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Arc-welded joints in steel maritime structures are typically identified as weakest links in terms of fatigue limit state performance. Multiaxiality can be involved, consisting of predominant mode-I and non-negligible mode-III components. Aiming to answer the question if a cracked geometry based fatigue strength parameter would outperform an intact geometry based one like the effective notch stress, the total stress is adopted. A von Mises type of criterion is defined at the critical fracture plane and includes mode specific and material characteristic strength and mechanism contributions. A lifetime dependent shear strength coefficient is introduced to cover the resistance curves intercepts and slopes, whereas the total stress parameter contains the mean stress contribution as well as the (mixed) mode dependent notch and crack tip elastoplasticity coefficients, reflecting an interaction mechanism. Cycle counting includes a cycle-by-cycle non-proportionality measure and damage accumulation is based on a linear model. Evaluating mid-cycle fatigue resistance data, the total stress and effective notch stress performance turns out to be similar. However, the total stress related elastoplasticity coefficients are an explicit and sensitive measure to incorporate the actual physics of the fatigue damage process, whereas the material characteristic lengths for the effective notch stress seem to be more implicit and less sensitive ones. • Far field stress intensity formulations for tubular structures have been developed. • Total stress intensity based crack growth is multiaxial response condition dependent. • Mixed mode notch/crack elastoplasticity improves the total stress performance. • The total stress and shear coefficient include strength and mechanism contributions. • Multiaxial fatigue resistance data fits the uniaxial mode-I data scatter band. [ABSTRACT FROM AUTHOR]

• Interrelation of defects, "as-built" surface and defect tolerance. • Nearly no impact of the "as-built" surface on HCF-life after T6 treatment. • Consideration of residual stresses on defect-based fatigue behavior. • Modified adaption of the √area approach for nonferrous metals. • Higher fatigue strength after T6 treatment due to an increased defect tolerance. As the fatigue strength of materials manufactured via lase-based powder bed fusion (PBF-LB) is highly influenced by process-induced defects and the "as-built" surface, in this work fatigue tests at specimens made of AlSi10Mg with "as-built" and polished surface condition were conducted. In this context also the defect tolerance of the material, and hence its ability to counteract process-induced notch effects, was analyzed. For this, specimens in a not heat-treated and an artificially aged (T6) condition were tested. For the not heat-treated specimens, a high reduction of the fatigue strength due to the "as-built" surface is observed, while the heat-treated specimens exhibit no influence of the surface condition. Using the √area approach, it is demonstrated that the smaller fatigue strength due to "as-built" surface observed for the not heat-treated condition results from process-related residual tensile stresses, which are removed by T6 heat treatment and polishing, respectively. Moreover, the results show that the "as-built" surface leads to similar stress concentrations than defects in the specimen volume, and thus, polishing had no influence on the fatigue performance after heat treatment. In addition to these findings, an increased fatigue strength after T6 heat treatment was observed, which is caused by an increased defect tolerance. [ABSTRACT FROM AUTHOR]

• Determining fatigue classes after TIG dressing for effective notch stress method. • Slope and fatigue strength after TIG dressing related to relative stress gradient. • S-N curve with m = 4.4 and FAT326 can well assess fatigue life after TIG dressing. TIG dressing significantly improves the fatigue strength of welded structures by enlarging the weld toe radius and thereby decreasing stress concentrations and stress gradients at the weld toe location. To accurately evaluate the fatigue life of welded members after TIG dressing, the present work proposes a new strategy by combining the relative stress gradient and effective notch stress method. For this purpose, 151 fatigue data containing various materials and weld types were collected to illustrate the applicability of the proposed strategy. The results show that the fatigue assessment classes adjusted by the relative stress gradient in the effective notched stress method are well suited for assessing the fatigue life of welded members after TIG dressing. Meanwhile, this study also determined the unified S-N curve of the effective notched stress method for assessing the fatigue life of welded members after TIG dressing, i.e., m = 4.4 and FAT326. [ABSTRACT FROM AUTHOR]

• The LCF tests of notched specimens under three temperatures after different degrees of initial overload treatment was conducted. • The initial overloads could significantly improve the crack initiation life, but could also introduce damage near the notch surface, especially for large overload situations. • The LCF life model considering the sensitivity of overload induced residual stress can give a more accurate prediction. • A recommendation to improve the fatigue life of GH4169 alloy by initial overload was given through comprehensive experimental results and finite element model analysis. In this paper, the effect of initial overload on the low cycle fatigue life of GH4169 alloy at different temperatures was studied. The plate specimen with a central hole was designed to perform low cycle fatigue tests at 20 °C, 500 °C and 650 °C with variable initial overloads. Results showed that the subsequent crack propagation rate depended on both overload ratio and following test temperature, and excessive overload treatment would also damage the surface or internal grains of the specimens through fracture characteristics under SEM. A finite element model was adopted to analyze the residual stress and stress-strain field at the edge of the central hole. Then, the stress and strain data were input to three strain-based model to predict the fatigue life of GH4169 alloy considering residual and mean stresses. Based on the combination of the experimental results and model prediction, the suggestion to improve the fatigue life of GH4169 alloy by initial overload can be put forward as following: a single initial overload makes the maximum local stress reach 90 %–93.5 % of the true ultimate stress of the material, so as to maximize its fatigue life. [ABSTRACT FROM AUTHOR]

• An investigation on the LCF behavior of Q345R steel with single-sided square notch was conducted. • A novel stress model optimized based on TCD and TSF was proposed for predicting fatigue lifetime. • Investigated the relationship between the characteristic length d* and its stress value of FPZ with the notch size Φ. The introduction of notches in engineering components often has detrimental effects on fatigue behavior. Current research on finite residual thickness is extremely limited, especially when dealing with more complex geometric shapes of notches. In this study, the low-cycle fatigue (LCF) life of cylindrical components with deep-rectangular notches were predicted based on the theory of critical distance (TCD) and the total stress field (TSF) method. The results indicated that the presence of stress concentration near the notch accelerated the initiation and propagation of cracks. Furthermore, finite element method (FEM) was conducted, revealing the need to comprehensively consider the stress distribution across the residual thickness zone (RTZ) for notched specimens. Through numerical method improvement and geometric modeling optimizations, the accuracy of fatigue lifetime prediction has been refined to within a margin of ± 1.5. Comparative simulations on notched specimens with various ratio of notch depth to residual thickness were conducted, characterizing the fatigue process zone (FPZ) near the notch root under LCF loading for Q345R steel, indicating that characteristic material parameter d* exhibits minimal fluctuations with changes in the notch size, while the corresponding local stress levels display a logarithmic upward trend, substantiating that fatigue lifetime decreases with larger notch sizes. [ABSTRACT FROM AUTHOR]

The integrity assessment of structural components under complex loading conditions relies on the evaluation of the fatigue damage typically arising from stress concentrations, such as geometric irregularities, notches, weld beads, grooves etc.. Various methodologies, including the Notch Stress Approach (NSA), the Theory of Critical Distances (TCD), the Strain Energy Density (SED), and the Critical Plane (CP) concept, have been pivotal in assessing fatigue strength for notched and welded components. Recent works combine some of the above mentioned methodologies, while other works propose to vary the embedded parameters accounting for the loading type or the fatigue lives, trying to improve the accuracy of the fatigue assessment process. This paper introduces a novel approach, the Effective Critical Plane (ECP), which is founded on the critical plane concept. The CP factor is, however, calculated starting from an averaged, over a small volume, stress–strain field. The size of the averaging volume is assumed to be a material parameter and is determined by a best fitting procedure over different experimental data sets. The novel approach is illustrated by means of the Fatemi-Socie and the Smith-Watson-Topper CP damage factors. Its potential application to other CP formulations is straightforward, as well. Literature experimental data for low carbon steel specimens possessing different notches and loading conditions are used to validate the method's capability in accurately determining the fatigue life and to set the radius of the averaging volume for the given material and CP parameter. A spherical volume or circular area are used in case of fully 3D or 2D numerical models, respectively. Results are compared to those of some of already existing methods, namely SED, TCD and the Modified Wöhler Curve Method. • A novel approach for fatigue assessment of notched components is presented. • The method integrates the critical plane concept and stress averaging around notches. • The calibration procedure requires few fatigue experiments. • The method was validated using low carbon steel specimens. • The method demonstrates excellent fatigue life predictions results. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel fatigue life prediction model is established by combining critical distance theory and crystal plasticity theory. • Probabilistic fatigue life prediction of GH4169 notched specimens is conducted. • Microstructural and geometrical size effects on critical distance and fatigue life are investigated. 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. [ABSTRACT FROM AUTHOR]

• An ESFI approach was proposed for notch fatigue life prediction. • The fatigue failure region was improved to consider actual stress distribution. • A novel weight function was proposed to consider all points relative stress gradient. • The impact of combining notch and size effect was characterized in the ESFI approach. The notch characteristics of engineering components significantly impact their fatigue lives. Accurately analyzing the notch effect and size effect is of great significance for structural integrity evaluation, which provides insights into the extent of fatigue failure in engineering structures. In this paper, a new method is proposed for determining the fatigue failure region based on actual elastic–plastic stress distribution under various notch geometries and applied loads. It considers the complex mechanical behavior of the plastic effect and stress relaxation near the notch root. Furthermore, a novel weight function is employed to consider the relative stress gradient at all points within the fatigue failure region. Finally, an enhanced stress field intensity approach is developed to predict notch specimens fatigue lives utilizing experimental lives of smooth specimens. Additionally, the effectiveness of the proposed method is analyzed by predicting the fatigue lives of three distinct materials notched specimens with varying geometries and applied loads. The prediction absolute errors of the proposed approach are compared with those of the other traditional notched fatigue analysis methods. The comparative results indicated that the performance of the proposed method outperforms the other methods. [ABSTRACT FROM AUTHOR]

• Development of a process to derive a fatigue spectrum from fuselage load factor data. • Finite element simulation of an endurance test on a novel fuselage panel experimental assembly. • Development of an analytical procedure that predicted the propagation of flaws inside the investigated fuselage panel. • The evolution of cracks depended heavily on initial notch orientation, and their position relative to welded stiffening elements. • Flaws affected only by pressure loads provided more scattered data than those loaded in both tension and pressure which achieved stage 2 stable growth. A mission profile of a passenger airplane consists of several phases during which its structural components experience complex sequences of alternating loads. When focusing on a fuselage section, these sequences are usually consistent in format during the cruise flight phase. In the present work a full-scale stiffened fuselage panel was tested in static and fatigue by a novel experimental arrangement for panel-testing, after its numerical simulation in FE environment. The panel was manufactured using a 4th generation AL-Li alloy and its stiffeners were assembled by means of Friction-Stir-Welding (FSW). For the fatigue test, flaws were machined on different areas and the panel was subjected to a combination of pressure and axial loads that recreated realistic fuselage loading conditions. Load factor data from the related open literature, which have been recorded during typical flights were organized in exceedance diagrams, from which a fatigue spectrum representative of fuselage loading conditions was derived. The resultant spectrum consisted of pressurization-depressurization cycles, on which bending cycles were superimposed. FE static analyses were conducted to calibrate the spectrum's values. Subsequently, analytical predictions of crack growth were performed and successfully compared to experimental measurements of through-cracks located at critical locations. [ABSTRACT FROM AUTHOR]

• A novel theory is developed for predicting total fatigue lives of notched structures. • Theory of Notch Structural Stress (TNSS) overcomes the life prediction limitations caused by slope differences in S-N curves. • TNSS constructs a life-predicting frame suitable for different notch types even including plain and sharp notches. • IR- p i relationship establishment is the key to comprehensively considering material and structural factors for finely predicting fatigue life. 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. [ABSTRACT FROM AUTHOR]

Simplified methods for elastoplastic calculations offer a cost-effective and time-saving solution to determine the mechanical state at the notch tip of a structure. Recently, a method using an Adjustable Localization Operator (ALO) has been developed, drawing inspiration from Eshelby's inclusion problem. This method has demonstrated its efficiency in addressing various engineering challenges owing to its robustness and inherent ability to handle multiaxial conditions. This paper aims to propose an alternative non-linear formulation of the ALO method to improve the accuracy of mechanical state predictions. To achieve this goal, Buczynski–Glinka's heuristic, which provides a non-linear plasticity correction, is expressed within the framework of the ALO method to highlight the ingredients required for a non-linear plastic correction. Subsequently, the parameters of the ALO method are adjusted based on these findings. To validate the approach, two cases are studied: a notched structure under plain strain conditions, and a more complex saddle notched structure subjected to local multiaxial proportional loadings. In both cases, the non-linear ALO method has shown a greater capability to reproduce the finite element reference results compared to the linear version of the method or to energetic methods. • Simplified methods based on an Adjustable Localization Operator are studied. • A nonlinear plastic correction is developed using Buczynski–Glinka's heuristic. • An original approach for Adjustable Localization Operator identification is proposed. • A simulation of multiaxial configuration is used to evaluate the proposed method. [ABSTRACT FROM AUTHOR]

• Closed-form analytical solutions for loading structural strain, unloading structural strain, and structural strain range for elastic-perfectly plastic material models. • Accurate results for structural strain range against those obtained with the numerical simulation (FEA) and the in-house modeling tools. • Similar results between the closed-form solutions and the numerical solutions obtained from the modified Ramberg-Osgood model in terms of structural strain range. • Excellent data correlation between predicted results and the test data available. • General analytical framework for future strain hardening and multiaxial fatigue analyses. A consistent and unified analysis framework is proposed in this paper for extending the early traction-based structural stress method to low-cycle fatigue regimes in fatigue evaluation of welded structures through a novel structural strain method. Instead of local strain or notch strain, the structural strain here refers to a through-thickness strain distribution that satisfies plane-remains-plan conditions under elastic–plastic deformation conditions. This paper provides closed-form solutions to structural strain distributions corresponding to loading and unloading, structural strain range, an equivalent structural strain range, and the structural stress–strain relation by considering elastic-perfectly plastic material behavior. Both finite element method (FEA) simulations and in-house modeling tools confirm the accuracy of these closed-form analytical solutions. Although these solutions are derived based on elastic-perfectly plastic material assumption, the resulting structural strain range, as a unique fatigue damage parameter for welded components, is shown essentially the same as those obtained numerically by considering a modified Ramberg-Osgood material model under pulsating loading condition, i.e., stress ratio equals zero. Good data correlation between the equivalent structural strain range predicted results and the test results obtained for a welded steel structure under the same loading condition is achieved. Finally, the structural strain range-based parameter is proven effective in correlating a large number of both low-cycle and high-cycle fatigue test data of welded structures made of several structural materials into a narrow scatter band demonstrating the universality of the structural strain method. [ABSTRACT FROM AUTHOR]

The response of maritime structures can be multiaxial, involving predominant mode-I and non-negligible mode-III components. Adopting a stress distribution formulation based effective notch stress as fatigue strength parameter for mixed mode-{I, III} multiaxial fatigue assessment purposes, a mode-I equivalent von Mises type of failure criterion has been established at the critical fracture plane. Counting includes a cycle-by-cycle non-proportionality measure and damage accumulation is based on a linear model. Distinguished mode specific and material characteristic strength and mechanism contributions in terms of respectively the resistance curve intercept and mean stress induced response ratio coefficient, resistance curve slope and material characteristic length, have been incorporated. Evaluating the mid-cycle fatigue resistance, the outperformance is impressive. The analysed multiaxial mode-{I, III} data fits the uniaxial mode-I reference data scatter band and a single resistance curve can be used for fatigue assessment. • Strength and mechanism contributions are mode specific and material characteristic. • The effective notch stress and shear stress coefficient cover strength and mechanism. • Incorporated strength and mechanism provides improved fatigue life time estimates. • Multiaxial fatigue resistance data fits the uniaxial mode-I data scatter band. • Assessment outperformance in terms of fatigue resistance scatter is impressive. [ABSTRACT FROM AUTHOR]

[Display omitted] • High frequency fatigue data were measured on smooth and notched samples made of 304L steel. • The critical length parameter was derived from experimental data based on critical distance theory. • The critical length parameter serves for lifetime predictions of notched samples and components. • The length parameter increases from 0 to 2.5 mm with increasing applied stress amplitude. In this paper, the fatigue data of smooth and notched specimens are evaluated using the method based on the theory of critical distances. This method is suitable for predictions of the fatigue lifetime of notched specimens using the axial stress distributions, the length parameter, and experimental data measured on smooth specimens. The critical length parameter l c r is determined from the relationship between two fatigue curves (one generated by testing smooth specimens and the other generated by testing specimens with a model notch). Experimental fatigue data are obtained using an ultrasonic fatigue pulsator at a frequency of 20 kHz and a load stress ratio R = −1. The paper describes the dependence of the critical length parameter for different notch radii on the number of cycles to fracture, as well as on the applied stress amplitude. The obtained results prove the possibility of using the critical length parameter for fatigue life prediction of notched specimens. [ABSTRACT FROM AUTHOR]

• Continuous flax fibers increase notched and plain specimens' tensile and fatigue strength. • Fatigue behavior at low and high cycles changes with notch type and fiber content. • The hysteresis of 3D-printed flax fiber composites differs in tension and compression. Nowadays, additive manufacturing is increasingly used for prototypes and low-volume products. Firstly, it offers great design freedom; however, the components must inevitably have certain design artifacts that cause stress concentrations. To this end, composites of biodegradable PLA (polylactic acid) matrix and continuous flax fibers were fabricated in this study using material extrusion 3D printing. Static characterization and mechanical fatigue tests between 103 and 2·106 load cycles were performed for plain specimens, three types of notches for the matrix, and two composite configurations. The notches only slightly affected the static performance of the tested materials. Accordingly, compared to the unnotched versions, the PLA matrix and composites exhibited a reduction in tensile strength of up to 8% and 19%, respectively. It was found that compared to the PLA matrix, the fatigue strength at high cycles increased by 1.8 and 2.3 times for the plain composite with a fiber volume fraction of 7.8% and 14% fibers, respectively. In the fatigue of notched composites, the notch effect is reduced by fibers in the tip. The fatigue strength is up to 1.9 and 2.9 times higher compared to the notched matrix for the composite with a smaller and larger volume fraction, respectively. [ABSTRACT FROM AUTHOR]

The fatigue crack growth behavior of friction stir welded joints of AZ31 Mg alloy is investigated for three different notch locations. The extended finite element method based simulation is developed to capture the effect of stir zone on fatigue crack growth behavior. The notch locations with asymmetric material domain experience an asymmetric stress field, which leads to crack path deviation from the orthogonal direction to the applied force. This leads to a longer crack path. The near-tip stress field from the simulation enhances the understanding of unusual crack path. The experimental results including crack path trajectories from experiments compare well against the numerical predictions. [ABSTRACT FROM AUTHOR]

• An analytical formulation to design blunt notched specimens to be tested in an ultrasonic machine is developed. • The analytical formulation is numerically verified and experimentally validated. • An experimental fatigue campaign of S690 steel with smooth and notched specimens is performed. • The fatigue behaviour of S690 steel in VHCF regime is assessed. • The mechanisms of fatigue failure are investigated. The presence of a notch or a detail which leads to stress concentration due to complex geometries has a considerable impact on the fatigue life of engineering structures. Therefore, on the one hand, the notch effect must be investigated and considered in the fatigue assessment of these structures. On the other hand, in the last decades, an interest in studying the very high-cycle fatigue regime (VHCF) has been perceived, due to the extension of the service life of engineering components beyond the 10 million cycles. Thus, the notch effect in the VHCF regime was analysed for the S690 structural steel. Therefore, an analytical approach to design notch specimens was developed and a numerical simulation was performed to assess it. Then, an experimental campaign with smooth and notched specimens with two different stress concentration factors was performed in an ultrasonic fatigue machine to assess the VHCF behaviour. The mechanisms of failure and fracture surfaces were also investigated. It was observed a fully notch sensitivity as well as the dominance of surface crack initiation for the S690 steel in the VHCF regime. [ABSTRACT FROM AUTHOR]

• A newly concept of energy field intensity (EFI) for notch fatigue analysis. • An energy field intensity-based procedure for LCF life prediction. • Modified Ellyin's model for multiaxial fatigue life prediction. • EFI approach yields better predictions of GH4169 and Al 7050-T7451 alloys than the other three criteria. • EFI approach provides a reference to deal with notch effect of arbitrary geometry features. Notched components with complex configurations are normally designed for engineering functional requirements. Geometrical discontinuities generally result into stress concentration and multiaxial stress states and eventually lead to fatigue failure. However, highly efficient methods for notch fatigue analysis of engineering components are still lacking. In this regard, by combining energy criteria with FE simulations, a new concept of energy field intensity (EFI) is proposed and an EFI-based framework is established for notch fatigue analysis of engineering components under multiaxial stress states. Better correlation of model predictions with experimental results of GH4169 and Al 7050-T7451 alloys are provided by the proposed model rather than that by Fatemi-Socie, Sun-Shang and energy criteria. [ABSTRACT FROM AUTHOR]

• Static and fatigue tests were conducted to understand the notch effect on strength and fatigue life of composite laminates. • Experiments show that the notch effect is dependent on fibre type, notch-depth, load type and load-sequence. • A modified model was proposed to consider the effects of the notch and the stress-ratio. • A progressive damage algorithm was developed to predict residual life of composites under spectrum-loading. • Good agreement between experiments and predictions has been achieved. This paper presents an experimental and numerical study of the notch effect on strength and fatigue life of double edge notched (DEN) composite laminates made of woven glass fibre lamina 3238A/EW250F and woven carbon fibre lamina 3238A/CF3052. Experimental results show that the notch effect is dependent on fibre type, notch depth, load type and load sequence. A s - n - R - d - r residual strength model was proposed to account for the effects of the notch and the stress ratio, and a progressive damage algorithm was developed to predict damage propagation and residual life of composites under spectrum fatigue load. Good agreement between the experimental results and the numerical predictions has been achieved. [ABSTRACT FROM AUTHOR]

• Highly stressed volume (HSV)-based method for modeling both notch fatigue and size effects. • Proposed relation of equivalent highly stressed region based on the maximum local stress. • A dynamic model coefficient to characterize different HSVs with similar stress state on fatigue life. • Proposed model predictions agree well with the probabilistic scatter bands of experiments. Modeling of the notch and size effects on fatigue behavior of materials is vital for ensuring structural integrity and reliability of engineering components. This study presents a methodology considering both effects of notch and size to analyze the fatigue life distribution of specimens with different geometries using the highly stressed volume approach. Specifically, a dynamic model coefficient considering the influence of different maximum local stresses is developed by modeling the size effect of highly stressed volumes with Weibull distribution. Experimental data of three aluminum and titanium alloys are utilized for model validation and comparison. Fatigue lives of three materials with different geometries are evaluated respectively, and predicted P-S-N curves indicate that proposed model predictions agree well with the probabilistic scatter band of experimental results. [ABSTRACT FROM AUTHOR]

• The fatigue-endurance strength of double-edge notches is assessed. • The stress state of quarter-elliptical and through cracks is evaluated by finite elements. • The notch life estimates agree well with experimental observations. • The shot-peening and applied stress affect the notch failure performance. This paper presents an investigation on the failure of plates with double-edge notches and fatigue damage. The main aim is to develop a computational model for estimating the behaviour of a quarter-elliptical corner crack or a through-the-thickness crack located at semicircular edge notch. In such an analytical/numerical analysis, driving forces are evaluated by means of an analytical model and the finite element method, whereas the fatigue life is assessed employing a stress ratio-dependent crack growth law and the stress intensity factor. [ABSTRACT FROM AUTHOR]

• The yield strength of notched structures is barely dependent on notch geometry. • Local plasticity leads to reduction of notch induced stress concentrations. • The lifetime to failure shows clear dependence on notch geometry. • Local defects can superimpose notch effects under cyclic load. • Filigree structures have lower strength and crack initiation resistance. Powder bed based additive manufacturing techniques enable for fabrication of filigree and geometrically complex structures. These structures typically are characterized by many cross section transitions, leading to notch induced stress concentration. Furthermore, the process induced surface roughness cannot be always mechanically removed as the surfaces are not accessible using conventional tools. Although these factors are critical for many different applications and materials, previous works mainly focused on notch effects in brittle materials, such as the common Ti-6Al-4V alloy. Systematic examination of notch induced stresses in ductile materials, especially under the constraints of process induced surface roughness and small part dimensions, was not conducted. Thus, the present study addresses these issues by evaluation of the mechanical performance of notched samples made of 316L stainless steel under quasi-static and cyclic loading. By means of digital image correlation insights into the local deformation mechanisms and the damage evolution, respectively, were obtained. Microstructural analysis by electron backscatter diffraction provided profound explanation for the phenomena observed. [ABSTRACT FROM AUTHOR]

• Experimental investigation of damage mechanism in AM metals. • Influence of heat treatment and process parameters on crack path. • Influence of surface roughness on crack path. • Correlation of multiaxial fatigue data based on criteria consistent with damage mechanism. Additive Manufacturing (AM) has recently gained much attention due to its many advantages. Nevertheless, design of critical load carrying parts via AM is still at its infancy because the damage mechanism and evolution of AM metals under cyclic loading are not yet understood, particularly under complex multiaxial loading conditions. This work investigates fatigue cracking behavior of two AM metals including Ti-6Al-4V and 17-4 PH Stainless Steel and their wrought counterparts using both unnotched and notched tubular specimens. Cracking mechanisms and paths under a variety of conditions including annealing and HIPing processes, surface roughness, and build orientation are evaluated. [ABSTRACT FROM AUTHOR]

Highlights • A fatigue analysis of a type of fillet welded joint is presented. • Four welded joint series for evaluating the fatigue performance were produced. • The fatigue behaviour combines an experimental campaign and numerical analysis. • In this fatigue analysis two main numerical tools – FEM and XFEM – were used. • Fatigue predictions based on crack initiation and propagation phases were made. Abstract This paper presents a fatigue analysis of a type of fillet welded joint representative of one main joint of the steel box girder of the Alcácer do Sal railway bridge. From previous studies, it was found that the welded joint between the box girder diagonal and the central hanger gusset is one of the most stressed details of the bridge. This welded joint was not fully manufactured according to current construction procedures, as regards the fillet weld end configuration. In order to assess the fatigue behaviour of such welded joint, the present study combines an experimental campaign and numerical analysis. A total of four welded joint series were produced in order to allow the comparison of the fatigue performance of similar type of welded joint of the Alcácer do Sal bridge with welded joints produced according to existing recommendations, such as EC3. Since scale-down specimens were considered, two different thicknesses were included in this study for each joint configuration, to allow the verification of any thickness effect. Concerning the numerical analyses, two main numerical tools were used: the standard Finite Element Method (FEM) with ANSYS and the eXtended Finite Element Method (XFEM) with ABAQUS. Fatigue life predictions were performed including both fatigue crack initiation and fatigue crack propagation phases. The number of cycles to initiate a fatigue crack was computed using local notch strain-life approaches, and the number of cycles for fatigue crack propagation was computed by integrating the Paris fatigue crack growth law with stress intensity factors computed with ANSYS (virtual crack closure technique) and ABAQUS (contour integral method, 3D XFEM model). Experimental tests demonstrated little influence of fillet weld end geometry on fatigue behaviour of welded joints and plate thickness effects were also reduced as also confirmed by the similar fatigue crack propagation rates. Both numerical simulations provided very accurate predictions of the experimental S-N curves, however the XFEM modelling opens new possibilities for mix-mode fatigue crack propagation simulations. [ABSTRACT FROM AUTHOR]

Highlights • Local linear-elastic stresses allow notch fatigue to be assessed accurately. • Material length parameters are successful in estimating uniaxial/multiaxial fatigue lifetime of notched metals. • The MWCM/PM and the SED approach can be applied by post-processing linear-elastic FE results. Abstract This paper assesses the accuracy and reliability of the Theory of Critical Distances (TCD) and the Strain Energy Density (SED) approach in estimating the lifetime of plain and notched specimens subjected to cyclic loading. To validate the two approaches for plain and notched components under uniaxial and multiaxial fatigue loading, a large bulk of experimental data taken from the literature was re-analyzed, with the state variables, i.e. the stress distributions and the strain energy density, being calculated via the Finite Element (FE) method. The results obtained demonstrate that both the TCD and the SED approach can provide highly accurate fatigue life estimation. In addition, these two approaches require little computational effort and little calibration information to be implemented and used for fatigue design in situations of practical interest. [ABSTRACT FROM AUTHOR]

• The effect of LSP on the fatigue life of notched specimens is studied. • Fatigue tests of LSP-treated aluminum alloys are carried out. • A life prediction method is proposed by introducing effective stress to TCD. • The effective stress is constructed by using initial and compressive stress. The effect of Laser shock peening (LSP) on the fatigue life of notched metallic specimens is studied. LSP is applied to notched specimens made of two aluminum alloys and the fatigue tests are carried out. The stress field at the notch is both measured by X-ray diffractometer and simulated through numerical methods. The highest compressive stress lies in the layer near but a little below the surface. A life prediction method is proposed by introducing the effective stress after LSP into the Theory of Critical Distances (TCD). The predicted results are in sound agreement with the test results. [ABSTRACT FROM AUTHOR]

• A new life prediction model based on the strain energy reformulated critical distance theory is proposed. • Experimental data of 4 materials verified the model predictions accuracy. • The influence of size effect on critical distance and fatigue life is quantified. Notch and size effects show significant impact on the fatigue performance of engineering components, which deserves special attention. In this work, strain energy reformulated critical distance theory was developed for fatigue life prediction of notched components under size effect. Experimental data of different notched specimens manufactured from GH4169, TC4, TC11 alloys and low carbon steel En3B were used for model validation and comparison. Results indicate the proposed model works better than the Yang's and the Shen's models. [ABSTRACT FROM AUTHOR]

• We performed a series of experiments to explore fatigue performance of smooth and notched specimens made of 34CrNi3Mo alloy steel under torsional loading. • Effect of notch on the fatigue performance of 34CrNi3Mo alloy steel under torsional loading is studied by comparing experimental results. • Micrograph and macrograph fractures of the 34CrNi3Mo specimens have studied to explore their failure mechanism and effect of notch on the fatigue performance. • Fatigue performance of 34CrNi3Mo specimens with v-notch at angles of angles of 45°, 90° and 135° was compared to exploit effect of notch by using validated numerical method. Joint of flange on a marine shaft subjected to torsional loading was more prone to occur fatigue fracture than the other section on the shaft due to stress concentration effect, so torsional fatigue performance in the vicinity of the joint should be payed more attention in terms of safe navigation of ships. The fatigue analysis of 34CrNi3Mo steel as a novel alloy material with a stress concentration factor of 1.45 under cyclic torsional loads was carried out based on the median S - N experimental method by simplifying the flange and shaft joint into 34CrNi3Mo specimens with u-notch subjected to torsional fatigue loading. The S - N curve and fatigue life of 34CrNi3Mo alloy steel were numerically calculated and compared with experimental fatigue life and the plotted S - N curve. It was found that fitting equation of the S - N curve was close to experimental results. Then, the fatigue performance of 34CrNi3Mo specimens with v-notch at angles of 45°, 90° and 135° was compared by means of the validated numerical method. The fracture macrographs and micrographs of the 14 specimens also were provided to explore the effect of notch on fatigue performance of the 34CrNi3Mo materials and to exploit the failure mechanism of fracture. [ABSTRACT FROM AUTHOR]

This paper presents the problems relating to the determination of the material constants and their assessment. The study demonstrated that the scatter of the values of the fatigue life and stresses applied for determination of the σ a -N f characteristics could have an effect on the coefficient m representing the slope of this characteristics. The phenomenon associated with the variability of a number of material parameters relating to the number of cycles to failure is discussed. In addition, the paper reports on the influence of the number of cycles to failure on the value of the notch factor. The paper also discusses the function accounting for the effect of the geometrical K W and structural K S notches based on the relation stating the dependence of K f coefficient on the number of cycle to failure. [ABSTRACT FROM AUTHOR]

The present paper focuses on the application of the total strain energy density approach to assess fatigue life in notched samples subjected to multiaxial loading. This approach requires, as a starting point, a fatigue master curve defined in terms of total strain energy density versus number of cycles to failure, which is usually determined from a set of uniaxial fatigue tests using smooth standard specimens. In order to reduce the time and cost associated with the generation of the fatigue master curve, a straightforward methodology based on the outcomes of only two uniaxial strain-controlled tests is proposed. The methodology is applied to round bars with U-shaped notches subjected to proportional bending-torsion loading histories. A very good correlation between the experimental and the predicted life of the notched specimens is observed. In addition, the theoretical predictions are nearly independent of the pairs of uniaxial strain-controlled tests selected to obtain the fatigue master curve, which indicates a good robustness of the suggested methodology. [ABSTRACT FROM AUTHOR]

While part I of this paper was focused on evaluating multiaxial variable amplitude fatigue life estimations for un-notched specimens, part II extends the same critical plane-based analysis procedures to situations involving notched specimens. In addition to the factors considered in the un-notched analyses, local stress concentrations, stress gradient effects, and changes in local stress state must also be accounted for in the presence of a notch. This was accomplished in the current study by coupling the Theory of Critical Distances point method with a pseudo stress-based plasticity modeling technique. Then, a modified version of the Fatemi-Socie parameter was used to calculate fatigue damage, and changes in life estimation accuracy were studied with respect to the consideration of transient material deformation behavior, crack initiation definition, and damage summation rule. Results from the notched specimen analyses were also compared to those for un-notched specimens, and some discussion is provided. While the effect of transient deformation behavior and crack initiation definition were found to be relatively small for the loading histories used in this study, changing the critical damage sum at failure had a much greater impact on life estimations. Although some of the analysis procedures investigated were able to estimate nearly all fatigue lives within a factor of 3 of experimental results, several areas were identified where there is potential for even further improvements to be made. These include issues related to the accuracy of life estimation curves, damage calculation models, and/or the modeling of material deformation behavior. [ABSTRACT FROM AUTHOR]

The metal additive manufacturing technology produces components of any geometrical complexity directly from a computer model. Full exploitation of metal AM technology may result in parts that can be lighter in weight, cheaper to produce, and have complex geometries that are difficult or impossible to produce with conventional methods. Two aspects significantly affecting the fatigue behavior of DMLS Ti6Al4V are investigated: (i) the as-built surface condition, which is known to introduce a significant knock-down factor in fatigue with respect to the machined condition, and (ii) the presence of a geometrical notch, which affect conventional materials but has not yet been studied in metal AM parts. After the preliminary determination of the reference unnotched fatigue response, the notch fatigue behavior of as-built DMLS Ti6Al4V specimens is determined. Four different notch fatigue factors are estimated including the influence of notch direction with respect to build direction and the notch fabrication conditions. [ABSTRACT FROM AUTHOR]

The critical distance length should in principle be deduced from the plain specimen fatigue limit and the threshold stress intensity factor range. However, the threshold range is difficult to measure experimentally, hence this length is usually obtained by means of a notched specimen. The critical distance inverse search, both according to the Line and the Point Methods, is presented in this paper referring to a relatively sharp V-notched specimen. Precise indications about the geometry parameters are given along with a complete analytical procedure to easily obtain the critical distance. A sensitivity analysis is discussed, providing evidence of a critical distance range for a well-posed inversion problem. [ABSTRACT FROM AUTHOR]

• A FFM model is developed to estimate the fatigue lifetime of notched components. • FFM is based on the simultaneous fulfilment of a stress and an energy condition. • Basquin's equations are employed to catch the variation of input parameters. • The number of cycles to failure and the critical crack advance represent FFM outputs. • The approach is validated against different data sets available in the literature. • FFM can be implemented for a characteristic length exceeding the ligament. A novel model is proposed to assess the fatigue lifetime of notched components under uniaxial loading conditions in the medium/high-cycle fatigue regime. It is based on the Finite Fracture Mechanics approach, previously proposed in static and fatigue limit frameworks, according to which failure occurs when a stress and an energy condition are simultaneously satisfied. Basquin's equations are employed to catch the variation of input parameters based on SN data of a plain and a notched sample. The criterion is validated against various experimental data sets from the literature, encompassing different notch shapes, loading conditions and materials. [ABSTRACT FROM AUTHOR]

• A shaker-based fatigue test is conducted to study the notch fatigue behavior. • A novel high-resonance-frequency notched specimen is designed for high-efficient test. • Different notch fatigue crack initiation mechanisms have been revealed. • A modified TCD method is proposed to predict the notch fatigue strength. The notch fatigue behavior of a titanium alloy TC17 in the very high cycle fatigue (VHCF) regime is investigated by a vibration-based bending fatigue test. A complete S - N relation of notched specimens with high resonance frequencies is obtained, covering the lifetime up to 109 cycles, and the fracture surfaces are observed. The results show the TC17 exhibits relatively high fatigue notch sensitivity. Several fatigue crack initiation mechanisms relevant to the notch are then revealed. Furthermore, a notch fatigue prediction model based on a modified theory of critical distance is proposed by introducing a special weight function, which is demonstrated to have good prediction accuracy. [ABSTRACT FROM AUTHOR]

Based on the theors of elasticity for linear elastic materials and strain partitioning, a method was developed to predict fatigue lives of load bearing Ti-6Al-4V ELI implants with surfaces modified in the micrometer scale to promote bioactive materials and accelerate osseointegration. Notched fatigue specimens were characterized by confocal laser scanning microscopy (CLSM) and subjected to axial fatigue tests at a constant maximum stress to determine the Siebel and Stieler’s C constant. The mean value obtained for C was employed to determine the fatigue resistance ( K f factor) allowing the life prediction of samples with different sized notches. The method was validated by performing fatigue tests with samples modified at two scales, 2 μm and 10 μm, typical values that can be produced in industrial conditions. It was confirmed that the prediction method was in agreement with the experimental results. In addition, the prediction method clarified the strong reduction in the fatigue resistance from 850 MPa to 594 MPa due to small differences in surface roughness, a result unexplained in the literature so far. [ABSTRACT FROM AUTHOR]

In a recent contribution by Tanaka, the fatigue behaviour of notched bars made of austenitic stainless steel, SUS 316L, and carbon steel, SGV 410, characterized by different values of the notch tip radius was investigated under torsion loading. Tanaka monitored both fatigue crack initiation and propagation phases by means of the potential drop technique. The crack initiation life is correlated here to the depth of the initiated fatigue crack by means of calibration curves derived from electrical finite element (FE) analyses. In the present contribution, the approach based on the strain energy density (SED) averaged over a structural volume embracing the notch tip is employed to re-analyse the original experimental results of each material, by taking into account the crack initiation life, in order to exclude all extrinsic mechanisms acting during the crack propagation phase, i.e. sliding contact, friction and meshing between crack mating surfaces. [ABSTRACT FROM AUTHOR]

Variable amplitude fatigue behavior of injection molded thermoplastics was investigated. A neat thermoplastic (polypropylene impact co-polymer) and a thermoplastic composite made of polybutylene terephthalate (PBT) with 30 wt% short glass fibers were used for the experimental study. Fatigue tests were conducted on both un-notched and notched specimens using plate-type specimens. For the notched specimens, a circular hole was drilled in the center of the specimens. Two-step loading (high-low and low-high) tests at two cycle ratios of 0.2 and 0.5 and stress ratios of R = 0.1 and −1 were conducted to investigate load sequence and mean stress effects, as well as the accuracy of linear damage rule. Periodic overload and service load history tests were also conducted to investigate the effect of overloads, cycling frequency, small stress cycles, and maximum stress level. A strengthening effect of increased frequency was observed for both materials and the effect of small stress cycles was found to be negligible. A general fatigue damage model along with the rainflow cycle counting method and linear damage rule were used for life predictions of variable amplitude service load history tests. More than 89% of the life predictions based on the modeling procedure used were within a factor of 3 of the experimental lives for both materials. [ABSTRACT FROM AUTHOR]

Notched AM50 sheet metal specimens with four different stress concentration factors are tested under quasi-static and cyclic uniaxial loading, and the results are analyzed and compared with those of unnotched specimens. Special attention is given to the formation of bands of twinned grains and the inhomogeneous local strain field in the vicinity of notches, which is measured by an in situ optical strain measurement technique. It is shown that the notch sensitivity is reduced, if bands of twinned grains occur. In the range between 500 and 1000 load cycles to fracture, the endurable load is only slightly reduced by the presence of a notch for completely reversed fatigue tests. In contrast, specimens are notch sensitive under loading conditions, where no twin formation occurs. [ABSTRACT FROM AUTHOR]

• A unified physics-informed machine learning (PIML) approach with simplicity, high-efficiency and high-accuracy is developed to predict the notch fatigue life of polycrystalline alloys. • Physics-informed parameters are introduced into the PIML model to reach a better predictive performance and generalization capability than physics-based models. • The key feature parameters for notch fatigue are accurately identified by the global sensitivity analysis method. • PIML model based on Latin hypercubic sampling achieves the prediction of probabilistic fatigue life and uncertainty assessment. Within this work, a unified physics-informed machine learning (PIML) framework is proposed for notch fatigue life prediction and key feature parameters identification of aerospace polycrystalline alloys. The unified PIML approach reaches a capable accuracy in notch fatigue life prediction of the polycrystalline alloys under a wide range of notch geometries and loading conditions compared with physics-based life models. In addition, the global sensitivity analysis method is used to accurately identify the key feature parameters that affect the notch fatigue life. The nominal maximum stress and unnotched specimen reference life are recognized as highly relevant features for notch fatigue life whereas the notch root radius is the key parameter among the notch parameters. Finally, life uncertainty induced by the notch geometry parameters is performed by using the proposed PIML model based on Latin hypercube sampling, which accomplishes the probabilistic estimation of notch fatigue life well. In practical applications, the PIML framework can be applied efficiently to notch fatigue life prediction of a new material dataset due to its powerful generalization ability without additional parameter fitting or finite element analysis. The well-trained PIML model and the uncertainty assessment method provide potential tools for notch fatigue evaluation under complex loading conditions. [ABSTRACT FROM AUTHOR]

The high cycle fatigue (HCF) life of DD6 at 760 ℃ was studied by performing a large number of experiments. A new thermodynamics-based life prediction method was proposed by considering the thermodynamic entropy change during the fatigue process. The model is not only suitable for smooth specimens but also can be applied to the life evaluation of notched specimens and those with different orientations. Finally, the life of an aeroengine blade with different orientations was qualitatively evaluated by the proposed model, which demonstrated that the model can be used to analyze the influence of crystal-axis deflection angle on blade life. [ABSTRACT FROM AUTHOR]

• Plastic notch effects in fatigue strength due to their tolerance to non-propagating short cracks. • Consequences of plastic fatigue stress concentration factor on SN curves. • The Theory of Critical Distances (TCD) as a simplification of the stress gradient method. Elastoplastic (EP) stress gradient factors (SGF) ahead of notch tips are used to evaluate the actual notch effects in fatigue strength due to their tolerance to non-propagating short cracks, which depends on the notch stress concentration factor, on the stress gradient ahead it, and on the material. Considering the significant role of local plasticity in the growth behavior of short cracks within the notch plastic zone, a sound mechanical methodology is proposed to account for the effect of EP stress and strain fields in the actual fatigue stress concentration factor value, which includes its tolerance to short cracks. To validate the proposed methodology, life predictions based on numerical EP SGF are compared to experimental SN data of notched specimens made of different materials tested under uniaxial loads collected from the literature. [ABSTRACT FROM AUTHOR]

This paper presents two advanced methods for modelling the fatigue lifetime of welded components with an irregular distributed geometry and compares the results achieved. The first approach, applies a deterministic method in order to analyse implicit gradient models, whereas the second one examines a Weibull-based model using a stochastic method. Both models can be combined in order to encompass the stress gradient and the statistical size effect. In order to determine the parameters for both models, an examination of specimens from five different test series with different macro-geometries and from different test laboratories is performed. After measuring the geometry of the welded components using a laser scanning system with high accuracy and resolution, the linear-elastic notch stresses on the real geometries of each specimen are calculated using FEM. Effective stresses are calculated with different parameters using the implicit gradient model. These notch stresses and effective stresses are used as input parameters for a Weibull-based method to model the crack initiation phase. This initiation phase includes cracks with a length of 3–5 mm. Both methods can be applied on a standalone basis or in combination. However, it has been observed that either combining the two methods or using exclusively the implicit gradient model other than the Weibull model leads to no better results. [ABSTRACT FROM AUTHOR]

This short technical note summarizes some recent data from hot dip galvanized steel bolted connections under fatigue loading. In particular the effect of a galvanizing coating on the fatigue strength of S355 structural steel is analyzed in detail showing that the decrease of the fatigue life is very limited if compared with that of uncoated joints and the results are in good agreement with Eurocode detail category, without substantial reductions. The procedure for the preparation of the specimens is systematically described in this note providing a useful tool for engineers involved in similar practical applications. The results are compared with previous data from notched galvanized specimens weakened by a central hole and not treated specimens characterized by the same geometry. [ABSTRACT FROM AUTHOR]

• Notch fatigue strength of 3D-printed Ti6Al4V is investigated experimentally. • Notched 3D-printed Ti6Al4V is modelled as a linear-elastic, homogenous, and isotropic material. • Length scale parameters are used to assess notch fatigue strength of 3D-printed Ti6Al4V. • The employed design methods can be used along with commercial FE codes. The accuracy of the Theory of Critical Distances, Gradient Elasticity and the Averaged Strain Energy Density criterion in estimating fatigue lifetime of notched additively manufactured Ti6Al4V is assessed against numerous experimental results generated under load ratios equal to −1 and 0.1. The 3D-printed Ti-alloy under investigation was tested by keeping the notches in the as-manufactured condition. The common feature of the considered design approaches is that they all make use of a material length scale. The validation exercise based on the generated experimental results demonstrates that the length scale concept can be extended successfully also to the fatigue assessment of notched 3D-printed metallic components. [ABSTRACT FROM AUTHOR]

• The macroscopic stress concentration is the main reason for as-welded specimens. • The GB cementite segregation is the main factor for smooth-welded specimens. • The calculated value of fatigue notch coefficient is consistent with measured value. • The fatigue risk factor model can evaluate the fatigue performance. • The R f model can predict the fatigue strengths of as-welded specimens with weld toe. The fatigue fracture mechanisms and the fatigue property evaluations of S355J2W steel butt-welded joints relieved welding residual stresses with and without welding reinforcement were systematically investigated by microstructure characterization and fatigue tests. Experimental results show that the complex evolution of microstructure at welded joint results in the notable degradation of mechanical properties. The fatigue performance of the as-welded specimen with welding reinforcement is significantly poorer than that of the smooth-welded specimen without welding reinforcement. Stress concentration under the fish-scale pattern at the weld toe caused by geometric dimension change is the major reason for the fatigue fracture of the as-welded specimen. By comparison, the cementite segregation at grain boundary in the partial recrystallization zone of the heat affected zone caused by the sharp change of microstructure should be responsible for the fatigue fracture of the smooth-welded specimen. The fatigue notch coefficient (K f = 1.89) calculated by the effective notch stress method is consistent with that (K f = 1.9) measured by the fatigue tests, which shows that the effective notch stress method can well evaluate the multiple notch effect under the coupling of the weld toe and the fish-scale pattern. Furthermore, the fatigue risk factor (R f) model was employed to evaluate the fatigue performance of welded joint, and the intuitive R f curves show that the fatigue fracture occurs at the fatigue life exceeding 2 × 106 cycles, i.e., R fmax exceeds 0.7. [ABSTRACT FROM AUTHOR]

• Mixed mode FCG on FPB and modified CT specimens was simulated using the VCTD criterion. • The VCTD criterion accurately captured the FCG behaviour of different specimen geometries. • The resulting data was used to train ANN with up to 5 input variables, and one hidden layer with 5 neurons. • Trained ANN were able to predict the miss or sink hole behaviour of different modified CT specimens. • Both FCG paths and specimen lives were predict by the trained ANN. The purpose of this paper is to analyse fatigue crack growth under mixed mode conditions using artificial neural networks. Mixed mode conditions were achieved using a four-point bending specimen, with different notch positions, and a modified compact tension specimen, with one hole using different diameters and centre coordinates. Finite element analysis simulations, using the vector crack tip displacement propagation criterion, were used to train the artificial neural networks. Once fully trained, the artificial neural networks were able to predict fatigue crack growth paths and lives under mixed mode conditions, showing good agreement with both experimental and numerical examples. [ABSTRACT FROM AUTHOR]

• Effect of artificial notches is studied in terms of fatigue crack non-propagation behaviors. • Non-propagation crack rate firstly increases and then decreases with the increased notch depth. • Gradient residual stress and microstructure are responsible for the crack non-propagation. • Crack traces of post-mortem specimen in the hardened layer and the matrix are compared. Abnormal damages in railway axles can lead to a significant hazard to running safety and reliability. To this end, a surface treatment was selected to effectively inhibit fatigue crack initiation and growth. In this study, a single-edge notch bending fatigue test campaign with artificial notches was conducted to elucidate the fatigue crack non-propagation behavior in railway S38C axles subjected to an induction hardening process. The fatigue cracking behavior in the gradient structure was revealed by optical microscopy, electron backscatter diffraction, and fractography. The microhardness distribution was measured using a Vickers tester. The obtained results show that the microhardness of the strengthening layer is nearly triple that of the matrix. Owing to the gradient microstructures and hardness, as well as compressive residual stress, the fatigue long crack propagates faster once it passes through the hardened zone (approximately 2.0 mm in the radial depth). Thereafter, local retarding (including deflection, branching, and blunting) of the long crack occurs because of the relatively coarse ferrite and pearlite in the transition region and matrix. Totally, this fatigue cracking resistance is reasonably believed to be due to the gradient microstructure and residual stress. These findings help to tailor a suitable detection strategy for maximum defects or cracks in railway axles. [ABSTRACT FROM AUTHOR]

• Models with realistic surface roughness profiles and single notches are generated. • Fatigue indicator parameters are computed from crystal plasticity simulations. • Different notch geometries, textures, and grain morphologies were examined. • Notch depth is more detrimental to fatigue resistance than notch radius. • Stress intensification field is unaffected by texture or number of grains sampled. Fatigue life knockdown due to surface roughness induces local stress concentrations. Physical fatigue experiments are crucial to qualify materials for use but are limited in their capability to examine the mechanisms underlying the fatigue crack formation driving force due to these stress concentrations. Microstructure-sensitive computational models can aid in this regard and are employed in this work to examine the effects of surface roughness in Al 7075-T6. Digital microstructure models are subjected to crystal plasticity finite element method (CPFEM) simulations to examine the combined effects of microstructure and grain scale asperities on the driving force for fatigue crack formation. Following elastic–plastic shakedown, mesoscale volume-averaged fatigue indicator parameters (FIPs) are computed within fatigue damage process zones of grains. An increase in the intensity of realistic surface roughness profiles corresponds accordingly to larger FIPs but it is difficult to precisely interpret the mechanisms and effects of this intensification. We thus parametrically investigate surface asperities that couple with microstructure and lack of constraint on slip at the free surface to describe microstructure-sensitive surface roughness knockdown effects on fatigue resistance. The depth of a single valley (i.e., notch) due to surface roughness is found to be more detrimental to fatigue resistance than the radius (i.e., notch width, not to be confused with notch acuity). We also find that different crystallographic textures, grain morphologies, and number of sampled grains have a limited effect on the depth of influence of these notches. [ABSTRACT FROM AUTHOR]

• TMF tests were investigated on specimens manufactured by precision casting with holes. • TMF deformational behaviors of the specimen with holes were studied. • The stress concentration at the hole led to crack nucleation and final failure. • A TMF life prediction method based on stress concentration was proposed. Thermomechanical fatigue behavior of a directional solidified superalloy was investigated. Circular holes were designed on the specimen manufactured by precision cast method. The results indicated that the TMF damage is mainly controlled by fatigue, creep and oxidation damage, and the stress concentration at the holes results in crack nucleation and final failure. A new TMF life prediction method for specimens with stress concentration is proposed. The stress gradient is introduced to consider the effect of stress concentration at hole and notch in this method, which is verified by TMF data of smooth, notched and hole specimens. [ABSTRACT FROM AUTHOR]