The role of grain bridging in affecting the initial rising portion of the R ‐curve and the transient, non‐steady‐state behavior of short cracks during (cyclic) fatigue‐crack propagation has been quantitatively examined in a 99.5% pure alumina. Fatigue‐crack growth properties for both long and short (Δ a f < 2 mm) cracks emanating from machined notches (root radius, ∼ 15–150 μm) were investigated, where Δ a f is the extension of the fatigue crack from the notch. Growth rates (d a /d N ) were far higher at the same applied stress‐intensity range (Δ K ) and fatigue thresholds, Δ K TH , were markedly lower for short cracks than for corresponding long cracks. Crack extension was measured at the lowest driving forces for short cracks emanating from razor micronotches with ∼ 15 μm. For growth rates <10 ‐8 m/cycle, d a /d N vs Δ K curves for short cracks merged with the demonstrably steady‐state curve for long cracks after ∼2 mm of crack extension. This length corresponds well to the extent of the measured crack‐bridging zone for a near‐threshold steady‐state fatigue crack. For d a /d N > 10 ‐8 m/cycle, however, non‐steady‐state behavior was observed at all crack sizes, indicating that achieving steady state at each Δ K level is difficult. The crack‐tip shielding contribution due to such grain bridging was determined using both direct compliance and the more accurate multi‐cutting/crack‐opening profile techniques. Bridging stress‐intensity factors were computed and subtracted from the applied stress intensities to estimate an effective (near‐tip) driving force, Δ K eff These results provided (i) a lower threshold (in terms of Δ K eff ) below which both long and short fatigue cracks should not propagate, and (ii) an estimate of the intrinsic toughness, K 0 , for the start of the R ‐curve. Such results quantitatively affirm that the reduced role of grain bridging is a primary source of the transient behavior of short cracks in grain‐bridging alumina‐based ceramics under cyclic loading.
Preface Acknowledgments Chapter One: The Problem and the Challenge Chapter Two: An Idealized Health Care System Chapter Three: Participants, Framework, and Approach Chapter Four: Functional Integration Chapter Five: Physician-System Integration Chapter Six: Clinical Integration Chapter Seven: Managing and Governing the Organized Delivery System Chapter Eight: Moving Forward: The Policy Issues and Implementation: Challenges: Resources References
Many useful and well-known image processing templates for cellular neural networks (CNN's) can be derived from neural field models, thereby providing a neural basis for the CNN paradigm. The potential ability of multitasking image processing is investigated by using these templates. Many visual illusions are simulated via CNN image processing. The ability of the CNN to mimic such high-level brain functions suggests possible applications of the CNN in cognitive engineering. Furthermore, two kinds of painting-like image processings, namely, texture generation and illustration style transformation are investigated.
Abstract Although memory processes play a central role in both psychological and neurobiological accounts of the development of posttraumatic stress disorder (PTSD), there has been little integration of the two literatures. This paper aims to consider the implications of an integrated account of trauma memory for pharmacological treatments that have been proposed for the prevention of PTSD. The idea of reprocessing trauma memories to bring about recovery, central to the psychological account of PTSD, is translated into terms more familiar in the biological literature using the concept of reconsolidation of active memories. It is suggested that physiological arousal enhances the reprocessing of trauma memories. Drugs that influence arousal may have effects after trauma which depend on the psychosocial context, helping to prevent the development of PTSD in some trauma victims, but impeding recovery in others who would do well without treatment.
The bridging tractions developed behind a crack tip are considered for a stationary crack under cyclic loading conditions at elevated temperatures in high-toughness, monolithic ceramics. Assuming a temperature range where the grain-boundary phases are sufficiently soft such that bridging can occur due to a viscous layer in the boundary, a viscoelastic model is developed in which bridging forces associated with the shear resistance of the grain-boundary phase are transmitted across the surfaces of a crack. Throughout the work, cyclic and static damage mechanisms which may be operating ahead of the crack tip (e.g. creep cavitation) are ignored in order to focus exclusively on the role of viscous grain bridging. A primary goal is to incorporate microstructural details like grain shape, grain-boundary thickness, and glass viscosity, as well as the effects of external variables such as loading rate and temperature. A fully self-consistent numerical approach is adopted, which does not require any prescribed assumptions as to the shape of the crack-opening profile. The self-consistent solution is compared to an analytical solution for a simplified parabolic approximation of the crack-flank opening displacements. The model is applicable to a wide range of ceramic materials at elevated temperatures, and rationalizes the frequency and temperature sensitivity not generally observed in ceramics at room temperature. Solutions identify a non-dimensional group associated with microstructure and external loading conditions, and solutions are presented over a range of this parameter.
The ability of a dendritic shell to afford site isolation to a porphyrin core was evaluated using electron-transfer experiments with a series of porphyrin-core dendrimers. Cyclic voltammograms show that surrounding a porphyrin site with even a small generation (G ∼ 2) dendrimer can significantly lower the rate of interfacial electron transfer, ostensibly by decreasing the proximity of the porphyrin core to the electrode surface. This inhibition of electron transfer is more pronounced when larger generation dendrimers are employed. While a significant measure of site isolation is achieved with respect to an electrode surface, no hindrance to penetration of a small molecule is afforded by the dendritic shell surrounding the porphyrin core, an encouraging result if dendrimers are to be designed as macromolecular hosts with a functioning catalyst at the core. Stern−Volmer analysis was used to investigate the accessibility of a small molecule, benzylviologen, to the porphyrin core. For generations 1−3, the den...