• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2001년도 춘계학술대회논문집A
• /
• pp.169-174
• /
• 2001

Low cycle fatigue tests are performed on high strength low alloy steels that be developed for submarine material. The relation between absorbed plastic strain energy and numbers of cycle to failure is examined in order to predict the low cycle fatigue life of structural steels by using plastic strain energy method. The cyclic properties are determined by a least square fit techniques. The life predicted by the plastic strain energy method is found to coincide with experiment data and results obtained from the Coffin-Manson method. Also the cyclic behavior of structural steels is characterized by cyclic softening with increasing number of cycle at room temperature. Especially, low cycle fatigue characteristics and microstructural changes of structural steels are investigated according to changing tempering temperatures. In the case of PFS steels, the $\varepsilon$-Cu is formed in 550C of tempering temperature and enhances the low cycle fatigue properties.

• Steel and Composite Structures
• /
• 제43권4호
• /
• pp.465-481
• /
• 2022

This paper presents a comprehensive integrity assessment of welded structural components, including uniform high- and low-cycle fatigue assessment of welded plate joints and fatigue-induced fracture assessment of welded plate joints. This study reports a series of fatigue and fracture tests of welded plate joints under three-point bending. To unify the assessment protocol for high- and low-cycle fatigue of welded plate joints, this study develops a numerical damage assessment framework for both high- and low-cycle fatigue. The calibrated damage material parameters are validated through the smooth coupon specimens. The proposed damage-based fatigue assessment approach describes, with reasonable accuracy, the total fatigue life of welded plate joints under high- and low-cycle fatigue actions. Subsequently, the study performs a tearing assessment on the ductile crack extension of the fatigue-induced crack. The tearing assessment diagram derives from the load-deformation curve of a single-edge notched bend, SE(B) specimen and successfully predicts the load-crack extension relation for the reported welded plate joints during the stable tearing process.

• 한국안전학회지
• /
• 제28권3호
• /
• pp.18-22
• /
• 2013

The Co-base super heat resisting alloy ECY768 is employed in gas turbine because of its high temperature strength and oxidation resistance. The prediction of fatigue life for superalloy is important for improving the efficiency. In this paper, low cycle fatigue tests are performed as variables of total strain range and temperature. The relations between strain energy density and number of cycle to failure are examined in order to predict the low cycle fatigue life of ECY768 super alloy. The lives predicted by strain energy methods are found to coincide with experimental data and results obtained from the Coffin-Manson method. The fatigue lives is evaluated using predicted by Coffin-Manson method and strain energy methods is compared with the measured fatigue lives at different temperatures. The microstructure observing was performed for how affect able to low-cycle fatigue life by increasing the temperature.

• 한국안전학회지
• /
• 제33권3호
• /
• pp.8-14
• /
• 2018

Ni-base superalloy has excellent resistance to extreme environments such as high temperatures and high stresses and are used as materials for large gas turbines. In this paper, the specimens were taken from the blade that were used for a long time, and their life span was studied by microstructure analysis and avoidance of cursing. The microstructural analysis of the specimens was carried out using a OM and SEM to observe the coarsening, carbides on gamma prime. Low-cycle fatigue tests were performed on new material and airfoil of long time-used blade. The test was conducted under various deformation conditions and temperature conditions of $760^{\circ}C$ and $870^{\circ}C$. The low cycle fatigue test was carried out using the Coffin-Manson equation and the fatigue life was predicted. After the test, crack path and fracture surface were analyzed using SEM.

• 소성∙가공
• /
• 제18권4호
• /
• pp.296-303
• /
• 2009

The effect of porosity on the high-cycle fatigue properties of Al-Si-Mg casting aluminum alloys was investigated in this study. Microstructure examination, tensile and high-cycle fatigue test were conducted on both Al-Si-Mg casted (F) and heat-treated (T6) conditions. Porosity characteristics on the fracture surfaces of fatigue-tested samples were examined using SEM and image analysis. The microstructure observation results showed that eutectic Si particles were homogeneously dispersed in the matrix of the Al-Si-Mg casting alloys, but there were porosities formed as cast defects. The high-cycle fatigue results indicated that the fatigue strength of the 356-T6 alloy was higher than that of the 356-F alloys because of the significant reduction in volume fraction of pores by heat treatment. The SEM fractography results showed that porosity affected detrimental effect on the fatigue life: 80% of all tested samples fractured as a result of porosity which acted as the main crack initiation site. It was found that fatigue life decreased as the size of the surface pore increased. A comparison was made between surface pore and inner pore for its effect on the fatigue behavior. The results showed that the fatigue strength with the inner pores was higher than that of the surface pore.

• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 2009년도 춘계학술대회 논문집
• /
• pp.350-352
• /
• 2009

The effect of porosity on the high-cycle fatigue properties of Al-Si-Mg casting aluminum alloys was investigated in this study. Microstructure examination, tensile and high-cycle fatigue test were conducted on both Al-Si-Mg casted (F) and heat-treated (T6) conditions. Porosity characteristics on the fracture surfaces of fatigue-tested samples were examined using SEM and image analysis. The microstructure observation results showed that eutectic Si particles were homogeneously dispersed in the matrix of the Al-Si-Mg casting alloys, but there were porosities formed as cast defects. The high-cycle fatigue results indicated that the fatigue strength of the 356-T6 alloy was higher than that of the 356-F alloys because of the significant reduction in volume fraction of pores by heat treatment. The SEM fractography results showed that porosity affected detrimental effect on the fatigue life: 80% of all tested samples fractured as a result of porosity which acted as the main crack initiation site. It was found that fatigue life decreased as the size of the surface pore increased. A comparison was made between surface pore and inner pore fur its effect on the fatigue behavior. The results showed that the fatigue strength with the inner pores was higher than that of the surface pore.

• 소성∙가공
• /
• 제30권4호
• /
• pp.172-178
• /
• 2021

The cold forging process induces material deformation in an enclosed space, generating a very high forging load. Therefore, it is mainly designed as a multi-stage process, and fatigue failure occurs in forging die due to cyclic load. Studies have been conducted previously to quantitatively predict the fatigue limit of cold forging dies, however, there was a limit to field application due to the large error range and the need for expert intervention. To solve this problem, we conducted a study on the introduction of a real-time forging load measurement technology and an automated system for quantitative prediction of die life cycle. As a result, it was possible to reduce the error range of the quantitative prediction of die life cycle to within ±7%, and it became possible to use the die life cycle calculation algorithm into an automated system.

• 한국표면공학회지
• /
• 제49권6호
• /
• pp.587-594
• /
• 2016

Various surface treatments and thin film coating processes on the surface of injection die steel have been developed to extend the life. Most of previous studies were mainly focused on investigating the wear and static bonding behavior of thin films. In this study complex surface treatments of DLC coating combined with ion nitriding were applied to increase fatigue life and wear resistance. Ion nitriding, DLC coating, and DLC coating following nitriding on the surface of Fe-3.0Ni-0.7Cr-1.4Mn-X steel were investigated to uncover the beneficial effect which is applicable to injection die. The effect of various surface treatments and coating conditions on high cycle fatigue resistance was studied. Surface morphology change during fatigue tests were observed with AFM. Fatigue life of the die steel increased by 10 to 1,000 times at the various level of stress amplitudes in the condition of DLC coating following the ion nitriding for 3 hrs comparing with the only DLC coated condition.

• 한국추진공학회:학술대회논문집
• /
• 한국추진공학회 2009년도 춘계학술대회 논문집
• /
• pp.18-21
• /
• 2009

높은 열하중하에 있는 부품의 수명은 일반적으로 다른 부품에 비해 짧은 수명을 가지고 있다. 액체 로켓의 터보펌프 터빈은 작동시간 동안 높은 회전속도로 인한 높은 원심력과 높은 온도와 같은 환경하에서 작동된다. 이와 같은 환경은 터보펌프 터빈의 저주기 피로를 야기한다. 우선 열응력을 해석하기 위해 ABAQUS/CAE가 사용되었으며 탄성변형률과 소성변형률을 고려하기 위해 Coffin-Manson 방정식을 사용하였다. 평균응력의 변화를 고려하기 위해 S.W.T법을 사용하였으며, 열응력해석 결과로 얻어진 변형률 이력을 이용하여 터보펌프 터빈의 취약지점에 저주기피로해석을 수행하였다. 본 연구에서는 저주기 수명을 해석하기 위해 변형룰 수명 방법이 적용되었다.

• Tribology and Lubricants
• /
• 제22권6호
• /
• pp.320-328
• /
• 2006

In this study, the simulation of rolling contact fatigue based on stress analysis is conducted under Elastohydrodynamic Lubrication state. To predict a crack initiation life accurately, it is necessary to calculate contact stress and subsurface stresses accurately. Contact stresses are obtained by contact analysis of a semi-infinile solid based on the use of influence functions and the subsurface stress field is obtained using rectangular patch solutions. And a numerical algorithm using Newton-Rapson method was constructed to calculate the Elastohydrodynamic lubrication pressure. Based on these stress values, several multiaxial high-cycle fatigue criteria are used and the critical loads corresponding to fatigue limits are calculated.