Fatigue Threshold Research Articles

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.

Evaluation of High-Intensity Interval Training and Beta-Alanine Supplementation on Efficiency of Electrical Activity and Electromyographic Fatigue Threshold.

Herda, AA, Smith-Ryan, AE, Kendall, KL, Cramer, JT, and Stout, JR. Evaluation of high-intensity interval training and beta-alanine supplementation on efficiency of electrical activity and electromyographic fatigue threshold. J Strength Cond Res 35(6): 1535-1541, 2021-The purpose of this study was to determine the effects of high-intensity interval training (HIIT) with or without β-alanine (BA) supplementation on the electromyographic fatigue threshold (EMGFT) and efficiency of electrical activity (EEA) in young women. Forty-four women (mean ± SD; age [yrs]: 21.7 ± 3.7; height [cm]: 166.3 ± 6.4; body mass [kg]: 66.1 ± 10.3) were randomly assigned to one of 3 treatment groups. The supplement groups performed HIIT on the cycle ergometer 3 times·wk-1 for 6 weeks. Electromyographic fatigue threshold and EEA were assessed at baseline (PRE), after 3 weeks of training (MID), and after 6 weeks of HIIT (POST). Two 2-way mixed factorial analyses of variance (time [PRE vs. MID vs. POST] × treatment (BA vs. PL vs. CON)] were used to analyze EMGFT and EEA with a predetermined level of significance α of 0.05. For EMGFT, there was no interaction (p = 0.26) and no main effect for time (p = 0.28) nor treatment (p = 0.86); thus, there were no changes in EMGFT regardless of training or supplementation status. For EEA, there was no interaction (p = 0.70) nor treatment (p = 0.79); however, there was a main effect for time (p < 0.01). Our findings indicated that neither training nor supplementation was effective in improving EMGFT in women. Efficiency of electrical activity was altered, potentially because of a learning effect. Coaches and practitioners may not use these tests to monitor training status; however, they may find EEA as a useful tool to track cycling efficiency.

Artificial aging time influencing the crack propagation behavior of the aluminum alloy 6060 processed by equal channel angular pressing

The combination of a severe plastic deformation (SPD) by equal-channel angular pressing (ECAP) with a subsequent heat treatment significantly influences the precipitation characteristics of age-hardenable aluminum alloys. The aging kinetics is accelerated and the morphology of the precipitates is affected by the induced strain. In the present study, the influence of the artificial aging time and the resulting precipitation morphology on the fatigue threshold was investigated for the aluminum alloy 6060 with conventional grain size (CG) and after ECAP processing. Four artificial aging conditions are compared for the as-extruded and ECAP processed material, respectively. The resulting microstructure is examined by transmission (TEM) and scanning transmission electron microscopy (STEM) and related to the processing condition and the aging time. The mechanical properties are characterized by tensile and hardness testing and crack propagation tests are conducted in order to determine the threshold ΔKth. The ECAP processed conditions exhibit a lower resistance against crack propagation regardless of the aging time, when compared to the as-extruded conditions. Therefore, the grain size is determined as the major influencing factor on the fatigue threshold. In contrast, the effect of the aging time and the precipitation morphology is only minor. When compared to incoherent precipitates, coherent ones promote planar slip resulting in an increased threshold against crack propagation for both CG and finer grained, bimodal microstructures. However, the size of the coherent precipitates and the resulting lattice distortion is decisive to achieve higher fatigue thresholds.

A modification to the two driving forces model for fatigue threshold prediction

The two driving forces (TDF) model was modified based on the dislocation dipoles mechanism of fatigue crack growth (FCG) and corrected by Kmax effect for a better prediction of the R dependent FCG thresholds ΔKth and Kmax,th. This model can well describe transformation of the crack growth mechanism from fatigue fracture to quasi-static fracture when R tends to 1, and agrees well with experimental data for a broad round of alloy systems. An empirical relationship between the fatigue threshold and the fracture toughness was proposed, and then an approximately material-independent fatigue threshold prediction model was established by normalization based on this relationship.

The effects of the toughening mechanism and the molecular weights between cross-links on the fatigue resistance of epoxy polymer blends

The fatigue resistance of epoxy polymer blends incorporating different types of preformed polymer particles was evaluated in terms of fatigue threshold and fatigue crack propagation (FCP). The fatigue thresholds of epoxies modified with polyamide-12 (PA12) particles were associated with higher stress intensity factor ranges (ΔK) than those of pure epoxies, irrespective of the molecular weights between cross-links (Mc). In contrast, the fatigue thresholds of epoxy blends with core-shell rubber (CSR) particles were affected by Mc. The FCP curve for a highly crosslinked CSR-toughened epoxy with Mc of 1110 g/mol was located at higher ΔK values compared with data for the corresponding pure epoxy. In contrast, the fatigue threshold of a moderately cross-linked CSR-modified epoxy with Mc of 2360 g/mol appeared in the lower ΔK range relative to the pure epoxy. Both cavitation of the CSR particles and plastic deformation of the latter epoxy were localized in a narrow area.

Female cancer survivors are more likely to be at high risk of malnutrition and meet the threshold for clinical importance for a number of quality of life subscales.

The present study aimed to explore malnutrition risk, handgrip strength and quality of life (QOL) in cancer survivors. In total, 232 individuals completed a demographic questionnaire, Patient-Generated Subjective Global Assessment Short Form and the European Organization for Research and Treatment of Cancer QOL Questionnaire (EORTC QLQ-C30). Handgrip strength was determined using a spring-loaded handgrip dynamometer and anthropometric measurements were taken by an oncology nurse. Frequencies and distribution data, analysis of variance and chi-squared tests were then conducted. The majority of the cohort were female (n=141; 60.8%) had breast cancer (n=62; 26.7%) and the mean±SD body mass index (BMI) was 26.6±6.2kgm-2 . Less than a one-third reported seeing a dietitian (n=68; 29.3%). Over one-third reported recent weight loss (n=88; 37.3%). Some 40.9% (n=95) were at moderate to high risk of malnutrition, with women more likely than men to be classified as high risk (p<0.05). Mean±SD handgrip strength was 25±15kg and this differed significantly by gender (p=0.00), cancer type (p=0.01) and BMI classification (p=0.01). One-fifth of individuals were classified as having dynapenia (n=48; 21.1%). Median (interquartile range) QOL score was 66.7 (33.3). The proportion of individuals meeting the threshold for clinical importance for QOL subscales ranged from 12.5% (constipation) to 42.7% (physical functioning). Females were more likely than males to meet the threshold for physical functioning (p=0.00), fatigue (p=0.02) and pain (p=0.01). Females are more likely than males to be at high risk of malnutrition and meet the threshold for clinical significance for several QOL subscales.

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.

Modelling the Variability and the Anisotropic Behaviour of Crack Growth in SLM Ti-6Al-4V.

The United States Air Force (USAF) Guidelines for the Durability and Damage Tolerance (DADT) certification of Additive Manufactured (AM) parts states that the most difficult challenge for the certification of an AM part is to establish an accurate prediction of its DADT. How to address this challenge is the focus of the present paper. To this end this paper examines the variability in crack growth in tests on additively manufactured (AM) Ti-6Al-4V specimens built using selective layer melting (SLM). One series of tests analysed involves thirty single edge notch tension specimens with five build orientations and two different post heat treatments. The other test program analysed involved ASTM standard single edge notch specimens with three different build directions. The results of this study highlight the ability of the Hartman–Schijve crack growth equation to capture the variability and the anisotropic behaviour of crack growth in SLM Ti-6Al-4V. It is thus shown that, despite the large variability in crack growth, the intrinsic crack growth equation remains unchanged and that the variability and the anisotropic nature of crack growth in this test program is captured by allowing for changes in both the fatigue threshold and the cyclic fracture toughness.

Short/small fatigue crack growth, thresholds and environmental effects: a tale of two engineering paradigms

Abstract This paper results from mutual discussions on the review ‘When do small fatigue cracks propagate and when are they arrested?’ in Corrosion Reviews, 2019; 37(5): 397–418. These discussions have arisen from the two engineering paradigms characterizing our fatigue research: (i) an aerospace research and technology remit for metallic airframes, and (ii) a materials science research programme supporting a methodology for steam turbine low pressure (LP) blade operations. In our opinion, this paper is of interest for other investigators of metal fatigue with respect to design requirements, life predictions and assessments. In more detail, the paper considers the fatigue design methodologies for airframes and steam turbine LP blades. This includes short/small fatigue cracks, fatigue crack growth thresholds, high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF), and the relevance of environmental effects (corrosion and corrosion fatigue).

Strong tough hydrogels via the synergy of freeze-casting and salting out

Natural load-bearing materials such as tendons have a high water content of about 70 per cent but are still strong and tough, even when used for over one million cycles per year, owing to the hierarchical assembly of anisotropic structures across multiple length scales1. Synthetic hydrogels have been created using methods such as electro-spinning2, extrusion3, compositing4,5, freeze-casting6,7, self-assembly8 and mechanical stretching9,10 for improved mechanical performance. However, in contrast to tendons, many hydrogels with the same high water content do not show high strength, toughness or fatigue resistance. Here we present a strategy to produce a multi-length-scale hierarchical hydrogel architecture using a freezing-assisted salting-out treatment. The produced poly(vinyl alcohol) hydrogels are highly anisotropic, comprising micrometre-scale honeycomb-like pore walls, which in turn comprise interconnected nanofibril meshes. These hydrogels have a water content of 70-95 per cent and properties that compare favourably to those of other tough hydrogels and even natural tendons; for example, an ultimate stress of 23.5±2.7 megapascals, strain levels of 2,900±450 per cent, toughness of 210±13 megajoules per cubic metre, fracture energy of 170±8 kilojoules per square metre and a fatigue threshold of 10.5±1.3 kilojoules per square metre. The presented strategy is generalizable to other polymers, and could expand the applicability of structural hydrogels to conditions involving more demanding mechanical loading.

A non-linear analysis of running in the heavy and severeintensity domains.

Altered movement complexity, indicative of system dysfunction, has been demonstrated with increased running velocity and neuromuscular fatigue. The critical velocity (CV) denotes a metabolic and neuromuscular fatigue threshold. It remains unclear whether changes to complexity during running are coupled with the exercise intensity domain in which it is performed. The purpose of this study was to examine whether movement variability and complexity differ exclusively above the CV intensity during running. Ten endurance-trained participants ran at 95%, 100%, 105% and 115% CV for 20min or to task failure, whichever occurred first. Movement at the hip, knee, and ankle were sampled throughout using 3D motion analysis. Complexity of kinematics in the first and last 30s were quantified using sample entropy (SampEn) and detrended fluctuation analysis (DFA-α). Variability was determined using standard deviation (SD). SampEn decreased during all trials in knee flexion/extension and it increased in hip internal/external rotation, whilst DFA-α increased in knee internal/external rotation. SD of ankle plantar/dorsiflexion and inversion/eversion, knee internal/external rotation, and hip flexion/extension and abduction/adduction increased during trials. Hip flexion/extension SampEn values were lowest below CV. DFA-α was lower at higher velocities compared to velocities below CV in ankle plantar/dorsiflexion, hip flexion/extension, hip adduction/abduction, hip internal/external rotation. In hip flexion/extension SD was highest at 115% CV. Changes to kinematic complexity over time are consistent between heavy and severe intensity domains. The findings suggest running above CV results in increased movement complexity and variability, particularly at the hip, during treadmill running.

Characterising crack growth in commercially pure titanium

One of the challenges in aircraft sustainment is to develop additively manufactured (AM) replacement parts for legacy aircraft. This is particularly important for fixed and rotary wing aircraft that operate in in aggressive environments, i.e. off carriers, in a marine environment, etc. On the other hand the United States Air Force (USAF) have now adopted the concept of using AM to rapidly field limited-life unmanned air platforms. Whilst the temptation is to use AM Ti-6Al-4 V for these purposes, Ti-6Al-4 V powder is both costly and its supply is somewhat restricted. This paper reveals that the yield and ultimate strengths, the strain to failure of commercially pure (CP) Titanium, which is highly corrosion resistant, and it’s resistance to crack growth is superior to that of the commonly used aluminium ally AA7050-T7451, which is used in the F/A-18 Hornet, Super Hornet and F-35 (Joint Strike Fighter). Interestingly, when allowance is made for these improved properties then, if the crack growth rate da/dN is expressed as per the Hartman-Schijve crack growth equation, the resultant crack growth curves for Grade 2, 3 and 4 Titanium, and AA7050-T7451 all fall onto (essentially) the same master curve. It is also shown that the effect of different aggressive environments on Grade 3 Titanium is merely to change the fatigue threshold and that, when allowance is made for this, the crack growth curves associated with these different environments also fall onto the same master curve determined for CP Titanium and AA7050-T7451 tested in a laboratory air environment. Consequently, the damage tolerance and durability analyses presented in this paper suggest that CP Titanium may be attractive both for use as replacement parts for many aircraft parts, and for unmanned aerial vehicles (UAV’s).

Temperature dependent fatigue crack growth in forged TiAl alloys with nearly-lamellar and triplex microstructure

Abstract Temperature dependency of fatigue crack growth behavior in β containing forged TiAl alloys is investigated. Two kinds of material with nearly-lamellar and triplex microstructures are employed in fatigue crack growth tests at room temperature, 400 °C and 760 °C. A series of ΔK increasing and decreasing tests exhibits that at room temperature the fatigue crack growth resistance of triplex microstructure is lower than that of nearly-lamellar microstructure, whereas the resistance of triplex becomes higher at 760 °C. This higher resistance of triplex microstructure at high temperature is mainly attributed to the higher volume fraction of ductile β phase. The values of fatigue crack growth threshold in two microstructures show almost comparable at three temperatures, and they reveal the same temperature dependency. The threshold is found to be the highest at 760 °C, while the lowest at 400 °C. This temperature-dependency of fatigue threshold should be related to the roughness-induced and oxidation-induced crack closure mechanisms.

Micromechanisms of a macrocrack propagation behavior affected by short to long fatigue microcracks

The propagation behavior of the macrocrack and microcracks from short to long is investigated based on the Muskhelishvili’s complex potential method and the equivalent crack method. The effect of microcracks on the macrocrack propagation is analyzed. The improved stress intensity factor at the crack tip and the theoretical solution of plastic zone are obtained. The equivalent crack method is applied to compute trajectories of the kinked microcracks. The direction of crack propagation is calculated based on the maximum circumferential tensile stress criterion. Three configurations of microcracks in front of the macrocrack tip are considered: arbitrarily oriented and located microcracks, parallel microcracks, and fan-shaped microcracks. Parallel distributed microcracks allow the macrocrack to propagate at the fatigue threshold ΔK th rapidly, then fan-shaped microcracks, followed by arbitrarily oriented and located microcracks. The results show that the physically short crack can propagate well below the ΔK th while the crack surface is closure-free. Moreover, the closure effect may delay the propagation process of long crack, taking longer for it to coalesce with the microcrack. The results are consistent with the experimental observations and can provide some useful information to predict the fracture or damage behaviors at the macrocrack tip for the material containing microcracks.

Fatigue-resistant adhesion II: Swell tolerance

In a previous paper (Zhang et al., 2020), we have demonstrated an adhesive of high fatigue threshold ∼300 J/m2 using an as-prepared hydrogel of long-chain polymer network. In most applications, however, hydrogel adhesives are in contact with water. The long-chain hydrogel swells and reduces the fatigue threshold. Here we demonstrate a type of swell-tolerant and fatigue-resistant hydrogel adhesive: a long-chain polymer network in topological entanglement with short-chain microgels. The short-chain microgels restrict swell, and the long-chain network retains the high fatigue threshold. After the hydrogel adhesives swell in water to equilibrium, the fatigue threshold is 61 J/m2 for an adhesive of a long-chain network without microgels, and is 262 J/m2 for an adhesive of a long-chain network with microgels.

On the functional connectivity between heart, muscle, and frontal brain cortex during exercise fatigability

We analyze the functional connectivity of the cortico-cardiac-muscular network during muscle fatigability due to exercise. For our experiments, we recruited ten volunteers who performed two cycling routines. Our volunteers were classified according to physical activity level as active or sedentary. First, we measured the electromyography (EMG) signal of the vastus lateralis muscle to find the volunteer’s individualized physical workload capacity at the fatigue threshold. Two to three days later, we measured the volunteers’ EMG, electrocardiography, and electroencephalography (EEG) data during the development of muscle fatigue with the individualized pedaling load previously obtained. We analyzed the EEG to identify the brain rhythm with highest power, then functional connectivity was assessed by computing the pairwise partial directed coherence of our measurements at such rhythm. Finally, we assessed significant changes in functional connectivity from start to finish of the exercise. Our results indicate that the sedentary group showed a significant decrease in connectivity, while the active group showed an increase in left-frontal connectivity. This might be explained in terms of frontal asymmetry phenomena which, for the case of active volunteers, may be a reflection of positive emotional states. Additionally, the active group showed increased self-regulatory processes in heart and muscle, while the sedentary group showed a decreased connectivity from heart to muscle together with a reduction in the muscle’s self-regulation. These last results may be explained by a preference of the body in the sedentary group to attend vital requirements instead of exercise.

Predicting lifetime of optical components with Bayesian inference.

Virtually all optical materials degrade over time when they are used in high average power or intensity optical systems. Extrapolation of optical components lifetime is crucial in such applications in order to avoid downtime or project failure. Measurements of the laser-induced damage threshold (LIDT) fatigue are usually done using the so-called S-on-1 test described in the ISO 21254-2 standard. The standard, however, suggests only rudimentary techniques for extrapolating LIDT, which are rarely used in practice, therefore, the goal of this work was to provide a framework for analyzing LIDT fatigue data using well established methods of Bayesian statistics. Numerical S-on-1 experiments (assuming constant fatigue) were performed for cases of online detection, interval detection and offline detection. Appropriate lifetime distributions were determined and used to fit simulated data taking into consideration data censoring. Credible intervals of lifetime predictions were determined using Markov chain Monte Carlo (MCMC) technique and compared with results from multiple experiments. The Bayesian lifetime analysis method was compared with technique described in the ISO 21254-2 standard for cases of low and high defect densities. Finally, the outlined extrapolation technique was applied to extrapolate lifetime of HR dielectric mirror.

Near-Threshold Fatigue Crack Growth Behavior of ECAPed Ultrafine-Grained Copper

Fatigue crack growth resistance of ultrafine grained Cu processed by equal channel angular pressing (ECAP) was investigated. Particular emphasis was devoted to the effects of microstructure evolution on fatigue crack growth in the near-threshold regime. The ultrafine grained Cu exhibits a lower fatigue threshold than coarse-grained Cu at stress ratios of 0.1 and 0.7. Fatigue induced coarsening of the UFG structure near the fatigue crack and intergranular fatigue crack growth are observed.

Damage-resistant and healable polyacrylonitrile-derived stretchable materials with exceptional fracture toughness and fatigue threshold

PAN-based stretchable materials with high fracture toughness and fatigue threshold.

An engineering approach to estimate fatigue thresholds of wrought and additively manufactured defective metallic materials

The threshold range of the stress intensity factor for long cracks, ΔKth,LC, is a material property of paramount importance in several fracture mechanics-based models to estimate the mechanical thresholds of materials and components affected by short cracks or defects. However, the experimental determination of ΔKth,LC is expensive in terms of time and costs, and the estimating model proposed in the literature often requires material parameters that are not readily available in the design phase or provides quite large approximations. Recently, the authors calibrated an empirical bi-parametric model for evaluating ΔKth,LC for load ratios equal to -1, 0 and 0.5 which is applicable to several wrought as well as additively manufactured alloys. where the only parameters involved are the Vickers hardness (HV) and a properly defined microstructural length l. The model was calibrated using data found in the literature and proved to be effective for estimations of the ΔKth,LC within an error band of ±20% of a quite wide range of wrought and additively manufactured metals.