Mechanical metamaterials at the theoretical limit of isotropic elastic stiffness.

A wide variety of high-performance applications require materials for which shape control is maintained under substantial stress, and that have minimal density. Bio-inspired hexagonal and square honeycomb structures and lattice materials based on repeating unit cells composed of webs or trusses, when made from materials of high elastic stiffness and low density, represent some of the lightest, stiffest and strongest materials available today. Recent advances in 3D printing and automated assembly have enabled such complicated material geometries to be fabricated at low (and declining) cost. These mechanical metamaterials have properties that are a function of their mesoscale geometry as well as their constituents, leading to combinations of properties that are unobtainable in solid materials; however, a material geometry that achieves the theoretical upper bounds for isotropic elasticity and strain energy storage (the Hashin-Shtrikman upper bounds) has yet to be identified. Here we evaluate the manner in which strain energy distributes under load in a representative selection of material geometries, to identify the morphological features associated with high elastic performance. Using finite-element models, supported by analytical methods, and a heuristic optimization scheme, we identify a material geometry that achieves the Hashin-Shtrikman upper bounds on isotropic elastic stiffness. Previous work has focused on truss networks and anisotropic honeycombs, neither of which can achieve this theoretical limit. We find that stiff but well distributed networks of plates are required to transfer loads efficiently between neighbouring members. The resulting low-density mechanical metamaterials have many advantageous properties: their mesoscale geometry can facilitate large crushing strains with high energy absorption, optical bandgaps and mechanically tunable acoustic bandgaps, high thermal insulation, buoyancy, and fluid storage and transport. Our relatively simple design can be manufactured using origami-like sheet folding and bonding methods.

Total cost comparison of 2 biopsy methods for nonpalpable breast lesions.

OBJECTIVE: To identify, quantify, and compare total facility costs for 2 breast biopsy methods: vacuum-assisted biopsy (VAB) and needle-wire-localized open surgical biopsy (OSB). STUDY DESIGN: A time-and-motion study was done to identify unit resources used in both procedures. Costs were imputed from published literature to value resources. A comparison of the total (fixed and variable) costs of the 2 procedures was done. PATIENTS AND METHOD: A convenience sample of 2 high-volume breast biopsy (both VAB and OSB) facilities was identified. A third facility (OSB only) and 8 other sites (VAB only) were used to capture variation. Staff interviews, patient medical records, and billing data were used to check observed data. One hundred and sixty-seven uncomplicated procedures (71 OSBs, 96 VABs) were observed. Available demographic and clinical data were analyzed to assess selection bias, and sensitivity analyses were done on the main assumptions. RESULTS: The total facility costs of the VAB procedure were lower than the costs of the OSB procedure. The overall cost advantage for using VAB ranges from $314 to $843 per procedure depending on the facility type. Variable cost comparison indicated little difference between the 2 procedures. The largest fixed cost difference was $763. CONCLUSIONS: Facilities must consider the cost of new technology, especially when the new technology is as effective as the present technology. The seemingly high cost of equipment might negatively influence a decision to adopt VAB, but when total facility costs were analyzed, the new technology was less costly.

Corrosion of stainless steel pipes in a hydrotherapy pool by a silver-copper disinfection system.

The silver-copper disinfection system has been shown to be effective for water purification. It emits silver ions that combine with bacteria, causing their death. While disinfecting the water, these silver ions exhibit adverse effects on the stainless steel pipes in hydrotherapy pools. In an oxidation-reduction reaction the silver ions are converted into solid silver that is deposited on stainless steel, causing a corrosion reaction. The corroded steel has a black deposit that readily adheres to the burned patient's skin.

Two epizootics of lymphocytic choriomeningitis virus occurring in laboratory mice despite intensive monitoring programs.

Two epizootics of lymphocytic choriomeningitis virus in mice occurred within two months in one research facility consisting of several widely separated rooms. These outbreaks developed despite intensive institutional monitoring policies designed to prevent introduction and spread of lymphocytic choriomeningitis virus. Evidence derived from serological and virological assays and interviews with the concerned investigators suggested that a single transplantable tumor carried in mice may have been responsible for spread of the virus. However, the tumor was not contaminated with lymphocytic choriomeningitis virus at the time of its introduction into the mouse facility. The origin of the virus responsible for the outbreaks was not definitively established although data supported an hypothesis that the virus was introduced into the research facility by a wild or feral mouse. Virus spread from infected mice to humans did not occur, as measured by serological tests. However, a large and valuable animal facility was depopulated for safety reasons. Absorption of sera with lymphocytic choriomeningitis virus antigen proved a necessary and reliable method for confirming specificity of lymphocytic choriomeningitis virus fluorescence-positive reactions.

Reconstructing Internal Temperature Distributions From Ultrasonic Time-of-Flight Tomography And Dimensional Resonance Measurements.

Two ultrasonic techniques for reconstructing the internal temperature distribution in metal bodies-time-of-flight tomography and dimensional resonance profiling-are described. An analysis of the tomographic reconstruction of temperature (including ray refraction effects) in a cylindrical body is presented together with initial experimental results. Dimensional resonance profiling is a new technique that allows the reconstruction of a one-dimensional distribution of temperature in a structure from measurements of its resonant frequencies. While time-of-flight tomography is well suited for measuring temperature in a cylindrical geometry, a combination of dimensional resonance and (a restricted form of) tomography is the best method for measuring temperature profiles in the more practically important rectangular slab geometry.

Acoustic Emission: Establishing the Fundamentals.

In the mid-1970's a program of fundamental research was initiated at NBS to improve the scientific understanding of acoustic emission. Many individual results of this research have been reported in the literature and are beginning to be incorporated in a new generation of acoustic emission instrumentation, in improved test methodologies, and in the analysis of data. Here, we summarize the problems faced by acoustic emission midway through the last decade, review the accomplishments of the NBS program and related researchs programs, and outline the research that will be required in future years.

Theory of Acoustic Emission From Phase Transformations.

A theoretical framework is developed within which it is possible to predict the dynamic elastic displacement field (acoustic emission) for a phase transformation in which there is a change of both crystal structure (elastic constants) and shape (density). An integral equation is presented for the acoustic emission displacement field due to formation of inhomogeneous inclusions. This integral equation is solved by expressing the source in multipolar form and using the Eshelby equivalent inclusion method to estimate the dynamic multipolar coefficients. Expressions for the source of elastic radiation are explicitly calculated for small isotropic spherical and ellipsoidal inclusions embedded in an isotropic matrix. These expressions are used for qualitative interpretation of recent experiments on martensitic transformations in steels and for identifying the information that may be deduced about transformation dynamics from quantitative measurements of acoustic emission.