Effective viscosity of grease ice in linearized gravity waves

Grease ice is an agglomeration of disc-shaped ice crystals, named frazil ice, which forms in turbulent waters of the Polar Oceans and in rivers as well. It has been recognized that the properties of grease ice to damp surface gravity waves could be explained in terms of the effective viscosity of the ice slurry. This paper is devoted to the study of the dynamics of a suspension of disc-shaped particles in a gravity wave field. For dilute suspensions, depending on the strength and frequency of the external wave flow, two orientation regimes of the particles are predicted: a preferential orientation regime with the particles rotating in coherent fashion with the wave field, and a random orientation regime in which the particles oscillate around their initial orientation while diffusing under the effect of Brownian motion. For both motion regimes, the effective viscosity has been derived as a function of the wave frequency, wave amplitude and aspect ratio of the particles. Model predictions have been compared with wave attenuation data in frazil ice layers grown in wave tanks.Comment: 13 pages, 3 eps figures included; one more section on inertia effect

Individuazione del danno su ponti in muratura mediante misure dinamiche

La diagnostica strutturale è un campo in continuo sviluppo e attualmente sono numerosi gli studi tendenti alla definizione di tecniche e metodi in grado di fornire parametri che possano identificare, in modo sempre più accurato, lo stato di sicurezza di una struttura. Stato di sicurezza è un termine globale che si riferisce alla capacità portante di una struttura e alla sua resistenza alle sollecitazioni esterne, siano esse statiche o dinamiche. Per tanto, un qualsiasi danneggiamento strutturale potrebbe in varia misura influenzare tale stato. Per quanto riguarda i ponti stradali, negli ultimi anni l’attenzione è stata focalizzata allo studio di metodi di individuazione del danno a partire dai risultati ottenuti dal monitoraggio delle vibrazioni mediante accelerometri. Questo è stato possibile grazie agli avanzamenti raggiunti nell’identificazione modale e nei sensori per il monitoraggio strutturale. Questo lavoro si pone come obiettivo quello di proporre una nuova metodologia di analisi dei dati ottenuti da indagini sperimentali che possono essere svolte in sito. La definizione di tale metodologia è affidata a simulazioni numeriche volte a rappresentare stati di danneggiamento avanzati su elementi strutturali. In una prima fase verranno analizzati elementi semplificati rappresentativi di strutture esistenti, come ad esempio travi da ponte in calcestruzzo armato. In una seconda fase, il metodo verrà testato su modelli più complessi che tengano conto della tridimensionalità del problema, oltre alle variazioni sezionali e di materiali degli elementi che lo compongono

Torque Penetrometric Test for the in-situ characterisation of historical mortars: fracture mechanics interpretation and experimental validation

The assessment of historical structures requires appropriate knowledge of the behaviour of the investigated materials. Concerning masonry, its mechanical characterisation is a challenging task, since its composite nature requires the careful evaluation of the behaviour of its material components. In particular, the experimental assessment of the strength of existing mortar in historical structures still encounters several difficulties. This study investigates a novel Minor Destructive Testing (MDT) technique virtually equivalent to the vane test used for soils. The instrumentation, called herein Torque Penetrometric Test (TPT), is composed of a steel nail with four protruding teeth and a torque wrench. The test consists in inserting the toothed nail into a mortar joint and then applying a torque by means of a dynamometric key, until reaching the failure of the material. This work presents a novel interpretation theory based on basic concepts of fracture mechanics and applied to the micro-mechanical analysis of the stress state induced by the instrument on the investigated mortar. The proposed interpretative theory is validated through the execution of experimental tests in the laboratory and in a historical masonry building. The test proves to be effective for a quick in-situ MDT evaluation of the strength of existing mortars.Peer ReviewedPostprint (author's final draft

Damage identification of cracked reinforced concrete beams through frequency shift

The safety evaluation of reinforced concrete (RC) bridges is of the outmost importance, both for the early warning of critical states below a given safety margin and owing to plan maintenance cycles of the infrastructural network. Structural health monitoring based on dynamic testing has become widespread in the last 20\ua0years, leading to very effective operational algorithms able to extract valuable structural features from the recorded signals. However, although in principle it is possible to identify position and severity of the damage by using a finite element model, still some identification issues are unresolved due to the non-linear nature of the oscillations of a cracked beam. In fact, the available experimental data show, for a given damage pattern, a significant underestimation of the natural frequencies given by cracked beam numerical models. This paper presents an approximate solution for the problem of a vibrating damaged RC beam with opening\u2013closing (breathing) cracks. The solution is based on the static equivalence of the kinetic energy and allows incorporating most of the features of a beam loaded above the cracking limit and oscillating under the self-weight with breathing cracks. The comparison with a wide data set collected in the literature points out the predictive capability of the developed analytical formulas. An independent test confirms the theoretical results

Static and Dynamic Investigation of the Taro Masonry Bridge in Parma, Italy

The Taro bridge has a very long history since was built by Antonio Cocconcelli in 1820. The bridge is composed of 20 arch spans and is arranged in the shape of three center Perronet arches with flood holes in the piers. The road over the bridge has two lanes and is set for the highest truck loading category. After the second World War has been used intensively for road transportation and due to some intrinsic features of the structure, in recent times many brick and stone detachments occurred. Recently, in order to assess the health of the bridge, static and dynamic testing was carried out, by using a set of 8 trucks filled with sand. A Finite Element Model was worked out for the data interpretation, and the parameters of the model were identified on the basis of the vibration frequencies. In order to test the effectiveness of the model, a comparison is made with the static load testing, finding a very good agreement. The main identified parameter are the compliance of the foundation blocks, and the elastic modulus of the fill. The analysis shows that models with different number of spans are all capable to identify the main frequencies of the bridge, if the correct value of the main parameters is introduced

Torque Penetrometric Test for the in-situ characterisation of historical mortars: fracture mechanics interpretation and experimental validation

The assessment of historical structures requires appropriate knowledge of the behaviour of the investigated materials. Concerning masonry, its mechanical characterisation is a challenging task, since its composite nature requires the careful evaluation of the behaviour of its material components. In particular, the experimental assessment of the strength of existing mortar in historical structures still encounters several difficulties. This study investigates a novel Minor Destructive Testing (MDT) technique virtually equivalent to the vane test used for soils. The instrumentation, called herein Torque Penetrometric Test (TPT), is composed of a steel nail with four protruding teeth and a torque wrench. The test consists in inserting the toothed nail into a mortar joint and then applying a torque by means of a dynamometric key, until reaching the failure of the material. This work presents a novel interpretation theory based on basic concepts of fracture mechanics and applied to the micro-mechanical analysis of the stress state induced by the instrument on the investigated mortar. The proposed interpretative theory is validated through the execution of experimental tests in the laboratory and in a historical masonry building. The test proves to be effective for a quick in-situ MDT evaluation of the strength of existing mortars.Peer Reviewe