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• This paper reports a damage-incorporated thermal–mechanical simulation method. • The proposed method predicts closely the welding residual stresses and fatigue life. • The proposed method is applicable for both high-cycle and low-cycle fatigue problems. • Welding residual stresses impose significant effects on high-cycle fatigue life estimation. Welding residual stresses impose significant influences on the fatigue behavior and structural integrity of welded connections in various engineering applications. Understanding the effects of these residual stresses on fatigue life is crucial for ensuring the reliability and safety of welded structures. This paper proposes a damage-incorporated thermal–mechanical simulation method to examine the effect of welding residual stresses on the fatigue life prediction of welded plates and circular hollow section (CHS) X-joints. Compared to the experimental X-ray diffraction measurements and the fatigue tests, the predicted residual stresses and fatigue life demonstrate good agreement with experimental results for both high-cycle and low-cycle fatigue. The location of fatigue cracks resembles closely the estimation from a uniform fatigue damage indicator. The fatigue life prediction without incorporating the welding residual stresses does not affect significantly the life estimation for low-cycle fatigue but can cause substantial over-predictions for high-cycle fatigue. [ABSTRACT FROM AUTHOR]

• Interaction between crack closure and in-plane constraint was examined. • ΔCTOD p can both account for the cyclic load and constraint effect. • Crack grows faster in structure that generates mild in-plane constraint. • Mechanism of in-plane constraint effects on CDF was analyzed. Nonlinear fracture mechanics parameters, ΔCTOD p and Δ J are implicitly used to characterize the crack driving force (CDF) in estimating Fatigue Crack Growth Rate (FCGR) in ductile material. However, in the presence of structure geometry effect, evaluation of effective CDF remains to be clarified. In this regard, a comparative study was conducted and the in-plane-constraint parameter was introduced to quantify geometry effect based on the constraint theory. Using modified boundary layer model (MBL) with multi-stage node releasing embedded, the effect of in-plane-constraint on ΔCTOD p and Δ J was investigated. Results indicate that ΔCTOD p can demonstrate geometry effect on CDF, as it correlates well with plastic deformation. Compartively, the effective cyclic J- integral (Δ J op) was found to have a significant dependency only on the crack closure level, thus incapable of capturing the effect of geometry on CDF. A humpbacked curve was found between in-plane constraint level and ΔCTOD p. Further investigation reveals that the gain of in-plane constraint on FCGR can be attributed to the interaction between in-plane constraint and crack closure, as well as the interplay between in-plane and out-of-plane constraint. This work not only reveals the mechanism of the in-plane constraint effect on CDF, but also provides a basis for establishing FCG model across different structure geometries. [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]

• The growth and coalescence of many cracks reduces the fatigue lifetime, but. • experiments using the replica technique and a new model suggest: • The reduction compared to a single crack can hardly be greater than a factor 2. In many metals and alloys, low-cycle fatigue (LCF) and thermomechanical fatigue (TMF) generate a large number of surface microcracks. Using the replica technique, we document their growth and coalescence in the austenitic steel 1.4550 and the nickel base alloy Inconel 100. A model based on elastic–plastic fracture mechanics is developed, which considers the growth and coalescence of many coplanar semi-elliptical surface cracks. The results of the model are consistent with the experiments. An important conclusion is that the fatigue lifetime predicted by the multi-crack model is not substantially shorter than that predicted by a corresponding model assuming only a single semi-circular surface crack. This means that lifetime assessment can still be based on single-crack data, if one considers a factor 2 to include scatter and multiple-crack effects. [ABSTRACT FROM AUTHOR]

The length-scale dependence of fatigue crack growth is evaluated for a set of metallic materials, namely titanium Ti-6Al-4V, ductile iron EN-GJS-500-7 and tool steel AISI H13, by performing fatigue crack growth tests on geometrically similar compact C(T) specimens of different sizes. With references to length-scale-invariant variables, notably the crack growth rate normalised by the specimen width, it is demonstrated that fatigue crack growth is not a length-scale-invariant process for the tested conditions. In particular, the length-scale dependence is less significant for larger specimens and at longer crack lengths. The test results also contradict the hypothesis of similitude, i.e., that the growth rate is uniquely related to the stress-intensity-factor range, as smaller specimens manifest a higher growth rate when compared at the same stress-intensity-factor range. The observations are in line with presented fracture-mechanical demonstrations, which show that the Paris–Erdogan law depends on the length scale whenever fatigue crack growth is anticipated to be scale invariant. • Paris–Erdogan law imposes an unexpected length-scale dependency. • Fatigue crack growth is generally length-scale dependent. • For large specimens, long cracks and high stress, the scale dependency is smaller. • The hypothesis of similitude is contradicted by varying the specimen size. [ABSTRACT FROM AUTHOR]

[Display omitted] • Y addition significantly increases the fatigue life of the IN713C alloy. • The decrease of porosity is attribute to the decrease of solidification temperature range. • Fatigue cracks in IN713C alloy mainly originate from pores and carbides. In order to improve the high temperature and low cycle fatigue performance of the IN713C alloy, trace amounts of yttrium (Y) element were added to such an alloy. The effect of Y on the microstructure and fatigue performance under push-pull loading condition of the IN713C alloy at 650 °C was investigated, and the fracture mechanisms of the IN713C alloy with different Y additions at strain amplitudes of 0.3%, 0.4% and 0.5% were also investigated. The results showed that addition of Y significantly increased the fatigue life of the IN713C alloy under different strain amplitude values. Compared with the traditional IN713C, the fatigue life of the alloy with 300 ppm Y was increased by 44.4%, 213.3%, and 61.4%, and that of the alloy with 500 ppm Y was increased by 184.6%, 66.1%, and 172.1%, respectively, at the strain amplitudes of 0.3%, 0.4%, and 0.5%. Shrinkage pores were found to be the main crack source at low strain amplitude (0.3%), while carbides and other interdendritic precipitates were the source of cracks at high strain amplitude (0.4% and 0.5%). The addition of Y increased the fatigue life of the IN713C alloy by reducing shrinkage and refining carbides and grain size. [ABSTRACT FROM AUTHOR]

• The fatigue life is explained by addressing the fatigue and ratchetting damages. • A non-proportionality factor is newly defined to consider the effect of mean stress. • Path-dependent fatigue and ratchetting damage parameters are defined by strain. • A multiaxial fatigue life-prediction model with addressing ratchetting is developed. Based on the strain measured in the SUS301L stainless steel tubular welded joint in the asymmetrically stress-controlled multiaxial low-cycle fatigue tests by using the digital image correlation (DIC) method, the dependence of fatigue life on the loading level and loading path is well explained by discussing the multiaxial ratchetting-fatigue interaction in the fatigue failure zone of welded joint. To reflect the contribution of mean stress to the non-proportionality of loading path, a new non-proportionality factor is defined. And then the path-dependent damage parameters of pure fatigue damage and additional ratchetting one are proposed in a strain form and validated. Finally, a multiaxial life-prediction model is constructed through addressing the concurrent efforts of additional ratchetting damage and pure fatigue one to predict the multiaxial fatigue life of welded joint. The results demonstrate that the predicted multiaxial fatigue lives are well expected and all life data fall in the twice error bands regarding to the experimental ones. [ABSTRACT FROM AUTHOR]

• The high-temperature fretting fatigue performance of nickel-based single crystal tenon attachment under different load conditions have been investigated. • Microstructure deformation of shot peened nickel-based single crystal superalloy is revealed. • Shot peening strengthening mechanism of nickel-based single crystal tenon is revealed. In order to reveal the shot peening (SP) strengthening mechanism on the high-temperature fretting fatigue performance of nickel-based single crystal (NBSC) tenon attachment, fretting fatigue tests under three load conditions were carried out at 600 ℃. The results showed that the fretting fatigue life of SP NBSC tenon attachment could be increased to 357.53 % of that of non-SP NBSC tenon attachment. SP changed the fretting fatigue crack initiation site from the contact surface to the subsurface. The larger the peak load was, the closer the crack initiation site was to the contact surface. SP changed the fretting fatigue crack propagation path, and the crack propagation deviated from the original direction approximately perpendicular to the contact surface. SP increased the microhardness by 28.1 %, induced −1098 MPa maximum compressive residual stress (CRS) and generated a mean geometrically necessary dislocation (GND) density of 7.55× 1014 m−2 near the surface. The micro convex bodies and greater microhardness alleviated the wear damage, and the increased dislocation density and CRS enhanced the inhibition of crack initiation and propagation, which were the strengthening mechanism of SP to improve the fretting fatigue performance of NBSC tenon. This work aimed to provide support for the long-life manufacturing of NBSC turbine blades. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fatigue loading induced microstructure evolutions were found in high-strength martensitic steels. • Dynamic precipitations of small carbides were observed in HCF and VHCF regimes. • Refinements of local grains were observed around stress singularities in LCF, HCF and VHCF regimes. • Local grain refinement contributes to crack initiation and early growth in low stress/long life case. Fatigue loading induced microstructure evolution featured by Fine Granular Area (FGA) in high-strength martensitic steels have close ties to the fatigue crack initiation and early growth, which are worthy of being investigated. On this issue, we conducted Very High Cycle Fatigue (VHCF), High Cycle Fatigue (HCF) and Low Cycle Fatigue (LCF) tests on three different types of specimens. Combined with post-mortem/quasi in-situ Scanning Electron Microscope (SEM) and Electron Back-Scattered Diffraction (EBSD) observations, as well as further Transmission Kikuchi Diffraction (TKD) and Transmission Electron Microscope (TEM) analyses, we captured two different types of microstructure evolution behaviors during the fatigue loading. One is the "dynamic precipitation", a kind of phase transformation from martensitic matrix (BCC) to Nb- and Mo-rich small carbides (MC, M 7 C 3) accelerated by repeated slight plastic deformation, whose formation does not rely on the stress concentration effect and also has no ties to the plastic strain localization and fatigue crack initiation. Another is the "local grain refinement" around those stress singularities (crack tip, or interior inclusion) generating substantial amounts of fine sub-grains in VHCF, HCF and LCF regimes, which can then promote fatigue crack initiation and early growth along sub-grain boundaries, fine/coarse grain boundaries and martensitic packet boundaries. [ABSTRACT FROM AUTHOR]

• Corrosion fatigue experiments of medical-grade biodegradable Mg alloy in organic fluid. • Long-term precorrosion significantly deteriorates the fatigue performance. • WE43MEO demonstrates good fatigue strength retention in DMEM at high stress amplitudes. • Fracture mode transition of environmentally assisted cracking. Rare-earth containing magnesium alloys are promising biomedical materials for a new generation of biodegradable orthopaedic implant systems due to their excellent biocompatibility, mechanical and biodegradation properties. However, chemo-mechanical interactions in aggressive physiological corrosion environments result in rapid degradation and early loss of mechanical integrity, limiting its broader application for orthopaedic implants. To date, only few studies have assessed the corrosion-fatigue behaviour of medical-grade magnesium alloys in an organic physiological corrosion environment, especially under sterile test conditions. In the present work, the corrosion-fatigue behaviour of fine-grained medical-grade magnesium alloy WE43MEO was systematically analysed under in vitro conditions using an organic physiological fluid DMEM. The experimental results showed that the fatigue strength of the alloy is nearly unaffected by a 1-day precorrosion, while a 7-day precorrosion resulted in a significant deterioration of mechanical integrity. In corrosion-fatigue experiments, the fatigue life was considerably reduced by interactions between corrosion and fatigue damages. The SEM analysis revealed that the mixed mode of intergranular and transgranular fracture in the crack propagation zone transits to intergranular cracking dominant mode under the corrosion-fatigue conditions due to hydrogen embrittlement. [ABSTRACT FROM AUTHOR]

• Component-like specimen reproducing EDM and STEM drilling on a turbine bucket. • Detrimental effect of EDM drilling on high-temperature HCF. • Competitive effects of surface roughness and microstructural alteration on HCF. • Non-propagating cracks from the EDM recast layer. The paper deals with the effects of electrochemical and electro-discharge deep hole drilling on the high-temperature fatigue behavior of a directionally solidified GTD-111 superalloy. High-temperature HCF tests were carried out at 800 °C on specimens having a coaxial hole drilled with the investigated processes. The hole diameter and length were 1.5 and 95 mm, respectively. The material's high-temperature HCF S-N curve was also obtained to provide a reference curve. Despite the scatter caused by the material microstructure in the fatigue crack nucleation and propagation regime, the fatigue strength was found to be affected by the investigated hole drilling techniques. The electro-discharge deep hole drilling was found to lower the fatigue strength by 8%. Fractographic analyses showed that it can be attributed to the different hole surface roughness and microstructural alteration in the underlying material. [ABSTRACT FROM AUTHOR]

• Fatigue properties of weld joints were investigated in 3.5 % NaCl at ∼ 0 °C and. 24 °C. • Fatigue lives were higher at 0 °C due to a lesser fraction of intergranular facets. • Low-stress amplitudes favored pit-to-pit type of crack initiation. • High-stress amplitudes favored pit-to-crack type of crack initiation. • Surrounding temperature dominantly controlled the crack growth mechanisms. This work investigates the fatigue behavior and damage mechanisms in the friction stir weld joints of AA-5086-H321 alloy under the combined effect of media: 3.5 % NaCl and air and its temperature: 24 °C and ∼ 0 °C. Sinusoidal loading tests were performed at different load ratios: 0.1 and 0.5, in load-controlled mode at 1 Hz in 3.5 % NaCl. The fatigue lives of the weld joints deteriorated notably under 3.5 % NaCl by selectively attacking its vulnerable nugget zone and heat-affected zone, while the effect of low temperature (∼ 0 °C) was beneficial in both air and 3.5 % NaCl. Two types of crack initiation mechanisms were identified irrespective of the other test conditions: pit-to-pit type, favored at low-stress amplitudes, and pit-to-crack type, favored at high-stress amplitudes. A few cases of crystallographic corrosion pitting were also observed and were accredited to the inherited crystal defects within the Al-matrix. The effect of surrounding temperature was apparent on the crack growth mechanism with limited or no impact on the crack initiation mechanism. The fatigue cracks grew in the intergranular mode under 3.5 % NaCl, while its quantitative presence was decreased at ∼ 0 °C, irrespective of the media, which was attributed as one of the key factors for the increased fatigue performance at ∼ 0 °C. [ABSTRACT FROM AUTHOR]

• The tensile properties and LCF life of LDED-ed Ti6Al4V alloys are improved through microstructure regulation. • The cyclic softening is attributed to the competition effect of back stress and friction stress. • The grain orientation strongly affects the fatigue crack propagation process. The application of the laser directed energy deposition (LDED) titanium (Ti) alloys has been severely impeded by their poor low cycle fatigue (LCF) properties in aerospace industry. Herein, we propose to improve the LCF properties of LDED-ed Ti6Al4V alloys via microstructure adjustment. A solid solution aging 850AA treatment is made to regulate the microstructure into a multi-scale composition dominated by discontinuous α GB and coarsened α P +(α S +β) phases. The LCF properties of LDED-ed Ti6Al4V with a multi-scale microstructure even outperform the wrought Ti6Al4V at high strain amplitudes. Furthermore, this multi-scale microstructure has special local plastic deformation behavior and dislocation activities under cyclic stress loading, which strongly affects the internal stress evolution and crack propagation. The fatigue softening behavior is attributed to the combined effect of back stress and friction stress. Fatigue cracks are deflected by the α/β boundary when the angle (φ) between the long axis of α lamellae and the crack direction is less than 35°. The transgranular fracture occurs when φ is close to 90° (70°–90°). Meanwhile, the fatigue crack usually propagates along the basal or prismatic planes with high Schmid factors. These findings provide novel insights into the microstructure design of LDED-ed Ti alloys with high fatigue damage tolerance. [ABSTRACT FROM AUTHOR]

• A novel fatigue cumulative damage model is proposed by integrating Manson-Halford model with machine-learning algorithms. • Two-level loading fatigue test is conducted, considering various loading conditions and damage accumulation scenarios. • Importance of different features on nonlinear fatigue cumulative damage model have been revealed. Machine learning serves as a potent tool for predicting fatigue damage. By integrating traditional physics-based models with machine learning techniques, prediction accuracy can be notably enhanced. Drawing from a substantial pool of experimental data, this article introduces a novel nonlinear fatigue cumulative damage model. This model combines the traditional Manson-Halford model with machine learning algorithms, enabling the prediction of residual fatigue damage across various materials. The predictive capabilities of this model are compared with those of traditional approaches. The article subsequently investigates the key parameters influencing the nonlinear fatigue cumulative damage model. The findings demonstrate that the proposed model exhibits significantly higher accuracy in residual fatigue damage prediction compared to traditional methods. Notably, the primary factor impacting the nonlinear cumulative fatigue damage model is identified as the first level loading damage. This study underscores the substantial potential of the proposed nonlinear fatigue cumulative damage model for predicting residual fatigue damage. [ABSTRACT FROM AUTHOR]

• Fatigue life of thick bonded DCB joints was investigated under displacement loading. • Grooved geometries were used to contain the crack path in the adhesive. • Fatigue crack growth curves depend on R d -ratio when plotted as da/dN vs G max. • Void content dominates the fatigue behavior and crack path geometry. This article investigates the influence of displacement ratio (R d -ratio) and void content on the fatigue behavior of Double Cantilever Beam (DCB) joints with thick adhesive layer (10 mm). Two R d -ratios and two different void contents were studied. To achieve cohesive failure, grooved DCB geometry was utilized to guide the crack propagation path into the adhesive layer, and the experimental conditions were adjusted to maintain stable propagation. The results showed that the crack propagation path under fatigue loading was mainly determined by the void size and distribution. Secondary cracks were also observed from digital image correlation (DIC) and X-ray tomography, which could lead to crack path switching on the lateral surface and influence the crack measurement. Data analysis using the total fatigue life model showed that the fatigue crack growth (FCG) depends on the R d -ratio, and void content. [ABSTRACT FROM AUTHOR]

• Very high cycle fatigue lives are predicted by a modified Smith-Waston-Topper model. • Very high cycle fatigue lives of 0°, 45°, to 90° specimens gradually decrease. • The maximum prospective defect size is used as the typical defect size. The ultrasonic fatigue tests on laser-powder bed fused (L-PBF) Ti-6Al-4V specimens manufactured at 0°, 45°, and 90° building orientations were conducted. It is found that the very high cycle fatigue (VHCF) lives decrease with the building orientation from 0°, 45°, to 90°. All the fatigue cracks initiate from internal pores or Lack of Fusion defects. Mode Ι cracks grow perpendicular to the direction of the maximum opening stress, regardless of the building orientation. The maximum prospective defect evaluated by the extreme value statistics (EVS) method is used as the typical defect for VHCF life prediction. A modified Smith-Waston-Topper (SWT) model was proposed and combined with the Critical Distance Method to predict ultra-high cycle fatigue life. The life prediction accuracy was validated by using both experimental data obtained from the experiments conducted in this paper and experimental data taken from the literature. The results show that 84% of the predicted fatigue lives are within a scatter band of 2 standard deviation, and 16% of the predicted lives are outside a scatter band of 2 standard deviation and within a scatter band of 3 standard deviation. [ABSTRACT FROM AUTHOR]

• Efficient excitation of focused narrow-band surface waves by arc-shaped grating mask. • Efficient detection of surface microcracks as small as 0.3 mm in length based on laser ultrasound surface wave acoustic spectroscopy. • Effective detection and characterization of actual submillimeter fatigue cracks. This paper introduces a novel method for detecting and characterizing initial micro-surface crack in metal structures and materials, with a particular focus on initial fatigue cracks. The proposed technique utilizes an arc-shaped double-spaced grating mask to spatially modulate laser beams, forming an arc-shaped bi-grating laser source on the surface of the specimen. This configuration excites two focused surface waves that have the same focus point but differ in frequency and propagate in opposite directions. By monitoring surface wave signals as they pass through defects and analyzing the relative changes in the amplitude of the frequency components in the spectrum of the reflected and transmitted waves, the initial defects can be identified. Both simulation and experimental results validate the effectiveness of this method in detecting tiny artificial surface cracks, including those shorter than 0.3 mm. In addition, this method has been successfully applied to the detection of actual initial fatigue cracks. Experimental findings demonstrate that focused surface waves enable highly sensitive detection of sub-millimeter fatigue cracks. [ABSTRACT FROM AUTHOR]

• The multi-axis load spectrum editing method of automotive components is studied. • A time–frequency editing method based on the generalized S-transform is proposed. • The fatigue damage of the lower swing arm bushing is analyzed numerically. Fatigue bench testing efficiency for automotive components can be significantly enhanced through load spectrum editing. However, current methodologies for editing uni-axial load spectra often overlook inherent phase correlations in the loading process, potentially leading to inaccurate test results. To address this issue, a novel time–frequency editing approach for multi-axis load spectra of automotive components has been introduced, utilizing the generalized S-transform (GST) theory. This approach identifies and eliminates load components with minimal impact on fatigue damage, generating an edited spectrum with a condensed duration without compromising the integrity of the original data. To evaluate the editing impact, the multi-axis load spectra were subjected to three distinct editing techniques: the time domain damage retention method, the wavelet transform method, and the GST-based method. Comparative analysis revealed that the GST-based method not only significantly reduced the load spectrum's duration but also preserved the original spectrum's phase relationships, statistical characteristics, power spectral density, and level counting. The efficacy of the GST-based editing technique was further validated through bushing fatigue simulations, confirming its superiority in enhancing the efficiency of automotive component fatigue testing. [ABSTRACT FROM AUTHOR]

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]

• Unveiling rotating bending corrosion fatigue behavior of laser powder bed fused 316L. • Proposing a fatigue life prediction model with high accuracy (less than 10% deviation). • Quantifying pitting defect/microstructure influence on corrosion fatigue performance. • Comparing the stress-life response of pre-corroded and non-corroded AM 316L. In this study, the ex-situ rotating bending corrosion fatigue behavior of laser powder bed fused stainless steel (SS) 316L has been studied incorporating experimental analyses and modeling. The findings revealed significant variations (almost 20 %) in fatigue lifetime in pre-corroded specimens relative to non-corroded (virgin) specimens because of corrosion-derived pitting phenomena on the surface. To this end, the fatigue strength of virgin (non-corroded) and pre-corroded SS 316L were recorded at 242 MPa and 203 MPa, respectively. Upon conducting the fatigue tests and extracting stress-life trends, advanced microstructural characterization and postmortem analyses were used to quantify fatigue crack behavior and fracture surface. An empirical model is also developed employing depth-sensing indentation testing, linear elastic fracture mechanics, and computer tomography scans to propose an optimum predictive trend. The analysis comparing existing models with our proposed model demonstrates reasonable agreement between experimental findings and the predicted life for the studied laser powder bed fused SS 316L material exposed to corrosion rotating bending fatigue test. The findings of this research carry important contributions to the structural integrity and longevity of marine engineering, ship construction, naval operations, and naval aviation where materials and components are exposed to cyclic loading and corrosion. [ABSTRACT FROM AUTHOR]

Metal structures often exhibit macroscopic defects from which cracks can nucleate during cyclic loading. The current work presents a two-scale approach to enable the prediction of crack nucleation from such defects by taking into account local microstructure features. The geometrical description of the defect and associated non-homogeneous strain fields are modeled using a macroscale model which employs a continuum elastoplastic material model for cyclic deformation. The cyclic deformation of the microstructure near the defect is modeled using a mesoscale model which employs a crystal plasticity material model and uses multiple realizations to address the statistical microstructure variability. The boundary conditions of the mesoscale model are extracted from the macroscale model. By simulating the deformation of the microstructure using the strain fields near the defect and by introducing a fatigue indicator parameter for crack nucleation, along with the weakest-link based upscaling methodology, the developed approach enables the prediction of the distribution of crack nucleation life. The approach is used for analyzing different defects for crack nucleation by considering local grain orientations. The predictions are shown to not only capture phenomena such as scatter, size effects, etc. qualitatively, but also agree with a classical engineering approach and experimentally reported data sets quantitatively. • Two-scale approach is proposed for macroscopic defects, including microstructure. • Continuum elastoplasticity is used at macroscale and crystal plasticity at mesoscale. • Statistical upscaling enables the incorporation of the length of the defects. • Different defects are compared for crack nucleation prediction in a metal structure. • Predictions agree with an engineering approach and experimental data. [ABSTRACT FROM AUTHOR]

• Significant equivalent creep damage occurs under typical cyclic loading. • Contribution of creep damage can be more pronounced than fatigue damage. • Strong creep-fatigue interaction exists in soft adhesives under various conditions. • A modified Chaboche damage model can accurately predict the shear fatigue life. Soft adhesives used in cutting-edge technological fields are susceptible to shear fatigue failure under complex in-plane loadings. Although the interaction or coupling effect between creep and fatigue damage have been studied, the effect of the equivalent creep damage during typical fatigue loading on the fatigue life of soft adhesives, as well as the interaction between the equivalent creep damage and fatigue damage of soft adhesives have never been characterized. This study systematically investigates the creep-fatigue interaction of the soft adhesive under various loading conditions. It was found that significant equivalent creep damage occurs and accumulates under typical cyclic loading, which seriously accelerates the shear fatigue failure of soft adhesives. Furthermore, the contribution of the equivalent creep damage to the final shear fatigue failure is even more pronounced than the fatigue damage. The quantitative construction of creep-fatigue damage diagrams has led to the determination of strong creep-fatigue interaction, which could be understood by the interactive mechanism between the stretching, friction, pulling-out, and rupture of molecular chains. In light of the distinctive creep-fatigue interaction mechanism, a modified Chaboche damage model has been developed, which could accurately predict the shear fatigue life of soft adhesives under a wide range of loading conditions. [ABSTRACT FROM AUTHOR]

• Mean stress would have a significant impact on the fatigue life of titanium alloy. • The conversion mean stress used for the pure fatigue damage calculation. • A thermo-mechanical fatigue life prediction method for titanium alloy was proposed. This paper presents a thermo-mechanical fatigue (TMF) life prediction method for titanium alloy TA15 under multiaxial random amplitude loading. Through strain-controlled TMF tests, it was found that under full-reversed cyclic loading, there was a significant presence of mean stress in the stress response. The induced mean stress led to the anomalous phenomenon where the fatigue life of thermal phase angle out-of-phase (TOP) was shorter than that of thermal phase angle in-phase (TIP). Therefore, a new damage calculation model was proposed, which can take into account the pure fatigue damage considering mean stress conversion and the cumulative environmental damage. Uniaxial TMF tests, constant amplitude multiaxial TMF tests, and random amplitude multiaxial TMF tests are conducted to verify the proposed method. The predicted results for all types of tests are in a good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Understanding the fatigue failure mechanisms in concrete is complex and remains an area of ongoing investigation. Material heterogeneity, along with the size effect, plays a significant role in specifying the crack propagation behaviour and fatigue life in case of repetitive loading conditions. In this study, the influence of material heterogeneity on the fatigue behaviour of concrete has been examined. Three different-size beam specimens with varying heterogeneity (aggregate sizes) have been considered to investigate various fracture characteristics under repetitive load conditions. Acoustic emission and digital image correlation techniques have been employed to analyse crack growth behaviour. The results revealed that stiffness degradation and crack propagation rates are faster for the specimens with smaller aggregate sizes compared to the larger aggregate size specimens of the same depth. Moreover, the fatigue life and effective crack length at failure are also higher in the case of larger aggregate sizes compared to smaller ones. Therefore, a heterogeneity-adjusted, analytical model of fatigue crack growth has been proposed. Further, the AE parameters have been utilized to characterize the tensile and shear crack dominance in different stages of fatigue crack propagation. The outcome of this study can be utilized to anticipate the crack propagation behaviour and residual fatigue life for any combination of specimen size and aggregate size. • The present study focuses on fatigue analysis in concrete considering material heterogeneity. • The work considers three different specimen sizes and aggregate sizes to demonstrate its influence. • The research proposes a heterogeneity adjusted fatigue crack growth model for concrete. • AE parameters have been utilized to characterize the tensile and shear crack dominancy. [ABSTRACT FROM AUTHOR]

• Creep-fatigue tests of P92 steel were conducted at 873 K with long holding times. • Cyclic response and creep-fatigue lives under various hold conditions were compared. • Effects of hold conditions and oxidation on creep-fatigue behavior were clarified. • Cracking mode and failure mechanism under various hold conditions were elucidated. • A three-dimensional creep-fatigue-oxidation damage interaction diagram was proposed. P92 steel has gained plenty of attention as a top contender for steam generator components and main steam tubes due to the growing need to increase the energy conversion efficiency. However, a major contributor to the premature failure of these components remains the interaction of creep and fatigue damage caused by typical dwell fatigue loading conditions. In this study, creep-fatigue tests of P92 steel were conducted under strain-controlled cyclic loading including hold durations up to 3600 s at 873 K in air. Special attention was given to the influence of hold conditions and oxidation on cyclic deformation and damage mechanisms. The results revealed that introducing hold time substantially decreased the failure life, with compressive hold causing more severe damage than tensile hold. Additionally, oxidation became severer with longer hold time, resulting in increased surface roughness, crack initiation sites and density of secondary cracks. Fatigue dominated the failure in the case of the short-term tensile hold, and the interaction of creep, fatigue and oxidation dominated the failure in long-term tensile hold tests. In contrast, the failure in compressive hold case was dominated by fatigue and severe oxidation. Furthermore, a three-dimensional oxidation-creep-fatigue damage interaction diagram was proposed for quantitative assessments in practice. [ABSTRACT FROM AUTHOR]

• Crack density growth in cross-ply laminates during fatigue loading is analyzed. • Transverse cracking is influenced by local stress ratio in the 90-layer. • Equivalent stress is introduced to include the effect of local stress ratio. • Equivalent stress concept is validated against two different cross-ply layups. Presented test results on transverse cracking in cross-ply laminates upon tension–tension cyclic loading show that the increase of crack density depends not only on the maximum transverse stress in the cycle but also on the local cyclic stress ratio R T loc in the analyzed layer. To include the effect of the R T loc in the model with statistical failure stress distribution for crack initiation (based on Weibull distribution) adapted for fatigue, an equivalent stress is introduced in a similar manner as the equivalent strain energy release rate has been used for delamination crack propagation. The equivalent stress in the layer is defined as a power function of the maximum stress and the stress ratio in the layer. It was found, testing laminates with two different fiber contents that higher the local stress ratio in 90-layer, higher the transverse cracking resistance. Transverse crack density simulation using the developed equivalent stress model has been validated against test results. [ABSTRACT FROM AUTHOR]

• A surface fatigue crack monitoring method for welds based on FMC-TFM is proposed. • The DIC technique reveals the non-consistent mechanical characteristics of the welds. • Revealing the mechanism of the surface fatigue crack in full- scale butt welded joints. • Using the traction structural stress to quantify the effect of angular misalignment on crack. • An analytical function for the evolution of cracks in welded joints is proposed. In this study, the digital image correlation technique is used to determine the inconsistent mechanical properties and microstructure non-uniformity in different regions of aluminum alloy butt welded joints. A state monitoring method is proposed based on phased-array ultrasonic total focus imaging for crack evolution. A series of surface fatigue crack propagation tests on full-scale aluminum alloy butt welds are investigated, which innovatively revealed the mechanism and evolution law of crack propagation at the surface of the weld. The analytical function for dynamic crack propagation in welded structures is proposed, which provides a new idea and data support for clarifying the state deterioration process of structures. The results show that the traction structural stress can effectively identify the critical fatigue position of the weld and effectively account for the effect of angular misalignment. The phased-array ultrasonic total-focus imaging monitoring analysis method for surface fatigue crack propagation is proposed to improve the completeness of the fatigue crack evolution law. [ABSTRACT FROM AUTHOR]

• The octahedral shear strain energy is proposed as a multiaxial fatigue damage parameter without introducing weight parameters. • The non-proportional energy hardening factor modified to the plastic stage on the right side of the life equation is more consistent with the physical properties of the material. • All parameters in the model can be derived from material strength properties, greatly simplify the multiaxial fatigue life prediction process. • On the basis of greatly simplifying the calculation process, better life prediction accuracy and adaptability to various materials are achieved. Present multiaxial fatigue models have achieved satisfactory performance in predicting multiaxial fatigue life of different loading paths; however, many of them require critical plane calculations and additional material constants. In this study, a novel multiaxial fatigue life prediction model based on octahedral shear strain energy is proposed. In the proposed model, the maximum octahedral shear strain energy serves as the damage parameter without requiring any additional material parameters. All material constants in this model can be derived from material strength constants and uniaxial fatigue constants, greatly simplifying the life prediction process. Moreover, based on the actual physical phenomenon of non-proportional additional hardening, the multiaxial non-proportional correction factor is incorporated into the plastic stage of the life equation, aligning more closely with fundamental principles in physics. Then, by utilizing the experimental data from the literature for four materials, a comparison is made between the prediction results of the novel model and those of four commonly employed models. The findings demonstrate that the new model exhibits superior simplicity and accuracy. Subsequently, through life prediction outcomes for DZ411 material across various paths, the stability and precision of the octahedral energy model are substantiated, rendering it suitable for practical engineering applications. [ABSTRACT FROM AUTHOR]

• The fatigue crack propagation (FCP) anisotropy mainly reflects in the near-threshold region. • Strength is the main influencing factor in the Paris region. • Fatigue crack deflection along the grain boundaries can reduce the FCP rate. • Larger grain orientation deviation on both sides of the crack can further reduce the FCP rate. The fatigue crack propagation (FCP) rate is an important performance index in the aerospace industry for the safe lifespan evaluation. In this study, the FCP anisotropy of an Al–Zn–Mg–Cu super-thick rolled plate was analyzed. The results show that as the depth increases, the FCP anisotropy gradually becomes significant especially in the low ΔK region. The specimens tested along the transverse direction exhibit lower FCP rate than those tested along the rolling direction in the low ΔK region, which is related to the difference in the grain shape and grain orientation. More specifically, the crack is easy to deflect along the grain boundaries in the deformed grains when loading along the transverse direction. Meanwhile, more obvious grain orientation deviation on both sides of the crack can cause larger angle deflection, longer FCP path, and lower FCP rate. In the Paris region, the FCP anisotropy is not obvious regardless of thickness. The reason is that the main influencing factor is controlled by strength in this region, while the strength anisotropy is very limited leading to this similarity. [ABSTRACT FROM AUTHOR]

• Mixed-mode loading significantly influences fatigue crack growth in SAPH440 material. • CTS specimen and multi-level loading technique were used to study mixed-mode fatigue. • Mode II loading increases threshold stress intensity factor in mixed-mode conditions. • Fatigue crack propagation rate depends on applied load angles in multi-level loading. In this study, the effect of mixed-mode loading on fatigue crack growth under varying applied loads for SAPH440 material was experimentally investigated. Previous research has either examined the effect of mixed-mode loading at a fixed angle or its effect on slanted cracks. This paper uses a CTS specimen and a loading device to apply varying angles of load during Mode I loading conditions. Initially, a numerical analysis was performed to validate existing equations for the stress intensity factor, which quantifies the crack driving force in the experiments. In the crack initiation region, load shedding techniques and load increasing techniques were used to confirm fatigue cracking considering mixed-mode effects. The change in the threshold value according to the loading angle was also evaluated. In the crack propagation region, a multi-level loading technique was employed to assess the mixed-mode effects under varying loading angles. This process verified the mixed-mode effect under conditions where the applied load angle varies. The fatigue crack fracture surfaces were examined using digital camera capture and SEM. It was found that when mixed-mode loading occurs during Mode I loading, the fatigue crack propagation rate varies depending on the mixed-mode loading angle. [ABSTRACT FROM AUTHOR]

• This study demonstrates that laser shock peening (LSP) effectively enhances the fatigue resistance of Q960 steel in structural engineering, as evidenced by both macroscopic mechanical properties and microstructural characterization. • The research shows that LSP can gently induce a high level of compressive residual stress (CRS) in structural steel for fatigue resistance, as evident from the minor defects on the surface of LSPed Q960 high-strength steel. • The study highlights the importance of CRS depth over surface peak value for enhancing fatigue resistance, with the enhancement positively correlated with CRS depth. Fatigue damage of steel structures is often likened to the "cancer" of structural engineering. Fatigue failure, characterized by the absence of apparent signs before occurrence, usually leads to significant losses. To investigate the potential of laser shock peening (LSP) as an effective method for improving the fatigue performance of steel structures, square plates and compact tension (CT) specimens made of Q960 structural steel were designed. The surface integrity, fatigue crack growth (FCG) performance, microstructural grain size, and fracture morphology of specimens were compared at different laser shock parameters. The results show that LSP implanted a high level of beneficial residual compressive stress in a mild way, and no obvious surface defects were introduced. A noticeable retardation effect was found in the FCG of LSPed specimens, and this effect becomes more pronounced with the increase of laser pulse energy. Compressive residual stress is a more dominant factor in enhancing fatigue performance compared to grain refinement for the small-grained metals studied. LSP can effectively reduce the FCG rate of Q960 high-strength steel, showing promising application prospects in strengthening the fatigue hazard zone of steel structures. [ABSTRACT FROM AUTHOR]

• A rapid fatigue test method based on compliance measurements and stepwise load is presented with simple data treatment. • Fatigue damage monitoring is related to different fatigue stages since incipient fatigue damage to macrocrack propagation. • The method allows an accurate assessment of the fatigue threshold, fatigue damage onset, crack propagation curve and onset curve by only using a few specimens. Characterising fatigue delamination in composite materials following standardised testing protocols is often associated with high costs and significant time investment. Therefore, it can be helpful to have a testing strategy to reduce the testing time and easily assess the fatigue delamination of multiple materials and conditions. This work shows how to apply one of the new rapid testing techniques known as the stiffness method on composite materials. The method has been developed to predict the fatigue delamination of composite materials by monitoring the fatigue damage through the compliance of the specimen. The obtained data in terms of displacement and force is used to obtain the fracture toughness, fatigue crack onset, fatigue crack threshold, and crack propagation parameters by using a reduced number of specimens and a few testing hours. The testing procedure has been developed on carbon fibre-reinforced polymer used in aerospace structures. The obtained results in a short time are promising, reporting a deviation below 10% compared to the values obtained in the standardised fatigue delamination tests. [ABSTRACT FROM AUTHOR]

• The cyclic deformation behavior and fatigue fracture mechanism of TC17 alloy under different strain amplitudes were investigated. • The microscopic causes of the cyclic softening/hardening phenomenon were analyzed. • The relationship between defects (secondary cracks, holes) and the basket-weave microstructure was explored. • The variations in microstructure surrounding the crack propagation path were elucidated. Room-temperature low-fatigue tests were conducted on TC17 titanium alloy, and the low-fatigue damage behavior of TC17 titanium alloy with a basket-weave microstructure at room temperature was investigated. The results show that TC17 titanium alloy exhibits different cyclic hardening/softening phenomena at different strain amplitude (ε ta ) levels, which was closely related to the magnitude of strain amplitude, the crack sources of all specimens generated on the surface, and the number of crack sources increases with the increase of strain amplitude level. The competitive relationship between back stress hardening and frictional stress softening caused by changes in different microstructures is the reason for the occurrence of different cyclic hardening/softening phenomena. The relationship between the holes and secondary cracks near the fracture surface and the basket-weave microstructure was further discussed. The secondary cracks/holes were more likely to initiate in the primary α phase where the solid solution strengthening effect was weak. In addition, the interaction between the secondary crack propagation path and the nearby α lamellae was studied by the Electron Backscatter Diffraction (EBSD) method. [ABSTRACT FROM AUTHOR]

[Display omitted] • Tensile and fatigue properties of CM and PBF-LB samples are compared. • Integrated characterization approaches are employed to reveal fatigue mechanisms. • The importance of cellular structures on fatigue properties is highlighted. • The difference in fatigue properties between PBF-LB and CM samples is explained. The mechanical properties of ASTM F1684 (Invar 36) alloy manufactured via laser beam powder-bed-fusion (PBF-LB) technique are strongly influenced by the non-equilibrium solidification microstructure. Due to the complexity of printed microstructure, e.g. , the presence of melted pools, nano-precipitates, and high-density dislocations, it is very challenging to disentangle the specific mechanisms responsible for the fatigue properties. Here, a comparative investigation of the fatigue properties between PBF-LB and conventionally manufactured (CM) samples was conducted. Dedicated characterizations, including electron channeling contrast imaging (ECCI), and high-angular resolution EBSD (HR-EBSD), were performed. The mid-cycle (104–105 cycles) fatigue behaviors of both samples are comparable, while the fatigue resistance of PBF-LB sample becomes inferior at high-cycle range (≥ 106 cycles). This disparity can be ascribed to the competing effect between crack nucleation and propagation. PBF-LB samples are more susceptible to fatigue crack initiation due to the printed macro-defects. Whereas, the homogeneous plastic deformation and the grain fragmentation, both triggered by the ultrafine cellular structure, efficiently mitigate crack propagation rates. Based on these findings, the current understanding of the underlying mechanisms governing the fatigue performance of PBF-LB Invar 36 alloy is deepened, emphasizing the significant advantages of non-equilibrium solidification microstructure in retarding fatigue crack propagation. [ABSTRACT FROM AUTHOR]

• A novel noniterative simplified method for vehicle-bridge interaction is proposed. • A combined corroded condition of pitting and uniform corrosion is considered. • The fatigue performance of corroded steel wires is estimated. • The hanger's fatigue life is predicted considering different failure criteria. Fatigue damage is a main concern during the life-cycle maintenance of bridges with corroded hangers under vehicle-bridge interaction (VBI). In this study, a novel noniterative simplified method for VBI is proposed to analyze the responses of hangers in a network arch bridge, considering the effects of vehicle types, driving lanes, and road surface roughness. Furthermore, the fatigue life of corroded hangers is investigated, considering the actual traffic flow and uneven distribution of stress in the hanger's cross-section. The results show that longitudinal and transverse bending moments of the hanger significantly affect the stress of steel wires in one hanger but have limited influence on stress amplitudes. The fatigue lives of steel wires are considerably reduced when subjected to both uniform and pitting corrosion. The fatigue life of steel wires is reduced under deteriorating road surface conditions. Moreover, as the fatigue life of steel wires in the hangers declines, the variations in fatigue life among individual steel wires within a single hanger gradually decrease. Consequently, this trend leads to a heightened likelihood of continuous wire breakage. [ABSTRACT FROM AUTHOR]

• The effect of FCHs on the vibration fatigue performance of DZ125L alloy was studied. • The crack propagation mechanism of FCH specimens under vibration was proposed. • Two new models enhance the applicability of frequency domain method to FCHs. Film cooling represents a critical protective measure for turbine blades, yet the presence of film cooling holes (FCHs) under vibrational loads can significantly impact structural strength and integrity. This study conducts random fatigue tests on DZ125L directionally solidified superalloy specimens with FCHs. It investigates how various FCH types and vibration signal intensities influence the vibration fatigue behavior of DZ125L alloy. Characterization of vibration fatigue fracture and surface cracks of FCH specimens utilizes ultra depth of field microscopy and scanning electron microscopy. Additionally, the crack propagation mechanism for FCH specimens under random processes is proposed based on finite element stress distribution and fracture morphology. Results reveal the generation of two types of cracks, namely hole cracks and edge cracks, under vibration load in FCH specimens. The crack propagation process produces water wave-like fatigue striations. Notably, a low stress zone exists between the two dangerous holes in the multi-hole specimen, mitigating the expansion trend of hole cracks between the FCHs compared to cracks expanding towards the edges. Furthermore, two novel models, New1 and New2, are introduced to enhance the applicability of the frequency domain method for predicting the fatigue life of FCH specimens under random processes. Accuracy and error analyses of the models suggest that New2, incorporating the FCH stress concentration coefficient K T and intensity function f (ξ) , exhibits superior accuracy and stability. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel approach to enhance the crack resistance in the inhomogeneous microstructure of remanufactured Ti6Al4V. • The crack resistance mechanism and the FCG behavior of inhomogeneous microstructure was investigated. • In-situ cooling process altered the fatigue crack propagation mechanism in RZ and HAZ, and diminish the anisotropy of RZ. The fatigue performance of remanufactured titanium alloy blade components has garnered significant attention in recent years. The repaired titanium alloy components using the direct energy deposition (DED) method exhibit structural differences, leading to an inhomogeneous microstructure that directly impacts the service life of the remanufactured component. The work presented here aimed to investigate novel approaches for enhancing the resistance to crack propagation and to gain a deeper insight into the fatigue crack propagation characteristics of the Ti6Al4V remanufactured interface with an inhomogeneous microstructure. The research focused on analyzing the microstructure evolution and fracture properties of remanufactured components using in-situ cooling. The results indicate that in-situ cooling has the potential to decrease the anisotropy of the repaired zone (RZ) and enhance the resistance to crack propagation in the heat-affected zone (HAZ) and base metal (BM). The high cooling rate associated with the in-situ cooling can elevate the presence of high-angle grain boundaries (HAGBs) in the RZ, diminish the phase transformation between the HAZ and BM, and decrease the size of the secondary α phase. The research contributes to a deeper comprehension of fatigue crack propagation in remanufactured components and provides a pathway for the improvement of fatigue performance of remanufactured titanium alloy. [ABSTRACT FROM AUTHOR]

The fatigue life model and fatigue stiffness degradation model are most crucial components in establishment of fatigue progressive damage model (FPDM). Based on numerous static and fatigue experiments of glass fiber reinforced polymer-matrix composites (GFRP) 3232A/Glass, a novel normalized FPDM for complete stress level based on artificial neural network (ANN) has been built: (1) By analyzing fatigue experimental data, the construction method of fatigue life model is deeply investigated. Furthermore, a normalized fatigue life model (NFLM) predicting fatigue life considering simultaneously tension–tension (T-T) fatigue and compression-compression (C–C) fatigue is developed, which has been validated by experimental data. It is expected to reduce experimental time and costs by nearly half; (2) To fully consider the influence of stress level and remove the constraint of the shape of stiffness degradation function, the traditional models are no longer suitable. Thus, a fatigue stiffness degradation model based on ANN (FSDM-ANN) has been constructed, which overcomes drawbacks of the traditional models. The results predicted by FSDM-ANN are consistent with fatigue data in different stress levels; (3) Due to distortion of the fatigue life prediction of extreme high-cycle and low-cycle, an empirical data extrapolation method is proposed here to construct a modified FSDM-ANN, which greatly improves prediction accuracy of stiffness degradation for complete stress levels. [ABSTRACT FROM AUTHOR]

[Display omitted] • A biaxial TTF spectrum with different frequencies and amplitudes is designed. • A method of synchronized acquisition of DIC and testing machine is proposed. • Damage behavior of plain weave composites under biaxial TTF loads is investigated. • Fatigue lives decrease rapidly with increasing torsional fatigue loads. • Superimposed torsional fatigue loads mainly promote matrix cracking and debonding. In this paper, the fatigue life and damage mechanism of plain weave composites under biaxial tension–torsion combined fatigue loading (TTF) are experimentally investigated. A biaxial TTF loading spectrum is designed to consider different fatigue frequencies and amplitudes, and an automated multi-module loading method with synchronized acquisition of the digital image correlation (DIC) and the testing machine is proposed. A comprehensive analysis of the experimental results, including fatigue lives, strain distribution, temperature variation, stiffness degradation, and typical damage morphologies at different fatigue cycles, has been conducted by utilizing DIC, infrared thermography (IRT) and computed tomography (CT). It is observed that the superimposed torsional fatigue load results in a rapid reduction in fatigue lives of plain weave composites, accompanied by more significant residual strain, energy dissipation, temperature rise and stiffness degradation. Particularly, the superimposed torsional fatigue loads mainly promote matrix cracking and debonding around the fiber/matrix interfaces, which then accelerates the damage initiations and accumulations of fiber yarns, ultimately leading to a rapid reduction in TTF fatigue lives. This study can provide valuable references for the design and safe application of plain weave composites under TTF loading. [ABSTRACT FROM AUTHOR]

Fatigue tests conducted on various types of materials often exhibit significant scatter, particularly in High Cycle Fatigue tests, attributed to imperfections located at the scale of the microstructure. In metallic materials, one contributor to this dispersion, but not the only one, is the variation in grain size, which cannot be uniform in a structure that has undergone a complex thermal history during its manufacturing process. This paper introduces a new model for predicting the evolution of the grain size in the nickel-based superalloy Inconel® 718 based on the applied thermal load and explores its impact on cyclic elasto-viscoplastic behaviour and fatigue life predictions. The proposed approach is applied to a complete numerical life analysis of an aircraft turbine disk. [Display omitted] • Grain size influences the mechanical behaviour of metallic materials. • Precise thermal treatments were developed to quantify the grain growth • Modelling the grain size evolution improved fatigue life predictions. • Hall Petch effect influenced in the same way the nonlinear behavior and fatigue. [ABSTRACT FROM AUTHOR]

Non-proportional stresses near welded joints are well known to cause increased fatigue damages as compared to proportional stresses. In this paper, a new method to remove noise from stress-time signals is developed and implemented in a principal component analysis-based approach for non-proportionality quantification. The noise removal method makes it possible to remove noise from stress-time signals causing low stress ranges, while still keeping the overall shape of the signal. When the signal shape is kept it is possible to accurately predict the levels of non-proportionality. The noise removal method is based on the standard rainflow counting method. By choosing a lower stress range limit, the stress ranges assumed to be caused by noise can be excluded and the original signal shape is kept by utilizing simple polynomial fitting. This makes the approach easy to implement and easy to control as it requires only two inputs. The noise removal method and non-proportionality quantification approach are then validated against simulated signals with noise and a simple experiment with proportional loading. The results show that the newly developed method for noise reduction accurately removes noise while keeping the signal shape. • New simple method for removing noise in stress-time signals. • Noise is efficiently reduced in signals while keeping the signal-shape. • Non-proportionality is quantified for noisy stress-time signals with high accuracy. [ABSTRACT FROM AUTHOR]

• Microstructures evolution and damage mechanisms of uncoated and coated superalloys were comparatively investigated. • Thermal barrier coating is beneficial to the fatigue lifetime of DD6 at high stresses, but negligible at low stresses. • The effect of TBC on DD6's fatigue life is related to its oxidation resistance, load - bearing capacity and interdiffusion. • Fatigue lifetime of coated DD6 superalloys was accurately predicted based on the modified Basquin model. Thermal barrier coatings (TBCs) are widely used to further extend the lifetime of turbine blades by protecting the blades from high temperature corrosion and oxidation. However, the mechanical behavior of turbine blades is obviously affected by the TBCs. In this study, microstructures evolution, damage mechanisms and life prediction of coated and uncoated nickel-based single crystal (NBSX) superalloy DD6 under isothermal fatigue load were investigated at 980 °C. The effects of TBCs on fatigue failure behavior and lifetime of DD6 were addressed. The results showed that the fatigue lifetime reduced with the increase of load. The effect of TBCs on the fatigue lifetime was related to the stress amplitude, as the effect was beneficial at high stress but almost negligible at low stress. Fracture morphologies showed that the cracks more likely initiated and propagated from basal defects for both coated and uncoated DD6, and the microstructure evolution also showed stress amplitude dependence. The crack density of uncoated DD6 increased first and then decreased with the increase of stress amplitude. However, the TBCs reduced the number of cracks that penetrate into the DD6 substrate, and the stress amplitude exerted a significant effect on crack propagation paths. In addition, the rafting behaviors of the DD6 substrate of coated and uncoated samples was compared, and results showed that TBCs could reduce the rafting degree of DD6. Finally, the fatigue lifetime of coated samples was predicted based on the modified Basquin model, and the prediction results fitted well with the experimental results. [ABSTRACT FROM AUTHOR]

• MT-CrackNet for automatic in situ fatigue micro-crack detection and quantification. • Self-attention mechanisms enhance precision in measuring micro-crack lengths. • Automates detection and measurement significantly reduces manual post-processing time. • MT-CrackNet outperforms existing deep learning methods in accuracy and generalization. Characterizing fatigue micro-cracks is crucial for understanding the mechanisms and behaviors of material damage. In-situ fatigue testing is an essential method for observing the evolution of fatigue micro-cracks; however, the process often requires significant time, making the measurement of micro-cracks a tedious task. This paper introduces a multi-task deep learning framework called MT-CrackNet, which enables automatic detection and quantification of in-situ fatigue micro-cracks. The framework is capable of recognizing or segmenting multiple tasks such as micro-cracks, text, and scales simultaneously, and its effectiveness is not limited by the magnification of in-situ images. By integrating attention mechanisms and multi-scale strategies, the model enhances its ability to handle long-range dependencies and preserve detail information, accurately identifying and measuring the length of micro-cracks. The effectiveness of the proposed MT-CrackNet is validated through three in-situ fatigue micro-crack propagation experiments. [ABSTRACT FROM AUTHOR]

• A new method is proposed for aircraft structural risk assessment. • Fatigue experiments on 2024 aluminum plates with center holes demonstrate the method. • The method can reduce costs and enhance risk assessment accuracy. An aircraft structural risk assessment method based on fatigue damage diagnosis and prognosis has been developed, considering fatigue crack propagation. The process is divided into three stages: initial crack diagnosis, crack diagnosis, and prediction, utilizing Monte Carlo simulation. Using 2024 aluminum alloy specimens with central holes, the study indicates that in the initial crack diagnosis stage, an inspection standard with a Single Flight Probability of Failure (SFPOF) less than 10-7 and a threshold method enhances structural fatigue crack diagnosis. In the crack diagnosis and prediction stages, iterative updates using Gaussian Process Regression (GPR) within a Dynamic Bayesian Network (DBN) improve crack propagation prediction and risk assessment accuracy. The diagnostic interval significantly impacts SFPOF, with an optimized interval balancing accuracy and computation time. Simplified and precise K value calculation methods enhance efficiency and accuracy. The method reduces costs and improves risk assessment accuracy, providing new insights for SPHM-based aircraft structural risk assessment. [ABSTRACT FROM AUTHOR]

• The tensile fatigue life of ECC under dry, half-saturated, and fully saturated conditions was systematically analyzed, focusing on the impact of free water on fatigue performance. • The tensile strain evolution, fatigue damage patterns, and crack propagation behaviors of ECC in varying saturation states were thoroughly investigated. • Using statistical models, the γ-P-S-N curves for ECC at different saturation levels were established to predict fatigue life and strength with high reliability. • The study confirmed that increased water saturation reduces ECC's fatigue strength, with a notable decrease in fatigue life across all stress levels. The degree of water saturation has a significant influence on the tensile fatigue properties of Engineered Cementitious Composites (ECC). The half-saturated and saturated state ECC can be rapidly prepared by a vacuum saturation equipment. The fatigue tensile mechanical tests under five tensile stress levels and corresponding studies on the damage morphology, fatigue life, S-N relationship, fatigue strain and fatigue damage on dry, half-saturated, and saturarted state were conducted. The mathematical and statistical analysis method was also utilized to predict ECC's fatigue life and strength with high confidence and high reliability, providing a reference for assessing structural safety. The study found that free water significantly reduces ECC fatigue strength, with fatigue life decreasing as water content increases. At 2 million cycles, fatigue strength reduction coefficients for 0%, 50%, and 100% water saturation are 0.623, 0.550, and 0.423, respectively. The three-parameter Weibull distribution best describes ECC fatigue life. Using this model, S-N and P-S-N curves were developed, incorporating confidence limits to derive γ-P-S-N relationships, ensuring high confidence and reliability. [ABSTRACT FROM AUTHOR]

• Full-scale induction-hardened axles were successfully cracked in a fatigue test. • S–N curve of induction-hardened axles was proposed for fatigue damage evaluation. • High-cycle fatigue limit of induction-hardened axles was examined. • Local fatigue-limit approach accurately predicted high-cycle fatigue limit. • Fatigue limit is constant in very-high-cycle regime. This study aimed to examine the stress–number of cycles (S–N) curve and fatigue limit of non-press-fitted parts to improve the sophistication of the design method of induction-hardened axles for medium-carbon steel high-speed railway cars. Macroscopic cracks were generated in the non-press-fitted parts of the axles by selecting appropriate fatigue test methods. Thus, the S–N curve was approximated, and the fatigue limit was obtained. The value of an index in the power-law expression of the S–N curve was 11, which was proposed to fatigue damage evaluation standard. Moreover, we constructed a prediction model for high-and very-high-cycle fatigue limits based on the local fatigue-limit approach and fatigue test results of cut-out specimens from several depth regions of induction-hardened axles. The fatigue limit predicted by the model agrees with the experimental high-cycle fatigue limit. The model estimated that the fatigue limit did not decrease in the very-high-cycle regime. [ABSTRACT FROM AUTHOR]

• Main crack is perpendicular to the maximum normal stress direction. • Increasing phase angle results in more secondary cracks merging with main cracks. • Crack perpendicular to RD dominated by prismatic slip exhibits faster propagation rates. • Increased activities of non-prismatic deformation modes reduce crack propagation rates. • Chapetti model incorporating Findley equivalent stress demonstrates high prediction accuracy. This study investigates the small fatigue crack propagation behavior of commercially pure titanium (CP-Ti) using thin-walled cruciform specimens under in-plane biaxial loading, considering the effects of biaxial ratio and phase angle. Increasing phase angle results in more secondary cracks merging with main cracks perpendicular to the rolling direction (RD) and transverse direction (TD), a phenomenon attributed to the rise in shear stress that accelerates main crack growth. Higher loading biaxiality or a lower phase angle leads to decreased crack propagation rates and increased biaxial fatigue life. Electron backscatter diffraction (EBSD) analysis reveals that when the maximum normal stress aligns with the RD, prismatic slip primarily governs crack propagation, thereby accelerating crack propagation rates. Conversely, alignment with the TD reduces prismatic slip activity and crack propagation rates. Under equi-biaxial loading, prismatic slip activity decreases further, and crack propagation is dominated by multiple slip and twinning, consequently resulting in the slowest propagation rates. Additionally, a higher proportion of prismatic slip under high phase angle also accelerates crack propagation. Finally, incorporating Findley equivalent stress into the Chapetti model, which considers the crack length-dependent threshold effect, a highly accurate biaxial small fatigue crack propagation rate model is proposed. [ABSTRACT FROM AUTHOR]

• Measurement of crack opening stress during fatigue loading and constraint-loss conditions. • Fatigue crack growth tests under constant amplitude, spike overload and representative spectrum loading. • Non-linear crack closure modelling for constraint-loss behavior. • Correlation of fatigue surfaces including development of shear lips and slant growth under constraint-loss conditions. The trend towards virtual testing and digital-twin assisted management means that the accurate and reliable simulation of fatigue crack propagation behaviour is more important than ever. Reliable but conservative approaches to support this are in widespread use in the aerospace industry. Nevertheless, the conservatism comes at a significant cost in terms of reduced structural life and an increased ongoing inspection requirement and, as such leads to questions about the economic burden of these approaches. Recent comparisons between blind predictions and test results revealed the extent of the issue for cracking in aluminium alloy 7075-T7351 coupons with configuration and loading representative of military transport aircraft wing skins. The current models were generally conservative by a factor of two or more in terms of crack propagation life. This suggested that there was significant scope to improve the modelling to better reflect all the complex contributing factors. The current work has investigated the issue of changes in the crack front constraint as the crack progresses from a state of high constraint (close to plane strain) to a lower constraint (approaching plane stress). This issue was investigated both experimentally and with the development of an improved analytical model. A test program was conducted on several specimens, loaded under constant-amplitude, constant-amplitude with spike-overloads and a variable amplitude spectrum. Crack-opening stress levels were measured at key points in the tests and the results were used to develop and evaluate improved modelling approaches. The improved model was generally able to predict crack growth within about ± 30 % of that demonstrated along with the correct form of the crack growth, which is a significant advance and will lead to reduced costs and increased safety. [ABSTRACT FROM AUTHOR]

• A novel prediction method for RCF damage was proposed based on the rough set theory. • The k e value of rail material and wheel-rail contact variables were both employed. • A 3D shakedown limit diagram for pearlite rail materials was constructed. • As the k e increases, the curved surface descends, indicating a lower shakedown limit. Evaluation and prediction of wheel-rail rolling contact fatigue (RCF) damage can provide important theoretical guarantees for the service safety of wheels and rails and help make maintenance easier to plan. This study aims to develop a novel method for evaluating and predicting RCF damage of the pearlite rail materials with various initial shear yield strengths (k e). Based on the rough set mathematical theory incorporated within the cloud model of the comprehensive evaluation index (P 0 / k e * μ t), a novel evaluation and prediction method for RCF damage states of various pearlite rail materials was constructed using the shakedown limits for pearlite rail materials with various initial shear yield strengths. To develop this novel prediction method, different evaluation indices for RCF damage states were designed. A comprehensive certainty approach was introduced to quantitatively analyze the actual measured values of distinct evaluation indices that corresponds to different RCF damage states, wherein the maximum value rule was applied. Moreover, the prediction results were confirmed after further verifying using the actual measured value of the P 0 / k e * μ t. The results indicated that the predicted results were consistent with the test outcomes. The key feature of this prediction method was that it involved both the intrinsic shear yield strength of evaluated pearlite rail materials and wheel-rail rolling contact variables. On the basis of the two-dimensional classical shakedown map, a three-dimensional shakedown limit diagram for rail materials with varying initial shear yield strengths was further constructed using this novel prediction method. The three-dimensional shakedown limit diagram featured an inclined curved surface. As the initial shear yield strength of the pearlite rail materials increased, the curved surface tilted downward, indicating that an increase in the initial k e value of the pearlite rail materials could result in a lower shakedown limit. [ABSTRACT FROM AUTHOR]

[Display omitted] • The interaction between scanning laser source-induced Rayleigh waves and open and closed cracks is studied. • Changes in peak-to-peak values exhibit similarities and differences for open and closed cracks. • Combining scanning laser source technology and self-reference peak-to-peak values enables integrated detection. • The proposed method is experimentally validated qualitatively and quantitatively. In this study, the integrated detection method for open and closed surface fatigue cracks is investigated. Firstly, the finite element simulation models are established to investigate the interaction between scanning laser source-induced Rayleigh waves and open and closed surface fatigue cracks, and the laser ultrasonic detection experiments are conducted for fatigue specimens containing fatigue cracks in different states. The simulation and experimental results indicate that the variation patterns in the peak-to-peak values of Rayleigh waves with horizontal scanning positions for open and closed cracks exhibit both similarities and differences. Subsequently, an integrated detection method based on self-referenced peak-to-peak features is proposed, which utilizes the similarities and differences to detect and distinguish open and closed cracks, respectively. Furthermore, this proposed method is experimentally validated, indicating that it can achieve accurate integrated imaging of open and closed cracks that have a high degree of agreement with the SEM images of the cracks. Additionally, the proposed method achieves a detection error of 2 % for the vertical lengths of the fatigue cracks. This study can provide guidance for integrated real-time detection of open and closed surface fatigue cracks of mechanical components in service. [ABSTRACT FROM AUTHOR]