177,071 research outputs found
TECNOLOGIE E PROPRIETA’ DEI MATERIALI COMPOSITI
No full textI contenuti di questo libro sono indirizzati principalmente agli studenti di ingegneria che si avvicinano per la prima volta al mondo di questi materiali tecnologicamente avanzati. Esso contiene però anche interessanti spunti per chi già lavora nel settore e vuole approfondire qualche argomento o razionalizzare le sue competenze. I contenuti dei primi Capitoli sono del tipo “di base” e sono quindi adatti anche a lettori che, pur non avendo solide conoscenze tecniche di tipo ingegneristico, siano mossi dalla semplice curiosità di approcciare questo mondo. I Capitoli relativi al comportamento meccanico richiedono invece una conoscenza seppure elementare di meccanica dei materiali e delle strutture. Di particolare interesse poi l’ultimo Capitolo che affronta la tematica dell’economia circolare, molto attuale in questo periodo, in modo semplice e divulgativo.
Il testo presentato non può e non vuole ovviamente essere completamente esauriente: nella parte di bibliografia sono però citati riferimenti utili per approfondire i diversi argomenti
Problems involved in fracture mechanics characterization of rubber-modified glassy polymers, using double torsion
No full textAn application of viscoelastic fracture criteria to steady crack propagation in a polymeric material under fixed deformation
No full textTensile stress relaxation and stable crack growth under fixed tensile deformation over an extended range of crack speeds were measured on a flexible thermosetting epoxy resin. The experimental results are analysed and compared with the predictions of two theoretical models of fracture kinetics in viscoelastic media, based on different approaches: one theory, developed by Schapery, postulates a criterion of material failure local to the crack tip; the second theory, evolved by Christensen, extends the overall statement of the conservation of energy to systems that dissipate energy due to viscoelastic effects. The observed behaviour is in qualitative agreement with the theoretical predictions. Quantitative discrepancies, however, call for further theoretical refinement
The effects of strain-rate and temperature on the interlaminar fracture toughness of interleaved PEEK/CF composites
No full textThe effect of interleaving on the interlaminar fracture behaviour of unidirectional and woven-fabric fibre composites based upon continuous carbon fibres (CF) and poly(ether ether ketone) (PEEK) has been investigated over a wide range of temperature and crack speed. The fracture data obtained from mode I and mode II tests have been analyzed using the time-temperature equivalence postulate and the results are discussed in terms of fracture micromechanisms. The insertion of a resin-rich layer between laminae was found to increase significantly the fracture resistance of the composites, without altering its dependence on crack speed
Mesoscale mechanical modelling and experimental measurements of textile composite reinforcements
No full textWoven fibrous textiles as reinforcement of composite materials have received considerable attention in many different industrial applications, due to their improved formability and mechanical properties. Predictive tools are mandatory to avoid time-consuming experimental measurements and to predict and assess the mechanical behaviour of such dry textiles upon variation of several parameters. Their mechanical features have considerable influence during complex shape forming process, in the initial step of the composite manufacturing, and on the mechanical behaviour of final composite components. In this context, the present study deals with the mechanical behaviour of fibrous textiles as reinforcements for composites. A glass plain weave and a hybrid PET/aramid/PET tetraxial textiles were considered. The tetraxial interlacement has warp, weft, and two diagonal yarns oriented at symmetrical angles (typically ±45°) with respect to the warp direction. The aims of the study were: (i) to experimentally observe the deformation and to measure some mechanical properties of the textile reinforcements; (ii) to numerically predict the nonlinear behaviour of the woven fibrous fabrics for any in-plane loading conditions; (iii) to compare the finite element (FE) modelling results to the experimental observations of biaxial tensile and inplane shear loadings. The experimental activities were focused mainly on uniaxial and biaxial extension tests, the latter with different ratio R of the warp to weft displacement rates. Besides, out of plane bending was investigated by cantilever beam test. The numerical study was based on a hyperelastic constitutive model for the yarns material [4]. Assuming the repetitive nature of the considered textiles, the simulation were focused on the Representative Volume (RV), namely meso-scale modelling. The comparison of experimental measurements and numerical predictions highlights the accuracy of the hyperelastic model to predict the nonlinear behaviour of the fabric for any loading condition and to provide the proper numerical model for further optimization investigations of woven fibrous reinforcement
Damage of concrete and bonding of FRP reinforcement
No full textDamage of reinforced concrete in structures like bridges and highways is related to factors like environmental aggression, service loads, construction technologies and, exceptionally, seismic events.
Chemicals and physical agents may induce a gradual increase in the concrete porosity and permeability, causing loss of the material integrity. In bridge decks, damage is also dependent on low fatigue level due to cyclic loads, on thermal loads and corrosion of steel bars.
The object of the present study is a viaduct of the Orte-Ravenna highway, built in the first seventies. The reinforced concrete slab of the viaduct exhibited serious structural deficiencies and, therefore, was destined to demolition and subsequent reconstruction. Parts of the removed slab were carried to the Ferrara University Laboratories in order to evaluate the concrete damage level and to assess a possible strengthening technique for repair.
In the paper, the investigation results are reported. The investigation consists of non destructive evaluation as well as experimental tests on concrete specimens [1]. At the structural scale a thermography is carried out in order to observe homogeneity of concrete and to point out anomalous zones with cracks, cavities or incoherent aggregates [2]. Thermography is based on the emission of thermic radiation and, according to
Wien law, the range of medium and far IR (2-15 μm) turns out to be suitable to characterise thermic emission of objects at ambient temperature. Surface temperature difference corresponds to different intensity of infrared rays and this produces, in the radiographic apparatus, contrast of the thermographic image. At the
specimen scale, the investigation is carried out by mercury porosimetry, compression test, three-point bending test, ultrasound measurements, scanning electron microscope equipped with an energy dispersive X-ray (EDX) analyser.
Concerning the strengthening techniques, the application of fibre reinforced polymers (FRP) is considered. In particular, the problem of fibre debonding and concrete cover separation [3], in the presence of damaged concrete, is addressed and experimental results are reported showing the effectiveness of different practical
approaches to improve the composite-concrete interface
The diluvium aquarum: geologic evidence and geoarchaeological constrains of extreme floods in northern Italy during early Middle Ages
No full textBetween the second part of the sixth century and the seventh century, many regions of Europe have been characterized by dramatic changes in the hydrographic setting, probably related to a strong cooling phase (Büntgen et al., 2016; Helama et al., 2017). In the Mediterranean region the period following the collapse of the Roman Empire is described by many authors as an interval characterized by important alluvial processes (e.g. floods, river avulsions and alluvial aggradation in the mountain valleys), strongly contrasting with the general geomorphological stability of Roman Age and Late Antiquity. Some important chronicles of early Middle Ages report the occurrence of very high-magnitude floods, often described by ancient historians as diluvium (deluge), contributing to create a sort of myth around an out-of-scale event or a longer meteo-climatic phase (Cremonini et al., 2013).
In the framework of the INQUA project “EX-AQUA: Palaeohydrological Extreme Events, evidence and archives” (1623P), a review of the traces of early Medieval floods occurred in Northern Italy was carried out. The study considered new information and critically re-analyses previous available data, supplied by geomorphological, stratigraphic and geoarchaeological evidence, ancient documents and written sources.
In Northern Italy a fast and strong sedimentary phase occurred between 5th and 9th century AD, leading many large Alpine rivers to avulse. In the system of Tagliamento, thanks to recent geoarchaeological excavations and geochronological analyses, a major extreme event is clearly constrained between the second half of the 6th and the first part of the 7th century, when the river avulsed and destroyed the ancient city of Concordia Sagittaria (Fontana et al., 2019). This episode overlaps with the one reported by the Lombard historian Paolo Diacono, dated to 589 AD, which strongly damaged Verona and, downstream of this city, possibly triggered the avulsion phase of Adige River near the so-called “Rotta della Cucca”. Important fluvial changes affected also the Piave and Livenza rivers, while an avulsion channel of Brenta River started to form in the 6th century AD.
Notwithstanding, for some minor alluvial systems the detailed chronology supported by archaeological and radiocarbon chronology allows to detect the existence of earlier flooding units, formed since the 2nd and 3rd century AD. Recent data in the Alpine valley of Adige River, around the city of Trento, point to the occurrence of some important flood events in the alluvial cones of the major tributary creeks already during the 3rd century AD. However, in the same area the floor of Adige valley experienced a vertical aggradation only since the 4th and 5th century AD, with an enhanced rate of deposition between 6th and 10th century AD. A rather comparable chronology characterizes part of the alluvial cones of the main Apennine streams flowing towards the Po Plain (e.g. near Modena), which aggraded during early Medieval, but this trend started already in the 3rd century AD (Cremonini et al., 2013).
This research supports new data for comparing the palaeoflood record of early Middle Ages with palaeoclimatic proxies, with the aim of distinguishing global forcing factors from regional constrains and anthropogenic disturbance.
Büntgen et al. (2016) – Cooling and societal change during the Late Antique Little Ice Age from 536 to around 660 AD. Nature Geoscience 9, 231–237.
Helama S., Jones P., Briffa K. (2017) - Dark Ages Cold Period: a literature review and directions for future research. Holocene 27, 1600–1606.
Cremonini S., Labate D., Curina R. (2013) - The late-antiquity environmental crisis in Emilia region (Po river plain, Northern Italy): Geoarchaeological evidence and paleoclimatic considerations. Quaternary International, 316, 162.178.
Fontana A., Frassine M., Ronchi L. (2019) - Geomorphological and Geoarchaeological Evidence of the Medieval Deluge in the Tagliamento River (NE Italy). In: Herget J., Fontana A. (Eds), Palaeohydrology, Traces, Tracks and Trails of Extreme Events. Springer, 97-116
Matrix toughness transfer and fibre bridging laws in acrylic resin based CF composites
No full textThe interlaminar fracture toughness, GIc, of unidirectional carbon fibre composite laminates produced by infusion moulding with two different thermoplastic acrylic matrices, one plain and one rubber toughened, was studied. For the composite with the plain matrix fracture was dominated by failure at the fibre-matrix interface. With the toughened matrix, fracture occurred within the matrix-rich layer, although the matrix toughness could not be fully exploited due to the interaction of the fibres with the development of the process zone. A fibre bridging model based on experimentally derived cohesive laws was developed and it confirmed that fracture occurs within the matrix
Fracture toughness of acrylic resins: Viscoelastic effects and deformation mechanisms
No full textThe time dependence of fracture toughness of two different acrylic resins, one plain and one toughened, intended to be used as continuous fiber composite matrices was studied. By performing fracture tests following the fracture mechanics approach, the energy release rate, GIc, was determined at different temperatures and displacement rates and by applying the time-temperature superposition it was possible to obtain GIc as a function of crack speed, math formula, over a wide range of speeds. The trends obtained for the two resins were different. For the plain resin it could be well described by J. G. Williams' viscoelastic fracture theory while for the toughened resin, the trend obtained was attributed to a change in the damage mechanism occurring at the crack tip during fracture. From measurements of the process zone size it was deduced that the damage mechanism at the crack tip for the plain resin was the same irrespective of time and temperature, for the toughened resin instead, different mechanisms seem to take place. This hypothesis was supported by results of volume strain measurements in tensile tests at different temperature and strain rates
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