Indentability of conventional and negative Poisson's ratio foams

The indentation resistance of foams, both of conventional structure and of re-entrant structure giving rise to negative Poisson's ratio, is studied using holographic interferometry. In holographic indentation tests, re-entrant foams had higher yield strengths sigma(sub y) and lower stiffness E than conventional foams of the same original relative density. Calculated energy absorption for dynamic impact is considerably higher for re-entrant foam than conventional foam

Biomimetic spatial and temporal (4D) design and fabrication

We imagine the built environment of the future as a ‘bio-hybrid machine for living in’ that will sense and react to activities within the space in order to provide experiences and services that will elevate quality of life while coexisting seamlessly with humans and the natural environment. The study of Hierarchical design in biological materials has the potential to alter the way designers/ engineers/ crafts-men of the future engage with materials in order to realise such visions. We are ex-ploring this design approach using digital manufacturing technologies such as jac-quard weaving and 3D printing

Damping Composite Materials: Effect of Structural Hierarchy

ABSTRACT: A combination of stiffness and loss (the product Etan ) is desirable in damping layer and structural damping applications. Composite materials of structure which gives rise to Reuss or Hashin-Shtrikman lower bound behavior can give rise to such properties. Hierarchical particulate morphologies attain the HashinShtrikman curve. We show that hierarchical composites give rise to complex Poisson's ratios which, however, have minimal effect on the stiffness-map. We show that structural hierarchy is useful in viscoelastic composites in that it enables the attainment of high concentrations of spherical inclusions, and that it facilitates the attainment of both stiffness and damping. A damping layer upon a substrate is considered as the top level of the structural hierarchy. We demonstrate that if the layer itself is a relatively stiff composite, the penalty usually associated with such a geometry for compliant layers is ameliorated

Negative stiffness and enhanced damping of individual multiwalled carbon nanotubes

The mechanical instabilities and viscoelastic response of individual multiwalled carbon nanotubes and nanofibers (MWCNTs/Fs) under uniaxial compression are studied with atomic force microscopy. Specific buckling events are evident by regimes of negative stiffness, i.e., marked drops in force with increasing compression. Uniaxial cyclic loading can be repeatedly executed even in initially postbuckled regimes, where the CNTs/Fs display incremental negative stiffness. Increases in mechanical damping of 145–600 % in these initially postbuckled regimes, as compared to the linear prebuckled regimes, are observed. Increased damping is attributed to frictional energy dissipation of walls in buckled configurations of the MWCNTs/Fs. This represents the extension of the concept of negative stiffness to the scale of nanostructures and opens up possibilities for designing nanocomposites with high stiffness and high damping simultaneously

Viscoelastic Properties of Crystals

We examine the question of whether fluids and crystals are differentiated on the basis of their zero frequency shear moduli or their limiting zero frequency shear viscosity. We show that while fluids, in contrast with crystals, do have a zero value for their shear modulus, in contradiction to a widespread presumption, a crystal does not have an infinite or exceedingly large value for its limiting zero frequency shear viscosity. In fact, while the limiting shear viscosity of a crystal is much larger than that of the liquid from which it is formed, its viscosity is much less than that of the corresponding glass that may form assuming the liquid is a good enough glass former.Comment: 13 pages, 3 figure

Post-yield Relaxation Behavior of Bovine Cancellous Bone

Relaxation studies were conducted on specimens of bovine cancellous bone at post-yield strains. Stress and strain were measured for 1000 s and the relaxation modulus was determined. Fifteen cylindrical, cancellous bone specimens were removed from one bovine femur in the anterior–posterior direction. The relaxation modulus was found to be a function of strain. Therefore cancellous bone is non-linearly viscoelastic/viscoplastic in the plastic region. A power law regression was fit to the relaxation modulus data. The multiplicative constant was found to be statistically related through a power law relationship to both strain (p \u3c 0.0005) and apparent density (p \u3c 0.0005) while the power coefficient was found to be related through a power law relationship, E(t, ε)= A(ε)t-n(ε), to strain (p \u3c 0.0005), but not apparent density

THE JOURNAL OF COMPARATIVE NEUROLOGY 308200-208 (1991) From Embryo to Adult: Anatomy and Development of a Leg Sensory Organ in Phormia regina, Meigen (Insecta: Diptera). 11. Development and Persistence of Sensory Neurons

ABSTRACT The imaginal leg disc of Phormia regina contains eight neurons that arise during embryogenesis. Five of the neurons are associated with Keilin's organ, and of these five, two persist to the adult fly. Two new neurons arise at about the time of pupariation and flank each of these persisting neurons, forming two triplets of cells. Both triplets can be followed throughout metamorphosis; in the late pupa they are situated anteriorly and posteriorly at the tip of the fifth tarsomere. Two triplets of cuticular specializations are found at corresponding positions in the adult fly, each consisting of two campaniform sensilla and a trichoid hair. The central member of each set of sensilla, a campaniform sensillum, is associated with the persisting cell. Key words: metamorphosis, imaginal disc, immunocytochemistry, blowfly, mechanoreceptor In the preceding paper (Lakes-Harlan et al., '91) we showed that five of the eight neurons found in the imaginal leg discs of blowflies are associated with a larval sense organ, Keilin's organ. Three of these neurons terminate at the bases of the three hairs of the organ and appear to be mechanoreceptors. The remaining two neurons terminate at the cuticle. Neither of these is associated with any obvious cuticular specialization, and one retracts from the cuticle before the larval stage is complete. Unlike the dendrites that are associated with hairs, neither of these has a well-developed tubular body. Thus, in contrast to the remaining three cells, the morphological features of these two cells do not strongly suggest that they serve as larval sensory neurons. These dendrites are morphologically similar, in the absence of a cuticular termination and of a well-developed tubular body, to dendrites of developing sensory neurons (Ameismeier, '85). It has previously been shown that some neurons of the imaginal disc remain in the developing leg (Jan et al., '85; Lakes et al., '89; Tix et al., '89b). Persistence of larval sensory neurons to the late pupa has also been reported in Manduca (Bate, '73; Levine et al., '85; Levine, '89). In the present paper we trace the neurons of the imaginal leg disc of the blowfly through development from the embryo through the adult, and show that two of the cells that are associated with Keilin's organ persist throughout development. MATERIALS AND METHODS The methods were described in the preceding paper '91). In order to stain neurons in late pupal legs (after formation of the antibody-impermeant adult cuticle) longitudinal incisions were made to allow antibodies access to the legs' interiors, and the durations of all incubation times were tripled. In order to be certain that we did not miss important developmental events, animals were collected at 6 hour intervals (6 hours = 2.7% of larval development) during the transition from larva to pupa. During the initial phases of pupal development, when changes occur rapidly, the intervals were less than 0.5 hours (Lakes and Pollack, '90)

Viscoelastic behavior of 80In15Pb5Ag and 50Sn50Pb alloys: Experiment and modeling

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