small artificial defect(미소인공결함)

이승용,정재강,홍석주 朝鮮大學校 機械技術硏究所 1998 機械技術硏究 Vol.1 No.1

본 실험에서는 자동차의 전륜 구동축재(S43C, S50C)에 대해서 미소 원공결함을 가공한 비열처리재와 고주파열처리재의 피로시험편에 회전굽힘응력을 가하여 피로강도와 피로크랙의 발생 및 전파거동에 관하여 실험한 결과 다음과 같은 결론을 내렸다. 비열처리 S43C재 및 S50C재의 피로한도는 각각 98.1MPa, 107.9MPa이었고, 열처리재의 경우 각각 147MPa, 156MPa로서 열처리재는 비열처리재보다 약 50Mpa 증가하였다. 응력이 피로한도의 110%에서 120%로 증가하는 경우 비열처리인 S43C재 및 S50C재에서 피로수명의 감소율은 각각 약 49%, 약 45%였으며 또한 열처리재의 경우는 각각 약29%, 약 22%로서 비열처리재보다 열처리재가 피로수명 감소율이 낮게 나타났다. 평활재에서 표면 미시크랙의 발생은 비열처리재인 S43C 및 S50C에서는 수명비(N/Nf) 0.2정도에서 처음 관찰되었으며, 동일한 수명비에서 관찰된 미시크랙의 수는 S43C가 많았고, 평균 크랙길이는 S50C가 길었다. 피로크랙길이와 파단수명비의 관계 log(2a)=A+B(N/Nf)식에 본 실험재료를 적용한 결과 응력수준에 관계없이 잘 일치하였다 In this paper, an experiment to investigate the behavior of fatigue crack propagation in induction hardened structural carbon steel(S43C, S50C) with artificial defect are conducted by the rotary bending fatigue test in the room temperature. The summarized results are as follows: The fatigue limits obtained from experiments of as-received S43C and S50C were 98.1MPa and 107.9MPa respectively. The fatigue limit of heat-treated materials were 147MPa and 156MPa respectively. It means that the heat-treated specimens showed about 50MPa higher fatigue limits than the as-received material. When the applied stress was increased from 110% to 120% of fatigue limit, the decreasing rate of fatigue life was around 49% and 45% respectably in the as-received S43C and S50C, in case of the heat-treated materials, the decreasing rate of fatigue life was around 29% and 22% respectively. It means that the heat-treated materials had lower decreasing rate of fatigue life than the as-received materials. The initiation of surface micro crack was observed at 0.2 of fatigue life ratio in as-received S43C and S50C. The number of micro cracks in S43C were more than that of S50C, but the average crack length in S50C was longer than that of S43C at the same fatigue life ratio. Experimental results obtained from fatigue test were evaluated with the equation log(2a)=A+B(N/Nr). The relationship between crack length and cycle ratio showed a good agreement to the experimental results.

Mean stress effects in fretting fatigue life estimation method using fatigue damage gradient correction factor

황동현,조성산 대한기계학회 2017 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.31 No.9

In our previous study, we developed a fretting fatigue life estimation method that considers stress gradient effect [Journal of Mechanical Science and Technology 28 (2014) 2153–2159]. In this method, fatigue damage value at the cracking location is corrected with the factor that is a function of fatigue damage gradient, and the corrected value is treated as the fatigue damage value in plain fatigue for life estimation. In the present study, we examined the effect of mean stress on fatigue damage gradient correction function, because the reliability of the developed method was only verified at a stress ratio (R) of −1 in previous studies. Fretting fatigue experiments were conducted to obtain the fatigue life data of three different fretting pad shapes with R values ranging from −1.0 to 0.3. Finite element analyses were then conducted to evaluate the fatigue damage parameter in the cracking region. The results revealed that fretting fatigue life decreases at increased stress ratio. Furthermore, the fatigue damage gradient correction function was unaffected by the stress ratio, although it is affected by plastic deformation at the cracking location. Thus, a correction function for the occurrence of plastic deformation and another for the absence of plastic deformation are necessary. The developed method was demonstrated to predict the fretting fatigue life at various levels of stress ratio with the use of plain fatigue data.

SS41 용접재의 잔류응력장내에서의 피로균열 성장거동에 관한 연구

최병기 한국산업안전학회 2002 한국안전학회지 Vol.17 No.1

SS41 material is welded automatically and is investigated some effects of the welding residual stress on the growth and propagation of fatigue crack, so as to study the fatigue behaviour in the welding residual stress field. The summarized results are as follows; 1) In case of the load amplitude is constant, as the stress ratio is changing to 0.1, 0.33 and 0.5 the propagation life is constant but the initiation life decreases. And than, when maximum load or minimum load is constant, as the stress ratio increases the initiation life and propagation life. 2) It was shown that the fatigue crack propagation ratio da/dN was almost constant regardless of the stress ratio change at constant load amplitude and that the larger stress ratio, the slower was the fatigue crack propagation ratio. 3) The opening ratio U is influenced by K_max but it isn't only the function of K_max because data range is very large. 4) The fatigue life of the specimens on tensile compressive residual stress field was decreased and increased respectably more than that of the base metal.

EFFECTS OF TEMPERATURE AND SHIELDING GAS MIXTURE ON FATIGUE OF ALUMINUM ALLOY

Chung, Jae-Kang 朝鮮大學校 機械技術硏究所 2004 機械技術硏究 Vol.7 No.1

A fatigue crack develops as a result of stress concentration and extends with each load cycle until failure occurs, or until the cyclic loads are transferred to redundant members. The fatigue performance of a member is more dependent on the localized state of stress than the static strength of the base metal or the weld metal. This is a study of fatigue strength of Al5083-O aluminum alloy to investigate influence of fatigue life on welding zone according to the mixing shield gas ratio and the test temperature. The GMAW was carried out for weld specimens with various mixing shield gas ratios (Ar 100% + He0%, Ar67% + He33%, Ar50% + He50%, and Ar33% + He67%). The test temperatures for the fatigue test were +25°C (+77°F), -30℃ (-22°F), -85℃ (-121°F), and -196℃ (-321°F). The welding zone was divided into four sub zones for analysis; weld metal. fusion line, HAZ, and base metal according to the notch position. Regarding the influence on the bead shape of the mixing shield gas ratio, the bead width was greatest in Ar100% + He0% mixture. But the bead depth and area were greatest in Ar33% + He67% mixture. The dilution was also best in the mixing shield gas ratio. In the influence of mixing gas ratio, fatigue life of weld metal specimens decreased with higher argon gas ratio, but fusion line and HAZ specimens were not influenced greatly. Also, the fatigue life increased at lower temperatures, particularly at -196℃.

Changes in the Central Fatigue by Manipulating the Level of Extrinsic Motivation

이재무,이동현,이지항 한국스포츠심리학회 2018 한국스포츠심리학회지 Vol.29 No.3

Purpose: The possibility of interaction between physical and mental fatigues was investigated in this study. Should any interaction between the physical and psychological fatigue takes place, it must accompany the changes in the brain. Therefore, the level of central activation ratio (central fatigue) has been measured as a function of monetary reward to elicit the extrinsic motivation. Methods: Twenty four healthy young male subjects performed quadriceps isotonic extensions until they experience the muscular fatigue. Then, the notification of the monetary reward depending on the level of extension force for the next 3 attempts were given to the experimental group, while no such information was given to the control group. During these post-fatigue isometric maximum voluntary contraction trials, the transcranial magnetic stimulation was delivered on the motor cortex to measure the central activation ratio. Two-way repeated ANOVA were performed. Results: Pre- vs. Post-fatigue comparisons confirmed that the subjects regardless of the group experienced the peripheral fatigue although the level of peripheral fatigue was less pronounced in the group with extrinsic motivation. On the other hand, the central activation ratio of the extrinsic motivation group did not displayed any decrease as the control group did. Conclusion: The results from this study provided an evidence showing the effects of motivation on the muscular fatigue. What’s intriguing is, however, the level of fatigue at the brain level showed no significant sign of fatigue when the monetary reward was suggested. It is presumable to suggest that the high motivation made our subjects prone to the central fatigue, and such effects might have decrease the peripheral muscle fatigue in turn.

Changes in the Central Fatigue by Manipulating the Level of Extrinsic Motivation

( Jae-moo Lee ),( Dong-hyun Lee ),( Ji-hang Lee ) 한국스포츠심리학회 2018 한국스포츠심리학회지 Vol.29 No.3

Purpose: The possibility of interaction between physical and mental fatigues was investigated in this study. Should any interaction between the physical and psychological fatigue takes place, it must accompany the changes in the brain. Therefore, the level of central activation ratio (central fatigue) has been measured as a function of monetary reward to elicit the extrinsic motivation. Methods: Twenty four healthy young male subjects performed quadriceps isotonic extensions until they experience the muscular fatigue. Then, the notification of the monetary reward depending on the level of extension force for the next 3 attempts were given to the experimental group, while no such information was given to the control group. During these post-fatigue isometric maximum voluntary contraction trials, the transcranial magnetic stimulation was delivered on the motor cortex to measure the central activation ratio. Two-way repeated ANOVA were performed. Results: Pre- vs. Post-fatigue comparisons confirmed that the subjects regardless of the group experienced the peripheral fatigue although the level of peripheral fatigue was less pronounced in the group with extrinsic motivation. On the other hand, the central activation ratio of the extrinsic motivation group did not displayed any decrease as the control group did. Conclusion: The results from this study provided an evidence showing the effects of motivation on the muscular fatigue. What’s intriguing is, however, the level of fatigue at the brain level showed no significant sign of fatigue when the monetary reward was suggested. It is presumable to suggest that the high motivation made our subjects prone to the central fatigue, and such effects might have decrease the peripheral muscle fatigue in turn.

Ultrasonic Fatigue Test Method for Stress Ratio Control

Hyo-Ri Kim(김효리),Seung-Hyeon Song(송승현),Ji-U Ha(하지우),Jeong-Eun Lee(이정은),In-Sik Cho(조인식) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11

Ultrasonic Fatigue Testing (UFT) is an accelerated fatigue test, typically at 20 kHz, which significantly shortens the application period of newly developed processes and materials in various mechanical parts and materials fields. In this paper, the system technology for controlling the stress ratio in ultrasonic fatigue testing is to be explained. This fatigue stress ratio control test is important for fatigue testing of aviation and power plant turbine parts, and is also an important method for crack propagation testing. Basically, an ultrasonic fatigue tester designed by applying the resonance analysis results to a 20KN tensile tester was attached to set the average stress and then enabled it at various stress ratios. Also, as a method of setting the corrected fatigue stress, the method of measuring the strain gauge at the center of the specimen and the method of simultaneously measuring the amplitude of the end of the specimen are described. Therefore, the results of this paper are intended to explain the construction of an ultrasonic fatigue tester optimized to suit various fatigue conditions and environments.

피로파면의 기하학적 형상을 바탕으로 한 응력진폭 변화와 혼합모드 하중관계에 관한 연구

서기정(Kijeong Seo),송삼홍(Samhong Song),이정무(Jeongmoo Lee) 한국자동차공학회 2004 한국자동차공학회 춘 추계 학술대회 논문집 Vol.- No.-

Most cracks in the structure occur under mixed mode loading and those fatigue crack propagation behavior depends on the stress ratio. Hence. it is necessary to study the fatigue behavior under mixed-mode loading as the stress ratio changes. In this paper, the fatigue crack behavior was respectively investigated at stress ratio 0.33, 0.50 and we changed the loading application angle into 0°, 30°, 45°, 60° to apply various loading mode. These experimental results were evaluated by the shape of fatigue fracture surface using 2D or 3D images obtained from the C-scan method. As stress ratio and mode mixity change, the change of effective stress intensity factor and crack closure have effect on the fatigue behavior of cracks. The variation of fatigue crack behavior caused by the change of stress ratio and loading application angle could be explained by using the shape of fatigue fracture surface.

순수티타늄 용접재의 피로수명 예측에 관한 연구

최병기,국중민,장경천 朝鮮大學校 機械技術硏究所 2001 機械技術硏究 Vol.4 No.1

본 연구에서는 순수티타늄를 TIG용접하여 모재와 용접재의 피로시험을 수행하여 피로크랙 진전거동을 고찰하고, Nisitani가 제안한 역학적 변수에 의해 재료상수를 구한 후 그 적용성을 검토해 보았으며 얻어진 결과는 다음과 같다. 1. 동일한 하중비 조건에서 피로수명은 모재에 비해 Specimen-A, B, C는 각각 약 481%, 751%, 562% 크게 조사되었다. 2. 동일한 조건의 시험편에서 피로수명은 하중비 0.01에 비해 0.1과 0.2는 평균적으로 129%, 180% 크게 나타났다. 3. 모재 시험편를 제외한 용접재 시험편에서 피로크랙이 3mm 진전되는데 소모된 반복수는 전체피로수명의 약 90%를 차지하였으며, 이후에는 급속파단이 일어났다. 4. 본 재료에 실험으로부터 얻어진 재료상수를 이용하여 Nisitani의 피로크랙 전파속도 예측식에 적용하여 피로수명을 예측한 결과 노치의 위치 및 하중비에 관계없이 약 5% 내외의 낮은 오차를 보이며 잘 일치함을 알 수 있었다. In this study, the fatigue test was carried out with pure titanium and TIG welded material using in chemical plants and airplane frames etc.. The fatigue the propagation behavior and the fatigue life were investigated at 190% (400 kgf) of fatigue limit of base metal. Then, we compared and reviewed the result of experimented values and predicted values with the fatigue crack propagation equation reported by Nisitani in the present study. The summarized results are as follows; 1. The fatigue life of specimen B was showed to be about 751% longer than that of the base material at same load ratio. 2. The average fatigue lives at R=0.1 and 0.2 were respectably showed to be about 129% and 180% longer than that at R=0.01 at the same condition. 3. The fatigue life of the welded specimen was consumed about 90% of the whole fatigue life to grow 3mm fatigue crack length and then, the rupture was occurred rapidly. 4. The fatigue lives were predicted with the formula of the fatigue crack propagation, da/dN = C_2a^p_2, and compared with the experimented results, the results were shown good and in agreement within 5.0% error regardless of the notch position.

Fatigue Crack Growth Evaluation of IMO Type B Spherical LNG Cargo Tank Considering the Effect of stress ratio and load history

이명섭,김명현 대한용접·접합학회 2022 대한용접·접합학회지 Vol.40 No.1

The crack propagation on a spherical IMO Type B LNG tank deployed on a 150K class LNG carrier is studied. IMO Type B tanks require a high level of safety . Therefore, analyses such as strength evaluation, fatigue analysis, fatigue crack propagation analysis, and LNG leak rate should be conducted to ensure safety. Various fatigue crack growth models were proposed for fatigue crack propagation analysis. The Forman model and the Walker model con- sider the stress ratio. Furthermore, the Huang model, and the Lee and Kim model consider the stress ratio and the ef- fects of overload and underload present in the past load history on crack propagation. To examine the effects of stress ratio and load history for the fatigue crack propagation models, the crack growth of the LNG tank was evaluated by considering the stresses under full load and ballast load operating conditions, and three stress amplitude sequence cases.