• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.2
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• pp.473-480
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• 1996

Automatic fatigue crack length measuring system using DC electrical potential method and the system control program for automatic fatigue testing under random load condition were made in this study. And using these system and control program, fatigue tests were executed under constant and random load condition. As the result, the propagation of crack in random loading can be represented Paris equaiton and log normal probability function. But constant and random load test show different crack propagation properties.

• Steel and Composite Structures
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• v.17 no.2
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• pp.145-157
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• 2014

This study aimed to investigate the fatigue crack growth behavior of a kind of fiber metal laminates (FML) under four different stress levels. The FML specimen consists of three 2024-T3 aluminum alloy sheets and two layers of glass/epoxy composite lamina. Tensile-tensile cyclic fatigue tests were conducted on centrally notched specimen at four stress levels with various maximum values. A digital camera system was used to take photos of the propagating cracks on both sides of the specimens. Image processing software was adopted to accurately measure the length of the cracks on each photo. The test results show that: (1) a-N and da/dN-a curves of FML specimens can be divided into transient crack growth segment, steady state crack growth segment and accelerated crack growth segment; (2) compared to 2024-T3 aluminum alloy, the fatigue properties of FML are much better; (3) da/dN-${\Delta}K$ curves of FML specimens can be divided into fatigue crack growth rate decrease segment and fatigue crack growth rate increase segment; (3) the maximum stress level has a large influence on a-N, da/dN-a and da/dN-${\Delta}K$ curves of FML specimens; (4) the fatigue crack growth rate da/dN presents a nonlinear accelerated increasing trend to the maximum stress level; (5) the maximum stress level has an almost linear relationship with the stress intensity factor ${\Delta}K$.

• Journal of the Korean Society of Safety
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• v.9 no.3
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• pp.13-21
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• 1994

Effects of tensile and compression residual stresses in the welded SS41 and A17075-76 on fatigue crack propagation behavior are investigated when a crack propagates from residual stresses region. We propose the fatigue crack growth equation on tensile and compression residual stresses in welded metal. The results obtained in this experimental study are summarized as follows . 1 ) A fatigue crack growth equation which applied fatigue fracture behavior of the welded metal is proposed. (equation omitted) where, $\alpha$, $\beta$, ${\gamma}$ and $\delta$ are constants, and R$_{eff}$ is effective stress ratio [R$_{eff}$=(Kmin+Kres)/(Kmax+Kres)], Kcf is critical fatigue stress intensity factor. The constants are obtained from nonlinear least square method. The relation between crack length and number of cycles obtained by integrating the fatigue crack growth rate equation is in agreement with the experimental data. 2) The experimental results confirmed that the cause of crack extension and retardation by residual stresses has relation to the phenomenon of crack closure. 3) The relaxing trend of residual stresses by the crack propagation was greater In case of compressive residual stress than that of tensile residual stress in the welded metal.tal.

• Journal of the Korean Society for Precision Engineering
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• v.27 no.3
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• pp.80-88
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• 2010

A simple computational model for modeling of subsurface crack growth under cyclic contact loading is presented. In this model, it is assumed that the initial fatigue crack will initiate in the region of the maximum equivalent stress at certain depth under the contacting surface. The position and magnitude of the maximum equivalent stress are determined by using the equivalent contact model, which is based on the Hertzian contact conditions with frictional forces. The virtual crack extension method is used for simulation of the fatigue crack growth from the initial crack up to the formation of the surface pit due to contact fatigue. The relationships between the stress intensity factor and crack length are then determined for various combinations of equivalent contact radii and loadings.

• Transactions of the Korean Society of Mechanical Engineers
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• v.6 no.3
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• pp.232-238
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• 1982

The behavior of corner crack propagation by unidirectional plane bending fatigue was investigated in the butt welded joints of SS41 and SM50 steel plates including an edge through-thickness notch. The properties of fatigue crack propagation were inspected in the weld metal, heat-affected zone, and base metal of the welded joints. Main results obtained are as follows; (1) When a plate with an edge through-thickness notch is loaded by plane bending fatigue in indirection, the 2 variant corner cracks on the upper and lower edge of the plate are initiated and propagated respectively from the notch. (2) In case of a specimen containing a corner crack, it is more reasonable to estimate the crack propagation rate by area of fracture surface than by crack surface length. (3) The rate of fatigue crack propagation becomes faster in the following order; weld metal, heat-affected zone, and base metal. (4) The specimen including reinforcement shape is rapidly failed throughout bond due to effect of its shape when the repeated load exceeds a certain cycle.

• Journal of Welding and Joining
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• v.6 no.2
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• pp.30-39
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• 1988

Generally, as the welded region of weld structures has the incomplete bead and welded deposit which are able to behave like the surface cracks occasinally, there is a high possibility that the fatigue fracture of the weld structures is due to the surface cracks on the wlded region. This study was done to investigate the effects of post weld heat treatment (PWHT) on the fatigue behaviors of the surface crack of the heat affected zone (HAZ) for the multi-pass welds under the repetitive pure bending moment. The obtained results are summarized as follows : 1. The crack grows to the depth direction initially as the number of cylces increase, the amount of crack length is increased for the surface dir3ction and cive versa for the depth direction. 2. The fatigue life is increased in a order of as weld, PWHT specimens and parent. 3. As the number of cycles increase, the crack length is increased to th surface direction. The increase of the depth length is blunted at the center of specimen thickness. 4. The fatigue crack growth of PWHT specimens to the surface direction is dependent upon the holding time and applied stress during PWHT. In order words, the crack growth rate decreases with the holding time and increases with the applied stress during PWHT. 5. As the crack grows, the aspect formed in the course of crack propagation approaches to semicircle for parent and ellipse with the largest semidiameter for PWHT ($1/4hr, 15kgf/mm^2$) 6. At depth direction, it is difficult to apply to the paris' equation because of the scattered data between the crack growth rate and the stress intensity factor range.

• Journal of Ocean Engineering and Technology
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• v.7 no.2
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• pp.68-78
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• 1993

Fatigue crack shape variation by a residual stress during crack growth and life predition are studied. An analytical method is presented to predict the influence of a residual stress due to heattreatment on crack shape variations. Computer simulation results using this me thod are graphically shown that crack growth rate to surface direction are decreased due to compressive residual stress exisiting in surface area. These results are commpared with experimental results. The fatigue life is also predicted by computer simulation of crack aspect ratio variation which is based on the surface crack length increment per unit cycle calculated from a-N diagram. Predited life is about 12 percent lower than experimental life.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.3
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• pp.603-609
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• 1990

Detection and non-propagating cracks of small fatigue crack for smooth and pre-cracked specimens were examined in a carbon steel. The fretting oxide induced crack closure triggered by the roughness induced crack closure has an important role in determing the length. The fatigue limit for the with no cracks or with a short pre-crack is lower at R=-1 than that at R=0. A non-propagating crack are quite different between points near the specimen's surface and those of deepest penetration.

• Proceedings of the KSR Conference
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• 1998.11a
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• pp.553-559
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• 1998

This thesis studied to evaluate the fatigue behavior and propagation of rail steel under welding line. Test of crack growth was performed by all member of rail under constant amplitude loading at the structures laboratory in Hongik University. The effect of the following parameters with initiated crack length on the bottom edge of rail were studied. Here, fracture mechanics mode is opening mode. and Testing Material is KS50N Rail. From analysis and experimented result on the three Point bending in the lab, This paper presented a effect of crack growth , shape and remaining service life. Further more, according to the variable crack length, variable section and the ratio of section the fatigue behavior and propagation were studied.

• Journal of Ocean Engineering and Technology
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• v.11 no.3
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• pp.76-88
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• 1997

Constant .DELTA.K fatigue crack growth rate experiments were performed by applying an intermediate single and multiple overload for structural steel, SM45C. The purpose of the present study is to investigate the influence of multiple overloads at various stress intensity factor ranges and the effect of statistical variability of crack retardation behavior. The normalized delayed load cycle, delayed crack length and the minimum crack growth rate are increased with increasing baseline stress intensity factor range when the overload ratio and the number of overload application were constant. The crack retardation under low baseline stress intensity factor range increases by increasing the number of overload application, but the minimum crack growth rate decreases by increasing the number of overload application. A strong linear correlation exists between the minimum crack growth rate and the number of overload applications. And, it was observed that the variability in the crack growth retardation behavior are presented, the probability distribution functions of delayed load cycle, delayed crack length and crack growth life are 2-parameter Weibull. The coefficient of variation of delayed load cycle and delayed crack length for the number of 10 overload applications data are 14.8 and 9.2%, respectively.