The largest single cause of failure in fan and compressor components in the cold frontal sections of commercial and military gas turbine engines has been attributed to high cycle fatigue (HCF). Additionally, both high-cycle fatigue (HCF) and lowcycle fatigue (LCF) loadings are widely recognized as unavoidable during operation of these components and because the classic Linear Damage Rule (LDR) neglects to account for the synergistic interaction between these damage contributors, dangerous over predictions of lifetime can result. Combined low-cycle fatigue / high-cycle fatigue (HCF/LCF) loadings were investigated in smooth Ti-6Al-4V. The specimens were subjected to a variable amplitude block loading history comprised of completely-reversed (R = -1) tensioncompression overloads followed by constant-amplitude zero-tension (R = 0) minor cycles. Axial specimens were excised from forgings representative of turbine engine fan blade forgings, and consisted of approximately 60% primary α in a matrix of lamellar α + β. Data are reported for smooth specimens of Ti-6Al-4V subjected to both constant amplitude and variable amplitude loadings. The axial specimens were prepared according to two distinct specimen conditions: low stress ground and longitudinallypolished (LSG+LP) and stress-relieved and chemically milled (SR+CM) conditions. Significantly longer lives were observed for the LSG+LP specimen condition under both constant and variable amplitude loading, due to the presence of a beneficial compressive surface residual stress. The presence of this residual stress was confirmed by x-ray diffraction, and its magnitude was of the order of 180 MPa (~20% of the yield stress). In either specimen condition, no appreciable effect of periodic overloads on the life of subsequent minor cycles was observed. / Master of Science

A study has been carried out to investigate the growth and closure behavior of short cracks in 2024-T351 aluminum alloy and four different conditions of 4340 steel using through thickness cracks of straight fronts. The experiments were carried out to study the influence of stress level, stress ratio, yield strength and prior austenitic grain A sizes in notched and unnotched specimens. The stereoimaging technique was developed and adapted to obtain crack closing and opening points, and also near tip displacement fields. Experimental results are presented with a general discussion. It was found that long cracks showed good correlation when analyzed in terms of effective stress intensity range. However,correlations were poor for short cracks. lt was found that short cracks show less closure behavior than long cracks. The estimates of initial crack lengths based on linear elastic data were made. These estimates differed significantly from the actual initial crack lengths for completely reversed cycling tests. Suggestions have been made to the equivalent initial flaw size approach and conclusions have been drawn. / Ph. D.

Includes photographs. / Digitized by Kansas Correctional Industries

A new design is presented for a wideband orthogonal interconnect between two perpendicular printed wiring boards, employing novel geometries and materials to minimize stress under cyclic loading. This will ensure fatigue survivability in high vibration environments, opening the door to vertical interconnection in RF circuit design. This is, to the best of knowledge, the first complete design and prototype for an orthogonal interconnect at the board level for broadband RF circuits. An analytical approach is used to define the driving parameters in the stress distribution within a smooth curve joining two perpendicular surfaces using analytical geometries, and Finite Element Analysis is used to finalize the design and ensure all constituent materials in the interconnect are subjected to stresses below their fatigue strength at 10 million cycles at full deflection. A manufacturing process is then proposed using thermoforming to shape the Liquid Crystal Polymer base material into the desired geometry, as well as an assembly solution to mount the interconnect to an RF signal feed card. Finally, a test setup is designed allowing for high cycle fatigue testing within the order of hours, including the capability to monitor performance of the interconnect by tracking DC continuity through a simulated application using a single post design. The prototype interconnect is tested to failure and is shown to survive 18 million cycles of a typical loading application before failure of the LCP springs occurs in the mode predicted by the initial FEA model.

Prediction of fatigue damage due to fretting is complex due to the number of influential factors and the competitive interaction between wear and fatigue. The majority of current fretting damage modeling approaches are limited to narrow ranges of conditions where little competition between damage mechanisms occurs. Recent models which account for damage interaction are largely phenomenological in nature and are still limited to a narrow range of applicability. A method to characterize and model the level of fatigue damage due to fretting was developed in this work to address the shortcomings of the current methods available by extending the range of conditions captured and enhancing the physical basis of the damage model. Baseline material properties for thin sheets of AISI 301 stainless steel in the full hard condition were determined as a function of temperature through tensile tests, fatigue tests, and metallography. Fretting experiments were performed for contact between 301 stainless steel and each ANSI A356 aluminum and AISI 52100 steel. Fretting experiments were performed over a range of material combinations, normal forces, displacement amplitudes, atmospheres, and temperatures. Subsequent characterization of the damage due to fretting was performed to determine the level of wear and fatigue damage incurred for each condition tested. A finite element model of the experiment was created to determine the cyclic stress-strain behavior and local frictional energy dissipation for each condition. Fatigue damage metrics were evaluated to determine the effects of the contact conditions on the driver for fatigue damage. A new model for fatigue damage due to fretting was developed which incorporates the wear behavior to describe the effect of wear on the level of fatigue damage caused by fretting. The level of fatigue damage is influenced using a function of frictional energy dissipation and wear rate to account for differences in wear mechanisms and changes in the severity of wear caused by changes in oxidation behavior and mechanical properties which result from changes in temperature or contacting materials.

This research is concerned with the effects of temperature and frequency on the fatigue crack propagation behaviour. The fatigue behaviour at elevated temperature has been reviewed with emphasis on the mechanisms of propagation proposed to explain the effects of temperature and frequency. The crack propagation characteristics of a single crystal superalloy have been investigated at various temperatures and frequencies. Determination of the fatigue crack propagation rates was combined with the measurement of crack tip plastic zone sizes and observation of the deformation behaviour. At elevated temperature, the fatigue crack propagation rates were determined to be dependent on the frequency of loading. At low and high crack growth rates increased crack propagation rates were associated with higher cycling frequencies. At intermediate crack growth rates, the highest propagation rates were associated with the lowest frequency. Selected area channelling pattern techniques were used to determine the plastic zone sizes and revealed that at high frequencies (>10Hz) the plastic zone sizes at elevated and ambient temperature are similar. With decreasing frequency at elevated temperature creep may lead to increased plastic zone sizes but may also affect the crack tip geometry thereby restricting further effects. At room temperature crack propagation is observed to occur by shear of the γ' precipitate on <110> {111} resulting in {111} crystallographic facets on the fracture surface. At elevated temperature (>6000 C) however, the crack growth mechanism changes and propagation takes place on {100}. Observations of the dislocation structure indicate that deformation is concentrated within the γ matrix. It is proposed that the observed crack propagation in the γ matrix on {100} arises by the constraints imposed by the surrounding γ' precipitate. A model is presented for the frequency dependence of the intermediate crack growth rate in terms of the observed strain gradients within the plastic zone and the achievement of a critical local fracture strain.

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