Browsing by Subject "fatigue"
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Item Characterization of Fatigue Cracking and Healing of Asphalt Mixtures(2012-07-16) Luo, XueFatigue cracking is one of the most common distresses of asphalt pavements, whereas healing is a counter process to cracking which alleviates cracking damage and extends fatigue life of asphalt pavements. Most of existing methods to characterize fatigue cracking and healing are generally empirical or phenomenological in nature, which does not satisfy the need to develop mechanistic-based pavement design methods. The objective of this study is to characterize fatigue cracking and healing of asphalt mixtures using an energy-based mechanistic approach. A controlled-strain repeated direct tension (RDT) test is selected to generate both fatigue cracking and permanent deformation in an asphalt mixture specimen. Fatigue cracking is separated from permanent deformation from a mechanical viewpoint. The development of fatigue cracking is described by the evolution of the damage density and the increase of the average crack size with the increase of loading cycles. A creep and step-loading recovery (CSR) test is designed to measure the internal stress in the recovery phase of an asphalt mixture specimen. The internal stress and the strain measured in the recovery phase are used to conduct the mechanistic analysis of recovery and healing of the asphalt mixture specimen. Then healing is described using the decrease of the damage density and average crack size with time. Different types of asphalt mixtures produce distinctly different fatigue cracking and healing characteristics. The effect of mixture composition, temperature, and aging are evaluated using the approach above. The entire series of tests for fatigue, permanent deformation and healing can be completed in one day, with the healing part requiring only a matter of minutes. The methods proposed in this study characterize fatigue cracking and healing of asphalt mixtures using its essential cause and effect relationship.Item Disaster relief: Fatugue and countermeasures(2007-09-04) Sean Andrew Hollonbeck; Sheryl Bishop; Richard Jennings; C. Joan RichardsonThe emphasis of this paper has been to educate disaster leaders on fatigue and fatigue management. The need for sleep is real, inescapable and often misunderstood. The impact of fatigue on performance is greatly magnified when individuals have to operate under conditions of high emotional, psychological or physiological stress – all inherent conditions for disaster response teams. Fatigue can clearly increase the risk of fatalities and injuries. Fatigue in disaster relief workers is an unstudied and critical safety issue in the complex process of disaster management and relief. This paper is designed for leaders in disaster agencies and management as guide to understanding the problem of fatigue in the austere uncontrolled chaos of a disaster event and to be able to implement effective scientific countermeasures to ensure mission success. \r\nThe National Transportation Safety Board (NTSB) has found that the incidence of fatigue is underestimated in virtually every transportation mode, because it is so hard to quantify and measure. Many accident investigations do not obtain the information necessary to determine the contribution of fatigue; namely, the condition of the workers, the extent to which they have been deprived of sleep, and their state of alertness. \r\nThis report will show through studied “best practices” in areas of industry (the military, medicine, the transportation industry and aviation) that the un-researched hazard of fatigue during disasters exists and more importantly by comparing and review these other areas the reader will be prepared address the challenge of severe decrements in cognitive and physical performance caused by fatigue. The outcome is to educate disaster relief leaders about fatigue, human fatigue physiology, the risks and hazards of fatigue as well as countermeasures to fatigue. Then armed with this new knowledge disaster leaders will be empowered to make effective decisions and establish policy and doctrine with a resulting positive impact on disaster relief safety.Item Effect of Zolpidem, Zaleplon, and Ramelton on cognitive functioning after awakening from napping(2008-04-21) RIchard W. Cole; Richard Jennings; Strahil Atanasov; James VanderploegNon-benzodiazepine sedative hypnotics and melatonin receptor agonist hypnotics induce the onset of sleep and are primarily prescribed for the treatment of insomnia. Although, non-benzodiazepine sedative hypnotic drugs zaleplon (Sonata) and zolpidem (Ambien) are not chemically like benzodiazepines, they induce sleep by binding to the same gamma-aminobutyric acid (GABA) receptors in the central nervous system. They may be less likely than benzodiazepine medications to disrupt natural sleep rhythm and patterns which may make sleep more restful. Ramelteon (Rozerem) is a new category of sleep medications that bind to the melatonin receptor in the suprachiasmatic nucleus. \r\nCognitive performance following a “full night’s” rest after taking these medications has been more thoroughly studied than performance decrements should return to duty be required. These hypnotics may be used to induce sleep in circumstances not ideal for rest (shift work, noisy environment, short period available during the day, etc.). Because these three drugs have a rapid onset and short half-life (one hour for zaleplon, 2.5 hours for zolpidem and 2.5 hours for ramelteon), they have the potential to be utilized in individuals that need assistance with sleep latency, but might need to wake up before a “full night’s” rest to perform critical tasks.\r\nThis project reviews the literature regarding the effectiveness of zaleplon, zolpidem, or ramelteon in inducing sleep and the effects on cognitive functioning and performance decrements within eight hours after use.Item Fatigue behavior of alpha-zirconium phosphate/epoxy nanocomposites(Texas A&M University, 2006-04-12) Varadharajan, Balaji R.Fatigue crack growth in ?-Zirconium phosphate/epoxy nanocomposites was investigated. A new fatigue testing technique was implemented for miniature samples. Two different methods ?strength of materials and Rayleigh-Ritz - were used in determining the bending stress. The fatigue stress and fatigue life of different nanocomposite specimens were plotted in a traditional stress-life (S-N) curve. It was inferred from the S-N plot that the values obtained from both the methods compare well. The experimental results showed that fatigue life of filled epoxy nanocomposite is more than that of the unfilled epoxy composite. A model for bending stresses, ultimate strength and the number of cycles to failure was obtained to predict a component service life without conducting elaborate tests. Scanning electron examination of the fractured surfaces revealed that the crack takes a tortuous path during its propagation course, indicating crack blunting and crack deflection roles of ZrP and CSR nanofillers, which consequently improve the fracture resistance. In case of the M-ZrP-epoxy systems, delamination of ZrP platelets from surrounding epoxy matrix was proposed as the reason behind crack growth. The improved fracture resistance of these nanocomposites was attributed to the delamination of ZrP platelets and deflection of crack direction. The superior behavior of CSR-ZrP-epoxy composites was attributed to the cavitation process and void coalescence due to CSR particles delamination.Item Fatigue resistance of hot-mix asphalt concrete (HMAC) mixtures using the calibrated mechanistic with surface energy (CMSE) measurements approach(Texas A&M University, 2006-10-30) Ofori-Abebresse, Edward KwameFatigue cracking is one of the fundamental distresses that occur in the life of a Hot Mix Asphalt Concrete (HMAC) pavement. This load induced distress leads to structural collapse of the entire pavement ultimately and can only be remedied by rehabilitation. There is the need, therefore, for a total understanding of the phenomenon to be able to counter its occurrence. The fatigue resistance of hot mix asphalt concrete (HMAC) has been estimated using approaches ranging from empirical methods to mechanistic-empirical methods to purely mechanistic methods. A continuum mechanics based approach called the Calibrated Mechanistic with Surface Energy (CMSE) measurements was developed at Texas A&M University and recommended after comparison with other approaches in predicting fatigue lives of two Texas HMAC mixtures. The CMSE approach which includes fundamental material properties such as fracture, aging, healing, and anisotropy has been shown to effectively model the parameters that affect the performance of HMAC pavements exposed to repetitive traffic loads. Polymer modified asphalt (PMA) improves pavement performance by providing additional resistance to the primary distresses in flexible pavements, including permanent deformation or rutting, thermal cracking, and fatigue cracking. In this research, the CMSE approach was utilized to estimate the fatigue resistance of HMAC fabricated with asphalts modified with Styrene-butadiene-Styrene (SBS) co-block polymer. These HMAC mixtures were fabricated from materials used on three different road sections in Texas and one test pavement in Minnesota. The CMSE approach was validated as an effective approach for estimating the fatigue resistance of HMAC mixtures with PMA. The effect of oxidative aging on the fatigue resistance of the HMAC mixtures was also verified. Oxidative aging of the mixtures resulted in a corresponding decrease in mixture fatigue resistance. In addition, for two HMAC mixtures with the same binder content and aggregate gradation, the mixture with the softer of the two Performance Grade (PG) binders exhibited greater fatigue resistance. The use of the Utility Theory revealed the possible effects of aggregate geometric properties on the HMAC mixture properties and consequently on their fatigue resistance.Item Structural Thermomechanical Models for Shape Memory Alloy Components(2014-04-18) Rao, AshwinThermally responsive shape memory alloys (SMA) demonstrate interesting properties like shape memory effect (SME) and superelasticity (SE). SMA components in the form of wires, springs and beams typically exhibit complex, nonlinear hysteretic responses and are subjected to tension, torsion or bending loading conditions. Traditionally, simple strength of materials based models/tools have driven engineering designs for centuries, even as more sophisticated models existed for design with conventional materials. In light of this, an effort to develop strength of materials type modeling approach that can capture complex hysteretic SMA responses under different loading conditions is undertaken. The key idea here is of separating the thermoelastic and the dissipative part of the hysteretic response by using a Gibbs potential and thermodynamic principles. The dissipative part of the response is later accounted for by a discrete Preisach model. The models are constructed using experimentally measurable quantities (like torque?twist, bending moment?curvature etc.), since the SMA components subjected to torsion and bending experience an in-homogeneous non-linear stress distribution across the specimen cross-section. Such an approach enables simulation of complex temperature dependent superelastic responses including those with multiple internal loops. The second aspect of this work deals with the durability of the material which is of critical importance with increasing use of SMA components in different engineering applications. Conventional S-N curves, Goodman diagrams etc. that capture only the mechanical loading aspects are not adequate to capture complex thermomechanical coupling seen in SMAs. Hence, a novel concept of driving force amplitude v/s number of cycles equivalent to thermodynamical driving force for onset of phase transformations is proposed which simultaneously captures both mechanical and thermal loading in a single framework. Recognizing the paucity of experimental data on functional degradation of SMAs (especially SMA springs), a custom designed thermomechanical fatigue test rig is used to perform user defined repeated thermomechanical tests on SMA springs. The data from these tests serve both to calibrate the model and establish thermodynamic driving force and extent of phase transformation relationships for SMA springs. A drop in driving force amplitude would suggest material losing its ability to undergo phase transformations which directly corresponds to a loss in the functionality/smartness of SMA component. This would allow designers to set appropriate driving force thresholds as a guideline for analyzing functional life of SMA components.Item The Effect of Dietary Starch Concentration on Glycogen Replenishment in Performance Horses(2013-12-12) Vonderohe, CaitlinFour Quarter Horses (2 to 3 yr; 401 to 432 kg BW) were used in a simple crossover design for a 49-d study to determine the effect of dietary starch levels on post-exercise glycogen replenishment. Horses were fed either high starch (HS) or low starch (LS) concentrates at 0.75% BW/d plus l.0% BW/d Coastal Bermudagrass hay for 14 d, and then worked to fatigue in a standardized exercise test (SET). After a 14-d washout period, horses were switched to the opposite diet for 14 d and then again performed the SET. The LS and HS concentrates were commercially available feeds. Total diets provided an average of 997.6 g of starch and 553.7 g of starch/d in the HS and LS diets, respectively. Throughout the trial, horses were lightly exercised for 30 min, 3 d/wk. The SET consisted of a 30-min warm-up period at a brisk trot in a panel exerciser, followed by 27 min of gradually ascending high-intensity work on a treadmill. Skeletal muscle biopsies were taken from the biceps femoris at rest, immediately after the SET, and again at 6-, 24- and 48-h post-exercise. Samples were flash frozen in liquid nitrogen and stored at -80?C until analysis for later muscle glycogen concentration using a commercial kit. Venous blood samples were taken at rest, immediately post exercise and every 15 min for 3 h post-exercise. Blood samples were analyzed for lactate, glucose, total protein and Ca concentration. Data were analyzed using Proc Mixed (SAS) procedure with main effects of sample time, horse, period, trt and time x trt interaction. Horses on the HS diet had a higher muscle glycogen concentration (P <0.05) at 48 h post exercise than the LS horses (18.1 vs. 10.6 ?g/mg wet wt). At 6 h, HS horses had a more rapid rate of repletion, as observed by the higher glycogen concentration (P<0.05) compared to the immediate post-exercise samples (15.4 vs. 7.9 ?g/mg wet wt); whereas, the LS horses did not return to normal levels until 24 h post SET. Results indicated that horses on the LS diet, which is representative of low-starch feeding programs commonly observed in the industry, replenish their skeletal muscle glycogen slower than horses on the HS diet. Based on this study, performance horses undergoing multiple bouts of intense exercise may benefit physiologically from receiving diets that contain more than 553.7 g of starch/day.Item Turn-of-the-nut tightening of anchor bolts(Texas A&M University, 2004-09-30) Richards, Jason HalbertDouble-nut anchor bolt systems are used in the erection of traffic signal poles, high-mast luminaries, and other highway appurtenances. An absence of a tightening standard for such systems decreases the confidence in their performance under fatigue loading. Past research has shown that a tightening standard should include the development of preload in the anchor bolt sufficient to provide adequate resistance to fatigue failure. Preload should be measured by a turn-of-the-nut method. Laboratory progressive tightening tests were performed in order to monitor the stress ranges occurring in the bolt at various locations of interest at various degrees of turn-of-the-nut tightness. Tests were performed on six diameters of anchor bolt ranging from 1 to 2-1/4 inches in diameter and two different categories of thread pitch: UNC and 8UN. Plots of stress range versus degree of tightness were developed for each test and evaluated to find the minimum degree of turn-of-the-nut at which stress range inside the nuts dropped below that outside the nuts. This shift was considered to be the principle theoretical indication of adequate performance. A fatigue test which saw failure outside the double-nut connection was set down as the practical indicator of adequate fatigue performance. The 2 inch 8UN bolt was chosen as the critical specimen due to its overall low generation of preload during tightening tests. Theoretical testing showed that 1/24 turn-of-the-nut would guarantee sufficient fatigue performance. Two practical fatigue tests of the bolt at that tightness saw one positive and one negative failure. After actual lab tests, finite element modeling was used to investigate the behavior of the bolt. It was found that performance did not see improvement until 1/12 turn-of-the-nut. After all results were considered, a standard of 1/6 turn-of-the-nut or refusal of tightening by specified methods was recommended, provided a minimum of 1/12 turn-of-the-nut was achieved. This value allows for ease of measurement, sufficient tightness, degree of safety, and has been shown in past testing not to cause failure through over-tightening. However, tightening to only 1/12 turn-of-the-nut still provided adequate performance.