A model for notch fatigue life (original) (raw)
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Fatigue & Fracture of Engineering Materials & Structures, 2009
Over the last three decades, a variety of models have been developed in order to predict the life of components under fatigue. Some of the models are based on the definition of the fatigue process as a combination of the phases of crack initiation and crack propagation, considering component life as the sum of the duration of each phase. Other models consider only one of the phases; some consider only initiation while others only propagation, though in this case, from cracks with lengths in the order of the microstructural dimensions. This article will carry out a comparative analysis of the methods that consider life as the sum of the duration of both phases. In this same line, it proposes yet another method, which simulates crack growth according to damage theories. In analysing the behaviour of each model, this paper will describe various elements: the prediction that each of them produces regarding notched specimens submitted to testing, the advantages and inconveniences of each, and lastly, the possibilities of applying each of the models to more realistic geometries.
Fatigue notch factor and short crack propagation
Engineering Fracture Mechanics, 2008
This paper addresses the problem of high cycle fatigue at notches and the role of short crack propagation in the fatigue notch factor k f. Ahead of a V-notched feature, the stress field is characterized by two parameters, i.e. the stress concentration factor k t and the normalized notch stress intensity factor k n. Whether fatigue strength at a given life is controlled by crack initiation (k f = k t) or by short crack propagation (k f < k t) depends on k t , k n and the material resistances to crack initiation and to short crack propagation. The analysis accounts for the effects of notch acuity, notch size, material and fatigue life on the fatigue notch factor k f. It opens the door to a new method for predicting fatigue life using two S-N curves for a given material; one being measured from a smooth specimen, the other from a severe V-notch.
Mode I fatigue crack growth evaluation at notches
An analytical elastic-plastic model describing the fatigue life of components with elliptical notches under constant amplitude loading has been proposed. The calculation occurs by integrating a crack growth law from a starting crack size of micro-structural dimension till up to the total fracture of the component. Plasticity induced crack opening and closure effects are explicitly taken into account. Thereby, calculated opening load levels for cracks growing in notch affected areas have been found out to be in good agreement with corresponding experimental values determined from notched specimens made of two different metallic materials. Furthermore, the comparison of experimentally determined and calculated crack growth curves for specimens with central notches confirm the calculation accuracy of the model.
Fatigue <html_ent glyph="@amp;" ascii="&"/> Fracture of Engineering Materials and Structures, 2000
The theoretical foundation of a micromechanical model that accounts for the fatigue crack growth threshold conditions at notches was described in Part I of this study. Strictly speaking, the proposed formulation is restricted to the analysis of a component with an elliptical notch under antiplane stress. In this section of the study, the expressions derived in Part I are generalized for application to axial stress states and non-elliptical notch geometries. The procedure is validated by comparing the model's predictions with reported experimental results.
Assessment of the Fatigue Failure Period of a Notched Component
2013
The total period, Nf, to failure is considered, which includes the periods offatigue macrocrack initiation, Ni, and propagation, Np, i.e.: Nf = Ni + Np. Fatiguefracture of materials has been modelled as a process of initiation of a macrocrack oflength ai = d* (the magnitude of d* is a material constant), which is successively repeated(step-by-step) during its growth. As a result the "local stress range, *Aoy , versusmacrocrack initiation period, Ni " relationship, which was established for notchedspecimens, might be applied to the determination of the "macrocrack growth rate, da/dN ,versus effective stress intensity factor range, AKeff" relationship and vice versa.
Prediction of fatigue crack initiation lives at elongated notch roots using short crack concepts
International Journal of Fatigue
Re-initiation lives of fatigue cracks departing from stop-holes roots, previously introduced at the tip of deep cracks on modified SE(T) specimens, have been satisfactorily predicted using their properly calculated notch sensitivity factor q, considering the notch tip stress gradient influence on the fatigue behavior of mechanically short cracks. This is an indispensable detail, since traditional q estimates are only applicable to semi-circular notches, whereas elongated slits can have q values which also depend on their shape, not only on their tip radius. Based on this experimental evidence, a criterion for acceptance of short cracks is proposed.
Fatigue crack initiation at a notch
International Journal of Fatigue, 2011
A short crack approach has been developed to determine the fatigue crack initiation life at a notch tip, in which the initiation life is assumed to correspond to the period during which an initial crack, equivalent to a microscopic defect, grows to attain a detectable size. This approach is compared to conventional local strain approach and it is shown that the short crack approach gives acceptable predictions as compared to experiments, in the 2024 T351 alloy. In the 7449 alloy, a different approach, using marker-band technique has been applied to determine the short crack growth kinetics at a notch. The advantages and disadvantages of the conventional strain based approach and the short crack model are discussed.
CRACK TIP PLASTICITY AND THE EFFECT OF STRESS RATIO ON EARLY NOTCH FATIGUE CRACK GROWTH RATES
This paper aims at investigating the ability of a previously proposed parameter to correlate the behaviour of a physical short mode I stage II crack initiated at and growing from the root of a notch due to constant amplitude cyclic load at different stress ratios. The parameter combined the extents of both monotonic and cyclic crack tip plasticity and a length-dependent crack resistance term. Low carbon steel plates having a single un-cracked U-shaped notch were tested under constant amplitude axial stresses. Four tests were performed at four stress ratios such that the applied maximum stress was kept the same. Each test was simulated with a cyclic elastic-plastic two-dimensional finite element analysis. The development of monotonic and cyclic plasticity accommodated at the tip of a mode I physical short crack artificially advancing from the notch root was traced. The numerical results demonstrated the capability of the proposed parameter in reflecting the effect of stress ratio and notch contribution on the observed fatigue crack growth rates.
In this work, short cracks emanating from circular holes are studied. For several combinations of notch dimensions, the smallest stress range necessary to both initiate and propagate a crack is calculated, resulting in expressions for the fatigue stress concentration factor K f and therefore the notch sensitivity q. A generalization of El Haddad-Topper-Smith's parameter, which better correlates with experimental crack propagation data from the literature, is presented.
Influence of notches on high cycle fatigue life
Materials Science and Engineering, 1981
A fatigue notch factor theory for the high cycle region is proposed. It is based on the quantitative description of the propagation of fatigue cracks nucleated at the notch root. The dependence of the fatigue notch factor on the number of cycles to fracture can be computed from the derived formula provided that the fatigue material parameters are known. The limitation to the high cycle region stems from the fact that the fatigue crack propagation rate is described by means of linear elastic fracture mechanics. For infinite life the theory yields a very simple expression for the fatigue limit of notched bodies, which can readily be ~sed in practice.