Papers by Keyword: Creep Damage

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Authors: Sang Guk Lee
Abstract: In this paper, artificial creep degradation test and ultrasonic measurement for their creep degraded specimens (Cr-Mo alloy steel) were carried out for the purpose of evaluation for creep damage. Absolute measuring method of quantitative ultrasonic measurement for material degradation was established, and long-term creep degradation tests using creep life prediction formula were carried out. As a result of ultrasonic tests for crept specimens, we conformed that both the sound velocity decreased and attenuation coefficient linearly increased in proportion to the increase of creep life fraction (fc). In frequency and noise analysis, it was conformed that the high frequency side spectra and central frequency components shift to low frequency band, and bandwidths decrease as increasing creep damage in backwall echo. And also, the ultrasonic noise linearly increased in proportion to the increase of creep degradation.
Authors: Masahiko Matsubara, A. Nitta, S. Sakai, N. Fujinawa
Authors: Hui Min Xie, Hang Shi, Peng Wan Chen, Feng Lei Huang, Dai Ning Fang
Abstract: In this paper, the creep deformation of PBX was measured using the moiré interferometry. The experimental results show a different creep process compared with pure high polymer and this phenomenon is preliminary analyzed from damage mechanics.
Authors: Sang Guk Lee, Sun Ki Lee, Jun Shin Lee
Authors: Krzysztof Nowak
Abstract: Polycrystalline materials like metals fail in creep conditions due to development of inter- or intra-granular voids. The model of creep damage is proposed which simulates voids growth on microscale using Cellular Automata (CA) technique at RVE level, coupled with creep deformation on macroscale. It is assumed that experimentally observed creep deformation is a result of interaction between hardening and softening of a material. The softening process is mainly due to voids development and it is built in deformation model by weakening of effective stress by damage parameter calculated by CA part of the model. Parameters of model are based on primary and secondary stages of creep experiments. The results of simulations show that multiscale model predicts quite well times to failure and strains at failure.
Authors: Zdeněk Kuboň, Lenka Pekařová, Jana Kosňovská, Pavel Poštulka
Abstract: Complex metallographic analysis was performed on the steam pipe elbow made of 0.5Cr-0.5Mo-0.3V steel after long-term (more than 240 000 hours) of operation at elevated temperature that revealed the extensive creep damage on the outer surface of the pipe elbow. Metallographic analysis confirmed pronounced creep damage at the outer surface but, at the same time, the non-uniform nature of the cavitation. The density of cavities continuously decreased from outer to inner pipe surface in the most damaged area and rapidly waned along the circumference as well as the length of the elbow. Parallel evaluation of actual extent of the cavitation damage made by metallography and replica methods in various parts of the pipe elbow confirmed that replica method is capable to describe and quantify the cavitation damage of this steel in the same way as metallography, including evaluation of creep damage according to Nordtest NT TR 302.
Authors: Guo Bin Zhang, Huang Yuan
Abstract: Creep damage is an important failure factor of high-temperature alloy. The fatigue crack growth under elevated temperature of the material is investigated for life prediction. In this paper, the numerical simulation of the crack propagation in nickel-based super alloy, IN718, was presented. A modified creep damage model was employed to accumulate the creep damage under cyclic loading conditions. The numerical results exhibit a reasonable agreement in the comparison with the experimental data. The cohesive zone approach, combining with the extended finite element method, has the ability to simulate the creep-fatigue crack propagation even for more complex loading conditions and specimen geometries.
Authors: Y.M. Baik, K.S. Kim
Abstract: Crack growth in compact specimens of type 304 stainless steel is studied at 538oC. Loading conditions include pure fatigue loading, static loading and fatigue loading with hold time. Crack growth rates are correlated with the stress intensity factor. A finite element analysis is performed to understand the crack tip field under creep-fatigue loading. It is found that fatigue loading interrupts stress relaxation around the crack tip and cause stress reinstatement, thereby accelerating crack growth compared with pure static loading. An effort is made to model crack growth rates under combined influence of creep and fatigue loading. The correlation with the stress intensity factor is found better when da/dt is used instead of da/dN. Both the linear summation rule and the dominant damage rule overestimate crack growth rates under creep-fatigue loading. A model is proposed to better correlate crack growth rates under creep-fatigue loading: 1 c f da da da dt dt dt Ψ −Ψ     =         , where Ψ is an exponent determined from damage under pure fatigue loading and pure creep loading. This model correlates crack growth rates for relatively small loads and low stress intensity factors. However, correlation becomes poor as the crack growth rate becomes large under a high level of load.
Authors: Zhi Xun Wen, Nai Xian Hou, Zhu Feng Yue
Abstract: Based on the microstructure change and damage characteristics of single crystal, a two-state-variable crystallographic creep damage constitutive model has been developed to investigate crack growth behaviors of single crystal compact tension specimen at 760 for two crack orientations: (001)[100] and (011)[100]. Numerical simulation results show the crack-tip stress fields are dependent on crack crystallographic orientation. Observations performed on the real single crystal specimens reveals that the macroscopic crack growth path appears as zigzag wave. The creep deformation at crack tip takes place in specific slip plane, and the deflection angles of crack initiation direction from the crack plane are 45º or 135 º and 53.7ºor 127.3º in the crack orientations (001)[100] and (011)[100]. A good agreement between experimental observations and numerical results is found.
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