6,332 publications from this institution
We introduce radial encoding of nanowires (NWs), a new method of differentiating and controlling NWs by a small set of mesoscale wires for use in crossbar memories. We describe methods of controlling these NWs and give efficient manufacturing algorithms. These new encoding and decoding methods do not suffer from the misalignment characteristic of flow-aligned NWs. They achieve comparable effective pitch and resulting memory density with axially encoded NWs, while avoiding potential cases of address ambiguity and simplifying NW preparation. We also explore hybrid axial/radial encodings and show that they offer no net benefit over pure codes.
Recent experimental evidence points to limitations in characterizing the critical strain in ductile fracture solely on the basis of stress triaxiality. A second measure of stress state, such as the Lode parameter, is required to discriminate between axisymmetric and shear-dominated stress states. This is brought into the sharpest relief by the fact that many structural metals have a fracture strain in shear, at zero stress triaxiality, that can be well below fracture strains under axisymmetric stressing at significantly higher triaxiality. Moreover, recent theoretical studies of void growth reveal that triaxiality alone is insufficient to characterize important growth and coalescence features. As currently formulated, the Gurson Model of metal plasticity predicts no damage change with strain under zero mean stress, except when voids are nucleated. Consequently, the model excludes shear softening due to void distortion and inter-void linking. As it stands, the model effectively excludes the possibility of shear localization and fracture under conditions of low triaxiality if void nucleation is not invoked. In this paper, an extension of the Gurson model is proposed that incorporates damage growth under low triaxiality straining for shear-dominated states. The extension retains the isotropy of the original Gurson Model by making use of the third invariant of stress to distinguish shear dominated states. The importance of the extension is illustrated by a study of shear localization over the complete range of applied stress states, clarifying recently reported experimental trends. The extension opens the possibility for computational fracture approaches based on the Gurson Model to be extended to shear-dominated failures such as projectile penetration and shear-off phenomena under impulsive loadings.
This account reviews the use of templates, fabricated by patterning paper, for the delivery of aqueous solutions of reactants (predominantly, ions) in the preparation of structured, thin materials (e.g., films of ionotropic hydrogels). In these methods, a patterned sheet of paper transfers an aqueous solution of reagent to a second phase—either solid or liquid—brought into contact with the template; this process can form solid structures with thicknesses that are typically ≤1.5 mm. The shape of the template and the pattern of a hydrophobic barrier on the paper control the shape of the product, in its plane, by restricting the delivery of the reagent in two dimensions. The concentration of the reagents, and the duration that the template remains in contact with the second phase, control growth in the third dimension (i.e., thickness). The method is especially useful in fabricating shaped films of ionotropic hydrogels (e.g., calcium alginate) by controlling the delivery of solutions of multivalent cations to solutions of anionic polymers. The templates can also be used to direct reactions that generate patterns of solid precipitates within sheets of paper. This review examines applications of the method for: (i) patterning bacteria in two dimensions within a hydrogel film, (ii) manipulating hydrogel films and sheets of paper magnetically, and (iii) generating dynamic 3-D structures (e.g., a cylinder of rising bubbles of O2) from sheets of paper with 2-D patterns of a catalyst (e.g., Pd0) immersed in appropriate reagents (e.g., 1% H2O2 in water).
Models for the debonding of a fiber embedded in a brittle matrix are proposed and analyzed. Attention is restricted to systems having a residual compressive stress acting across the fiber/matrix interface. Debonding, as well as pullout after the fiber breaks, is accompanied by frictional sliding. Fiber—matrix interaction is modeled by a cylindrical cell with two sets of boundary conditions: one modeling an isolated fiber—matrix unit and the other a matrix containing an array of unidirectional fibers. The elastic properties of the fiber are taken to be transversely isotropic about the fiber axis, while the matrix is assumed to be isotropic. The debonding process is treated within the framework of fracture mechanics as a mode 2 crack. Two idealizations of friction are considered: a constant friction stress independent of normal compression across the interface, and Coulomb friction. Approximate closed form solutions to the model are presented. These are assessed using results from an accurate numerical analysis.
No abstract is provided for this article.
Numerical simulations of slip instabilities on a vertical strike‐slip fault in an elastic half‐space are performed for various models belonging to two different categories. The first category consists of inherently discrete cellular fault models. Such are used to represent fault systems made of segments (modeled by numerical cells) that can fail independently of one another. Their quasi‐independence is assumed to provide an approximate representation of strong fault heterogeneity, due to geometric or material property disorder, that can arrest ruptures at segment boundaries. The second category consists of models having a well‐defined continuum limit. These involve a fault governed by rate‐ and state‐dependent friction and are used to evaluate what types of property heterogeneity could lead to the quasi‐independent behavior of neighboring fault segments assumed in the first category. The cases examined include models of a cellular fault subjected to various complex spatial distributions of static to kinetic strength drops, and models incorporating rate‐ and state‐dependent friction subjected to various spatial distributions of effective stress (normal stress minus pore pressure). The results indicate that gradual effective stress variations do not provide a sufficient mechanism for the generation of observed seismic response. Strong and abrupt fault heterogeneity, as envisioned in the inherently discrete category, is required for the generation of complex slip patterns and a wide spectrum of event sizes. Strong fault heterogeneity also facilitates the generation of rough rupture fronts capable of radiating high‐frequency seismic waves. The large earthquakes in both categories of models occur on a quasi‐periodic basis; the degree of periodicity increases with event size and decreases with model complexity. However, in all discrete segmented cases the models generate nonrepeating sequences of earthquakes, and the nature of the large (quasi‐periodic) events is highly variable. The results indicate that expectations for regular sequences of earthquakes and/or simple repetitive precursory slip patterns are unrealistic. The frequency‐size (FS) statistics of the small failure episodes simulated by the cellular fault models are approximately self‐similar with b ≈ 1.2 and b A ≈ 1, where b and b A are b values based on magnitude and rupture area, respectively. For failure episodes larger than a critical size, however, the simulated statistics are strongly enhanced with respect to self‐similar distributions defined by the small events. This is due to the fact that the stress concentrated at the edge of a rupture expanding in an elastic solid grows with the rupture size. When the fault properties (e.g., geometric irregularities) are characterized by a narrow range of size scales, the scaling of stress concentrations with the size of the failure zone creates a critical rupture area terminating the self‐similar earthquake statistics. In such systems, events reaching the critical size become (on the average) unstoppable, and they continue to grow to a size limited by a characteristic model dimension. When, however, the system is characterized by a broad spectrum of size scales, the above phenomena are suppressed and the range of (apparent) self‐similar FS statistics is broad and characterized by average b and b A values of about 1. The simulations indicate that power law extrapolations of low‐magnitude seismicity will often underestimate the rate of occurrence of moderate and large earthquakes. The models establish connections between features of FS statistics of earthquakes (range of self‐similar regimes, local maxima) and structural properties of faults (dominant size scales of heterogeneities, dimensions of coherent brittle zones). The results suggest that observed FS statistics can be used to obtain information on crustal thickness and fault zone structure.
