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Axially loaded cyclic tests of a nickel-based alloy were conducted at 550 °C, 630 °C and 650 °C and stress ratios of −1, −0.3 and 0.5 up to high cycle fatigue regime. Multiscale characterizations and quantitative analyses of microstructural damage were carried out to illustrate the physics and mechanics of interior microstructure induced fatigue strength degradation and cracking. Results showed that the fatigue strength increased with the decreases of twin spacing, the size and aspect ratio of carbide, and γ′ phase size. Twin collapse due to slip band-twin interaction was found for the first time and confirmed as a form of crack initiation process, followed by short crack growth forming facets with significant contribution from shear stress. A new mechanistic fatigue life model of interior microstructure induced cracking was established with sound agreement with experimental data. All these combined into a design-manufacturing integrated approach which are capable of ensuring the safe operation and maintenance of service structures in long-life regime. • A shorter twin spacing, a reduced size and aspect ratio of carbide, and a smaller γ′ phase size were found beneficial for higher fatigue strength. • Slip bands-twin interaction induced twin collapse was found for the first time, which was the first step to interior facet induced cracking. • Dislocation slip, the size and location of facet was combined into a fatigue life model to evaluate interior facet induced fatigue failure. • A design-manufacturing integrated approach to anti-fatigue for long-life service structures was established. [ABSTRACT FROM AUTHOR]

Nonlinear ultrasonic analysis has been proposed to characterize the thermal degradation of HP40Nb steel associated with microstructure evolution. Measurements of ultrasonic nonlinear signals were performed in the thermally degraded damaged HP40Nb specimens and the results showed a clear change of increase-plateau-decrease tendency of the normalized nonlinear parameter with respect to the different thermal loading time. Based on metallographic studies, the variation in the measured acoustic nonlinearity reveals that the normalized acoustic nonlinearity increases due to the increases of the second phase precipitates and dislocation density in the early stage and it decreases as a combined result of the reduction of dislocation density, coarsening of precipitates and initiation of micro-voids after long-term high temperature exposure. Moreover, an analytical model calculation of precipitate-dislocation interaction has been performed to interpret the correlation between microstructural evolutions and the measured ultrasonic nonlinearity. Consequently, ultrasonic nonlinearity is found to be strongly sensitive to the microstructure evolutions during thermal degradation of HP40Nb steels. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this paper was to address the fatigue behavior and mechanism of the laser cladded WC/Ni composite coatings under rolling contact condition. The coating was deposited by a 4kW continuous wave CO2 laser system. Rolling contact fatigue tests were conducted under lubrication condition at room temperature. The surface morphologies of the entire wear track for different rolling cycles were observed and recorded. Experimental results showed that the surface crack behaviors were strongly dependent on the location on the wear track. It was identified that spalling was the main fatigue failure mode of the coating under rolling cycles. The joining of surface cracks and the subsurface branched cracks led to the formation of the spall. The depth of the main subsurface crack was influenced by the distribution of the WC particles in the coating. [Copyright &y& Elsevier]

Abstract: Evolution of microstructure, micro-hardness and micro-tensile strength behavior was investigated in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint after the artificial aging at 350°C for 3000h. After detailed characterization of microstructures in optical microscopy, scanning electron microscopy and transmission electron microscopy, it is revealed that the change of martensite–bainite constituent promotes more homogeneous microstructure distribution. The aging treatment facilitates redistribution of carbon and chromium elements along the welded joint, and the micro-hardness is increased slightly through the welds due to enrichment of carbon. The types of precipitates in the weldment mainly include M3C, MC, M2C and M23C6. The carbides in base metal, weld metal and coarse-grained heat-affected zone are prone to change from ellipsoidal to platelet form whereas more uniform spherical carbides are observed in the fine-grained zone. Precipitation and coarsening of M23C6 near the fusion line, and formation of MC and M2C, are responsible for the tensile strength decrease and its smooth distribution in the aged heat-affected zone. This implies that the thermal aging can relieve strength mismatch in the weldments. [Copyright &y& Elsevier]

Abstract: The creep rupture life of 9–12% chromium ferritic steel is predicted as a function of alloy composition, creep stress, and creep temperature. A database is made up from data in previous publications. A novel abductive network is constructed to predict creep rupture life. With a four-layer architecture, the network shows a precise prediction of creep rupture life of 9–12% chromium ferritic steel. Performance is examined and compared with backpropagation algorithm (BP) neural network. Results indicate that the proposed approach is more accurate than Larson–Miller parametric method and more efficient than that of BP neural network. Automatic relevance determination reveals that the influence of Cr and W on creep rupture life of 9–12% chromium ferritic steel are the greatest amongst the alloying elements. [Copyright &y& Elsevier]

• Nonlinear Lamb wave imaging of micro-damage is achieved based on wavefield data. • A new approach is proposed for weak nonlinear feature enhancement. • Demonstrated capability of accurately imaging the fatigue crack tip. • Visualize the wave field of the harmonics. The traditional nonlinear ultrasonic technique, as typified by the second-harmonic generation and the frequency mixing response, can be employed to identify and characterize the micro-damage. However, the research on micro-damage characterization using nonlinear Lamb wave imaging technique remains an ongoing challenge and is rarely reported. A method called standardized amplitude difference is proposed for nonlinear feature enhancement, and further for fatigue crack imaging based on the wavefield data. Wavefield data contain abundant information on the spatial and temporal variation of propagating waves in the damaged structure. The nonlinearity index β ′ of the signal difference under the high and low incident wave amplitudes is calculated for fatigue crack imaging. Two scanning methods, including local scanning and global scanning, are introduced to image the fatigue crack tip and visualize the wave field of the harmonics respectively. The experimental validation, based on the imaging results of an aluminum alloy plate specimen with a barely visible fatigue crack and a steel plate with a blind hole, manifests that the proposed method can be used to enhance and extract the nonlinear features and suppress the fundamental frequency, so as to improve the signal-to-noise ratio (SNR) of the micro-damage imaging results. [ABSTRACT FROM AUTHOR]

• Physics-informed neural network (PINN) is built for creep-fatigue life prediction. • Physics-informed feature engineering and physics-informed loss function are applied. • PINN exhibits better prediction capacity than conventional machine learning models. Physics-informed neural network has strong generalization ability for small dataset, due to the inclusion of underlying physical knowledge. Two strategies are enforced to incorporate physics constraints to a deep neural network in this work. One is to obtain extended features through physics-informed feature engineering, and the other is to incorporate physics-informed loss function into deep neural network as constraints. Conventional machine learning models, deep neural network and physics-informed neural network are applied to predict creep-fatigue life of 316 stainless steel. Results show that physics-informed neural network presents better prediction accuracy than deep neural network and conventional machine learning models. [ABSTRACT FROM AUTHOR]

The total focusing method (TFM) is considered as the standard in ultrasonic phased array imaging and plays a vital role in industrial non-destructive testing (NDT). By utilizing the full matrix capture (FMC) dataset, the TFM can focus on every point within the specified imaging region, and is more accurate than the traditional ultrasonic phased array imaging methods. However, the TFM is essentially a delay and sum technique that often operates linearly on the time-domain signals and takes no prior information into account, so its image quality remains inadequate when dealing with defects in close proximity or scattering materials. To address this problem, this paper formulates the imaging principle of the TFM as a Boolean matrix and establishes the discretized tensor-based model. Subsequently, the model is addressed by employing the sparse regularization strategy, taking into some characteristics of industrial NDT. Regarding the solution algorithm, due to the generation of negative values by the fast iterative shrinkage threshold algorithm (FISTA), this paper introduces the rectified linear unit (ReLU) function as a non-negative constraint and presents a dedicated solution algorithm (ReLU-FISTA) for acquiring detection results. Through verification of simulation and experiment, the proposed approach exhibits superior capabilities of defect characterization and noise suppression when compared to the TFM, leading to substantial enhancements in image quality. [ABSTRACT FROM AUTHOR]

Abstract: A mathematical model for simulating the active dissolution of a pit on stressed metal surface had been developed. Based on active dissolution mechanism, dissolution current density on the pit surface was assumed and extended through accounting for the thermal activation energy and the multiaxial stress state in pit bottom. The influence of applied tensile stress, pit radius and temperature was addressed. The distribution of solution potential and species concentration was predicted for different applied tensile stresses based on finite element calculations. [Copyright &y& Elsevier]

Abstract: The aim of this paper was to address the effect of WC content on the tensile properties of WC/Ni composite coatings deposited by laser cladding on 45# steel substrates. The tensile tests were performed in the chamber of a scanning electron microscope. Experimental results showed that the mechanical properties and fracture behavior of WC/Ni coatings were strongly dependent on the volume fraction of WC reinforcing particles in the coatings. The tensile properties of the coating decreased with increasing content of WC particle. When the WC content was relatively high, multiple through-width cracks (TWC) in the coating can be found. The number of through-width cracks in the coating as a function of applied strain was measured. From the in-situ observation of coating surfaces, most of cracks were initiated from the WC particle, and then propagated throughout the coatings. This phenomenon was attributed to the stress concentration in the WC particle or near the WC particle, which was confirmed by using finite element analysis. The present research may provide some insights on the development of designing methodology of composite coatings. [Copyright &y& Elsevier]

Abstract: In this paper, a computationally attractive damage index method is proposed for structural damage detection of cylindrical shell. It provides an insight to the extent of structural damage. Strain energy values computed for undamaged and damaged states are used in the corresponding damage index algorithms. It is derived from two parameters as moment response power spectral density and curvature response power spectral density. For all measured mode shapes, the damage index is defined by using the statistical parameter of relative root mean-square error of case before and after damage. The algorithm is applied to two numerical examples and its efficiency is demonstrated through damage simulations. [Copyright &y& Elsevier]

The creep damage in high temperature resistant titanium alloys Ti60 is measured using the nonlinear effect of an ultrasonic Lamb wave. The results show that the normalised acoustic nonlinearity of a Lamb wave exhibits a variation of the “increase-decrease” tendency as a function of the creep damage. The influence of microstructure evolution on the nonlinear Lamb wave propagation has been analyzed based on metallographic studies, which reveal that the normalised acoustic nonlinearity increases due to a rising of the precipitation volume fraction and the dislocation density in the early stage, and it decreases as a combined result of dislocation change and micro-void initiation in the material. The nonlinear Lamb wave exhibits the potential for the assessment of the remaining creep life in metals. [ABSTRACT FROM AUTHOR]

An analysis was made of the diffusional creep in polycrystalline materials when subjected to simultaneous action of tensile stress and electric current. Incorporating the contribution of electromigration to atomic diffusion in grain boundaries, the problem of grain boundary diffusion was solved. A simple relationship was derived between diffusional creep rate and local electric field intensity (current density). The results showed that electromigration can enhance the diffusional creep in polycrystalline materials. [ABSTRACT FROM AUTHOR]

Abstract: The thermal degradation in ferritic Cr–Ni alloy steel plates is measured using the nonlinear effect of Lamb wave propagation. Experiments were carried out to introduce controlled levels of thermal damage to determine the nonlinear response of Lamb waves. A “mountain-shape” change in the normalized acoustic nonlinearity of Lamb wave versus the level of thermal degradation in the specimens has been observed. The variation in the measured acoustic nonlinearity reveals, based on metallographic studies, that the normalized acoustic nonlinearity increases due to the second phase precipitates in the early stage and it decreases as a combined result of dislocation change and micro-void initiation in the material. The results show a potential application of the nonlinear Lamb waves for the quantitative assessment of thermal damage in metallic plates or pipes. [Copyright &y& Elsevier]

Abstract: The cumulative second-harmonic analysis of ultrasonic Lamb wave has been performed to study the precipitation kinetics and microvoid initiation of ferritic Cr–Ni alloy steel during the ageing process. Ageing of ferritic Cr–Ni alloy materials have been done at 1223K and 1173K for different degradation time intervals and air cooled. The results show that the normalized acoustic nonlinearity of Lamb wave increases with the formation of fine precipitates at the early stage of ageing till about 1000h and keeps as a plateau with the precipitates dynamic balance for a long-term ageing, and then decreases gradually at the final holding time with the coarsening of precipitates and initiation of microvoids. The results also show that the variation of nonlinear Lamb wave follows the same trend as that of hardness in materials. Therefore, the cumulative second-harmonic of ultrasonic Lamb waves has been found to be strongly sensitive to the precipitates behavior and microstructure evolution during the thermal ageing of ferritic Cr–Ni alloy steel. [Copyright &y& Elsevier]

Abstract: The miniaturization of the process equipment has lead to the development of micro-chemical and thermal systems for high tech applications. With the increase of temperature and pressure, the design of the miniature components against creep failure becomes a primary concern. However, the stress analysis of the structure is highly complicated and time-consuming by general finite element methods if a network of inter-connected dodecahedral-like cells is involved. In the present paper, the short-term mechanical properties and the viscoelastic parameters are measured by uniaxial tension experiments. Based on these parameters and small deformation theory, the time-dependent deformation and stresses of the plate-foam structures are analyzed by assuming the metal foams as elastic foundation. Using the numerical inversion of Laplace transform and the standard linear solid model, the analytic expressions of the deformation and stresses are derived. By comparison, the results show that the time-dependent deflection and bending stress along the normalized distance from the analytical method were consistent with those from the finite element method. It indicates that the proposed analytical method was feasible and accurate. Additionally the effect of the viscoelasticity on the deflection will disappear gradually if is much larger than 600GPa. [Copyright &y& Elsevier]

• A general machine learning life prediction method is proposed for creep, fatigue and creep-fatigue conditions. • Creep, fatigue and creep-fatigue data are integrated into a unified dataset. • DNN exhibits better prediction accuracy than conventional machine learning models. Deep learning is a particular kind of machine learning, which achieves great power and flexibility by a nested hierarchy of concepts. A general life prediction method for components under creep, fatigue and creep-fatigue conditions is proposed. Fatigue, creep and creep-fatigue data of a typical austenitic stainless steel (i.e., 316) are integrated. Conventional machine learning models (e.g., support vector machine, random forest, Gaussian process regression, shallow neural network) and deep learning model (e.g., deep neural network) are applied for life predictions. Results show that deep learning model exhibits better prediction accuracy and generalization ability than conventional machine learning model. [ABSTRACT FROM AUTHOR]

Abstract: In the present paper the crack behavior of multi-component material systems is investigated under extensive creep condition. The validation of the creep fracture parameters C * and C(t) is firstly examined at the microscale level. It is found that the C * value is no longer path-independent when mismatch inclusions are embedded into the matrix. To characterize the crack fields in inhomogeneous material the integral value defined at the crack tip as is introduced to reflect the influence of the inclusion. The interaction effects between microcrack and inclusion are systematically calculated with respect to different mismatch factors, various inclusion locations and inclusion numbers. The analysis results show that the value is not only influenced by the inclusion properties but also depends on the microstructure near the crack tip. [Copyright &y& Elsevier]

Abstract: A probabilistic model for prediction of bonding time in diffusion bonding is presented based on the stochastic characteristics of surface finish of bonded area. The probabilistic distribution of surface roughness is introduced into a deterministic model of diffusion bonding by taking the parameters of void radius and height as random variables. Monte Carlo simulation technique in conjunction with numerical integration is used to calculate the bonding time and its probabilistic distribution. The sensitivity analysis of parameters is then performed to evaluate the effects of creep exponent and surface roughness on the bonding time. Comparison is also made between the theoretical predictions and available experimental results from previous work on Ti–6Al–4V alloy in order to validate the probabilistic model. The proposed model showed a good agreement with the experimental results. [Copyright &y& Elsevier]

Fatigue damage and micro-cracks growth have been proved to be closely related to the nonlinear effect of ultrasonic Lamb waves propagation in materials. In this paper, the relationship among fatigue life, length and width of micro-cracks, and the acoustic nonlinearity parameter was established. The variation of the acoustic nonlinearity parameter caused by fatigue damage was analyzed based on micro-cracks growth. Considering the multimodal nature of nonlinear Lamb waves, we selected the low-frequency mode pairs S0–s0 and S1–s2 to evaluate fatigue damage in 7075 aluminum alloy plates. Both FE simulations and experiments show that the mode pair S1–s2 is more suitable than S0–s0 for the assessment of fatigue damage. It was found that with increasing fatigue cycles, the relative acoustic nonlinearity parameter A2/A12 firstly increases because the micro-cracks length extends while the micro-cracks width does not change much. The relative acoustic nonlinearity parameter A2/A12 reaches its peak at about 60% of the fatigue life. Then the relative acoustic nonlinearity parameter A2/A12 decreases because of the rapid increase in the micro-cracks width. Our findings clarify the mountain-shape curve between the relative acoustic nonlinearity parameter A2/A12 and the fatigue cycles. [ABSTRACT FROM AUTHOR]

• A new compounding imaging algorithm integrating amplitude and phase information of signal based on Lamb wave phased array is proposed. • Sigmoid function with variable gradient is used for establishing the nonlinear mapping between imaging index and phase difference. • The proposed imaging algorithm is validated on plates of different materials and dimensions with multiple types of defects. • The proposed imaging algorithm can effectively improve the signal-to-noise ratio of the images and suppress artifacts, implementing imaging without threshold. Aimed at improving the imaging performance of Lamb wave phased array based on Delay-and-Sum (DAS) of signal amplitude, a compounding imaging algorithm integrating amplitude and phase information of signal is proposed in this paper. By applying back propagation dispersion compensation, the pulse echoes caused by defects will be in-phase with the excitation signal, implementing signal focusing. In the proposed imaging algorithm, the amplitude imaging index is calculated by classical total focusing method (TFM) and the phase imaging index is calculated by phase coherence factor (PCF), which establishes the nonlinear mapping between imaging index and phase difference by sigmoid function with variable gradient. The final imaging results are the compound of two imaging indexes. The experimental validation is carried out on plates of different materials and dimensions with multiple types of defects based on uniform linear array. Compared with the imaging results of conventional TFM, the proposed imaging algorithm can effectively improve the signal-to-noise ratio (SNR) of the images and suppress artifacts, implementing threshold-free imaging. [ABSTRACT FROM AUTHOR]

[Display omitted] • A multiscale method to predict mechanical properties of polymer composite is proposed. • The contributions include polymer matrix, particle filling and surface adhesion. • The comparisons between predictions and experimental measurements validate the model. • Further theoretical analysis on matrix composite and particle size distribution is performed. Owing to the multiscale structural characteristics and complex internal coupling, the evaluation of mechanical properties of a polymer composite upon its microstructural information is challenging. Herein, we propose a multiscale method for predicting the mechanical properties of polymer composites by accounting for the contributions of the polymer matrix and those from particle filling and particle–matrix interactions. The former contributions are addressed with the reference polymer matrix by means of molecular dynamic simulation, while the latter are described by a revised continuum model with the input from basic experimental data. The proposed model, validated with the corresponding experimental measurements, indicates that a small difference in the matrix composition can lead to a significant deviation of 60% in the ultimate stress at room temperature, and that the particle size distribution has a synergistic effect, resulting in a nonmonotonic dependence of the mechanical properties on the average particle size. This study provides a feasible engineering tool for evaluating the mechanical properties of polymer composites. [ABSTRACT FROM AUTHOR]

Thermographic inspection is considered an effective and promising nondestructive testing tool because of its intuitiveness, wide range and noncontact property. Despite this, the detection of weak defects and the recovery of their shape remain difficult, particularly when the surface being inspected is the opposite of the surface being drilled. This study proposes a new tensor robust principal component analysis method based on Bayesian Tucker decomposition to improve the spatial resolution of thermography. A hierarchical form of a generalized Student-t prior is imposed on the model parameters in the Bayesian framework so as to approximate the low-rank component related to the defect feature. Through variational Bayesian inference, all model parameters are adaptively estimated. Based on two experimental data, it appears that the proposed method is capable of improving the spatial resolution and detection accuracy of the thermographic inspection system. [ABSTRACT FROM AUTHOR]

• The stress transfer mechanism for overload effect was verified successfully. • The level of crack closure after overload was found varied periodically. • Overload plastic zone and overload affected zone were linearly correlated with overload ratio. Cyclic plastic finite element modeling was carried out to investigate effect of overload on fatigue crack growth under blunting and no-blunting mechanisms at different overload ratios. The stress transfer mechanism was verified and was found more obvious in the case of crack blunting though its effect was time-limited. The level of crack closure varied periodically due to gradual relaxation of compressive residual stress during crack propagation. Both the overload plastic zone and the overload affected zone were linearly correlated with overload ratio, and an overload ratio value of 1.1 was the best choice for studying overload effect. [ABSTRACT FROM AUTHOR]

Reliability and safety factor based methods are usually employed to characterize uncertainty of creep rupture properties, but the correlation between these two methods is rarely reported. In this work, a framework of correlation between reliability and safety factor based methods was proposed, and application of this framework in representation of minimum stress to creep rupture was reported. Effect of standard deviation on correlation results was discussed and comparisons of minimum stress to creep rupture in codes and standards were conducted. Results indicated that for a given reliability, the safety factor included in the minimum stress to creep rupture is higher at a small stress level and a high temperature. Accordingly, the minimum stress to creep rupture by reliability based method in ASME code (i.e. 95 %) is lower than that by safety factor based method in RCC-MRx code (i.e. 1.25), presenting a smaller allowable stress value and more conservative evaluation. Contrarily, a smaller safety factor is gained at a high stress level and a low temperature for a given reliability. The opposite conclusion is drawn when the reliability at a given safety factor is analyzed. In addition, the safety factor of 1.5 is larger than that at the reliability of 99 % for a wide range of stress levels, inducing an over-conservative design and excessive manufacturing costs. • Framework of correlation between reliability and safety factor based methods is proposed. • Effect of standard deviation on correlation results is discussed. • Comparisons of minimum stress to creep rupture in codes and standards are conducted. • Safety factor is higher at a small stress level and a high temperature for a given reliability. [ABSTRACT FROM AUTHOR]

• The reliability of expanded data based on Z- parameter was confirmed. • Data expansion compensated for the shortage of original experimental data. • A Z -parameter neural network integrated model was successfully established. • The training dataset size and physical model affected prediction accuracy of PINN model. • The PINN model was superior to traditional physical model for life prediction. The defects created in metallurgical and manufacturing processes generally play a decisive role in very high cycle fatigue life of engineering structures. By taking stress level, defect size and location into account, the physical Z -parameter model on fatigue life prediction was combined with artificial neural network as a new data-physics integrated approach for fatigue life prediction of 15Cr and FV520B-I steels in this work. The original data from tests were expanded based on the Z -parameter model, and the physics-informed loss function featuring Z-parameter was integrated into artificial neural network as the constraint. Results showed that the physics-informed neural network established in this work could be applied for life prediction in the very high cycle fatigue regime, and the model came with higher predictive accuracy than the physical Z -parameter model and the Mayer's model did. [ABSTRACT FROM AUTHOR]

• The high cycle fatigue property of FGH96 inertia friction welded joint was studied. • Interior crack initiation transferred from facet to twin-assisted facet to inclusion. • The twin-assisted crack initiation was observed with direct evidence. • The Z-parameter fatigue lifing model was applied to facet-induced cracking. High cycle fatigue behavior of FGH96 alloy inertia friction welded joint under axially push–pull cyclic loading was studied at 650 °C. Results showed fatigue strength of welded joint was comparable to pure base metal. Larger grain size, more twin boundaries and high angle grain boundaries induced a higher crack initiation potential at the base metal. Interior crack initiation was dominant while it was transferred from large facet, to twin-assisted facet, and finally to inclusion with stress level decreasing. The Z -parameter model was applied to fatigue life prediction in the case of facet-induced crack initiation for the first time, and good agreement was obtained between prediction and experimental data. [ABSTRACT FROM AUTHOR]

[Display omitted] • VHCF life prediction was explored with SVM, ANN and Z -parameter based PINN models. • Datasets were extended by the Z -parameter model with sound reliability. • An evaluation method was constructed to analyze and compare model performance. • A larger dataset was found having more desirable implications for model prediction. • Z -parameter based PINN model outperformed others in terms of predictive accuracy and reliability. The research on life prediction for mechanical structures in very high cycle fatigue regime is pivotal to improve structure service, but it can be costly and time-consuming to collect fatigue data. In response, the data-driven approach of machine learning emerged as a solution to data insufficiency. In this work, after extracting a small dataset of GCr15 bearing steel subjected to very high cycle fatigue tests from open literature, the Z -parameter model was applied to obtain extended datasets to establish models driven by support vector machine, artificial neural network, and Z -parameter based physics-informed neural network, respectively. With training on extended datasets and the original data as test set, fatigue life prediction for GCr15 steel was carried out and evaluated between these models. Results showed that the physics-informed neural network calibrated by Z -parameter model trained on a larger dataset featured more accurate and reliable prediction than other models did, which demonstrated effectiveness of Z -parameter in data extension and model construction as priori physics knowledge for a data-driven approach. Looking into the future, Z -parameter model deserves more attention to its employment in life prediction for more engineering materials and structures serving in the very high cycle fatigue regime. [ABSTRACT FROM AUTHOR]

• 3D synchrotron and 2D in-situ SEM were used for crack closure determination. • FCG retardation was correlated with crack-tip straining, plastic zone and COD. • Crack-tip shielding effect and strain transfer was observed for the first time. • Load history dependence of FCG and residual compressive strain relaxation were evidenced directly. • Load transfer was proposed as the physics of near-tip straining, crack opening and FCG. • FCG retardation was mainly governed by residual compressive stress. Fatigue crack growth retardation of AZ31 alloy under a single tensile overload was investigated by 3D synchrotron radiation, 2D in-situ SEM and digital image correlation techniques. Results showed a consistent evolution of crack closure level by both 3D and 2D methodologies. The straining and plastic zone ahead of crack-tip, and crack opening displacement behind, which were quantitative indicators of crack-tip shielding, residual stress formation and relaxation, were correlated well with the retardation behavior. Mechanistically, compressive residual stress played a dominant but time-limited role while crack closure was secondary, based on an intrinsic load transfer concept during crack growth. [ABSTRACT FROM AUTHOR]

In this paper, the guided wave propagation behavior in damaged 30CrMo steel curved plates was investigated experimentally and numerically. The effects of the notch orientation, depth in the curved plate, as well as its radius, on the wave propagation characteristics were mainly analyzed by the amplitude distribution curves and the directivity diagrams of A0/S0 (zero-th order of the symmetric/antisymmetric Lamb wave) modes. An ellipse-based algorithm was compiled to locate the notches in the curved plates. Results show that the normalized S0 wave amplitude in the circumferential orientation was the largest, and it increases as notch depth increases in the axial orientation. The A0 wave amplitude in axial orientation was the largest, while it decreases with the increasing of notch depth in the other orientations. The normalized A0 wave amplitude in axial orientation increases with the increasing of radius. With the increasing of radius, the other normalized A0/S0 amplitudes linearly decreased for the other paths. The ellipse-based algorithm has high notch localization accuracy, and the notch localization error increase from 0.005% to 1.47% with the notch depth decreasing from 5 mm to 1 mm in the curved plates. For the curved plates with different radius, the maximum notch localization error is 1.20%. These satisfactory results demonstrate the effectiveness of the developed algorithm in locating damages in the researched structure. [ABSTRACT FROM AUTHOR]

Fatigue crack growth behavior of Inconel 740H and Haynes 282 nickel-based alloys at 20 °C, 700 °C and 750 °C was studied. Results showed increased fatigue crack growth rate for both the two alloys, while increased fatigue threshold in Inconel 740H at high temperatures. The size and distribution of MC carbides, serrated grain boundaries and twins deflected crack growth path. The strengthening γ' phase was interacted with dislocations by Orowan bypass and shear mechanisms at room temperature, while γ' was sheared in Haynes 282 and Orowan bypassed in Inconel 740H alloy at elevated temperatures. The cross slip, dislocation climbing and entanglement reduced the resistance of dislocation movement and accelerated the accumulation of damage. The higher content of Cr in Haynes 282 alloy promoted formation of M 23 C 6 carbide while the lower content in Inconel 740H alloy promoted thicker oxide layer. The Cr content needed careful tuning as it was superior to microstructural factors for higher fatigue threshold at elevated temperatures. [ABSTRACT FROM AUTHOR]

To understand the stress state dependent creep damage, following creep tests in 9–12% Cr steel notched components with different root radii, a study of associated time dependent microstructural evolution is presented in this work. Particular attention is paid to the dependence of creep damage evolution on the stress state. The average creep strain of notched component exhibits a typical three-stage characteristic (transient, steady and tertiary stages). During the transient stage, remarkable re-distribution in stress state (equivalent stress and stress triaxiality) is found. The rapid relaxation of equivalent stress near the notch root, associated with the initial coarsening of substructure would delay the nucleation or growth of cavity. By contrast, the time-dependent increase of equivalent stress combined with the relatively high stress triaxiality seems to be responsible for the higher cavity density at the center of notched component. The precipitate coarsening is heterogeneous and promoted by the high level of equivalent stress or stress triaxiality. During the steady stage, the creep damage behavior is related to the magnitude of stress state. Higher cavity density and greater reduction in hardness are mainly found in the region where both the equivalent stress and stress triaxiality are high. As a whole, both the redistribution process of stress states and their levels exhibit significant influence on microstructural evolution and creep damage behavior of notched components at elevated temperatures. • Dependence of time-dependent creep damage on evolution of stress states is clarified. • Both the redistribution of stress states and their levels influence the evolution of microstructural features. • The relaxation of equivalent stress accompanied by the initial coarsening of substructure would delay the growth of cavity. • The magnitudes of stress states rather than their redistribution play an important role on precipitate coarsening. [ABSTRACT FROM AUTHOR]

As a result of the high specific strength/stiffness to mass ratio, filament wound composite pressure vessels are extensively used to contain gas or fluid under pressure. The ability to in-situ monitor the composite pressure vessels for possible damage is important for high-pressure medium storage industries. This paper describes an in-situ monitoring method to permanently monitor composite pressure vessels for their structural integrity. The sensor is made of a multi-walled carbon nanotube (MWCNT) that can be embedded in the composite skin of the pressure vessels. The sensing ability of the sensor is firstly evaluated in various mechanical tests, and in-situ monitoring experiments of a full-scale composite pressure vessel during hydraulic fatigue cycling and pressurization are performed. The monitoring results of the MWCNT sensor are compared with the strains measured by the strain gauges. The results show that the measured signal by the developed sensor matches the mechanical behavior of the composite laminates under various load conditions. In the hydraulic fatigue test, the relationship between the resistance and the strain is built, and could be used to quantitative monitor the filament wound pressure vessel. The bursting of the pressure vessel can be detected by the sharp increase of the MWCNT sensor resistance. Embedding the MWCNT sensor into the composite pressure vessel is successfully demonstrated as a promising method for structural health monitoring. [ABSTRACT FROM AUTHOR]

Highlights: • Different loading mode results in different crack initiation mechanism. • The mismatch between galvanic corrosion and strength affects fracture location. • Stress corrosion cracking sensitivity must be assessed by multiple test methods. Abstract Stress corrosion cracking (SCC) behaviors of 30Cr2Ni4MoV welded joint in simulated environment of low pressure nuclear steam turbine were addressed with different fracture positions in the welded joint by constant loading test (CLT) and slow strain rate test (SSRT). The fracture of the welded joint happened near the fusion line which was ascribed to the interaction between stress and galvanic effect for CLT, while in welded metal with the weakest strength for SSRT. The mismatch between galvanic corrosion and strength of welded joint determined the fracture location and SCC sensitivity of the welded joint when the two different loading modes were applied. [ABSTRACT FROM AUTHOR]

The nonlinear interaction of Lamb waves has attracted considerable attention because of its potential for localizing micro-damage in plate-like structures. Most recent studies have focused on the nonlinear mixing of Lamb waves propagating in the same direction, but this makes it difficult to determine the exact position of micro-damage because of the relatively large mixing zone. To overcome these shortcomings, we consider the nonlinear interaction of counter-propagating Lamb waves in this letter. An obvious resonance phenomenon is first observed in both experiments and simulations, which agrees well with our previous theoretical analysis [Li et al., J. Appl. Phys. 124, 044901 (2018)]. Also, analyzing the time-domain signal shows that the precision of damage localization is a function of the frequencies, the number of cycles, and the phase and group velocities of the primary waves. [ABSTRACT FROM AUTHOR]

Highlights • Comparative study on cyclic softening evolutionary characters in different subzones. • Comparison of fatigue strength of different subzones in CrNiMoV welded joint. • Study the ratcheting behavior of welded joints under different stress ratios. • Comparison of cyclic and fatigue features for joints under different control mode. • Explanation of the failure location shifting behavior in the welded specimen. Abstract The cyclic plasticity and the low cycle fatigue failure behavior of the weld metal (WM) and base metal (BM) of the CrNiMoV steel welded joint under the strain and stress-control modes were investigated respectively. Significant cyclic softening was observed for both the WM and BM under the low cycle fatigue tests with the two control modes. Besides, obvious ratcheting happened in the WM and BM under the stress-controlled cyclic loading conditions. It is shown that both the WM and BM exhibited lower fatigue strength at the stress control mode than that at the strain control mode due to the influence of tension-compression asymmetry. Meanwhile, the WM showed larger cyclic softening rate, lower ratchetting deformation and fatigue strength than the BM under the same loading levels. The failure location of the WM specimens shifted from BM region (nearby the heat affected zone) to the center of WM with the increasing of strain amplitude under the strain-controlled tests, which can be explained with the similar maximum equivalent plastic strain amplitude location shifting behavior observed from the corresponding finite element simulations. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

The feasibility of fatigue damage evaluation has been investigated using nonlinear Lamb waves with group-velocity mismatching. To choose an efficient mode pair, a parameter is proposed to quantify the efficiency of cumulative second-harmonic generation (SHG) of Lamb waves based on the normal modal analysis. Experiments and simulations are performed to verify the proposed parameter, which demonstrates that whether the matching condition of group velocity is satisfied or not, the efficiency of cumulative SHG increases with the order of Lamb mode for the five low-order Lamb waves investigated. Then, S3-s6 mode pair with group-velocity mismatching is chosen to characterize the fatigue damage of an aluminium alloy for the high efficiency of cumulative SHG. Results show that S3-s6 mode pair is sensitive to fatigue damage evolution and the integrated amplitude of second harmonics increases by nearly 300% with fatigue cycles. Nonlinear Lamb waves with group-velocity mismatching are validated to be a candidate to efficiently evaluate the fatigue damage. [ABSTRACT FROM AUTHOR]

Creep-cyclic plasticity of a benchmarked holed plate subjected to thermo-mechanical loading is investigated by means of nonlinear finite element analysis. From the analyses, a structural creep recovery response is found within a dwell period, which has serious repercussions on structural integrity. The structural creep recovery can take place by reversing the creep stress in sign during the stress relaxation due to the creep stress redistribution, consequently enhancing unloading plasticity which causes a substantial increase of total strain range within a cycle. Based on this critical observation, further analyses and discussions are provided to investigate the root cause of this precautious structural response. Various cyclic loadings with a dwell at the peak thermal load are analysed to define factors influencing the structural creep recovery mechanism, and to investigate how the mechanism affects the lifetime of the structure. To show the effectiveness of the structural creep recovery mechanism under cyclic loading, Chaboche nonlinear kinematic hardening model is adopted. Limitations of applying elastic follow-up in predicting creep strains and appropriate creep-fatigue damage calculation methods are discussed in the presence of this structural creep recovery mechanism. This research work confirms that when a structure experiences the structural creep recovery it can reduce creep damage, nevertheless the structure may experience significant fatigue damage due to creep enhanced plasticity. [ABSTRACT FROM AUTHOR]

Cross-weld and pure base metal specimens machined from dissimilar welded joints of 9%Cr and CrMoV steels were cyclically loaded up to very high cycle fatigue at 500 °C under load ratio of zero. Results showed fatigue strength of the welds was greatly weakened, lower than base metals. A transition of failure mechanisms from geometrical soft zone in heat-affected zone of CrMoV steel to micro-defects induced cracking at weld metal was responsible for decreasing fatigue strength reduction factor with the increasing of fatigue lifetime. A sound fatigue design requires careful selection of filler metal and consideration of soft zone and micro-defects. [ABSTRACT FROM AUTHOR]

• The application of machine learning in fatigue life prediction is reviewed. • The shift from data-driven to physics-based hybrid approaches is discussed. • The approaches of combining physics-based and data-driven models are described. • The future challenges and development directions of fatigue life prediction are discussed. Fatigue life prediction is critical for ensuring the safe service and the structural integrity of mechanical structures. Although data-driven approaches have been proven effective in predicting fatigue life, the lack of physical interpretation hinders their widespread applications. To satisfy the requirements of physical consistency, hybrid physics-informed and data-driven models (HPDM) have become an emerging research paradigm, combining physical theory and data-driven models to realize the complementary advantages and synergistic integration of physics-based and data-driven approaches. This paper provides a comprehensive overview of data-driven approaches and their modeling process, and elaborates the HPDM according to the combination of physical and data-driven models, then systematically reviews its application in fatigue life prediction. Additionally, the future challenges and development directions of fatigue life prediction are discussed. [ABSTRACT FROM AUTHOR]

• Compared with the traditional nonlinear ultrasound technology. The combination of linear and nonlinear responses in the time–frequency domain of nonlinear ultrasound can improve the characterization accuracy of grain size distribution. • Convolutional neural network (CNN) can establish the implicit relationship between nonlinear ultrasonic Lamb wave and grain size distribution, and construct a grain size distribution prediction model. • As the input of CNN, the Short-Time Fourier diagram of nonlinear Lamb wave can provide multi-dimensional information for the network. • Machine learning can solve the multi parameter inverse problem and ill posed problem in inversion of microstructure with a single nonlinear ultrasonic parameter. • Remove the fundamental frequency and second harmonic respectively to verify the performance of the network, and also confirm that linear and nonlinear ultrasonic information is essential. Nondestructive characterization of grain size distribution is significant for revealing the mechanical properties of metal materials. Traditional linear ultrasonic methods have been applied to estimate mean grain size but are less sensitive to small grain size. Nonlinear ultrasonic technology shows high sensitivity to microstructures, while it unfortunately encounters underdetermined problem in the characterization of microstructures across multiple scales. Combining linear and nonlinear ultrasonic responses would be a quite attractive way to achieve comprehensive observation of microstructural evolution. This work presents a data-driven approach for characterizing grain size distribution using nonlinear Lamb waves. The short-time Fourier transform (STFT) of detected ultrasonic signals were input into a convolutional neural network (CNN) to comprehensively learn the implicit linear and nonlinear dynamics involved with grain size distribution. Multiple convolution kernels slide across the STFT images with multiscale receptive fields to collectively model hierarchical representations and capture local correlation of interesting from time-frequency domain. The trained model achieved 94.7% accuracy in predicting mean grain size, and also achieved 95.4% and 86.3% accuracy in predicting the expectation and standard deviation of the lognormal distribution of grain size, respectively. The visual activation maps demonstrate that local interesting features are successfully captured. In particular, the prediction accuracy was demonstrated to reduce greatly after removing the fundamental frequency and second harmonic from the STFT images, confirming that incorporating linear and nonlinear ultrasonic information is indispensable for accurate characterization of grain size distribution. [ABSTRACT FROM AUTHOR]

In this paper, nonlinear Lamb waves with phase-velocity mismatching are proposed to detect accurately closed cracks by excluding the intrinsic material nonlinearity. Simulations and experimental studies were conducted to analyze double frequency Lamb waves (DFLWs) induced by the closed cracks and the intrinsic material nonlinearity. The results show that DFLWs induced by the material nonlinearity are negligible as compared with that caused by the contact acoustic nonlinearity (CAN) of the closed cracks. The closed cracks can be accurately detected using the proposed method, and the acoustic nonlinearity parameter increases monotonically with the crack evolution. The findings of this study provide a feasible method for detection and characterization of closed cracks. • Closed cracks are effectively detected using phase-velocity mismatching method. • Material nonlinearity is excluded in the detection of closed cracks. • Acoustic nonlinearity parameter increases monotonously with crack length. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulation (MD) of the observed stepwise oxidation of core-shell structured Al/Al 2 O 3 nanoparticles is presented. Different from the metal ion hopping process in the Cabrera-Mott model, which is assumed to occur only at a certain distance from the oxide layer, the MD simulation shows that Al atoms jump over various interfacial gaps directly under the thermal driving force. The energy barrier for Al ionization is found to be increased along with the enlargement of interfacial gap. A mechanism of competition between thermal driving force and ionization potential barrier is proposed in the interpretation of stepwise oxidation behavior. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulation, local plasticity of bcc Fe (0 0 1) is studied at different density of Fe–H cluster. H-induced softening and hardening of Fe substrate are observed along with the tensile elongation at low and high density, respectively. The two contradictory phenomena are ascribed to H behaviours-related plastic deformation. At high H partial pressure, initial H aggregation would lead to the formation of many H-enriched clusters similar to hydride. Tensile strain-induced dislocations (TSID) prefer to be generated and grow at the weakening interface of clusters and iron substrate. At low H partial pressure, TSIDs are uniformly distributed in the whole substrate. Owing to the affinity between H and dislocations, the diffusion of H appears to be distinct under different spatial distribution of TSIDs. H aggregation and dispersion can be enhanced and produce nonuniform and uniform plastic deformation during the continuous tensile process at high and low Fe–H cluster density, respectively. The former can stimulate local failures and accelerate the degradation of mechanical property. The results are helpful for better understanding of Fe–H cluster-related hardening and softening considering external strain-altered H behaviours except for the mechanism of H-dislocation interaction. [ABSTRACT FROM PUBLISHER]

Non-collinear wave mixing recently has been proposed to detect and localize micro-damage in materials. It is proved sensitive to the interaction angle α of the incident waves. In this work, the relationship between the acoustic nonlinearity parameter χ and the α is studied by numerical simulations and experimental measurements based on the nonlinear interaction of two shear waves. A single-peak change in the normalized acoustic nonlinearity parameter χ ′ of the mixing wave versus the incident angle is observed from numerical simulations and then is verified by experiments. The results show a -6 dB decrease of χ ′ corresponds to a deviation of about 4° in the incident angle. Meanwhile, the detection and localization of plastic deformation is also conducted on an aluminum alloy based on the scanning of non-collinear wave mixing, in which the distribution of the acoustic nonlinearity is similar to that of the plastic strain. [ABSTRACT FROM AUTHOR]

There are many power plant components operating at elevated temperatures, subjected to the combined mechanical and thermal loading conditions. The shakedown problem is an important topic for the safe operation of these high temperature components. This work mainly addresses the shakedown analysis of the pressure pipeline with an oblique nozzle at elevated temperatures using the Linear Matching Method (LMM). Parametric studies on main factors affecting the shakedown boundary are conducted. The results indicate that the LMM analysis tool is adequate to identify the shakedown boundary of the component, verified by the ABAQUS step-by-step analysis. Regarding the angle of the oblique nozzle, the ratchet limit is enhanced with the increasing angle, while the reverse plasticity limit increases until a maximum value is reached, and then presents a certain decrease. This implies that designers should make some compromises on the limit load and reverse plasticity limit when determining an economic angel of the oblique nozzle. In addition, parametric studies demonstrate that the diameter-to-thickness ratios of the oblique nozzle and the main pipe have a remarkable effect on the reverse plasticity and ratchet limit of the component. [ABSTRACT FROM AUTHOR]

• A probabilistic framework of creep life assessment for components at elevated temperature is provided. • Effect of standard derivation of material parameters on creep reliability assessment is discussed. • Comparisons between probabilistic and deterministic creep design methods are conducted. Current creep life assessment methods of components at elevated temperatures are mainly based on deterministic analysis strategies, which could not achieve the goal of probabilistic evaluation on creep failure. Based on this, a probabilistic framework of creep life assessment for components at elevated temperatures was provided. A method of determining distribution characteristics of material parameters was provided by randomly selecting a group of results at each stress level. Monte Carlo simulation was combined with finite element analysis technology to capture the distribution characteristic of creep rupture life of one typical structural component. Effect of standard derivation of material parameters on creep reliability assessment was discussed. Comparisons between probabilistic and deterministic creep design methods were made. Results indicated that the probabilistic analysis strategy can calculate the specific value of failure probability at various loading conditions, not two values of failure probability (i.e. 100% and 0%) by deterministic analysis strategy. The effect of standard derivation on mean values of effective stress and creep rupture life of the component is dependent on distribution characteristics of material parameters and related variables. A small standard derivation reduces the data scatter of effective stress and creep rupture life of the component. [ABSTRACT FROM AUTHOR]

In this work, experimental and numerical studies are conducted to explore the buckling behaviours of cylindrical–conical–cylindrical assembly shells under axial compression at elevated temperature. Firstly, bucking experiments of six test specimens at 600 °C are carried out. Then, numerical method considering initial geometric imperfection is used to predict buckling behaviours of these assembly shells. Furthermore, the effects of key factors on buckling behaviours of cylindrical–conical–cylindrical shells are also discussed. Results indicate a good agreement between experimental data and numerical result based on the model with measured geometric imperfection, and the shell model with measured outer surface imperfection is adequate for buckling analysis. Comparatively, the buckling load predicted by the model with eigenmode imperfection is reasonably conservative compared with experimental result only when the amplitude of imperfection is close to the measured imperfection amplitude. In addition, the effects of material properties and geometry parameters, e.g., yield stress, ratio of radius to shell thickness and semi-vertex angle, on the load-carrying capacity and buckling mode of cylindrical–conical–cylindrical shells are investigated systematically. Finally, a strategy for buckling analysis of cylindrical–conical–cylindrical shells is proposed. • Buckling mechanism of cylindrical–conical–cylindrical shells under axial compression at elevated temperature is elaborated. • Shell model with measured outer surface imperfection or eigenmode imperfection is recommended for buckling analysis. • Key factors effects on load-carrying capacity and buckling mode of cylindrical–conical–cylindrical shells are discussed systematically. • A strategy for buckling analysis of cylindrical–conical–cylindrical shells is proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • An atomic engineering strategy was reported to tune the humidity sensing performance of Mo 2 TiC 2. • An ultrahigh sensitivity of 1.75 was achieved on Mo 2 TiC 2 toward humidity sensing. • We report a strategy to explore the adsorption dynamics of electrified interfacial water. Understanding and manipulating the ubiquitous electrified interfacial water adsorption dynamics are highly important in surface science, sensing, catalysis and energy storage. However, the interfacial water is extremely hard to probe due to the interference from bulk water and complex interfacial environments. Herein we report a strategy to explore the adsorption dynamics of electrified interfacial water through combing a dynamic humidity sensing system, the in situ diffuse reflectance infrared Fourier transform spectroscopy, and the density functional theory calculations. By atomically engineering the Ti 3 C 2 T x with atomic Mo substitution of Ti, the water adsorption dynamics on the obtained Mo 2 TiC 2 T x were dramatically enhanced. The Mo 2 TiC 2 T x was demonstrated as an excellent humidity-sensing material with an ultrahigh sensitivity of 1.75 ± 0.06, fast response (35.9 s), and hysteresis-free properties in 2.7–93 % relative humidity range. This work provides universal strategies to in situ probe and manipulate the electrified interfacial water adsorption dynamics. [ABSTRACT FROM AUTHOR]