• Journal of Electrical Engineering and Technology
• /
• 제10권1호
• /
• pp.299-307
• /
• 2015

Reduction in pollution performance of polymeric insulators, aged due to water absorption stress and thermal stress, is a major threat to the reliable operation of power transmission and distribution system. Formation of partial discharges on the surface of wet polluted insulator plays a major role in determining the life time and pollution performance of outdoor polymeric insulators. However, reports on partial discharge characteristics of water absorption stress aged and thermal aged polymeric insulators are scanty. This paper discusses the pollution performance characteristics of accelerated aged polymeric insulators using the advanced ultra wide band PD signal measurement and analysis. Laboratory experiments on accelerated aged polymeric insulators were carried out as per IEC 60507 under AC voltage, at different humidity and contamination levels using NaCl as a contaminant. PRPD pattern and Time-Frequency map analysis of PD signals were carried out. From the results, it can be speculated that PD analysis is a well suited technique to understand the pollution performance of aged polymeric insulators.

• Journal of Mechanical Science and Technology
• /
• 제20권12호
• /
• pp.2178-2188
• /
• 2006

The stress collapse in the formation of shear bands in elasto-thermo-viscoplatic materials is systematically studied within the framework of one-dimensional formulation via analytical and numerical methods. The elastic energy released in a domain is found to play an important role in the collapse behavior of shear localization. A non-dimensional parameter named the stability indicator is introduced to characterize the collapse behavior, with approximate forms of the incremental governing equations. The stability indicator offers useful information regarding the degree of severity of an abrupt change of deformations during the stress collapse. Numerical experiments are carried out to verify the stability indicator by varying material properties.

• 대한기계학회논문집
• /
• 제18권6호
• /
• pp.1448-1454
• /
• 1994

Data which gathered and used in the field of fatigue and fracture mechanics have a lot of uncertainties. In this case, those uncertainties will make scatter band in evaluation of fatigue life and fracture toughness. Thus, the probabilistic analysis of these data will be needed. For determining the fatigue life in mixed mode, using crack direction law and fatigue crack growth law, the problem is studied as a constrained life minimization. Stress intensity factor(SIF) is computed by approximate solution table(Ewalds/Wanhill 1984) and 0th order PFEM. The variance of fatigue life and SIF are computed by differentiation of tabulated approximate solution and 1st order PFEM. And these are used for criterion of design values, principal parameter determination and modelling. The problem of center cracked plate is solved for checking the PFEM model which is influenced by various parameters like as initial crack length, final crack length, two fatigue parameters in Paris Equation and applied stress.

• 한국해양공학회지
• /
• 제1권1호
• /
• pp.104-110
• /
• 1987

Fatigue tests by axial loading (R = 0.05) were carried out to investigate fatigue crack growth characteristics of small surface cracks in 2 1/4 Cr-1 Mo steel at room temperature by using flat specimens with a small artificial pit. All the data of the fatigue crack growth rate obtained in the present test are determined as a function of the stress intensity factor range about a semi-elliptical crack, so that the application of linear fracture mechanics to the surface fatigue crack growth and to the fatigue crack growth into depth, and all the data obtained from tests were discussed in comparison with the data of Type 304 stainless steel and two type of mild steel under the same test conditions. The obtained results are as follows: 1)When the cycle ratios are same, surface fatigue crack length and its depth are almost same and fall within a narrow scatter band in spite of different stress levels. 2)Relations of the surface fatigue crack growth rate (da/dN) and fatigue crack growth rate into depth (db/dN) to its stress intensity factor range ($\Delta K_{Ia}, \Delta K_{Ib}$) can be plotted as a straight line at log-log diagram without dependence of stress level and coincide with the data of part-through crack in various steels.

• 대한조선학회지
• /
• 제12권2호
• /
• pp.43-58
• /
• 1975

There are many reports on fatigue crack of metallic materials but most of them relate crack propagation rate to stress intensity factor. The problem of crack propagation is not yet clarified, especially the bridge between micro and macro phenomena In this experiment rotating bending fatigue tests have been carried out with smoothed specimen of rolled steel plates including 0.2% carbon under application of three stress conditions to investigate the slip band and the crack propagation behaviour. The results obtained are as follows; 1) The length of cracks which have grown at initial crack tips can be expressed as follows; $l=Ae^{BNr}$(A,B: constant, $N_r$: cycle ratio) $\frac{dl}{dN}=\frac{AB}{N_f}{\cdot}e^{BNr}$($N_f$:fatigue life) 2) The ratio of slipped grain number to total grain number is $S_f=7{\sigma}-5.6$-5.6{\sigma}_c$($\sigma$: stress amplitude) (${\sigma}_c$: fatigue limit) 3) When the fatigue process transfers from Stage I to Stage II, the crack which propagates into specimen changes its direction from that of the maximum shear stress to the direction of perpendicular to principal stress and this is same in the circumferential direction of specimen. the crack propagation behaviors of both sides of a crack are different each other when they approach to the grain boundary.

• KCI Concrete Journal
• /
• 제12권1호
• /
• pp.57-68
• /
• 2000

The effect of structure size on the nominal strength of unidirectional fiber-polymer composites, failing by propagation of a kink band with fiber microbuckling, is analyzed experimentally and theoretically. Tests of novel geometrically similar carbon-PEEK specimens, with notches slanted so as to lead to a pure kink band (without shear or splitting cracks), are conducted. The specimens are rectangular strips of widths 15.875, 31.75. and 63.5 mm (0.625, 1.25 and 2.5 in and gage lengths 39.7, 79.375 and 158.75 mm (1.563, 3.125 and 6.25 in.). They reveal the existence of a strong (deterministic. non-statistical) size effect. The doubly logarithmic plot of the nominal strength (load divided by size and thickness) versus the characteristic size agrees with the approximate size effect law proposed for quasibrittle failures in 1983 by Bazant This law represents a gradual transition from a horizontal asymptote, representing the case of no size effect (characteristic of plasticity or strength criteria), to an asymptote of slope -1/2 (characteristic of linear elastic fracture mechanics. LEFM) . The size effect law for notched specimens permits easy identification of the fracture energy of the kink bandand the length of the fracture process zone at the front of the band solely from the measurements of maximum loads. Optimum fits of the test results by the size effect law are obtained, and the size effect law parameters are then used to identify the material fracture characteristics, Particularly the fracture energy and the effective length of the fracture process zone. The results suggest that composite size effect must be considered in strengthening existing concrete structural members such as bridge columns and beams using a composite retrofitting technique.

• International Journal of Automotive Technology
• /
• 제7권4호
• /
• pp.485-492
• /
• 2006

The AA5182/polypropylene/AA5182(AA/PP/AA) sandwich sheets have been developed for application to automotive body panels in future lightweight vehicles with significant weight reduction. It has been reported that the AA5182 aluminum sheet shows $L\"{u}ders$ band because of dissolved Mg atoms that cause fabrication process problem, especially surface roughness. The examination of serration behavior has been made after the tensile deformation of the AA/PP/AA sandwich sheets as well as that of the AA5182 aluminum skins at room and elevated temperatures. All sandwich sheets and the AA5182 aluminum skin showed serration behavior on their flow curves. However, the magnitude of serration was significantly diminished in the sandwich sheet with high volume fraction of the polypropylene core. According to the results of the analysis of the surface roughness following the tensile test, $L\"{u}ders$ band depth of the sandwich sheet evidently showed lower than that of the AA5182 aluminum skin. The strain rate sensitivity, m-value, of the AA5182 aluminum skin was -0.006. By attaching these skins to the polypropylene core, which has relatively large positive value of 0.050, m-value of the sandwich sheets changed to the positive value. The serration mechanism of the sandwich sheets was quantitatively investigated in the point of the effect on polypropylene thickness variation, that on the strain rate sensitivity and that on the localized stress state.

• 전력전자학회논문지
• /
• 제23권3호
• /
• pp.168-173
• /
• 2018

In this paper, an auxiliary power supply (APS) for railroad cars is proposed. The APS can reduce the number of devices required to supply power through structural modification and operates at a high switching frequency by application of a SiC device. The voltage stress on the device in the proposed circuit can be reduced to less than half of the input voltage of the system; thus, a device with low breakdown voltage can be designed. By adapting a SiC device instead of an IGBT device, the proposed circuit can reduce switching and conduction losses and operate at a high switching frequency, thereby reducing output voltage and inductor current ripples in the proposed circuit. The theoretical analysis results of the proposed APS are verified with a 40 kW computer-based simulation and a 2 kW experiment.

• 한국지반공학회논문집
• /
• 제32권3호
• /
• pp.49-60
• /
• 2016

일반적으로 흙의 파괴는 전단 변형이 집중되는 영역인 전단면의 형성과 발달에 의해 발생한다. 정규압밀 점성토의 파괴거동을 확인하기 위해 시료 내부의 변형거동 분포에 대해 평가가 필요하다. 본 연구에선 재성형된 카올리나이트 시료에 대해 평면변형률 시험을 수행하였으며, 전단 과정에서 일정 변형률 간격에서 디지털 이미지 해석을 수행하였다. 시험 결과로 도출된 응력-변형률 결과를 통해 4개의 단계를 결정하여 시료의 변형거동과 전단면 특성을 평가하였다.

• 한국해양공학회지
• /
• 제9권1호
• /
• pp.142-151
• /
• 1995

A simulation program concerned with multi-surface fatigue cracks which initiated at the semi-circular surface notches has been developed to predict their growth and coalescence behaviors at the elevated temperature. Three kinds of coalescence models such as SPC(surface point connection), ASME and BSI(British Standards Institution) conditions were applied, and the results of the simulation were compared with those of the experiment. This simulation is able to enhance the reliance and integrity of structures especially under the elevated temperature which have lots of difficulties in experiments and applications. This shows that the simulation result has utility for fatigue life prediction. Even though all the specimens were the same shape, the error rate was increased in accordance with the applied stress to the specimen. Among the material constants C and m in the narrow band, the results applied upper values of the band to the simulation has shown quite small error compared with the experiment results.