Technical report writing

This manual covers the fundamentals of organizing, writing, and reviewing NASA technical reports. It was written to improve the writing skills of LeRC technical authors and the overall quality of their reports

Structural-Damage Detection by Distributed Piezoelectric Transducers and Tuned Electric Circuits

A novel technique for damage detection of structures is introduced and discussed. It is based on purely electric measurements of the state variables of an electric network coupled to the main structure through a distributed set of piezoelectric patches. The constitutive parameters of this auxiliary network are optimized to increase the sensitivity of global measurements- as the frequency, response functions relative to selected electric degrees of freedom-with respect to a given class of variations in the structural-mechanical properties. Because the proposed method is based on purely electric input and output measurements, it allows for accurate results in the identification and localization of damages. Use of the electric frequency-response function to identify the mechanical damage leads to nonconvex optimization problems; therefore the proposed sensitivity-enhanced identification procedure becomes computationally efficient if an a priori knowledge about the damage is available.Comment: 18 page

Multiparameter actuation of a neutrally-stable shell: a flexible gear-less motor

We have designed and tested experimentally a morphing structure consisting of a neutrally stable thin cylindrical shell driven by a multiparameter piezoelectric actuation. The shell is obtained by plastically deforming an initially flat copper disk, so as to induce large isotropic and almost uniform inelastic curvatures. Following the plastic deformation, in a perfectly isotropic system, the shell is theoretically neutrally stable, owning a continuous manifold of stable cylindrical shapes corresponding to the rotation of the axis of maximal curvature. Small imperfections render the actual structure bistable, giving preferred orientations. A three-parameter piezoelectric actuation, exerted through micro-fiber-composite actuators, allows us to add a small perturbation to the plastic inelastic curvature and to control the direction of maximal curvature. This actuation law is designed through a geometrical analogy based on a fully non-linear inextensible uniform-curvature shell model. We report on the fabrication, identification, and experimental testing of a prototype and demonstrate the effectiveness of the piezoelectric actuators in controlling its shape. The resulting motion is an apparent rotation of the shell, controlled by the voltages as in a "gear-less motor", which is, in reality, a precession of the axis of principal curvature.Comment: 20 pages, 9 figure

Discrete approximations of the Föppl–Von Kármán shell model: From coarse to more refined models

AbstractThe problem of deducing, from the Föppl–Von Kármán energy functional, a sequence of reduced discrete models having few degrees of freedom is analyzed. Similar discrete models have been recently intensively studied to analyze the multistable behavior of shallow shells, the bifurcations of composite laminates under temperature loads or the wrinkling in soft tissues.In particular three relevant examples are discussed and compared among them, where the curvature is assumed uniform, linearly and quadratically varying through the shell. While the uniform-curvature assumption dates back to Mansfield (1962), linear variations of the shell curvatures can describe smooth transitions between everted configurations, while quadratic variations can account for the, usually disregarded, bending boundary conditions.For their deduction we revisit the Maxwell–Mohr method: accordingly, a sequence of auxiliary elliptic problems of plane elasticity is solved to determine the statically unknown membranal stresses. This is a key ingredient for the presented models to compare extremely well with Finite Element approximations or with literature models with far more degrees of freedom

Vibration control in plates by uniformly distributed PZT actuators interconnected via electric networks

In this paper a novel device aimed at controlling the mechanical vibrations of plates by means of a set of electrically-interconnected piezoelectric actuators is described. The actuators are embedded uniformly in the plate wherein they connect every node of an electric network to ground, thus playing the two-fold role of capacitive element in the electric network and of couple suppliers. A mathematical model is introduced to describe the propagation of electro-mechanical waves in the device; its validity is restricted to the case of wave-forms with wave-length greater than the dimension of the piezoelectric actuators used. A self-resonance criterion is established which assures the possibility of electro-mechanical energy exchange. Finally the problem of vibration control in simply supported and clamped plates is addressed; the optimal net-impedance is determined. The results indicate that the proposed device can improve the performances of piezoelectric actuationComment: 22 page

Formal cooperation and the performance of Italian firms operating inside and outside industrial districts

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Investigating space-time patterns of regional industrial resilience through a micro-level approach:An application to the Italian wine industry

This paper introduces a new methodology to identify space-time patterns of regional resilience using a micro-level approach. The novel empirical tool combines geographically weighted regression with panel stochastic frontier analysis with endogenous covariates. The analysis is implemented on a panel of farm holdings operating in the Italian wine industry, focusing on the impact of a major institutional change. The results show the effectiveness of the new procedure in identifying geographical clusters of wine producers who reacted to the shock in similar ways. The responses are found to be homogeneous within specific territories and heterogeneous between regions

A phase-field model for fracture in beams from asymptotic results in 2D elasticity

We propose a derivation of a damage model in slender structures, focusing on the particular case of a rod. The peculiarity of the model is that it takes into account the changes in rigidity of the body, distinguishing between bending, traction and the possible mixed interactions between the two. The approach is based on a matched asymptotic expansion, taking the recent work of Baldelli et al [1] as starting point. Choosing the slenderness of the rod as small parameter for the asymptotic expansion, we determine the first order at which a correction occurs with respect to the Saint-Venant solution of the elastic problem, due to the presence of a crack. The results highlight that the presence of a defect affects in different ways the bending and traction rigidities of the rod, and that a coupling between the two deformation modes might occur, depending on the geometry of the crack. Moreover, the derivation allows to explicitly calculate the coefficients of this correction, for any given depth of the crack, by means of a simple numerical procedure. Application to the classic three-point bending problem is considered in order to highlight the predictive capabilities of the model. These results suggest ways in which state of the art phasefield models (e.g. [2]) for damage could be refined. This work goes in the direction of developing phase-field models suitable for application to slender structures, where the use of reduced dimensional models has proved promising [3]

Towards a phase-field model for thin structures: a coarse-grained constitutive law for brittle fracture of beams

Damage gradient models approximate fracture mechanics using a modulation of the material stiffness. To this aim a single scalar field, the damage, is used to degrade as a whole the elastic energy. If applied to the structural models of beams and shells, where the elastic energy is the sum of the stretching and bending contributions, a similar approach is not able to capture some important features. For instance, the coupling between axial and bending strains induced by cracks non-symmetric with respect to the center line is completely missed. In this contribution, we deduce a constitutive law for a beam having a crack non-symmetric with respect to the center line. To achieve this, we perform an asymptotic coarse-grained procedure from a 2D problem, using a sharp interface model. We deduce a homogenized 1D elastic energy, coupling the axial and bending strains, the constitutive parameters of which depend in a different way on the crack depth and we state how precisely they do. This should pave the way to a rational development of a phase-field gradient model for thin structures

The city and the grid : building beauty at large scale

The enduring popularity of “A City is not a Tree” (Alexander, 1965) for scholars in different areas of knowledge does not seem to give signs of receding. Quite on the contrary, a quick search on Google Scholar reveals that its annual rate of citations in the past five years is about 3.5 times that of the overall period since its first publication in 1965. In this paper Alexander proposes a focus on the complex nature of cities that, along the same line of Jacobs’ chapter 22 of “Death and Life”, entitled “The kind of problem a city is” (Jacobs, 1961), challenges to the hart the conventional approach to urban planning and design; this challenge is all the more relevant today, when the call for a profound renovation of the foundations of the discipline comes not just by planning scholars, but also governmental and educational bodies (Bothwell, 2004; Farrell, 2014; U.N., 2015; U.N.HABITAT, 2009). The urgency of this problem is obvious in an age characterized by both unprecedented urbanization, predominantly involving the poorest parts of human population in the weakest planning systems (U.N.DESA, 2014), and the unprecedented impact of human activities on the fundamental forces of nature (Steffen, Broadgate, Deutsch, Gaffney, & Ludwig, 2015). In the light of our failure in the post-WWII urbanization of the Global North, the question is simple: can we planners help at all with that of the Global South, which occurs far faster and at higher scale? How can we become part of the solution, rather than the problem? In a rapidly urbanizing world, patronizing a niche cannot suffice; we need a new mainstream, one that works