Increased Fatigue Resistance Research Articles

Gestational age at initial exposure to in utero inflammation influences the extent of diaphragm dysfunction in preterm lambs.

In utero infection may critically influence diaphragm development and predispose preterm infants to postnatal respiratory failure. We aimed to determine how frequency and gestational age (GA) at time of intra-amniotic (IA) lipopolysaccharide (LPS) exposure affects preterm diaphragm function. Pregnant ewes received IA injections of saline or 10-mg LPS at 7 days or 21 days or weekly injections 21, 14 and 7 days before delivery at 121-day GA. Foetal lambs were killed with pentobarbitone (150 mg/kg; intravenous). Diaphragm contractile function was measured in vitro. Muscle fibre type, activation of protein synthesis and degradation pathways, pro-inflammatory signalling and oxidative stress were evaluated using immunofluorescence staining, RT-qPCR, ELISA, Western blotting and biochemical assay. In utero LPS exposure significantly impaired diaphragm contractile function. LPS exposure 7 days before delivery caused maximum specific twitch and tetanic forces 30% lower than controls. When the initial LPS exposure occurred 21 days before delivery maximum specific forces were 40% lower than controls. Earlier LPS exposure also prolonged twitch contraction time, increased fatigue resistance and elevated protein carbonyl content. Despite increased white blood cell counts and interleukin-6 mRNA expression following weekly LPS exposure, there were no significant differences in contractile properties between exposure 21 days before delivery and repeated LPS groups suggesting that frequency of inflammatory exposure does not influence the severity of contractile dysfunction. GA at time of initial LPS exposure, rather than frequency of exposure, determines the extent of inflammation-induced diaphragm dysfunction.

Interpreting the stress ratio effect on delamination growth in composite laminates using the concept of fatigue fracture toughness

This paper provides a study on fatigue delamination growth in composite laminates using energy principles. Experimental data has been obtained from fatigue tests conducted on Double Cantilever Beam (DCB) specimens at various stress ratios. A concept of fatigue fracture toughness is proposed to interpret the stress ratio effect in crack growth. The fatigue fracture toughness is demonstrated to be interface configuration independent but significantly stress ratio dependent. An explanation for this phenomenon is given using SEM fractography. Fracture surface roughness is observed to be similar in different interfaces at the same stress ratio. But it is obviously more rough for high stress ratio in comparison with that for low stress ratio, causing the fatigue resistance increase. Therefore, the stress ratio effect in fatigue crack growth can be physically explained by a difference in resistance to crack growth.

Fatigue limit of induction hardened railway axles

AbstractA new surface induction hardening technology was designed for the purpose of increasing the resistance of railway wheelsets to fatigue damage. This paper gives a detailed presentation of the technological aspects of induction hardening of axles. The increased fatigue resistance in hardened surfaces compared with standard heat treatment of EA4T steel was verified using tensile test specimens, press‐fitted wheel seat/axle joints at 1:3 scale and press‐fitted wheel/axle joints at actual size. The 70% increase in the fatigue limit of induction hardened EA4T steel specimens compared with material subjected to standard heat treatment clearly demonstrates the effectiveness of this technology.

Improvement of Endurance Based on Muscle Fiber-Type Composition by Treatment with Dietary Apple Polyphenols in Rats.

A recent study demonstrated a positive effect of apple polyphenol (APP) intake on muscle endurance of young-adult animals. While an enhancement of lipid metabolism may be responsible, in part, for the improvement, the contributing mechanisms still need clarification. Here we show that an 8-week intake of 5% (w/w) APP in the diet, up-regulates two features related to fiber type: the ratio of myosin heavy chain (MyHC) type IIx/IIb and myoglobin protein expression in plantaris muscle of 9-week-old male Fischer F344 rats compared to pair-fed controls (P < 0.05). Results were demonstrated by our SDS-PAGE system specialized for MyHC isoform separation and western blotting of whole muscles. Animal-growth profiles (food intake, body-weight gain, and internal-organ weights) did not differ between the control and 5% APP-fed animals (n = 9/group). Findings may account for the increase in fatigue resistance of lower hind limb muscles, as evidenced by a slower decline in the maximum isometric planter-flexion torque generated by a 100-s train of electrical stimulation of the tibial nerve. Additionally, the fatigue resistance was lower after 8 weeks of a 0.5% APP diet than after 5% APP, supporting an APP-dose dependency of the shift in fiber-type composition. Therefore, the present study highlights a promising contribution of dietary APP intake to increasing endurance based on fiber-type composition in rat muscle. Results may help in developing a novel strategy for application in animal sciences, and human sports and age-related health sciences.

A statistical approach to understand the role of inclusions on the fatigue resistance of superelastic Nitinol wire and tubing

Superelastic wires and diamond-shaped stent surrogates were manufactured from Nitinol rods and tubing, respectively, from five different mill product suppliers – Standard VAR, Standard VIM, Standard VIM+VAR, Process-Optimized VIM+VAR, and High-Purity VAR. High-cycle fatigue tests up to 107 cycles were conducted under tension–tension conditions for wires and bending conditions for diamonds. These materials were compared under both testing methods at 37°C with 6% prestrain and 3% mean strain (unloading plateau) with a range of alternating strains. The High-Purity VAR material outperformed all alloys tested with a measured 107-fatigue alternating strain limit of 0.32% for wire and 1.75% for diamonds. Process-Optimized VIM+VAR material was only slightly inferior to the High Purity VAR with a diamond alternating bending strain limit of 1.5%. These two “second generation” Nitinol alloys demonstrated approximately a 2× increase in 107-cycle fatigue strain limit compared to all of the Standard-grade Nitinol alloys (VAR, VIM, and VIM+VAR) that demonstrated virtually indistinguishable fatigue performance. This statistically-significant increase in fatigue resistance in the contemporary alloys is ascribed to smaller inclusions in the Process-Optimized VIM+VAR material, and both smaller and fewer inclusions in the High-Purity VAR Nitinol.

HyFlex EDM: superficial features, metallurgical analysis and fatigue resistance of innovative electro discharge machined NiTi rotary instruments.

To evaluate the surface and microstructural alterations of new and used HyFlex EDM prototypes and to test their fatigue resistance. Fifteen HyFlex EDM prototypes were used for in vitro instrumentation of severely curved root canals. Surface and microstructural characteristics of new and used files were compared by ESEM analysis equipped with energy dispersive X-ray spectrophotometry (EDS) and optical metallographic imaging. Usage-induced degradation was assessed. Thirty additional HyFlex EDM prototypes and 20 standard manufactured HyFlex CM files were subjected to cyclic fatigue tests. Time to fracture was recorded, and results were validated using the Kruskal-Wallis test (α-level 0.05). Fatigued files were analysed by ESEM for fractographic evaluation. Surface and microstructural characterization of EDM prototypes revealed the typical spark-machined surface of a NiTi EDM alloy. No fractures were registered during root canal instrumentation. No evident surface alterations and minor degradation were observed between new and used instruments. The metallographic analysis of new and used files disclosed a homogeneous structure, mostly composed of lenticular martensite grains, and some residual austenite. The cyclic fatigue test showed an increase of fatigue resistance up to 700% on the EDM compared to CM files. Spark-machined peculiar surface is the main feature of HyFlex EDM. Low degradation was observed after multiple canal instrumentations. Prototypes exhibited surprising high values of cyclic fatigue resistance and a safe in vitro use in severely curved canals.

Analysis of vacuum tower asphalt extender and effect on bitumen and asphalt properties

Interest has increased related to the impact on asphalt performance of bitumen containing vacuum tower asphalt extender (VTAE), the non-distillable fraction from re-refining used engine oil. This laboratory study provides information on bitumen and asphalt characteristics with and without VTAE. VTAE was blended with PG 64-22 to achieve bitumen grade of PG 58-28. To serve as control a second PG 58-28 was formulated using PG 52-28 with the same PG 64-22. The bitumens were analysed for Superpave PG grade, X-ray fluorescence and polycyclic aromatic compound content, and also using gel permeation chromatography and thermogravimetric analysis. Grade results showed no appreciable difference in ageing between the two PG 58-28 bitumens after Rolling Thin-Film Oven and Pressure Ageing Vessel ageing. The second portion of the study compares asphalt prepared with the two PG 58-28 bitumens including volumetric properties, resistance to moisture damage and rutting, stiffness and fatigue resistance. Fatigue specimens were subjected to short-term and long-term ageing. Asphalt with the VTAE bitumen had equal or increased fatigue resistance in both the short-term and long-term aged conditions. Bitumen was recovered from each ageing condition and tested for binder grade. Comparing the two PG 58-28 bitumens showed no significant difference in stiffness or low temperature properties.

Influence of z-pin length on the delamination fracture toughness and fatigue resistance of pinned composites

The effect of z-pin length on the mode I and mode II delamination toughness and fatigue resistance of z-pinned carbon-epoxy composites is investigated. Experimental testing and mechanical modelling reveals that both the mode I fracture toughness and fatigue resistance increase with the z-pin length due to increased bridging traction loads generated by elastic stretching and pull-out of the pins. The opposite trend occurs for mode II toughness, which decreases with increasing z-pin length due to lower traction loads arising from restrictions on the shear-induced rotation and pull-out of the pins. The mode II fatigue resistance is increased by z-pinning, although it is not dependent on the z-pin length. Increasing the z-pin length beyond a critical size also changes the mode I and mode II delamination fracture and fatigue processes from single to multiple cracking. The effect of z-pin length on the delamination toughening and fatigue strengthening mechanisms is determined.

Effect of equal channel angular extrusion on Al-6063 bending fatigue characteristics

The purpose of this investigation was to refine the grains of annealed 6063 aluminum alloy and to improve its yield stress and ultimate strength. This was accomplished via the equal channel angular extrusion (ECAE) process at a temperature of 200°C using route A, with a constant ram speed of 30 mm/min through a die angle of 90° between the die channels for as many as 6 passes. The experiments were conducted on an Avery universal testing machine. The results showed that the grain diameter decreased from 45 μm to 2.8 μm after 6 extrusion passes. The results also indicated that the major improvement in fatigue resistance occurred after the first pass. The subsequent passes improved the fatigue life but at a considerably lower rate. A maximum increase of 1100% in the case of low applied stresses and an approximately 2200% increase in fatigue resistance in the case of high applied stresses were observed after 5 passes. The improvement of fatigue resistance is presumed to be due to (1) a reduction in the size and the number of Si crystals with increasing number of ECAE passes, (2) the aggregation of Cu during the ECAE process, (3) the formation and growth of CuAl2 grains, and (4) grain refinement of the Al-6063 alloy during the ECAE process.

Glucose concentration and streptomycin alter in vitro muscle function and metabolism.

Cell culture conditions can vary between laboratories and have been optimised for 2D cell culture. In this study, engineered muscle was cultured in 5.5 mM low glucose (LG) or 25 mM high glucose (HG) and in the absence or presence (+S) of streptomycin and the effect on C2C12 tissue-engineered muscle function and metabolism was determined. Following 2 weeks differentiation, streptomycin (3-fold) and LG (0.5-fold) significantly decreased force generation. LG and/or streptomycin resulted in upward and leftward shifts in the force-frequency curve and slowed time-to-peak tension and half-relaxation time. Despite changes in contractile dynamics, no change in myosin isoform was detected. Instead, changes in troponin isoform, calcium sequestering proteins (CSQ and parvalbumin) and the calcium uptake protein SERCA predicted the changes in contractile dynamics. Culturing in LG and/or streptomycin resulted in increased fatigue resistance despite no change in the mitochondrial enzymes SDH, ATPsynthase and cytochrome C. However, LG resulted in increases in the β-oxidation enzymes LCAD and VLCAD and the fatty acid transporter CPT-1, indicative of a greater capacity for fat oxidation. In contrast, HG resulted in increased GLUT4 content and the glycolytic enzyme PFK, indicative of a more glycolytic phenotype. These data suggest that streptomycin has negative effects on force generation and that glucose can be used to shift engineered muscle phenotype via changes in calcium-handling and metabolic proteins.

PO2 Cycling Reduces Diaphragm Fatigue by Attenuating ROS Formation

Prolonged muscle exposure to low PO2 conditions may cause oxidative stress resulting in severe muscular injuries. We hypothesize that PO2 cycling preconditioning, which involves brief cycles of diaphragmatic muscle exposure to a low oxygen level (40 Torr) followed by a high oxygen level (550 Torr), can reduce intracellular reactive oxygen species (ROS) as well as attenuate muscle fatigue in mouse diaphragm under low PO2. Accordingly, dihydrofluorescein (a fluorescent probe) was used to monitor muscular ROS production in real time with confocal microscopy during a lower PO2 condition. In the control group with no PO2 cycling, intracellular ROS formation did not appear during the first 15 min of the low PO2 period. However, after 20 min of low PO2, ROS levels increased significantly by ∼30% compared to baseline, and this increase continued until the end of the 30 min low PO2 condition. Conversely, muscles treated with PO2 cycling showed a complete absence of enhanced fluorescence emission throughout the entire low PO2 period. Furthermore, PO2 cycling-treated diaphragm exhibited increased fatigue resistance during prolonged low PO2 period compared to control. Thus, our data suggest that PO2 cycling mitigates diaphragm fatigue during prolonged low PO2. Although the exact mechanism for this protection remains to be elucidated, it is likely that through limiting excessive ROS levels, PO2 cycling initiates ROS-related antioxidant defenses.

Comparative study of the mode I and mode II delamination fatigue properties of z-pinned aircraft composites

This paper compares improvements to the mode I and mode II delamination fatigue resistance of an aerospace composite material achieved by z-pin reinforcement. Mode I (cyclic crack opening) and mode II (cyclic crack sliding) interlaminar fatigue tests were performed on a carbon fibre reinforced epoxy composite reinforced in the through-thickness direction with different volume contents and diameters of z-pins. Paris curves obtained from displacement-controlled fatigue tests reveal that z-pins are more effective at resisting the initiation and growth of delamination cracks under mode I than mode II conditions. Both the mode I and II fatigue resistance increase with the z-pin content due to the formation of a large-scale extrinsic crack bridging toughening zone, although fatigue strengthening is greater for mode I. Improvements to the mode I and mode II delamination fatigue resistance are also dependent on the z-pin diameter. Similarities and differences in the mode I and mode II fatigue properties are related to the fatigue strengthening mechanisms induced by z-pins for the two load conditions.

Residual Stress Gradients in AISI 9254 Steel Springs Submitted to Shot Peening and Heat Treatment for Increased Fatigue Resistance

The presence of surface and subsurface residual stresses in steel components has a significant influence on fatigue resistance. In the present work, surface modification of AISI 9254 steel coil springs by heat treatment and multiple shot-peening procedures was investigated. Samples were characterized in the as-coiled, quenched, quenched and tempered, as well as submitted to single and double shot peening treatments. Depth resolved residual stress profiles were determined by X-ray diffraction combined with electrolytic dissolution of the steel. Fracture analysis was performed subsequent to fatigue tests by scanning electron microscopy. It was possible to show that double shot peening led to an increase in compressive stresses in the immediate sub-surface region, which improved fatigue resistance relative to the other tested conditions.

Improvements in the microstructure and fatigue behavior of pure copper using equal channel angular extrusion

In this study, annealed pure copper was extruded using equal channel angular extrusion (ECAE) for a maximum of eight passes. The fatigue resistance of extruded specimens was evaluated for different passes and applied stresses using fatigue tests, fractography, and metallography. The mechanical properties of the extruded material were obtained at a tensile test velocity of 0.5 mm/min. It was found that the maximum increase in strength occurred after the 2nd pass. The total increase in ultimate strength after eight passes was 94%. The results of fatigue tests indicated that a significant improvement in fatigue life occurred after the 2nd pass. In subsequent passes, the fatigue life continued to improve but at a considerably lower rate. The improved fatigue life was dependent on the number of passes and applied stresses. For low stresses (or high-cycle fatigue), a maximum increase in fatigue resistance of approximately 500% was observed for the extruded material after eight passes, whereas a maximum fatigue resistance of 5000% was obtained for high-applied stresses (or low-cycle fatigue). Optical microscopic examinations revealed grain refinements in the range of 32 to 4 μm. A maximum increase in impact energy absorption of 100% was achieved after eight passes. Consistent results were obtained from fractography and metallography examinations of the extruded material during fatigue tests.

Increase of fatigue resistance of sheet welded joints of aluminum alloys using high-frequency peening

The Paton Welding Journal, 2014, №05. International Scientific-Technical and Production Journal «The Paton Welding Journal» «The Paton Welding Journal» has been published monthly since 2000 in English, ISSN 0957-798X. «The Paton Welding Journal» is a cover-to-cover English translation of the «Avtomaticheskaya Svarka» (Automatic Welding) journal. The «Avtomaticheskaya Svarka» journal has been published monthly since 1948 in Russian, ISSN 005-111X.

Regulation of glycogen breakdown and its consequences for skeletal muscle function after training.

Repeated bouts of physical exercise, i.e., training, induce mitochondrial biogenesis and result in improved physical performance and attenuation of glycogen breakdown during submaximal exercise. It has been suggested that as a consequence of the increased mitochondrial volume, a smaller degree of metabolic stress (e.g., smaller increases in ADP and Pi) is required to maintain mitochondrial respiration in the trained state during exercise at the same absolute intensity. The lower degree of Pi accumulation is believed to account for the diminished glycogen breakdown, since Pi is a substrate for glycogen phosphorylase, the rate-limiting enzyme for glycogenolysis. However, in this review, we present an alternative explanation for the diminished glycogen breakdown. Thus, the lower degree of metabolic stress after training is also associated with smaller increases in AMP (free concentration during contraction at specific intracellular sites) and this results in less activation of phosphorylase b (the non-phosphorylated form of phosphorylase), resulting in diminished glycogen breakdown. Concomitantly, the smaller accumulation of Pi, which interferes with cross-bridge function and intracellular Ca(2+) handling, contributes to the increased fatigue resistance. The delay in glycogen depletion also contributes to enhanced performance during prolonged exercise by functioning as an energy reserve.

Laser Hardening Parameters Influencing Component Lifetime and Residual Stresses

Laser surface hardening is an advanced method of surface treatment of structural steels with a great potential for wide industrial applications. The technology is quite new and so, investigations have to be performed in order to gain a comprehensive knowledge about effects on microstructure, hardness, surface properties of treated materials, but also mechanical an particularly fatigue properties. Concerning fatigue resistance of material treated with this technology, results and knowledge recently published in the literature indicate that fatigue resistance can be either reduced or increased, even considerably, depending on numerous parameters of basic material, laser hardening parameters etc. This contribution contains results of a partial study of effect of laser hardening of relatively small specimens on fatigue resistance of 42CrMo4 steel. Two different parameters of the treatment were used, namely two speeds of laser beam on the material surface at constant beam energy. Unlike the lower speed, when fatigue resistance was slightly reduced, higher speed of laser beam resulted in a slight increase of fatigue resistance and fatigue limit. The results are discussed considering an occurrence of residuals stresses. Key words: Laser hardening, residual stress, lifetime, fatigue, fracture, microstructure, surface

ANGLE-SPECIFIC CHANGES IN ECCENTRIC HAMSTRING TORQUE AND HAMSTRING TO QUAD RATIO FOLLOWING SIMULATED SOCCER

BackgroundSoccer-specific fatigue decreases eccentric peak torque and the functional peak hamstring to quadriceps torque ratio (fHecc:Qcon), changes thought to increase the risk of hamstring strain (HS). Inadequate eccentric muscle strength...

Structure and Properties of Irradiation Crosslinked UHMWPE

Irradiation crosslinking is one of the most important methods to modify UHMWPE, it can effectively improve the hardness, creep resistance and abrasion resistance of this material. In order to eliminate free radical, increase material strength, abrasion resistance and fatigue resistance of creep, this study uses Co60-γ to modify UHMWPE under vacuum atmosphere, with the irradiation dose of 30, 60, 90, 120 Kgy respectively. Its mechanical properties, microstructure, degree of crosslinking and crystallinity are analyzed, the results show that the modified UHMWPEs impact strength decreased, elongation basically remain unchanged, tensile fracture strength and tensile yield strength increased.

Age-induced muscle atrophy and increase in fatigue resistance

Age-induced muscle atrophy and increase in fatigue resistance