• Proceedings of the Materials Research Society of Korea Conference
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• 2003.11a
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• pp.86-86
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• 2003

주편은 1차 냉각 지역인 수냉 몰드를 통과한 후, 2차 냉각 지역에서 guide roll, pinch roll 그리고 driven roll등에 의해 반복적인 압축하중을 받고 있으며, roll과 roll사이에서는 철정압에 의한 주편 bulging 현상이 발생하고 주편의 표면은 인장응력을 받게 된다. 특히 연속주조 중 주편의 변형기구가 단순 탄소성 변형 이 아닌 creep에 의한 변형임을 고려할 때, 2차 냉각 지역에서 주편의 표면은 전술한 압축 및 인장변형 이 반복되는 저주기 고온 피로 환경을 거침을 알 수 있다. 본 연구에서는 탄소함량에 따른 주편의 bulging시의 크랙 발생에 미치는 저주기 고온 피로의 효과를 조사하였다. 또한, 용체화 처리 온도에서 시험 온도까지의 냉각 속도의 영향을 조사하기 위하여 1$^{\circ}C$/s 및 1$0^{\circ}C$/s로 냉각 속도를 변화시켜 열간 연성 곡선을 작성하였다. 본 연구에서 얻어진 결과는 다음과 같다. 저탄소강의 경우는 저주기 피로의 영향이 관찰되지 않았으며, 중탄소강의 경우, 저온에서는 저주기 피로로 인해 열간 연성이 증가하였으나, 고온에서는 변형유기 페라이트의 생성으로 인해 열간 연성 이 감소하였다. 고탄소강의 경우는 저주기 피로로 인하여 열간 연성이 모든 온도 구간에서 증가하였다. 또한 용체화 처리후 시험 온도까지의 냉각 속도가 감소함에 따라 열간 연성이 증가하였는데, 이는 입 계 석출물의 조대화로 인해 열간 연성이 증가하는 것으로 판단된다.

• Korean Journal of Materials Research
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• v.7 no.11
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• pp.1007-1011
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• 1997

영활화혁발전소 보일러헤다 재질인 1Cr0.5Mo강의 파형에 따른 저주기 피로특성을 규명하고자 상온(298K) 및 고온(177K)의 삼각파와 사인파형 저주기 피로시험을 수행하였고 소성에너지법을 이용하여 파형에 따른 소성변형에너지와 피로수명과의 +관계를 분석하였다. 저주기 피로시 재료내부의 소성변형에너지를 히스테리시스루프의 면적으로 계산하여 구하였으며 이를통해 저주기 피로수명을 예측하였고 Coffin-Manson법 및 변형률분할법을 이용한 저주기 피로수명 결과와 서로 잘 일치하였다. 또한 상온 및 고온에서 피로반복수의 증가와 함께 재료가 반봅연화됨을 알 수 있었다.

• Journal of Advanced Marine Engineering and Technology
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• v.40 no.7
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• pp.593-598
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• 2016

Static tests and low-cycle fatigue tests were conducted to analyze the characteristics of the bellows for LNG vessels according to the forming methods. The cycle life of bellows was tested based on the specified cyclic life, 80000 cycles, to analyze the difference in characteristics between pre-and post-test data by measuring the strain and stress of each convolution of formed bellows. The low-cycle fatigue test was conducted using a strain gauge that was attached to the convolution of bellows. Formed bellows were placed on the structural test device which was equipped with a hydraulic system and was capable of moving in the x-y direction. Data was measured and processed by a multi recorder. Through the static test and low-cycle fatigue tests results, the difference between the cycle life of bellows formed by mechanical methods and of those formed by hydraulic methods was investigated. Moreover, the cause of difference in cyclic life according to forming methods was performed.

• Composites Research
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• v.31 no.6
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• pp.332-339
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• 2018

Fiber metal hybrid laminate (FML) can be used as an economic material with superior mechanical properties and light weight than conventional metal by bonding of metal and FRP. However, there are disadvantages that it is difficult to predict fracture behavior because of the large difference in properties depending on the type of fiber and lamination conditions. In this paper, we study the failure behavior of hybrid materials with laminated glass fiber reinforced plastics (GFRP, GEP118, woven type) in Al6061-T6 alloy. The Al alloys were coated with GFRP 1, 3, and 5 layers, and fracture behavior was analyzed by using a static test and a low cycle fatigue test. In the low cycle fatigue test, strain - life analysis and the total strain energy density method were used to analyze and predict the fatigue life. The Al alloy did not have tensile properties strengthening effect due to the GFRP coating. The fatigue hysteresis geometry followed the behavior of the Al alloy, the base material, regardless of the GFRP coating and number of coatings. As a result of the low cycle fatigue test, the fatigue strength was increased by the coating of GFRP, but it did not increase proportionally with the number of GFRP layers.

• Journal of the Korean Society of Propulsion Engineers
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• v.26 no.5
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• pp.1-10
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• 2022

For Cu-Cr alloy developed for rocket engines, estimated fatigue lives were calculated using various fatigue life prediction methods and compared with fatigue life acquired from low-cycle fatigue tests. The utilized methods for fatigue life prediction are as follows: Coffin-Manson relation, plastic/total strain energy density relations, Smith-Watson-Topper relation, Tomkins relation, and Jahed-Varvani relation. As results of estimation of fatigue lives, it satisfied within scatter band two compared to the test fatigue lives in all methods. The quantitative calculation of the deviation of predicted fatigue lives gives that the total strain energy density relation presents the best result.

• Proceedings of the Korean Nuclear Society Conference
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• 1996.11b
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• pp.463-468
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• 1996

질소의 함량을 0.04%~0.15% 까지 변화시킨 316L 스테인레스 강으로 공기 중에서 저주기 피로시험을 수행하였다. 전체변형범위 1%, 변형속도 2$\times$$10^3$/sec, 삼각파로 상온 ~$600^{\circ}C$ 온도범위에서 시험을 수행하였다. 상온에서는 사이클이 진행됨에 따라서 연화가 계속해서 발생하지만 온도가 증가하면 초기에 경화가 발생한다. 피로시험 초기에 경화되는 정도와 saturation 응력은 온도가 증가하면 증가한다. 이러한 경화현상은 동적변형시효에 의해서 발생되는 것으로 판명되었다. 질소를 첨가하면 강도는 증가하지만 경화는 감소되었다. 질소에 의한 경화의 감소는 질소가 동적변형시효를 억제하기 때문이다.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.7
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• pp.753-758
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• 2011

The Ni-base super-heat-resistant alloy, GTD-111, is employed in gas turbines because of its high temperature strength and oxidation resistance. It is important to predict the fatigue life of this superalloy in order to improve the efficiency of gas turbines. In this study, low-cycle fatigue tests are performed as variables of total strain range and temperature. The relationship between the strain energy density and number of cycles to failure is examined in order to predict the low-cycle fatigue life of the GTD-111 superalloy. The fatigue life predicted by using the strain-energy methods is found to coincide with that obtained from the experimental data and from the Coffin-Manson method.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.41 no.8
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• pp.765-770
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• 2017

GTD111 DS of nickel base superalloy has been used for gas turbine blades. In this study, low cycle fatigue test was conducted on the GTD111 DS alloy by setting conditions similar to the real operating environment. The low cycle fatigue tests were conducted at room temperature, $760^{\circ}C$, $870^{\circ}C$, and various strain amplitudes. Test results showed that fatigue life decreased with increasing total strain amplitude. Cyclic hardening response was observed at room temperature and $760^{\circ}C$; however, tests conducted at $870^{\circ}C$ showed cyclic softening response. Stress relaxation was observed at $870^{\circ}C$ because creep effects occurred from holding time. A relationship between fatigue life and total strain range was obtained from the Coffin-Manson method. The fratography using a SEM was carried out at the crack initiation and propagation regions.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.2
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• pp.252-256
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• 1989

In the low cycle fatigue fracture testing with KS(or JIS) SS41, crack growth rate, AE count rate and J-integral range are measured to get empirical relations between crack growth rate and J-integral range, AE count rate and J-integral range as well as AE count rate and crack growth rate. All the relations are shown to be linear on the log-log graphs. It is also shown that the linear relations can be formulated by using Dunegan's assumption and elastic-plastic fracture mechanics along with the well-known relation of crack growth rate and J-integral range. It is concluded that the differences between experimental and theoretical values are due to Dunegan's assumption.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.10
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• pp.1419-1425
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• 2010

Boron steel sheet is suitable for fabricating automobile parts because it is very strong and has low weight. Recently, many car makers are investigating the feasibility of fabricating the chassis part of automobiles using boron steel. In order to use boron steel sheets to fabricate the chassis part of automobiles, much better material property of low cycle fatigue life as well as high formability during hot stamping is required. Therefore, the low-cycle fatigue life of hot-stamped quenched boron steel was investigated in this study. The fatigue life observed at low strain amplitude was longer than that of an as-received boron steel sheet. However, the fatigue life reduced at high strain amplitude because of the low ductility and low fracture toughness of martensite, which was produced as a result of hot stamping.