• Journal of Ocean Engineering and Technology
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• v.5 no.1
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• pp.88-96
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• 1991

Fatigue life and retardtion behavior after through-thickness were examined experimentally by using a CT specimen and surface-cracked specimen. The material used was 3% Ni-Cr-Mo steel. The fatigue crack shape before through-thickness is almost semicircular, and the measured aspect ratio is larger than the value obtained by calculation using the K value proposed by Newman-Raju. It is found that the crack growth behavior on the back side after through-ghickness is unique and can be divided into three stages a, b and c. A retardation parameter has been used successfully to predict the growth of cracks in specimen, and in this time, retardation factor is 4.3. By using the crack propagation rule considering on retardation state and the K value proposed by the authors, the remarkable crack growth behavior and the change in crack shape can be evaluated quantitatively.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.14 no.3
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• pp.105-111
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• 2015

Ultrasonic machining (USM) does not involve heating or any electrochemical effects, and subsequently causes low surface damage, has small residual stress, and does not rely on the conductivity of the workpiece. These characteristics are suitable for the machining of brittle materials, such as glass or ceramics. However, USM for brittle materials generates cracks on the workpiece while machining, especially at the hole exit with a small diameter. In this study, wax coating was used to deposit wax on the back side of the workpiece to decrease the occurrence of cracks at the exit holes in USM, and it was finally removed with a cleaning process. The experimental results show that this technique is beneficial for restricting the occurrence of cracks in glass or ceramics.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.10a
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• pp.804-807
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• 1997

Recently equal-channel angular pressing (ECAP) has been employed to produce ultra-fine grain size materials. In this paper pure-Zirconium is considered due to its applicability to nuclear reactors. Among many process parameters of ECAP frictional effect on the plastic deformation of die has been investigated. The back pressure effect due to friction increases the stress level of die especially at the crossing area of channels, which may result in plastic deformation of die. The finite element method has been employed to investigate this issue.

• Journal of Mechanical Science and Technology
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• v.16 no.1
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• pp.24-31
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• 2002

The leak-before-break (LBB) behaviors of a structural component under high and low fatigue loads are an important problem in nuclear power plants, liquid nitrogen gas tankers and chemical plants. This paper is an experimental study to evaluate the crack opening behavior after penetration for plate and pipe specimens. Crack opening displacement after penetration under low fatigue load could be satisfactorily determined at the center of the plate thickness regardless of the specimen size. In the case of high fatigue load, it is shown that the crack opening displacement at the center of a penetrated crack carl be derived using the gross stress, $\sigma$/sug G/, and the front surface crack length, a$\_$s/, together with the back surface crack length, a$\_$b/.

• Journal of Korean Society for Quality Management
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• v.40 no.3
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• pp.270-277
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• 2012

Purpose: In this paper, storage lifetime of delay system in the fuse of 81MM illuminating projectile is estimated. Methods: Accelerated degradation testings of tungsten delay system using both temperature and humidity stresses were performed, and then delay time increase of the systems were analyzed as degradation data based on distribution-based degradation processes. Results: The estimated storage lifetime of detonator is between 11.8 years and 17.6 years with each stress-life relationship. Conclusion: Comparing with field data, storage lifetime of 90% reliability is about 12 years.

• Journal of the Korean Society for Nondestructive Testing
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• v.25 no.6
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• pp.475-482
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• 2005

The iterative calculation by least square method was used to determine the source location of acoustic emission in rock, as so called "traditional method". The results were compared with source coordinates infered from the application of neural network system for new input data, as so called "new method". Input data of the neural network were based on the time differences of longitudinal waves arrived from acoustic emission events at each transducer, the variation of longitudinal velocities at each stress level, and the coordinates of transducer as in the traditional method. The momentum back propagation neural network system adopted to determine source location, which consists of three layers, and has twenty-seven input processing elements. Applicability of the new method were identified, since the results of source location by the application of two methods were similarly concordant.

• Journal of the Korean Society for Precision Engineering
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• v.18 no.3
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• pp.115-121
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• 2001

In micro automation technology, the concurrent realization of a high resolution and a large operating rage has been achieved by a dual actuator, usually called by piggy-back system, conventionally. But, because of its manufacturing cost, the complexity of control, and the limit of overall bandwidth, the contract-free and single servo actuators have been suggested with specific applications. In this paper, we suggest a novel non-contact surface actuator suing combination of the Lorentz force and the magnetized force, and discuss the actuating principles including an analytical approach. Differently from the existing planar system, an operating range of the suggested system can be expanded by an additional attachment of active elements. Therefore, it is estimated to be suitable for the next-generation moving system.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1995.03a
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• pp.252-263
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• 1995

It is visualized that dislocations move straightly in polycrystalline structure and the trans-grain dislocation moving occur from yield point to ultimate tensile stress. Some fracturemodes in uniaxial tensile test are ilustrated in order to explain that after the ultimate point the grains deforms by twins and the rotations of grains make cracks at the grain-boundaries by the incompatibility . The luders banks. which propagates along the axis of the specimen, are twin bands whcih are formed by rearrangement of the atoms within the structure of three-dimensional crystallizing $\pi$-bondings. The fatigue limit can be found through the atom's rolling back motion during elastic deformation inthe uniaxial tensile test by the change of the gradient.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1995.03a
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• pp.19-30
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• 1995

The analysis had been already completed to detect forming defects for the forming processes of C/R bearing rings. But some unpredicted problems were found through the experiments. So expert redesigned new forming processes to prevent the problems and new analysis was began according to the new processes to find faults for the processes. The forming processes consist of 1 for the outer ring. 6 inner ring. The thickness of metal sheet used is changed to 1.5mm from 1.6mm. Elasto-plastric finite element method is applied to involve the effect of spring back . The most representative alteration is forming of two predents to assist later forming . Thining and distribution of high residual stress are derived from the results of simulations. It is confirmed that the industry expert agree the possiblilty of defects dervied from the new FEM results.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2000.04a
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• pp.88-94
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• 2000

A winding bend rig for measurement of springback ratio is made. It measures the bending angles before and after release of bending load which bends specimens with keeping a constant curvature. Thus the springback ratio can be obtained by using this bend rig. Analytical explanations for the spring back are tried by employing simple beam theory. For the analytical calculations with the theory, Young's modulus, fracture strain and stress-strain curve are necessary and these data are obtained from a tensile test. Using both of the beam theory and the results of tensile test, the springback ratio is also calculated. Comparisons of the two springback ratios, one is obtained from bending test and the other from tensile test, show a good agreement.