Characterization of microstructural effects on small fatigue crack growth mechanisms in Ti-6242S

Design and life management of fracture critical components are made difficult by small scale deformation responses to low applied stresses in the very high cycle fatigue (VHCF) regime, where the number of cycles exceeds 107 and fatigue life is dominated by crack initiation and the growth of microstructurally small cracks. Furthermore, fatigue crack formation is extremely sensitive to microstructural features. Existing models do not fully capture small scale microstructural influences on early fatigue deformation behavior. An understanding of the interactions between the local microstructure and the plastic zone present at the crack tip of a microstructurally small fatigue crack is critical to modeling fatigue crack growth mechanisms. Plastic zone sizes at this scale are highly influenced by local microstructural characteristics such as grain boundary misorientation, phase boundaries, and the presence of precipitates. The use of multiple experimental techniques that enable study of the interactions between the crack-tip plastic zone and local microstructural features in-situ will help develop a quantitative understanding of the underlying small crack growth mechanisms. The role of microstructure on small fatigue crack growth mechanics in the near a titanium alloy Ti6242S has been investigated in the VHCF regime. An experimental methodology for in-situ ultrasonic fatigue (20 kHz) in a scanning electron microscope (SEM) has been developed to investigate mechanisms of fatigue crack formation and the growth of microstructurally small cracks in vacuum and in varying partial pressures of water vapor. In-situ observations of small crack growth behavior from focused ion beam machined micronotches located at key microstructural sites indicated a significant microstructural dependence on crack growth rates. Fatigue crack propagation behavior is also correlated with crack opening displacement and cyclic strain accumulation as measured by advanced in-situ scanning electron microscopy digital image correlation techniques. The evolution of small-scale strain fields at crack tips and in the microstructural neighborhood of the advancing crack is also examined. The specifics of the ultrasonic fatigue SEM instrumentation and the influence of microstructure on small fatigue crack propagation, crack-tip plasticity, and strain localization in the VHCF regime will be described

Effects of Colony Creation Method and Beekeeper Education on Honeybee (Apis mellifera) Mortality

The two-part study reported here analyzed the effects of beekeeper education and colony creation methods on colony mortality. The first study examined the difference in hive mortality between hives managed by beekeepers who had received formal training in beekeeping with beekeepers who had not. The second study examined the effect on hive mortality between hives that were initiated as nucleus or package colonies. Colonies created from package bees were more likely to survive for 1 year than nucleus colonies. Colonies managed by beekeepers who had received formal education also exhibited better survival rates than those managed by non-educated beekeepers

Microstructural strain memory and associated plasticity in superelastic niti under low cycle fatigue

When cyclically loaded in tension, superelastic Nickel Titanium (NiTi) undergoes a characteristic shakedown behavior which dramatically changes its hysteretic stress–strain response. As many uses of superelastic NiTi involve cyclic loading, a detailed understanding of the interaction between phase transformation and associated plasticity is necessary to predict the lifetime behavior of NiTi devices. Earlier macroscopic studies have dealt with this phenomenon on a bulk material level, but its microstructural origin and small scale analogues remain largely uninvestigated. To that end, low cycle, low strain-rate fatigue tests were performed on superelastic NiTi sheet to examine the local damage and accumulation of plastic deformation that contribute to the evolution of its stress strain response. Local strain measured in situ with Scanning Electron Microscopy Digital Image Correlation was matched with individual microstructural features – such as individual parent grains and grain neighborhoods – measured with Electron Backscatter Diffraction. Martensitic transformation associated with superelasticity was inferred from the full-field strain maps captured each load cycle. Special attention was paid to the particular martensite variants and twinning modes that nucleate in the first cycle and their similitude to subsequent martensite transformation. In addition, cyclic behavior such as martensite retention and ratcheting, strain memory of both martensite and austenite configurations, and damage accumulation are also considered

Residual stresses in amorphous alumina films synthesized by ion beam assisted deposition

A set of experiments was conducted to determine the origin of residual stresses in amorphous Al2O3 films formed by ion beam assisted deposition. Samples were deposited during bombardment by Ne, Ar, or Kr over a narrow range of energies, E, and a wide range of ion‐to‐atom arrival rate ratios, R. Films were characterized in terms of composition, thickness, density, crystallinity, microstructure, and residual stress. Film composition was independent of ion beam parameters and residual stress was independent of thickness over the range 200–1200 nm. Stress varied strongly with ion beam parameters and gas content. Residual stress and gas content saturated at a normalized energy of ∼20 eV/atom or an R of ∼0.05. Where residual stress varied linearly with RE1/2, results are consistent with an atom peening model, but saturation at high R or RE1/2 is inconsistent with such a model. Stress due to gas pressure in existing voids explains neither the functional dependence on gas content nor the magnitude of the observed stress. A probable explanation for the behavior of stress is gas incorporation into the matrix, where the amount of incorporated gas is controlled by trapping. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70865/2/JAPIAU-77-7-3029-1.pd

Effect of ion bombardment on in-plane texture, surface morphology, and microstructure of vapor deposited Nb thin films

Niobium films were deposited by physical vapor deposition (PVD) and ion-beam-assisted deposition (IBAD) using ion energies of 0, 250, 500 and 1000 eV, and RR ratios (ion-to-atom arrival rate ratio) of 0, 0.1, and 0.4 on (100) silicon, amorphous glass, and (0001) sapphire substrates of thickness 50–1000 nm. Besides a {110} fiber texture, an in-plane texture was created by orienting the ion beam with respect to the substrate. The in-plane texture as measured by the degree of orientation was strongly dependent on both ion-beam energy and the RR ratio. In fact, the degree of orientation in the films followed a linear relationship with the energy per deposited atom, En.En. The grain structure was columnar and the column width increased with normalized energy. The surface morphology depended on both the normalized energy of the ion beam and the film thickness. All films had domelike surface features that were oriented along the ion-beam incident direction. The dimension of these features increased with normalized energy and film thickness. Surface roughness also increased with normalized energy and film thickness, with the root-mean-square roughness increasing from 1.6 nm for the PVD sample (100 nm thick) to 36.7 nm for the IBAD film (1000 eV, R = 0.4,R=0.4, 800 nm thick). Both the surface morphology evolution and in-plane texture development in these films were the result of the different ion sputter rates among differently oriented grains. © 1997 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69738/2/JAPIAU-81-10-6754-1.pd

Effects of task complexity on dynamic retinoscopy observations

Effects of task complexity on dynamic retinoscopy observation

Near-threshold fatigue crack growth behaviour of a ferritic stainless steel at elevated temperatures

Near-threshold fatigue crack propagation (FCP) behaviour has been studied in an 18%Cr---Nb stabilized ferritic stainless steel at temperatures ranging from room temperature to 700 [deg]C. At a stress ratio of 0.1 increasing the test temperature from room temperature to 500 [deg]C resulted in an increase of the growth rates in the midrange growth regime and a sharply defined threshold at a [delta]K level higher than the room temperature threshold, giving rise to a crossover type of behaviour. At temperatures higher than 500 [deg]C increased crack tip plasticity predominates and the fatigue crack growth rates decrease smoothly with a decreasing value of [Delta]K to thresholds lower than the room temperature value. Crack closure measurements suggest that asperity-induced closure dominates at room temperature but transitions to plasticity-induced closure dominate at 500 [deg]C. A constant-Kmax, increasing R-ratio (CKIR) test procedure was utilized at room temperature and at 500 [deg]C in an attempt to identify near-threshold FCP data in the absence of crack closure. However, the type of crossover behaviour identified with constant R-ratio tests at room temperature and 500 [deg]C was also observed in the CKIR tests. This is attributed to a change in the closure mechanism from a roughness-induced one to one involving crack tip plasticity.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30165/1/0000549.pd

Effects of temperature and frequency on fatigue crack growth in 18% Cr ferritic stainless steel

The fatigue crack growth behaviour of a ferritic stainless steel has been investigated as a function of test temperature, thermal exposure and frequency at intermediate growth rates. In general, fatigue crack growth rates increased with increasing temperature and in the temperature range 500-700 [deg]C growth rates were described by a kinetic process with an activation energy of 48 kJ/mole. Higher than normal growth rates at 475 [deg]C were observed and attributed to an embrittlement process which is known to occur in this temperature regime in high-chromium ferritic stainless steels. The influence of frequency on fatigue crack growth rates was examined at 500 and 655 [deg]C for a load ratio of 0.1 and over four decades of frequency. A transition from time-independent to time-dependent behaviour was observed at each temperature as frequency was lowered. The frequency at which this transition occurred was dependent on temperature. For all temperatures investigated, near threshold crack propagation occurred by a crystallographic or faceted propagation mechanism. At high crack growth rates, crack-tip plasticity was significant and propagation proceeded by a ductile striation formation process. At intermediate growth rates a mixed-mode fatigue crack growth mechanism was observed where some intergranular fracture occurred.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30806/1/0000464.pd

The influence of ion beam mixed Ni---Al surface layers on fatigue in polycrystalline nickel

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26250/1/0000331.pd