Influence of notch sensitivity and crack initiation site on low cycle fatigue life of notched components under multiaxial non-proportional loading (original) (raw)
Related papers
Physical Mesomechanics, 2020
Real mechanical assemblies favor the initiation and propagation of fatigue cracks due to stress concentration phenomena arising from the geometrical features such as notches, corners, holes, welding toes, etc. Classical fatigue analysis of notched specimens is done using an empirical formula and a fitted fatigue strength reduction factor, which is experimentally expensive and lacks physical scene. In the present paper, a simple and meaningful methodology is proposed to assess notched components against multiaxial fatigue. In this method, by precisely defining a finite-size volume surrounding the fatigue crack initiation site (notch tip), over which the strain energy is averaged, the morphological effect on the process zone is fully addressed. Such a method takes into account the effect of combination of different modes (I, II, III) and the load ratio. In order to implement it for components with a sharp or blunt notch, it is enough to analyze a linear elastic finite element model and to know only the properties of materials obtained from simple uniaxial tests. New relationships for determining an effective (tensile-type) stress and fatigue strength reduction factors are derived for notched specimens. The accuracy of the proposed model is validated by experimental data available in the literature, related to tubular specimens weakened with sharp/blunt notches under combined bending-torsion loading. Such a situation widely appears in equipment used for various branches of industry such as piping, automotive, power plant, drilling, etc.
Fatigue Failure of Notched Specimen—A Strain-Life Approach
Materials Sciences and Applications, 2011
Failure cycles of notched round specimens under strain controlled cyclic loading are predicted using strain-life relations obtained from experiment for plain fatigue round specimens. For notched specimens, maximum strain occurs at notch root and is different from applied controlled strain. The maximum strain is computed by appropriate Finite element analysis using the FE software ABAQUS. FE model and material parameters are validated by comparing the FE results and experimental results of LCF tests of round specimens. This value of maximum strain is used for prediction of failure cycles. Prediction is compared with the experimental results. The results show good matching.
Notch effect on multiaxial low cycle fatigue
International Journal of Fatigue, 2011
This paper discusses the notch effect on multiaxial low cycle fatigue. Neuber's rule was firstly introduced to estimate the local strain at the notch root in proportional tension and torsion loading. The Neuber's rule was applied to estimating crack initiation and for propagation lives in tension-torsion low cycle fatigue. The rule conservatively estimated the crack initiation life in tension low cycle fatigue and appropriately in torsion low cycle fatigue. A simple method for estimating the local strain at the notch root was proposed in tension and torsion loading. The notch effect in nonproportional low cycle fatigue was discussed in two materials. The local strain at the notch root obtained by finite element analysis underestimated the crack initiation lives for the additional hardening material but that obtained by the Neuber's rule overestimated for the non-additional hardening material.
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.
IMPACT OF NOTCH GEOMETRY ON THE FATIGUE LIFE OF BS 970-4 GRADE 349S52 STAINLESS STEEL
IJRAME PUBLICATIONS, 2022
Components in engineering applications often have discontinuities and abrupt change in cross sections which may be present owing to their functional requirements like oil holes, grooves, keyways etc. This would result in localization of high stresses when these components are subjected to loading. Situation becomes more hazardous to the material if the loading is not static and varying in magnitude with time. This fatigue phenomenon reduces the resistance of the material under fluctuating stresses. Fatigue can be defined as a failure taking place by the formation and growth of cracks due to repeated stresses. Fatigue design is considered to be complex as the failure sometimes occurs abruptly without any indication about the initiation of the failure. It is evident from experience that, around 80% of structural failures is due to insufficient fatigue design. Prodigious work is done in the field of fatigue design but there is still lot of scope in this area. C.S.Yen et.al. Reviewed lot on the literature and concluded that, the fatigue notch-sensitivity of a metal member depends upon three different factors namely, the basic material characteristics, the degree of material homogeneity, and the geometry of the member (C.S. Yen, 1952). M.Makkonnen showed that when the notch gets sharper, the magnitude of the plastic portion of the strain starts to play an important role in the fatigue crack initiation, and the fatigue limit is lower than that prediction is by statistical and geometric size effects. In those cases, fatigue limits should be arrived, by assuming the notch to be an initial crack with the notch depth being considered as the depth of the crack and the fatigue limit is computed to this crack by banking upon linear elastic fracture mechanics and the stress intensity factor range threshold (M. Makkonen, 2003). Yoshiaki Akiniwa et. al. demonstrated that, for specimens with circumferential notch, fatigue fracture starts from the surface or very near the surface. The slip deformation is often responsible for the crack initiation in high cycle and very high cycle regimes (Yoshiaki Akiniwa, 2006). A.J.McEvily et.al. proved that, for holes of radii less than 1 mm in the steel investigated, the notch fatigue factor, KF, is dependent upon crack closure and for holes of radii in the range of 1–5 mm in the steel investigated; the analysis indicates that KF is constant and dependent upon the ratio σmax/σy (A.J. McEvily, 2008). M. Zehsaz et.al. Showed that the volumetric approach gives good results in predicting the fatigue life of the notched specimens. The effect of notch radius for different notched specimens was investigated to observe the stress concentration factor, notch strength reduction factor, and fatigue life of the specimens (M. Zehsaz, 2010). G.H.Majzoobi et.al. demonstrated that notch geometry has profound effect on fatigue life of materials. For high strength steel this reduction is roughly about 50%. For low strength -steel alloy, however, the reduction depends on fatigue life and varies from 20% for low cycle fatigue tests up to 75% for high cycles fatigue tests. The maximum and minimum fatigue life reduction occurs for the V-shape and U-shape notches, respectively (G.H. Majzoobi, 2010). Baohua Nie et. al. concluded that fatigue life improves with the increase in the crack initiation depth. The scatter of the fatigue property should be carefully considered in fatigue design (Baohua Nie, 2018). M.L. Aggarwal et. al. developed a numerical model using stress approach to predict the fatigue life of a shot-peened mechanical component (M.L. Aggarwal, 2006). The current research work aims at finding the pressure that the selected material can sustain in tension at a fixed stress level and fatigue life. Specimens are fabricated with different geometries of notches on them. The width, depth and central angle of the notch are varied and the fatigue life is found out using finite element method (R. Marimuthu, 2014) (J. Jagadesh Kumar, 2017). The design of experiments using Taguchi L9 orthogonal array is selected to know the impact of each parameter on the pressure bearing capacity and thereby on the fatigue life. Numerous finite element method runs are conducted in an iterative approach using ANSYS 18.1 to get the target values of stress and fatigue life.
International Journal of Fatigue, 2009
This paper attempts to improve the understanding of the multiaxial high cycle fatigue response of micro sized stress concentrations or notches of different geometries. The investigation is composed of an experimental part and a numerical part. In the former, three types of micro-notches or "artificial defects" are compared: spherical, elliptical and circumferential. All types have the same basic dimensions, the difference being the 3D geometry. The notches were machined on the surface of smooth cylindrical specimens made of mild steel. The fatigue limits under reversed tension (push-pull) and reversed torsional loading conditions for different micro-notch sizes have been experimentally determined. In the numerical part, finite elements simulations using a cyclic elasto-plastic material behaviour law show that the mechanical state ahead of the different stress concentrations change drastically with the loading mode and the geometry of the artificial defect. From a fatigue point of view, it is shown that a stress gradient correction is required for all the loading, size and geometry configurations. Once the gradient correction is made and a proper multiaxial criterion is used, it appears that the size effect due to increasing the loaded surface area at the notch tip for the different geometries is negligible compared to the gradient effect.
KnE Engineering, 2020
This paper presents a methodology to predict the fatigue lifetime in notched geometries subjected to multiaxial loading on the basis of the cumulated strain energy density. The modus operandi consists of defining an energy-based fatigue master curve that relates the cumulated strain energy density with the number of cycles to failure using standard cylindrical specimens tested under low-cycle fatigue conditions. After that, an elastic-plastic finite-element model representative of the material behaviour, notched geometry and multiaxial loading scenario is developed and used to account for the strain energy density at the crack initiation site. This energy is then averaged using the Theory of Critical Distances and inserted into the energy- based fatigue master curve to estimate the lifetime expectancy. Overall, the comparison between the experimental and predicted fatigue lives has shown a very good agreement. Keywords: Multiaxial fatigue, Fatigue life prediction, Strain energy density
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.
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.
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.