Strain controlled fatigue of P92 steel with various strain hold dwells introduced at the peak loading point were conducted at 625 °C. Two features which depend on the cycle and strain range level ...were observed under the fatigue-creep condition for the viscous and cyclic softening material. The first one is the accelerated cyclic softening response which is ascribed to the accumulated inelastic strain transformation from the creep mechanism during the strain dwell period and becomes more significant with the decrease of strain ranges. The second one is the decelerated stress relaxation behavior which is caused by the reduced viscous stress related to the continuous cyclic softening and fades with the decrease of cyclic strain ranges. Accordingly, a new unified viscoplastic constitutive model within the framework of Chaboche model was developed by improving the nonlinear isotropic hardening rule and the kinematic hardening rule with a cyclic softening parameter. As a result, the accelerated cyclic softening and decelerated stress relaxation response of fatigue-creep interaction was finely reproduced by the proposed model.
•Strain controlled fatigue of P92 with various strain hold dwells introduced at the peak loading point were conducted at 625 °C.•The accelerated cyclic softening response increases with the decrease of strain ranges due to the strain dwell.•The decelerated stress relaxation behavior fades with the decrease of cyclic strain ranges due to the continuous cyclic softening.•A unified viscoplastic constitutive model was developed by improving the kinematic hardening rule with a cyclic softening parameter.
Axially loaded push-pull cyclic tests of a precipitation-hardened stainless steel with different sampling orientations were conducted in high cycle and very high cycle fatigue regimes. Results showed ...apparent fatigue anisotropy with non-metallic inclusions dominating crack initiation behavior. A fatigue lifing model was developed by combining size, location and shape of inclusions into a new form of Z parameter to rationalize the orientation effect. Using a multi-scale and full-field approach, the inclusion-induced interior cracking mechanisms were found to be associated with inclusion-microstructure interaction resulted plasticity. Micro-hardness at the cracking site was the lowest on the fracture surface, and surrounding microstructures showed formation of small grains with clear interfaces. The fine granular area was characteristic of several nano-scale fine grains formed in terms of dislocation cell structures by martensitic laths breakdown. The coalescence of interfaces or micro-crackings finally became interior early fatigue cracks. The mechanistic modeling of “fragmentation of martensitic laths and formation of dislocation cells” revealed a microstructure-dependent crack initiation and stage I growth for interior fatigue cracking. All these inform the significance of combining metallurgical and processing factors in designing against fatigue of engineering materials.
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•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.
Single‐atom catalysts (SACs) have garnered enormous interest due to their remarkable catalysis activity. However, the exploitation of universal synthesis strategy and regulation of coordination ...environment of SACs remain a great challenge. Herein, a versatile synthetic strategy is demonstrated to generate a series of transition metal SACs (M SAs/NC, M = Co, Cu, Mn; NC represents the nitrogen‐doped carbon) through defect engineering of metal‐organic frameworks (MOFs). The interatomic distance between metal sites can be increased by deliberately introducing structural defects within the MOF framework, which inhibits metal aggregation and consequently results in an approximately 70% increase in single metal atom yield. Additionally, the coordination structures of metal sites can also be facilely tuned. The optimized Co SAs/NC‐800 exhibits superior activity and excellent reusability for the selective hydrogenation of nitroarenes, surpassing several state‐of‐art non‐noble‐metal catalysts. This study provides a new avenue for the universal fabrication of transition metal SACs.
A general metal–organic framework defect engineering strategy is proposed to increase the yield of single‐atom catalysts. This strategy enlarges the distance between metal active sites, effectively hindering the aggregation of metal atoms and affording a 70% improved yield of metal single atoms. The optimized Co SAs/NC‐800 exhibits superior activity and reusability in nitroarene hydrogenation.
•Physical origin of cyclic softening discrepancy between stress and strain cycling are clarified.•A cyclic softening model is developed involving the special dislocation annihilation and storage ...events.•Various cyclic responses under different loading modes were predicted very well by the proposed model with a single parameter set.
Stress and strain controlled low cycle fatigue of the modified 9–12% Cr steel with the hierarchical arranged lath martensitic structure were conducted. We found that, apart from microstructure recovery in lath structure, an additional mechanism of reverse avalanche of low angle boundary, associated with the burst-like plastic deformation is responsible for the accelerated softening during stress cycling. These microstructural evolutions can be explicitly represented via deriving different dislocation evolution laws in terms of dislocation annihilation and storage events. Accordingly, a new model is proposed involving the microstructural evolution. Results indicate that the accelerated softening behavior under stress cycling can be reproduced very well by the present model with a very limited number of adjustable parameters. In addition, the model can capture the features of cyclic response and microstructural evolution under both the strain and stress cycling over a wide range of amplitudes.
It is known that fine granular area morphology is often formed around interior micro‐defect in the case of very high cycle fatigue failure of high strength steel. The mechanism of “fragmentation of ...martensitic laths and formation of dislocation cells” (Zhu's model), which revealed a microstructure‐dependent crack initiation and early growth for micro‐defect induced internal fatigue cracking, was discussed in several lathy martensitic or similar steels, verified under environmental fatigue, and applicable to white etching crack in rotating contact fatigue. A machine learning‐Z parameter integrated approach was developed for fatigue life, which is promising for design against fatigue of engineering structures for long life.
Highlights
The FGA model of martensitic steels in air (Zhu's model) was illustrated.
Zhu's model was confirmed in environmental media and similar steels.
Zhu's model was applicable to white etching crack in rotating contact fatigue.
Machine learning‐Z parameter was integrated for fatigue life prediction.
The local creep properties of a CrMoV weldment were acquired by using miniature specimens of in-situ creep test. The creep performances of heat-affected zone (HAZ), weld pass metal in the welding ...direction (WP) and multiple weld passes through the thickness direction (MWP) were compared with each other. It is shown that the magnitudes of creep strength order satisfy WP > MWP > HAZ, while the creep ductility of these three samples exhibit an inverse order: WP < MWP < HAZ. In addition, the typical three-stage creep deformation curves are observed in both WP and MWP samples. However, no significant difference between the primary and the secondary stages is detected for HAZ sample.
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The defect classification task is of great benefit to evaluating the safety performance of equipment and providing useful feedback information for discovering production process problems. In this ...article, we present a semisupervised learning (SSL) framework for transient thermography detection to employ the temporal and spatial information encoded into the three-dimensional transient thermal tensor data and provide pixel-level classification results for defect types. The time- and frequency-domain physical models for the transient thermal evolution of different kinds of defects are established to illustrate the theoretical foundation of defects classification based on transient thermography. The semisupervised multiclass Laplacian support vector machine is proposed to enable involving the abundant unlabeled data for enhancing learning performance in practical industrial applications where labeled samples are insufficient and labeling work is costly and laborious. A case study on silicone insulating materials with various types of artificial simulated internal defects validates the stronger generalized ability of the proposed method. This work, for the first time, proposes an SSL framework in transient thermography-based defect detection studies. It is believed that our proposed method is quite inspired for introducing SSL techniques to transient thermography for preferable performance in practical industrial applications.
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•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.
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.
