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Abdullah, A.B.M.Development of a Closed-loop Resonant Fatigue Testing Methodology and Experimental Life Test of Aluminum Alloy
Master of Science, University of Akron, 2010, Civil Engineering
A vibration-based testing methodology is presented that assesses fatigue behavior of material for metallic structure. To minimize the testing duration, the test setup is designed for a base-excited multiple-specimen arrangement driven in a high-frequency resonant mode, which allows completion of fatigue testing in an accelerated period. A high performance electro-dynamic exciter (shaker) is used to generate harmonic oscillation of cantilever beam specimens, which are clasped on the shaker armature with specially-designed clamp fixtures. The shaker operates in closed-loop control with dynamic specimen response feedback provided by a scanning laser vibrometer. A test coordinator function is developed to synchronize the shaker controller and the laser vibrometer, and to complete the closed-loop scheme: the test coordinator monitors structural health of the test specimens throughout the test period, recognizes change in specimen dynamic behavior due to fatigue crack initiation, terminates test progression, and acquires test data in an orderly manner. Topological design is completed by constructing an analytical model and performing finite element analysis for the specimen and fixture geometry such that peak stress does not occur at the clamping fixture attachment points. Experimental stress evaluation is conducted to verify the specimen stress predictions. A successful application of the experimental methodology is demonstrated by validation tests with aluminum specimens subjected to fully-reversed bending stress.

Committee:

Gun Jin Yun, Dr. (Advisor); Craig C. Menzemer, Dr. (Committee Member); Wieslaw K. Binienda, Dr. (Committee Member)

Subjects:

Engineering

Keywords:

High-cycle fatigue; Vibration-based testing; Scanning laser vibrometer; Shaker controller; Closed-loop testing; Bending fatigue; Experimental fatigue testing; Resonant fatigue

Eftekhari, MohammadrezaCreep, Fatigue, and Their Interaction at Elevated Temperatures in Thermoplastic Composites
Doctor of Philosophy, University of Toledo, 2016, Mechanical Engineering
Thermoplastic composites are suitable alternatives to metals in some load-bearing applications such as in the automotive industry due to a large number of advantages they present. These include light weight, ease of processing for complex geometries at high production rate, outstanding cost to performance ratio, ability to reprocess, and corrosion resistance. Addition of fillers such as talc or reinforcements such as short glass fibers can improve the mechanical performance of unreinforced thermoplastics to a high degree. Components made of thermoplastic composites are typically subjected to complex loadings in applications including static, cyclic, thermal, and their combinations. These applications may also involve environmental conditions such as elevated temperature and moisture which can dramatically affect their mechanical properties. This study investigated tensile, creep, fatigue, creep-fatigue interaction, and thermo-mechanical fatigue (TMF) behaviors of five thermoplastic composites including short glass fiber reinforced and talc-filled polypropylene, short glass fiber reinforced polyamide-6.6, and short glass fiber reinforced polyphenylene ether and polystyrene under a variety of conditions. The main objectives were to evaluate aforementioned mechanical behaviors of these materials at elevated temperatures and to develop predictive models to reduce their development cost and time. Tensile behavior was investigated including effects of temperature, moisture, and hygrothermal aging. Kinetics of water absorption and desorption were investigated for polyamide-6.6 composite and Fickian behavior was observed. The reductions in tensile strength and elastic modulus due to water absorption were represented by mathematical relations as a function of moisture content. In addition to moisture content, aging time was also found to influence the tensile behavior. A parameter was introduced for correlations of normalized stiffness and strength with different aging times and temperatures. Higher strength and stiffness were obtained for re-dried specimens after aging which was explained by an increase in crystallinity. Mechanisms of failure were identified based on fracture surface microscopic analysis for different conditions. Creep behavior was investigated and modeled at room and elevated temperatures. Creep strength decreased and both creep strain and creep rate increased with increasing temperature. The Larson-Miller parameter was able to correlate the creep rupture data of all materials. The Monkman-Grant relation and its modification were successfully used to correlate minimum creep rate, time to rupture, and strain at rupture data. The Findley power law and time-stress superposition principle (TSS) were used to represent non-linear viscoelastic creep curves. Long-term creep behavior was also satisfactory predicted based on short-term test data using the TSS principle. Effect of cycling frequency on fatigue behavior was investigated by conducting load-controlled fatigue tests at several stress ratios and at several temperatures. A beneficial or strengthening effect of increasing frequency was observed for some of the studied materials, before self-heating became dominant at higher frequencies. A reduction in loss tangent (viscoelastic damping factor), width of hysteresis loop, and displacement amplitude, measured in load-controlled fatigue tests, was observed by increasing frequency for frequency sensitive materials. Reduction in loss tangent was also observed for frequency sensitive materials in dynamic mechanical analysis tests. It was concluded that the fatigue behavior is also time-dependent for frequency sensitive materials. A Larson-Miller type parameter was used to correlate experimental fatigue data and relate stress amplitude, frequency, cycles to failure, and temperature together. Effects of temperature and mean stress on fatigue behavior were also investigated by conducting load-controlled fatigue tests under positive stress ratios and at room and elevated temperatures. Larson-Miller parameter was used and a shift factor of Arrhenius type was developed to correlate fatigue data at various temperatures. Effect of mean stress on fatigue life was significant for some of the studied materials, however, for the polyphenylene ether and polystyrene blend no effect of mean stress was observed. Modified Goodman and Walker mean stress equations were evaluated for their ability to correlate mean stress data. A general fatigue life prediction model was also used to account for the effects of mean stress, temperature, anisotropy, and frequency. Creep-fatigue tests were conducted using trapezoidal load signal with hold-time periods. Effects of temperature, frequency, load level, mean stress, and hold-stress position on creep-fatigue interaction behavior were studied. In-phase TMF tests were conducted on polyamide-based composite for the temperature variation between 85 to 120 °C. Significant non-linearity was observed for the interaction of creep and fatigue damage. The applicability of Chaboche non-linear creep-fatigue interaction model to predict creep-fatigue and TMF lives for thermoplastic composites was investigated. A frequency term was added to the model to consider the beneficial effect of increased frequency observed for some the studied materials. The Chaboche model constants were obtained by using pure fatigue, pure creep, and one creep-fatigue interaction experimental data. More than 90% of life predictions by the Chaboche model were within a factor of 2 of the experimental life for both creep-fatigue and TMF test conditions.

Committee:

Ali Fatemi, Dr. (Advisor); Mohamed Samir Hefzy, Dr. (Committee Member); Saleh Jabarin, Dr. (Committee Member); Joseph Lawrence, Dr. (Committee Member); Efstratios Nikolaidis, Dr. (Committee Member)

Subjects:

Engineering; Mechanical Engineering

Keywords:

Tensile Behavior; Creep Behavior; Fatigue Behavior; Thermo-Mechanical Fatigue Behavior; Creep-Fatigue Interaction; Short Fiber Reinforced Thermoplastic Composite; Talc-Filled Thermoplastic Composite

Liu, Mu-HsinMultiaxial Fatigue Testing Machine
Master of Science (MS), Ohio University, 2002, Mechanical Engineering (Engineering and Technology)
Multiaxial Fatigue Testing Machine

Committee:

Hajrudin Pasic (Advisor)

Subjects:

Mechanical Engineering

Keywords:

fatigue testing; multiaxial fatigue; fatigue

Lavvafi, HosseinEFFECTS OF LASER MACHINING ON STRUCTURE AND FATIGUE OF 316LVM BIOMEDICAL WIRES
Doctor of Philosophy, Case Western Reserve University, 2013, Materials Science and Engineering
Recent advances in minimally invasive surgical techniques and an increasing need to miniaturize medical devices has led to a surge in developing advanced manufacturing techniques. In order to meet the functional needs of such small devices as cardiovascular stents, guide wires, and needles, the use of new materials and delicate geometries has increased creating a new challenge for manufacturing and machining. Some applications often include fine details that are impossible to achieve with rotary tool machining. Laser machining is one tool harnessing an enormous potential for the manufacture of such finely detailed devices as well as providing a means for improving the local material effects as a result of processing. However, thermal damage caused by laser machining can affect the performance of the components. As devices continue shrinking in size, there is a greater need for “athermal” manufacturing methods that have no adverse effect on performance. In this study, Nd:YAG and femtosecond lasers with different pulse widths were used to machine AISI 316LVM biomedical grade wires. The mechanical behavior of these materials were evaluated in uniaxial tension, and in cyclic strain-controlled fatigue with the use of a flex tester operated to provide fully reversed bending fatigue. All the fatigue testing was conducted in air over a range of cyclic strains to determine both the high-cycle and low-cycle fatigue regimes. The effects of laser input energy and pulse width on surface quality, heat affected zone (HAZ), and subsequent mechanical response are reported Baseline fatigue data on 316LVM wires in the annealed and hard conditions revealed that the hard wires exhibited better high cycle fatigue behavior than exhibited by the annealed wires. However, the low cycle fatigue behavior of the annealed wires was better than that obtained on the hard wires. This was successfully modeled using the Coffin-Manson-Basquin approach. Mixed results were obtained on the fatigue behavior of laser-treated wires. Low power Nd:YAG laser-treated 316LVM annealed wires exhibited better fatigue performance in both high cycle fatigue and low cycle fatigue compared to the annealed 316LVM wires. In contrast, high power Nd:YAG laser-treated annealed wires exhibited poorer fatigue performance in both high cycle and low cycle fatigue. Nd:YAG laser-treated 316LVM hard wires exhibited poorer fatigue performance in both high cycle fatigue and low cycle fatigue compared to the 316LVM hard wires. The femtosecond laser-machined annealed wires exhibited better fatigue life in both high cycle and low cycle fatigue compared to the annealed wires. In contrast, the femtosecond laser-machined hard wires exhibited lower fatigue life in both the high cycle fatigue and low cycle fatigue regimes compared to hard wires. Attempts at modeling the fatigue behavior of the laser-treated wires was unsuccessful using the Coffin-Manson-Basquin approach, likely due to localized surface conditions that dominated the fatigue behavior of the laser-treated wires.

Committee:

John J. Lewandowski, PhD (Committee Chair); David Schwam, PhD (Committee Member); Gerhard Welsch, PhD (Committee Member); Malcolm Cooke, PhD (Committee Member)

Subjects:

Materials Science

Keywords:

316LVM; medical device; laser machining; microfabrication; flex bending fatigue; femtosecond laser; stent; Nd:YAG nanosecond laser; Coffin-Manson-Basquin; fatigue-life; heat exchanger; heat-affected zone

Williams, Petra S.Neural Mechanisms of Task Failure During Sustained Submaximal Contractions
Doctor of Philosophy (PhD), Ohio University, 2013, Biological Sciences (Arts and Sciences)
Fatigue is an expected and normal physiologic reaction to intense and to sustained activity. As fatigue develops during sustained isometric submaximal contractions, the amount of excitatory descending drive from supraspinal regions to the spinal motorneuron pool increases to compensate for the decline in spinal excitability by recruiting additional motor units in order to prolong task performance. However, despite the compensatory mechanisms from supraspinal inputs, task failure remains inevitable. Therefore, it remains largely unknown whether supraspinal mechanisms that could alter the amount of descending drive, including changes in motor cortex excitability and voluntary drive upstream to the motor cortex, also contribute to task failure. The focus of this dissertation research was to delineate the specific contributions that supraspinal circuits have in determining the time to task failure. Experiment 1 compared adjustments in multiple neurophysiologic measures of supraspinal and spinal excitability taken throughout the performance of two different fatigue tasks (i.e. force-matching and position-matching) to determine the functional significance of the changes to task duration. Although no task-specific differences were found, task failure occurred for both tasks after a similar mean decline in motorneuron excitability developed coupled with a similar mean increase in corticospinal excitability. Additionally, the amount of intracortical inhibition dropped while the amount of intracortical facilitation and upstream excitation of the motor cortex remained unchanged. Together the data for these two tasks indicate that, in general, the motor cortex is able to compensate for changes in spinal excitability until a critical amount of change in both regions develops. This suggests that unless more drive is provided to the motor cortex to sustain or strengthen the descending drive, failure occurs. Experiment 2 examined whether delivering anodal transcranial direct current stimulation (tDCS), a non-invasive neurostimulation known to transiently increase cortical excitability, to the motor cortex during the performance of a sustained submaximal contraction would increase task duration compared to a sham tDCS condition. Anodal tDCS increased the time to task failure by more than 30% and also increased the amount of muscle fatigue by 6% in individuals whose time to task failure occurred prior to the termination of the anodal stimulation. Additionally, the stimulation increased the duration of time that the subjects were able to exert a high amount effort. These finding suggests that the anodal tDCS provided the additional excitatory input to the motor cortex needed when task failure was eminent in order to overcome the increase in spinal resistance that could not otherwise be met by volitional drive. Together the results from these two experiments provide complimentary evidence to support the conclusion that the capacity of supraspinal inputs to endlessly override the decline in spinal motorneuron excitability is eventually limited by the failure to increase intracortical facilitation as well as upstream drive delivered to the motor cortex and not the development of intracortical inhibition. The experience of fatigue in healthy populations is both physical and perceptual; however the clinical symptom of perceived fatigue may not be associated with changes in motor performance. The application of these findings to clinical examination and treatment of physical performance fatigue and the symptom of perceived fatigue will benefit from both clinical research as well as further research into the mechanisms of tDCS induced enhancements in task performance and also into the mechanisms behind the difference in time to task failure for the force-matching and position-matching tasks.

Committee:

Brian Clark, PhD (Advisor); Thad Wilson, PhD (Committee Member); Robert Staron, PhD (Committee Member); Roger Gilders, PhD (Committee Chair)

Subjects:

Neurology; Neurosciences; Physiology; Rehabilitation

Keywords:

neural mechanisms of fatigue; sustained submaximal contractions; transcranial direct current stimulation tDCS; transcranial magnetic stimulation TMS; cervicomedullary evoked potentials CMEP; supraspinal mechanisms of fatigue

Liu, Mu-HsinMultiaxial Fatigue Testing Machine
Master of Science (MS), Ohio University, 2002, Mechanical Engineering (Engineering and Technology)
Multiaxial Fatigue Testing Machine

Committee:

Hajrudin Pasic, PhD (Advisor)

Subjects:

Computer Science; Electrical Engineering

Keywords:

fatigue; testing; multiaxial fatigue

Restrepo-Velez, Ana M.Long-Term Performance of Asphalt Concrete Perpetual Pavement WAY-30 Project
Master of Science (MS), Ohio University, 2011, Civil Engineering (Engineering and Technology)
This thesis analyzes the performance of perpetual pavements in use by the Ohio Department of Transportation. The pavement responses, collected from the Controlled Vehicle Loading Test, conducted on the perpetual section AC 664, of the WAY-30 project, were the basis of the study. The effects of several factors on the tensile strains and the loading pulse durations of the pavement, including speed, temperature, applied load, and lateral wheel offset, were evaluated. It was found that, even though the maximum longitudinal tensile strain, measured at the bottom layer of the pavement section, was greater than the recommended Fatigue Endurance Limit of 70 µε, no major distresses have appeared since the road was opened to traffic five years ago. It was also found that temperature and speed have a significant effect on the tensile strains and pulse durations. Greater strains were measured for the higher temperatures and the lower speeds, whereas greater pulse durations occurred at the lowest values of speed and temperature. Additionally, these results were compared with pavement responses predicted using the Mechanistic-Empirical approach described in the Mechanistic-Empirical Pavement Design Guide (MEPDG) and the multi-layer elastic analysis software, JULEA. The MEPDG procedure led to an over-prediction of the strain pulse durations of around 80% compared to those measured in the field. Contrarily, longitudinal tensile strains measured in the field were approximately 1.5 times those predicted using JULEA. Furthermore, an analysis of the pavement performance was conducted based on visual distress surveys and a pavement performance simulation using the MEPDG software. It was found that, according to annual inspections of the conditions of the Section AC 664, no major fatigue cracking signs have been observed in the five year operation period of the road. Additionally, according to the MEPDG simulation, the pavement design will not present significant fatigue cracking distresses over a period of 50 years. Consequently, if this prediction is accurate, it can be concluded that, for this specific pavement design, a higher FEL threshold could be used.

Committee:

Shad Sargand, PhD (Advisor); Deborah McAvoy, PhD (Committee Member); Patricia Toledo-Torres, PhD (Committee Member); Munir Nazzal, PhD (Committee Member)

Subjects:

Civil Engineering

Keywords:

Perpetual pavements; Fatigue cracking in pavements; Fatigue endurance limit; MEPDG; Strains and pulse durations; Pavement performance

Wertz, John NicholasIsothermal Fatigue Life Prediction Techniques
Doctor of Philosophy, The Ohio State University, 2013, Aero/Astro Engineering
Substantial progress has been made in advancing a pre-existing energy-based fatigue life prediction method into a powerful tool for real-world application through three distinct analyses, resulting in considerable improvements to the fidelity and capability of the existing model. First, a torsional fatigue life prediction method with consideration for the identification and incorporation of loading multiaxiality was developed and validated against experimental results from testing of Aluminum 6061-T6 specimens at room temperature. Second, a unique isothermal-mechanical fatigue life testing capability was constructed and utilized in the development of an isothermal-mechanical fatigue life prediction method. This method was validated against experimental data generated from testing of Aluminum 6061-T6 specimens at multiple operating temperatures. Third, alternative quasi-static and dynamic constitutive relationships were applied to the isothermal-mechanical fatigue life prediction method. The accuracy of each new relationship was verified against experimental data generated from testing of two material systems with dissimilar properties: Aluminum 6061-T6 at multiple operating temperatures and Titanium 6Al-4V at room temperature. Each investigation builds upon a previously-developed energy-based life prediction capability, which states: the total strain energy dissipated during both a quasi-static process and a dynamic process are equivalent and a fundamental property of the material. Through these three analyses, the energy-based life prediction framework has acquired the capability of assessing the fatigue life of structures subjected to unplanned multiaxial loading and elevated isothermal operating temperatures; furthermore, alternative constitutive relationships have been successfully employed in improving the fidelity of the life prediction models. This work represents considerable advancements of the energy-based method, and provides a firm foundation for the growth of the energy-based life prediction framework into the thermo-mechanical fatigue regime. This future work will utilize many of the models developed for isothermal-mechanical fatigue; additionally, the isothermal-mechanical testing capability will be readily modified to perform thermo-mechanical fatigue.

Committee:

Mo-How Herman Shen, Ph.D. (Advisor); Noriko Katsube, Ph.D. (Committee Member); Jack McNamara, Ph.D. (Committee Member); Datta Gaitonde, Ph.D. (Committee Member); Tommy George, Ph.D. (Other); Onome Scott-Emuakpor, Ph.D. (Other)

Subjects:

Aerospace Engineering; Aerospace Materials

Keywords:

fatigue; fatigue lifing; life prediction; energy-based; isothermal

Gates, Nicholas RFatigue Behavior under Multiaxial Stress States Including Notch Effects and Variable Amplitude Loading
Doctor of Philosophy, University of Toledo, 2016, Engineering
The central objective of the research performed in this study was to be able to better understand and predict fatigue crack initiation and growth from stress concentrations subjected to complex service loading histories. As such, major areas of focus were related to the understanding and modeling of material deformation behavior, fatigue damage quantification, notch effects, cycle counting, damage accumulation, and crack growth behavior under multiaxial nominal loading conditions. To support the analytical work, a wide variety of deformation and fatigue tests were also performed using tubular and plate specimens made from 2024-T3 aluminum alloy, with and without the inclusion of a circular through-thickness hole. However, the analysis procedures implemented were meant to be general in nature, and applicable to a wide variety of materials and component geometries. As a result, experimental data from literature were also used, when appropriate, to supplement the findings of various analyses. Popular approaches currently used for multiaxial fatigue life analysis are based on the idea of computing an equivalent stress/strain quantity through the extension of static yield criteria. This equivalent stress/strain is then considered to be equal, in terms of fatigue damage, to a uniaxial loading of the same magnitude. However, it has often been shown, and was shown again in this study, that although equivalent stress- and strain-based analysis approaches may work well in certain situations, they lack a general robustness and offer little room for improvement. More advanced analysis techniques, on the other hand, provide an opportunity to more accurately account for various aspects of the fatigue failure process under both constant and variable amplitude loading conditions. As a result, such techniques were of primary interest in the investigations performed. By implementing more advanced life prediction methodologies, both the overall accuracy and the correlation of fatigue life predictions were found to improve for all loading conditions considered in this study. The quantification of multiaxial fatigue damage was identified as being a key area of improvement, where the shear-based Fatemi-Socie (FS) critical plane damage parameter was shown to correlate all fully-reversed constant amplitude fatigue data relatively well. Additionally, a proposed modification to the FS parameter was found to result in improved life predictions in the presence of high tensile mean stress and for different ratios of nominal shear to axial stress. For notched specimens, improvements were also gained through the use of more robust notch deformation and stress gradient models. Theory of Critical Distances (TCD) approaches, together with pseudo stress-based plasticity modeling techniques for local stress-strain estimation, resulted in better correlation of multiaxial fatigue data when compared to traditional approaches such as Neuber’s rule with fatigue notch factor. Since damage parameters containing both stress and strain terms, such as the FS parameter, are able to reflect changes in fatigue damage due to transient material hardening behavior, this issue was also investigated with respect to its impact on variable amplitude life predictions. In order to ensure that material deformation behavior was properly accounted for, stress-strain predictions based on an Armstrong-Frederick-Chaboche style cyclic plasticity model were first compared to results from deformation tests performed under a variety of complex multiaxial loading conditions. The model was simplified based on the assumption of Masing material behavior, and a new transient hardening formulation was proposed so that all modeling parameters could be determined from a relatively limited amount of experimental data. Overall, model predictions were found to agree fairly well with experimental results for all loading histories considered. Finally, in order to evaluate life prediction procedures under realistic loading conditions, variable amplitude fatigue tests were performed using axial, torsion, and combined axial-torsion loading histories derived from recorded flight test data on the lower wing skin area of a military patrol aircraft (tension-dominated). While negligible improvements in life predictions were obtained through the consideration of transient material deformation behavior for these histories, crack initiation definition was found to have a slightly larger impact on prediction accuracy. As a result, when performing analyses using the modified FS damage parameter, transient stress-strain response, and a 0.2 mm crack initiation definition, nearly all variable amplitude fatigue lives, for un-notched and notched specimens, were predicted within a factor of 3 of experimental results. However, variable amplitude life predictions were still more non-conservative than those observed for constant amplitude loading conditions. Although there are numerous factors which could have contributed to this non-conservative tendency, it was determined that some of the error may have resulted from inaccuracies in life prediction curves, the modeling of material deformation behavior, the consideration of normal-shear stress/strain interaction effects, and/or linear versus nonlinear damage accumulation. In addition to crack initiation, fatigue crack growth behavior was also of interest for all tests performed in this study. Constant amplitude crack growth in notched specimens was observed to be a primarily mode I process, while cracks in un-notched specimens were observed to propagate on maximum shear planes, maximum tensile planes, or a combination of both. Specialized tests performed using precracked tubular specimens indicated that the preferred growth mode was dependent on friction and roughness induced closure effects at the crack interface. As a result, a simple model was proposed to account for frictional attenuation based on the idea that crack face interaction reduces the effective stress intensity factor (SIF) by allowing a portion of the nominally applied loading to be transferred through the crack interface. Crack path/branching, growth life, and growth rate predictions based on the proposed model were all shown to agree relatively well with the experimentally observed trends for all loading conditions considered. For notched specimen fatigue tests, although crack growth was observed to be mode I-dominated, constant amplitude crack growth rates under multiaxial nominal stress states were observed to be higher than those for uniaxial loading at the same SIF range. While T-stress corrections were able to account for this difference in some cases, growth rates for pure torsion loading still had the tendency to be higher than those for uniaxial loading. Additionally, using short crack models to account for stress concentration and initial crack geometry effects was found to improve growth rate correlations in the notch affected zone. For 90° out-of-phase loading conditions, small crack growth appeared to have been dominated by the mode I loading from the axial component of the applied stress, but as cracks grew, they turned, and mode I SIF range alone was unable to successfully correlate crack growth rate data. Finally, for variable amplitude crack growth, two state-of-the-art analysis models, UniGrow and FASTRAN, were used to predict crack growth behavior for the notched specimens tested in this study. UniGrow is based on the idea that residual stress distributions surrounding the crack tip are responsible for causing load sequence effects, while FASTRAN attributes these effects to varying degrees of plasticity induced closure in the crack wake. While both models were able to predict nearly all uniaxial constant amplitude crack growth lives within a factor of 3 of experimental results, they both produced conservative predictions under uniaxial variable amplitude loading conditions. For variable amplitude torsion and combined axial-torsion crack growth, however, the degree of conservatism in these predictions was found to reduce. This was attributed to an increase in experimental growth rates due to multiaxial stress states effects, which are not accounted for in either UniGrow or FASTRAN. By comparing differences in crack growth life between tests performed using full and edited versions of the same loading history, it was found that FASTRAN was generally better able to account for the effects of small cycles and/or changes in loading history profile. Additionally, initial crack geometry assumptions were found to have a fairly significant impact on analysis results for the specimen geometry considered in this study.

Committee:

Ali Fatemi (Advisor); Mohamed Samir Hefzy (Committee Member); Efstratios Nikolaidis (Committee Member); Lesley Berhan (Committee Member); Darrell Socie (Committee Member); Nima Shamsaei (Committee Member)

Subjects:

Aerospace Engineering; Aerospace Materials; Engineering; Mechanical Engineering; Mechanics

Keywords:

multiaxial fatigue; variable amplitude; service loading; non-proportional; notch effects; critical plane; fatigue crack growth; mixed-mode; crack closure; cyclic plasticity; 2024-T3 aluminum alloy

Blank, Jonathan PEffect of boron additions on microstructure and mechanical properties of titanium alloys produced by the armstrong process
Doctor of Philosophy, The Ohio State University, 2008, Materials Science and Engineering
The beneficial influence of boron additions on processing, microstructure, physical and mechanical properties of various titanium alloys has been recognized since 1950’s. However, boron additions to titanium alloys to obtain specific microstructures and mechanical properties for several niche applications, including automotive and aerospace, have been actively studied during the past 25 years. The addition of boron concentrations greater than 0.05 wt.% to titanium alloys creates a dispersion of TiB. The presence of TiB enhances the tensile and fatigue strengths as well as the wear resistance as compared to the original titanium alloy. Although these improvements in mechanical properties are attractive, there are still two major obstacles in using these alloys: (1) relationship of microstructure and mechanical properties in Ti-B alloys needs further investigation to optimize the alloys for specific commercial applications; and (2) cost to benefit ratio of producing these alloys is high for a given application(s). The Armstrong process is a novel process that can produce commercially pure (CP) titanium and titanium alloy powder directly from TiCl4 (and other metal halides or as required, to obtain the desired alloy composition). The Armstrong process uses sodium as a reducing agent, with similar reactions as the Hunter process using sodium as a reducing agent and Kroll process using magnesium as a reducing agent. The Armstrong process forms CP-Ti and titanium alloyed powder, which can be directly consolidated or melted into the final product. In comparing the downstream processing steps required by the Kroll and Hunter processes with direct consolidation of Armstrong powder, several processing features or steps are eliminated: (1) restriction of batch processing of material, (2) blending of titanium sponge and master alloy material to create titanium alloys, (3) crushing of the sponge product, (4) melting, and (5) several handling steps. The main objective of this research was to characterize structure and properties of CP-Ti and Ti-B alloys produced by the Armstrong process. Particular emphasis has been placed on improved understanding of the strengthening mechanisms associated with the addition of boron to titanium alloys.

Committee:

James Williams (Advisor)

Subjects:

Textile Technology

Keywords:

Titanium; Titanium-boron; TiB; Commercially Pure Titanium; CP-Ti; Titanium microstructure; Titanium-boron microstructure; Tensile; Notched Fatigue; Fatigue Crack Growth

Yablinsky, Clarissa A.Characterization of Fatigue Mechanisms in Ni-based Superalloys
Doctor of Philosophy, The Ohio State University, 2010, Materials Science and Engineering

Ni-based superalloys are important for turbine engine airfoil applications. Historically, creep has been the main failure mode and thus creep mechanisms have been the subject of numerous studies. However, modern airfoil designs maintain cooler temperatures, and consequently creep is no longer the primary failure mode. Rather, in the cooled components, experience and experimental studies have shown that fatigue is the life-limiting factor. The changing cause of failure highlighted the need for a comprehensive study of fatigue deformation mechanisms. Information about crack propagation and the associated deformation mechanisms has allowed appropriate design changes based on fatigue as a life-limiting factor.

The focus of the study will be on a monocrystalline Ni-based superalloy, René N5, which is currently used for airfoils. Compact tension specimens were tested under cyclic loading conditions to determine the influence of microstructure and material properties on crack propagation and fatigue failure. The crack growth rate as a function of temperature, environment, frequency, and crystallographic orientation was determined. High resolution scanning electron microscopy was used to examine the fracture surface on length scales from nano to macro. Deformation mechanisms in the plastic zone ahead of the crack tip and within the plastic wake of the crack were studied using TEM and FIB techniques.

Environment and frequency seem to have a larger effect on fatigue crack growth rates and threshold stress intensity factor ranges, while temperature and orientation effects are present, but not as dramatic. In the normal blade orientation, (001)[100], mode I crack propagation was prevalent, with mode II crack propagation found at higher ΔK values. Interdendritic particles appear to be slowing crack growth rates in the threshold region of specimens tested in air. Microstructural analysis showed no change in γ’ precipitate size or morphology with temperature or stress. From TEM investigations, it is theorized that a combination of mechanisms is occurring during testing, which is the reason there is no universal trend with temperature for the threshold stress intensity factor ranges. The mechanisms discussed include Kear-Wilsdorf locking, oxide-induced crack closure, and crack tip softening due to γ’ depletion.

Committee:

Katharine Flores, PhD (Advisor); James Williams, PhD (Advisor); Michael Mills, PhD (Committee Member)

Subjects:

Aerospace Materials; Engineering; Materials Science; Metallurgy

Keywords:

Ni-based Superalloys; Fatigue Crack Growth; Rene N5; Fatigue; Temperature; Environment; Frequency; Orientation; Characterization; TEM

Varadarajan, RavikumarOn the Nature of Static and Cyclic Fracture Resistance of Ultra High Molecular Weight Polyethylenes Used in Total Joint Replacements
Doctor of Philosophy, Case Western Reserve University, 2007, Mechanical Engineering

In 2005, about 629,000 total joint replacement (TJR) surgeries were performed in the United States alone and the number is expected to increase by 343% by 2030. In addition, the average age of the patient receiving TJR is decreasing. Therefore, there is an immediate need to enhance the material properties of the implants. Fracture of ultra high molecular weight polyethylene (UHMWPE) components used in total joint replacements is a clinical concern. In this work, static and cyclic fracture resistance of conventional and highly crosslinked and post-processed UHMWPE materials were evaluated in ambient air and physiologically relevant environmental conditions.

Applicability of a compliance based automated system for crack length measurement during fatigue crack propagation (FCP) tests was demonstrated for UHMWPE materials. The Standard compliance calibration coefficients were found to accurately predict the fatigue crack growth only in the low da/dN regime (da/dN < 10-4 mm/cycle). New compliance calibration coefficients that can accurately predict the fatigue crack growth were computed for different UHMWPE materials. FCP studies were conducted in ambient air and in 37°C PBS environments to evaluate the cyclic fracture resistance of UHMWPE materials. In a 37°C PBS environment, the resistance to fatigue crack inception and propagation of sterilized and highly crosslinked UHMWPE materials were found to be reduced compared to ambient air. This findings suggests that under in-vivo conditions UHMWPE implants are more likely to be susceptible to fatigue fracture than might be expected from tests conducted in ambient air. The presence of crack closure overestimates the FCP resistance in the near threshold regime. Crack closure was not observed for any of the UHMWPE materials under the testing conditions selected for this study. Under in-vivo conditions, UHMWPE components may be subjected to overloads. On application of an overload, some test specimens exhibited crack retardation while others exhibited crack acceleration on application of a single overload. On application of multiple overloads crack arrest was observed.

To evaluate the static fracture resistance of UHMWPE materials, J-R curves were obtained in ambient air and 37°C PBS environments using a single specimen normalization method. The single specimen method based on power law based deformation function was demonstrated to predict J-R curves accurately for UHMWPE materials. Significantly lower J-R fracture resistance was observed in the 37°C PBS environment as compared to that in an ambient air environment. A novel experimental method based on CTOD was developed to evaluate fracture initiation in UHMWPE materials. This method predicted conservative and more reliable J-initiation fracture toughness estimates as compared to the traditional blunting line approach.

Committee:

Clare Rimnac (Advisor)

Keywords:

UHMWPE; Polyethylene; Fatigue crack propagation resistance; Compliance calibration; Polymer fatigue; Fracture initiation; J-R curves

McKelvey, Sean AmbroseInfluence of Surface Finish on Bending Fatigue of Forged Steel Including Heating Method, Hardness, and Shot Cleaning Effects
Master of Science in Mechanical Engineering, University of Toledo, 2011, Mechanical Engineering
The overall objective of this study was to conduct a systematic and comprehensive experimental investigation to evaluate and quantify forged surface finish effect at several hardness levels (19 HRC, 25 HRC, 35 HRC, and 45 HRC) on bending fatigue specimens of a commonly used forged steel (10B40 steel). Specimens were subjected to reverse cantilever bending and rotating bending fatigue. Two surface conditions were evaluated, a smooth-polished surface finish to be used as the reference surface, and a hot-forged surface finish. The heating methods used for forging were gas furnace heating as well as induction heating, to allow comparison of the two heating methods, as decarburization depth differs between the two methods. Since shot blasting is commonly used as a forged surface cleaning process with the additional benefit of inducing compressive residual stress, the hot-forged surface finish was evaluated with and without shot blasting. Some testing was also conducted to investigate the effect of the flash left by the forging process. In addition, the effect of grain flow resulting from the forging process was evaluated by testing smooth specimens machined from the same rolled bar used for forging. Fatigue test results in this investigation confirm that the old data commonly used for the as-forged surface condition are too conservative. New forged surface finish factors and curves as a function of hardness or tensile strength and fatigue life were developed based on experimental data. A fracture mechanics-based approach was also used to predict fatigue life for the as-forged fatigue specimens.

Committee:

Dr. Ali Fatemi (Committee Chair); Dr. Phillip White (Committee Member); Dr. Lesley Berhan (Committee Member)

Subjects:

Engineering

Keywords:

Fatigue; Surface Finish Effects; Forged Steel; Bending Fatigue

Hotait, Mohammad AdelA Theoretical and Experimental Investigation on Bending Strength and Fatigue Life of Spiral Bevel and Hypoid Gears
Doctor of Philosophy, The Ohio State University, 2011, Mechanical Engineering

The tooth bending strength characteristics of spiral bevel and hypoid gears are investigated in this study both experimentally and theoretically, focusing specifically on the impact of gear alignment errors. On the experimental side, a new experimental set-up is developed for operating a hypoid gear pair under typical load conditions in the presence of tightly-controlled magnitudes of gear misalignments. The test set-up allows application of all four types of misalignments, namely the shaft offset error (V), the horizontal pinion position error (H), the horizontal gear position error (G) and the shaft angle error (γ). An example face-hobbed hypoid gear pair from an automotive axle unit is instrumented with a set of strain gauges mounted at various root locations of multiple teeth and incorporated with digital signal acquisition and analysis system for collection and analysis of strain signals simultaneously. A number of tests covering typical ranges of misalignments and input torque under both drive and coast conditions are performed to quantify the influence of misalignments on the root stress distributions along the face width.

On the theoretical side, the computational model developed in earlier by Kolivand and Kahraman [31] is expanded to generate the root surfaces of spiral bevel and hypoid gears cut by using either face-milling or face-hobbing processes. A new formulation is proposed to define the gear blank and a numerically efficient cutting simulation methodology is developed to compute the root surfaces from the machine settings, the cutter geometry and the basic design parameters, including both Formate and Generate motions. The generated surfaces are used to define customized finite element models of N-tooth segments of the pinion and the gears via an automated mesh generator. Toot contact loads predicted by a previous load distribution model of Ref. [31] is converted to nodal forces based on the same shape function used to interpolate for nodal displacements. A skyline solver is used to compute the nodal displacements and the resultant stresses at the Gauss points. An extrapolation matrix based on the least-square error formulation is applied to compute the stresses at the root surfaces. Predicted gear root stresses are shown to compare well with the measurements, including not only the extreme stress values but also the stress time histories. Through the same comparisons, the model is also shown to capture the impact of misalignments on the root stress distributions reasonably well.

At the end, a multiaxial, crack nucleation fatigue model of tooth bending is proposed; the model accounts for the multiaxial and non-proportional nature of the stress states predicted. Fatigue lives predicted by the proposed model are compared to those estimated by using a conventional uniaxial failure criterion to show that the predicted multiaxial fatigue lives are significantly lower. The fatigue model is also used to quantify the influence of the misalignments as well as certain key cutting tool parameters on the bending fatigue life of the hypoid gear pair.

Committee:

Ahmet Kahraman, PhD (Advisor); Gary Kinzel, PhD (Committee Member); Dennis Guenther, PhD (Committee Member); Anthony Luscher, PhD (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Hypoid Gears; Spiral Bevel Gears; Tooth Bending Fatigue Failure; Gear Root Stresses; Gear Misalignments; Multiaxial Fatigue

Sirimamilla, Pavana AbhiramMECHANISTIC STUDY OF CRACK INITIATION AND PROPAGATION IN CROSSLINKED ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENES (UHMWPE) SUBJECTED TO STATIC AND CYCLIC LOADING
Doctor of Philosophy, Case Western Reserve University, 2013, EMC - Mechanical Engineering
Many total joint replacement (TJR) designs incorporate a hard bearing material articulating against a soft bearing (polymeric) material. The bearing polymer, typically made of ultra high molecular weight polyethylene (UHMWPE) has been successfully used for the last four decades. However, these bearing materials can fail as a result of in vivo mechanical loads. Reports suggest that the fracture resistance of UHMWPE could be improved which can help the longevity of the components in vivo. Fatigue crack propagation behavior of two crosslinked UHMWPE formulations was investigated to estimate the mechanical governing factor for stable crack propagation. Frequency, waveform and R-ratio were varied between test conditions to determine the governing factor for fatigue crack propagation. It was found that the crack propagation velocity in crosslinked UHMWPE is driven by peak stress intensity (Kmax) in a loading cycle. The findings suggest that stable crack propagation can occur in a static mode rather than in a cyclic mode. Crack initiation from a notch under fatigue conditions is also investigated for remelted 100 kGy material and compared to crack initiation under constant loading conditions. Crack initiation times for fatigue loading conditions were found to be substantially lower compared to static loading conditions. The results suggest that the crosslinked UHMWPE material is resistant under static loading conditions compared to fatigue loading conditions. Fracture resistance of two crosslinked UHMWPE formulations was also investigated. Fracture research utilizing the traditional LEFM and EPFM approaches has not yielded a definite failure criterion for UHMWPE. Therefore, an advanced viscous fracture model has been applied to various notched compact tension specimen geometries to estimate the fracture resistance. Results suggest that the viscous fracture model can be applied to the crosslinked UHMWPE materials and a single value of critical energy (Jc) governs crack initiation and propagation in these materials. This is the first report of a mechanistic approach to crack initiation and propagation in UHMWPE for a range of clinically relevant stress-concentration geometries. A combination of structural analysis of components and material parameter quantification is a path to effective failure prediction in total joint replacement bearings.

Committee:

Clare Rimnac (Committee Chair); Davy Dwight (Committee Member); Joseph Mansour (Committee Member); John Lewandowski (Committee Member)

Subjects:

Engineering

Keywords:

fatigue crack propagation; fatigue crack initiation; crosslinked UHMWPE; viscous fracture model; initiation time;

Hsu, Jui-PoInfluence of Low-Temperature Carburization on Fatigue Crack Growth of Austenitic Stainless Steel 316L
Master of Sciences, Case Western Reserve University, 2008, Materials Science and Engineering
A series of horizontal fatigue tests were performed in air or NaCl solution at room temperature on a 316L type austenitic stainless steel according to ASTM E647-93, “Standard test method for measurement of fatigue crack growth rates”. Direct observation of the carburization effect on fatigue crack growth in air was made on low-temperature carburized and non-carburized specimens under cyclic uniaxial tension. The threshold stress intensity factor range (ΔKth) increases from 8.1 ± 0.3 to 10.0 ± 0.3 MPa√m after low-temperature carburizing, a 23% improvement in retarding crack growth initiation. The resistance to fatigue crack propagation is also improved by decreasing the fatigue crack growth rate at a given value of the stress intensity factor range (ΔK ). Fatigue results in NaCl solution indicate that carburization process does not change the resistance to corrosion fatigue. In addition, SEM fractographic investigation shows no obvious significant difference on fracture mechanisms observed in carburized and non-carburized specimens.

Committee:

Arthur Heuer, PhD (Committee Chair); Frank Ernst, PhD (Committee Member); Gary Michal, PhD (Committee Member); John Lewandowski, PhD (Committee Member)

Subjects:

Materials Science

Keywords:

316L; fatigue; fatigue crack growth rate; WOL; FCGR; low-temperature carburization; LTCSS;

Letcher, Todd M.Structural reliability through robust design optimization and energy-based fatigue analysis
Doctor of Philosophy, The Ohio State University, 2012, Mechanical Engineering

This dissertation analyzes structural reliability through the study of mechanical fatigue and design optimization. This dissertation is a compilation of four separate studies.

First, an energy-based critical fatigue life prediction method is developed and analyzed. The original energy-based fatigue life prediction theory states that the number of cycles to failure is estimated by dividing the total energy accumulated during a monotonic fracture by the strain energy per cycle. Because the accuracy of this concept is heavily dependent on the cyclic behavior of the material, a precise understanding of the strain energy behavior throughout each failure process is necessary. Examination of the stress and strain during fatigue tests shows that the cyclic strain energy behavior is not perfectly stable as initially presumed. It was discovered that fatigue hysteresis energy always accumulates to the same amount of energy by the end of the stable energy region, which has led to a new “critical energy” material property. Characterization of strain energy throughout the fatigue process has thus improved the understanding of an energy-based fatigue life prediction method.

Next, a multi-objective robust optimization framework that incorporates a robustness index for each objective was developed. The top level of the framework consists of the standard optimization problem formulation with the addition of a robustness constraint for each objective. The bottom level uses the Worst Case Sensitivity Region (WCSR) concept to solve a single objective optimization problem to estimate sensitiviy. A separate robustness index for each objective allows the designer to choose the importance of each objective. Example problems display the capabilities of this concept, and the results of these problems demonstrate the effectiveness and usefulness of the multiple robustness index capabilities.

The robust optimization concepts from Chapter 3 are used to develop a new curve fitting technique to find experimental constants for the energy-based fatigue lifing approach. Due to variations in the empirically measured hysteresis loops and monotonic fracture area, fatigue life prediction with the energy-based method shows some variation. In order to account for these variations, a robust design optimization technique is employed. The robust optimization framework ensures that the difference between the predicted lifetime at a given stress amplitude and the corresponding experimental fatigue data point is minimized. Accounting for these experimental variations boosts confidence in the energy-based fatigue life prediction method despite a limited number of test specimens.

Finally, the energy-based lifing method is further examined by studying the effects of varying strain rates and loading waveforms. Previous studies have focused on developing this method for a sine wave loading pattern only – a variable strain rate. In order to remove the effects of a variable strain rate throughout the fatigue cycle, a constant strain rate triangle wave loading pattern was tested. Testing was conducted at various frequencies to evaluate the effects of multiple constant strain rates. Hysteresis loops created with sine wave loading and triangle loading were compared. The effects of variable and constant strain rate loading patterns on hysteresis loops throughout a specimens’ fatigue life are examined.

Committee:

Herman Shen, PhD (Advisor); Rebecca Dupaix, PhD (Committee Member); Brian Harper, PhD (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

fatigue; robust-optimization; hysteresis; energy-based fatigue;

Wolcott, Paul JosephToward Load Bearing Reconfigurable Radio Frequency Antenna Devices Using Ultrasonic Additive Manufacturing
Master of Science, The Ohio State University, 2012, Mechanical Engineering

Ultrasonic additive manufacturing (UAM) is a low temperature, solid-state manufacturing process that enables the creation of layered solid metal structures with designed anisotropies and embedded materials. As a low temperature process, UAM enables the creation of composites using smart materials or other components that would otherwise be destroyed in fusion-type processes. The process uses ultrasonic energy to bond metallic foils to one another under an applied load through a scrubbing action at the foil interface. This scrubbing action creates the nascent surfaces necessary for solid state bonding. To be able to take full advantage of the UAM process, the mechanical properties of composites made therein must be fully characterized. In this study, scanning electron microscopy is utilized to investigate the bonding behavior at the foil interface of samples tested in tension. Findings show a relationship between the amount of ductile fracture and the strength of UAM samples. In addition, fatigue testing was conducted on UAM Al samples to determine their lifetime under cyclic loading conditions. The results indicate a flat S-N behavior between the loading and number of cycles to failure. This indicates that lifetime prediction of UAM samples is difficult at this time due to the inconsistent bonding at the interface. It is theorized that the unbonded areas at the interfaces grow into one another and eventually lead to fast fracture.

Along with the developments made in understanding UAM mechanical properties, the design and manufacture of reconfigurable antennas was conducted with an eventual goal of developing a structural reconfigurable antenna using UAM. The reconfigurable antenna design concept uses shape memory alloy switches to electrically connect to an antenna structure to create discrete shifts in the antenna natural frequency. Using this design concept, three sets of shape memory alloy switches were made to connect with three different antenna structures. The first switch proved the concept of reconfiguration with a monopole antenna, yielding a frequency shift of 85 MHz. The second switch design used smaller dimensions to work in conjunction with a microstrip line, an antenna-like device. With this switch, the transmission of a radio frequency signal was tested to confirm the operation of the switches in both the on and off positions. This setup showed that the metallization of the antenna could effectively change the natural frequency while maintaining significant signal strength. A final set of switches was made for implementation into a planar antenna structure. The planar antenna was designed and constructed with free segments within the structure where switches are connected create reconfiguration. This antenna provides tunable frequency shifts from 2.43 GHz with no switches connected to 2.25 GHz when both switches are connected while maintaining a high gain and repeatable radiation pattern.

Committee:

Marcelo Dapino, Ph.D. (Advisor); S. Suresh Babu, Ph.D. (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Ultrasonic Additive Manufacturing; Ultrasonic consolidation; Metal matrix composites; Fatigue; Reconfigurable antennas; Smart materials

Whitney-Rawls, Ashley WinfieldImpact of Induced Defects on Rotor Life Assessment
Master of Science in Engineering (MSEgr), Wright State University, 2010, Mechanical Engineering
There is an economic need to reduce the conservatisms of current lifing methods and extend component life. Extending component usage increases the probability of failure during operation. Therefore, the risk of continued service must be quantified before life extension concepts can successfully be implemented. The current FAA approved software for the certification of new rotor designs, only accounts for defects present prior to service. Defects due to the handling of components during inspection and material fatigue will induce defects during service and need to be included in any analysis of component life extension. Component life extension analysis of an Inconel 718 late stage turbine disk was conducted which accounted for manufacturing, handling, and fatigue defects. The probability of fracture due to manufacturing defects has a large effect prior to the first inspection. After the first inspection, these defects have a negligible effect due to the significant sensitivity and reliability of component inspection methods. It is shown that the effect of handling induced defects on the probability of fracture is dependent on their occurrence rate and size. It was concluded that handling has a limited effect on the probability of fracture. Past the low cycle fatigue life limit, fatigue of the material will continue to induce defects. The cycles to failure of the defects present at this limit will determine the first, post low cycle fatigue life limit inspection interval. Fatigue defects that initiate shortly before an inspection have a low probability of detection. If future inspection intervals are not adjusted, these undetected defects will grow to failure and have a large impact on the probability of fracture. To account for these undetected defects, future inspection intervals must be shorted to prevent such failures. The effect of applying an inspection timing distribution and percentage of components inspected is also evaluated.

Committee:

Bor Zeng Jang, PhD (Committee Chair); Tarun Goswami, DSc (Committee Co-Chair); Ravi Penmetsa, PhD (Committee Member); Raghavan Srinivasan, PhD (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Rotor Life Extension; Fatigue Induced Defects; Handling Induced Defects

Joiner, Josiah W.Investigation into the Impact of Hold Time, Thermal Mechanical Fatigue, Shotpeen, and Retardation on Fatigue Crack Growth in Inconel Dovetail Slots in Jet Engines
MS, University of Cincinnati, 2011, Engineering and Applied Science: Mechanical Engineering
Current jet engine industry studies are ongoing to develop a generic Inconel dovetail slot test case that will be used for calibrating a manufacturing-induced surface damage anomaly distribution curve for future probabilistic life assessments. The stress and temperature profile during the mission have been defined. This analysis will consist of a design of experiments on the Inconel dovetail slot test data. The test case includes thermal and mechanical stresses, as well as variations in hold time, stress and temperature regimes. Several DOEs will be created and run to help assess the impact of four crack growth mechanisms on the damage tolerance life for the different mission profiles: hold time, thermal mechanical fatigue, shotpeen, and retardation. For the sake of this study a parametric study is considered to be a DOE. Calculations will be completed for both surface and corner cracks. For surface cracks, a 2:1 aspect ratio semi-circular initial flaw size of 15 x 30 mils will be used. For corner cracks, a 1:1 aspect ratio semi-circular initial flaw size of 15 x 15 mils will be used. The calculations will be completed using a proprietary crack propagation code.. The results of this study will reveal the mission profile at which each of the aforementioned effects begins to have a significant impact on the damage tolerance life. These studies are critical to ensuring the final test case adequately addresses each of these critical crack propagation drivers.

Committee:

Janak Dave, PhD (Committee Chair); Janet Dong, PhD (Committee Member); David Thompson, PhD (Committee Member)

Subjects:

Engineering

Keywords:

fracture mechanics;thermal mechanical fatigue;hold time;retardation;tmf

Yang, QiBIO-SIGNAL ANALYSIS IN FATIGUE AND CANCER RELATED FATIGUE: WEAKENING of CORTICOMUSCULAR FUNCTIONAL COUPLING
Doctor of Engineering, Cleveland State University, 2008, Fenn College of Engineering
Fatigue is a common experience that reduces productivity and increases chance of injury, and has been reported as one of most common symptoms with greatest impact on quality-of-life parameters in cancer patients. Neural mechanisms behind fatigue and cancer related fatigue (CRF) are not well known. Recent research has shown dissociation between changes in brain and muscle signals during voluntary muscle fatigue, which may suggest weakening of functional corticomuscular coupling (fCMC). However, this weakening of brain-muscle coupling has never been directly evaluated. More important information could be gained if fCMC is directly detected during fatigue because a voluntary muscle contraction depends on integration of the entire chain of events and is a complex interaction of different components from the central nervous system to peripheral systems. This research, first, evaluated the effect of muscle fatigue on fCMC in healthy people by determining electroencephalography (EEG)-electromyography (EMG) coherence during two stages of a sustained voluntary muscle contraction, one with minimal fatigue and the other with severer fatigue. The obtained results suggest that despite an elevation of the power for both the EEG and EMG activities with muscle fatigue, the fatigue weakens strength of fCMC between the two signals. Secondly, given the fact that there is larger discrepancy between central and peripheral fatigue in CRF, the effect of cancer related fatigue on fCMC was evaluated by comparing EEG-EMG coherence during a muscle fatigue task in CRF patients with healthy controls. CRF patients showed significantly lower fCMC compared to healthy controls during minimal fatigue stage which may be caused by possible pathophysiological impairments in the patients. Finally, to better understand dynamic fatigue effect on fCMC, a single trial coherence estimation based on Morlet wavelet was developed and applied to investigate fatigue effect on fCMC in single trial during repetitive maximal muscle contractions. It was revealed that the decreasing pattern of the fCMC varied among the subjects but the overall decreasing trend was consistent across subjects. The results from the single-trial study suggest it is possible to detect more dynamic fCMC adaptations under acute neuromuscular instability conditions, such as muscle fatigue. This research reveals that muscle fatigue impairs normal coupling between the central and peripheral neuromuscular systems, which could be a major factor contributing to worsened performance under fatigue influence. In general, cancer patients with fatigue symptom exhibit substantially weakened fCMC, even without influence of muscle fatigue. The findings are potentially important in understanding neural mechanisms of muscle fatigue and cancer related fatigue, and in guiding development of new methodologies to improve diagnosis and treatment of fatigue symptoms in clinical populations.

Committee:

Guang Yue (Committee Chair); George Chatzimavroundis (Committee Member); Jingzhi Liu (Committee Member); Fuqin Xiong (Committee Member); Andrew Slifkin (Committee Member)

Subjects:

Biomedical Research

Keywords:

EEG EMG Coherence Fatigue

Daily, Jeremy SDissipated Energy at a Bimaterial Crack Tip Under Cyclic Loading
Doctor of Philosophy (PhD), Wright State University, 2006, Engineering PhD
A new theory of fatigue crack growth in ductile solids has recently been proposed based on the total plastic energy dissipation per cycle ahead of the crack. This, and previous energy-based approaches in the literature, suggest that the total plastic dissipation per cycle can be closely correlated with fatigue crack growth rates under mode I loading. The goal of the current research is to extend the dissipated energy approach to steady-state crack growth under mixed-mode I/II loading conditions, with application to cyclic delamination of ductile bimaterial interfaces. Such systems can occur in brazing, soldering, welding, and a variety of layered manufacturing applications, where high-temperature material deposition can result in a mismatch in mechanical properties between the deposited material and the substrate. The total plastic dissipation per cycle is obtained by 2-D elastic-plastic finite element analysis of a stationary crack in a general mixed-mode specimen geometry under constant amplitude loading. Numerical results for a dimensionless plastic dissipation per cycle are presented over the full range of relevant material combinations and mixed-mode loading conditions. Results suggest that while applied mode-mix ratio is the dominant parameter, mismatches in yield strength and hardening modulus can have a significant effect on the total plastic dissipation per cycle, which is dominated by the weaker/softer material. Results extended to general elastic-plastic mismatches provide substantial insight into the effects of crack-tip constraint, material hardening behavior and applied mode-mix ratio on the dissipated energy during fatigue crack growth. A consistent definition of the mode mix ratio is presented based on minimizing the elastic strain energy at a crack tip. Next, application of the current theory is demonstrated for thermomechanical fatigue of bonded bimaterials. Finally, the plastic dissipation computations are erformed in a probabilistic framework in an attempt to assess the variability of the fatigue crack growth rate based on variation in bulk properties.

Committee:

Nathan Klingbeil (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

fatigue; fracture; bimaterials; plastic dissipation; crack growth rate

Tarar, Wasim AkramA New Finite Element Procedure for Fatigue Life Prediction and High Strain Rate Assessment of Cold Worked Advanced High Strength Steel
Doctor of Philosophy, The Ohio State University, 2008, Mechanical Engineering
This dissertation presents a new finite element procedure for fatigue life prediction and high strain rate assessment of cold worked Advanced High Strength Steel (AHSS). The first part of this research is related to the development of a new finite element procedure from an energy-based fatigue life prediction framework previously developed for prediction of axial, bending and multi-axial fatigue life. The framework for the prediction of fatigue life via energy analysis consists of constitutive laws which correlate the cyclic energy to the amount of energy required to fracture a material. In this study, the energy expressions that construct the new constitutive laws are integrated into a minimum potential energy formulation to develop new finite elements for fatigue life prediction for structural components subjected to axial, bending and multi-axial cyclic loads. The comparison of finite element method (FEM) results to the existing experimental fatigue data verifies the new finite element method for fatigue life prediction. The final output of this finite element analysis is in the form of number of cycles to failure for each element. The performance of the fatigue finite element is demonstrated by the fatigue life predictions from Al 6061-T6 and Ti-6Al-4V. In addition to developing new fatigue finite elements, a new equivalent stress expression and a new finite element procedure for multiaxail fatigue life prediction are also proposed. The new procedure is applicable to biaxial as well as multiaxial fatigue applications. The second part of this research is related to the development of LSDYNA material model for vehicle crash simulation based on high strain rate assessment of cold worked AHSS. In order to simulate actual crash using software like LSDYNA, it is desirable to have accurate stress/strain data for materials. The material models available in the literature ignore the effect of cold working on the material and present data only for flat sheets. In this research, the cold worked AHSS with curved cross-section is tested at strain rates of 1000 (in/in)/s and the data is used to develop a corresponding LSDYNA material model for vehicle crash simulation.

Committee:

Dr. M.-H. Herman Shen (Advisor); Dr. Brian Harper (Committee Co-Chair); Dr. Noriko Katsube (Committee Member); Dr. John Yu (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Fatigue; Life Cycles; Finite Element Methods; Material Models

Scott-Emuakpor, Onome EjaroDevelopment of a novel energy-based method for multi-axial fatigue strength assessment
Doctor of Philosophy, The Ohio State University, 2007, Mechanical Engineering
An accelerated method for determining the fatigue stress versus cycle life (S-N) behavior of isotropic materials is developed for prediction of axial (tension-compression), bending, shear, and multi-axial fatigue life at various stress ratios. The framework for this accelerated method was developed in accordance with a previous understanding of a strain energy and fatigue life correlation, which states: the total strain energy dissipated during a monotonic fracture and a cyclic process is the same material property, where each can be determined by measuring the area underneath the monotonic true stress-strain curve and the area within a hysteresis loop, respectively. The developed framework consists of the following six elements: (1) New experimental procedures used to acquire more sufficient uniaxial and multi-axial test results than conventional methods, (2) an analytical representation for the effect of the stress gradient through the fatigue zone, thus providing capability for bending fatigue prediction, (3) the effect of mean stress on fatigue life for tension/compression and bending, (4) development of an improved energy-based prediction criterion for shear loading at various stress ratios, (5) fatigue life prediction for materials experiencing the endurance limit phenomenon, and (6) the development of a multi-axial fatigue life prediction method. Validation of this accelerated fatigue life determination framework is achieved based on comparison with numerous experimental results acquired from Aluminum 6061-T6 and Titanium 6Al-4V. The results of the comparison are extremely encouraging, thus providing justification that the future direction for the strain-energy based fatigue life prediction method is very promising.

Committee:

Mo-How Shen (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

fatigue; multiaxial; prediction; energy; failure; bending; uniaxial

Bathini, UdaykarA Study of Microstructure, Tensile Deformation, Cyclic Fatigue and Final Fracture Behavior of Commercially Pure Titanium and a Titanium Alloy
Master of Science in Engineering, University of Akron, 2010, Civil Engineering

Rapid industrial growth and advances in the domains of engineering and related technologies during the last fifty years have led to the extensive use of traditional metals and their alloy counterparts. Titanium is one such metal which has gained wide popularity in the aerospace and defense related applications owing to a wide range of impressive mechanical properties like excellent specific strength (σUTS/ρ), stiffness, corrosion and erosion resistance, fracture toughness and capability to withstand significant temperature variations.

Two materials, namely commercial purity titanium (Grade 2), referred to henceforth as Ti- CP (Grade 2) and the “work-horse” alloy Ti-6Al-4V have been chosen for this research study. The intrinsic influence of material composition and test specimen orientation on the tensile and fatigue behavior for both Ti- CP (Grade 2) and Ti-6Al-4V have been discussed. Samples of both Ti- CP (Grade 2) and Ti-6Al-4V were prepared from the as-provided plate stock along both the longitudinal and transverse orientations. The specimens were then deformed to failure in uniaxial tension for the tensile tests and cyclically deformed at different values of maximum stress at constant load ratio of 0.1 for the high cycle fatigue tests. The microstructure, tensile properties, resultant fracture behavior of the two materials is presented in the light of results obtained from the uniaxial tensile tests. The conjoint influence of intrinsic microstructural features, nature of loading and specimen properties on the tensile properties is discussed. Also, the macroscopic fracture mode, the intrinsic features on the fatigue fracture surface and the role of applied stress-microstructural feature interactions in governing failure for the cyclic fatigue properties for both the materials under study Ti- CP (Grade 2) and the “work-horse” alloy Ti-6Al-4V have been discussed in detail.

Careful study of the microstructure for Ti-CP (Grade 2) material at a low magnification revealed the primary alpha (α) grains to be intermingled with small pockets of beta (β) grains. Observation at the higher allowable magnifications of the optical microscope revealed very fine alpha (α) phase lamellae located within the beta (β) grain. The microhardness and macrohardness measurements were consistent through the sheet specimen for Ti- CP (Grade 2) and slightly lower compared to Ti-6Al-4V. However, the macrohardness was marginally higher than the microhardness resulting from the presence of a large volume fraction of the soft alpha phase. The hardness values when plotted reveal marginal spatial variability. Tensile fracture of Ti-CP (Grade 2) was at an inclination to the far field tensile stress axis for both longitudinal and transverse orientations. The overload region revealed a combination of fine microscopic cracks, microscopic voids of varying size and randomly distributed through the surface, and a large population of shallow dimples, features reminiscent of locally brittle and ductile failure mechanisms. The maximum stress (σmaximum) versus fatigue life (Nf) characteristics shown by this material is quite different from those non-ferrous metals that exhibit a well-defined endurance limit. When compared at equal values of maximum stress at a load ratio of 0.1, the fatigue life of the transverse specimen is noticeably greater than the longitudinal counterpart. At equivalent values of maximum elastic strain, the transverse specimens revealed noticeably improved fatigue life as compared one-on-one to the longitudinal counterparts.

Careful observations of the Ti-6Al-4V alloy microstructure over a range of magnifications spanning very low to high magnification revealed a duplex microstructure consisting of the near equiaxed alpha (α) and transformed beta (β) phases. The primary near equiaxed shaped alpha (α) grains (light in color) was well distributed in a lamellar matrix with transformed beta (dark in color). The microhardness and macrohardness values recorded for the Ti-6Al-4V alloy reveal it to be harder than the commercially pure (Grade 2) material. However, for the Ti-6Al-4V alloy the microhardness is noticeably higher than the corresponding macrohardness value that can be ascribed to the presence of a population of processing-related artifacts and the hard beta-phase. Tensile fracture of the Ti-6Al-4V alloy was macroscopically rough and essentially normal to the far field stress axis for the longitudinal orientation and cup-and-cone morphology for the transverse orientation. However, microscopically, the surface was rough and covered with a population of macroscopic and fine microscopic cracks, voids of varying size, a population of shallow dimples of varying size and shape, features reminiscent of locally brittle and ductile failure mechanisms. When compared at equal values of maximum stress at a load ratio of 0.1, there is a marginal to no influence of microstructure on high cycle fatigue life of both orientations of the alloy.

Committee:

Anil Patnaik, Dr. (Advisor)

Subjects:

Civil Engineering; Materials Science

Keywords:

Titanium Alloy; Commercially Pure Titanium; Microstructure, Tensile Properties; Tensile Fracture; Titanium; Materials tests; Cyclic tests; Fatigue life; Fracture

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