Fatigue Process Research Articles

Effects of Cardiopulmonary Rehabilitation on the Muscle Function of Children with Congenital Heart Disease: A Prospective Cohort Study.

Critical medical and surgical advances have led to a shift in the care and management of children with congenital heart disease (CHD). These patients present with muscle deconditioning, which negatively influences their response to exercise, functional capacities, and quality of life. This study evaluates the influence of a cardiopulmonary rehabilitation program (CPRP) on the function of peripheral musculature of children with CHD. A single-center prospective cohort study was designed. Fifteen CHD subjects, between 12 and 16 years of age, with reduced aerobic capacity on a cardiopulmonary exercise test, were included in a three-month, 24-session CPRP. Measurements of the subjects’ handgrip strength, biceps brachii and quadriceps femoris strength, and triceps surae fatigue process were collected at the beginning of the program, after completion, and six months after the end of the intervention. A substantial and statistically significant improvement was observed in the subjects’ handgrip strength (kg) (p < 0.001), biceps brachii and quadriceps femoris strength (N) (p < 0.001), as well as triceps surae fatigue process (repetitions) (p = 0.018), with a maintenance of the results six months after the intervention. These results suggest that a CPRP could potentially improve the peripheral muscle function of children with CHD. Additional research is needed to confirm and expand on this hypothesis.

Corrosion fatigue life prediction of crude oil storage tank via improved equivalent initial flaw size

Corrosion fatigue is identified as the main failure mechanism for structures working in severe corrosive medium subjected to cyclic rotating bending fatigue, for example crude oil storage tank (COST). Emphasis is placed on the study of corrosion pit formation and the development of cracks from pits. An improved equivalent initial flaw size (I-EIFS) is proposed for corrosion fatigue life prediction. Based on the concept of equivalent initial flaw size (EIFS), pitting corrosion and small crack growth are equivalent to a part of long crack growth process in corrosion fatigue process. Pitting and crack propagation are quantified throughout the fatigue loading thereby allowing a model to be developed that included the stages of pit development, pit-to-crack transition and crack growth in order to predict the fatigue life. A corrosion fatigue life prediction case is adopted to demonstrate the effectiveness of the proposed model. Based on the proposed model, failure analysis and stress calculation are performed to predict the corrosion fatigue life of COST, which provides a method for the life prediction of COST. The validity of proposed method is verified by comparing the service life of COST.

Effect of thermo-mechanical ageing on materials and interface properties in flexible microelectronic devices

The effect of thermal ageing, fatigue and thermo-mechanical ageing on flexible microelectronic devices is studied and reported from a materials and functional durability perspective. The degradation mechanisms of encapsulant and substrate were both studied. Property changes in encapsulant and substrate materials are analyzed and their relationships in the failure mechanisms of the flexible devices determined. Stiffening of the resin under thermal ageing conditions is associated with delamination in test vehicles, which leads to a loss of functional electrical properties. Degradation was due to the prominence of crosslinking reactions occurring during thermal oxidation at 120 °C. A moderate stiffening of the resin occurs after fatigue stress testing. Whereas this latter stiffening is similarly associated with crosslinking reactions, here, stiffening could not be induced by thermal degradation of the resin, because the stress frequency employed was low. Instead thermo-mechanical coupling occurred in two stages. Under mild conditions, the degradation mechanisms correspond to the combined effect of thermal ageing and fatigue processes. Under harsher thermo-mechanical conditions, the chain-scission mechanisms become more efficient and induce a softening of the resin.

Damage-entropy model for fatigue life evaluation of off-axis unidirectional composites

This research is devoted to studying the capability of the damage-entropy model for fatigue life prediction of the off-axis unidirectional composites. The damage process in composites was characterized by the micro-continuum damage mechanics (MCDM) and an available reformulated form of the Mori-Tanaka model was used. In the present study, the fracture fatigue entropy (FFE), which is the accumulated entropy generation up to final failure during the fatigue process, was considered as a fatigue failure criterion for the off-axis unidirectional composites. One of the outstanding characteristics of the damage-entropy model is considering the self-heating and temperature evolution during the fatigue life that is due to the damage evolution, irreversible work, and internal friction in composites. Moreover, the dissipated heat due to the viscoelastic intrinsic property of the polymeric matrix was considered. The proposed model was validated by investigating the fatigue behavior of unidirectional T800H/2500 and E-glass/epoxy composites under tension-tension cyclic loading. Results show that the fatigue life prediction of the damage-entropy model is in good agreement with the experimental data.

Effects of Machined Surface Integrity on High-Temperature Low-Cycle Fatigue Life and Process Parameters Optimization of Turning Superalloy Inconel 718

Machined surface integrity characteristics, including surface stresses, physical-mechanical properties and metallographic structures, play important roles in the fatigue performance of machined components. This work aimed at investigating the effects of machined surface integrity on high-temperature low-cycle fatigue life. The process parameters were optimized to obtain required surface integrity and fatigue life of the turning superalloy Inconel 718. The relationships between low-cycle fatigue life and machined surface integrity characterization parameters were established based on the low-cycle fatigue tests at a high temperature (650 °C). The sensitivities of turning process parameters to high-temperature low-cycle fatigue life were analyzed, and the optimization parameters were proposed with the goal of antifatigue manufacturing. Experimental results indicated that the impact order of the characterization parameters of machined surface integrity on the high-temperature low-cycle fatigue life were the degree of work hardening RHV, the residual stress in the cutting speed direction S22, the fatigue stress concentration factor Kf, the degree of grain refinement RD and the residual stress in the feed direction S33. In the range of turning parameters of the experiments in this research, the cutting speeds could be 80~110 m/min, and the feed rate could be 0.10~0.12 mm/rev to achieve a longer high-temperature low-cycle fatigue life. The results can be used for guiding the fatigue-resistant manufacturing research of aeroengine superalloy turbine disks.

Compressive Fatigue Behavior of Gum and Filled SBR Vulcanizates.

The influence of carbon black on physical mechanical properties, compressive fatigue life, and the temperature changes during compression fatigue process of styrene-butadiene rubber (SBR) vulcanizates were explored. A series of unfilled and filled SBR compounds were prepared, and the compressive fatigue behaviors of the vulcanizates were performed on a mechanical testing and simulation (MTS) machine. The top surfaces of the filled SBR were imaged using scanning electron microscopy (SEM) after 105 cycles of compressive fatigue. The filled SBR shows greater compressive fatigue resistance than the unfilled SBR. The incorporation of carbon black into SBR improves the creep resistance. The best compressive fatigue resistance for the filled SBR was achieved by the addition of 30 phr carbon black. With the increase of carbon black content, the energy dissipation and the heat build-up increase simultaneously. Furthermore, SEM images of the vulcanizates suggest that the crack propagation mechanism of the unfilled and the filled SBR was different. For the unfilled SBR, due to periodical compressive stress, the polymer chains can be destroyed, and the cracks can be easily initiated and propagated, showing serious damage on the top surfaces of the specimen. However, for the filled SBR, the carbon black agglomeration around the cracks can greatly delay the generation of the cracks, decrease the fatigue damage, and ultimately improve the fatigue resistance.

A molecular dynamics study on tensile and low cycle fatigue behaviors of Ti single crystal nanowire

It is essential to reveal the fatigue mechanism of the main elements in alloys at the nano scale for the material design against fatigue failure. In the present study, molecular dynamics (MD) simulations have been employed to investigate the static tensile and low cycle fatigue (LCF) behaviors of a defect free single crystal Ti nanowire with hexagonal close-packed (HCP) structure at room temperature (300 K). Results showed that the explosive initiations of body centered cubic and disordered atoms led yielding of Ti nanowire under static tensile load. The following stress response to constant amplitude cyclic strain was found strongly relatived to the instantaneous microstructure configuration at the first unloading. Variation of potential energy was used to characterize the competitions between face-centered cubic (FCC) atoms, body-centered cubic (BCC) atoms and disordered atoms. A conclusion was drew that the cyclic characteristics of Ti nanowire were heavily dependent on the dominant atom configuation during the fatigue process. Additionally, this research also found that static tensile yield strength of Ti nanowire may vary notably with respect to crystal direction, e.g. for [100] (or for [0001]) it is about 1.5 times of that for [110].

Understanding the damage mechanism of asphalt binder under controlled-stress fatigue loads in dynamic shear rheometer

Fatigue damage is one of the major distresses of asphalt binder. Most of the previous studies have focused on the crack growth in asphalt binder while cracking is not the only type of damage that occurs during fatigue process, which means that the damage mechanism of asphalt binder under fatigue loads is not well understood. This study characterized and compared the fatigue damage features of asphalt binder under controlled-stress time sweep tests with different amplitudes of fatigue loads. Cohesive failure appearances of all the involved asphalt binder were collected and employed to analyze the failure process. These failure appearances showed that there were primarily two types of pattern in the failure appearance: factory roof morphology and the wave-shaped pattern. Factory roof morphology was related to the cracking of asphalt binder, and the wave-shaped pattern was linked to the plastic flow of asphalt binder. Combining the failure appearance and the collected rheological properties proved that two types of damage occurred in asphalt binder in the time sweep test: elastic damage and plastic flow damage. The former and the latter one respectively dominated the failure process of asphalt binder under small- and large-amplitude fatigue loads. During the elastic damage process, phase angle of asphalt binder held constant while the complex shear modulus kept decreasing. During the plastic flow damage process, obvious variations on both complex shear modulus and phase angle can be observed. It addition, it was found that the profile of the failure surface was concave rather than horizontal, which proved the non-negligible effect of normal force on the cracking behavior of asphalt binder.

Nonlinear Ultrasonic Guided Wave Method Using Semi-Analytical Finite Element (SAFE) Technique on a Damaged SWO-V Spring Coil

This work presents a non-destructive method for quantitative essessment of fatigue damage of materials with linear elastic properties using nonlinear ultrasonic techniques. A nonlinear study was conducted on these materials with fatigue and shot peening processing using a nonlinear ultrasonic technique. A numerical method based on the semi-analytical finite element (SAFE) technique, was used to obtain the phase-matching modes of the specimens. Experiments confirm that the nonlinearity for shot peening and samples with a certain level of fatigue shows a tendency to increase with levels of fatigue.

Performance of Al2O3-CBN materials and perspective of using hyperspectral imaging during cutting tests

The performance of cutting tool materials (CTMs) influences the quality and lifetime of parts produced with their help. Unexpected fracturing or other failures of the tools lead to defects of parts that exaggerate materials fatigue and fracture processes. For the purpose of Industry 4 and future generations of factories it is important to enable in-situ monitoring of cutting processes while hyperspectral imaging can serve as a powerful tool. Cubic boron nitride (cBN) has extreme hardness and can provide improved wear resistance if mixed with other CTMs. Besides, such materials can be used without cooling liquids that helps to mitigate workplace health risks. The aim of the current work was to understand how well current hyperspectral imaging technologies can track the changes in performance of CTMs with addition of cBN. The paper presents the results of multiple in-situ (obtained during cutting with real lathe) and static (before or after cutting) tests performed with hyperspectral camera. The wear rate of CTMs and roughness of workpieces were measured with the help of scanning electron microscope and 3D optical profiler respectively. The effect of cBN content and effect of TiN or ZrO2 additives on performance of alumina-based CTMs produced by spark plasma sintering technique is presented.

Fatigue behaviors of physical hydrogels based on hydrogen bonds

Physical hydrogels, composed of non-covalently crosslinking three dimensional polymer networks and a large amount of water, have drawn much attention due to their excellent mechanical properties such as high toughness, self-healing, self-recovery, and shape-memory. But the fatigue behaviors of physical hydrogels, especially the role of non-covalent bonds in the fatigue process, have not been well studied. Here we study fatigue damage and fatigue fracture of two kinds of physical hydrogels based on hydrogen bonding (H-bonding): poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) gels with weak H-bonding (weak gels) and PVA/PAA-LiCl gels with strong H-bonding (strong gels). For the fatigue damage of uncut samples, both the weak and strong gels are fatigue strengthened in small deformation by stretch-induced H-bonding rearrangement, but are fatigue damaged in large deformation by chain sliding. For the fatigue fracture of precut samples, both the weak and strong gels show no measurable fatigue threshold, but the strong gel can mitigate crack growth. These results reveal the intrinsically weak non-covalent bonds cannot effectively increase the fatigue resistance of physical hydrogels. This work provides a deep perspective for developing novel anti-fatigue hydrogels.

Fatigue property and failure mechanism of TC4 titanium alloy in the HCF and VHCF region considering different forging processes

Based on the self-built three point bending ultrasonic fatigue test system, fatigue behavior of TC4 titanium alloy after different forging processes in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes was discussed in this paper. The experimental results showed that the fatigue S-N curves of TC4 titanium alloy with three different structures presented different characteristics: continuous decline; double platform; linear decline. The fatigue property of TC4 titanium alloy is closely related to the content of primary α phase and β-transformed microstructure. For TC4 titanium alloy, grain refinement and certain volume fraction of the primary α phase contributed to enhancing fatigue property. The fatigue performance of bimodal structure by near β forging was obviously better than two structures by α + β forging in HCF and VHCF regime. It was found that failure mode shifted from the surface at relatively high stress to the subsurface at relatively low stress. For three kinds of structures, crack initiations were observed at the surfaces of specimens in HCF regime. Meanwhile, cracks of the three structures all originated from primary α cleavage planes in the interior. The fatigue life of VHCF is dominated by the crack initiation stage.

Measurement and Analysis of Inadequate Friction Mechanisms in Liquid-Buffered Mechanical Seals Utilizing Acoustic Emission Technique

Mechanical seals play an important role in the reliability of a process. Currently, the condition monitoring of mechanical seals is restricted due to the limitations of the traditional monitoring methods, including classical vibration analysis. For this reason, the objective of the present work is the detection and analysis of friction mechanisms inside a mechanical seal that are unfavorable and induce fault conditions using the acoustic emission technique, which allows the measurement of high-frequency vibrations that arise due to material fatigue processes on a microscopic scale. For this purpose, several fault condition modes were induced on a test rig of an agitator vessel system with a double-acting mechanical seal and its buffer fluid system. It was possible to detect the presence of inadequate friction mechanisms due to the absence and limited use of lubrication, as well as the presence of abrasive wear, by measuring a change in the properties of the acoustic emissions. Operation under fault condition modes was analyzed using the acoustic emission technique before an increase in the leakage rate was evaluated using traditional monitoring methods. The high friction due to the deficient lubrication was characterized by a pattern in the high-frequency range that consisted of the harmonics of a fundamental frequency of about 33 kHz. These results demonstrate the feasibility of a condition monitoring system for mechanical seals using the acoustic emission technique.

Probing the Brain-Body Connection Using Transcranial Magnetic Stimulation (TMS): Validating a Promising Tool to Provide Biomarkers of Neuroplasticity and Central Nervous System Function.

Transcranial magnetic stimulation (TMS) is a non-invasive method used to investigate neurophysiological integrity of the human neuromotor system. We describe in detail, the methodology of a single pulse TMS protocol that was performed in a large cohort of people (n = 110) with multiple sclerosis (MS). The aim was to establish and validate a core-set of TMS variables that predicted typical MS clinical outcomes: walking speed, hand dexterity, fatigue, and cognitive processing speed. We provide a brief and simple methodological pipeline to examine excitatory and inhibitory corticospinal mechanisms in MS that map to clinical status. Delayed and longer ipsilateral silent period (a measure of transcallosal inhibition; the influence of one brain hemisphere’s activity over the other), longer cortical silent period (suggestive of greater corticospinal inhibition via GABA) and higher resting motor threshold (lower corticospinal excitability) most strongly related to clinical outcomes, especially when measured in the hemisphere corresponding to the weaker hand. Greater interhemispheric asymmetry (imbalance between hemispheres) correlated with poorer performance in the greatest number of clinical outcomes. We also show, not surprisingly, that TMS variables related more strongly to motor outcomes than non-motor outcomes. As it was validated in a large sample of patients with varying severities of central nervous system dysfunction, the protocol described herein can be used by investigators and clinicians alike to investigate the role of TMS as a biomarker in MS and other central nervous system disorders.

Low-Cycle Fatigue Life of Continuously Cyclic-Hardening Material

Abstract A general fatigue life equation is derived by modifying the Tanaka-Mura-Wu dislocation pile-up model for variable strain-amplitude fatigue processes, where the fatigue crack nucleation life is expressed in terms of the root-mean-square of plastic strain range. Low-cycle fatigue tests were conducted on an austenitic stainless steel. At 400 °C and 600 °C, the material exhibits continuously cyclic-hardening behavior. The root-mean-square of plastic strain ranges is evaluated from the experimental data for each test condition at strain rates ranging from 0.0002/s to 0.02/s. The variable-amplitude Tanaka-Mura-Wu model is found to be in good agreement with the low-cycle fatigue (LCF) data, which effectively proves Miner's rule on the stored plastic strain energy basis.

Fatigue deformation and damage characteristics of bolting system under stress-controlled cyclic pullout

This study explored the fatigue mechanical properties and acoustic emission (AE) characteristics of bolting system under cyclic pullout tests controlled by different upper stress levels. A novel compression-pull load conversion device was designed to precisely apply cyclic pullout load on bolting system. Strain gauges and AE sensors were employed to capture the surface strain variations of rock bolts and microfracture development during static and cyclic testing. The fatigue strength, deformation, axial/shear stress distributions and AE characteristics of the bolting system were comprehensively investigated. Fatigue threshold, an important value determining the service life of a bolting system, was supposed to be lower than 85% of the static strength in this study. The bolt residual slip strain curve was S-shaped and comprised three stages: the initial stage, constant-velocity stage and acceleration stage. The constant-velocity stage accounted for most of the fatigue life and was the main stage of fatigue development. The fatigue failure of the anchorage system was presumably controlled by the entire static stress–strain process, and the residual strain at fatigue failure corresponded to the peak strain at the peak stress under static loading. The anchorage fatigue damage development completely coincided with the residual strain evolution, and the turning points were identical between the different deformation stages. Based on the residual slip strain development, nonlinear inverted S-shaped cumulative fatigue damage models were proposed for different upper stress levels. A comparison of the results from different samples revealed that the damage evolution was highly dependent on the upper stress level. The fatigue life of the bolting system decreased as the upper stress level increased due to the increased steepness of the second stage and the decreased proportion of the second stage to the entire fatigue process.

Fatigue and its effect on the mechanical and thermal transport properties of polycrystalline graphene

The fatigue failure of graphene plays an important role in determining the service life of many graphene-based nanodevices. In this paper, the fatigue behaviours of polycrystalline graphene are investigated by using molecular dynamics (MD) simulations. The results show that the fatigue process of polycrystalline graphene contains three stages, which, successively, are the initiation of microvoids, formation of large cracks and rapid propagation of cracks. The loading amplitude, grain size and temperature can greatly affect the fatigue properties. Evolutions of the mechanical and thermal transport properties of polycrystalline graphene during the fatigue process are also investigated by MD simulations. No significant changes are found in these material properties in the first two stages of fatigue. However, a dramatic decrease in the Young’s modulus, tensile strength and thermal conductivity is found in the last stage. The large cracks occurring in the last stage are responsible for the reduced Young’s modulus and tensile strength, because they can reduce the effective stiffness of polycrystalline graphene and induce the stress concentration in graphene. Meanwhile, large cracks also can reduce the heat flux and, meanwhile, increase the phonon scattering in polycrystalline graphene, both of which account for the reduced thermal conductivity observed in the last stage of fatigue.

Fatigue resistance of reinforced UHPFRC beams

This work investigates the fatigue resistance of reinforced Ultra High Performance Fiber Reinforced Cementitious composite (R-UHPFRC) beams with special consideration of steel rebar – UHPFRC interaction under tensile stresses due to bending. Experimental testing of full-scale beams reveals that the fatigue resistance depends on both minimum and maximum loadings in the cycle. The increase of stress range in the rebar during the fatigue process is discussed and quantified. Finally, recommendations for fatigue verification of R-UHPFRC beams are given.

Can a chloride channel blocker mitigate muscle fatigue?

Can a chloride channel blocker mitigate muscle fatigue?

Analysis of fatigue in the biceps brachii by using rapid refined composite multiscale sample entropy

The analysis of human muscle fatigue is of great significance to human physiological activities. Surface electromyography (sEMG) is the widely used technique to analyze muscle fatigue due to its non-invasiveness. However, sEMG signals are complex, non-linear, and multiple time scales. Hence, multiscale entropy is an effective method to quantify the process of muscle fatigue. In this study, we proposed rapid refined composite multiscale sample entropy (R2CMSE) and used it to analyze the process of muscle fatigue. Firstly, R2CMSE was utilized to characterize the complexity and validity of noise signals on different scales and lengths. Secondly, isometric contraction activities of the biceps brachii were recorded by using sEMG from ten subjects. Then, combined with the sEMG signals of all subjects, a three-dimensional map of scale-length-entropy was constructed to determine an appropriate time scale and data length. Meanwhile, a two-way repeated-measures ANOVA was operated, and the results showed that non-fatigue and fatigue conditions exist significant differences under all algorithms. Finally, R2CMSE was used to quantify the fatigue process to analyze the reliability of its application to different subjects. It was shown that compared with other multiscale entropy algorithms, R2CMSE was faster in the calculation, less dependent on different data lengths, and more robust at different time scales. The proposed algorithm can also extract the hidden information of sEMG signals and investigate the process of muscle fatigue more effectively.