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Daily, Jeremy S.Plastic Dissipation Energy in Mixed-Mode Fatigue Crack Growth on Ductile Bimaterial Interfaces
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 and the substrate.

Committee:

Nathan Klingbeil (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

Fatigue crack growth; energy methods; finite elements; fracture mechanics; bimaterials; mixed-mode.

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;

Verma, DhirendraStochastic modeling of fatigue crack growth
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

Keywords:

Stochastic modeling 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

Brosi, Justin KeithMechanical Property Evolution of Al-Mg Alloys Following Intermediate Temperature Thermal Exposure
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

Keywords:

aluminum alloys; aluminum-magnesium; 5xxx; 5083-H116; 5456-H116; solid solution strengthening; sensitization; mechanical properties; thermal exposure; thermal treatment; materials science; fatigue crack growth; delamination; splitting

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

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

Olsen, Kirk WilliamFatigue Crack Growth Analyses and Experimental Verification of Aerospace Threaded Fasteners
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) solution produces conservative crack growth life estimates. Based on test bolt fatigue data, greater accuracy may be possible with this Y(a/d) solution.

Committee:

Clare Rimnac (Advisor)

Keywords:

Fatigue Crack Growth; Aerospace Threaded Fasteners; FRANC3D; Bolts