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고온피로균열성장 수명평가 모델 개선을 위한 크리프-역전 변수 도입
마영화(Young-Wha Ma),백운봉(Un-Bong Baek),윤기봉(Kee-Bong Yoon) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.8
Crack growth at elevated temperatures becomes an important consideration in estimating the remaining life, in determining the operating conditions and in deciding the inspection criterion and intervals. Fracture mechanics approaches that have been developed over the past few decades have proven useful in providing a rigorous framework for accomplishing these goals. In this paper, for improvement of creep-fatigue crack growth behavior model, it was performed to study for creep-reversal parameter measurement. Because conventional models to evaluate values of (Ct)avg become unuseful for creep-ductile materials with expensive ranges of the effect of cyclic plasticity for creep deformation. creep-fatigue crack growth tests for CR(creep-reversal parameter) measurements were tested by using trapezoidal loading waveforms with hold times of 0, 60, 600 and 3600 seconds. Measurements of the load-line deflection change and degree of reversal during hold periods were made using an elevated temperature clip-on capacitance gage. These reversed deflection changes were then used to calculate the experimental values of CR without any assumptions with regards to the creep deformation behavior of the materials or effects of cyclic plasticity. thus, The experimentally defined creep-reversal parameter, CR, will provide a direct means to accurate calculating (Ct)avg.
횡방향으로 등방성인 재료에서 균열선단 크리프 변형 거동
마영화(Young Wha Ma),윤기봉(Kee Bong Yoon) 대한기계학회 2009 大韓機械學會論文集A Vol.33 No.12
Theoretical mechanics analysis and finite element simulation were performed to investigate creep deformation behavior at the crack tip of transversely isotropic materials under small scale creep (SCC) conditions. Mechanical behavior of material was assumed as an elastic-2<SUP>nd</SUP> creep, which elastic modulus (E), Poisson’s ratio (ν) and creep stress exponent (n) were isotropic and creep coefficient was only transversely isotropic. Based on the mechanics analysis for material behavior, a constitutive equation for transversely isotropic creep behavior was formulated and an equivalent creep coefficient was proposed under plain strain conditions. Creep deformation behavior at the crack tip was investigated through the finite element analysis. The results of the finite element analysis showed that creep deformation in transversely isotropic materials is dominant at the rear of the crack-tip. This result was more obvious when a load was applied to principal axis of anisotropy. Based on the results of the mechanics analysis and the finite element simulation, a corrected estimation scheme of the creep zone size was proposed in order to evaluate the creep deformation behavior at the crack tip of transversely isotropic creeping materials.
마영화(Young-Wha Ma),이지원(Ji-Won Lee),구지호(Ji-Ho Gu),송전영(Jeon-Young Song),이연수(Yeon-Soo Lee) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11
An experimental study was conducted on the evaluation of fatigue crack growth behavior under various temperatures ranging from room temperature to 950 °C for a polycrystalline nickel-based superalloy, which was produced by investment casting. From the results, the fatigue crack growth rate showed a tendency to increase with increasing temperature up to 800 ℃. However, the crack growth rate at 950 °C was lower than that at 800 °C. The crack growth behavior was significantly fluctuated, and the crack front showed an irregular growth amount, and it grew as a zigzag-shaped slanted crack. This is considered to be because cracks grew along the slip plane with different directions for each grain.
등통로각압축법으로 성형가공된 알루미늄 2024 합금의 이방성 강도 평가
마영화(Young Wha Ma),윤기봉(Kee Bong Yoon) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5
To assess anisotropic tensile strength of material produced by ECAP, small punch (SP) testing method was adopted as a substitute for the standard uniaxial tensile test because ECAP processing sample is usual1y limited to approximately 10 ㎜ length in the transverse direction perpendicular to the direction of ECAP extrusion. SP tests were performed with specimens taken out along both the longitudinal and transverse directions of over-aged aluminum 2024 alloy subjected to three ECAP passes at 150℃. Failure surfaces of SP specimens tested showed that failure was caused by shear deformation and the ECAP processed material had strength anisotropy. Based on the aspect of the failure surface, a method of assessing tensile strength was proposed.