Despite the recognized importance of the in vivo fatigue properties of Nitinol, there is still limited understanding of how fatigue cracks propagate in this material. This study represents an initial approach to gain such understanding via a combination of fracture-mechanics testing and synchrotron x-ray (micro) diffraction, by providing insight into the role of transformational and local strain fields on the progression of fracture in a stent-like material structure. The results presented here compare the fracture-mechanics predicted transformation-zone size and shape with the actual zones measured by micro-diffraction. Tests were conducted with compact-tension specimens, laser-cut from Nitinol tube that was shape-set flat; this configuration mimics the microstructure and texture observed in Nitinol medical devices. Fatigue cracks were grown ex situ at near-threshold conditions (ΔK = 3 MPa√m) to a crack length to sample width ratio of a/W = 0.5. Specimens were then loaded in situ with a miniature straining rig to various stress intensities for multiple fatigue cycles. Thousands of local diffraction patterns (1 μm 2 spot area) spanning hundreds of micrometers surrounding the crack tips, were combined to produce contour maps of phase volume and local strain. The differences in monotonic and cyclic loading conditions can be deduced from these tests and can be used to differentiate in vivo single-event versus cumulative-damage fractures.
We present CellIQ, a real-time cellular network analytics system that supports rich and sophisticated analysis tasks. CellIQ is motivated by the lack of support for realtime analytics or advanced tasks such as spatio-temporal traffic hotspots and handoff sequences with performance problems in state-of-the-art systems, and the interest in such tasks by network operators. CellIQ represents cellular network data as a stream of domain specific graphs, each from a batch of data. Leveraging domain specific characteristics--the spatial and temporal locality of cellular network data--CellIQ presents a number of optimizations including geo-partitioning of input data, radius-based message broadcast, and incremental graph updates to support efficient analysis. Using data from a live cellular network and representative analytic tasks, we demonstrate that CellIQ enables fast and efficient cellular network analytics--compared to an implementation without cellular specific operators, CellIQ is 2× to 5× faster.
On the basis of constraints from reported experimental observations and density functional theory simulations, in this paper we propose a mechanism for the reduction of CO<sub>2</sub> to C<sub>2</sub> products on copper electrodes. To model the effects of an applied potential bias on the reactions, calculations are carried out with a variable, fractional number of electrons on the unit cell, which is optimized so that the Fermi level matches the actual chemical potential of electrons (i.e., the applied bias); an implicit electrolyte model allows for compensation of the surface charge so that neutrality is maintained in the overall simulation cell. Our mechanism explains the presence of the seven C<sub>2</sub> species that have been detected in the reaction, as well as other notable experimental observations. Furthermore, our results shed light on the difference in activities toward C<sub>2</sub> products between the (100) and (111) facets of copper. Finally, we compare our methodologies and findings with those in other recent mechanistic studies of the copper-catalyzed CO<sub>2</sub> reduction reaction.
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An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
