Fatigue Process Research Articles

The investigation on the bending fatigue properties of Ti-6Al-4V lattice sandwich structures fabricated EB-PBF

In this study, lattice sandwich structures of Ti-6Al-4V alloy with simple cubic (SC) and rhombic dodecahedron (RD) unit cells were prepared by electron beam powder bed fusion (EB-PBF) technique. Through a combination of finite element simulation and experimental investigation, the effect of lattice cell shape and heat treatment on the bending fatigue performance of these lattice sandwich structures was studied. The results show that the underlying fatigue mechanism of the RD lattice sandwich structure is primarily dominated by cyclic ratcheting of the lattice. In contrast, for the SC lattice sandwich structure, the fatigue mechanism involves an interaction of cyclic ratcheting and fatigue crack initiation and propagation of the lattice. During the cyclic deformation process, the struts of the lattice unit cells in the SC lattice sandwich structure mainly undergo buckling deformation. Under the same bending deformation strain, the struts experience much higher stress in the SC lattice sandwich structure, making them more susceptible to crack initiation and resulting in a quick fatigue failure. Therefore, the fatigue performance of the RD lattice sandwich structure is significantly superior to that of the SC lattice sandwich structure. Heat treatment result in the enhanced plasticity of parent materials of the lattice sandwich structure, leading to improved resistance to cyclic strain accumulation during fatigue processes and an increase in fatigue strength.

Damage evolution simulation and lifetime prediction for composite blades under continuous droplet impacts

Offshore wind power generation is a promising technology in renewable energy applications due to its high reserves of wind energy in sea areas. To improve the energy transformation efficiency, the blade length of the offshore wind turbines has become larger and larger, and it has made rain erosion be one of the most frequent failures during the turbine operation. As the natural rainfall is stochastic in spatial and time domains, it is difficult to depict the damage evolution process caused by rain impact exactly. Therefore, regular and continuous droplet impact simulation and experiments present an alternative methodology for this issue. With the composite structure of the blade and deformability of the liquid droplet in consideration, a fluid solid interaction model will be established to investigate the impact response and subsequent damage evolution. In which, the Smooth Particle Hydrodynamics (SPH) model is utilized to depict the constitutive relationship within the droplet, and Finite Element Method (FEM) is used to construct the Representative Volume Element (RVE) model of the blade leading edge. The impact process is simulated first to obtain the impact pressure distribution at the contact center and velocity field in the droplet. Furthermore, the stress wave propagation in the blade multilayer structure can be analyzed. Owing to the multiaxial fatigue feature of the continuous droplet impact, the continuum damage mechanics is integrated with the fatigue criterion and the Jump-in-Cycle procedure is used to simulate the high-cycle fatigue process. The damage factor distribution on the blade coating surface and its influence on mechanical properties are analyzed. Thereafter, the droplet impact fatigue life can be accumulated based on Miner’s linear rules. The theoretical achievements are validated by experimental data provided by Rain Erosion Testing (RET), which shows a good agreement between each other. As a result, V-N curves and D-N curves, i.e. quantitative relationship between droplet falling conditions and impact fatigue life, are established. The achievements in this study can provide an effective tool for rain erosion mechanism analysis and life prediction in industrial applications.

Enhancing strength, ductility, and thermal fatigue resistance in Stellite 21 coatings via Mn alloying and Transformation-Induced Plasticity

This study investigates the effect of Mn on the microstructure and mechanical properties of Stellite 21 cobalt-based coatings prepared by laser cladding on 24CrNiMo steel substrates, focusing on utilizing the Transformation-Induced Plasticity (TRIP) mechanism to enhance toughness and thermal fatigue performance. The results indicate that the microstructure of the Stellite 21 alloy coating is primarily composed of γ-Co (FCC) with a minor presence of ε-Co (HCP), M23C6, and M7C3 carbides, which predominantly precipitate at or near the grain boundaries. Upon the addition of Mn, it dissolves into the γ-Co matrix, causing lattice distortion and increasing dislocation density, thereby contributing to solid solution strengthening. Compared to the Mn-free Stellite 21 alloy coating, the coating with 5% Mn exhibits an approximately 21% increase in tensile strength (∼1135 MPa) and a 74% increase in elongation (∼14.5%). This improvement is mainly attributed to the solid solution strengthening caused by lattice distortion due to Mn addition and the synergistic enhancement from the TRIP mechanism facilitated by the lower stacking fault energy. Moreover, the thermal fatigue crack propagation rate in the coating with 5% Mn is reduced by approximately 42.5% compared to the Mn-free Stellite 21 alloy coating. The enhancement in thermal fatigue performance is primarily due to the Mn addition, which stabilizes the ε-Co phase during thermal fatigue processes, enhancing plastic deformation capacity. The phase transformation process alleviates stress concentration, effectively reducing crack propagation rates and ultimately improving the material's thermal fatigue performance.

Research on brain functional network property analysis and recognition methods targeting brain fatigue

At present, researches on brain fatigue recognition are still in the stage of single task and simple brain region network features, while researches on high-order brain functional network features and brain region state mechanisms during fatigue in multi-task scenarios are still insufficient, making it difficult to meet the needs of fatigue recognition under complex conditions. Therefore, this study utilized functional near-infrared spectroscopy (fNIRS) technology to explore the correlation and differences in the low-order and high-order brain functional network attributes of three task induced mental fatigue, and to explore the brain regions that have a major impact on mental fatigue. Self-training algorithms were used to identify the three levels of brain fatigue. The results showed that during the fatigue development, the overall connection strength of the endothelial cell metabolic activity and neural activity frequency bands of the low-order brain functional network first decreased and then increased, while the myogenic activity and heart rate activity frequency bands showed the opposite pattern. Network topology analysis indicated that from no fatigue to mild fatigue, the clustering coefficient of endothelial cell metabolic activity and myogenic activity frequency bands significantly decreased, while the characteristic path length of myogenic activity significantly increased; when experiencing severe fatigue, the small-world attribute of the neural frequency band significantly weakened. However, each frequency band maintained its small-world attribute, reflecting the self-optimization and adaptability of the network during the fatigue process. During mild fatigue, neuronal activity bands’ node degree, cluster coefficient, and efficiency rose in high-order brain networks, while low-order networks showed no significant changes. As fatigue progressed, the myogenic activity bands of high-order network properties dominated, but neural bands gained prominence in mild fatigue, approaching the level of myogenic bands in severe fatigue, indicating that brain fatigue orchestrated a shift from myogenic to neural dominance in frequency bands. In addition, during the process of fatigue, the four network attributes of the high-order network cluster composed of low-order nodes related to the prefrontal cortex region, left anterior motor region, motor assist region, and left frontal lobe eye movement region significantly increased, indicating that these brain regions had a significant impact on brain fatigue status. The accuracy of using both high-order and low-order features to identify fatigue levels reached 88.095%, indicating that the combined network features of both high-order and low-order fNIRS signals could effectively detect multi-level mental fatigue, providing innovative ideas for fatigue warning.

Experimental Characterization of the Mechanical Properties of Filter Media in Solid-Liquid Filtration Processes.

Nonwoven filter media are used in many industrial applications due to their high filtration efficiency and great variety of compositions and structures which can be produced by different processes. During filter operation in the separation process, the fluid flow exerts forces on the filter medium which leads to its deformation, and in extreme cases damage. In order to design or select a reliable filter medium for a given application, it is essential to have a comprehensive understanding of the mechanical properties of the nonwoven material. In general, the properties of the filter material are influenced by temperature and can be changed during loading due to irreversible deformation, fatigue, and aging processes. In order to gain a deeper comprehension, the presented study examines the influence of temperature and repeated tensile stress on the filter medium properties. The focus is on fuel and oil filters employed in automotive applications. The characteristic properties of the samples, including thickness, porosity, and permeability as well as Young's modulus and Poisson's number, are measured. Young's modulus is determined for both new and aged samples. In addition, the viscoelastic behavior is investigated via a dynamic mechanical thermal analysis. The results demonstrate a significant dependence of mechanical properties on the material composition and the aging effects.

The evaluation of wear processes of the rolling-sliding contact by means of flake wear debris in dry and wet contact

One of the tasks of this paper is to explain the cause-and-effect relationship of the influence of operating parameters on the wear of the wheel-rail system where fatigue processes are initiated. In this way, learning about and defining the nature of the phenomenon, makes it possible to determine the reliability of component operation. The aim of this study was to determine the influence of operational parameters on tribological properties of the rolling-rail contact representing the wheel-rail association in real operating conditions. Determining the form of wear on the basis of laboratory tests carried out on an Amsler-type wear testing machine in the wheel-roller system makes it possible to determine the nature of changes occurring as a result of cyclic interaction of normal and tangential forces on the surface of the tested material, without the need for long-term and costly observation of the tested object in normal operation. Quantitative metallographic analysis of the morphology of wear debris was used to determine the intensity of surface wear. Determination of the influence of operational parameters on the stereological characteristics of wear debris makes it possible to predict the regenerative grinding of rails in service in the track and allows practical application by the sections of operation and diagnosis of the railway system.

Investigation on fatigue performance and residual stress release mechanism of split sleeve cold expansion holes of heterogeneous 7050-T7451 aluminum alloy

7050-T7451 aluminum alloy is widely applied in aircraft structural components due to good properties, these components majority connected with hole structures. However, stress concentration occurs around structural holes during application, which leads to fatigue cracking and ultimately resulting in low fatigue performance of these parts. The residual stress generated by cold expansion can provide sufficient strength to prevent fatigue crack initiation. However, this protective effect is lost when the residual stress is released during fatigue. Consequently, it is crucial to investigate the release of residual stress in hole structures during the fatigue process. In this work, the residual stress release process and microstructure evolution under to different cyclic loads were investigated. The back stress induced by the heterogeneous structure on residual stress release and the effect of heterogeneous structure on crack propagation were discussed. The findings indicate that the back stress generated by GNDs of heterogeneous structures can resist the applied load in early stages, delay residual stress release. As the cycle number increases, potential mechanisms for the release of residual stress include dislocation annihilation, dislocation rearrangement, and dislocation shearing. The heterogeneous grain structure with high tortuosity and the deflection and branching of cracks can reduce the driving force of crack growth.

A phase field framework for corrosion fatigue of carbon steel

Corrosion fatigue damage occurs when metallic materials are subjected to cyclic loading in a corrosive medium. In this study, a phase field framework is proposed to predict the corrosion fatigue of carbon steels. The coupling effect of fatigue and corrosion is explicitly implemented in the proposed phase field framework by coupling the displacement field, electrochemical field and phase field. A degradation function of the interface free energy density with the consideration of elastic and plastic strain energies is introduced to account for the fatigue damage accumulated during the corrosion fatigue process. The applicability of this framework is validated by accurately capturing the pure fatigue and corrosion fatigue behaviors of compact tension specimens, particularly the acceleration effect of corrosion on the fatigue crack growth. The propagation morphology and rate of the corrosion fatigue crack in single pit and multiple pit models are studied. The distribution of stress state and strain energy density induces the directionality of crack propagation. The influence of loading frequency on the corrosion fatigue process is discussed in detail. Due to the corrosion-fatigue coupling effect, the corrosion rate increases with increasing of the loading frequency, resulting in an accelerated corrosion fatigue process. Moreover, the significance of the plasticity in the prediction of corrosion fatigue is emphasized.

The Metal Magnetic Memory Method and Its Application for Early Detection of Stress Zones in NPP Piping

The policy of each NPP is to ensure safe production of electrical energy as well as security of supply. It is known that stress concentration (SC) zones, in which corrosion, fatigue and brittle processes develop most intensively, are the main sources for the occurrence of failures in operational structures. In this paper the method of magnetic memory of metal (MMM method) is discussed. This method was developed in order to conduct an early diagnosis of the condition of the metal of the technological equipment. The method is new and has not yet been studied in depth. The article presents the essence of the method, equipment and scheme of scanning the object. The method was applied to perform a pipeline test of NPP equipment. The results are given and verified. The aim of the article is to present the application of the Metal magnetic memory method in practical terms for performing technical diagnostics of the equipment in NPPs.

Fatigue analysis of selective laser melting Inconel 718 and the mechanism of ductile–brittle continuous alternating steps during fatigue process

In this paper, Inconel 718 powber fabricated by selective laser melting were studied to explore the mechanical characteristics and microstructures. The results of microstructure show that the material exhibits the columnar, honeycomb, and mixed crystal structures. The microhardness is very uniform, with a average value of 317 HV. Tensile test shows the ultimate tensile strength of 785Mpa, elongation of 4.93%, which is high stress brittle fracture. The fatigue strength of 180 MPa was obtained from the fatigue test. The fatigue life was sensitive to the effect of stress. In the scanning electron microscope observation, small crack propagation was found in the fracture, and a large number of step-like fracture characteristics of repeated transformation of toughness and brittleness were found. The mechanism analysis of the step-like ductile–brittle fracture is carried out, which is divided into four stages. Mathematically analyzing the tough-brittle fracture of the step type, where the crack is subjected to both the stress field around the pore and the stress field at the crack tip on the way to extension, a stress field model for crack propagation around the pore and a fracture toughness formula for transverse and longitudinal propagation of tough-brittle steps applicable to this paper are proposed.

Fretting fatigue of 42CrMo4+QT steel: Experimental and numerical assessment

This paper focuses on an experimental and numerical assessment of fretting fatigue in cyclically loaded specimens from 42CrMo4+QT steel in contact with bridge pads of the same material. Four different pad configurations were employed, varying the radii of the pad legs, to achieve various stress gradient conditions in the fatigue process zone. A qualitative change in terms of the number of contact initiation locations observed in relation to fatigue life served as one of the aspects of the evaluation of a set of 11 fatigue prediction methods. It was observed that elastic–plastic stress analysis is crucial for capturing this phenomenon. Among the tested methods, Papuga’s quadratic parameter criterion (QCP) performs best in terms of risk assessment of crack initiation and prediction of its location.

Model‐based analysis of the influence of surface roughness on fatigue processes

AbstractIn the last decade, the phase field method was developed to simulate fatigue fracture processes. The biggest advantage of phase field modeling is its unified framework, in which the entire fracture evolution from nucleation to propagation is covered. Even though it is known that surface roughness has an essential impact on fatigue life, it has rarely been considered directly in phase field models for fatigue processes due to the required computational effort. In this work, we provide a simulation setup to incorporate roughness profiles into a phase field model for fatigue cracks. We have used a roughness model for our studies and also present initial results that show agreement between simulation and experiments.

Investigation of fatigue crack growth behavior and crack tip plastic zone characteristics in welded structures based on local displacement fields

The fatigue crack growth behavior in welds is difficult to evaluate due to complex internal stresses and microstructural differences. This paper uses the CJP model based on displacement field variations to describe the fatigue crack growth behavior and the size of the crack tip plastic zone in welds. First, fatigue crack growth rate tests were conducted on base metal and as-welded specimens. The digital image correlation technique was used to capture the speckle pattern variations on the specimen surface during crack growth, obtaining the crack tip displacement field required by the CJP model. Based on this, by calculating the stress intensity factor ranges ΔK and ΔKCJP, two da/dN–ΔK(ΔKCJP) curves defined by the traditional and CJP models were obtained, proving the model’s applicability for describing fatigue crack growth behavior in welded joints. Finally, the CJP model was used to calculate and compare the crack tip plastic zones of base metal and as-welded specimens. It was found that the maximum error for the as-welded specimen was 37.84%, occurring at the initial stage of crack propagation and gradually decreasing with the fatigue process.

Neural network integrated with symbolic regression for multiaxial fatigue life prediction

In the realm of structural engineering, accurately predicting multiaxial fatigue life presents a formidable challenge, stemming from the complex interplay of stress and strain across multiple directions. This research introduces the Symbolic Regression-Neural Network (SR-NN) framework, a novel integration of symbolic regression-derived expressions with neural networks aimed at enhancing predictive accuracy in this field. Initiated by selecting salient features through SHAP-informed Recursive Feature Elimination, our approach effectively minimized dimensionality while pinpointing key feature groups. Symbolic regression, newly applied to guide neural network training in the prediction of alloy material fatigue life, facilitated the generation of meaningful expressions that describe the fatigue processes. These expressions were incorporated into the network’s loss function alongside MSE loss, refining the model’s learning dynamics. Our proposed SR-NN framework demonstrated substantial improvements over traditional models in rigorous testing, notably reducing MSE by 55.4% compared to RF and 45% against SVM models, underscoring the potential of merging symbolic regression with machine learning to tackle the inherent variability and complexity of fatigue behavior across diverse materials and loading conditions. This study highlights the transformative impact of symbolic regression in improving the interpretability and accuracy of predictive models within structural engineering.

Theoretical prerequisites for planning the content and focus of training sessions in the pre-season preparation of hockey players

Importance. The rapid growth in the number of student sports teams in recent years has predetermined the issue of combining sports and academic activities. In this regard, the considered aspects of the processes of fatigue and recovery in amateur sports become relevant and require analysis and systematization of the views of domestic and foreign scientists on the issues of localization and mechanisms of development of fatigue, as well as the patterns and time of recovery of the athlete’s body in the process of training activities. The study purpose: analysis and systematization of scientific articles on the problem of fatigue and recovery of the athlete’s body after physical activity, determination of the fundamental scheme for the formation of the content and focus of training sessions based on the analysis of the theoretical component of the problem of fatigue and recovery during physical activity of various nature, direction and magnitude in different types of sports.Materials and Methods. The study is conducted on the basis of studies concerning the problem of interaction of the body’s life-supporting systems in the development of fatigue mechanisms, published in scientific journals, monographs of famous physiologists covering a large time period. The study is based on a systemic-structural approach, which allows, during the analysis of scientific publications, to identify the main patterns of the development of mechanisms and localization of fatigue, to put forward hypotheses regarding the prospects for developing an appropriate scheme for the formation of the content and focus of training sessions, to determine the range of issues that require instrumental confirmation of the recovery time of the athlete’s body during combining various characteristic aspects of loads in the training process. Within the framework of this study, the following research methods are used: scientific and methodological literature analysis, systematization, pedagogical observation, generalization.Results and Discussion. In the course of the study, the main factors are identified that influence the mechanisms of the development of fatigue during work of various sizes and directions, in different intensity zones; The interrelationships of various localization systems and mechanisms of fatigue development, as well as energy supply to support the athlete’s vital activity during physical activity, are revealed. An attempt is made to substantiate the strategy of expediently combining types of training activities, the content and focus of training sessions in the preparatory period of pre-season training for students involved in hockey at Hockey club “Derzhava”.Conclusion. Research prospects include issues of experimental substantiation of the construction of a system of training sessions, taking into account the registration of the main indicators of lifesupport systems, allowing the trainer to expediently distribute and quickly regulate training loads and ensure the effectiveness of recovery processes in the period under study.

Study of the relationship between the strength limit and long-term fatigue of steel reinforcement of reinforced concrete structures with a long servise life in aggressive environments

It is known that the fatigue process begins with plastic deformation of the surface layers of the reinforcement metal. Moreover, the movement of dislocations in the conditions of repeatedly alternating stresses is observed at loads below the elastic limit of the metal. The rate of local plastic deformation during cyclic deformation is several orders of magnitude higher than the rate of deformation under static loading. Slip of dislocations begins in grains with a favorable orientation near the stress concentrators. With an increase in the number of cycles in the surface layers, the density of dislocations and the number of vacancies increases. When the basic number of NR cycles is reached, a surface hardened metal layer is formed with a large number of seed cracks, the size of which does not reach a critical value. An increase in the number of cycles cannot cause further development of destruction in such a layer. Only when the stresses exceed the endurance limit do the cracks reach a critical length, after which the process of their draining into the main crack begins with the propagation of the latter. The results of experimental studies indicate a strong influence of diffusible hydrogen on the static and cyclic parameters of crack resistance. It was established that with the increase of flooding, especially when the hydrogen content exceeds 5 cm3100g, both the static strength and long-term strength (fatigue-laziness) decrease sharply. Moreover, these areas of the hydrogen solution in the reinforcing steel are characterized by a viscous nature of fracture, while for heavily flooded reinforcement (from 5 to 12 cm3100g, brittle fracture by the mechanism of micro-split in the hardened (martensitic or troostite structure) is inherent. The analysis of the obtained experimental results made it possible to determine the optimal content of hydrogen in reinforcing steel (3-5 cm3 100g), the excess of which will cause a decrease in the crack resistance of the reinforcement in the process of long-term operation, especially in corrosive and aggressive environments. The issue of fatigue of reinforcing steels is very relevant today.

How sleep and fatigue shape statements in evidence: A psycho-legal perspective

Testimonial evidence in the form of verbal accounts by victims, witnesses, and suspects plays a critical role in investigations and judicial proceedings, often serving as the only evidence during a trial. The psychological nature of testimonies causes this form of evidence to be inherently limited, motivating psycho-legal scholars to identify both risk factors and solutions necessary to improve its reliability. To this end, the current perspective argues that sleep-related fatigue is a formative factor that influences the fidelity of statements and confessions provided during legal interactions. Specifically, it considers the prevalence of sleep disruption among subjects interacting with the criminal justice system, its likely impact on memory of victims and witnesses, and the role of sleep deprivation in confessions. In view of legal doctrines relevant to both evidentiary and constitutional considerations, this analysis is meant to motivate future work at the intersection of sleep-related fatigue and legal processes.

Numerical investigation to the effect of wear on fretting fatigue life using CDM and fracture mechanics

Fretting fatigue is a common type of problem in the engineering fields. Due to its multiaxial characteristics, it leads to a shorter overall fatigue life compared to plain fatigue. Fretting fatigue is caused by the micro slip at the contact surface, which is always accompanied by interface wear. This paper presents a refined elastic numerical simulation method combining interfacial wear with continuum damage mechanics (CDM) and fracture mechanics to predict fatigue life. The crack initiation life is determined by damage evolution law and the initiation position is determined by the total dissipated energy. Crack propagation life and path can be predicted using stress intensity factor. The stress redistribution and geometric change of contact surface caused by wear are considered in the whole fatigue process. Results indicate that the fretting fatigue simulation method incorporating wear has higher accuracy compared with the experimental results in the literature. It is noteworthy that wear retards nucleation damage process and crack propagation. As stress amplitude decreases, fatigue life increases, and delay effect of wear becomes more obvious. Additionally, wear causes the crack initiation position to move out, but crack initiation region is still near the slipping zone and propagation path does not change.

Unique damage process in micro-sized copper single crystal with double-slip orientation in response to near-[112] tension-compression fatigue

In order to clarify the fatigue process for a micro-sized copper (Cu) single crystal with a double-slip orientation, a tension-compression cyclic loading test was carried out with in situ FE-SEM observation. Although the primary slip system B4 finally brought about a crack from eminent intrusions, the process was complex due to characteristic interactions among multiple slip systems. The process was approximately divided into three stages; early slipping, cave-in and damaging. In the early stage, two slip systems with high Schmid factor (SF), B4 and C1, worked at first, however both immediately stopped working. Extrusions/intrusions were caused by the survived slip system B5 (third-highest SF). In the second stage, a triangular depression was brought about by the cross-slip from B5 to C5 and localized wavy undulation was formed by the continuous cross slips. In the final stage, though both of B4 and B5 worked, the slip system with the highest SF, B4, dominated to form the eminent extrusions/intrusions, resulting in the fatigue cracking. The applied resolved shear stress amplitude was a few megapascals, indicating significantly lower fatigue strength than that of the bulk counterpart while it was in same range for that of micro-sized Cu with a single slip orientation. The complex fatigue damage morphology of the specimen was explained by accounting for the interaction of dislocations between the slip systems.

Thermo-mechanical creep-fatigue damage evolution and life assessment of TiAl alloy

Thermo-mechanical creep-fatigue (TMCF) behaviors of a TiAl alloy were investigated by conducting various creep, creep-fatigue, and thermo-mechanical tests and performing detailed microstructural analysis. A new creep model was proposed to unify primary strain hardening and tertiary accelerating damage effects. The TMCF failure of TiAl alloys is accompanied by the crack initiation from surface oxides/carbides and mixed transgranular and intergranular cracking. It revealed that introducing thermo-mechanical cycles significantly reduces the creep damage rate during the creep dwell stage and the fatigue process, i.e., the creep-delaying effect, which is attributed to the non-proportional strengthening effect induced by thermal cycles. A life model based on the average minimum creep strain rate was proposed to predict the creep-dominated TMCF life. This model incorporates the complex creep-fatigue interactions on creep damage and demonstrated good agreement with experimental results under varying loading conditions. The present work provides new insights into understanding the creep-dominant thermo-mechanical damage evolution of TiAl alloys.