• International Journal of Automotive Technology
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• v.6 no.3
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• pp.229-234
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• 2005

The dynamic crack growth in ductile steel is investigated by means of the impact loaded 3 point bending (3PB) specimens. Results from experiments and numerical simulations are compared to each other. A modified 3PB specimen designed with the reduced width at its ends has been developed in order to avoid the initial compressive loading of the crack tip and also to avoid the uncertain boundary conditions at the impact heads. Numerical simulations of the experiments are made by using a finite element method (FEM) code, ABAQUS. The high speed photography is used to obtain the crack growth and the data of the crack tip opening displacement (CTOD). The direct measurements of the relative rotations of two specimen halves are made by using the Moire interference pattern.

• International Journal of Concrete Structures and Materials
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• v.7 no.4
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• pp.253-264
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• 2013

A major difference between high-strength reinforcing steel and conventional steel in concrete is that the service-load steel stress is expected to be greater. Consequently, the service-load steel strains are greater affecting cracking behavior. A parametric study investigating crack widths and patterns in reinforced concrete prisms is presented in order to establish limits to the service-load steel stress and strain. Additionally, based on the results of available flexural tests, crack widths at service load levels were evaluated and found to be within presently accepted limits for highway bridge structures, and were predictable using current AASHTO provisions. A limitation on service-level stresses of $f_s{\leq}414$ MPa (60 ksi) is nonetheless recommended.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.10 no.5
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• pp.110-117
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• 2001

In the weldments, the crack propagation rate is changed due to the residual stress. The crack propagation rate is high in the region with the residual stress. However it shows the same behavior with the base metal in the region that does not include the residual stress. The fatigue crack growth rate for the material with residual stresses can be predicted more precisely by using the effective stress ratio. The difference between experimental results and prediction results seems to be due to the redistribution of the residual stresses and microstructural change.

• Journal of the Korean Institute of Gas
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• v.4 no.1 s.9
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• pp.55-62
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• 2000

A surface acoustic wave method for the evaluation of small fatigue crack initiated from a pit-type surface flaw is presented. In-situ ultrasonic experiments are performed for aluminum 2024-T3 alloy samples under the fatigue test. During the fatigue test, the surface acoustic wave reflection signal from the pit and crack is measured under different hold-stress levels. From the measured and predicted surface wave reflections the depths of fully and partially open cracks are determined and results are verified by comparing with SEM fractography The crack opening behavior of the fatigue crack is evaluated from the predicted effective crack depths. The method developed in this study can be applied to monitor and characterize crack initiation and propagation from pit-type surface flaws in the early stage of fatigue life.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.5
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• pp.740-745
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• 1987

For the prediction of the crack growth initiation from a blunt notch or a precrack in a prestrained material under plane strain tension and small-scale yielding conditions, a microscopic fracture criterion is proposed in terms of the crack tip opening displacement(COD) needed for the attainment of fracture strain at a microstructural distance. Smooth blunting of a crack tip with an initial root radius is assumed, and strain distributions on the crack-line axis are calculated at each deformation stage until the distributions against an original distance normalized to the COD are insensitive to an initial root radius. This case of no initial-root-radius effect is taken as for a sharp crack tip, on which the criterion is applied to determine the characteristic length of material from a critical COD for a fatigue-precracked specimen. The predicted COD at the fracture initiation from a crack with an initial root radius or a prestraining shows reasonable agreement with experimental values.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.2
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• pp.158-164
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• 2004

Steam generator tubes experience widespread degradations such as stress corrosion cracking, wear, tube rupture, denting, fatigue and so on. The resulting damages can cause tube bursting or leak of the primary water which contains radioactivity Therefore the allowable size of the damage is required to be determined on the maintenance purpose. The burst pressure of a tube with a T-type combination crack consisting of longitudinal and circumferential cracks is obtained experimentally and analytically. Fracture parameters such as stress intensity factor and crack opening angle are investigated. Also the burst pressure for a T-type combination crack is compared with that of a single longitudinal crack to develop a length-based criteria.

• KIEAE Journal
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• v.15 no.1
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• pp.139-146
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• 2015

This is an experimental study on the engineering characteristics of perforated reinforced concrete beams with shells. In the material matter of this study, the water cement ratio put 60%, the ratio of substitution of oyster shells to fine aggregate 30%. And in the structural matter, the form of opening put circle and square, the size of opening as the radius and the length of it changed from one to three times of the beam depth with a change presence and absence of reinforced steel around opening. All thirteen reinforced concrete beam tests composed one standard beam and twelve six beams with the circle and square opening were tested in shear strength under two points loading and compared and analyzed the characteristics of test beams under the same conditions one another. The results of the study showed as followed. 1) The initial crack load value of the opening test beams is similar the standard beam but the maximum load value decreased with increase in proportion of the opening size, in the square opening than the circle opening and in the absence than the presence of reinforced steel. 2) As the difference between the circle opening and the square opening beams is represented 2.17~9.8% in the maximum load value and the load capacity of the square opening suddenly decrease than it of the circle opening, it is judged because of the shortage of concrete section, the concentration of the stress in the corner of the square opening and material influence of shell substitution. 3) The failure figure such as the pattern of the crack and so on is represented brittle failure as the opening size is the bigger and the ratio of substitution is higher because of the lack material properties.

• Journal of Korean Association for Spatial Structures
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• v.15 no.2
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• pp.69-77
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• 2015

Ultra High Performance Fiber Reinforced Concrete (UHPFRC) has a outstanding tensile hardening behaviour after a crack develops, which gives ductility to structures. Existing shear strength model for fiber reinforced concrete is entirely based on crack opening behavior(mode I) which comes from flexural-shear failure, not considering shear-slip behavior(mode II). To find out the mode I and mode II behavior on a crack in UHPFRC simultaneously, maximum shear strength of cracked UHPFRC is investigated from twenty-four push-off test results. The shear stress on a crack is derived as variable of initial crack width and fiber volume ratio. Test results show that shear slippage is proportional to crack opening, which leads to relationship between shear transfer strength and crack width. Based on the test results a hypothesis is proposed for the physical mechanics of shear transfer in UHPFRC by tensile hardening behavior in stead of aggregate interlocking in reinforced concrete. Shear transfer strength based on tensile hardening behavior in UHPFRC is suggested and this suggestion was verified by comparing direct tensile test results and push-off test results.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.20 no.3
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• pp.24-32
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• 2021

The size of container ships is increasing to increase the cargo loading capacity. However, container ships are limited in terms of the hull longitudinal strength. To overcome this limitation, brittle crack arrest steel can be used. This study was aimed at examining the influence of the heat input on the welding procedures of flux cored arc welding and submerged arc welding. In the experiment, the crack tip opening displacement test, which pertains to a parameter of fracture mechanics, was performed, and a 3-point bending tester was adopted. Based on the results, the crack measurement method was presented, and the stress expansion coefficient value for the pre-fatigue crack length was derived according to the heat input after the pre-cracking length was measured. It was noted that the heat input affected the crack tip opening displacement of brittle crack arrest steel.

• Structural Engineering and Mechanics
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• v.19 no.3
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• pp.347-359
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• 2005

This paper attempts to develop the analytical model of estimating the fatigue damage using a linear elastic fracture mechanics method. The stress history on a welding member, when a truck passed over a bridge, was defined as a block loading and the crack closure theory was used. These theories explain the influence of a load on a structure. This study undertook an analysis of the stress range frequency considering both dead load stress and crack opening stress. A probability method applied to stress range frequency distribution and the probability distribution parameters of it was obtained by Maximum likelihood Method and Determinant. Monte Carlo Simulation which generates a probability variants (stress range) output failure block loadings. The probability distribution of failure block loadings was acquired by Maximum likelihood Method and Determinant. This can calculate the fatigue reliability preventing the fatigue failure of a welding member. The failure block loading divided by the average daily truck traffic is a predictive remaining life by a day. Fatigue reliability analysis was carried out for the welding member of the bottom flange of a cross beam and the vertical stiffener of a steel box bridge by the proposed model. Results showed that the primary factor effecting failure time was crack opening stress. It was important to decide the crack opening stress for using the proposed model. Also according to the 50% reliability and 90%, 99.9% failure times were indicated.