• Proceedings of the Korean Society For Composite Materials Conference
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• 2000.04a
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• pp.20-25
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• 2000

In order to study the impact fracture behavior of brittle materials, a steel-ball-impact experiment was Performed. Five kinds of materials were used in this study : soda-lime glass plates, glass/epoxy prepreg-one layer-bonded and unbonded glass plates, glass/epoxy prepreg-three layers-bonded and unbonded glass plates. Fracture patterns, the maximum stress and absorbed fracture energy were observed according to various impact velocities 40-120m/s. With increasing impact velocity, ring crack, cone crack, radial crack and lateral crack took place in the interior of glass plates. The generation of such cracks was largely reduced with glass/epoxy prepreg coating. Consequently, it is thought that the characteristics of the dynamic Impact fracture behavior could be evaluated using the absorbed fracture energy and the maximum stress measured at the back surface of glass plates.

• Journal of the Korean Society for Precision Engineering
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• v.16 no.6
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• pp.100-107
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• 1999

This paper describes a probabilistic remaining life assessment program for the creep crack growth. The probabilistic life assessment program is developed to increase the reliability of life assessment. The probabilistic life assessment involves some uncertainties, such as, initial crack size, material properties, and loading condition, and a triangle distribution function is used for random variable generation. The resulting information provides the engineer with an assessment of the probability of structural failure as a function of operating time given the uncertainties in the input data. This study forms basis of the probabilistic life assessment technique and will be extended to other damage mechanisms.

• Journal of Mechanical Science and Technology
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• v.19 no.5
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• pp.1116-1122
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• 2005

A finite element method was used to simulate the wave propagation of laser-generated ultrasound and its interaction with surface breaking cracks in an elastic material. Thermoelastic laser line source on the material surface was approximated as a shear dipole and loaded as nodal forces in the plane-strain finite element (FE) model. The shear dipole- FE model was tested for the generation of ultrasound on the surface with no defect. The model was found to generate the Rayleigh surface wave. The model was then extended to examine the interaction of laser generated ultrasound with surface-breaking cracks of various depths. The crack-scattered waves were monitored to size the crack depth. The proposed model clearly reproduced the experimentally observed features that can be used to characterize the presence of surface-breaking cracks.

• 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.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2017.11a
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• pp.175-176
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• 2017

Many problems in various aspects such as generation of plastic/dry contraction cracks and cold joints can be caused unless proper quality control measures are established in hot weather circumstances. Therefore, this study aimed to compare the crack patterns of concrete by applying a change in 3 surface curing methods such as a mono aluminum-deposited bubble sheet developed to reduce the temperature and cracks through reflection of heat in summer and a PE film and a surface exposure used generally to an actually constructed apartment slab. The study result confirmed that the best concrete crack reduction effect can be obtained with a mono aluminum-deposited bubble sheet.

• Journal of the Korean Society for Precision Engineering
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• v.27 no.12
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• pp.99-106
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• 2010

Gas turbines operation for power generation increased rapidly since 1990 due to the high efficiency in combined cycle, relatively low construction cost and low emission. But the operation and maintenance cost for gas turbine is high because the expensive superalloy hot gas path parts should be repaired and replaced periodically This study analyzed the initiation and propagation of the crack at the gas turbine blades which are coated with MCrAIY as a bond coat and TBC as a top coat. The sample blades had been serviced at the actual gas turbines for power generation. Total 7 sets of blades were analyzed and they have different EOH(equivalent operation hour). Blades were sectioned and the cracking distribution were measured and analyzed utilizing SEM(scanning electron microscope) and optical microscope. The blades which had 52,000 EOH of operation had cracks at the substrate and the maximum depth was 0.2 mm. Most of the cracks initiated at the boundary layer between TBC and bond coat and propagated down to the bond coat. Once bond coat is cracked, the base metal is exposed to the oxidation condition and undergoes notch effect. Under this environment, the crack branched at the inter-diffusion layer and propagated to the substrate. Critical cracks affecting the blade life were analyzed as those on suction side and platform.

• Journal of the Korean Society for Railway
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• v.12 no.3
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• pp.396-404
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• 2009

With developing the next generation high speed railway, there need to be plans to make sure of running safety though researchs on the crack and break of rail by rolling contact fatigue. Therefore, this study performed the parametric analysis effecting on the fatigue life of rail using simplified equations. It analyzed the internal stress of rail according to the track quality, train velocity, wheel radius, track stiffness, sleeper space, wheel load. For the more, via the finite element method, it analyzed shear force on the rail head which could be changed by the early length of crack, angle of crack and temperature. As a result, this study continued the main parameter effecting on the fatigue life of rail.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.4
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• pp.296-309
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• 2009

Thick-walled cylinder with high pressure have had wide application in the armament industry. In the thick-walled cylinder, fatigue crack is generated at inner radius and developed toward the outer radius. To prevent generation of fatigue crack, the autofrettage process had been used. The compressive residual stress induced by the autofrettage process extends loading pressure and fatigue life of the thick-walled cylinder. In this study, the residual stress of single and compound cylinder by the autofrettage process was evaluated. The analytical compressive residual stress of single cylinder was good agreement with experimental result at inner radius. The analysis on the residual stress of compound cylinder was conducted. The compressive residual stress at inner radius was increased with the overstrain level. And fatigue life of the compound cylinder with initial crack was evaluated. The considered initial crack shape was straight and semi-elliptical. The fatigue life was extended with the overstrain level. The fatigue life of the compound cylinder with semi-elliptical crack was longer than straight crack. The suitable way to extend fatigue life of the compound cylinder was proposed.

• Transactions of Materials Processing
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• v.26 no.4
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• pp.210-215
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• 2017

In this paper, utilizing the theory of ductile fracture a chevron crack in a 4-stage open cold extrusion process is predicted by the finite element methods and then compared with previous experiments. The normalized Cockcroft-Latham damage model is employed and the material is identified using a tensile test based material identification technique that gives fracture information as well as flow stress at large strain. A large difference between the predicted cracks and actual experiments is observed, specifically narrower width and greater maximum height of the crack. This reveals the limitation of this approach based on the conventional theory of ductile fracture. Based on the observations and the related criticisms, a new approach for predicting the chevron crack is proposed, suggesting that either the critical damage should not be a fixed material constant, or that the conventional fracture theory should be considered with the effects of embrittlement due to accumulated plastic deformation while the duration of crack generation and plastic deformation should be reduced.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.4
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• pp.745-753
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• 2002

It is required to evaluate nondestructively the crack depth of surface-breaking cracks for the assurance of safety of structure. Optical generation of ultrasound produces well defined pulses with a repeatable frequency content, that are free of any mechanical resonances; they are broad band and are ideal for the measurement of attenuation and scattering over a wide frequency range. Self-calibrating surface signal transmission measurement is very sensitive and practical tool for surface-breaking crack depth. In this paper, the self-calibrating technique by laser-based ultrasound is used to evaluate the depth of surface-breaking crack of material. It is suggested that the relationship between the signal transmission and crack depth can be used as a practical model for predicting the surface-breaking crack depths from the signal transmission measured in structure.