• KSCE Journal of Civil and Environmental Engineering Research
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• v.11 no.3
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• pp.29-36
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• 1991

This experimental research evaluated the length of crack extension with the measured compliances as the mutual comparison factors instead of the method proposed in ASTM E561-80. And this research measured the R-curves with the application to the concept of the strain energy release rate that was formulated from the inelastic energy absorbed during the crack growth. With the interpretation of R-curves, this research obtained the starting point of the unstable crack growth, and compared the values of critical fracture toughness with each other, and then examined the effects of variations of the maximum size of coarse aggregate and the thickness of specimen on the values of the critical fracture toughness.

• Journal of the Korean Society of Safety
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• v.16 no.3
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• pp.7-11
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• 2001

In this study, specimens were classified four welded specimens and a base metal to investigate fatigue life and crack growth rate of pure titanium welding materials, and Ti was used in turbine equipment of nuclear power generation, etc. The summarized results are as follows; 1) Specimen-2 was bigger 712% than base metal, when it was compared with other welding materials, 2) As the result of specimens data, specimen-2 crack behavior rate res lower 30 times than base metal, and so total fracture life was very influenced by it, 3) Notch tip of Specimen-2 was offsetted 6.7mm from boundary H.A.Z, and if formed 25% in total fracture length, 4) As the considering of da/dN and $\Delta$K, Paris' law is incongruous in this study, because fro inclines nsf on one date.

• Journal of the Korean Society of Safety
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• v.17 no.4
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• pp.11-18
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• 2002

We proposed a new experimental method which is required to easily observe the growth behavior of fatigue cracks. In the proposed method, 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. 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.

• Journal of KSNVE
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• v.11 no.3
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• pp.437-442
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• 2001

In this study, the fracture phenomena of optical disks are discussed by theoretical and experimental approaches and then some recommendations are presented to prevent the fracture. Linear equations of motion are discretized by using the Galerkin approximation. From the discretized equations, the dynamic responses are computed by the generalized- time integration method. As a fracture criterion for optical disks, the critical crack length is presented. From experimental methods, the fracture procedure is analyzed. The fracture occurs when disks have crack on the inner radius of the disks. Since the crack growth and the fracture result from the stress concentration on the tip of the crack, a measure should be taken to overcome the stress concentration. This problem can be resolved by the structural modification of a disk. This study proposes 3 types of improved optical disks.

• 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.

• Magazine of the Korea Concrete Institute
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• v.9 no.3
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• pp.127-135
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• 1997

In this study, the wedge splitting tcst (WST) was carried out for the fatigue wack growth behavior of concrete. Selected test variables were concrete compressive strength of 28, 60 and 118 MI%, and stress ratio with 2 levels (6. 13%). In oder to make the designed stress ratio, the maximum and thr minimum fatigue loading level were 75-85% and 5- 10% of ultimate static load, respectively. Fatigue testing was preceded by crack mout.h opening displacement (CMOI)) compliance calibration tcst, and then the fatigue crack growth was computed by crack lcngth vs. (lMOI) compliance relations acquisited by the CMOD compliance calibration technique. To evaluate thc validity of CMOD compliancc calibration techniquc, the crack length p~mlicted by this method was cornpard with the crack length by linear elastic fracture mechanics(LEFIbl) and dyeing test. On the basis of the experimental results, a LRFhl-based c.mpirica1 model for f'at,igue crack growth rate(da/dN-AKI relationships) was presented. The fat,igut. crack growth ratc increased with the strength of concwtc. It appcars that t.he da/tiN-AKI relationships was influenced by stress ratio, however, the effect is diminished with an increase of strength. The comparisons between CblOl) compliance calibration technique anti the other. methods gave the validity of' ('MOD compliance calibration technique for the LZXT.

• Proceedings of the KSME Conference
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• 2000.11a
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• pp.328-332
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• 2000

Following the method described in ASTM E1737, J resistance curves are measured for Cr-Mo steel SA387, and Cr steel A240 which are used as piping materials in nuclear industry. Crack driving force diagrams are generated in order to find out instability points in crack growth. The $J_{appl}$ curves, which are used in the crack driving force diagram, are obtained from EPRI J estimation method and the finite element analysis. Crack growth instability points are plotted in load-crack length plane and the results are discussed.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2011.04a
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• pp.572-577
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• 2011

The acoustic emission (AE) technique is a well-known non-destructive test technique, both in research and for industrial applications. It is mainly used to monitor the onset of cracking processes in materials and components. Predicting and preventing the crack phenomenon has attracted the attention of many researchers and has continued to provide a large incentive for the use of condition monitoring techniques to detect the earliest stages of cracks. In this research, goal is in grasping features of AE signal caused by crack growth. The envelope analysis with discrete wavelet transform (DWT) is used to find the characteristic of AE signal. To estimate feature of divided into three by crack length, the time waveform and the power spectrum were generated by the raw signals and the transferred signal processed by envelope analysis with DWT.

• Tunnel and Underground Space
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• v.8 no.2
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• pp.96-106
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• 1998

Rock contains discontinuities at all scales. These discontinuities make rock behave in a complex way. This paper discusses a new approach to underground design based on the theory of rock fracture mechanics. The mechanism of deformation and failure of coal was studied by observing the distributions of length, orientation and spacing of the pre-existing as well as stress-induced cracks. Different types of crack information. The crack information is dependent on the scale used. The cracks propagate along the intersections of the pre-existing cracks, and both extensile and shear crack growth occur depending on the direction of the load relative to the bedding planes. An analytical model that takes into account both shear and extensile crack growth was developed to predict the nonlinear stress-strain behavior of coal including strain-hardening and strain-softening.