• Journal of Ocean Engineering and Technology
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• v.5 no.2
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• pp.76-84
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• 1991

본 연구는 피로수명 평가를 위한 새로운 파괴역학적 parameter의 확립에 관한 연구이다. 실질적으로 피로파괴가 일어나는 피로 균열선단의 국소영역에서 변형분포를 미소원형격자측정법을 이용하여 실험적으로 명확히 밝혀내었다. 그리고 이 결과를 기초로 하여 국소피로 변형율장을 대표할 수 있는 피로변형율 확대계수 $\Delta$A를 제안하였다. 또한 새로운 parameter $\Delta$A의 유효성을 여러 피로조건에서 검토한 결과, 균열선단 국소 영역에서 피로 변형율 확대계수 $\Delta$A에 의하여 피로 균열전파 속도평가를 일의적으로 나타낼 수 있음을 확인하였다.

• Journal of Ocean Engineering and Technology
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• v.5 no.2
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• pp.216-216
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• 1991

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.1
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• pp.209-217
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• 1991

We have carried out a preliminary study to search for the new fracture mechanics parameter which can effectively estimate the fatigue life. In this study, the distribution of local fatigue strains near a fatigue crack tip was detailedly revealed using by fine dot grid strain measurement method. From these results, a single parameter (.DELTA.A), which characterize local fatigue strain field, was nearly proposed by the authors.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.6 no.3
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• pp.53-62
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• 1986

For the research on the fatigue fracture behavior in the welded joints of steel structures, base metal specimens and welded ones were selected, and the direct fatigue tests were carried out. Thereafter, fatigue-life (S-N) curves, plastic strain-number of cycles (${\varepsilon}_p$-N) curve, the extrapolated fatigue-life (${\varepsilon}_p$-$N_c$) curve, and da/dN-${\Delta}K$ curves were plotted. By these results the followings were obtained. It was shown that the ratio of fatigue strength at $2{\times}10^6$ cycles of the welded specimen to that of the base metal one was 0.6, and that 0.72 for the base metal and 0.65 for the welded one were the ratio of fatigue strength at $2{\times}10^6$ cycles to yielding stress. The S-N curve for the welded specimen was separated into two sections, the low gradient section and the steep section. As this result, it was shown that the more stress became to reduce, the more the reduction of fatigue strength became to be great. It was shown that fatigue strength at $2{\times}10^6$ cycles from this case was about 83 % of that from the S-N curve plotted with one section. It was thought that the reason was that weld flaw acted greatly on the fatigue strength within the low stress range. It was shown that at the instart of crack initiation plastic strain increased abrupt1y in the case of the welded specimen more than the case of the base metal specimen, and increased abruptly in the upper stress range in both cases. It was shown that the experimental constant ${\alpha}$, 0.42, in the base metal nearly accorded with Manson-Coffin's result, but this made a great difference with the case in the welded specimen. It was thought that it was due to the abrupt change of plastic strain and the influence of weld flaw.

• Journal of the Korean Society of Safety
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• v.11 no.2
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• pp.3-8
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• 1996

Fatigue fracture is the cyclic fracture phenomena at a very small local area near a crack tip. Therefore, the detailed quantitative experimental analysis about local cyclic strain distribution near a crack tip is prerequisite In order to make an effective parameter able to account for fatigue fracture problems. However, there are few reports on detailed quantitative experimental analysis of a local cyclic strain distribution near a crack tip, because of experimental difficulties. In this study, the distribution of local fatigue strains near a fatigue crack tip was in detail studied using by fine dot grid strain measurement method. From these results, a single parameter, which characterizes local fatigue strain field, was proposed. In addition, this parameter was applied to evaluate the fatigue crack propagation rate.

• Korean Journal of Materials Research
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• v.3 no.3
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• pp.253-260
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• 1993

High strength and fracture toughness of Al-Li-Cu alloy(2090 Al alloy) have been achieved by the improvement of melting and casting, extrusion and heat treatment techniques. To establish the sucessful process for semi-industrial scale ingot(20Kg) the following areas have been investigated: (1) Improvement of melting and casting techniques for ingot by introducing atmospheric modifications, vacuum and rotary degassing, and deslagging. (2) The effect of heat treatment on mechanical properties (3) Mechanical characterization by tensile test, fracture toughness test and fatigue crack propagation test. High mechanical properties were found to be intimately related with ingot soundness. Tensile strength of final products varied from 534MPa to 566MPa in peak aged condition while elongation/ductility ranged from 9.0% to 11.9%. From the fracture toughness test with using compact tensile specimen, plane strain fracture toughness($K_{Ic}$) appeared to be 39MPa${\surd}$m in peak aged condition and 23MPa${\surd}$ m in underaged condition. When load ratios of 0.1, 0.3 and 0.5 were given ${\Delta}K_{th}$ was 6.0MPa${\surd}$ m, 5.3MPa${\surd}$ m and 4.3MPa${\surd}$ m respectively.