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【期刊论文】Ratchetting behavior of advanced 9-12% chromium ferrite steel under creep-fatigue loadings
轩福贞, Peng Zhao, Fu-Zhen Xuan
Mechanics of Materials,-0001,():
-1年11月30日
The ratchetting behavior of advanced 9-12% chromium ferrite steel was investigated by cyclic loading tests with various hold times and stress ratios at elevated temperature of 873 K. Particular attention was paid to the effect of hold time on the whole-life ratchetting deformation and failure mechanism. Results indicate that the total ratchetting strains under creep-fatigue loading can be decomposed into two parts, i.e., cyclic accumulated creep strain produced during the peak stress hold time ( ), cyclic accumulated inelastic strain produced during the stress change process ( ). A transition in ratchetting components and rupture behavior with the increase of hold time was observed. In the long hold time domain, a quick shakedown of ratchetting strain occurs after the very first few cycles and the rupture behavior is fully controlled by the time-dependent creep damage. In the short hold time domain, ratchetting strain increases till the specimens fails and a mixed damage mode is responsible for the failure. An attempt is made to explain the existence of these two domains in terms of the evolutions of three internal stress components (back stress, isotropic stress and viscous stress) measured at the end of the holding period.
Ratchetting, creep-fatigue loading, internal stress
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轩福贞, Yanxun Xiang, Mingxi Deng, Fu-Zhen Xuan, and Chang-Jun Liu
Journal of Applied Physics,-0001,():
-1年11月30日
An analytical model is presented for the effect of the interactions of dislocations with precipitate coherency strains on the generation of second-harmonic of Lamb waves in metallic alloys. The cumulative second-harmonic of Lamb wave propagation is shown to depend dominantly on the dislocation density, pinning dislocation length, internal stress due to the coherency strain, volume fraction of the precipitates, and the phase matching degree between the primary Lamb wave and the double frequency Lamb wave (DFLW). Experiments were carried out to introduce controlled levels of creep-induced damage to determine the nonlinear response of Lamb waves in titanium alloy Ti60 plates. A like mountain-shape change in the normalized acoustic nonlinearity of Lamb wave versus the creep loading time has been observed. Microscopic image analyses were performed to interpret the variation of the measured acoustic nonlinearity and to obtain the microstructure parameters of the Ti60 specimens with different creep damages. The analytical model was applied to these creep damaged Ti60 specimens, which revealed a good accordance with the measured results of the nonlinear Lamb waves. These results indicate that the acoustic nonlinearity of Lamb wave increases due to the rising of the precipitation volume fraction and the dislocation density in the early stage, and it decreases as a combined result of the reduction of the precipitation volume fraction and the dislocation density and the increasing mismatch of the phase velocity between the primary Lamb wave and the DFLW after a further creep loading
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【期刊论文】A review of creep analysis and design under multi-axial stress states
轩福贞, Hua-Tang Yao, Fu-Zhen Xuan∗, Zhengdong Wang, Shan-Tung Tu
Nuclear Engineering and Design 237(2007)1969-1986,-0001,():
-1年11月30日
The existence of multi-axial states of stress cannot be avoided in elevated temperature components. It is essential to understand the associated failure mechanisms and to predict the lifetime in practice. Although metal creep has been studied for about 100 years, many problems are still unsolved, in particular for those involving multi-axial stresses. In this work, a state-of-the-art review of creep analysis and engineering design is carried out, with particular emphasis on the effect of multi-axial stresses. The existing theories and creep design approaches are grouped into three categories, i.e., the classical plastic theory (CPT) based approach, the cavity growth mechanism (CGM) based approach and the continuumdamage mechanics (CDM) based approach. Following above arrangements, the constitutive equations and design criteria are addressed. In the end, challenges on the precise description of the multi-axial creep behavior and then improving the strength criteria in engineering design are presented.
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轩福贞, Fu-Zhen Xuan, Jun Si, Shan-Tung Tu
Int J Fract 153(2008)117-123 ,-0001,():
-1年11月30日
Based on the comprehensive finite element (FE) creep analyses, the influence of free surface on the time dependent fracture mechanics parameter of a crack near the free surface in plates under tension has been investigated. It is found that the time dependent fracture parameter C∗ increases as the crack tip closes to the free surface. Such an increment is related not only to the crack configurations but also to the material properties, especially the creep exponent n of power creep law. In addition, more pronounced interaction is observed between the C∗ of subsurface crack and that of a single isolated crack compared to that denoted by SIF under the linear elastic fracture condition. Under the framework of reference stress method, we also developed a closed form solution for creep interaction factor. Overall good agreement is achieved between the proposed method for the C∗ of subsurface crack and the FE results which provides us confidence in practical application.
Subsurface cracks, Creep, C∗ integral, Interaction
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轩福贞, Qing-Qi Chen, Fu-Zhen Xuan *, Shan-Tung Tu
Thin Solid Films 517(2009)2924-2929,-0001,():
-1年11月30日
A general theoretical model was developed to predict the creep deformation and its effect on the stress relaxation and distribution in the multilayer systems under residual stress and external bending. Based on the proposed solution, a simplified solution for the special case of one film layer on a substrate is also presented. Finite element analysis was carried out to validate the presented model. Good agreements were observed between the finite element simulation and the prediction of the proposed model. In addition, the effects of film thickness on creep strain and stress distribution, the creep effect on neural axis location in the bilayer assembly subjected to the combination of residual stress and external bending were also discussed.
Creep, Multilayer, Residual stress, Externalbending, Modeling
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