Master of Sciences (Engineering), Case Western Reserve University, 2022, Materials Science and Engineering

The effects of changes in process parameters, heat treatment, build orientation, and resulting defects on fatigue behavior of selective laser melting-processed AlSi10Mg have been determined. Samples were prepared under distinct P-V parameters and heat treatments for the following classifications: A- nominal with SR+HIP+T6, B- large defects with SR+HIP+T6, G- small to medium defects with SR+HIP+T6, D- many defects with SR+HIP+T6, E- large defects with SR+T6, and F- small to medium defects with SR+T6. Fatigue crack growth (FCG) testing of bend bars and high cycle fatigue (HCF) fatigue testing of cylindrical samples occurred at a stress ratio R = 0.1 and 20 Hz according to appropriate ASTM standards. This is reviewed along with ASTM-standardized tension testing of cylindrical samples. Increased defects typically reduced UTS, ductility, and fracture toughness particularly in the Z orientation. The S-N performances of X/Y orientation were improved or similar to the Z orientation in HCF as a result of having smaller or similar fatigue-initiating defect sizes, that were quantified for all failed S-N samples. HIP-processing generally reduced fatigue-initiating defects sizes and improved most S-N performances. Compared to SLM AlSi10Mg from literature, the nominal (A) build had an increased HCF performance. HCF sample life was estimated by using fracture surface defect measurements and FCG data obtained for those process conditions.

Committee: John Lewandowski (Committee Chair); Sunniva Collins (Committee Member); Clare Rimnac (Committee Member) Subjects: Materials Science

Master of Science, The Ohio State University, 2021, Mechanical Engineering

Teeth of a gear undergo cyclic forces as they rotate in and out of the gear mesh contact zone. The resultant contact and bending stresses produce fatigue damage, which can lead to failure through contact surface degradation and tooth breakage through the root fillet. The fatigue process is not instantaneous, and damage accumulates at different rates throughout the fatigue life. Many proposed diagnostic techniques to detect the onset of gear failure rely on changes in mechanical properties due to crack growth. Moreover, theoretical studies to predict fatigue crack growth rate rely on only a few experimental measurements for their validation with little extension to gears. The focus of this study is to develop an optics-based system for measuring surface crack length coupled with an acoustic emission system for measuring the release of elastic stress waves generated in the tooth root of a fatiguing spur gear, which may be related to early life fatigue damage. A gear single-tooth bending machine, a high-speed camera, and an acoustic emission sensor are utilized in unison to demonstrate the methodology. A digital image correlation technique is used to compute crack length during each load cycle to obtain cyclic crack growth rate, and acoustic emission signals are examined for signature behaviors.

Committee: Ahmet Kahraman PhD (Advisor); Isaac Hong PhD (Advisor) Subjects: Engineering; Mechanical Engineering

Doctor of Philosophy, University of Akron, 2021, Civil Engineering

Heavy duty riveted steel grating is commonly used in bridge construction as it is relatively lightweight and is readily installed. One particular design concern is fatigue, which may affect overall durability and service life of the deck. In this particular study, the fatigue behavior of the deck focused on the riveted connection between the bearing, intermediate and reticuline bars. Fatigue tests previously conducted on sections of riveted deck were compared to more recent results from tests on both open-hole and riveted coupons. Scanning electron microscopy (SEM) allowed observation of the fracture surfaces of several test specimens and striation spacing was used to estimate the stress intensity range at several locations. Lower bound S-N curves are derived for each condition and compared. While test results from the open-hole coupons were consistent with the behavior of the deck sections, fatigue tests of the riveted coupons exhibited superior performance. Sub-scale tensile specimens fabricated from actual rivets were tested and stress-strain curves were obtained. The stress – strain response was utilized in a model of the riveting process to determine the approximate residual stress distribution. A fracture mechanics model was used to examine fatigue cracking around a typical rivet hole and predict fatigue life. Fatigue crack growth curves were developed and compared with the material library in AFGROW. Fracture toughness tests were conducted on specimens with a single edge notch to evaluate the toughness of structural materials used in components of the deck. Fracture toughness values were obtained and compared with AASHTO requirements. In second part of this study, an analysis of a typical riveted steel deck under a standard AASHTO fatigue truck with a 15% impact factor was conducted. Hand calculations were compared with the results of a finite element model using SAP2000 v19.2.1. Bending moments and stresses were evaluated and compared. Stresses at (open full item for complete abstract)

Committee: Craig Menzemer Professor (Advisor) Subjects: Civil Engineering

Master of Science in Engineering, University of Akron, 2020, Mechanical Engineering

Ti-6Al-4V is extremely used and still a promising in aerospace and turbine engines, where fracture toughness and fatigue crack growth resistance are considered as primary important mechanical properties. As repair of damaged turbine blade are challenging with conventional techniques based on criticality of structural parts; directed energy deposition additive manufacturing (DED AM) is in use to repair the damaged parts without total replacement. This has given an interest to have clear ideas in studying the properties of the AM materials in the growth and perpendicular to the growth direction. In this study the use of compact specimens to determine the anisotropy of fracture toughness and fatigue crack growth of DED AM Ti-6Al-4V were investigated. The specimens were originally part of an additive manufacture (AM) repaired study where AM material was added to an already existing commercial wrought Ti-6Al-4V bar. In order to assess the AM material and compare it with the wrought material, a smaller geometry specimen was required. Mode 1 fracture toughness and fatigue crack growth tests were performed under loading conditions to compare the fracture toughness and fatigue crack growth properties and to study the effect of crack geometry. The modes of fractures associated with microstructures were analyzed based on observation of the fracture surfaces. To have a clear insight on causes of failure mechanisms, the mirror-like polished fractured surfaces were subsequently subjected to optical and scanning electron microscopical examinations and the influences of the microstructures on mechanical properties were studied and characterized. The full-field displacements were measured and recorded using 5M DIC equipment. The mechanical testing was supported using electrical resistance (ER) as non-destructive technique to monitor and sensitively capture the formation and growth of cracks in chosen specimens and the data obtained was appropriately analyzed to measure the (open full item for complete abstract)

Committee: Gregory Morscher (Advisor); Manigandan Kannan (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy (PhD), Wright State University, 2020, Engineering PhD

In many structural applications void-like defects cause significant performance debits which call for component redesign or post-processing to account for or remove the defects. For laser powder bed fusion (LPBF) processes, it has been shown that many of these features and their size and shape characteristics are controllable through LPBF process parameter manipulation. For design efforts, however, it is necessary to understand the direct influences of processing on the formation of porosity and the role that individual pores and porosity distributions have on the properties and performance of AM components. Additionally, design criteria must be established to facilitate implementation of AM components into structurally critical applications. To this end, the investigations that have been performed here relate the AM material processing of alloy 718 to the pore structure, crack growth properties and fatigue performance. This dissertation first explores the influence of four key process parameters and scan strategies on the formation and characteristics of porosity distributions in AM material. Then, based on the porosity distributions observed via non-destructive inspection techniques, a crack-growth based life prediction method was developed to accurately predict fatigue lives of AM components. Additionally, fatigue limit models were modified based on experimental data to explore the interactions of defect size and applied stress with respect to both finite and "infinite" fatigue life which enables defect tolerant design for components manufactured via AM. Finally, a novel compliance-based method for crack initiation detection was developed and used to assess some of the assumptions made in the prior investigations. The connections made through the work presented herein link AM processing to potential design requirements which will facilitate faster, safer design efforts for implementation of AM components into structurally critical applications.

Committee: Joy E. Gockel Ph.D. (Advisor); Nathan W. Klingbeil Ph.D. (Committee Member); Ahsan Mian Ph.D. (Committee Member); Onome Scott-Emuakpor Ph.D. (Committee Member); Anthony Rollett Ph.D. (Committee Member) Subjects: Materials Science; Mechanical Engineering; Mechanics; Metallurgy

Master of Science (M.S.), University of Dayton, 2019, Mechanical Engineering

Experimental study accomplished the characterization of fatigue crack growth rates and mechanisms in both hand and die forged Aluminum 7085-T7452. Testing was conducted at various positive and negative loading ratios, primarily focused on L-S and T-S orientations to discover a correlation between crack tip branching or turning mechanisms and stress intensity. Interior delaminations were found to originate in the interior of the specimen and propagate outward to the surface and manifested as splitting cracks parallel to the loading direction. Stress intensity ranges have been correlated with the onset of crack deviation from the primary crack plane, as well as, the transition to branching dominated fatigue crack growth.

Committee: David Myszka (Committee Chair); James Joo (Committee Member); Thomas Spradlin (Committee Member); Mark James (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science; Mechanical Engineering

Master of Science (M.S.), University of Dayton, 2018, Materials Engineering

Aircraft turbine engines, especially military engines, experience variable amplitude loading (mission loading) during operation. Predicting the impact of overloads in turbine engines is key in interpreting fatigue damage and assessing the reliable lifetime of components. The objective of this study was to understand the effects of single and repeated overloads during fatigue crack growth in Ti-6Al-2Sn-4Zr-2Mo used in aircraft turbine engine rotor components. Experiments were conducted using compact tension specimens in a servo-hydraulic testing machine to measure the fatigue crack growth rates during the application of single overloads under stress intensity factor and load control. Additional experiments were conducted having, variable amplitude loading consisting of a controlled number of constant amplitude baseline cycles between periodic overloads. Single overload experiments revealed crack growth acceleration and not the classic retardation typically expected. Repeated overloads experiments demonstrated that Miner's rule accurately predicted realistic overload behavior. Overall, the crack growth rates during single overload or repeated overloads resulted in consistent behavior, and significant crack growth retardation was not observed throughout testing in this material. In addition, crack growth rates were similar for overload and underload block fatigue conditions. The understanding of this behavior and the impact on aircraft turbine engine life tracking using Total Accumulated Cycles (TACs) was discussed. It appeared that minor cycles were generally more damaging than currently accounted for in military turbine engine life tracking.

Committee: Charles Browning (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science

Master of Science in Mechanical Engineering (MSME), Wright State University, 2016, Mechanical Engineering

Ambient temperature dwell sensitivity is known to be deleterious to the fatigue response of near-alpha titanium alloys. Dwell fatigue refers to the presence of a sustained hold at peak stress as opposed to the continuous variation of normal cyclic fatigue loading. This reduction in failure life-times from dwell loading is attributed to early crack nucleation and faster crack propagation. The degradation is the result of plastic anisotropy on the microstructural scale along with tendency of titanium alloys to creep at low temperatures at stresses well below the 0.2% offset yield strength. Despite being the most widely used titanium alloy, Ti-6Al-4V has not been the subject of most dwell fatigue research. Generally, dwell sensitivity is microstructurally dependent and believed to only affect Ti-6Al-4V when severe crystallographic texture is present and under high peak stress loading. Recent studies, however, have suggested that small clusters of preferred crystal orientations, known as micro-textured regions (MTR), can have a significant effect on the dwell sensitivities in Ti-6Al-4V even without severe overall texture in the material. In this study, smooth-bar fatigue specimens were subjected to uniaxial fatigue at 20 Hz cyclic and 2-min dwell loading conditions under load-control at stresses representative of service conditions, until failure occurred. A reduction in specimen life-times by approximately a factor of three was observed under dwell conditions, which was less than for the highly susceptible near-a titanium alloys such as Ti-6Al-2Sn-4Zr-2Mo, where the dwell debit is often in excess of a factor of ten. Measurement of fatigue and dwell fatigue crack growth rates revealed a significant acceleration of the dwell crack growth rates in certain cases. Backscattered electron imaging and electron backscattered diffraction were utilized to quantify the interaction between the cracks and local microstructure. Though no correlation was found between crack growth (open full item for complete abstract)

Committee: Raghavan Srinivasan Ph.D. (Advisor); Adam Pilchak Ph.D. (Committee Member); Joy Gockel Ph.D. (Committee Member) Subjects: Materials Science; Mechanical Engineering

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 fa (open full item for complete abstract)

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

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 (open full item for complete abstract)

Committee: Nathan Klingbeil (Advisor) Subjects: Engineering, Mechanical

Master of Science in Engineering (MSEgr), Wright State University, 2003, Mechanical Engineering

Daily, Jeremy S., M.S. Egr., Department of Mechanical and Materials Engineering, Wright State University, 2003. Plastic Dissipation Energy in Mixed-Mode Fatigue Crack Growth on Ductile Bimaterial Interfaces. 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 study is to extend the dissipated energy approach to steady-state crack growth under mixed-mode loading conditions, with application to cyclic delamination of ductile interfaces in layered materials. 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. Both elastic-perfectly plastic and bi-linear kinematic hardening constitutive behaviors are considered, and numerical results for a dimensionless plastic dissipation per cycle are presented over the full range of relevant mechanical properties and mixed-mode loading conditions. In addition, numerical results are presented for the case of fatigue crack growth along a bonded interface between materials with identical elastic, yet dissimilar plastic properties, including mismatches in both kinematic hardening modulus and yield strength. Finally, the approach is generalized to include mismatches in both elastic and plastic properties, and results for the dimensionless plastic dissipation per cycle are reported over the complete design space of bimaterial interfaces. The results of this thesis are of interest in soldering, welding, coating, electronic packaging, and a variety of layered manufacturing applications, where mismatches in both elastic and plastic properties can exist between the deposited material (open full item for complete abstract)

Committee: Nathan Klingbeil (Advisor) Subjects: Engineering, Mechanical

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, Rene 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 i (open full item for complete abstract)

Committee: Katharine Flores PhD (Advisor); James Williams PhD (Advisor); Michael Mills PhD (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science; Metallurgy

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 res (open full item for complete abstract)

Committee: James Williams (Advisor) Subjects: Textile Technology

Master of Science (M.S.), University of Dayton, 2012, Materials Engineering

Dwell loaded components are common in the turbine engine section of gas turbine engines. The turbine components are subjected to high stresses and high loads for up to 2 min during a pilots take-off, cruise and landing. Understanding the mechanisms of crack initiation while the turbine components are subjected to dwell loading is essential for fatigue life predictions. Previously conducted research includes the fatigue testing and life prediction of dwell specimens with an evenly distributed stress that is representative of a smooth location on the turbine disk. However, turbine disks have notches and fillets that change the stress concentration. This thesis utilizes the techniques and testing strategies that have been successful in predicting fatigue life in smooth bar dwell specimens and applies them to dwell notched specimens that are more representative of actual fielded components. The notch fatigue study indicates that the minimum fatigue life prediction method is applicable for various geometries and therefore, its applicability to actual fielded components holds reasonable promise.

Committee: Charles Browning PhD (Advisor); Andrew Rosenberger PhD (Committee Member); James Snide PhD (Committee Member) Subjects: Engineering; Materials Science

Master of Science (M.S.), University of Dayton, 2010, Mechanical Engineering

It has been observed that the life limiting fatigue behavior in numerous superalloys is dominated by small crack growth behavior. While environmental effects on crack growth behavior of Ni-base superalloys are well documented within the literature, the published research is largely limited to long crack behavior due to the difficulty of measuring small cracks in a vacuum chamber. A testing capability for optical measurement of small cracks under ultra-high vacuum and at elevated temperatures has been developed. Optical measurement capabilities have been evaluated on a lab air machine to determine crack measurement accuracy. Vacuum tests were then run at 650°C on a sub-solvus IN100 specimen to quantify the effect of vacuum on the propagation life within the small crack regime. The effectiveness of this test capability and the role of environment on small crack growth behavior will be discussed.

Committee: Robert Brockman PhD (Advisor); Steven Donaldson PhD (Committee Member); Gerald Shaughnessy (Committee Member) Subjects: Materials Science; Mechanical Engineering

Master of Sciences (Engineering), Case Western Reserve University, 2010, Materials Science and Engineering

Experience has shown that 5xxx-series alloys become susceptible to chemical segregation when exposed to temperatures as low as 50°C for long times. In thecurrent study, 5083-H116 and 5456-H116 alloys were exposed to temperatures of 80°C, 100°C, and 175°C for up to 10,000 hours. After exposure, room temperature hardness, tensile, and L-T direction fatigue bend tests were conducted. Reductions in 0.2% yield strength up to 28.8% were observed, with greater reduction as temperature and time increase. Rockwell B hardness decreased up to 20.7% after 100 hours, and then returned to near-as-received levels. Elastic modulus, ultimate tensile stress, and L-T direction fatigue threshold were not measurably affected by thermal exposure. In L-T direction fatigue testing, lateral splitting appeared on the fatigue fracture surfaces, becoming more evident with increasing thermal exposure. Limited fatigue testing in the direction of splitting (S-T direction) suggests that toughness is significantly lower than in the L-T direction.

Committee: John Lewandowski PhD (Committee Chair); Gary Michal PhD (Committee Member); David Schwam PhD (Committee Member) Subjects: Engineering; Materials Science; Metallurgy

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

Doctor of Philosophy, Case Western Reserve University, 2004, Mechanical Engineering

Because fatigue crack growth in a threaded fastener can cause the loss of an aircraft, damage tolerant analyses are required. Therefore, aerospace designers must be able to perform accurate crack growth analyses on fasteners. However, threaded fasteners are difficult to analyze and fastener fatigue crack growth data is scant, especially for non-dimensionalized crack depths of (a/d) < 0.1. The objective of this research is to determine the stress intensity multiplication factor (Y), as a function of a/d, in the threads of a nut loaded, aerospace, roll-threaded bolt under tensile fatigue conditions as a/d approaches zero. Y(a/d) can then be used to improve the accuracy of fatigue crack growth life estimations. The research objectives were achieved through bolt material characterization, cyclic testing, and numeric modeling. X-ray diffraction was used to determine the residual stress within the thread root of the test bolts. Unflawed and flawed aerospace bolts were fatigue tested at a maximum stress (S) ranging from the ultimate tensile strength (UTS) to the surface endurance limit of the test bolt and loading ratios of 0.1 < R < 0.9. The following data was collected: cycles to failure (Nf), fracture surface striation spacing, and crack front shape. The numeric studies accounted for residual stress. The fracture analysis code, FRANC3D, was used because it could predict crack front shape and stress intensity factor (K). The thread root, residual compressive stress reached 65% of the material UTS. The S- Nf plots showed test bolt fatigue strength decreased as R decreased and 10% reduction in allowable fatigue stress due to flaws. The shape of the crack front in the unflawed and flawed stainless steel, test bolts were different and both changed as the crack grew. The developed numeric models also predicted a changing crack front and the stress intensity factor. By curve fitting the numeric and experimental data, a new Y(a/d) solution was determined. The use of this Y(a/d) (open full item for complete abstract)

Committee: Clare Rimnac (Advisor) Subjects:

Doctor of Philosophy, Case Western Reserve University, 1993, Macromolecular Science

The present work has made gains in relating the kinetics of fatigue testing to that of creep failure. It has been found that fatigue loading produces brittle crack growth similar to that observed in long term field failures. Furthermore, the fatigue testing is able to rank different polyethylene copolymers in the same order as does creep. This is very promising in light of the fact that fatigue can accelerate slow crack growth to orders of magnitude faster than creep. In support of this, international round robin testing strongly recommends the use of fatigue to rank brittle crack resistance in polyethylene. These results establish a basis for using fatigue to predict long term creep behavior in relatively short testing times. The approach taken was to use a modified Paris equation to obtain kinetic parameters (A and m) as a function of R ratio and then extrapolate these parameters to creep conditions (R = 1). These extrapolated parameters were in good agreement with those found in the literature. This predictive method is an order of magnitude faster and can even be used with newer, tougher (Category II) materials that are not amenable to the accelerated creep method. So far, the concern has been with the propagation of the crack. We know however, that initiation can comprise a large part of the material lifetime. Kinetics parameters of craze evolution preceding fatigue crack initiation were therefore determined and analyzed. Growth of the craze ahead of the crack tip with respect to the number of fatigue cycles was found to follow a power law. Then, once the craze had grown to its fully developed configuration, initiation occurred. It is also instructive to note that the craze length after a single cycle correlated with the number of cycles needed for initiation. That is, the shorter this single cycle craze, the greater the number of cycles needed for initiation. Prior to initiation, we have seen that the damage zone ahead of the crack tip must grow to some critic (open full item for complete abstract)

Committee: A. Moet (Advisor) Subjects:

Doctor of Philosophy, Case Western Reserve University, 1990, Civil Engineering

Fatigue of metals has been recognized as an important cause of failure of engineering structures. The experiments show that the fatigue life of real mechanical components is characteristically random. The random nature of the fatigue process is most obvious if a structure is subjected to time-varying random loading. This work develops a stochastic phenomenological model for crack growth which incorporates the effects of material inhomogeneity and random loading as well as including deterministic models which try to explain experimentally observed behavior, thus removing a majority of the shortcomings in existing stochastic models.

Committee: Fred Moses (Advisor) Subjects: Engineering, Civil