Papers by Author: Nicola Maria Pugno

Abstract: In this paper the self-buckling of nanostructures, such as nanotubes, fullerenes and peapods, is analytically treated; this surprising phenomenon is due to the interaction among the nanostructures caused by the surface energy; it is peculiar of the nanoscale and has not a macroscopic counterpart. The influence of the surrounding nanostructures on one of them in a crystal is nearly identical to that of a liquid having surface tension equal to the surface energy of the solid. For the beneficial implications of the self-buckling on the overall mechanical strength see Pugno (2010; The design of self-collapsed super-strong nanotube bundles, Journal of the Mechanics and Physics of Solids, Available Online).

Abstract: This paper models the elastic properties of 2-D woven hierarchical tissues, assuming an orthotropic material of warp and fill yarns at level 0. Considering matrix transformation and stiffness averaging, stiffness matrices of warp and fill yarns of the tissue at level i are employed to calculate those of the tissue at level i+1. We compare our theory with another approach from the literature on tendons and experiments on leaves performed by ourselves. The result shows the possibility of designing a new class of hierarchical 2-D scaffolds with desired elastic anisotropy, better matching the anisotropy of the biological tissues and thus maximizing the regeneration.

Abstract: In a recent letter, Xiao et al. [3] interpreted experimental results on the failure of nanotube bundles using Weibull Statistics. The prediction of the force versus strain curve was a smooth curve, only partially able to capture the observed discrete behavior of the bundle. In particular, abrupt jumps in the force, at nearly constant strains, were clearly observed experimentally, each of them corresponding to the failure of a sub-bundle. Accordingly, we have developed a simple modification of the Weibull Statistics able to treat the observed catastrophic failure of the bundle, considering a linear or nonlinear elastic constitutive law.

Abstract: Theoretical van der Walls gloves could generate an adhesion force comparable to the body weight of ∼500 men. Even if such a strength remains practically unrealistic (and undesired, in order to achieve an easy detachment), due to the presence of contact defects, e.g. roughness and dust particles, its huge value suggests the feasibility of Spiderman gloves. The scaling-up procedure, from a spider to a man, is expected to decrease the safety factor (body weight over adhesion force) and adhesion strength, that however could remain sufficient for supporting a man. Scientists are developing fascinating new biomimetic materials, e.g. gecko-inspired. Here we complementary face the problem of the structure rather than of the material, designing and fabricating a first prototype of Spiderman gloves, capable of supporting ∼10 kilograms each, thanks to new Adhesive Optimization Laws.

Abstract: Many biological materials exhibit a hierarchical structure over more than one length scale. Understanding how hierarchy affects their mechanical properties emerges as a primary concern, since it can guide the synthesis of new materials to be tailored for specific applications. In this paper the strength and stiffness of hierarchical materials are investigated by means of a fractal approach. A new model is proposed, based both on geometric and material considerations and involving simple recursive formulas.

Abstract: The results of an experimental research on plain concrete are presented. The non-linear behavior of both virgin and damaged samples is investigated by means of ultrasonic tests: recent theoretical models, indeed, have pointed out that mono-frequency ultrasonic excitations bring to light such phenomena as harmonic generation and sidebands production, which are essentially due to the material classical or hysteretic non-linearity. The estimation of the harmonic components parameters (amplitudes and phases) is achieved through a signal processing technique based on MUltiple SIgnal Classification (MUSIC) system, which reveals to be optimal for the specific signal model here considered. The experiments described in this paper show that the material non-linear features increase with increasing level of internal micro-cracking, thus suggesting the possibility to use the ultrasonic signal analysis in the frequency domain as a valuable tool for damage assessment.

Abstract: In this contribution some characteristics and predictive capabilities are discussed of a recently introduced model for damage progression and energy release, in view of modelling Acoustic Emission. The specimen is discretized in a network of connected springs, similar to a Fibre Bundle Model approach, with the spring intrinsic strengths statistically distributed according to a Weibull distribution. Rigorous energy balance considerations allow the determination of the dissipated energy due to crack surface formation and kinetic energy propagation. Based on results of simulations, the macroscopic behaviour emerging from different choices at “mesoscopic” level is discussed, in particular the relevance of model parameters such as the distribution of spring cross sections, Weibull modulus values, and discretization parameters in determining results like stressstrain curves and energy scaling versus time or specimen size.

Abstract: In this paper the damage assessment of nanostructures is discussed. As an example we assess the damage of nanobeams with non destructive dynamical resonance or destructive tensile tests: a small number of nanocracks, i.e., ~10, with length of ~1nm, is accordingly estimated.