• 대한기계학회논문집
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• 제19권1호
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• pp.122-131
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• 1995

The research trends for metal matrix composites have been on basic mechanical properties, fatigue behavior after aging and fractographic observations. In this study, the fatigue crack initiation as well as the fatigue crack growth behavior and the fracture mechanism were investigated through observations of the fracture surface on silicon carbide particles reinforced aluminum metal matrix composites(SiCp/Al). Based on the fractographic study done by scanning electron microscope and replica, crack growth path model and fracture mechanism are presented. The mechanical properties, such as the tensile strength, yield strength and elongation of SiCp/Al composites are improved in a longitudinal direction, however, the fatigue life is shorter than the basic Al6061 alloys. From fractographic observations, it is found that the failure mode is ductile in basic Ai6061 alloys. And because some SiC particles were pulled out from the matrix and a few SiC particles could be seen on the fracture surface of SiCp/Al, crack growth paths are believed to follow the interface of the matrix and its particles.

• International Journal of Precision Engineering and Manufacturing
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• 제3권2호
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• pp.95-101
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• 2002

The propagation rate(da/dt) prediction parameter and the microstructure properties of creep crack in domestic 3.3NiCrMov steel were investigated at 550$\^{C}$ by using 0.5" CT specimen under constant load(4090N) and constant Ct(300∼4000N/mhr) condition that was maintained during crack growth of 1mm distance. C＊ usually increased with crack length though load was reduced in order to maintain constant Ct value as crack growth and considerably showed the scatter band, but Ct depended on load line displacement rate and represented a good relation with da/dt. At constant toad and Ct region, crack growth slope was 0.900 and 0.844 each, in the other hand C＊ slope was 0.480. Fully coalesced area(FCA) ahead of crack tip was increased as Ct value increase to the critical value, and after that value FCA decreased. The average diameter ditribution of cavity in FCA showed the greatest value about 1.5 ㎛ when Ct=2000N/mhr. The increasing of Ct in FCA view point enlarged the size of damage area and the size reached to maximum 800 ㎛ when Ct=2000N/mhr.

• 한국안전학회지
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• 제17권4호
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• pp.5-10
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• 2002

For fitness-for-service evaluation of high temperature plant components with defects, crack growth life must be assessed properly as indicated in the recent draft of API 579 code. Type 308 stainless steel has been widely used as a field weld material in the petrochemical industry. In this study, creep crack data of type 308 stainless steel are collected and re-analyzed using $C_t$ as a characterizing fracture parameter. A unique da/dt versus $C_t$ relationship was obtained despite of difference of creep deformation constant of the reviewed materials and specimen geometry of the tested specimens. The obtained results can be employed for crack growth life assessment and fitness-for-service evaluation for the cracks in high temperature components. It is also argued that since the effect of creep properties and other material variability on the creep crack growth behavior would be minor the obtained model may be applied for most of the 308 stainless steels.

• 대한기계학회논문집
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• 제11권3호
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• pp.355-361
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• 1987

본 연구에서는 고온기기의 부재로 널리 사용되는 304스테인리스강을 사용하여 기기의 사용조건을 고려한 538.deg. C(1000.deg. F), 593.deg. C(1100.deg. F)및 650.deg. C(1200.deg. F)의 세가지 온도레벨하의 대기중에서 크리이프-피로시험을 실시하여 작은 인공표면결함으로부터 발생.성장하는 표면균열의 성장거동을 조사 연구하였다. 또한 주파수의 영향을 조사 하기 위하여 538.deg. C에서 고온피로시험을 실시하여 이 결과를 크리이프 및 크리이프피로 시험의 결과와 비교.검토하였다.

• 대한기계학회논문집A
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• 제21권10호
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• pp.1674-1682
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• 1997

Fatigue tests were performed to investigate the effect of ceramic coatings as in TiN and TiCN on fatigue crack growth properties of ceramic coated 1Cr-1Mo-0.25V steel with different coating thickness in laboratory air conditions. The experimental results are described with respect to a Paris equation, da/dN=C(.DELTA.K)$^{m}$ , where the crack growth rate of coated specimens provided as similar growth rate as that of the uncoated specimen regardless of coating thickness. Furthermore, it was observed that the type of coating layer had virtually no effect on crack growth rate in the full region of stress intensity factor range. And it was also appeared that the final crack length of TiCN coated specimens was short compared to that of TiN coated, and the substrate specimens, in which it was inferred due to lowering the toughness of coated material from high hardness of TiCN coating layer itself.

• 한국자동차공학회논문집
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• 제4권4호
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• pp.130-139
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• 1996

The silicon carbide particles reinforced aluminium 6061($SiCp/A\ell$) composites are generally known have wild range of applications from automobiles to airospaces. But, by the results of existing study for $SiCp/A\ell$ composites, there are reports that the fatigue life of $SiCp/A\ell$ composites has improved than $A\ell$matrixes and has not improved then $A\ell$ matrixes. Consequently, in order to perform the reliable life prediction for $SiCp/A\ell$, the properties of probability distribution of fatigue crack initiation life & fracture life, crack growth length in constant number of cycles, crack growth rate in constant stress intensity factor range and m & C value in Paris's fatigue crack growth law and the estimation of statistical parameters have been evaluated by the statistics method.

• Steel and Composite Structures
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• 제17권2호
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• pp.145-157
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• 2014

This study aimed to investigate the fatigue crack growth behavior of a kind of fiber metal laminates (FML) under four different stress levels. The FML specimen consists of three 2024-T3 aluminum alloy sheets and two layers of glass/epoxy composite lamina. Tensile-tensile cyclic fatigue tests were conducted on centrally notched specimen at four stress levels with various maximum values. A digital camera system was used to take photos of the propagating cracks on both sides of the specimens. Image processing software was adopted to accurately measure the length of the cracks on each photo. The test results show that: (1) a-N and da/dN-a curves of FML specimens can be divided into transient crack growth segment, steady state crack growth segment and accelerated crack growth segment; (2) compared to 2024-T3 aluminum alloy, the fatigue properties of FML are much better; (3) da/dN-${\Delta}K$ curves of FML specimens can be divided into fatigue crack growth rate decrease segment and fatigue crack growth rate increase segment; (3) the maximum stress level has a large influence on a-N, da/dN-a and da/dN-${\Delta}K$ curves of FML specimens; (4) the fatigue crack growth rate da/dN presents a nonlinear accelerated increasing trend to the maximum stress level; (5) the maximum stress level has an almost linear relationship with the stress intensity factor ${\Delta}K$.

• 대한기계학회논문집A
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• 제29권3호
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• pp.462-469
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• 2005

Single phase bainite structure which is obtained by the conventional austempering treatment reduces the ductility of ductile cast iron. Because of the reduction of ductility it is possible to worsen the fatigue properties. Therefore, semi austempered ductile iron which is treated from ${\alpha}+{\gamma}$ is prepared to investigate the static strength and fatigue properties in comparison with fully austempered ductile iron (is treated from ${\gamma}$). In spite of semi austempered ductile iron shows the $86{\%}$ increase of ductility. Also, semi austempered ductile iron shows the higher fatigue limit and lower fatigue crack growth rate as compared with fully austempered ductile iron. By the fractographical analysis, it is revealed that the ferrite obtained by semi austempering process brings about the plastic deformation(ductile striation) of crack tip and gives the prior path of crack propagation. The relatively low crack growth rate in semi austempered specimen is caused by above fractographical reasons.

• 동력기계공학회지
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• 제17권4호
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• pp.86-93
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• 2013

This paper deals with the effects of driving force and material properties on statistical distribution of fatigue crack growth rate (FCGR) for the friction stir welded joints of Al 7075-T651 aluminum plate. In this work, the statistical probability distribution of fatigue crack growth rate was analyzed by using our previous constant stress intensity factor range controlled fatigue crack growth test data. As far as this study are concerned, the statistical probability distribution of fatigue crack growth rate for the friction stir welded (FSWed) joints was found to evaluate the variability of fatigue crack growth rate for base metal (BM), heat affected zone (HAZ) and weld metal (WM) specimens. The probability distribution of fatigue crack growth rate for FSWed joints was found to follow well log-normal distribution. The shape parameter of BM and HAZ was decreased with increasing the driving force, however, the shape parameter of WM was decreased and increased with increasing the driving force. The scale parameter of BM, HAZ and WM was increased with the driving force.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회A
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• pp.264-268
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• 2008

An assessment of creep-fatigue crack behavior is required to ensure the structural integrity for high temperature components such as fast breeder reactor structures or thermal power plant components operating at an elevated temperature. In this study, an evaluation of creep-fatigue crack growth has been carried out according to the French assessment guide of the RCC-MR A16 for austenitic stainless steel structures. The assessment procedures for creep-fatigue crack growth in the recent version of the A16 (2007 edition) have been changed considerably from the previous version (2002 edition) and the material properties (RCC-MR Appendix A3) have been changed as well. The impacts of those changes on creep-fatigue crack growth behavior are quantified from the assessments with a structural model. Finally the assessment results were compared with the observed images obtained from the structural tests of the same structural specimen.