• The Transactions of The Korean Institute of Electrical Engineers
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• v.59 no.8
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• pp.1497-1505
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• 2010

The present study was performed to investigate the effect of aging on muscular activity of older subjects under the condition of fixed load muscle contraction. SEMG signals were recorded from old (46 women) and young (45 women) groups performing sustained isometric elbow flexion contraction with a fixed load during 30 seconds. Linear regression and mean square error (MSE) analysis with four characteristic variables (ARV, RMS, MDF, MNF) were used to compare the age-related difference (of local muscle fatigue and fluctuation of the amplitude and frequency) in the SEMG signal. The main results can be summarized as follows: During sustained muscle contraction with a fixed load: i) the MSE values of amplitude (ARV, RMS) and frequency (MDF, MNF) variables were more than 30% higher for the young than for the old adults; ii) the measures of local muscle fatigue (slope of the MDF and MNF) indicated greater fatigue in the old; and iii) the rate of increase of the SEMG amplitude was higher for the young than the old.

• Journal of Mechanical Science and Technology
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• v.16 no.9
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• pp.1084-1092
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• 2002

The frequency dependency of Rayleigh surface wave is investigated indirectly by measuring the angular dependency of the backward radiation of the incident ultrasonic wave in two kinds of degraded specimens by scuffing or corrosion. Then, the frequency dependency is compared with the residual stress distribution or the corrosion-fatigue characteristics for the scuffed or corroded specimens, respectively. The width of the backward radiation profile increases with the increase of the variation in residual stress distribution for the scuffed specimens. In the corroded specimens, the profile width decreases with the increase of the effective aging layer thickness and is inversely proportional to the exponent, m, in the Paris' law that can predict the crack size increase due to fatigue. The result observed in this study demonstrates high potential of backward radiated ultrasound as a tool for nondestructive evaluation of subsurface gradient of material degradation generated by scuffing or corrosion.

• Proceedings of the KSME Conference
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• 2007.05a
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• pp.7-10
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• 2007

In case of large steel water pipe, it have been observed that its fracture mostly occurs due to the complicated outside fatigue load on the pipe in the underground. It is also well known that its damage and leakage happen mainly in a weld zone. In this study we evaluated the fatigue characteristics based on size effect and residual stress by comparing the test results on the standard specimen collected from real pipe with those on full scale pipe.

• Transactions of the Korean Society of Automotive Engineers
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• v.9 no.3
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• pp.181-189
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• 2001

For the Tailor Welded Blank sheet used fur automobile body panel, the characteristics of fatigue crack propagation behavior were studied. The thickness of specimen was machined to be same (0.9+0.9mm) and different (0.9+2.0mm). As a base test, mechanical properties around welding zone were examined. The results indicated that there were no significant decreases in mechanical properties, but hardness around welding bead is 2.3 times greater than base material. The crack propagation rate was noticeably decreased around welding line and rapidly increased as it passed by welding line. Reviewing the shape of the crack propagation, crack width around welding line was wide around the welding zone due to retardation of crack growth, but it became narrow passing welding line due to decreased toughness. Elasto-Plastic analysis was performed by finite element analysis fur explaining the test results.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.10a
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• pp.829-832
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• 1997

It is difficult to explain the effects of complex variable loading experienced by the machine and the structure only with the studies of the single-overload itself. Hence, it is thought that the variation of overload-holding time are required to explain the effects more clearly. The effects of the overload were analyzed by means of the crack retardation, and the fractography on retardation zone. A characteristic of the fractography on retardation zone was that striation distribution did not appear due to decreased crack driving force. Rotary bending fatigue tests were performed with the circular shaft which has two hole defects.

• Journal of Advanced Marine Engineering and Technology
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• v.23 no.1
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• pp.62-68
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• 1999

The strength data on AlBC3 and HBsC1 which are used for materials of marine propeller blade are insufficient as used of material limited and even rules of KR(Korean Register of Shipping) describe only a point of view that the chemical composition and men values of mechanical proper-ties have to be certain level. In this study distribution characteristics as well as mean values of mechanical properties are investigated through the tensile test and the characteristics of fatigue strength are investigated through the fatigue test.

• Proceedings of the Korean Reliability Society Conference
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• 2006.05a
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• pp.357-361
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• 2006

Fatigue fracture behavior of a hybrid joint part with bolting was evaluated in comparison to the case of static fracture. Hybrid joint part specimens for bending test were made with layers of CFRP and aluminum honeycomb. Characteristic fracture behaviors of those specimens were obviously different under static and cyclic loads. Static bending load showed the shear deformation at the honeycomb core, whereas cyclic bending load caused the delamination between CFRP skin layers and honeycomb core. Experimental results obtained by static and fatigue tests were considered in modifications of design parameters of the hybrid joint.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2009.11a
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• pp.382-386
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• 2009

The life of a component decreases when it was exposed at the extreme condition. A turbine blade of a turbopump used for a liquid rocket engine is operated under the environment of high temperature and pressure, and experienced high centrifugal force. Thus the durability of the turbopump operated under the these conditions become lower than expected because of the severe fatigue and creep influence. The damage of the turbine being considered the fatigue and the creep influence is estimated to ensure the durability of turbopump turbine. ABAQUS/CAE and MSC.Fatigue are used for the fatigue analysis, and Larson-Miller parameter and robinson's rule are used for the creep analysis. In this paper, comparison and analysis of the fatigue and the creep influence were performed to ensure the life expectancy of turbopump turbine.

• Korean Journal of Materials Research
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• v.7 no.1
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• pp.62-68
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• 1997

In the first part of the paper, the crack growth process in rolling contact fatigue has been investigated on ring type plate specimens, in which crack growth is two dimensional and cracks are observed on the side surface of the specimens. The results have shown that cracks are initated from the contact surface in tensile mode in the direction approximately normal to the contact surface and after some short length of growth, shear mode growth occurs from the tip of the crack and it grows until the separation of the surface layer, namely flakung type failure, occurs. In the second part, mode U fatigue crack growth tests have been made by using an apparatus designed based on the concept that the subsurface fatigue crack growth in rolling contact fatigue is the mode U fatigue crack growth under the stress state where the tensile mode growth is suppressed by compression stress. The rest results have shown that the mode U fatigue crack growth occurs if the superposed compression stress is enough to suppress the tensile mode growth.

• Advances in materials Research
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• v.3 no.3
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• pp.163-174
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• 2014

In this study, fatigue properties and crack growth characteristics of a polycarbonate (PC) were examined during cyclic loading at various mean stress (${\sigma}_{amp}$) and stress amplitude (${\sigma}_{mean}$) conditions. Different S vs. N and da/dN vs. ${\Delta}K$ relations were obtained depending on the loading condition. The higher fatigue strength and the higher resistance of crack growth are seen for the PC samples cyclically loaded at the higher mean stress and lower stress amplitude due to the low crack driving force. Non-linear S - N relationship was detected in the examination of the fatigue properties with changing the mean stress. This is attributed to the different crack growth rate (longer fatigue life): the sample loaded at the high mean stress with lower stress amplitude. Even if the higher stress amplitude, the low fatigue properties are obtained for the sample loaded at the higher mean stress. This was due to the accumulated strain energy to the sample, where severe plastic deformation occurs instead of crack growth (plasticity-induced crack closure). Shear bands and discontinuous crack growth band (DGB) are observed clearly on the fracture surfaces of the sample cyclically loaded at the high stress amplitude, where the lower the ${\sigma}_{mean}$, the narrower the shear band and DGB. On the other hand, final fracture occurred instantly immediately after the short crack growth occurs in the PC sample loaded at the high mean with the low ${\sigma}_{amp}$, i.e., tear fracture, in which the shear bands and DGB are not seen clearly.