1,721,370 research outputs found
Model calibration of 3D printed lattice structures
No full textNowadays cellular materials are receiving great attention for their excellent mechanical properties, being applied in energy absorbers or in structural components having optimized mass distribution. In this paper stretch-dominated lattice structures have been considered. A 3D periodic lattice structure of different cell size, TPMS (triply periodic minimal surface), made of epoxy resin by DLP technology was studied. Compression tests at different strain rate (10-3 to 103 1/s) have been performed and a constitutive model to assess the experimental findings has been calibrated
Study of high strain rate effect on sheet formability based on Nakazima test
No full textThe knowledge of the sheet metals formality is important for the success of the manufacturing processed based on plastic deformation. Even though many industrial processes take place at relatively high speeds, higher than the quasi-static regime ones, few studies are present in the literature on the evaluation of the formability at high speed. In this framework, the present paper aims at studying the effect of the high strain rate on the formability of AA6082 aluminium alloy, characterized by a thickness of 1 mm. An experimental equipment, based on the Nakazima test, mounted on a direct tension-compression Split Hopkinson Bar, has been developed in order to define the formability under dynamic loading conditions. The high strain rate behaviour of the alloy has been compared with the one obtained in quasi-static condition
2D DIC-based inverse procedures for the plastic identification of sheet metals in high strain rate tests
No full textIn this work, dynamic tension tests have been conducted by an SHB on sheet metals in order to characterize the plastic behaviour of the materials. Pictures of the samples were acquired during the test by means of a fast camera and were post-processed by a global approach 2D DIC software. On the one hand, this permitted to validate the strain measurement by the classical SHB theory formulas; on the other hand, strain distribution maps were computed with the approximation of small out-of-plane displacements. Different analytical methods have been adopted to exploit quantitatively these strain maps and to extract the parameters of a strain hardening constitutive model: the most simple method uses only the data extracted from the necking region and accounts for the cross section reduction; the most complex is represented by the VFM which uses the information provided by all the pixels in the acquired images
Identification of constitutive model parameters in Hopkinson bar tests
No full textIn this work, tension and compression tests have been carried out on aluminium samples at low and high strain rate, the latter performed by means of a direct tension Hopkinson bar equipment. The parameters of the Johnson-Cook constitutive model have been identified using different approaches; the first method consists in the classical Finite Element Model Updating, where numerical simulations are repeated with different material parameters until the mismatch between the experimental and numerical load–displacement curves falls below an acceptable threshold. The second method is based on the analysis of the digital images acquired by a fast camera during the tests; this permitted to calibrate the JC model by an analytical minimization procedure, without any FE simulation. A third inverse technique was also implemented, consisting in applying the FE model updating but using an enriched cost function, where also the mismatch between the numerical and acquired specimen shape profiles is included and minimized. The advantages and drawbacks of the different techniques are assessed and compared
Evaluation of Friction at High Strain Rate using the Split Hopkinson Bar
No full textThe present work aims at studying the influence of strain rate on the frictional behaviour of AA7075 aluminium alloy in the O-annealed temper state. To this purpose, ring compression tests were performed both under quasi-static and dynamic loading conditions. The high strain rate tests were carried out by means of the Split Hopkinson Tension-Compression Bar in the direct version. In both cases, hollow cylindrical samples, characterised by an initial outer diameter to inner diameter to height ratio of 6:3:2, were tested under dry condition and by lubricating with molybdenum disulphide grease. The different frictional behaviour exhibited by AA7075-O under quasi-static and dynamic loading conditions can be attributed to the strain rate effect both on the plastic flow behaviour of the deformed material, and on the thickness of the lubricant film
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Studio del comportamento di gallerie sottoposte ad azioni sismiche mediante approcci numerici avanzati
No full textIl contributo dell’Università di Bologna è indirizzato allo studio del comportamento di gallerie sottoposte ad azioni sismiche mediante approcci numerici avanzati. L’attività di ricerca prevede una parte di lavoro teorico di validità generale, riferito a condizioni geologico, geotecniche e strutturali ideali, ed una parte dedicata all’applicazione della ricerca allo studio di casi reali in contesti sismotettonici diversi con particolare riferimento alle strutture in sotterraneo realizzate nelle formazioni strutturalmente complesse dell’Appennino Settentrionale e Meridionale (argille scagliose del Complesso Caotico, argille varicolori, flysh, ecc..). Gli obiettivi dell’attività di ricerca possono quindi essere sintetizzati nei seguenti punti: (A) ESECUZIONE DI ANALISI DI TIPO DISACCOPPIATO ED ACCOPPIATO ADOTTANDO APPROCCI CARATTERIZZATI DA CRESCENTI LIVELLI DI COMPLESSITA' E LORO CONFRONTO: a1) Definizione di uno o più casi ideali di riferimento in termini di input sismico, distanza bedrock – struttura in sotterraneo, caratteristiche geotecniche dei terreni attraversati e caratteristiche geometriche e strutturali della galleria; a2) Studio dei diversi casi ideali mediante analisi di tipo disaccoppiato e modelli costitutivi per il terreno “semplici”. L’accelerogramma alla quota della galleria verrà valutato mediante l’analisi della risposta sismica locale, adottando il codice di calcolo Eera e schematizzando il terreno come un mezzo mono-fase viscoelastico non lineare. Le azioni sul rivestimento della galleria verranno valutate assimilando la galleria ad una trave elastica sottoposta alle deformazioni imposte dal terreno circostante secondo come indicato nelle sintesi di Wang (1993) e Hashish et al. (2001); a3) Studio dei diversi casi ideali mediante analisi di tipo accoppiato e modelli costitutivi per il terreno “semplici”. Le analisi numeriche verranno realizzate con i codici di calcolo Abaqus e Plaxis in condizioni di deformazione piana, valutando in questa sede le ipotesi dei due programmi circa l’interazione fra lo scheletro solido ed il fluido interstiziale in condizioni dinamiche. Per descrivere il comportamento meccanico dei terreni si adotteranno uno o due modelli costitutivi disponibili nelle librerie dei programmi di calcolo. Questa fase, pur con i limiti rappresentati dall’utilizzo di modelli costitutivi “semplici”, permetterà di approfondire gli aspetti legati all’interazione terreno-galleria in condizioni dinamiche ed alla loro modellazione nell’ambito di analisi agli elementi finiti; a4) Scelta di un modello costitutivo appropriato a descrivere il comportamento dei terreni naturali sotto l’azione di carichi ciclici e dinamici Data la precedente attività del responsabile dell’Unità di Ricerca ed avendo a mente le tipologie di terreni che verranno presi in esame durante l’applicazione della ricerca allo studio di casi reali, verrà preso a riferimento il modello proposto da Kavvadas & Amorosi (2000), apportando a questo lievi modifiche per quanto pertiene alla descrizione del comportamento reversibile secondo quanto proposto recentemente da Houlsby et al. (2005), alla legge di incrudimento isotropo relativa alla componente di destrutturazione in accordo con Amorosi & Boldini (2003), ed eventualmente all’espressione della legge di variazione del modulo plastico per problematiche legate alla sua non facile implementazione secondo una particolare classe di algoritmi di integrazione; a5) Integrazione del modello costitutivo selezionato nel codice di calcolo commerciale Abaqus, e dopo sua messa a punto, nel codice Plaxis di più semplice utilizzo L’implementazione del modello costitutivo sarà realizzata mediante un algoritmo di tipo completamente implicito in grado di assicurare soluzioni accurate del primo ordine, stabilità incondizionata e convergenza asintoticamente quadratica a livello globale, come già dimostrato in precedenti esperienze di integrazioni di versioni più semplificate (i.e.,..
Improvements of an unconventional desiccant air conditioning system based on experimental investigations
No full textDesiccant-based HVAC systems are nowadays a convenient alternative to conventional systems based on dehumidification by cooling, because they allow the reduction of equivalent CO2 emissions, electric peak loads and black-outs, as well as energy savings, interesting payback periods in several cases, better indoor humidity control and air quality, separate control of thermal sensible and latent loads. Their coupling with small scale natural gas-fired cogenerators is also suitable as thermal energy required for desiccant regeneration can be conveniently recovered from the microcogenerator. In this paper, the main improvements of an innovative desiccant-based HVAC system, located in Southern Italy (Mediterranean climate), are presented. To this aim, three different desiccant system configurations are experimentally analysed, while a conventional HVAC system based on dehumidification by cooling is evaluated by means of numerical approach. Firstly, the primary energy savings (up to 20-25%) and the reductions of equivalent CO2 emissions (up to 40-50%) of the desiccant systems compared to the conventional one are calculated when considering chillers with different energy efficiency ratio values. Successively, the above mentioned indices are evaluated as a function of the effectiveness of the heat exchanger and the cooling air humidifier installed in the proposed desiccant system configuration: primary energy savings up to 25-28% and reductions of equivalent CO2 emissions up to 35-40% are obtained
- …
