Browsing by Subject "Fatigue"

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An investigation of the practices used in airline crew scheduling and their impact on the physical and mental health of airline crew
(2020-08-17) Benson, Taylor Yvette; Fleischmann, Kenneth R.
The literatures surrounding the topics of crew scheduling optimization and crew health are often researched separately. The models are disconnected from the people they affect and are often seen as separate and neutral entities, despite being intrinsically connected with the mental and physical health of flight crews, in the same way that a typical work schedule impacts the physical and mental health of traditional full-time employees. With this in mind, this report gives a high-level overview of popular scheduling models and also, more importantly examines how the use of these models and current schedule optimization practices directly and indirectly impact the health of flight crews.
Assessment of remaining fatigue performance of high mast illumination poles
(2014-05) Belivanis, Konstantinos Victor; Helwig, Todd Aaron, 1965-
Failures of high mast illumination poles (HMIP) around the US have raised the concerns of officials because of their location close to areas with important human activity. Previous research, conducted at the University of Texas proved that those failures were fatigue type failures and that cracking initiated at the shaft to baseplate connection, specifically at the bends of the shaft. However, no research has been conducted on the remaining fatigue life of poles after they have been in service. This thesis investigates the remaining fatigue life of a pole, removed from service after severe cracking has been observed on it, via laboratory testing. Moreover, nondestructive and destructive testing procedures were implemented to investigate the initiation and evolution of fatigue cracks. The results validated the procedures for the in-situ validation of the remaining fatigue life of those poles. Last, the destructive test results enhanced the arguments that initial cracking at the toe of the weld at the shaft to baseplate connection is a result of the galvanizing process.
Changes in maximal power output and perceptual fatigue responses during a Division I female collegiate soccer season
(2011-05) McLean, Blake David; Coyle, Edward F., 1952-; Tanaka, Hirofumi
The purpose of this study was to investigate how maximal power output (P[subscript max]), as measured via the inertial load technique, changes throughout a collegiate soccer season in relation to training load completed. Furthermore, this study also aimed to describe the time course of recovery of [maximal power output] following two collegiate soccer matches in one weekend. The current investigation involved two experimental periods. The first took place throughout the 2010 Big XII soccer season. Nineteen Division I female collegiate soccer players (19.9 ± 1.2 yrs, 165.1 ± 6.6 cm, 61.0 ± 6.8 kg) from the same collegiate soccer team completed regular inertial load testing and perceptual fatigue questionnaires throughout this study period. These athletes' training was not altered in any way by the investigators throughout the 2010 season. During the second experimental period, two off-season matches were played in one weekend during the 2011 off-season. Training was prescribed by coaches, with the aim of simulating an in-season training week. [Maximal power output] and perceptual fatigue responses were monitored every day throughout the second study period. All training and matches throughout the study were monitored via the session RPE method. The results demonstrated that STARTERS experience much greater load throughout the season than NON-STARTERS (p<0.05, 2247 ± 176 AU and 1585 ± 174 AU), accounted for by increased load experienced during matches. This increased training load throughout the season lead to a decline in [maximal power output] in STARTERS (to 92.3 ± 6%, p<0.05) whilst [maximal power output] was maintained in NON-STARTERS (p>0.05) for the duration of the season. Furthermore, STARTERS experienced greater muscle soreness throughout the in-season period compared to NON-STARTERS. During the second experimental period [maximal power output] declined (p<0.05, 93.3 ± 7%) two days following matches played on the weekend and remained reduced for three days. Notably, two heavy training days in the middle of the training week appeared to have detrimental effects on both [maximal power output] and perceptual fatigue responses. The main finding of this study is that [maximal power output] declined throughout the middle and latter parts of the season in STARTERS, after experiencing significantly greater training loads than NON-STARTERS throughout the season. The current findings, combined with previous investigations, suggest that the inertial load technique may be useful in monitoring fatigue in team sport athletes.
Changes in muscle activity and kinematics of highly trained cyclists during fatigue
(2006-12) Joubert, Jason E.G.; Dingwell, Jonathan B.
Up to 85% of cyclists experience repetitive strain injuries (RSI's). During long bouts of repetitive tasks, muscle fatigue may cause mal-alignments in kinematics, having cumulative effects, leading to an RSI. Purpose: The study's purpose was to examine how changes in localized muscle fatigue relate to changes in movement kinematics in highly trained cyclists throughout a full fatigue protocol. Methods: Seven highly trained cyclists participated in a 2 session experiment. Session 1 included a VO2 max test and familiarization trial and Session 2 was the fatigue protocol. Kinematic angles measured were trunk lean, hip, knee, ankle, and knee splay angle. Mean angle (MA) and range of motion (ROM) was calculated for each revolution thought the trial. Muscles monitored were the quadriceps, hamstring, gastrocnemius, and tibialis anterior. EMG median frequency (MDF) for each muscle was calculated for each revolution by averaging MDF for the two halves of each revolution. Cross-correlation analysis was done on MDF and MA data and MDF and ROM data. Results: All subjects exhibited increases in trunk lean and decreases in ankle angle. Non-monotonic changes were observed in trunk lean, ankle, knee splay angle, and among ROM results for all 5 angles. A 1-tailed T-tests for all subjects, revealed that HAM (p = 0.020) and GAS (p = 0.018) exhibited significant muscle fatigue. One-tailed T-tests yielded significantly negative cross-correlation time lags [Greek small letter tau] for trunk lean MA, ROM, and hip MA. Conclusions: Non-monotonic changes are present in kinematics and MDF. Therefore pre vs. post experimental designs cannot quantify fatigue processes. Shifts in trunk lean MA, ROM and hip MA are significantly correlated with preceding decreasing shifts of MDF (indicative of onset of fatigue).
Cross-frame forces in a straight bridge with normal supports
(2018-05) Moore, Matthew Eric; Helwig, Todd Aaron, 1965-; Engelhardt, Michael D.
Cross-frame and diaphragm systems are important structural elements in steel I-girder bridges. These members enhance the lateral-torsional buckling resistance in straight girder systems by reducing the unbraced length. The critical stage for the braces from a stability perspective often occurs during construction of the concrete bridge deck when the non-composite steel girders must resist the entire construction load. The braces not only enhance the lateral-torsional buckling resistance of the girders, but are also necessary to resist the torsion applied to the girders due to the deck overhang construction, and distribute lateral loads across the structure from sources such as wind. In horizontally curved bridges, the braces are primary structural elements in the superstructure and engage the girders across the width of the bridge to behave as a structural system to resist the torsion that develops as a result of the curved geometry. For straight girder systems, the specifications of the American Association of State Highway and Transportation Officials (AASHTO) have generally provided little guidance in the sizing of the braces other than recommended connection plate (web stiffener) details or slenderness limits. While there have been many advances in recent years towards improving the understanding of the behavior of cross-frame systems, there has not been sufficient research carried out on the proper loading conditions for assessing the fatigue performance of cross-frames. The work outlined in this thesis is part of a larger study on the behavior of cross-frames in steel bridge system. The study is funded by the National Cooperative Highway Research Program (NCHRP 12-113). The fundamental goals of the research investigation are to produce methodologies and design guidelines for the following: evaluation of fatigue design stresses in cross-frames in straight and horizontally curved steel I-girder bridges; calculation of minimum cross-frame strength and stiffness requirements for stability bracing of I-girders during construction and in-service; development of improved methods to account for the influence of end connection details on cross-frame stiffness that extend beyond and improve upon the suggested guidance currently provided in Article C4.6.3.3.4 of the AASHTO LRFD Bridge Design Specifications. This work includes field monitoring and parametric FEA studies. The field studies are focused on three bridges: 1) a straight bridge with normal supports, 2) a straight bridge with skewed supports, and 3) a horizontally curved bridge. The field studies include rainflow monitoring of fatigue induced stresses in select cross-frames and the girders for a period of approximately 1 month as well as live load tests using trucks of known weights. This thesis focuses on some of the background studies, a survey of bridge owners around the United States, as well as the instrumentation and live load tests on the straight bridge with normal supports
Defining structurally acceptable mechanical properties of high-strength reinforcing steel bars through low-cycle fatigue testing
(2017-12-11) Hogsett, Graham Stephen; Engelhardt, Michael D.; Ghannoum, Wassim M.
Low-cycle fatigue tests were performed on high-strength reinforcing bars currently being developed in the United States, to quantify mechanical properties and fatigue life under simulated seismic conditions. Reinforcing bars with yield strengths ranging for about 60ksi to over 100ksi were tested. The high-strength bars with yield strengths exceeding 80ksi were obtained from three manufacturers that produce high-strength bars using the main three manufacturing techniques in use in the United States. Primary variables considered also include chemical composition, geometric deformations, bar grade, clear gripping span, loading protocol, and manufacturing process. The results of monotonic and cyclic tests are presented and comparisons are made based on the variables listed. A previously proposed fatigue model is considered and recalibrated for the new testing data.
Defining structurally acceptable properties of high-strength steel bars through material testing
(2015-09-15) Slavin, Chase Meany; Ghannoum, Wassim M.; Engelhardt, Michael D.
Low-cycle fatigue tests were performed on reinforcing bars in order to assess the acceptability of newly developed high-strength reinforcing bars in seismic applications. The steels tested are classified as grade 60 A706, grade 80 A706, grade 80 A615, and grade 100. The high-strength reinforcing bars tested represent the two most common manufacturing processes used today: microalloying and quenching-and-tempering. The results of these tests are presented along with comparisons between the fatigue life of bars based on steel grade and other bar properties. A statistical analysis of the test results is presented in order to assess the impact of many parameters on the low-cycle fatigue performance of grade 60 A706 and higher-strength reinforcing bars.
Development and analysis of stretchable electronics in biopotential monitoring
(2016-05) Nicolini, Luke Robert; Lu, Nanshu; Djurdjanovic, Dragan
In this Thesis, stretchable electronics are studied and developed, with a focus on the epidermal monitoring of biopotentials for healthcare and other applications. Conventional manufacturing processes for stretchable electronics are time and cost intensive. A novel manufacturing method to create stretchable electronics is developed, in which a cutter plotter is used to directly shape thin metal films, in order to produce stretchable designs. The limits of the manufacturing process are investigated and recorded, both in terms of the cutter plotter capabilities and in the use of different materials and substrates for the thin-film device. The thin, stretchable devices are also tested in a wide variety of data collection situations, including measuring of a variety of biopotentials including ECG, EMG, and EEG. The new Epidermal Sensor Systems created with the Cut-and-Paste manufacturing method perform equal to or better than conventional electrodes. Thus the cut-and-paste method is determined to be a novel and more cost effective method to produce stretchable sensors. Stretchable sensors we produce can match the thickness and mechanical stiffness of human epidermis, and can hence be laminated and fully conformed on human skin like a temporary transfer tattoo for long-term biopotential monitoring including electrocardiogram (ECG), electromyogram (EMG), and electroencephalogram (EEG). Such epidermal sensors have enabled the investigation of muscle fatigue and recovery over time. An Autoregressive Moving Average (ARMAX) model is developed in order to map between forearm flexor muscle EMG and the grip force of the corresponding hand. The fit of this model is tracked over the course of fatigue, and changes in the model are analyzed to provide useful trends with which to measure and follow muscle fatigue patters. The epidermal sensor is found to be equivalent to conventional electrodes for muscle fatigue monitoring, while being more comfortable and durable. Further, the ARMAX modeling procedure is proven to have useful results in terms of modeling of forearm muscle fatigue. Overall, this research contributes to the field of stretchable electronics and their applications for biopotential monitoring.
Effect of galvanization on the fatigue strength of high mast illumination poles
(2010-05) Pool, Charles Stephen; Frank, Karl H.; Helwig, Todd A.; Liljestrand, Howard M.
This research investigation studied the effects of galvanization on the fatigue life of high mast illumination poles. Reports that galvanization of high masts caused initial cracks to form at the toe of the weld connecting the base plate to the shaft of the pole were first validated. The effects of these initial cracks on fatigue strength were then checked through experimental testing. A variety of variables were tested for both their effects on the occurrences of the initial cracks and effects on fatigue life. These variables included testing galvanized against ungalvanized specimens, testing of varying fabricators and galvanizers, and testing of various types of connection details. These test results were compared against inspection results provided by Texas Department of Transportation inspectors. Also, methods of mitigating the effects of toe cracks on the fatigue life of poles were investigated. A method for repairing specimens both in the fabrication shop and in the field were developed and tested. Both methods showed strong improvement in fatigue life of the specimens providing a possible repair solution.
Effect of laser polishing on fatigue behavior of additively manufactured IN718
(2022) Lee, Seungjong; Bureš, Martin; Shao, Shuai; Wells, Douglas N.; Zetek, Miroslav; Kepka, Miloslav; Shamsaei, Nima
This study investigates the effect of laser-polishing on the fatigue behavior of Inconel 718 fabricated using laser powder bed fusion process. Three different conditions including as-built and laser-polished using two different process parameters are considered. Uniaxial tension- compression fatigue tests are conducted in strain-controlled mode to examine the fatigue behavior for each condition. In addition, surface roughness measurements and fractography using optical microscopy and porosity measurements using the X-ray computed tomography are also performed for all conditions. The results indicate that laser-polishing alone does not improve fatigue performance, even though it can significantly reduce surface roughness. The beneficial effects of the smoother surfaces may have been countered by the volumetric defects close to the surface induced by laser-polishing. The fracture surfaces also reveal that fatigue cracks are initiated from the defects close to the surface, and therefore, fatigue behavior is not improved.
The effects of carbohydrate-protein supplementation on glycogen utilization and fatigue during a simulated soccer match
(2010-12) Dessard, Benjamin; Ivy, John, 1945-; Tanaka, Hirofumi
The purpose of this study was to examine if the addition of protein to a carbohydrate supplement (CHO+PRO), provided during a simulated soccer match, would reduce fatigue and muscle glycogen utilization in comparison to an isocaloric carbohydrate only supplement (CHO). Two female and eight male (n = 10) trained soccer players performed a modified version of the Loughborough Intermittent Shuttle Test (LIST) on two separate occasions, followed by a run to exhaustion (RTE). Supplements were provided 10 minutes before the simulated match and at the beginning of half-time, but not during exercise in order to create real-match conditions. Supplements were composed of 2.8% protein + 7% carbohydrate (CHO+PRO) or 9.8% carbohydrate (CHO). Muscle biopsies were performed before and at the end of the LIST, after which iv participants ran to exhaustion. No differences were found between treatments for RTE (489 ± 121 sec for CHO and 589 ± 186 sec for CHO+PRO) or glycogen utilization (37.9 ± 7.6 µmol•g wet wt-1 during the CHO and 29.1 ± 6.0 µmol•g wet wt-1 during the CHO+PRO). No differences were found for the other measurements such as sprint times, heart rate, RPE, blood glucose, lactate, and insulin. Blood Creatine kinase (CK), and overall muscle soreness were measured 24 hours after each trial in order to evaluate muscle damage but no differences between treatments were found. In accordance with these findings, the phosphorylation state of the protein FOXO3a was not altered differently by the treatments. These results suggest that the addition of protein to a traditional carbohydrate-only supplement provided immediately prior to and at the half of a simulated soccer match does not further improve the benefits of a CHO supplement.
Estimating the remaining fatigue life of steel bridges using field measurements
(2013-05) Fasl, Jeremiah David; Helwig, Todd Aaron, 1965-; Wood, Sharon L.
As bridges continue to age and budgets reduce, transportation officials often need quantitative data to distinguish between bridges that can be kept safely in service and those that need to be replaced or retrofitted. One of the critical types of structural deterioration for steel bridges is fatigue-induced fracture, and evaluating the daily fatigue damage through field measurements is one means of providing quantitative data to transportation officials. When analyzing data obtained through field measurements, methods are needed to properly evaluate fatigue damage. Five techniques for evaluating strain data were formalized in this dissertation. Simplified rainflow counting, which converts a stress history into a histogram of stress cycles, is an algorithm standardized by ASTM and the first step of a fatigue analysis. Two methods, effective stress range and index stress range, for determining the total amount of fatigue damage during a monitoring period are presented. The effective stress range is the traditional approach for determining the amount of damage, whereas the index stress range is a new method that was developed to facilitate comparisons of fatigue damage between sensors and/or bridges. Two additional techniques, contribution to damage and cumulative damage, for visualizing the data were conceived to allow an engineer to characterize the spectrum of stress ranges. Using those two techniques, an engineer can evaluate whether lower stress cycles (concern due to electromechanical noise from data acquisition system) and higher stress ranges (concern due to possible spike from data acquisition system) contribute significantly to the accumulation of damage in the bridge. Data from field measurements can be used to improve the estimate of the remaining fatigue life. Deterministic and probabilistic approaches for calculating the remaining fatigue life were considered, and three methods are presented in this dissertation. For deterministic approaches, the output of the equations is the year when the fatigue life has been exceeded for a specific probability of failure, whereas for probabilistic approaches, the probability of failure for a given year is calculated. Four different steel bridges were instrumented and analyzed according to the techniques outlined in this dissertation.
Evaluation of fatigue design load models for cross-frames in steel I-girder bridges
(2020-09-10) White, Joshua Benjamin; Engelhardt, Michael D.; Helwig, Todd Aaron, 1965-; Ravi-Chandar, Krishnaswa; Williamson, Eric B
There have been a number of advances in the level of understanding of cross-frame systems for steel I-girder bridges; however, very little work has focused on the proper loading conditions to produce an adequate estimate of the fatigue load in cross-frames. The goal of this research is to provide an improved definition of the fatigue loading for cross-frames in straight, horizontally-curved, and skewed steel I-girder bridges which will be analyzed using refined analysis techniques. In order to compare load effects, three bridges were instrumented and monitored. The bridges include: i) a straight bridge with normal supports, ii) a straight bridge with skewed supports, and iii) a horizontally curved bridge with radial supports. Data gathered from the field instrumentation was used to validate three-dimensional finite element analysis (FEA) models that were used to carry out extensive parametric analyses to improve the understanding of the behavior of cross-frame stresses as a function of truck position on the bridge. A wide range of geometrical parameters of straight and horizontally curved bridges were used to understand the general behavior of the bridges. The primary objectives of this research include the following: 1) Investigate the adequacy of the current AASHTO (American Association of State Highway and Transportation Officials) fatigue load model for the design of cross frames in steel I-girder bridges. 2) Investigate the effects of multiple presence on the design of cross-frames in steel I-girder bridges. 3) Investigate the reliability of the developed load model and identify the gaps in knowledge of cross-frame detail resistance data as it relates to the reliability of current design practices. These objectives were accomplished by examining recently collected, high-resolution, multi-lane weigh-in-motion (WIM) data, which represent actual truck traffic records in the US. The current AASHTO fatigue design load model was evaluated by comparing cross-frame load effects caused by the fatigue load model to load effects caused by simulated truck traffic representing actual live load. Influence surfaces generated from three-dimensional FEA models provided information on the stresses in select cross-frame members as a function on truck position on the bridge deck. WIM data representing real truck traffic (tens of millions of truck records) were filtered and analyzed; multi-lane data were analyzed using a cluster analysis. The statistical parameters of this WIM study were used to simulate actual live load on the three-dimensional bridge models and compare load effects to those generated by a fatigue design truck. The outcome of this study indicates the current fatigue design truck axle and weight configuration and placement of the fatigue design truck to maximize design-controlling fatigue effects for both the Fatigue I and Fatigue II AASHTO limit states is overly conservative. Stochastic techniques were used to investigate the implications of new load factors in the context of reliability-based fatigue design
Experimental Analysis of Functionally Graded Materials produced by Fused Filament Fabrication
(2022) Alkunte, Suhas; Fidan, Ismail; Hasanov, Seymur
Multi-material additive manufacturing has grabbed tremendous attention in the research community. In this investigation, a multi-material single extrusion system was used to fabricate the combination of chopped carbon fiber reinforced Polyethylene Terephthalate Glycol (CF- PETG) and Thermoplastic Polyurethane (TPU) materials with gradient transition for a more robust material interface. Various patterns such as the 20, 40, 60, and 80% by volume blend of CF-PETG and TPU materials are designed, printed, and analyzed to understand their tensile and fatigue behaviors. Tensile–tensile fatigue tests with a stress ratio of 0.1 were performed on each specimen at 80% of UTS. The characterization of functionally gradient material interface and direct transition patterns were conducted for comparison. The results showed that gradient change in material concentrations from soft to hard material has significantly enhanced the interface strength.
Experimental and analytical evaluation of cross-frame fatigue behavior in steel I-girder bridges
(2020-09-09) Reichenbach, Matthew Craig; Helwig, Todd Aaron, 1965-; Engelhardt, Michael D; Salamone, Salvatore; Kyriakides, Stelios
Cross-frames are important structural components that serve many functions throughout the service life of steel I-girder bridge systems. They primarily act as stability braces to enhance the lateral-torsional buckling resistance of the girders during erection and deck construction, but also distribute live loads in the final composite condition. Under repetitive load cycles caused by heavy truck passages, cross-frames and their connections are susceptible to load-induced fatigue cracking if not properly designed. Cross-frames have historically been detailed and fabricated based on general rules-of-thumb and experience. In recent years, however, developments in bridge design specifications have necessitated the modernization of cross-frame design and analysis practices. Cross-frames are now designed and detailed based on rational analysis for all stages of construction and service life, which has further emphasized the importance of accurate and reliable analysis techniques and design criteria. Although considerable research over the past several decades has improved cross-frame design and analysis, the design industry has generally lacked quantitively based guidance on load-induced behavior of cross-frames in composite, in-service bridges. As such, this dissertation explores two major concepts: (i) the influence of skewed and curved superstructure geometry on the fatigue response of cross-frames and (ii) the limitations of simplified analysis techniques commonly utilized in commercial software programs with respect to estimating cross-frame force effects. Field experiments were performed on three steel I-girder bridges in the greater Houston area, and the stress ranges induced in key cross-frame members from truck traffic were monitored for one month each. Upon validating a finite-element approach with the measured data, an extensive analytical parametric study was conducted to expand the breadth and depth of knowledge gained from the limited field studies. In general, the load-induced fatigue behavior of conventional X- and K-type cross-frames were examined for a variety of bridge geometries commonly found in the United States. These analyses were performed with different levels of computational refinement, ranging from sophisticated three-dimensional approaches to simplified two-dimensional approaches. Based on the data collected and processed from the experimental and analytical studies, recommendations are proposed to improve the design and analysis of cross-frames in composite bridge structures. Because cross-frames represent a costly component of fabrication and erection, these recommendations ultimately lead to improved efficiency and economy of new steel bridge construction
Extension of phase-field modeling to fatigue and large structures
(2022-05-06) Lo, Yu-Sheng, Ph. D.; Landis, Chad M.; Mear, Mark E; Ravi-Chandar, Krishnaswa; Huang, Rui; Foster, John T
The phase-field approach to fracture naturally captures the emergence of complex crack patterns and behaviors, including crack nucleation, turning, branching, and intersection in both two and three dimensions without any additional constitutive assumptions for these behaviors. What follows will present the study of fatigue crack growth using the phase-field approach. A crack growth viscosity parameter is introduced into the standard phase-field model for brittle fracture to account for rate- or cycle-dependent crack growth phenomena. A modified J-integral is developed to demonstrate how the phasefield approach can be used to generate Paris-law type crack growth rates. In order to model more general crack versus applied loading behavior that are not fit by a single simple Paris-law, steady-state finite element calculations are performed to calibrate fits of the phase-field model to measure crack growth rates found in da/dN versus ∆Kcurves. Transient time- or cycle-dependent calculations are performed and compared to experimental measurements on samples where crack turning is induced by the presence of a hole in the vicinity of the crack. A three-dimensional example with turning of the crack front is also computed illustrating the capabilities of the phase-field approach for complex crack paths. The phase-field approach to fracture replaces sharp crack surfaces with a diffuse fracture zone that represents the traction-free crack faces. The diffuse zone is characterized by a length scale that appears prominently in the governing partial differential equation that governs the evolution of the phase-field variable. Within numerical calculations the diffuse fracture zone must be resolved with a sufficient number of degrees of freedom in order to obtain accurate solutions. Additionally, all material fracture problems possess a physical process zone length scale that scales with the ratio of the fracture energy to the square of the process zone strength. For the vast majority of problems governed by linear elastic fracture mechanics this physical process zone length scale is small, for example it is on the order of microns for aluminum alloys. The most commonly used formulation of the phase-field approach to fracture ties the diffuse phase-field crack length scale to the physical process zone length scale. This presents a challenge for extending such models to large structures that may be on the order of meters in size due to the prohibitive meshing requirements for the phase-field crack length scale. This is especially the case for three-dimensional problems. To address this problem, we present a new form of the phase-field degradation function that allows for a decoupling of the phase-field length scale from the physical process zone length scale. The behavior and limitations of this new formulation are discussed and illustrated with a series of numerical test cases.
Fatigue assessment of high mast illumination poles using field measurements
(2011-08) Magenes, Luca; Helwig, Todd Aaron, 1965-; Frank, Karl
Failures of high mast illumination poles (HMIPs) in recent years have raised concerns on the long-term fatigue performance of the poles by various transportation officials around the US. The thesis documents a study sponsored by the Texas Department of Transportation focused on the fatigue behavior of in-service HMIP systems. This study is an extension of previous investigations on the fatigue behavior of the poles that have demonstrated that many poles have poor performance and fail in fatigue before the AASHTO category E' limit. Galvanized specimens were also tested and some of them showed evidence of initial cracking, impacting the fatigue performance such that the galvanized poles behaved worse than the uncoated specimens. Ultrasonic Testing (UT) has shown several poles around the state of Texas contain cracks in the welds between the shaft and base plate. To further investigate the performance of the poles in-service, a field study was initiated to measure the wind speed and direction, as well as the corresponding stresses in the pole shaft. This thesis presents results from the field investigation. A data acquisition system was developed to gather wind data and induced stresses. The system was powered by a solar panel and can be remotely accessed via a wireless modem. Data collected throughout the year details the intensity and number of stress cycles experienced by the poles, and could be correlated with the measured wind velocity. Using the field data, more accurate estimates of expected fatigue life for the poles were made. The study provides TxDOT with valuable data on the performance of in-service poles so that the most critical fatigue cases can be identified and proper decisions can be made on the appropriate inspection or repair schedule.
Fatigue behavior of post-installed shear connectors used to strengthen continuous non-composite steel bridge girders
(2016-08) Ghiami Azad, Amir Reza; Engelhardt, Michael D.; Williamson, Eric B., 1968-; Helwig, Todd A; Jirsa, James O; Taleff, Eric M
Many older bridges in Texas are constructed with floor systems consisting of a concrete slab over steel girders. A potentially economical means of strengthening these floor systems is to connect the existing concrete slab and steel girders using post-installed shear connectors to change the behavior of the beam from non-composite to partially-composite. Since fatigue is one of the main concerns in designing bridges, investigating the fatigue properties of these post-installed shear connectors becomes crucial. Results from direct-shear testing show that post-installed shear connectors have a better fatigue life compared to conventional welded shear studs. However, based on currently available data from direct-shear tests, fatigue life of post-installed shear connectors is still inadequate for economical retrofit in some cases. Furthermore, it is unclear if direct-shear tests provide an appropriate means of evaluating fatigue performance. The objective of this dissertation is to develop new and more accurate approaches for evaluating the fatigue characteristics of post-installed shear connectors. This objective is addressed through large-scale beam fatigue tests and computational studies. The focus of the work is on evaluating fatigue life of shear connectors based on both slip and stress demands.
Fatigue characterization of asphalt binders using a thin film poker chip test
(2016-05) Hajj, Ramez Muhammad; Bhasin, Amit; Motamed, Arash
Asphalt binder is the adhesive that holds together aggregate particles of different sizes of an asphalt mixture. The tensile properties of an asphalt binder can greatly affect the performance of the asphalt mixture under repeated traffic loading. While the current performance grade specification has been in use for a long time to characterize the asphalt binders with regards to fatigue, it has been shown to be largely ineffective. This study was performed with the goal of investigating a strength-based measure to evaluate the fatigue cracking resistance of the asphalt binder. The poker chip geometry was used for this purpose. The test involved tensile loading of a thin film of asphalt binder between two rigid substrates. The first part of this study focused on determining failure criteria for the test. The second part was a study of the binders that have a similar grade based on the current performance grade specification but are expected to perform differently due to difference in their chemical makeup. Finally, the third part involved a study of the effects of nanomaterials as additives on the strength of the binder based on poker chip test results. The results demonstrated that failure strain criteria is promising as a material property, but still needs further study for validation. It was also observed that binders with similar performance grade had significantly different tensile strength. Finally, it was observed that nanomaterials had a significant impact on the test results of unaged binder, but had less effect on aged asphalt binders.
The fatigue performance of cross frame connections
(2010-08) Wahr, Andrew Scott; Engelhardt, Michael D.; Helwig, Todd A.
A new method of connecting cross-frames to bridge girders had been proposed to alleviate concerns with current design practices. This new, half-pipe detail needs to be examined for fatigue issues that may exist which would make it infeasible as a replacement candidate for the current bent-plate design. A program of laboratory testing was carried out to determine the comparative performance between the half-pipe and the bent-plate designs. These tests were then translated into a finite element model which was examined to determine behavior over a wide range of designs scenarios. Finite element results, along with the laboratory testing data, were used to determine the appropriate use of the half-pipe stiffener.