• Korean Journal of Materials Research
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• v.6 no.6
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• pp.561-567
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• 1996

The grain size effect on grain boyndary cracking in Pb-Sn eutectic during isothermal fatigue was investigated. Fatigue experiments were confined to two conditions : (1) 0.4% total strain range(approximetely 0.2% plastic strain range), 1.67$\times$10$^{-3}$/s frequency; and (2) 1.5% total strain rante(approximately 1.2% plastic strain range), 8.33$\times$10$^{-4}$/s frequency. Fatigue specimens were cross-sectioned to monitor the depth of crack growth continuosly and then, the maximum crack depths in units of the number of boundaries were plotted as functions of number of cycles for these two different strain ranges. The results revealed that the rate of crack growth(per cycle at fixed rate of crosshead motion) can be expressed as dc/dN=($\Delta$$\varepsilon$$_p$)$^n$c where n is typically 2, c is the crack length, $\Delta$$\varepsilon$$_p$ is the plastic strain range, and A is a "constant" that depends on whether the crack is deeper or shallower than its first triple point of the grain boundary, A decrdases by about a factor of three after the crack hits the first triple point, indecating that the fatigue crack is trapped at the triple point of the grain boundaries.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.5
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• pp.1097-1103
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• 1988

This paper deals with surface crack behavior and the fatigue life prediction of notched specimens using the relation between surface crack length, a, and the cycle ratio, $N/N_{f}$. From the $a-N\;/\;N_{f}$ curves, UC(the upper limit curve), LC(the lower limit curve) and MC(the middle limit curve) were assumed and utilized to predict the fatigue life and crack growth rate. The data computed from the three assumed curves were compared with the experimental data. It has been found that in the stable crack growth region ($N/N_{f}=0.3-0.8$) fatigue life can be predicted within 20% errors. Using the characteristics of $a-N\;/\;N_{f}$ curve, it is possible to predict the $da/dN-K_{max}$ curve, the $da/dN-{\Delta}K_{{\varepsilon}_t}$ curve, and the $S-N_{f}$ curve.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.6 no.3
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• pp.65-72
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• 1997

The horizontal corrosion fatigue tester has been developed for investigating environmental strength. Using this tester, we investigated about corrosion fatigue caracteristics for A106-Gr B steel weldments in 3.5% synthetic seawater and room temperature. Considered parameter is only frequency of 1, 3 and 5Hz.. and Corrosion fatigue crack length was measured by DC potential difference method. From the results, we could find that the horizontal corrosion fatigue tester could be well applied to estimation of fatigue strength. and, In case of 5Hz., corrosion fatigue crack growth pate of A106-Gr B steel weldment was transgranular, and of 1 and 3Hz. showed that transgranular and interfranular was mixed. Also, Material constants of corrosion fatigue crack growth estimated in each frequency were C=9.33$\times$$10^{-9}$ and m=2.93 in 1Hz., C=9.77$\times$$10^{-10}$ and m=3.47 in 3Hz., C=1.02$\times$$10^{-10}$ and m=4.05 in 5Hz

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.1
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• pp.87-95
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• 1990

Rotating bending fatigue tests of an authentic steel 304 were performed at various temperatures such as room temperature, $538^{\circ}$ and $593^{\circ}C$. The plastic replica method was also applied in order to estimate the fatigue life on the basis of serial observation of small fatigue crack initiation and growth on the pit specimen surface. The fatigue crack growth behavior of 304 stainless steel was investigated within the frame work of elastic-plastic fracture mechanics within a narrow scatterband in spite of different stress levels at elevated temperature as at room temperature. The growth law of small surface crack is determined uniquely by the term. $\DELTA\sigma^{n}a$ where $\DELTA\sigma$ is the stress amplitude, a is the crack length, and n is a constant. It is found that the small crack growth behavior is basically equivalent to the S-$N_{f}$ relationship, where S and $N_{f}$ are stress and number of cycles to failure, and the fatigue life prediction is in good agreement with the experimental results.

• Journal of the Society of Naval Architects of Korea
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• v.43 no.4 s.148
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• pp.494-503
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• 2006

Inherent damage zone model is presented to explain the fatigue properties near the fatigue limit and the crack growth threshold consistently Inherent damage zone model assumes that the stress at a point which is located at a small distance, $r_0$, an inherent length of the material that represents the size of effective damage zone, from the crack initiation position governs the fatigue characteristics regardless of the geometric configuration of the specimen; smooth specimen, notched specimen or cracked specimens with short and long crack length. A special feature of the paper is using the exact stress distributions of notched and cracked specimens at the strength evaluations. Analytical elastic solutions by Neuber and Westergaard are employed for this purpose Relationship between fatigue limit of smooth specimen and threshold stress of cracked specimen, occurrence condition of non-propagating crack at the root of elliptic notch and circular hole and relationship between stress concentration factor and fatigue notch factor are discussed quantitatively based on the proposed model.

• Journal of Welding and Joining
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• v.14 no.5
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• pp.69-77
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• 1996

This study aims to analyze the S. I. F. K value upon Mode I cracks in a finite-width plate of varying thickness, which is expressed in terms of width ratio ($\omega$), thickness ratio ($\beta$) and non-dimensional crack length (λ) by using the 2-dimensional finite element method. Then, by comparing the effectiveness of the results obtained by the two finite element methods, it is seen that the 2-dimensional finite element method can be used in order to analyse the S. I. F. K values upon a various thickness model. A model is developed in order to analyze the effects of initial residual stress upon the fatigue crack growth behavior in various thickness welded specimens. In this model, crack growth rate da/dN appears to be come small as the thickness ratio with the same ΔK is increased. Also, in the initial step, as ΔK is increased with crack growth rate is decreased and then increased because the repeated compressive residual stress retards crack growth rate.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1997.04a
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• pp.143-149
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• 1997

the horizontal corrosion fatigue tester has been developed for investigating environmental strength. Using this tester, we investigated about corrosion fatigue caracteristic for A106 GrB steel weldments in 3.5% synthetic seawater and room temperature. Considered parameter is only frequency of 1,3 and 5Hz. and Corrosion fatigue crack length was measured by DC potential difference method. From the results, we could find that the horizontal corrosion fatiued tester could be well applied to estimation of fatigue strength. and In case of 5Hz, corrosion fatigue crack growth path of A106 GrB steel weldment was transgranular, and of 1 and 3Hz showed that transgranular and intergranular was mixed. Also, Corrosion fatigue crack growth caracteristic values estimated in each frequency were C=9.33 x 10-9 and m=2.93 in 1Hz, C=9.77x10-10 and m=3.47 in 3Hz, C=1.02x10-10 and m=4.05 in 1Hz

• Journal of the Society of Naval Architects of Korea
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• v.45 no.6
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• pp.669-678
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• 2008

The fatigue life of ship structure under cyclic loading condition is made up of initiation and propagation stages. In this study, crack growth test is carried out on large scale structure test specimens and fracture mechanical analysis is performed. The fatigue lives measured from fatigue tests are compared with DNV, Matsuoka and BS 5400 S-N curve. And to predict the crack initiation life, S-N curve, corresponding to crack length 20mm at welded joint, is developed based on hot spot stress range. Also crack propagation life is calculated using crack growth equation. Consequently, computed crack propagation life is compared with experiment results.

• Nuclear Engineering and Technology
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• v.27 no.6
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• pp.825-832
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• 1995

The effect of loading variables and temperature on fatigue crack growth rate in SA508 Cl.3 nuclear pressure vessel steel was investigated in air environment Crack growth rate tests on compact tension specimen of thickness 12mm were conducted by using sinusoidal waveform. The crack length was monitored by compliance method. Test conditions were at 0.1 and 0.5 of load ratio, at 1 and 10 Hz of loading frequency, and at room temperature to 40$0^{\circ}C$. At the lower temperatures, the fatigue crack propagation was not affected by the frequency and temperature, while at the higher temperatures above 12$0^{\circ}C$, fatigue crack growth rate increased with decreasing loading frequency and increasing temperature. This accelerated fatigue crack propagation was associated with the increase of oxidation rate at the ahead of crack tip. Fatigue crack growth rate increased with in-creasing the load ratio. The effect of load ratio was more significant at the lower temperature, while the dependence on load ratio decreased with increasing temperature. The sensitivity of load ratio to temperature can be explained by crack closure with the oxidation process.

• Journal of the Korean Society for Nondestructive Testing
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• v.22 no.1
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• pp.14-21
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• 2002

The development of a new experimental method is required to easily observe the growth behavior of fatigue cracks. To satisfy the requirement, an image processing technique was introduced to fatigue testing. The length of surface fatigue crack could be successfully measured by the image processing system. At first, the image data of cracks were stored into the computer while the cyclic loading was interrupted. After testing, crack length was determined using an image processing software which was developed by authors. Various image processing techniques like a block matching method was applied tc the detection of surface fatigue cracks. By comparing the data measured by the image processing system with those by the manual measurement with a microscope, the effectiveness of the image processing system was established. If the proposed method is used to monitor and observe the crack growth behavior automatically, the time and efforts for fatigue test could be dramatically reduced.