177,561 research outputs found
Hierarchical Fibers with a Negative Poisson’s Ratio for Tougher Composites
No full textIn this paper, a new kind of hierarchical tube with a negative Poisson’s ratio (NPR) is proposed. The first level tube is constructed by rolling up an auxetic hexagonal honeycomb. Then, the second level tube is produced by substituting the arm of the auxetic sheet with the first level tube and rolling it up. The Nth ( ) level tube can be built recursively. Based on the Euler beam theory, the equivalent elastic parameters of the NPR hierarchical tubes under small deformations are derived. Under longitudinal axial tension, instead of shrinking, all levels of the NPR hierarchical tubes expand in the transverse direction. Using these kinds of auxetic tubes as reinforced fibers in composite materials would result in a higher resistance to fiber pullout. Thus, this paper provides a new strategy for the design of fiber reinforced hierarchical bio-inspired composites with a superior pull-out mechanism, strength and toughness. An application with super carbon nanotubes concludes the paper
Elastic and Transport Properties of the Tailorable Multifunctional Hierarchical Honeycombs
No full textIn this paper, we analytically studied the in-plane elastic and transport properties of a peculiar hexagonal honeycomb, i.e., the multifunctional hierarchical honeycomb (MHH). The MHH structure was developed by replacing the solid cell walls of the original regular hexagonal honeycomb (ORHH) with three kinds of equal-mass isotropic honeycomb sub-structures possessing hexagonal, triangular and Kagome lattices. Formulas to calculate the effective in-plane elastic properties and conductivities of the MHH structure at all densities were developed. Results show that the elastic properties of the MHH structure with the hexagonal sub-structure were weakly improved in contrast to those of the ORHH. However, the triangular and Kagome sub-structures result in substantial improvements by one or even three orders of magnitude on Young’s and shear moduli of the MHH structure, depending on the cell-wall thickness-to-length ratio of the ORHH. The present theory could be used in designing new tailorable hierarchical honeycomb structures for multifunctional applications
Systematic numerical investigation of the role of hierarchy in heterogeneous bio-inspired materials
No full textIt is well known that hierarchical structure is an important feature in biological materials to optimise various properties, including mechanical ones. It is however still unclear how these hierarchical architectures can improve material characteristics, for example strength. Also, the transposition of these structures from natural to artificial bioinspired materials remains to be perfected. In this paper, we introduce a numerical method to evaluate the strength of fibre-based heterogeneous biological materials and systematically investigate the role of hierarchy. Results show that hierarchy indeed plays an important role and that it is possible to “tune” the strength of bio-inspired materials in a wide range of values, in some cases improving the strength of non-hierarchical structures considerably
Nanoscale Weibull statistics
No full textIn this paper a modification of the classical Weibull statistics is developed for nanoscale applications. It is called nanoscale Weibull statistics. A comparison between nanoscale and classical Weibull statistics applied to experimental results on fracture strength of carbon nanotubes clearly shows the effectiveness of the proposed modification. A Weibull's modulus of similar to 3 is deduced for nanotubes. The approach can treat (also) a small number of structural defects, as required for nearly defect-free structures (e.g., nanotubes) as well as a quantized crack propagation (e.g., as a consequence of the discrete nature of matter), allowing to remove the paradoxes caused by the presence of stress intensifications
Policy-makers and the R&D-patent relationship. Bruegel Policy Contribution/May 2008
No full textThis policy contribution summarises a communication entitled “A policy insight into the R&D-patent relationship” presented at Industry Canada in their Distinguished Speakers in Economics Series, Ottawa, Canada, 18 April, 2008. It argues that the number of priority filings should be used as a patent-based measure of Europe’s innovation performance. It also identifies several policies that may affect the R&D-patent relationship. Patent-based indicators at the country level are frequently used to assess countries’ innovation performance or effort. Yet they are often said to reflect the propensity to patent rather than actual research productivity. We argue that patent-based indicators can rightly be used to measure research productivity, as witnessed by the influence of several policy tools on the R&D-patent relationship. We also put forward a new counting methodology, less subject to ‘home bias’
Quantized fracture mechanics and related applications for predicting the strength of defective nanotubes
No full textMechanics of plant fruit hooks
No full textHook-like surface structures, observed in some plant species, play an important role in the process of plant growth and seed dispersal. In this study, we developed an elastic model and further used it to investigate the mechanical behaviour of fruit hooks in four plant species, previously measured in an experimental study. Based on Euler–Bernoulli beam theory, the force–displacement relationship is derived, and its Young's modulus is obtained. The result agrees well with the experimental data. The model aids in understanding the mechanics of hooks, and could be used in the development of new bioinspired Velcro-like materials
Nanoscale Weibull Statistics for nanofibers and nanotubes
No full textIn this paper a modification of the classical Weibull statistics is applied to nanostructures. A comparison is presented of "nanoscale" versus classical Weibull statistics in treating recent experimental results on the fracture strength of C nanofibers and nanotubes, and WS2 nanotubes. "Nanoscale" Weibull moduli of 3.8 for electrospun and then heat-treated carbon nanofibers, 2.7 for arc-discharge synthesized multiwalled carbon nanotubes, 1.8 for chemical vapor deposited multiwalled carbon nanotubes, and 3.0 for multiwalled WS2 nanotubes, are deduced
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