作者: Yi Sun, Rui Zhang, Jun Ma

205

作者: Tomáš Denk, Vladislav Oliva, Aleš Materna

摘要: A two-parameter constraint-based fracture mechanics approach is used to explain the effect of the constraint on the apparently anomalous behavior of short fatigue cracks. The different levels of stress constraint are quantified by the T-stress, and microstructurally as well as mechanically short cracks are discussed. Short cracks generally behave more sensitively to the constraint than the long ones. It is shown that in most cases, the existence of short cracks goes hand in hand with an intrinsic loss of the constraint, which contributes to a decrease of their fatigue
threshold values and accelerates their growth. In this paper, the above effect is quantified and conclusions concerning the applicability of the fracture mechanics parameters and approaches to the estimation of the residual fatigue life of structures are discussed.

307

作者: Jia Zhen Zhang, Xiao Dong He, Shan Yi Du

摘要: In-situ SEM observations have revealed that fatigue crack propagation in aluminium
alloys is caused by the shear band decohesion around the crack tip and the formation and cracking
of the shear band is mainly caused by the plasticity generated in the loading part of the load cycle.
This shear band decohesion process has been observed to occur in a continuous way over the time
period during the load cycle. Based on this observation, in this study, the transient fatigue crack
growth rate, da/dt, has been used to obtain the relationship between the conventional used parameter
da/dN and the applied driving force. It is proven that two parameters are necessary in order to
accurately describe fatigue crack propagation rate per stress cycle, da/dN. The well known stress
ratio effects on fatigue crack propagation rate can be correlated by this model.

293

作者: Stanislav Seitl, Pavel Hutař, Zdeněk Knésl

摘要: The formulations of fatigue crack growth prediction are still mostly based on
phenomenological models. A commonly used formula in the field of high cycle fatigue is the Paris-
Erdogan law. For given experimental conditions (such as temperature, stress ratio or environmental
conditions) the parameters C and m have to be experimentally determined and considered as
material constants. Thus, for a given material, the fatigue crack growth rate (FCGR) depends only
on the applied range of the stress intensity factor. In a threshold region a significant shift in the data
of the fatigue crack propagation rate can be observed. The shift is induced by different test specimen
geometry. To analyses it the authors will present their own laboratory fatigue crack growth rate test
data measured on two different specimens with different levels of constraint and for different steels.
It is demonstrated that fatigue characteristics (i.e. C, m and Kth) obtained from different specimen
geometries are not only properties of the materials but depends on the specimen geometry.

557

作者: Li Hong Gao, Ge Ning Xu, Ping Yang

摘要: The random formula on fatigue crack growth is deduced by the fatigue crack data and the improved Taguchi method, and the sample estimates of random variables are received by the least square method in the random formula. Fatigue fracture life and reliability of structure are analyzed by the random model. The result show the model is correct and practical, and get the same result with Monte Carlo simulation, moreover its calculation is very simple.

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