• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.19 no.12
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• pp.1281-1288
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• 2009

Variation of dynamic characteristics of a low pressure turbine blade with crack length is studied in this paper via both experiments and finite element model. Since most of the turbine blades used in domestic power plants are imported from abroad, it is necessary to understand their dynamic behavior in advance. When experimentally obtained natural frequencies and mode shapes are compared with those from FEM results, they are close to each other in their magnitude. Then, it is more feasible to use finite element model for analyzing the dynamic characteristics of a blade under various operation conditions (rotation speed, temperature, etc) as well as with a crack in the blade.

• Journal of the Korean Society of Safety
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• v.23 no.2
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• pp.14-20
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• 2008

The objective of this study is to examine fatigue strength of the welded details. In order to attain the goal of this study, the bending fatigue tests was performed for four kinds of welded details used in steel bridges, such as in-plane gusset, out-of-plane gusset, cruciform, and cover plate. The effect of the length of welded attachment on fatigue strength was greater in out-of-plane gusset than in in-plane gusset. The fatigue strength of welded details with short attachment was superior to that with long attachment. Fatigue strength of welded details with transversely loaded welds was lower than that with longitudinally loaded welds, and those results were not satisfied with AASHTO specifications. For the fatigue strength of cover plate, cover plate with rectangular section was superior to that with tapered section. It was found that the fatigue crack initiates at the points of stress concentration which are the boundary between the base metal and the bead of weld in the part of the longitudinal edge of attachment, and propagates first along the boundary and along the perpendicular to the direction of the principle stress in the base metal of welded tip.

• Transactions of the Korean Society of Mechanical Engineers
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• v.8 no.3
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• pp.195-200
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• 1984

In-plane tension fatigue tests(R=0.1) were carried out to investigate the initiation and growth behaviors of very small surface fatigue cracks on smooth unnotched surfaces of type 304 stainless steel at room temperature. The present paper deals with the unification of two approaches to the analysis of fatigue: the one approach is based on fracture mechanics concept and the other on low-cycle fatigue concept. The results are;(1)Maximum crack length, 2 $a_{max}$, initiated at a very small surface scratch not exceeding 20 .mu.m which can exist on the surface after buffing. And the density of small surface crack is remarkably low compared to that of mild steel. (2) The growth rate, d(2a)/dN, of very small fatigue cracks can be represented by one straight line as a function of either stress intensity factor range, .DELTA. $K_{I}$ or cyclic total strain intensity factor range, .DELTA. $K_{\epsilon}$$_{I}$/, for various values of the nominal stress range.e.e.e.e.

• 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.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.25 no.5
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• pp.386-392
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• 2016

The sizes of last stage blades (LSB) in a low-pressure steam turbine have been getting larger for the development of high-capacity power plants. They are also larger than other blades in the same system. As a result, crack propagation in an LSB is caused by the large centrifugal force, low natural frequency, and repeated turbine startups. In this study, the critical crack length for a fracture and vibration characteristics, in accordance with crack propagation, were analyzed using a finite element method to calculate the stress intensity factor (SIF) and the natural frequency that was affected by the stress-stiffening effect. It was calculated that the frequency of the third and fifth modes passed the excited harmonic resonance (5X and 10X) and the observed calculated critical crack length matched that of the real fractured surface.

• Journal of the Korean Society of Propulsion Engineers
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• v.20 no.2
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• pp.11-17
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• 2016

Powder metallurgy nickel based superalloy has been used in a high temperature part of turbine engine for airplane. The fatigue crack growth behavior was investigated using CT specimens for the materials at room temperature(R.T.), $600^{\circ}C$ and $700^{\circ}C$. The direct current potential drop(DCPD) method suggested by ASTM E647 was used to measure the crack length during fatigue crack growth at various stress ratios. The fatigue crack growth rate at R=0.5 was faster than that at R=0.1 for all temperature conditions and increased with the increase of stress ratio and temperature. Fractography was conducted for analysis of fracture mechanism.

• Journal of Korean Society of Steel Construction
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• v.15 no.6 s.67
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• pp.629-637
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• 2003

This s paper presented the results of the three phases of fatigue tests to determine the fatigue strength of in-plane welded gusset joints, which are the most common details inevitably existing in the region of high stress range. A total of 57 fatigue specimens with varying thickness and strengths were made and tensile fatigue tests performed. One full-scale beam fatigue test was also performed. The validity of the fatigue strength of those details in the specification was confirmed, with the effects of thickness of flanges and welded attachments, length of attachments, and strength of applied steel examined. The characteristics of crack initiation and propagation were also observed. The test result was evaluated by comparing it with other test data and fatigue criteria of other countries. To determine the degree of stress concentration in the weld toe depending on geometric configuration such as attachment length and transition radius, analyses were performed. Compared to the present specification, analytical results indicate the need to revise and subdivide the detail categories.

• Transactions of Materials Processing
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• v.26 no.6
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• pp.356-364
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• 2017

In this study, submerged arc cladded Fe-Cr-Ni-Mo-CuWNbV-C stainless steels containing various Cr contents between 11.2 wt.% and 16.7 wt.% were prepared with fixed C content at about 0.14 wt.%. Using these alloys, changes in microstructure, tensile property, and thermal fatigue property were investigated. Phase fraction of delta-ferrite was increased gradually with increasing Cr content. However, tensile strength, hardness, and thermal fatigue resistance appeared to be decreased. When the microstructure of delta-ferrite was observed, it was revealed that the mesh structure retained up to about 15% Cr content. Although thermal fatigue resistance was almost the same for Cr contents between 11.0 and 14.5 wt.%, it was significantly decreased at higher Cr contents. This was evident from mean value of crack lengths of 10 largest ones. Evaluation of thermal fatigue resistance on alloys with various Cr contents revealed the following important results. First, the reproducibility of ranking test was excellent regardless of the number of cycles. Second, thermal fatigue resistance was increased in proportion to true tensile fracture strength values of overlay materials. Finally, the number of thermal fatigue cracks per unit length was increased with increasing true tensile fracture strength.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.150-155
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• 2004

The base material of optical disc is the amorphous polymer, polycarbonate. So crack growth on the rotating optical disc could happen by the static centrifugal force. This phenomenon is called static fatigue. Today's CD-R disc drive is being operated over 10,000 RPM. This is increasing the possibility of the disc fracture when operating. In this reason, new method to measure the static fatigue threshold quality and the way to calculate the threshold J-integral value and the safe crack length of the optical disc are studied. Finally the environmental effect to optical disc is also studied in this paper.

• Proceedings of the KSR Conference
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• 2008.11b
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• pp.874-882
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• 2008

In a domestic, HSR-350x which has the maximum speed 350km/h was developed and then next, the next generation high speed train which has the maximum speed 400km/h has still been developing. With developing the next generation high speed railway, there need to be a general plan to make sure of dynamic safety though the a study on the crack and failure of rail by rolling contact fatigue. Therefore, this study investigated occurring stress of rail according to the track quality, train velocity, wheel radius, track stiffness, distance between sleepers, axial force using Eisenmann's equations. For the more, via the finite element method, it investigated shear force on the rail head which could be changed by the early crack length, angle and temperature. As a result, this study confirmed the main elements which effect on the fatigue life cycle of rail.