Stages Of Fatigue Research Articles

Evolution of the fracture mechanics parameters in the vicinity of the hole in conditions of low-cycle fatigue according to the data of modeling a crack with narrow notches

We propose a new method for studying the effect of low-cycle fatigue on the evolution of the fracture mechanics parameters in conditions of loading plane specimens with stress concentrators. Three programs of loading with a constant value of the stress range and different values of the stress ratio, as well as two programs with a constant value of the stress ratio but different stress ranges are considered. One program is common for both cases. All the programs include uniaxial tension-compression. Each program was implemented by testing a batch of the same specimens from seven to nine in each. One specimen from each batch was assigned to assess the durability. The other specimens were brought to various stages of low-cycle fatigue in each program. Experimental data were obtained for cracks of different length which were modeled by a sequence of three narrow notches launched from a through hole in a rectangular specimen. Each notch was exposed to constant external load of the same level. The deformation response to a small increment in the notch length at a constant external load was measured at different stages of low-cycle fatigue using electronic speckle pattern interferometry. The interference fringe patterns used as initial experimental data provided determination of the tangential components of in-plane displacements directly on the notch sides and the values of notch opening were thus determined from the results of direct measurements. The transition from measured displacement components to the values of the stress intensity factor and T-stress was performed using the relationships of a modified version of the crack compliance method based on Williams formulation. Distributions of the fracture mechanics parameters along the notches were obtained at various stages of cyclic loading. The dependences of the crack mouth opening displacement, the stress intensity factor and the T -stress on the number of loading cycles are constructed for the notches of a fixed length at different stages of low-cycle fatigue. It is shown that experimental distributions of the stress intensity factor values over the life time practically coincide for all four combinations of the loading cycle parameters.

Effect of fatigue loading-confining stress unloading rate on marble mechanical behaviors: An insight into fracture evolution analyses

Rocks in underground works usually experience rather complex stress disturbance. For this, their fracture mechanism is significantly different from rocks subjected to conventional triaxial compression conditions. The effects of stress disturbances on rock geomechanical behaviors under fatigue loading conditions and triaxial unloading conditions have been reported in previous studies. However, little is known about the dependence of the unloading rate on fatigue loading and confining stress unloading (FL-CSU) conditions that influence rock failure. In this paper, we aimed at investigating the fracture behaviors of marble under FL-CSU conditions using the post-test X-ray computed tomography (CT) scanning technique and the GCTS RTR 2000 rock mechanics system. Results show that damage accumulation at the fatigue stage can influence the final fracture behaviors of marble. The stored elastic energy for rock samples under FL-CSU tests is relatively larger compared to those under conventional triaxial tests, and the dissipated energy used to drive damage evolution and crack propagation is larger for FL-CSU tests. In FL-CSU tests, as the unloading rate increases, the dissipated energy grows and elastic energy reduces. CT scanning after the test reveals the impacts of the unloading rate on the crack pattern and a fracture degree index is therein defined in this context to represent the crack dimension. It shows that the crack pattern after FL-CSU tests depends on the unloading rate, and the fracture degree is in agreement with the analysis of both the energy dissipation and the amount of energy released. The effect of unloading rate on fracture evolution characteristics of marble is revealed by a series of FL-CSU tests.

Fatigue evolution characteristic and self-healing behaviour of asphalt binders

ABSTRACT The self-healing ability of asphalt material has a significant effect on improving fatigue life of asphalt pavement. The objective of this paper was to investigate the stage characteristics of fatigue damage of asphalt binder and its associated self-healing behaviours. Firstly, the rheological behaviour and fracture surface morphology were discussed to investigate the fatigue evolution characteristics of asphalt binders. Secondly, an effective method for interpreting the self-healing capability of asphalt binder at different fatigue stages was proposed by conducting fatigue-healing-refatigue tests and Fourier transform infrared spectroscopy analysis. Finally, the effect of temperature on the self-healing properties of different aging asphalt binders at different fatigue damage stages was explored. The results show that the stage characteristics of fatigue damage of asphalt binders could be distinguished effectively by correlating complex shear modulus with ratio of dissipation energy change under load time history. The fatigue cracking morphologies of asphalt binders under cyclic shear loading are susceptible to the aging degree of asphalt binder. An index associated with initial dissipative energy and number of cycles could effectively evaluate the self-healing properties of different aging asphalt binders. After the occurrence of macrocracks, the mechanical properties of asphalt binder cannot be completely restored by increasing the temperature.

Optimal imaging of multi-channel EEG features based on a novel clustering technique for driver fatigue detection

Fatigue may cause a decrease in mental and physical performance capacity, which is a serious safety risk for the drivers in the transportation system. Recently, various studies have demonstrated the deviations of electroencephalogram (EEG) indicators from normal vigilant state during fatigue in time and frequency domains. However, when considering spatial information, these feature descriptors are not satisfying the demand for reliable detection due to the well-known challenge of signal mixing. In this paper, we propose a novel approach based on clustering on brain networks (CBNs) to alleviate the problem to improve the performance of driver fatigue detection. The clustering algorithm was employed to extract the spatial nodes with distinct connectivity attributes throughout the EEG-based brain networks. Then, the temporal features of wavelet entropy from the extracted nodes were transformed to spatio-temporal images so that the image edge detection method (pulse-coupled neural networks) to distinguish different stages of fatigue can be used. The experimental results demonstrated the temporal features from the extracted nodes reduced signal mixing and showed clearer deviations. The detected fatigue based on the imaging method was to an extent consistent with self-reported subjective feelings and most of the critical fatigue was detected before the subjective feelings of fatigue. For all the subjects, 21 of 29 accidents happened after detected fatigue in the simulated driving task. Therefore, the proposed method owns potential value for early warning and avoidance of traffic accidents caused by driver fatigue.

Evaluation of Dark Etching Regions for Standard Bearing Steel under Accelerated Rolling Contact Fatigue

Subsurface microstructural alterations are formed in the later stages of rolling contact fatigue (RCF) under high contact pressure. The subsurface changes observed as a dark contrast under optical microscopy are classified as Dark Etching Regions (DERs). Despite the fact that DERs have been presented for several decades, the understanding of its development and growth is yet to comprehend. Current research employed a modified high-speed microprocessor rotary tribometer to conduct systematic RCF study under accelerated testing conditions with variable temperatures and contact pressures. Comprehensive RCF data has been acquired, analysed and is reported for the very first time with ball-on-ball point contact loading conditions. The subsurface microscopic investigations have shown the ongoing progression and development of DER extent and are reported to be associated with the accumulation of plasticity during RCF. The comparison of the DER with the responsible stress components have revealed that DER formation is more closely related to the von Mises stresses when superposed with residual stresses. The experimentally observed area fraction of dark etching zones has been evaluated in terms of DER% and compared with the dislocation assisted carbon diffusion model for DER formation. The overprediction of the numerical model in comparison with the presented results in current research manifests its limitations which can be improved with the incorporation of cyclic plasticity governed by evolved von Mises stresses. Detailed evaluated DER results are presented as 3D DER% maps incorporating the combined effects of contact stress, temperature and rolling cycles simultaneously which enables an in-depth RCF understanding within microstructural context and therefore can be used as guidelines for DER formation models.

Fatigue damage evaluation of carbon fiber reinforced composites using nonlinear resonance spectroscopy

The present work explores the use of nonlinear resonance spectroscopy to detect early fatigue damage in carbon fiber-epoxy composite laminates. The objective is to test composite samples at mid to high cycle fatigue (>10∧3 cycles), so that the damage evolution could be tracked. The nondestructive nature of the testing method ensures that the sample can be loaded and tested till failure, i.e. interrupted fatigue testing. The acoustic nonlinear response tracked over the fatigue life of the sample includes, the nonlinear frequency shift and modal damping ratio. Additionally, slow dynamic diagnostics was also carried out to investigate the recovery rate and recovery time. The samples that were tested were all quasi-isotropic laminates under 4-point bending load. These included a pristine sample, and two samples with impact damage to introduce variability. Tracking the nonlinear parameters over fatigue life showed a non-monotonic increase in nonlinearity. Conjectures based on acoustic response to microdamage and different stages of fatigue were used to explain the observed variation.

Cumulative fatigue damage in thin aluminum films evaluated non-destructively with lasers via zero-group-velocity Lamb modes

In this paper, a non-contact non-destructive evaluation of cumulative fatigue damage in 75μm-thick aluminum plates is conducted by using the first symmetric zero-group-velocity Lamb mode and taking benefit of its local and long-lasting resonance feature. The tested aluminum sheets are subjected to fatigue loading, in a two sides clamped compression configuration inducing buckling. For understanding the experimental observations, we propose and establish an empirically-inspired theoretical modeling based on the cumulative damage theory, completed with a finite element simulation, for comparison with the experimental measurements. The observed phenomena along fatigue cycles show the potential for the prediction of the fatigue lifetime and the quantitative assessment of different stages of the fatigue damage in solid plate structures. Good agreement is found between the proposed theory/simulation and the experiment on zero-group-velocity resonance frequency. The quality factor of zero-group-velocity resonance is also experimentally studied and compared with numerical calculations, a disagreement is observed after ∼30% of fatigue lifetime. This point elucidates the start time of the change of mechanical properties during the early fatigue stage and is identified as a potential path for the improvement of the proposed empirical model in the future.

Metal magnetic memory inspection of Q345B steel beam in four point bending fatigue test

The metal magnetic memory (MMM) method is a nondestructive testing method to inspect the early damage of steel structures. A four-point bending fatigue beam was inspected by this method during the whole fatigue life. The self-magnetic-leakage field (SMLF) values were measured during the test of the Q345B steel beam. Frequency of 3 Hz and stress ratio of 0.2 contributed to the fatigue life of 100,180 cycles. Test results indicate that both the normal and tangential component of the SMLF values can identify the stress concentration and the crack zones. The gradient can reflect the degree of damage. Furthermore, in order to study the change law of the SMLF values in the crack zone, an improved magnetic dipole model considering the crack dimensions and shape is proposed. The model predictions are in very good agreement with the corresponding test results. Lastly, three characteristic parameters, i.e. average ΔHp(y) field values, resultant magnetic field value H and magnetic parameter m, were extracted from the test data. These parameters can judge the fatigue stage and the damage degree of the specimen. Therefore, the research lays the experimental and theoretical foundation for using the MMM method to inspect steel structures under fatigue bending loads.

Theoretical and experimental investigation on the magnetomechanical effect of steel bars subjected to cyclic load

This paper combines Zheng-Liu (Z-L) model and modified Jiles-Atherton-Sablik (J-A-S) model to propose a magnetomechanical model, to investigate the relationship between the fatigue damage of the steel bar and the surface magnetic memory signal, and to analyze and predict the variation trends of the magnetization of steel bar subjected to cyclic loading. Then tensile fatigue tests on HRB400 steel bars were conducted and the surface magnetic flux density of the specimens under cyclic load was measured. The experimental results demonstrate that the magnetic flux density of the specimens showed three stages in the fatigue process. The magnetic flux density changed rapidly in the early and last stages of the cyclic loading, and developed slightly in the middle stage of fatigue. The magnetization of the specimens during the first two stages was mainly affected by the magnetomechanical effect, and the magnetization during the last stage was dominated by the magnetic flux leakage effect. According to the proposed model, the magnetization of the specimens during the fatigue was simulated. Combined with the fatigue damage formula based on the magnetic signal, the magnetic signal curve reflecting the first two stages of fatigue damage was obtained. By comparing the simulation results with the experimental results, the proposed model can effectively describe the variation trends of the magnetization of the steel bar during the fatigue.

On the Importance of Structural and Functional Fatigue in Shape Memory Technology

The present work provides a brief overview on structural and functional fatigue in shape memory alloys (SMAs). Both degenerative processes are of utmost technological importance because they limit service lives of shape memory components. While our fundamental understanding of these two phenomena has improved during the last two decades, there are still fields which require scientific attention. NiTi SMAs are prone to the formation of small cracks, which nucleate and grow in the early stages of structural fatigue. It is important to find out how these micro-cracks evolve into engineering macro-cracks, which can be accounted for by conventional crack growth laws. The present work provides examples for the complexity of short crack growth in pseudoelastic SMAs. The importance of functional fatigue has also been highlighted. Functional fatigue is related to the degeneration of specific functional characteristics, such as actuator stroke, recoverable strain, plateau stresses, hysteresis width, or transformation temperatures. It is caused by the accumulation of transformation-induced defects in the microstructure. The functional stability of SMAs can be improved by (1) making phase transformations processes smoother and (2) by improving the material’s resistance to irreversible processes like dislocation plasticity. Areas in need of further research are discussed.

Estimation of remaining useful life of fatigued plate specimens using Lamb wave‐based nonlinearity parameters

The present study focuses on estimating material nonlinearity and consequently remaining useful life of fatigued specimens using the amplitude- and physics-based material nonlinearity parameters evaluated for Lamb wave motion in plate specimens. The amplitude-based nonlinearity parameter depends on amplitudes of the Lamb wave harmonics generated due to material nonlinearity. Here, it is employed to estimate the inherent and dislocation induced material nonlinearities for different stages of fatigue. The cumulative effect is obtained from the strict matching of phase and group velocities of the S1 − S2 mode pair. The physics-based nonlinearity parameter is obtained from the higher order elastic coefficients, plastic coefficients, and substructural evolution parameters. It does not depend on wave propagation distance; however, it depends on percent fatigue life. Thus, it is used to construct a theoretical nonlinearity curve. A spectral amplitude normalization technique is given to systematically evaluate the material nonlinearity, once the Lamb wave data over different wave propagation distances are known either from experiments or from simulations. The values of amplitude-based nonlinearity parameter thus estimated through the simulation and experiments for different fatigue stages are then plotted onto the obtained theoretical nonlinearity curve. A reasonably good agreement is seen between the fatigue life estimations given by both the nonlinearity parameters. Thus, the amplitude-based nonlinearity parameter obtained from the Lamb wave response can be effectively used to estimate the remaining useful life of the fatigued plate specimens.

Изучение суточной активности студентов вуза физической культуры

In this article, we have identified the aspect of violation of the functional state of the body and a reduced level of daily activity in students. The topic of daily activity for students of a sports University is relevant, since it is not easy to combine training and sports without breaking the correct daily routine. The final results can help to improve the effectiveness of the educational and training process, students will be able to make adjustments to their usual daily routine to improve the functioning of life support systems and as a result of improving performance.  To get information about the General condition and performance, we used the method of visual-motor reaction, and also analyzed the data during the tapping test for students of the Ural state University of physical culture, which was used to evaluate the work of the nervous system.  Based on the results of a simple visual-motor reaction, we determined that the majority of students have a tendency to decrease performance, are at the "border of the norm" (62%) have an early stage of fatigue. But in General, the situation is stable, with 22% having absolutely normal indicators. Students need adequate exercise, a balanced diet, reduced emotional stress, and rest. The results of the tapping test indicate that a large number of subjects have marked the severity of a weak nervous system, also in a small group, the result indicates a medium-weak nervous system, and only a couple of people have a strong nervous system. The study revealed the following: the majority of participants in the experiment during the tapping test marked the severity of a weak nervous system (72%), a medium-weak nervous system occurs in 24% of the subjects, a medium-strength nervous system is noted in 3% of athletes, and a strong nervous system is noted in only 1 person from the number of participants in the experiment (1%)

Fatigue Crack Initiation and Propagation Mechanism of FGH96 PM Superalloy

FGH96 is regarded as a promising powder metallurgy superalloy for high-temperature / pressure turbine disc in aerospace industries due to its high resistance / defect tolerance and high working temperature up to 750 °C. The initiation and propagation mechanism of FGH96 PM superalloy is researched by in-situ fatigue experiments. The result shows that the crack of gap samples initiates and propagates at the same stress level at 25 °C and 300°C. At 300°C, there are no creep characteristics such as the intergranular characteristics found in the sample fracture. The crack at 300°C has a faster initiation and propagation process compared to the crack at 25°C, especially in the late stage of fatigue. The increase of the crack propagation rate of the sample is unrelated to the creep, mainly due to oxidation. Oxidation reduces the fatigue life of materials because it accelerates the initiation and propagation of transgranular cracks.

Determining the fatigue process in ribbed steel bars using piezomagnetism

The research presented here investigated the relationship between piezomagnetic signals and fatigue damage in ribbed steel bars. Two groups of HRB400 ribbed steel bars were separately tested using either uniaxial static or cyclic tensile loading. Subsequent piezomagnetic signals were recorded in real-time. Results demonstrated that there was a relatively extreme—but stable—value ratio for the piezomagnetic curves obtained under both static and cyclic stress. The microstructure and surface features presented by ribbed steel bars resulted in different theoretical analyses when examining stress-induced magnetization. There were significant differences in the time-varying piezomagnetism trace and piezomagnetic hysteresis loop for different stages. These differences revealed the evolution of fatigue damage, allowing for signs of impending terminal failure. Piezomagnetic parameters developed in parallel with the accumulation of fatigue damage and the transitions between three fatigue stages were determined using piezomagnetic signals. The percentages from three stages of fatigue were approximately 2.1%, 95.2%, and 2.7%. Collectively, these results may suggest the effectiveness of this nondestructive technique for analyzing the fatigue process in ribbed steel bars.

Effects of Mental Fatigue on Small-World Brain Functional Network Organization.

Brain functional network has been widely applied to investigate brain function changes among different conditions and proved to be a small-world-like network. But seldom researches explore the effects of mental fatigue on the small-world brain functional network organization. In the present study, 20 healthy individuals were included to do a consecutive mental arithmetic task to induce mental fatigue, and scalp electroencephalogram (EEG) signals were recorded before and after the task. Correlations between all pairs of EEG channels were determined by mutual information (MI). The resulting adjacency matrices were converted into brain functional networks by applying a threshold, and then, the clustering coefficient (C), characteristic path length (L), and corresponding small-world feature were calculated. Through performing analysis of variance (ANOVA) on the mean MI for every EEG rhythm, only the data of α1 rhythm during the task state were emerged for the further explorations of mental fatigue. For a wide range of thresholds, C increased and L and small-world feature decreased with the deepening mental fatigue. The pattern of the small-world characteristic still existed when computed with a constant degree. Our present findings indicated that more functional connectivities were activated at the mental fatigue stage for efficient information transmission and processing, and mental fatigue can be characterized by a reduced small-world network characteristic. Our results provide a new perspective to understand the neural mechanisms of mental fatigue based on complex network theories.

Experimental study on rate-dependent uniaxial whole-life ratchetting and fatigue behavior of polyamide 6

Rate-dependent whole-life ratchetting and fatigue failure of polyamide 6 were explored by conducting a series of stress-controlled uniaxial fatigue tests at room temperature. Meanwhile, the temperature rises on the surface of specimens were recorded to reflect the self-heating occurred during the fatigue tests. The experimental results demonstrate that ratchetting strain rate is sensitive to the variation of prescribed stress rate, and the whole-life ratchetting of PA6 presents a tri-staged evolution feature at high stress rate but a four-staged one at low stress rate. Two failure modes (i.e., the necking controlled by unstable self-heating and resultant large ratchetting strain; and the fracture controlled by the low-cycle fatigue damage) are experimentally observed in the tests at different stress rates. By considering the relationship between ratchetting evolution and temperature rise, it is found that whether increasing the stress rate retards or promotes the ratchetting is dependent on the magnitude of self-heating produced at each fatigue stage, and whether it will be beneficial or detrimental to the total fatigue life is determined by the competition of resultant strain hardening and self-heating softening.

αs dissolving induced mechanical properties decay in Ti-55511 alloy during uniaxial fatigue

The properties and microstructure evolution of Ti-5Al-5Mo-5V-1Cr-1Fe alloy with bimodal microstructure during uniaxial fatigue were investigated. The results showed that, the elastic modulus, ultimate strength and elongation decreased by 14.8%, 15.7%, and 92% respectively at the late stage of fatigue, under conditions of frequency f = 10 Hz and stress ratio R = 0 (σmax = 0.75σ0.2) at room temperature. Dislocation gliding along {1 0 1}β cut secondary α phase (αs) into fragments through twin, stacking fault and shear band, resulting in αs dissolving. This induced the decay of mechanical properties. The dissolving of αs inferred the microstructural design strategy in high strength titanium alloys by introducing αs phase.

A Sequential Cohort Study Comparing Kappamab Alone to Kappamab, Lenalidomide and Low Dose Dexamethasone in Kappa-Restricted Relapsed Refractory Multiple Myeloma (AMaRC 01-16)

A Sequential Cohort Study Comparing Kappamab Alone to Kappamab, Lenalidomide and Low Dose Dexamethasone in Kappa-Restricted Relapsed Refractory Multiple Myeloma (AMaRC 01-16)

A More Realistic Study on the Mechanical Properties of CFRC Composite Grouting Material under a Long-Term Water Immersion Process

The long-term effect of water immersion on the mechanical properties of CFRC composite grouting materials was studied by using five different carbon fiber contents (0, 0.25%, 0.50%, 0.75%, and 1.00%). The direct shear and long-term immersion tests were performed based on the specified and optimum values of carbon fiber content, respectively. The results showed the following: (1) the application of carbon fiber significantly improved the shear resistance of CRFC composite grouting material by using “reinforcing” and “anchoring” actions. The shear strength of the specimen was increasing by 5.66%∼43.41% when the carbon fiber content increased from 0.25% to 1.00%. After a comprehensive analysis, the optimum carbon fiber content was found to be 0.75%. (2) The degradation in the compressive and tensile strength of CRFC composite specimens exhibited a consistent trend (i.e., a steep gradient was gradually followed by a gentle slope) under a long-term water immersion process. About 90% of the total degradation in the compressive and tensile strength has occurred in 90 immersion days (i.e., 16.05% and 18.45%, respectively). In comparison, the degradation in the tensile strength (20.05%) was slightly higher than the compressive strength (18.16%). (3) Under the long-term water immersion process: the properties of the specimens were gradually deteriorating, the carbon fibers were gradually reaching a fatigue stage, and the bonding properties of carbon fiber was decreasing, which resulted in a reduction in the compressive and tensile strength. The uniaxial compression failure mode changed from brittle to ductile, and the development of local failure was very noticeable. Based on the findings of this paper, groundwater has a significant impact on the mechanical properties of grouted rock mass such as dam foundations and abutments. Therefore, the degradation in the grouting materials has to be considered in practical cases.

Fatigue Failure Characteristics of Single-Crystal Fe-3%Al Alloy

To obtain a better understanding of the mechanical properties and failure characteristics of the single-crystal Fe-3%Al alloy (SC), tensile and fatigue properties of the SC and polycrystalline Fe-3%Al alloy (PC) have been investigated. The tensile strength and fracture strain for the SC samples were different depending on the loading direction, i.e., 〈111〉, 〈110〉 and 〈100〉, which could be explained by the Schmid factor. Fatigue strength for the SC samples was basically correlated with their tensile strength. Different fatigue properties were obtained for the PC sample, e.g., the high fatigue strength was obtained in the early fatigue stage, and significant reduction in the fatigue strength was appeared in the later fatigue stage, e.g., low endurance limit at 107 cycles. The reason for this was caused by the material brittleness due to the accumulated dislocation at the grain boundaries. Because the high ductile SC samples made severe plastic deformation ahead of the crack tip, the crack growth rate did not enhance even if the high stress intensity factor range was applied in Region II (da/dN versus ΔK). The crack growth rate dropped just before the final fracture. Effective stress intensity factor range was estimated using the crack closure model, but the crack growth rate is found to be affected strongly by the slip system of SC sample.