• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.354-366
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• 2020

This work mainly focuses on determination of the fully plastic J-integral solutions for welded center cracked plates subjected to remote tension loading. Detailed three-dimensional elasticeplastic Finite Element Analyses (FEA) were implemented to compute the fully plastic J-integral along the crack front for a wide range of crack geometries, material properties and weld strength mismatch ratios for 900 cases. According to the database generated from FEA, Back-propagation Neural Network (BPNN) model was proposed to predict the values and distributions of fully plastic J-integral along crack front based on the variables used in FEA. The determination coefficient R2 is greater than 0.99, indicating the robustness and goodness of fit of the developed BPNN model. The network model can accurately and efficiently predict the elastic-plastic J-integral for weld centerline crack, which can be used to perform fracture analyses and safety assessment for welded center cracked plates with varying strength mismatch conditions under uniaxial loading.

• Journal of Ocean Engineering and Technology
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• v.4 no.2
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• pp.250-250
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• 1990

In this paper, fatigue tests were carried out at stress test levels of 461 MPa, 441 MPa, and 431 MPa by using smooth specimen of$2_{1/4}$ Cr-1 Mo steel with the stress ratio(R) of 0.05. The initiation, growth and coalescense process of the major cracks and sub-cracks among the fatigue cracks on the smooth specimen are investigated and measured under each stress level at a constant cycle ratio by the replica technique with optical microscope. Some of the important results are as follows: In spite of the difference of stress levels, the major crack data gather into a small band in the curve of surface crack length and crack depth against cycle ratio N/Nf. The sub-crack data, however, deviate from the band of the major crack. The growth rates, da/dN, of major and sub-crack plotted against the stress intensity factor range, ${\Delta}K$, have the tendency to be compressed on a relatively small band. But it is more effective to predict fatigue life through major cracks. The propagation behavior of surface microcracks on the smooth specimens coincides with that of the specimen having an artificial small surface defect or through crack.

• Journal of the Korean Society for Nondestructive Testing
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• v.22 no.5
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• pp.532-538
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• 2002

This study was performed to classify the fatigue crack opening and closure for three kinds of aluminum alloy using principal component analysis (PCA). Fatigue cycle loading test was conducted to acquire AE signals which come from different source mechanisms such as crack opening and closure, rubbing, fretting etc. To extract the significant feature from AE signal, correlation analysis was performed. Over 94% of the variance of AE parameters could accounted for the first two principal components. The results of the PCA on AE parameters showed that the first principal component was associated with the size of AE signals and the second principal component was associated with the shape of AE signals. An artificial neural network (ANN) an analysis was successfully used to classify AE signals into six classes. The ANN classifier based on PCA appeared to be a promising tool to classify AE signals for fatigue crack opening and closure.

• Transactions of the Korean Society of Mechanical Engineers
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• v.7 no.4
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• pp.425-433
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• 1983

Fatigue tests by axial loading (R=0.1) were carried out to investigate fatigue crack growth properties of small surface cracks in mild steel at room temperature, 250.deg. C and 400.deg. C, by using flat specimens with a small artificial pit. All the data of the fatigue crack growth rate obtained in the present tests are determined as a function of the stress intensity factor range, so that the applicability of liner fracture mechanics to the fatigue crack growth of surface cracks at elevated temperatures is investigated and discussed in comparison with the data of type 304 stainless steel at room temperature and elevated temperature. The obtained results are as follows: 1) Relations of both surface fatigue crack length and its depth to cycle ratio fall within a narrow scatter band in spite of different stress levels. 2) The .DELTA. .sigma. .root. .pi. a-da/dN relation of surface fatigue crack growth at room temperature is independent of the stress level and can be plotted as a straight line at log-log diagram, but the relation at 400.deg. C depends partly on the stress level. 3) Relations of the fatigue crack growth into depth d(2b)/dN and is stress intensity factor range .DELTA. $K_{I}$, accounted for the aspect ratio variation, fall within a narrow scatter band for wide range of the applied stress levels. And .DELTA. $K_{I}$E-d(2b)/dN relations of mild steel at different stress level coincide relatively well with the data of type 304 stainless steel. 4) The value of aspect ratio obtained by a beach mark method and a temper coloring method approaches about 0.9 in common with crack growth and it is independent of stress level and temperatures. 5) The equi-crack length curve is parallel to S-N$_{f}$ curve at elevated temperatures.s.s.s.

• Geomechanics and Engineering
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• v.14 no.3
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• pp.283-291
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• 2018

Cracks in soil provide significant preferential pathways for contaminant transport and rainfall infiltration. Water exchange between the soil matrix and crack is crucial to characterize the preferential flow, which is often quantitatively described by a water exchange ratio. The water exchange ratio is defined as the amount of water flowing from the crack into the clay matrix per unit time. Most of the previous studies on the water exchange ratio mainly focused on cracked sandy soils. The water exchange between cracks and clay matrix were rarely studied mainly due to two reasons: (1) Cracks open upon drying and close upon wetting. The deformable cracks lead to a dynamic change in the water exchange ratio. (2) The aperture of desiccation crack in clay is narrow (generally 0.5 mm to 5 mm) which is difficult to model in experiments. This study will investigate the water exchange between a deformable crack and the clay matrix using a newly developed experimental apparatus. An artificial crack with small aperture was first fabricated in clay without disturbing the clay matrix. Water content sensors and suction sensors were instrumented at different places of the cracked clay to monitor the water content and suction changes. Results showed that the water exchange ratio was relatively large at the initial stage and decreased with the increasing water content in clay matrix. The water exchange ratio increased with increasing crack apertures and approached the largest value when the clay was compacted at the water content to the optimal water content. The effective hydraulic conductivity of the crack-clay matrix interface was about one order of magnitude larger than that of saturated soil matrix.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.3 s.258
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• pp.344-354
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• 2007

The objective of this study is to investigate the effect of arbitrarily located defect around the circular hole in the aircraft structural material such as Al/GFRP laminates and monolithic Al alloy sheet under cyclic bending moment. The fatigue behavior of these materials may be different due to the defect location. Material flaws in the from of pre-existing defects can severely affect the fatigue crack initiation and propagation behavior. The aim of this study is to evaluate effects of relative location of defects around the circular hole in monolithic Al alloy and Al/GFRP laminates under cyclic bending moment. The fatigue behavior i.e., the stress concentration factor($K_t$), the crack initiation life($N_i$), the relationship between crack length(a) and cycles(N), the relationship between crack growth rate(da/dN) and stress intensity factor range(${\Dalta}K$) near a circular hole are considered. Especially, the defects location at ${\theta}_1=0^{\circ}\;and\;{\theta}_2=30^{\circ}$ was strongly effective in stress concentration factor($K_t$) and crack initiation life($N_i$). The test results indicated the features of different fatigue crack propagation behavior and the different growing delamination shape according to each location of defect around the circular hole in Al/GFRP laminates.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.3
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• pp.355-361
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• 1987

Creep and fatigue tests were carried out on crack growth properties of small surface cracks in 304 stainless steel at 538.deg.C, 593.deg. C and 650.deg. C in air, by using small plate specimens with a small artificial pit. All the data of the crack growth rate per hour obtained in the present tests were correlated with the maximum stress intensity factor, so that the applicability of linear fracture mechanics to the crack growth of surface cracks at elevated temperature was investigated. In the creep test, relatiion of .sigma.$\^$n/.t$\_$f/=C is obtained between failure time and nominal stress at each temperature level, where n has the value of 11-14 depending on the temperature level. In the creep and fatigue crack growth properties of surface cracks at the elevated temperatures, the maximum stress intensity factor, $_{4}$$\_$max/, is some extent applicable parameter to describe the surface crack growth rate under the present experimental conditions. The crack growth rate per hour increases when the holding time decreases, and creep crack growth rate per hour becomes the lowest limit of crack growth rate per hour in this tests.

• Journal of the Korean Society for Nondestructive Testing
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• v.31 no.5
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• pp.508-515
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• 2011

Bovine incisor was investigated using active infrared thermography(IRT) to visualize crack on bovine teeth. An artificial crack was carefully created in bovine incisor sample by compression load of universal tensile machine. While applying a sinusoidal heat wave to the cracked bovine incisor through halogen lamp, consecutive digital infrared images was captured from the sample surface at a frequency synchronized with heat excitation. Phase information of thermal image was calculated by four-point correlation method and processed to produce the phase image of bovine incisor. This phase image showed clearly the crack on the incisor, which was hardly detected in traditional passive thermography.

• Nuclear Engineering and Technology
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• v.47 no.1
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• pp.102-114
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• 2015

Background: A technique developed to produce artificial intergranular stress corrosion cracks in structural components was applied to thick, forged alloy 600 base and alloy 182 weld metals for use in the qualification of nondestructive examination techniques for welded components in nuclear power plants. Methods: An externally controlled procedure was demonstrated to produce intergranular stress corrosion cracks that are comparable to service-induced cracks in both the base and weld metals. During the process of crack generation, an online direct current potential drop method using array probes was used to measure and monitor the sizes and shapes of the cracks. Results: A microstructural characterization of the produced cracks revealed realistic conformation of the crack faces unlike those in machined notches produced by an electrodischarge machine or simple fatigue loading using a universal testing machine. Conclusion: A comparison with a destructive metallographic examination showed that the characteristics, orientations, and sizes of the intergranular cracks produced in this study are highly reproducible.

• The Journal of Engineering Geology
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• v.16 no.2 s.48
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• pp.179-187
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• 2006

Two specimens of mortar containing artificial slit and Geochang granite containing the straight notch were selected to be used in this research. Source mechanism of micro-crack by radiation pattern based on dislocation the-ory was estimated by the first motion of longitudinal wave and spatial distribution between the location of transducers for monitoring acoustic emission and source coordinates determined by the application of the least square method. Result of analysis showed that the orientation of dislocation surfaces due to shear dislocation and tensile dislocation squares considerably with crack direction visually observed. The ultimate goal of this study is to provide fundamental information for source mechanism of micro-crack within materials.