Long-term tensile and bending strength of natural building stones

The results of experimental long-term tension and bending tests carried out on four natural stones formely widely used in Italian traditional building (a white-veined Carrara marble, the “pietra Serena” and “Pietraforte” sandstones, and the Piobbico-Cesana limestone) are presented. The tests consisted of applying continually different fractions of the “instantaneous” fracture loads and monitoring the strain evolution of the specimens until a time-delayed fracture was reached. A typical three-stage creep response was obtained in all the tests. At the same time, a 60% reduction in tensile and bending strength was observed in relation to testing periods not much longer than one year. A phenomenological constitutive model is proposed for analytically reproducing the experimental viscoplastic behaviour. Furthermore, a creep time-to-rupture function giving direct estimates of the strength decay effects is provided, with a view to technical applications. The results of the identification analyses carried out to calibrate the characteristic parameters of the above laws are also reported.     

Grafting Single Molecule Magnets on Gold Nanoparticles

The chemical synthesis and characterization of the first hybrid material composed by gold nanoparticles and single molecule magnets (SMMs) are described. Gold nanoparticles are functionalized via ligand exchange using a tetrairon(III) SMM containing two 1,2‐dithiolane end groups. The grafting is evidenced by the shift of the plasmon resonance peak recorded with a UV–vis spectrometer, by the suppression of nuclear magnetic resonance signals, by X‐ray photoemission spectroscopy peaks, and by transmission electron microscopy images. The latter evidence the formation of aggregates of nanoparticles as a consequence of the cross‐linking ability of Fe₄ through the two 1,2‐dithiolane rings located on opposite sides of the metal core. The presence of intact Fe₄ molecules is directly proven by synchrotron‐based X‐ray absorption spectroscopy and X‐ray magnetic circular dichroism spectroscopy, while a detailed magnetic characterization, obtained using electron paramagnetic resonance and alternating‐current susceptibility, confirms the persistence of SMM behavior in this new hybrid nanostructure.     

Digital-to-RF conversion for a vector modulator

Digital technology lacks sufficient speed to support many high data rate applications at microwave frequencies. This is unfortunate since areas such as higher order modulation, predistortion, equalization, and demodulation could benefit in flexibility, modularity, and performance from digital architectures. However, use of radio frequency microwave technology permits the implementation of digital functions at these higher speeds and frequencies. This paper describes the implementation of a nonrecursive (finite-impulse response) filter in microwave technology     

Non-linear dynamic design procedure of FV spring-dampers for base isolation — frame building applications

The non-linear dynamic design procedure of fluid viscous spring-dampers proposed in the accompanying paper is applied to two selected case studies, represented by a reinforced concrete and a steel five-storey frame building with identical global dimensions. The fundamental vibration periods of the two structures in fixed-base conditions, equal to 0.58 s and 1.08 s, respectively, fall within the range of technical interest for use of base isolation. The reliability of the analytical relationship by which the damping coefficient is estimated in the preliminary design phase is further verified by comparing its predictions with the loss factor values calculated from the results of numerical integration of the equations of motion. The final verification phase of the procedure is then developed with regard to a double design performance objective, for which immediate occupancy and life safety levels are targeted under the “basic design” and “maximum considered” earthquakes, respectively.     

Distribution of generalist and specialist predators along urban gradients

The distribution of predators in urban areas is overlooked by urban planners. Due to the homogenizing power of urbanization, towns are expected to favour the settlement of generalist predators. We compared the frequency of specialist and generalist predators along an urban gradient (centre, inner periphery, outer periphery, and scarcely built areas) in towns of different size (small, medium, large). We obtained data on specialist (diurnal and nocturnal raptors, Dendrocopos major) and generalist (Larus michahellis, corvids) predators breeding in each square of the grid of 27 urban atlases. The number of species per square of specialist predators decreased in more urbanised sectors, whereas the number of generalist predators either did not change or increased in the same sectors. However, some specialist (nocturnal raptors except Athene noctua) and generalist (Garrulus glandarius, Pica pica) predators did not follow such a pattern of distribution. Town size may affect the distribution of predators. Some species (D. major, G. glandarius, P. pica) were more frequent in the more urbanised sectors in small towns. Data gathered in few towns on a limited number of predator species may not fully represent the complex response by predators to urban gradients. Urban planners should consider the distribution of predators in towns to limit risks for urban wildlife.     

Time-delayed rupture of stones under low tensile and bending stress states

The advancements of research dealing with the time-delayed behaviour of building stones are presented. In preceding papers, the results of long-term tensile and bending tests conducted on one type of Carrara marble, two sandstones, and one limestone were reported. A composite phenomenological model was also defined which permitted to suitably reproduce the recorded responses. In this paper, the tests carried out by applying the minimum percentile rate of the fracture loads (equal to 35%), having in the meantime arrived at completion, are described. The outcomes of these tests confirm the basic features of the creep response observed in the previous sections of the experimental programme. By updating the experimental time-to-rupture curves with reference to the new data, an asymptotic trend towards the elastic limit values of the materials is noted. A modified version of the time-to-rupture law associated with the tertiary branch of the phenomenological model is supplied, which allows to further improve the relevant prediction potentialities.

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Résumé On présente les progrès des recherches ayant comme objet le comportement à long terme des pierres naturelles à batir. Dans des articles précédents, on avait présenté les résultats des assais à long terme à la flexion et à la traction conduits sur un marbre de Carrare, deux grès et une pierre calcaire. Un modèle constitutif du genre phénoménologique a également été défini qui permet de reproduire analytiquement les réponses observées. Dans cet article, on présente des essais récemment terminés qui appliquent le taux minimal des charges de rupture (égal à 35%). Les résultats de ces essais confirment les plus importantes propriétés de la réponse au fluage précédemment observées. Une mise à jour, sur la base des nouvelles données, des courbes expérimentales du temps de rupture a permis de démontrer une démarche asymptotique vers les limites élastiques des matériaux. Une formulation modifiée, proposée ici, pour la loi des temps de rupture associée à la branche tertiaire du modèle phénoménologique permet d’améliorer ses capacités de prévision.

     

Operation and Optimization of Silicon-Diode-Based Optical Modulators

An optical modulator in silicon based on a diode structure has been operated in both forward and reverse bias. This modulator achieves near state-of-the-art performance in both modes, thereby making this device ideal for comparing the two modes of operation. In reverse bias, the device has a V $_pi$L of 4.0 V·cm and a bandwidth of 26 GHz. In forward bias, the device is very sensitive, a V$_{pi}$ L as low as 0.0025 V·cm has been achieved, but the bandwidth is only 100 MHz. A new geometry for a reverse-bias device is proposed, and it is predicted to achieve a $V_{pi}L$ of 0.5 V·cm.      

Effects of initial creep conditions and temporary unloading on the long-term response of stones

A special part of a research programme concerning the tensile and bending creep behaviour of natural building stones is presented in this paper. The new experimental studies were aimed at inquiring the effects of temporary viscoplastic conditions on the time-delayed response produced by subsequent variations of the applied stress. Long-term tests were performed which consisted of unloading the specimens from different stages of the secondary creep branch and reloading them with the same, or decreased, shares of the ultimate-stress values. Further appreciable reductions in the delayed strengths were observed compared to the results of the continuous tests developed in the preceding phases of this research. At the same time, a series of instantaneous collapse reloading-tests highlighted a little influence of the initial deformation time-history on the relevant rupture-loads. Within the context of structural-diagnostic problems, these last values cannot be regarded as exhaustive references for estimating the safety limits of stone members, unless coherent interpretation criteria of the results deriving fromin-situ extracted specimens are adopted. A simple methodology is herein suggested to this aim. The delayed elasticity effects following load-removal were also analysed, and reproduced by means of a proper phenomenological law that allowed to complete the global creep model proposed in previous works.

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Résumé On présente une phase d'une étude sur le comportement de fluage des pierres naturelles à batir soumises à la flexion et à la traction. Le but de ces nouvelles expériences était d'étudier les effets d'états temporaires de viscoplasticité sur le comportement à long terme résultant des variations des efforts appliqués. Des essais à long terme consistaient à décharger les éprouvettes à partir de différentes étapes de fluage secondaire et à les recharge jusqu'aux mêmes valeurs, ou à des valeurs moindres, des efforts de rupture ultime. On a constaté des réductions significatives de la résistance en comparaison avec les résultats des essais effectués lors des précédentes phases de recherche. En même temps, une série d'essais à rupture instantanée a montré le peu d'influence exercée par l'historique de la déformation initiale sur les charges de rupture. Dans le contexte de problèmes de diagnostic de structures, ces dernières valeurs ne peuvent pas servir de références complètes pour estimer les limites de sécurité des éléments en pierre, à moins d'adopter des critères d'interprétation cohérents déduits des résultats obtenus sur des éprouvettes extraitesin situ. Pour ce faire, on présente une méthodologie. On analyse également les effets retardés d'élasticité après le déchargement, interprétés au moyen d'une loi phénoménologique qui permet de compléter le modèle général du fluage précédemment présenté.

     

Quinonoid metal complexes: toward molecular switches

The peculiar redox-active character of quinonoid metal complexes makes them extremely appealing to design materials of potential technological interest. We show here how the tuning of the properties of these systems can be pursued by using appropriate molecular synthetic techniques. In particular, we focus our attention on metal polyoxolene complexes exhibiting intramolecular electron transfer processes involving either the ligand and the metal ion or the two dioxolene moieties of a properly designed ligand thus inducing electronic bistability. The transition between the two metastable electronic states can be induced by different external stimuli such as temperature, pressure, light, or pH suggesting the use of these systems for molecular switches.