• 대한기계학회논문집
• /
• 제18권7호
• /
• pp.1722-1729
• /
• 1994

At early stage of creep-fatigue crack growth tests, initial transient behavior which implies high crack growth rate has been generally observed by some researchers. Since the influence of the initial transient crack growth behavior on the remaining life of components is significant, cause of it should be further studied. In this study, characteristics of the initial transient behavior of 1Cr-1Mo-0.25V steel is studied experimentally by performing creep-fatigue crack growth tests at $538^{\circ}C$ in air under trapezoidal waveshapes. It is verified that the cause of the initial transient behavior is not high ${(C_t)}_{avg}$ values due to the small scale creep condition at the early stage of test, but oxidation-dominated crack growth mechanism during the transient period which is different from the creep-dominated crack growth mechanism in steady crack growth period.

• 한국정밀공학회:학술대회논문집
• /
• 한국정밀공학회 1996년도 추계학술대회 논문집
• /
• pp.928-932
• /
• 1996

Single overload fatigue tests with overload sizes ranging from 50% and 100% have been performed to investing ate the fatigue crack growth retardation behavior. A modified and experimental method of Willenborg's model for prediction of crack growth retardation behavior has been developed, based on evaluations of equivalent plastic zone size (EPZS) changing its size along the overload plastic zone boundary. The minimum crack growth rates of each overload size are linearly decreased with overload size increasing, but fatigue lives extended by single overload are increasing much more unlike the crack growth rates. Comparisons of crack growth behavior predicted by EPZS model and Willenborg model have shown that the EPZS model accounts for overload effects better than Willenborg model. These effects include delayed retardation, large retardation region, minimum crack growth rate, and the increase rate of crack growth rate in the region crack growth rate recovered.

• 한국기계기술학회지
• /
• 제13권2호
• /
• pp.31-37
• /
• 2011

Sheet aluminum alloys have been used in manufacturing of machine structures. In fatigue crack propagation behavior of thin sheet aluminum alloys, it is important that fatigue crack growth rate is affected by crack closure phenomenon. In this work, we analyzed the characteristics of fatigue crack propagation behavior in experiment of constant stress condition for thin sheet Al 2024-T3 alloys, and identified the retardation behavior of crack growth by comparing experimental results of thin and thick plate specimen. We attempt to operate the fatigue life estimating process using the fatigue related material constants from referred fatigue crack propagation analysis. And we analyzed the experimental and prediction results of fatigue life of thin sheet aluminum alloy in order to identify the relation between retardation behavior of fatigue crack growth and crack closure phenomenon.

• 한국세라믹학회지
• /
• 제32권1호
• /
• pp.51-56
• /
• 1995

Variation of sintered density of BaTiO3 powder calcined at 120$0^{\circ}C$ and 135$0^{\circ}C$ was investigated with respect to the grain growth behavior. It was found that BaTiO3 powder, which was calcined at 120$0^{\circ}C$, showed abnormal grain growth behavior during sintering process. At initial stage of sintering process, the densification rate of specimen was accelerated with rapid grain growth caused by the abnormal grain growth. But with the increase of sintering time, abnormally grown grain met each other and the density of specimen decreased drastically due to coalescence of pores located in triple junction. On the contrary, BaTiO3 powder calcined at 135$0^{\circ}C$ showed normal grain growth behavior and gradually densified with the increase of sintering time.

• 무역학회지
• /
• 제41권4호
• /
• pp.41-59
• /
• 2016

한 국가의 기업가정신이 경제성장에 기여하는 영향요인으로 주목받고 있는 상황이다. 본 연구는 기업가정신을 기업가적 행동과 환경으로 세분화하여 이들이 국가의 경제성장에 미치는 영향에 대하여 분석하고자 하였다. 기존 연구에서는 기업가정신이라는 변수를 중심으로 경제성장과의 관계성을 분석하였다면, 본 연구에서는 기업가적 활동과 기업가적 환경이라는 변수를 이용하여 극가들의 경제성장에 미치는 영향을 설명하고자 하였다. 더 나아가, 무역과 혁신이 제시된 독렵변수들과 경제성과와의 관계에서 조절적 역할을 수행하는 지를 검증하고자 하였다. GEM의 Adult Population Survey(APS)와 National Expert Survey(NES), World bank에서 데이터를 수집하여 패널분석을 실시한 결과, 기업가적 행동과 기업가 환경요인은 모두 혹은 부분적으로 국가별 경제성장에 유의한 정의 영향을 미치는 것으로 분석되었다. 특히, 기회추구형 창업활동이 경제성장에 정의 영향을 주는 것으로 나타나 창업활동의 질적인 측면이 경제성장의 성과로 이어지는데 중요한 변수임을 도출하였다. 또한, 정부 창업 지원프로그램과 경제성장의 관계에 대하여 무역과 혁신이 유의하게 조절효과를 갖는 것을 확인하였으며, 본 연구 결과에 따른 시사점에 대하여 논의하였다.

• 대한기계학회논문집
• /
• 제15권6호
• /
• pp.1990-2001
• /
• 1991

본 연구에서는 평활부 및 응력집중부에 표면균열을 갖는 표면균열재의 피로수 명과 균열관통 거동을 검토하고, 그 특성을 해명하였다.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2000년도 추계학술대회논문집A
• /
• pp.141-146
• /
• 2000

Crack closure and growth behavior of physically short fatigue cracks under random loading are investigated by performing narrow- and wide-band random loading tests for various stress ratios. The importance of the crack closure phenomenon is examined by predicting the growth lives of short cracks using obtained crack opening behavior. Artificially prepared two-dimensional, short through-thickness cracks are used. The crack opening load of short cracks is much lower under random loading than under constant-amplitude loading corresponding to the largest load cycle in a random load history. This result indicates that the largest load cycle in a random load history has an effect to enhance crack opening of short cracks. Most of the life prediction ratios are within the factor of 2 scatter band except several data at very short crack sizes, indicating that crack growth predictions based on the measured crack opening data are excellent. From the results obtained in this study, it can be concluded that crack closure is the primary factor governing fatigue crack growth of short cracks under random loading as well as under constant-amplitude loading.

• 한국분말재료학회지
• /
• 제29권2호
• /
• pp.110-117
• /
• 2022

The grain growth behavior in the (1-x)K_0.5Na_0.5NbO₃-xCaZrO₃ (KNNCZ-x) system is studied as a function of the amount of CZ and grain shape. The (1-x)K_0.5Na_0.5NbO₃-xCaZrO₃ (KNNCZ-x) powders are synthesized using a conventional solid-state reaction method. A single orthorhombic phase is observed at x = 0 - 0.03. However, rhombohedral and orthorhombic phases are observed at x = 0.05. The grain growth behavior changes from abnormal grain growth to the suppression of grain growth as the amount of CaZrO₃ (CZ) increases. With increasing CZ content, grains become more faceted, and the step-free energy increases. Therefore, the critical growth driving force increases. The grain size distribution broadens with increasing sintering time in KNNCZ-0.05. As a result, some large grains with a driving force larger than the critical driving force for growth exhibit abnormal grain growth behavior during sintering. Therefore, CZ changes the grain growth behavior and microstructure of KNN. Grain growth at the faceted interface of the KNNCZ system occurs via two-dimensional nucleation and growth.

• 비파괴검사학회지
• /
• 제29권1호
• /
• pp.43-48
• /
• 2009

The fatigue crack growth behavior of high strength X80 pipeline steel was investigated with compact tension specimens that crack growth directions were aligned either parallel or normal to the rolling direction of the pipeline. Also, the fatigue crack growth rates for welded joint of X80 pipeline steel were investigated with compact tension specimens that crack growth directions were aligned either parallel or normal to the welding line. The experimental results indicated the fatigue crack growth behavior was markedly different in three zones, weld metal, heat affected zone and base metal of welded joints. There was a trend toward increment in the fatigue life of weld metal and heat affected zone as compared with the X80 pipeline steel.

• 한국분말야금학회:학술대회논문집
• /
• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part2
• /
• pp.891-892
• /
• 2006

To test the correlation between grain shape and growth behavior we prepared WC-TiC-Co samples with rounded (Ti, W)C grains and faceted WC grains. The growth of rounded (Ti, W)C grains was normal. In contrast, the growth of faceted WC grains was abnormal or suppressed depending on the initial size of WC particles. These observations were explained using growth theories of crystals in a liquid and were also confirmed by a simulation using their growth equations. The present results thus demonstrate that the growth behavior of carbide grains in a liquid is governed only by their shape, irrespective of the presence of another phase.