields were compared with the solution obtained from the finite element analysis with fine mesh. It is noted that in the region ahead of the crack front the stress values from FEAM are close to the values from FEM. But large discrepancy between two values is observed near the crack surfaces.

• Journal of the Korean Society for Precision Engineering
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• v.24 no.3 s.192
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• pp.91-99
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• 2007

Fractures of galvannealed coating layer during actual press forming in automotive applications were observed by scanning electron microscopy in order to understand fracture mechanism. Fracture behaviors of galvannealed coating layer in extra deep drawing quality steels and high strength steels have been studied by performing the tests describing the representative plastic deformation in sheet metal forming such as uni-axial tensile test, compression test, bi-axial test and plane strain test. Growth and direction of cracks were deeply related to the plastic deformation modes and history. The material properties of galvannealed coating layer were investigated by nano-indentation test equipped with Berkovich diamond indentor for the specimens. Hardness and elastic modulus of the coating layer were higher than bared steels and that was the reason for crack of coating layer. Flat friction test and drawbead friction test were performed to observe the effect of the surface morphology on the frictional characteristics. The micro-plasto hydrodynamic lubrication were appeared and played an important role in reducing the coefficient of friction.

• Structural Engineering and Mechanics
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• v.58 no.5
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• pp.745-755
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• 2016

A three-dimensional finite element method is used for analysis of repairing cracks in plates with bonded composite patch in elastic and elastic plastic analysis. This study was performed in order to establish an analytical model of the J-integral for repair crack. This formulation of the J-integral to establish models of fatigue crack growth in repairing aircraft structures. The model was developed by interpolation of numerical results. The obtained results were compared with those calculated with the finite element method. It was found that our model gives a good agreement of the J-integral. The arrow shape reduces the J integral at the crack tip, which improves the repair efficiency.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.23 no.7 s.166
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• pp.1112-1119
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• 1999

Fracture resistance(J-R) curves, which are used for the elastic-plastic fracture mechanics analyses, are known to be dependent on the cyclic loading history. The objective of this paper is to study the effect of reverse cyclic loading on J-R curves in CT specimens. The effect of two parameters was observed on the J-R curves during the reverse cyclic loading. One was the minimum-to-maximum load ratio(R) and the other was the incremental plastic displacement(${\delta}_{cycle}/{\delta}_i$), which is related to the amount of crack growth that occurs in a cycle. Fracture resistance test on CT specimens with varying load ratio and incremental plastic displacement were performed. For the SA 516 Gr. 70 steel, the results showed that the J-R curves were decreased with decreasing the load ratio and the incremental plastic displacement. When the load ratio was set to -1, the results of the J-R curves and the $J_i$ value were about $40{\sim}50$ percent of those for the monotonic loading condition. Also on condition that the incremental plastic displacement reached 1/40, the J-R curves and the $J_i$ value were about $50{\sim}60$ percent of those for the incremental plastic displacement of 1/10.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 1999.04a
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• pp.42-49
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• 1999

Because the linear elastic tincture analysis has been proved to be insufficient in predicting the failure of cracked bodies, in recent years, a number of fracture concepts have been studied which remain applicable in the presence of large-scale plasticity near a crack tip. This work thereby presents a new finite element model, as accurate as possible, to analyze plane problems of ductile fracture under large-scale yielding conditions. Based on the incremental theory of plasticity, the p-version finite element analysis is employed to account for the values of J-integral, the most dominant fracture parameter, and the shape of plastic zone near a crack tip by using the J-integral method and equivalent domain integral method. The numerical results by the proposed model are compared with the theoretical solutions in literatures and the numerical solutions by the i,-version of F.E.M.