A Study on the Damage Behavior and Life Estimation of High Strength Materials under Creep and Corrosion Fatigue Loading Conditions

Abstract

In this dissertation, the unified modeling methodology for materials behavior and life estimation was investigated for high strength materials under complex loading conditions. A continuum damage mechanics based scheme in large deformation was proposed, and two different engineering cases were employed to verify the model. Firstly, a continuum damage mechanics based creep-fatigue interacted life estimation procedure of the turbine disk material, namely, Nickel-based super-alloy working under high-speed rotation and high temperature was investigated. Secondly, a corrosion fatigue behavior of the structural material of high performance steel which is applicable to very long span bridges on the seaside environment was investigated by using a corrosion-fatigue interacted regime. Further investigations on this case, the corrosion-fatigue crack propagation behaviors and its modeling procedure were also presented. Firstly, in the modeling of the creep-fatigue combined behavior for Waspaloy, creep and low cycle fatigue tests were conducted at high temperature environment. Here, the visco-plasticity behavior together with complex isotropic-kinematic hardening for the material was analyzed. Furthermore, a continuum damage mechanics based creep-fatigue interacted life prediction equation was developed and implemented into the constitutive model. The interaction laws between creep and fatigue damage were also discussed with linear and nonlinear interaction rules. The material parameters used in the model were obtained from the experiments by using least square method through a curve fitting technique. Finally, using the developed unified model, the cyclic stress-strain behaviors of the material were properly predicted. Furthermore, the proportion of creep and fatigue damages in the low cycle fatigue was evaluated quantitatively. Secondly, the corrosion fatigue behavior of the high performance steel for bridges, namely HSB800 was investigated by using cycles to failure fatigue test in air and seawater environments. Three different types of materials based on the HSB800 such as base metal (BM), coarse-grained (CG) heat affected zone (HAZ) and inter-critically reheated coarse-grained (ICCG) heat affected zone were considered in this work because the structural material generally used in these types on conventional applications. The uniaxial cyclic tension-unloading tests were conducted for the fatigue test, and S-N curves (W?hler curve) were obtained for the material types. As loading conditions, levels of stress range were set to 600~950 MPa with the load ratio of R=0 (minimum stress was zero) in air and seawater environments. A corrosion-fatigue damage model which enabling the life estimation was then developed by considering the corrosion mechanisms. By comparing the experimentally obtained fatigue and corrosion fatigue lives with the predicted lives using the developed equations, the accuracy of the model was validated. Furthermore, the three point single edge notched bending (3-SENB) fatigue tests were conducted for the material types in air and seawater environments as the fatigue crack propagation and the corrosion fatigue crack propagation tests, respectively. The fatigue crack propagation rate graphs were obtained in each test. A fracture model which widely describes the fatigue crack propagation behavior including the effects of load ratio, frequency and environment, and also covering the overall regions of the behavior was proposed by modifying the conventional models such as Paris and Forman equations. By comparing the fatigue crack propagation behavior and the life predictions of the developed equation with the several conventional equations to the experimental results in every case, the accuracy of the developed model was validated.|본 논문에서는 복합하중 조건 고강도 소재의 재료거동 및 수명예측 모델링 기술에 관한 연구를 수행하였다. 대변형 이론과 손상역학을 기반으로 한 모델링 기술을 제시하고자 하였으며 이의 검증을 위해 두 가지 형태의 복합하중 조건을 사용하였다. 우선, 고속-고온의 복합하중에서 사용되는 항공기용 엔진 핵심부품인 터빈 디스크 소재의 변형거동 및 수명예측을 위해 손상역학 기반 크리프-피로 모델링 기술이 연구되었으며, 아울러 해수환경에서 사용되는 고성능강재의 거동 및 수명예측을 위해 부식-피로 모델링 기술을 개발하였다. 아울러, 동종 소재의 부식피로 균열전파거동에 관한 연구를 수행하였다. 첫 번째, 크리프-피로 모델링 연구에서 터빈 디스크 소재는 니켈기 초내열합금인 Waspaloy가 사용되었으며, 고온에서 크리프 시험 및 저주기 피로시험을 실시하였다. 등방경화와 이동경화 모델을 적용하여 탄-점소성 재료거동 모델을 개발하였으며, 손상역학 기반 크리프-피로 수명예측 모델과 통합하였다. 크리프와 피로 손상의 상호작용은 선형 및 비선형 모델을 비교, 분석하였다. 개발된 모델에 사용된 재료상수는 크리프시험 및 피로시험의 결과로부터 최소자승법을 통해 도출하였다. 개발된 재료거동-손상 통합모델을 이용하여 저주기 피로시험에서 나타난 소재의 반복 응력-변형률 거동을 비교적 정확히 예측하였으며 아울러 피로수명도 예측하였다. 나아가, 저주기 피로시험의 다양한 시험조건에서 나타나는 크리프 및 피로손상의 누적 경향을 비교 분석하였다. 두 번째, 교량용 고성능강재인 HSB800 소재의 부식피로거동 연구를 위해 공기조건과 해수환경조건을 설정하였다. 피로시험은 균열발생과 전파거동을 거쳐 파단에 이를 때까지 실시하였다. 실제 현장에서 사용되는 소재특성의 고려를 위해 HSB800 모재 및 용접열영향부 (HAZ) 두 종을 포함하여 모두 세가지 소재가 사용되었으며 각 소재에 대한 피로시험을 통해 S-N 선도를 도출하였다. 하중조건은 600~950MPa 기준으로 하였으며 응력비는 0으로 설정하였고 공기조건과 해수조건으로 구분하여 시험을 실시하였다. 부식손상 및 피로손상 모델을 접목하여 부식피로손상 모델을 개발하였으며 이를 통해 수명예측을 실시하였다. 피로수명과 부식피로수명에 대한 실험결과와 예측결과를 비교하여 개발된 모델을 검증하였다. 아울러 3점굽힘 피로시험을 통해 공기조건과 해수환경에서의 피로균열전파 거동을 고찰하였다. 이를 통해 각 조건에서의 피로균열전파속도 선도를 구하였으며, 부식메커니즘을 적용하여 Paris 및 Forman 모델을 수정하여 응력비, 하중주기, 환경 효과를 포함하고 균열전파영역을 총괄적으로 모사할 수 있는 피로 및 부식피로 균열전파모델을 개발하였다. 이를 통해 피로 및 부식피로균열전파속도를 비교적 정확히 예측할 수 있었으며 여타 모델과의 비교를 통해 정확성을 입증하였다.; In this dissertation, the unified modeling methodology for materials behavior and life estimation was investigated for high strength materials under complex loading conditions. A continuum damage mechanics based scheme in large deformation was proposed, and two different engineering cases were employed to verify the model. Firstly, a continuum damage mechanics based creep-fatigue interacted life estimation procedure of the turbine disk material, namely, Nickel-based super-alloy working under high-speed rotation and high temperature was investigated. Secondly, a corrosion fatigue behavior of the structural material of high performance steel which is applicable to very long span bridges on the seaside environment was investigated by using a corrosion-fatigue interacted regime. Further investigations on this case, the corrosion-fatigue crack propagation behaviors and its modeling procedure were also presented. Firstly, in the modeling of the creep-fatigue combined behavior for Waspaloy, creep and low cycle fatigue tests were conducted at high temperature environment. Here, the visco-plasticity behavior together with complex isotropic-kinematic hardening for the material was analyzed. Furthermore, a continuum damage mechanics based creep-fatigue interacted life prediction equation was developed and implemented into the constitutive model. The interaction laws between creep and fatigue damage were also discussed with linear and nonlinear interaction rules. The material parameters used in the model were obtained from the experiments by using least square method through a curve fitting technique. Finally, using the developed unified model, the cyclic stress-strain behaviors of the material were properly predicted. Furthermore, the proportion of creep and fatigue damages in the low cycle fatigue was evaluated quantitatively. Secondly, the corrosion fatigue behavior of the high performance steel for bridges, namely HSB800 was investigated by using cycles to failure fatigue test in air and seawater environments. Three different types of materials based on the HSB800 such as base metal (BM), coarse-grained (CG) heat affected zone (HAZ) and inter-critically reheated coarse-grained (ICCG) heat affected zone were considered in this work because the structural material generally used in these types on conventional applications. The uniaxial cyclic tension-unloading tests were conducted for the fatigue test, and S-N curves (W?hler curve) were obtained for the material types. As loading conditions, levels of stress range were set to 600~950 MPa with the load ratio of R=0 (minimum stress was zero) in air and seawater environments. A corrosion-fatigue damage model which enabling the life estimation was then developed by considering the corrosion mechanisms. By comparing the experimentally obtained fatigue and corrosion fatigue lives with the predicted lives using the developed equations, the accuracy of the model was validated. Furthermore, the three point single edge notched bending (3-SENB) fatigue tests were conducted for the material types in air and seawater environments as the fatigue crack propagation and the corrosion fatigue crack propagation tests, respectively. The fatigue crack propagation rate graphs were obtained in each test. A fracture model which widely describes the fatigue crack propagation behavior including the effects of load ratio, frequency and environment, and also covering the overall regions of the behavior was proposed by modifying the conventional models such as Paris and Forman equations. By comparing the fatigue crack propagation behavior and the life predictions of the developed equation with the several conventional equations to the experimental results in every case, the accuracy of the developed model was validated.