응력비의 영향을 고려한 표면피로균열의 균열성장식

강용구(Yong-Goo Kang),김대석(Dea-Suk Kim) 한국해양공학회 1998 韓國海洋工學會誌 Vol.12 No.1

In this work, fatigue tests by axial loading were carried out to investigate the effect of stress ratio on the growth behaviors of surface fatigue crack for SM45C steel and Al 2024-T4 alloy. The growth behaviors of surface crack have been monitored during fatigue process by measuring system attached CCTV and monitor. When the growth rates of surface crack were investigate by the concept of LEFM based on Newman-Raju's ΔK, the dependence of stress ratio appears both SM45C steel and Al 2024-T4 alloy.<br/> Therefore, modified stress intensity factor range, ΔK [=(1+R)<SUP>n</SUP>ΔK] are introduced to eliminate the dependence of stress ratio.<br/> Using ΔK, it is found that the dependence of stress ratio disappears both SM45C steel and Al 2024-T4 alloy.

암반 응력조건에 따른 원형수직구 굴착시의 응력경로 및 응력모드 분석

박상찬,문현구 한국자원공학회 2014 한국자원공학회지 Vol.51 No.6

수직구는 심도에 따른 초기응력 조건이 계속 변화하는 특징을 갖는다. 암반내 다양한 초기응력 조건을측압계수와 수평응력비를 이용한 초기주응력모드로 분류하고 초기주응력비가 갖는 응력상태의 특징을 분석하였다. 굴착으로 재형성되는 수직구 벽면의 2차주응력을 크기와 방향에 따라 2차주응력모드로 분류하고, 원형공동에 대한 이론해와 3차원 수치해석법을 이용하여 다양한 초기응력 조건에서의 원형수직구 굴착에 따른 응력경로와 응력모드 분석을 수행하였다. 주응력 평면에서의 응력경로 분석을 통하여 초기주응력모드와 2차주응력모드와의 상관성을 평가하였다. 또한, 암반조건과 보강조건이 수직구 굴착시의 응력경로와 응력모드에 미치는영향을 분석하였다. In-situ stresses change continuously as the depth of the shaft increases. Various in-situ stress conditions are classified using In-situ principal stress mode defined by the ratio of horizontal to vertical stress and the ratio of horizontal stresses. And the characteristics of in-situ principal stress ratio relating to stress condition are analysed. Secondary principal stress mode is used to classify the principal stresses states acting on the shaft wall after excavation according to the magnitude and direction of the principal stresses. And stress-path and stress-mode analyses are performed by using analytic solution and 3-dimensional numerical method for a circular-shaped shaft excavation with various in-situ stress conditions. With analysis of stress-path, correlation between In-situ principal stress mode and Secondary principal stress mode is estimated. Also, the effects of rock mass condition and support state on the stress-path and stress-mode during the shaft excavation are analysed.

Aluminum Alloy 용접재의 피로균열성장거동 연구

정영석(Young-Seok Chung),최용식(Yong-Shick Choy),이환우(Hwan-Woo Rhee) 한국해양공학회 1989 韓國海洋工學會誌 Vol.3 No.1

The fatigue crack growth behavior in a type Al-Alloy 5052-H38 welded joint was investigated. In particular experimental results on the effects of redistributed compressive residual stress over various stress(R=0.l, 0.3, 0.5) on the mode I fatigue crack growth behavior were reviewed. At the low ΔK region, the increased ΔK compared with ΔKe?? at the same da/dN in the welds is attributed to compressive residual stress introduced by welding. This effect was analyzed in the present paper by means of superposition concept of stress intensity factor whereby the applied stress intensity factor range was decreased to some lower effective value. As a rule, the value of Kres decreased gradually with increasing ΔKapp values, which is most pronounced at lower ΔK values. This behavior is attributable to a redistribution of residual stress in the specimen as the crack advances, and to residual stress relaxation. The higher applied tensile mean load(that is, higher values or the stress ratio, R)diminished the influence of compressive residual stress.

용접 잔류응력장 중에서의 Aluminum-Alloy 용접재의 피로균열 성장거동 연구(Ⅱ) : 잔류응력 재 분포 Redistribution of the Residual Stress

최용식,정영석 成均館大學校 科學技術硏究所 1988 論文集 Vol.39 No.1

It is well known that welding residual stress has a great influence on fatigue crack growth rate in welded structure. In the general area of fatigue crack growth in the presence of residual stress, it is noted that the correction of stress intensity factor(K) to account for residual stress is important for the determination of both stress intensity factor range(ΔK) and stress ratio (R) during a loading cycle. The superposition technique can be applied generally for determination of K. The crack growth rate of the welds correlated with the effective stress intensity factor range, ΔK_eff, which was estimated by superposition of the respective stress intensity factors for the residual stress fields and for the applied stresses. However, redistribution of residual stress occurs during crack growth and its effect is not negligible. In this study, fatigue crack growth characteristics of the weld joint were examined by using superposition of redistributed residual stress and examined in comparison with that of the initial residual stress superposition.

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.

잔류응력으로 인한 응력비 변화를 고려한 U자형 구조의 피로특성 평가

김상영(Sang Young Kim),구재민(Jae Mean Koo),석창성(Chang Sung Seok),모진용(Jin Yong Mo) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11

Mechanical structures with power sources experience repeated force produced by motors. As a result, the life of the pipes reduces and ultimately, the pipes collapse. In most cases, the U-shape pipe is made from a straight pipe by complicated bending work. During this work process, plastic deformation of the pipe produces residual stress in the pipe. This residual stress significantly affects the fracture behavior of the pipe and induces the change of the stress ratio (min. stress/Max. stress = R). For this reason, residual stress has to be evaluated. In this paper, the residual stress of a U-shaped pipe was evaluated by FEM analysis. In addition, fatigue tests of the U-shaped pipe were performed by using a uniaxial fatigue testing machine. The results of the fatigue test were modified with the results of FEM (Finite Element Method) analysis for residual stress. The modified fatigue test results of the U-shaped pipe were compared with those of a straight pipe.

압력용기용 강의 응력비에 따른 저온 피로균열 진전특성에 관한 연구

박경동(KYUNG-DONG PARK),하경준(KYUNG-JUN HA),박형동(HEYOUNG-DONG PARK) 한국해양공학회 2001 韓國海洋工學會誌 Vol.15 No.3

In this study, CT specimens were prepared from Pressure Vessel Steel which was used for pressure vessel plates for room and low temperature service. And we got the following characteristics from fatigue crack growth test carried out in the environment of room and low temperature at 25℃, -30℃, -60℃, -80℃, -100℃ and -120℃ and in the range of stress ratio of 0.05 and 03 by means of opening mode displacement. At the constant stress ratio, the threshold stress intensity factor range △Kth in the early stage of fatigue crack growth (Region Ⅰ) and stress intensity factor range △K in the stable of fatigue crack growth (Region Ⅱ) was increased in proportion to descend temperature. It assumed that the fatigue resistance characteristics and fracture strength at low temperature is considerable higher than that of room temperature in the early stage and stable of fatigue crack growth region. The straight line slope relation of logarithm da/dN -△K in Region Ⅱ, that is, the fatigue crack growth exponent m increased with descending temperature at the constant stress ratio. It was assumed that the fatigue crack growth rate da/dN is rapid in proportion to descending temperature in Region Ⅱ and the cryogenic-brittleness greatly affect a material with decreasing temperature.

SB41용접재의 피로크랙 진전거동에 관한 연구

최병기,장경천 한국공작기계학회 2002 한국생산제조학회지 Vol.11 No.2

SB41 material is welded automatically and is investigated some effects of the welding residual stress in 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 ration is changing to 0.1, 0.33 and 0.5 the propagation life is constant but the growth life decreases. And than, when maximum load or minimum load is constant, as the stress ration increases the growth life and propagation life. 2) It was shown that the fatigue crack propagation ratio da/dN was almost constant regardless of the stress ration 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.

스프링강의 피로파괴에 미치는 압축잔류응력의 영향

진영범(Young-Beom Jin),박경동(Keyung-Dong Park) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.4

Recetly the steel parts used for automiles and trains are required to be used under higher stress than ever before in need of the weight down. However, there are a lot of problems with developing such materials that require expensive tools, and a great deal of time and effort. Therefore, the improvement of fatigue strength and fatigue life are mainly focused on by adopting residual stress. And got the following characterisics from crack growth test carried out stress ratio. Fatigue life shows more improvement in the Un-peening material. And compressive residual stress of surface on the Shot-peening processed operate resistance force of fatigue. So we cam obtain fallowings. (1) The fatigue crack growth rate on stage Ⅱ is conspicuous with the size of compressive residual stress and is dependent on Paris equation. (2) Although the maximum compressive residual stress is deeply and widely formed from surface, fatigue life does not improve than when maximum compressive residual stress is formed in surface. (3) The threshold stress intensity factor range is increased with increasing compressive residual stress.

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.