CRACK TIP PLASTICITY AND THE EFFECT OF STRESS RATIO ON EARLY NOTCH FATIGUE CRACK GROWTH RATES (original) (raw)
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Fatigue & Fracture of Engineering Materials & Structures, 2015
paper presents a damage mechanics method applied successfully to assess fatigue life of notched specimens with plastic deformation at the notch tip. A damage-coupled elasto-plastic constitutive model is employed in which nonlinear kinematic hardening is considered. The accumulated damage is described by a stress-based damage model and a plastic strain-based damage model, which depend on the cyclic stress and accumulated plastic strain, respectively. A three-dimensional finite element implementation of these models is developed to predict the crack initiation life of notched specimens. Two cases, a notched plate under tension-compression loadings and an SAE notched shaft under bending-torsion loadings including non-proportional loadings, are studied and the predicted results are compared with experimental data.
Influence of notch (TIP) radius on fatigue crack growth rate
JOURNAL …, 2006
The paper deals with the problem of fatigue crack propagation from notches in plates made of FeP04 steel and AA356-T6 aluminium alloy. The tests were performed under different stress ranges by keeping the nominal load ratio (R= 0. 1) constant. The specimens were ...
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.
Frattura ed Integrità Strutturale, 2019
A series of strain controlled multiaxial low cycle fatigue (LCF) tests under proportional and non-proportional loading conditions have been conducted on notched specimens. Cylindrical bars of Al 6061 aluminum alloy and AISI 316L stainless steel with four values of stress concentration factors referred to the net section Kt,n were employed. The experimental results evidenced a reduction of fatigue life due to non-proportional loading. Furthermore, the crack initiation site has been detected to be moved from the notch tip in the case of steel for high values of notch radius under non-proportional loading. Stress concentration factor evaluated in the elastic field K t,n has been included in the Itoh-Sakane parameter to evaluate the fatigue life, returning a general underestimation of fatigue life especially for high values of Kt,n. Material notch sensitivity and crack initiation position have been taken into account to further modify the model, improving the original results and showing a better assessment.
Prediction of notch sensitivity effects in fatigue and in environmentally assisted cracking
Fatigue & Fracture of Engineering Materials & Structures, 2014
A B S T R A C T Semi-empirical notch sensitivity factors q have been used for a long time to quantify notch effects in fatigue design. Recently, this old concept has been mechanically modelled using sound stress analysis techniques, which properly consider the notch tip stress gradient influence on the fatigue behaviour of mechanically short cracks. This mechanical model properly calculates q values from the basic fatigue properties of the material, its fatigue limit and crack propagation threshold, considering all the characteristics of the notch geometry and of the loading, without the need for any adjustable parameter. This model's predictions have been validated by proper tests, and a criterion to accept tolerable short cracks has been proposed based on it. In this work, this criterion is extended to model notch sensitivity effects in environmentally assisted cracking conditions. a = crack size a 0 = short crack characteristic size at R = 0 a R = short crack characteristic size at R ≠ 0 b = notch depth E = Young's modulus gr = grain size K f = 1+q(K t À 1), actual value of the stress concentration factor under fatigue loads K max , K min = maximum and minimum values of the stress intensity factor K t = stress concentration factor K tSCC = 1+ q SCC (Kt À 1), actual value of stress concentration factor under stress corrosion cracking conditions K ISCC = resistance to crack propagation under stress corrosion cracking conditions pz = plastic zone q = notch sensitivity factor in fatigue q SCC = notch sensitivity factor under stress corrosion cracking conditions R = load ratio, R = K min /K max = σ min /σ max S L = fatigue limit amplitude S SCC = resistance to crack initiation under stress corrosion cracking conditions S U = ultimate strength S Y = yield strength γ = Bazant's exponent ΔK = stress intensity range ΔK th0 = ΔK th (R = 0), long crack fatigue crack growth threshold at R = 0 ΔK thR = ΔK th (R), long crack fatigue crack growth threshold at R ≠ 0 ΔK thR (a) = short crack fatigue crack growth threshold at R, a function of the crack size a ΔS L0 = range of the fatigue limit at R = 0 ΔS LR = ΔS L (R) = 2S LR , range of the fatigue limit at R ≠ 0
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.
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.
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.
https://www.preprints.org/manuscript/202308.0788/v1, 2023
Fatigue failure remains a critical concern in structural engineering and material science, prompting extensive research to understand and predict the behaviour of materials under cyclic loading conditions. The present study aims to investigate the fatigue life of carbon steel specimens containing opposite semicircular edge notches through a comprehensive experimental and numerical analysis. In this study, stress concentration factors (SCF, Kt) of rectangular plate with opposite semicircular notches are considered under uniform tensile stress to analyse the notch deformation because of stretching of plate. Furthermore, the research focuses on quantifying stress concentration factors (SCFs) for these notches based on S-N curves of carbon steel. The study employs a combination of experimental and numerical techniques to understand the influence of these notches on the fatigue performance of carbon steel structures. A plate with opposite semicircular edge single notches under the axial load creates stress concentration near the notch and it is much larger than the average stress on the plate. Both analytical and finite element methods are used to calculate the maximum stress around the notch. SOLIDWORKS Premium Student Edition 2023 has been employed for modelling and SOLIDWORKS Simulation Premium Student Edition 2023 has been used for stress analysis and fatigue notch factor of rectangular plate of size 31 mm x 25.4 mm x 6.35 mm. The uniform tensile load with a magnitude of 20195 N is applied on one sides of rectangular plate normal to the sides of notches with ratio h/r = 1, for the semicircular notch. The result obtained on both analytical and finite element methods are compared and the percentage of error has been evaluated. Subsequently, these specimens undergo fatigue testing under varying loading conditions to capture their fatigue behaviour. The acquired fatigue data is then plotted against stress amplitude to construct S-N curves, forming the foundation for assessing the fatigue life of the notched specimens. To complement the experimental findings and to gain a deeper understanding of the stress concentration phenomenon, numerical simulations are conducted using advanced finite element analysis (FEA) techniques. The finite element models are carefully calibrated against the experimental results to ensure their accuracy and reliability. The FEA simulations enable the determination of stress concentration factors at critical locations within the notched specimens, further validating the experimental observations. The investigation reveals crucial insights into the effect of opposite semicircular edge notches on the fatigue life of carbon steel structures. The obtained S-N curves allow engineers and designers to predict the fatigue life of components with similar notches, aiding in the development of reliable and durable structures in practical applications. Moreover, the stress concentration factors determined from the numerical simulations provide valuable data to assess the potential failure modes and to optimise designs, effectively mitigating fatigue-related failures. The combination of experimental and numerical approaches ensures a comprehensive and rigorous analysis of the fatigue behaviour in notched specimens, offering a reliable basis for making informed engineering decisions. The comparison between the analytical method and the Finite Element Method (FEM) demonstrated good agreement, with an error percentage of 4.272%.