Material fatigue properties for assessing mechanical components weakened by notches and defects (original) (raw)
Related papers
Total Fatigue Life Estimation of Notched Structural Components Using Low-Cycle Fatigue Properties
Strain, 2010
In this investigation, an efficient fatigue life computation method under variable amplitude loading of structural components has been proposed. Attention in this study is focused on total fatigue life estimation of aircraft structural components. Flat specimens with central hole made of quenched and tempered steel 13H11N2V2MF were tested as representatives of different structural components. Total fatigue life of these specimens, defined as sum of fatigue crack initiation and crack growth life, was experimentally determined. Specimens were tested by blocks of positive variable amplitude loading. Crack initiation life was computed using theory of low-cycle fatigue (LCF) properties. Cyclic stress-strain curve, Masing's curve and approximate Sonsino's curve were used for determining stress-strain response at critical point of considered specimens. Computation of crack initiation life was realised using Palmgren-Miner's linear rule of damage accumulation, applied on Morrow's curves of LCF properties. Crack growth life was predicted using strain energy density method. In this method, the same LCF properties were used for crack initiation life and for crack growth life computations also. Computation results are compared with own experimentally obtained results.
A non-linear model for the fatigue assessment of notched components under fatigue loadings
International Journal of Fatigue, 2016
This paper presents a general theory for the estimations of an entire fatigue curve in ductile materials based on the implicit gradient approach. In order to modify the slope of the Woehler curves, the material was considered non-linear. The average stress of the hysteresis loop was taken into account by means of Walker's model. Subsequently, the implicit gradient method was adopted for the numerical evaluation of the effective stress and strain at low-and medium-cycle fatigue life and was then related to the fatigue strength of the material. The characteristic length, relating to the fatigue behaviour of the material, was considered constant for the fatigue lifetime. In order to confirm the proposed method, new experimental data were obtained, relating to axisymmetric notched specimens loaded with nominal stress ratio R=-1 and R=0. In terms of the effective strain amplitude, evaluated by means of the implicit gradient approach, the different Woehler curves of notched specimens were summarised in a unique fatigue curve as a function of Walker's cycle parameter.
An iterative technique to assess the fatigue strength of notched components
Procedia Structural Integrity, 2020
The present work provides an efficient formulation to assess the growth of short fatigue cracks in metallic components. The proposed technique consists on the iterative combination of a micromechanical short-crack growth model and the Finite Elements Method. The interaction of the crack with the microstructure of the material is evaluated through the dislocations distribution technique. The finite elements analysis of the problem is needed to obtain the stress gradient ahead of the notch. The division of the main problem into simpler scenarios makes the resolution of the method easier since cases with known solutions are required exclusively. The iterative method formulation is properly described and application examples are given in order to show its usefulness.
Prediction of the fatigue limit of blunt-notched components
International Journal of Fatigue, 2001
A prediction of the fatigue limit of blunt-notched components of a low carbon steel was made on the basis that the fatigue limit of polycrystalline metals represents the critical conditions for the propagation of nucleated cracks. An expression for the material resistance to crack propagation as a function of the crack length is obtained for the first part of the short crack regime, which defines the blunt notch sensitivity to fatigue. The material resistance curve is modeled from a depth d, given by the position of the strongest microstructural barrier to microstructurally short crack propagation, which defines the plain fatigue limit. A microstructural threshold, ⌬K d , is suggested as an intrinsic material resistance to microstructurally short crack propagation, defined by the plain fatigue limit ⌬s e0 and the position of the strongest microstructural barrier d. The modeled notch sensitivity fits reasonably well the experimental results for a low carbon steel.
The present paper is concerned with the formulation of an elasto-plastic strain based approach suitable for assessing fatigue strength of notched components subjected to in-service variable amplitude cyclic loading. The hypothesis is formed that the crack initiation plane is closely aligned with the plane of maximum shear strain amplitude, its orientation and the associated stress/strain quantities being determined using the Maximum Variance Method. Fatigue damage is estimated by applying the Modified Manson-Coffin Curve Method (MMCCM) along with the Point Method (PM). In the proposed approach, the required critical distance is treated as a material property whose value is not affected either by the sharpness of the notch being assessed or by the profile of the load spectrum being applied. The detrimental effect of non-zero mean stresses and degree of multiaxiality of the local stress/strain histories is also considered. The accuracy and reliability of the proposed design methodology was checked against several experimental data taken from the literature and generated under different uniaxial variable amplitude load histories. In order to determine the required local stress/strain states, refined elasto-plastic finite element models were solved using commercial software ANSYS®. This preliminary validation exercise allowed us to prove that the proposed approach is capable of estimates laying within an error factor of about 2. These preliminary results are certainly promising, strongly supporting the idea that the proposed design strategy can successfully be used to assess the fatigue lifetime of notched metallic components subjected to in-service multiaxial variable amplitude loading sequences.
Short crack threshold estimates to predict notch sensitivity factors in fatigue
International Journal of Fatigue, 2007
The notch sensitivity factor q can be associated with the presence of non-propagating fatigue cracks at the notch root. Such cracks are present when the nominal stress range Dr n is between Dr 0 /K t and Dr 0 /K f , where Dr 0 is the fatigue limit, K t is the geometric and K f is the fatigue stress concentration factors of the notch. Therefore, in principle it is possible to obtain expressions for q if the propagation behavior of small cracks emanating from notches is known. Several expressions have been proposed to model the dependency between the threshold value DK th of the stress intensity range and the crack size a for very small cracks. Most of these expressions are based on length parameters, estimated from DK th and Dr 0 , resulting in a modified stress intensity range able to reproduce most of the behavior shown in the Kitagawa-Takahashi plot. Peterson or Topper-like expressions are then calibrated to q based on these crack propagation estimates. However, such q calibration is found to be extremely sensitive to the choice of DK th (a) estimate. In this work, a generalization version of El Haddad-Topper-Smith's equation is used to evaluate the behavior of cracks emanating from circular holes and semi-elliptical notches. For several combinations of notch dimensions, the smallest stress range necessary to both initiate and propagate a crack is calculated, resulting in expressions for K f and therefore for q. It is found that the q estimates obtained from this generalization, besides providing a sound physical basis for the notch sensitivity concept, better correlate with experimental data from the literature.
International Journal of Fatigue, 2020
Classical fatigue limit prediction models and material-dependent characteristic length parameters for notched components are critically reviewed before a modified stress gradient based approach is proposed to integrate the non-propagation behavior of microstructurally small crack. Assuming fatigue limit (strength) as a general function of root surface stress and stress gradient defined over a characteristic length, an effective stress parameter is derived to characterize the resistance of a notch to fatigue limit (high cycle fatigue) loading. The model is validated by comparing with reviewed models through a large amount of test data. Physical interpretation and perspectives on the model are also discussed.
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.
Frattura ed Integrità Strutturale, 2019
In the present paper, the approach based on the strain energy density (SED) averaged over a structural volume is reformulated to estimate the lifetime of notched components subjected to variable amplitude (VA) uniaxial fatigue loading. The accuracy and reliability of the proposed reformulation of the SED approach was checked against a large number of data taken from the literature and generated, under two different load spectra, by testing specimens of carbon steel C40 containing notches of different sharpness. Such a validation exercise allowed us to demonstrate that the extension of the SED approach as proposed in the present paper is capable of accurately estimating fatigue damage in notched components subjected to in-service VA fatigue loading.
This paper describes three new models that can be used (1) to determine the stress-strain response in stress concentration zones for components submitted to complex multiaxial loading paths, (2) to extract relevant cycle sequences from a general threedimensional loadin path, (3) to find the hypersphere enclosing any type of cyclic load history. These three models can be combined to post-process an elastic Finite Element Analysis, and provide a fast estimation of the fatigue life of the components.