Fatigue Notch Research Articles

Notch fatigue behaviour of friction stir welded AA6061-T651 aluminium alloy joints: Role of microstructure, and residual stresses

Notch fatigue behaviour of friction stir welded AA6061-T651 aluminium alloy joints: Role of microstructure, and residual stresses

Fatigue notch strengthening effect of nickel-based single crystal superalloys under different stress ratios

Nickel-based single crystal superalloys (Ni-SXs) are widely used in turbine blades of aircraft engines. With the increasing demand for higher temperature-bearing capacity, the introduction of film cooling holes, impingement holes, and trailing edge slots for cooling induces stress concentration, which compromises the structural integrity of the blades, generates complex multiaxial stress states, and adversely affects their operational performance. In this study, Ni-SXs, DD6, was utilized to investigate the influence of stress ratios on fatigue performance under complex stress states. Low cycle fatigue (LCF) tests were carried out at different stress ratios with stress concentration coefficient Kt = 1.0, 2.0 and 3.0. The results indicate that the notched specimens exhibit a significant fatigue notch strengthening effect under high stress ratios. The fracture surface and microstructure also indicate that under high stress ratios loading, the notches exhibit significant creep failure characteristics. This means that the main cause of fatigue notch strengthening is similar to creep notch strengthening effect. A macroscopic anisotropic constitutive model coupled with damage was developed and applied to finite element analysis of notched specimens. The results demonstrate that as the stress ratio rises, the stress relaxation effect at the notch becomes more pronounced, yet the level of damage diminishes. Additionally, a stress-equivalent model based on Bridgman stress was proposed, which effectively unifies the lifetime trends of notched and smooth specimens, predicting lifetimes within a threefold error band.

Crystal plasticity-driven evaluation of notch fatigue behavior in IN718

The aim of this study was to establish a method for evaluating notch fatigue behavior through crystal plasticity finite element (CPFE) simulations based on the actual microstructure of the nickel-based alloy Inconel 718 (IN718). Initially, the equivalent plastic strain, εeps, which reflects the comprehensive slip at the grain scale, was employed to analyze the fatigue crack initiation mechanism of IN718, revealing that twinning and triple junctions of grain boundaries were high-risk locations for fatigue crack initiation. Next, fatigue simulations were performed on notched specimens using the CPFE model, with a material-level CPFE model particularly employed at the notch root. The increment of εeps, Δεeps, in a stable cycle was used as the fatigue damage control parameter and correlated with the fatigue life, Nf, revealing that the Δεeps-Nf relationship at material-level satisfied the form of the Mason-Coffin model. Finally, fatigue life prediction of IN718 notched specimens was carried out based on the Δεeps-Nf relationship, with the predicted results falling within the 2-fold scatter band, demonstrating good prediction accuracy.

Non-Propagating Cracks at the Fatigue Limit of Notches: An Analysis of Notch Sensitivity and Size Effect.

The issues of the high-cycle fatigue resistance of notches and the role of non-propagating short cracks in defining the fatigue notch sensitivity and fatigue limit of the configuration are addressed. A fracture mechanics approach is employed to determine the threshold configuration that defines the associated fatigue limit. The influence of notch sharpness, notch size, intrinsic fatigue limit, microstructural dimensions, and the threshold for crack propagation is examined. A simple expression is proposed to estimate the maximum fatigue notch factor, kfMax, which incorporates the influence of these non-propagating cracks. The fatigue limits for both blunt and sharp elliptical notches are analyzed and predicted based on experimental results reported in the literature. Additionally, shallow notches or small defects are analyzed, where it is found that the same hypothesis may not be applicable.

Experimental and numerical determination of the fatigue notch factor in as‐built wire arc additive manufacturing steel components

AbstractParts manufactured by wire arc additive manufacturing (WAAM) are characterized by a peculiar surface morphology, namely, surface waviness, that negatively affects the fatigue performance. To exploit the full potential of WAAM and minimize the need for postproduction work, it is crucial to utilize the components in the as‐built state. This is because conventional machining techniques, typically employed for postprocessing operations, severely curtail the freedom of geometry of the components. This study focuses on an experimental and numerical characterization of the notch effect of the surface waviness for an AISI 308 LSi stainless steel. This is done by quantifying the fatigue notch factor in a probabilistic fashion, considering the results of ad‐hoc designed fatigue tests. A finite element model is developed by considering a 3D scan of the geometry of WAAMed plates, allowing to determine the theoretical stress concentration factor. The fatigue notch factor is also estimated from the numerical model by making use of the gradient correction according to the FKM guidelines. To validate the numerical approach, test data produced for different testing conditions are correlated by using the local stress approach, showing that classical methods are also applicable to additive manufactured parts.

Multiaxial low-cycle fatigue life model for notched specimens considering small sample characteristics

PurposeSufficient sample data are the necessary condition to ensure high reliability; however, there are relatively poor fatigue test data in the engineering, which affects fatigue life's prediction accuracy. Based on this, this research intends to analyze the fatigue data with small sample characteristics, and then realize the life assessment under different stress levels.Design/methodology/approachFirstly, the Bootstrap method and the principle of fatigue life percentile consistency are used to realize sample aggregation and information fusion. Secondly, the classical outlier detection algorithm (DBSCAN) is used to check the sample data. Then, based on the stress field intensity method, the influence of the non-uniform stress field near the notch root on the fatigue life is analyzed, and the calculation methods of the fatigue damage zone radius and the weighting function are revised. Finally, combined with Weibull distribution, a framework for assessing multiaxial low-cycle fatigue life has been developed.FindingsThe experimental data of Q355(D) material verified the model and compared it with the Yao’s stress field intensity method. The results show that the predictions of the model put forward in this research are all located within the double dispersion zone, with better prediction accuracies than the Yao’s stress field intensity method.Originality/valueAiming at the fatigue test data with small sample characteristics, this research has presented a new method of notch fatigue analysis based on the stress field intensity method, which is combined with the Weibull distribution to construct a low-cycle fatigue life analysis framework, to promote the development of multiaxial fatigue from experimental studies to practical engineering applications.

Implementation of modelled surface roughness in the accelerated lifetime prediction of a 20MnMoNi5-5 steel

Fatigue of metals is a complex, multi-parametric phenomenon with a high amount of influencing factors. In order to ensure a reasonable lifetime prediction of specimens, structures or components, the impact of these factors need to be understood in detail. Since fatigue cracks mostly initiate at the surface, especially in the high cycle fatigue regime, the surface condition is a very important factor, which has to be taken into account. The presented research proposes a method to model the surface roughness using a Fourier transformation to calculate a fatigue notch factor, which can be included in the accelerated lifetime prediction method MiDAcLife. Therefore, it is possible to generate virtual S–N curves for different surface conditions based on only one fatigue test. The investigated material is a ferritic-bainitic 20MnMoNi5-5 steel, which is mainly used in spray-lines of German nuclear power plants. In addition, a method for calculating the fatigue strength of various materials is presented in which a newly defined parameter, the specific hardness, is introduced.

Fatigue Behavior of Medium Entropy Alloys AlCrFe<sub>2</sub>Ni<sub>2</sub> and AlCrFe<sub>2</sub>Ni<sub>2</sub>Mo<sub>0.1 </sub>- A Comparison with Super Duplex Steel 1.4517

In the present study the notched fatigue behavior of two multi-phase medium entropy alloys (MEAs) AlCrFe2Ni2 and AlCrFe2Ni2Mo0.1 was characterized by three-point-bending (3-PB), along with a super-duplex steel 1.4517 as a reference material. An analytical approach for characterizing the fatigue notch factor (kf), based on grain size analysis in combination with finite element modelling (FEM) was used, relating the theory of critical distances (TCD) to the grain size of the material. To validate the approach, for the reference steel, the fatigue notch factor was also determined experimentally by comparing the fatigue behavior of notched and smooth specimens, resulting in an experimentally determined fatigue notch factor (kf) ~ 1.07. The numerically and analytically estimated notch effects increase with decreasing average grain size and vary between ~ 1.07 for the coarse-grained reference material – in very good agreement with the experimental results – and ~ 1.35 for the much more fine-grained AlCrFe2Ni2Mo0.1 medium entropy alloy. Note that these values are significantly lower than the stress concentration factor (kt) ~ 1.58, associated with the notch geometry. Fatigue endurance limits were measured at a fatigue stress ratio R ~ 0.1 (unidirectional stress), but were converted to fatigue amplitudes at R = -1 (σa, R-1, fully reversed stress), to be able to make due comparisons with available literature data, by using the elliptical relationship. The resulting fatigue endurance limit amplitudes for specimens surviving at least 2E+06 cycles for a minimum of three tested samples and including notch effects are σa, R-1 ~ 508 MPa for the AlCrFe2Ni2 alloy, σa, R-1 ~ 540 MPa for the AlCrFe2Ni2Mo0.1 alloy modification and σa, R-1 ~ 400 MPa for the reference super-duplex steel, putting the analyzed MEAs into a very competitive position compared to Cobalt containing multi-phase high or medium entropy alloys as well as commercially available steels.

Notch corrosion fatigue behaviour and simplified strain energy density-based notch fatigue life assessment of Grade-A shipbuilding steel

Corrosion fatigue is a key challenge in lifecycle management of ship hull structures. Although sea wave fatigue stresses are linear-elastic, highly-damaging elastic–plastic conditions occur at geometrical discontinuities or notches. Estimation of notch stress-strains to characterize notch fatigue behaviour conventionally requires large computational efforts. Also, constant strain energy density (SED), as a driving force, is far more effective in realistic applications versus constant stress and strain fatigue life approaches. Identified research gaps were the development of a simplified SED approach for fatigue life assessment of hole-notched structures. Fatigue studies were conducted on hole-notched specimens of Grade-A shipbuilding steel in air and corrosion mediums. Strong synergistic interactions of corrosion and fatigue damage mechanisms were studied by scanning electron microscope (SEM) fractography. Notch corrosion fatigue life was reduced by 70% from notch air fatigue life due to environmental effect. Two new parameters are proposed to improve the efficiency of notch fatigue life assessment, namely “fatigue notch strain energy density concentration factor (K_SED) and “fatigue notch toughness”. This improved fatigue life assessment method was demonstrated as equally reliable but much simpler to implement, eliminating the complex computational analysis. This improved method has promising applications in real-time structural health monitoring (SHM) of ship hull structures.

Equivalent characterization of initial crack size of blunt notched components based on test data and physical model

The precise definition of initial crack size applicable to fracture mechanics is crucial for predicting the total fatigue life of structural components. Given that the demarcation between crack formation and crack propagation depends not only on the material but also on the stress level, theoretical and experimental procedures for defining the crack boundary point are developed. Underlying the procedure is that the designed notched members have the same local stress distribution as the structure. Consequently, the fatigue life and initial crack size of S355J2 steel notched components with different scales but similar stress fields were predicted and compared with experimental data. Results show that fatigue life predictions fall within ±2 factor band, and the crack initiation size obtained by AM is closest to the experimental results, with a difference of less than 30.2%, which proves that the approach of treating the initial crack size as a function related to fatigue life and fatigue notch factor is reasonable.

Study of the Point Method for fretting fatigue life prediction

With the improvement and success of the Theory of Critical Distances (TCD) in notch fatigue problems, the TCD, especially the Point Method (PM), combined with multiaxial fatigue criteria, has been used over the last few years to predict the fretting fatigue life. However, little attention is paid to the effect of different damage mechanisms, especially stress distribution, between the two conditions on the application of the PM. This paper aims to select a suitable path for the PM, considering various stress distributions, for the fretting fatigue life prediction of complete contacts. A new method is developed to select the suitable path of the PM for fretting fatigue life prediction. To validate the analysis, corresponding tests were carried out using an improved proving ring-like rig in a Ti-6Al-4V alloy. Predicted lives on all paths were calculated using the PM combined with different multiaxial fatigue criteria, without incorporating wear effects, and compared with experimental lives using an error bar graph. Results indicate that when the fretting wear is not considered, the minimum predicted life path is more suitable for the PM than the perpendicular and maximum predicted life paths. This suitable path is compared to the path selection in the uniaxial symmetric V-notch fatigue condition. Analysis shows that the conventional path (the notch bisector) of the PM produces the maximum predicted life for the notch fatigue condition. The cause of this different path selection of the PM in the two fatigue conditions is assumed to be the effect of the asymmetric stress distribution and fretting wear damage in the fretting fatigue condition.

Assessing notch fatigue performance of aluminium alloy AA2024-T4 after uni-axial and equi-biaxial tensile pre-straining: Experimental observations and predictive models

This study delves into the effect of different tensile pre-straining (uni-axial and equi-biaxial pre-strain) modes on the notch fatigue performance of AA2024-T4. By conducting a series of fully reversed stress-controlled high cycle fatigue (HCF) experiments, pre-strained samples demonstrated notable improvements in HCF resistance, surpassing those without pre-strain. A stress-based fatigue life prediction approach, namely the theory of critical distance (TCD), is applied. We propose an experimentally validated physics-based method for predicting notch fatigue life in the HCF regime, utilising the cyclic plastic zone (CPZ) concept and TCD’s point method. The CPZ commonly plays a significant role in both crack initiation and propagation at the notch roots, thereby altering the stress in the vicinity under cyclic loading. However, TCD considers a linear-elastic stress field around the notch root for fatigue life calculation. The model successfully predicts HCF life based on experimental results and gives better prediction than the conventional TCD approach in terms of prediction accuracy, which falls within a range of ± 2 the experimental results for all the pre-strain conditions.

Experimental research on mechanical, material, and metallurgical properties of Inconel 600: Application in elevated temperature environment

Experimental research on mechanical, material, and metallurgical properties of Inconel 600: Application in elevated temperature environment

Probabilistic notch fatigue assessment under size effect using micromechanics-based critical distance theory

Geometrical discontinuities are inevitable in engineering structures due to various functional requirements, which often yield complex stress–strain state and thereby significantly impact their fatigue performances. In this study, crystal plasticity theory is coupled with traditional critical distance theory for notch effect modelling considering microstructural heterogeneity. Specifically, microstructural and geometrical size effects on critical distance and fatigue performance are studied based on Sub-modelling approach. In addition, critical distance theory is coupled with Weibull distribution for probabilistic fatigue life evaluation of notched components. The developed method is verified and compared by using experimental data of GH4169 alloy. The findings demonstrate that the evaluated P-S-N curves exhibit strong agreement with tested data and geometrical size effect poses larger influence than the grain size effect on critical distance and fatigue life.

Stress redistribution and stress triaxiality effect on the fatigue life of notched Ni-based single crystal superalloy at 760℃

The multiaxial stress state caused by geometric discontinuities will result in significant differences in the fatigue performance compared to the uniaxial stress state. It is crucial importance to research the notch fatigue behaviors of nickel-based single crystal (Ni-SX) superalloys at high temperature. This study investigated the impact of stress redistribution and stress triaxiality on the fatigue life of notched Ni-based single crystal superalloy at 760℃. The high temperature fatigue tests were carried out on the specimens under different stress ratios and the stress amplitudes under the same life conditions were obtained. A novel anisotropic damage model, incorporating the influence of stress triaxiality is proposed coupled with crystal plasticity theory. Based on experimental findings, an evaluation of the test parameters was conducted.The results showed that Mises stress, maximum principal stress, normal stress of principal stress along the loading direction, maximum principal strain, and strain along the loading direction were inadequate in reflecting the significant variations of samples observed among different stress ratios. However, the stress triaxiality at the unloading valley exhibited a high sensitivity to changes in fatigue life. To evaluate the fatigue life of notched specimens under different stress ratios, an improved Basquin’s type life prediction model was proposed, utilizing the stress triaxiality at the unloading valley. The proposed model demonstrates a favorable normalization trend for the fatigue life of notched specimens subjected to various stress concentration factor (SCF) conditions.

Notch Fatigue Property of 7050-T7451 Aluminum Alloy under Bending Resonance Environment

Based on the electromagnetic shaker table, the resonant fatigue testing system and the corresponding conventional fatigue testing system were designed respectively. The fatigue life (S-N) curves of smooth and notched 7050-T7451 aluminum alloy specimens were measured in the range of 105-107 cycles, and the notch fatigue property of 7050-T7451 aluminum alloy under bending resonance environment was studied. The results indicate that the fracture mechanism under bending resonant fatigue environment is roughly the same as that under bending conventional fatigue loading. All the S-N curves obtained through resonant and conventional fatigue tests show a continuously downward trend, and there are no obvious fatigue limits. Notch fatigue property under bending resonance environment presents “stage characteristics”, which increases first and then decreases with the increase of cycles. There is a critical cycle number Nc at 8×105 cycles, corresponding to the maximum fatigue notch factor Kf-max (or the maximum notch sensitivity factor qmax). There are two critical cycles N1 and N2. When N1<N<N2, the notch fatigue property of the material and the corresponding structures under resonance environment requires more consideration than conventional fatigue loading.

Notch structural stress theory: Part II predicting total fatigue lives of notched structures

Notch fatigue problems are among the most common topics for the reliability analysis and structural design of mechanical engineering components. In previous work, we proposed the Notch Structural Stress (NSS) method based on fracture mechanics for reflecting the notch effect on fatigue crack propagation lives. However, like other regular fatigue assessing procedures, NSS also cannot accurately predict the fatigue life of notched structures with slower stress gradients due to the inconsistent slope of their S-N curves. This paper presents the Theory of Notch Structural Stress (TNSS) for predicting total fatigue lives of notched structures, which overcomes the life prediction limitations caused by slope differences in S-N curves and constructs a life-predicting frame suitable for different notch types even including plain and sharp notches. Compared with the often-used stress-concentration-assessing procedures, TNSS provides a more effective consolidation of the S-N data from different notch types, mostly since it takes both Initiation and Propagation state resistance into account at the same time.

Research on fatigue curve fitting methods based on the notch stress approach

PurposeThis study investigates the fitting techniques for notch fatigue curves, seeking a more reliable method to predict the lifespan of welded structures.Design/methodology/approachBuilding on the fatigue test results of butt and cruciform joints, this research delves into the selection of fitting methods for the notch fatigue curve of welded joints. Both empirical formula and finite element methods (FEMs) were employed to assess the notch stress concentration factor at the toe and root of the two types of welded joints. Considering the mean stress correction and weld misalignment coefficients, the notch fatigue life curves were established using both direct and indirect methods.FindingsAn engineering example was employed to discern the differences between the direct and indirect approaches. The findings highlight the enhanced reliability of the indirect method for fitting the fatigue life curve.Originality/valueWhile the notch stress approach is extensively adopted due to its accurate prediction of component fatigue life, most scholars have overlooked the importance of its curve fitting methods. Existing literature scantily addresses the establishment of these curves. This paper offers a focused examination of fatigue curve fitting techniques, delivering valuable perspectives on method selection.

Experimental evaluation of the fatigue notch factor in as-built specimens produced by Wire and Arc Additive Manufacturing

The notch Effects created by the rough surface of as-built Wire and Arc Additive Manufactured (WAAMed) products may negatively affect the fatigue resistance. This can be prevented by post-production treatments such as machining, but it implies an additional manufacturing step involving costs and time. There is a need to quantify the actual notch Effects of as-built surfaces of WAAMed parts, which can be accomplished via a parameter called fatigue notch factor. This paper focuses on the experimental evaluation of the fatigue notch factor in as-built WAAMed specimens made of AISI 308LSi stainless steel tested with a load ratio of 0.1. Two test series consisting of as-built and machined plain specimens, respectively, have been tested by applying the load perpendicularly to the deposition plane. In this loading configuration, the as-built surface can be regarded as made of periodic notches. A dedicated specimen geometry based on the indications given in the International Standard ASTM E466-21 has been used for the tests. This innovative design allows to induce crack initiation in the periodic notches of the as-built surface while preventing the final fatigue fracture at the first notch root. After the fatigue tests, the fracture surfaces resulting from some specimens have been analyzed using the Scanning Electron Microscope (SEM) to determine the location of crack initiation. As a result of the study, the S-N curves in terms of nominal stress range have been derived and the fatigue notch factor has been determined.

Microstructural size effect on the notch fatigue behavior of a Ni-based superalloy using crystal plasticity modelling approach

Fatigue performances of critical structures are strongly affected by the microstructural (e.g. grain, defect, inclusion, etc.) size effect, and it is thus important to quantify their detrimental effect. In this work, a numerical procedure is constructed to quantify the influence of microstructure on the mechanical and fatigue behaviors of Ni-based superalloy GH4169. Specifically, by combining sub-modelling approach with crystal plasticity constitutive model, a dual-scale modelling approach is developed for studying grain-level mechanical behavior of Ni-based superalloy GH4169 notched components. In addition, the dislocation-based Tanaka-Mura-Wu model is applied for fatigue crack initiation life prediction. The Paris law is then utilized for fatigue crack propagation analysis based on the simulated short crack. To study the microstructural size effect on fatigue crack initiation and propagation behaviors, sub-models containing various grain orientation, grain size, defect size/shape are built and analysed. Finally, a series of fatigue tests on notched specimens of Ni-based superalloy GH4169 were carried out for method validation. Results indicate that the established dual-scale modelling approach and fatigue life prediction framework yields good agreement with experimental results.