Enhanced Fatigue Resistance Research Articles

Neurogranin is expressed in mammalian skeletal muscle and inhibits calcineurin signaling and myoblast fusion.

Calcineurin is a Ca2+/calmodulin (CaM)-dependent phosphatase that plays a critical role in promoting the slow fiber phenotype and myoblast fusion in skeletal muscle, thereby making calcineurin an attractive cellular target for enhancing fatigue resistance, muscle metabolism, and muscle repair. Neurogranin (Ng) is a CaM-binding protein thought to be expressed solely in brain and neurons, where it inhibits calcineurin signaling by sequestering CaM, thus lowering its cellular availability. Here, we demonstrate for the first time the expression of Ng protein and mRNA in mammalian skeletal muscle. Both protein and mRNA levels are greater in slow-oxidative compared with fast-glycolytic muscles. Coimmunoprecipitation of CaM with Ng in homogenates of C2C12 myotubes, mouse soleus, and human vastus lateralis suggests that these proteins physically interact. To determine whether Ng inhibits calcineurin signaling in muscle, we used Ng siRNA with C2C12 myotubes to reduce Ng protein levels by 60%. As a result of reduced Ng expression, C2C12 myotubes had enhanced CaM-calcineurin binding and calcineurin signaling as indicated by reduced phosphorylation of nuclear factor of activated T cells and increased utrophin mRNA. In addition, calcineurin signaling affects the expression of myogenin and stabilin-2, which are involved in myogenic differentiation and myoblast fusion, respectively. Here, we found that both myogenin and stabilin-2 were significantly elevated by Ng siRNA in C2C12 cells, concomitantly with an increased fusion index. Taken together, these results demonstrate the expression of Ng in mammalian skeletal muscle where it appears to be a novel regulator of calcineurin signaling.

Fatigue strength prediction methodology of shot-peened materials

Shot peening is used to enhance fatigue strength, but it is usually neglected by manufacturing design. The main purpose of the present work is to suggest strategies capable of making connection between the process conditions and the fatigue strength. In the present paper, fatigue lives of shot-peened parts were predicted. A new multiaxial HCF model, based on the Crossland criterion, was proposed; it integrates the favourable effects of shot peening that are the combined effect of compressive residual stresses and strain hardening. The plastic strain field accounts for the strain hardening. In order to compute residual stress and equivalent plastic strain profiles due to the repeated impacts, we have modelled and simulated, through a finite element method, the impact of the shot dynamics resulting from the process. The relaxation of induced compressive residual stresses, over the fatigue life, has been considered. For the purpose of spotlighting the capability of the present methodology, the predictions were compared with existing experimental data. This study demonstrates that the single effect of stabilized residual stress is not sufficient to explain the full enhancement of fatigue resistance. The proposed fatigue criterion provides a conservative prediction in terms of endurance limit.

Effect of ultrasonic shot peening on the surface defects of thin struts built by electron beam melting: Consequences on fatigue resistance

Ti-6Al-4V thin struts, built by selective electron beam melting have been submitted to hot isostatic pressing and further processed with ultrasonic shot peening to investigate the effect of this surface treatment on surface defects. The consequence of those surface defects on the fatigue resistance of thin struts has been evaluated before and after treatment. X-ray microtomography has been used to track the defect population with repeated scans before and after ultrasonic shot peening as well as after fatigue tests conducted to failure. Our results show that the fatigue resistance of surface-treated struts (measured at 105 cycles) is doubled compared to the fatigue resistance of struts with an as-built surface. This enhancement in fatigue resistance is attributed to a refinement of the sub-surface microstructure, to the introduction of local compressive residual stresses and to a significant surface smoothing effect induced by ultrasonic shot peening. Ultrasonic shot peening fails to “heal” the most tortuous surface defects observed in the as-built condition, leaving residual defects in the sub-surface region. The presence of those residual surface defects limits the fatigue resistance when compared to struts with a machined surface.

Aesthetic and Mechanical Suitability of a Clear Synthetic Resin as a Unconventional Binder for Road Pavements

Current environmental awareness interests several aspects of civil engineering, including road construction. Indeed, new challenges related to environmental pollution and landscape preservation must be faced. In this sense, clear road pavement surfaces represent an effective technology aimed at guaranteeing environmental-friendly aesthetic pavements. The use of clear synthetic resin as a binder involves several benefits for the mitigation of in-service reached temperatures and the heat distribution within pavements (with appreciable effects on pavement mechanical performance too). The present paper illustrates an experimental study aimed at analysing the chromatic and mechanical properties of a clear synthetic resin and thus its suitability as a binder for road pavement mixes. Chromatic characteristics were assessed through digital image analysis at different aging conditions. A dynamic shear rheometer was used to evaluate the linear viscoelastic properties as well as fatigue and rutting potential of the binder in a wide range of temperatures and frequencies. A conventional 35/50 penetration grade bitumen was also investigated for comparison purposes. The clear resin exhibited limited changes in colour (darkening effects), mainly in the case of short-term aging. On the other hand, a low temperature-dependency of such a binder was observed up to 58°C. Slightly increased aptitude to rutting at the higher temperatures was detected, even if it is worth noting that clear in-service mixtures would achieve lower temperatures than traditional “black” materials at a given environmental condition (air temperature, solar radiation, etc.). The resin also exhibited a softer behaviour, along with an enhanced fatigue resistance. Overall, the studied innovative binder showed promising results in view of its effective use in road paving.

Formation of a hydroxyapatite layer on Ti–29Nb–13Ta–4.6Zr and enhancement of four-point bending fatigue characteristics by fine particle peening

A hydroxyapatite (HAp) surface layer was generated at room temperature on a beta-titanium alloy (Ti-29Nb-13Ta-4.6Zr, TNTZ) using fine particle peening (FPP), as a means of enhancing fatigue resistance. This material is commonly employed in bio-implants, but has a suboptimal Young's modulus. Treated specimens were assessed using scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and electron backscatter diffraction, as well as by measuring the Vickers hardness. Performing FPP using shot particles made of HAp was found to transfer the particles to the TNTZ surface to rapidly produce a HAp layer that became thicker as the FPP process progressed. Room-temperature four-point bending fatigue tests in air showed that the HAp layer resulting from the FPP process increased the endurance limit of test specimens. This improved performance is ascribed to the appearance of a hardened surface layer having a significant degree of compressive residual stress.

Short-Term Back Squat Protocol Effect on 5km Run Performance

PURPOSE: Previous research indicates conflicting data on concurrent training. For instance, high intensity strength training, an uncommon training method for runners, has the potential to be beneficial as studies show it allows for enhanced fatigue resistance during high intensity endurance performance. Conversely, some evidence suggests high intensity strength training is harmful for endurance running performance and may alter fuel substrate utilization. Therefore, this study sought to determine the effects of a short-term, high repetition back squat training protocol on 5km run performance as well as on carbohydrate and fat oxidation rates. METHODS: Fifteen runners [4 men, 11 women; 150+ minutes of endurance exercise per week; age = 22 + 5.1y; 20.4 + 5.2 body mass index] completed two weeks of a high repetition back squat training protocol consisting of three sets of 15-24 repetitions at 60% of one-repetition max (1RM), three times per week. Pre- and post-tests included a 5km timed run on an outdoor track, respiratory exchange ratio (RER) through indirect calorimetry during two different intensities of steady-state treadmill exercise (60% and 70% heart rate max (HRmax)), and 1RM for back squats. RESULTS: Back squat 1RM significantly increased by 15% with training (48.0+27.9 to 56.7+30 kg, P = <0.001). However, no statistically significant differences were found in 5km times adjusted for heat-stress of outdoor conditions (Pre: 21.6+8.3 vs. Post: 21.1+8.0 minutes, P = 0.20), which potentially had a significant confounding effect on post-training outcomes. RER at 60% HRmax (Pre: 0.81+0.04 vs. Post: 0.79+0.05, P = 0.45) and 70% HRmax (Pre: 0.84+0.04 vs. 0.85+0.04, P = 0.85) was unchanged. CONCLUSIONS: Performing a high repetition back squat protocol for two weeks does not appear to impact 5km running time or alter the body’s fuel utilization during exercise but is an effective way to improve lower body strength. Further studies are necessary in a temperature-regulated testing environment to determine whether high repetition strength training alters 5km running performance and fuel utilization.

Extremely high fatigue resistance in an ultrafine grained high entropy alloy

Fatigue of structural materials is a leading cause of failure and has motivated fatigue-resistant design to eliminate risks to human lives. Majority of conventional materials, including advanced steels and Ni-based superalloys, exhibit limited fatigue endurance limits at ambient temperature. With this in mind, we designed a Cu-containing FeMnCoCrSi high entropy alloy which exhibits a normalized fatigue ratio of ∼0.62UTS (ultimate tensile strength). Our design approach was based on (a) engineering the γ phase stability to attain sustained work hardening (WH) through delayed γ (f.c.c.)→ɛ (h.c.p.) transformation to hinder fatigue crack propagation and (b) an ultrafine grained (UFG) microstructure to delay crack initiation. We verified that a UFG γ dominant microstructure could provide opportunities for enhanced fatigue resistance, as sustained WH activity strengthened the material locally in the crack plastic zone, thereby validating our expectation that the combination of UFG and transformation induced plasticity (TRIP) is a path to design the next generation of fatigue-resistant alloys.

Excellent superelasticity of Cu-Al-Mn-Cr shape memory single crystal obtained only through annealing cast polycrystalline alloy

The polycrystalline Cu-13.0Al-7.7Mn-0.6Cr cast alloy consists of the Heusler L21 (Cu2AlMn) parent and the disordered body-centered cubic A2(Cr) precipitate due to bcc phase separation. These two phases are completely coherent. When annealing cast alloy, the abnormal grain growth occurs. It is closely related to the dissolution of A2(Cr) particles and the formation of a continuous misorientation gradient within the matrix grains. The results show that the single crystal close to [100] orientation shows large superelastic strain of 8.5%. Furthermore, the single crystal almost remains full superelastic characteristic after 1.2 × 103 cycles, showing enhanced fatigue resistance.

Enhancement of fatigue resistance using the accelerated diffusion/sintering phenomenon near beta transus temperature in Ti-6Al-4V powder metallurgy alloy

Rapid sintering at the beta-transus temperature (1010 °C) of a titanium alloy (Ti-6Al-4V), by exploiting the accelerated self-diffusion, is shown here to yield a high density (~98%), and a refined microstructure, relative to that obtained at a higher sintering temperature in powder-metallurgical processing. While saving energy and cost, this approach is shown to produce a nearly equivalent tensile stress-strain behavior, and, strikingly, a significant improvement in overall SN (stress-life) fatigue resistance, relative to that of the alloys sintered at a relatively higher temperature. This new approach may be broadly applicable to all titanium alloys exhibiting alpha-to-beta phase transformation.

Self‐recovery and fatigue of double‐network gels with permanent and reversible bonds

ABSTRACTDouble‐network (DN) gels subjected to cyclic deformation (stretching up to a fixed strain followed by retraction down to the zero stress) demonstrate a monotonic decrease in strain with time (self‐recovery). Observations show that the duration of total recovery varies in a wide interval (from a few minutes to several days depending on composition of the gel), and this time is strongly affected by deformation history. A model is developed for the kinetics of self‐recovery. Its ability to describe stress–strain diagrams in cyclic tests with various periods of recovery is confirmed by comparison with observations on several DN gels. Numerical simulation reveals pronounced enhancement of fatigue resistance in multi‐cycle tests with stress‐ and strain‐controlled programs when subsequent cycles of deformation are interrupted by intervals of recovery. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2019,57, 438–453

The HDAC3 enzymatic activity regulates skeletal muscle fuel metabolism.

Histone deacetylase 3 (HDAC3) is a major HDAC, whose enzymatic activity is targeted by small molecule inhibitors for treating a variety of conditions. However, its enzymatic activity is largely dispensable for its function in embryonic development and hepatic lipid metabolism. HDAC3 plays a pivotal role in regulating muscle fuel metabolism and contractile function. Here, we address whether these muscular functions of HDAC3 require its enzymatic activity. By mutating the NCoR/SMRT corepressors in a knock-in mouse model named NS-DADm, we ablated the enzymatic activity of HDAC3 without affecting its protein levels. Compared to the control mice, skeletal muscles from NS-DADm mice showed lower force generation, enhanced fatigue resistance, enhanced fatty acid oxidation, reduced glucose uptake during exercise, upregulated expression of metabolic genes involved in branched-chain amino acids catabolism, and reduced muscle mass during aging, without changes in the muscle fiber-type composition or mitochondrial protein content. These muscular phenotypes are similar to those observed in the HDAC3-depleted skeletal muscles, which demonstrates that, unlike that in the liver or embryonic development, the metabolic function of HDAC3 in skeletal muscles requires its enzymatic activity. These results suggest that drugs specifically targeting HDAC3 enzyme activity could be developed and tested to modulate muscle energy metabolism and exercise performance.

Microscopic Insight into Electric Fatigue Resistance and Thermally Stable Piezoelectric Properties of (K,Na)NbO3-Based Ceramics.

Pb-Based piezoelectric materials such as Pb(Zr,Ti)O3 have been the mainstay for electromechanical devices; however, they encounter the challenge of sustainable environmental development, which requires Pb-free piezoelectric counterparts. The foremost obstacles in developing Pb-free piezoceramics are their low piezoresponse and inferior temperature stability. In this work, we reported Mn-modified ⟨00 l⟩c-textured (K,Na)NbO3-based ceramics to achieve thermally stable piezoelectric properties and enhanced fatigue resistance in conjunction with high piezoelectricity of d33 ∼ 560 pC/N. The in situ d33 measurement reveals that the temperature stability of the small signal d33 is mainly dependent on temperature-induced phase transition. However, the local piezoresponse force microscopy measurements imply that the excellent temperature stability of the large signal d33* (field-induced strain) benefits from the stable domain response to applied electric field at elevated temperatures. Moreover, the good fatigue resistance is proposed to be associated with the decreased defect concentration by Mn doping, based on the analyses of dielectric loss, leakage current, and thermally stimulated depolarization current.

Effect of heat treatment on the phase transformation and mechanical properties of Ti6Al4V fabricated by selective laser melting

In this work, Ti6Al4V ELI samples were fabricated with previously optimized parameters by selective laser melting (SLM) and then heat-treated under vacuum (HT) or hot isostatic pressed (HIP). Optical microscopy (OM) and scanning electron microscopy (SEM) were used to characterize the microstructure. As-built microstructure was found to be dominated by columnar grains and acicular α′ martensite. Thermo-mechanical treatments allowed to modify the microstructure in the size and shape of the grains, as well in the volume fraction and chemical composition of the phases. Apart from the phase transformation, HIP treatment also allowed to significantly reduce the interior defects, such as pores and internal cracks. Tensile and fatigue properties of the Ti6Al4V ELI samples corresponding to the different microstructures were subsequently investigated. As-built samples exhibited a high tensile strength but a poor ductility with an elongation generally smaller than 6% and low fatigue resistance. HT samples displayed a reduced tensile strength but improved elongation behavior and relatively enhanced fatigue resistance versus as-built samples. The HIP treatment also resulted in a reduction in strength but improved furthermore the ductility and the fatigue behavior due to the elimination of internal defects.

Enhanced fatigue resistance of suppressed hysteresis in perovskite solar cells by an organic crosslinker

With record power conversion efficiencies of hybrid perovskite solar cells now exceeding 20% under laboratory conditions, improvements in stability of the cells under real-world working conditions are now key requirements for their commercial success. Here, we present a novel strategy to reduce penetration of humidity and oxygen into perovskite films via incorporation of a diammonium glycol. The two ammonium groups of this molecule allow it to serve as a crosslinker in the structure, bridging two unit cells within a crystallite or even across a grain boundary. In a planar heterojunction solar cell containing PCBM as an electron transport layer, the power conversion efficiency of the cell with ~0.1% diammonium glycol in the absorber layer was 13.96%, slightly exceeding that of the glycol-free device (13.53%). Most importantly, the glycol-free device exhibited the typical growth in hysteresis with performance degradation, but hysteresis remained suppressed in the device doped with diammonium glycol, even as its overall performance deteriorated. Futhermore, the chemical stability of the unpackaged device under continuous AM1.5G illumination at ambient conditions was substantially improved relative to the glycol-free device. Formation of PbI2 was significantly suppressed, which could minimize release of toxic Pb ions.

Enhancement of fatigue resistance for 316L welds produced by magnetic field assisted laser-MIG hybrid welding

Crack growth rate (CGR) in air for welds produced by 24mT magnetic field assisted laser-MIG hybrid welding was decreased by 33%. CGR in 3.5% NaCl solution for welds produced by 16 mT magnetic field assisted welding was reduced by 20%. Texture variation in welds was investigated through electron backscattered diffraction (EBSD). External magnetic field lowered thermal gradient and changed orientations of maximum thermal gradient (∇Tmax), resulting in weaker texture intensity and less ferrite. Reduced intensity of {100} planes and 〈100〉 orientations improved resistance to cyclic plastic deformation. Reduced ferrite increased the volume of strain-induced martensite at crack tip, improving fatigue cracking resistance.

Enhanced bipolar fatigue resistance in PMN-PZT ceramics prepared by spark plasma sintering

The bipolar fatigue behaviors of lead magnesium niobate-lead zirconate titanate (PMN-PZT) ceramics sintered by conventional sintering (CS) and spark plasma sintering (SPS) were systematically investigated. Significantly enhanced bipolar fatigue resistance was observed for SPS samples by a comparative analysis of the evolution of both large signal (polarization, strain) and small signal (piezoelectric coefficient, permittivity) hysteresis curves. The enhanced fatigue resistance is not only attributed to the suppressed development of microcracks, which is due to the decrease in grain size and porosity, but is also related to the reduction of domain wall pinning effect induced by migratory point defects, especially oxygen vacancies. Besides, the different phase structure and its evolution upon poling and fatigue are also responsible for the enhanced fatigue resistance.

Role of Graphene in Reducing Fatigue Damage in Cu/Gr Nanolayered Composite.

Nanoscale metal/graphene nanolayered composite is known to have ultrahigh strength as the graphene effectively blocks dislocations from penetrating through the metal/graphene interface. The same graphene interface, which has a strong sp2 bonding, can simultaneously serve as an effective interface for deflecting the fatigue cracks that are generated under cyclic bendings. In this study, Cu/Gr composite with repeat layer spacing of 100 nm was tested for bending fatigue at 1.6% and 3.1% strain up to 1,000,000 cycles that showed for the first time a 5-6 times enhancement in fatigue resistance compared to the conventional Cu thin film. Fatigue cracks that are generated within the Cu layer were stopped by the graphene interface, which are evidenced by cross-sectional scanning electron microscopy and transmission electron microscopy images. Molecular dynamics simulations for uniaxial tension of Cu/Gr showed limited accumulation of dislocations at the film/substrate interface, which makes the fatigue crack formation and propagation through thickness of the film difficult in this materials system.

Enhancing fatigue resistance and damage characterisation in adhesively-bonded composite joints by carbon nanofibres

In the present work we report on the use of carbon nanofibres (CNFs) to simultaneously improve the cyclic fatigue resistance and the detectability of disbonding in adhesively-bonded structures made of carbon-fibre reinforced-plastic (CFRP) composites. The effects of the concentration of the CNFs (i.e. 0.4, 0.7 and 1.0 wt%) and their orientation (i.e. random versus aligned) in the epoxy-adhesive layer between two CFRP substrates are investigated. The results show that increasing the concentration of randomly-oriented CNFs (a) improves greatly the mode I fatigue resistance of the adhesive layer, including raising the crack growth threshold of the cyclic strain-energy release-rate, and (b) increases the quasi-static fracture toughness. Further improvements in the fatigue resistance occur when the CNFs are aligned perpendicular to the plane of the joint, i.e. normal to the crack plane, as opposed to being randomly-oriented in the adhesive layer. In addition, the CNFs form a conductive network that makes it possible to detect and characterise fatigue-induced disbonding using an electrical-resistance technique. A simple model is developed for the relationship between the disbond (i.e. crack size) and the electrical resistance of a bonded joint with conductive substrates. Finite element analyses are carried out to quantify the applicability of this model as a function of the conductivity of the adhesive from 10−4 S/m to 1 S/m. The results confirm that the proposed simple model is highly accurate for joints where the composite substrates have a through-thickness electrical conductivity exceeding a hundred times that of the adhesive. This research paves the way for new multi-functional adhesives with greatly enhanced fatigue resistance and disbond detection capability.

Multicolor Photochromism of Fulgide Mixed Crystals with Enhanced Fatigue Resistance

Two photochromic mixed fulgide single crystal systems showing multicolor photochromism and enhanced fatigue resistance were prepared with isostructural molecular pairs containing methyl-chloro and methyl-bromo groups. Parent crystals p-methylacetophenylisopropylidenesuccinic anhydride (1E), p-chloroacetophenylisopropylidenesuccinic anhydride (2E), and p-bromoacetophenylisopropylidenesuccinic anhydride (3E) showed light magenta, brownish orange, and orange colors, respectively, upon irradiation at 365 nm. Each two component mixed crystal MIX-1E and MIX-2E composed of 1E/2E and 1E/3E in a 0.39:0.61 and 0.88:0.12 ratio, showed four different colors upon irradiation with UV light and visible light of appropriate wavelengths. Further, it was found that colors of mixed crystals can be fine-tuned to impart different shades of colors over a range of four distinct colors shown by each, upon changing the length of the time irradiated at selected wavelengths. Such multicolored photochromic systems can be used to dev...

Structural life enhancement on friction stir welded AA6061 with optimized process and HFMI/PIT parameters

This novel study presents an unconventional approach to find the best governing process parameters of high frequency mechanical impact technique based on multi-objective optimization method. In this investigation, the post-weld mechanical treatment is aimed to enhance fatigue resistance of structural friction-stirred weld subjected to fluctuating loads by obtaining nominal sub-surface hardness. The experimental study was conducted on aluminium alloy AA 6061 with thickness of 6 mm under varied parameters centred on indenter diameter, air pressure and impact frequency. The investigation began with obtaining optimum parameters for single response by using conventional Taguchi method with L9 orthogonal array. Next, advanced optimization approach by means of multi-objective Taguchi method attempts to consider the multiple responses simultaneously which are sub-surface hardness and structural life. As the final results, the optimum value was acquired by calculating the total normalized quality loss and multiple signal-to-noise ratios based on unequal desirability. The significant level of the parameters was evaluated by using analysis of variance. Furthermore, the second-order model for predicting the objectives was derived by applying response surface methodology. It can be summarized that, first, the affecting parameters to obtain superior structural life can be ordered at significant level of ca. 65, 25 and 10% for air pressure, impact frequency and indenter diameter, respectively. Secondly, using subsequent post-weld mechanical treatment, the life cycle number can be extended up to 12 times on friction-stirred weld. Finally, based on the experimental confirmation test, the proposed method can effectively estimate the structural life and surface hardness within the acceptable range of relative error.