662 research outputs found
Design and simulation of porous lightweight structures built by selective laser melting
Get PDFI materiali porosi in acciaio sono una classe di materiali alleggeriti che presentano un notevole potenziale in diverse applicazioni che spaziano dal settore automotive a quello aerospaziale. Le proprietà di tale tipologia di materiali sono fortemente influenzate dal tipo, dalla dimensione e dalla distribuzione dei pori in essa presenti. La fusione laser selettiva (selective laser melting o SLM) è una delle più importanti tecnologie di additive manufacturing e consente di controllare in modo appropriato i principali parametri che influenzano il comportamento dei materiali porosi realizzati attraverso tale processo produttivo. Lo scopo del presente lavoro è simulare il comportamento a compressione a temperatura ambiente di provini in acciaio, ottenuti mediante selective laser melting, caratterizzati da pori sferici di grandi dimensioni. Tali strutture rappresentano un concetto di produzione altamente innovativo. La simulazione numerica sviluppata è in grado di coprire l’intero range di deformazione a cui sono soggetti i provini, vale a dire fino al 80% di nominal strain. A tal riguardo, sono stati sviluppati tre modelli agli elementi finiti dei provini sottoposti a compressione uniassiale includendo differenti livelli di dettaglio. Il modello costitutivo selezionato per la realizzazione delle simulazioni agli elementi finiti è il modello di Hollomon e, sebbene i test sperimentali abbiano evidenziato un comportamento altamente non lineare che manifesta il collasso plastico dei campioni porosi, le simulazioni numeriche hanno riprodotto accuratamente i risultati sperimentali. Il presente studio è inoltre in grado di quantificare fenomeni e aspetti caratteristici del comportamento delle architetture porose come, ad esempio, la distribuzione delle tensioni e deformazioni, l’evoluzione dello stress concentration factor in prossimità dei pori e l’effetto dell’aspect ratio dei pori e del comportamento deformativo sulla localizzazione degli stress. Vengono in aggiunta discusse tematiche inerenti la modellazione che contribuiscono a limitare il costo computazionale delle simulazioni pur garantendo un buon livello di accuratezza dei risultati numerici. Infine, nella sezione finale del lavoro si incominciano a porre le basi per le future investigazioni: viene proposto un algoritmo sviluppato “in house” capace di realizzare distribuzioni spaziali di pori sferici al fine di progettare strutture in acciaio caratterizzate da porosità con disposizione random compatibili con le limitazioni imposte dal processo produttivo di fusione laser selettiva.Steel porous materials are multifunctional lightweight materials with a great potential in several applications ranging from automotive to aerospace sector. Their properties are strongly influenced by type, dimension and distribution of pores. Selective Laser Melting (SLM) is one of the most important additive manufacturing technologies, which allows to properly control the main parameters that affect the behavior of manufactured porous materials. The purpose of this study is to simulate the room temperature compression behavior of steel specimens with large spherical pores built via SLM. Such a construction is a highly innovative manufacturing concept. The numerical simulation covers the whole deformation range undergone by specimens, that is up to 80% nominal strain. In this regard, three finite element models of the specimens subjected to uniaxial compression are developed including different levels of detail. The Hollomon model is selected as the constitutive model given in input to the FE simulations. Although experimental tests highlight a highly nonlinear behavior entailing plastic collapse of porous samples, numerical simulations can accurately reproduce experimental results. Moreover, the present study is able to quantify phenomena and characteristic aspects of porous architectures behavior, such as stress–strain distribution, the evolution of stress concentration factor near pores and the influence of pore’s aspect ratio and deformation behavior on stress localization. Modelling issues that may contribute to limit the computational cost of simulations yet preserving good level of accuracy of FE results also are discussed in the article. Finally, bases for future investigations are traced in the last Section of the work: a developed in-house algorithm capable of realizing spherical pores spatial distribution is proposed in order to design steel structures characterized by random porosity compatible with constraints imposed by SLM manufacturing process
F.W. Angel memorial lecture ; 1970; F.W. Angel memorial lecture, 1970
No full textThe Third Annual F.W. Angel Memorial Lectur
Il nichilismo tra Jacobi e il giovane Hegel. Paradigmi di una critica del rappresentazionalismo
No full textVerifica a fatica dei giunti saldati sulla base di misure di deformazione locale
Get PDFLo studio della resistenza a fatica delle giunzioni saldate rappresenta un campo di prova molto interessante dal punto di vista scientifico, con ricadute pratiche altrettanto importanti. Si tratta in generale di fornire dei metodi di verifica e progetto deterministici, come è proprio dell’ingegneria, per prevedere il comportamento meccanico di un materiale che, quando è sotto forma di saldatura, cambia le sue proprietà meccaniche e microstrutturali rispetto al materiale base, si dispone in una geometria locale del cordone estremamente variabile e non definibile a priori, è infine affetto da campi di tensione residua non proprio trascurabili. In questo lavoro, partendo da una breve panoramica sui principali indicatori e metodi che sono stati utilizzati nel corso degli anni per la stima dello stato di sollecitazione in un giunto saldato e della vita residua a fatica, si presenta l’approccio basato sulla misura della deformazione locale che è stato seguito da diversi autori nel corso degli anni, evidenziandone i vantaggi ma anche le limitazioni rilevate attraverso le numerose attività sperimentali direttamente eseguite
Fatigue damage evolution of fiber reinforced composites with digital image correlation analysis
Get PDFAbstractSeveral studies on Digital Image Correlation technique allowed this methodology to be recognised in the last decade as a welloptimised tool for mechanical experimental measures, but much work is to be done to apply DIC based analyses on advanced applications, such as fatigue damage studies and composite materials failure investigations. DIC based methodology gives direct measure of the displacement field using a well defined formulation of matching the images of loaded and load free component, so that displacement vectors can be evaluated for a certain number of grid reference points on the analysed surface.In this work, the authors present the latest research with this method, and analyse fatigue damage phenomena in GFRC laminates under tensile fatigue load employing notched and notch-free specimens. Damage analysis based on optical DIC technique has been performed to detect the damaged areas on the specimen surface and to follow failure mechanisms during lifetime; the damage evolution in composites was performed monitoring two parameters: the local hysteresis area of stress strain cycles, the local stiffness variation. These parameters are evaluated on a grid by means of DIC strain measurements during the entire fatigue life showing encouraging results
Fatigue life prediction of notched components based on a new nonlinear continuum damage mechanics model
No full textAbstractA CDM (continuum damage mechanics) model for damage evaluation is here considered and applied to the study of two different typologies of notched and cylindrical specimens. The model presents some parameter and authors determined them in a previous work testing cylindrical and smooth specimens.Firstly fatigue characterization was conducted and the SN curves found. In order to evaluate the capability of CDM model to predict the sequence effect and to simulate a more realistic loading condition, tests with various loading blocks were carried on and in particular high-low, low-high and random blocks were applied to the three specimens considered. Model previsions showed good agreement with results for each geometry considered
Temperature harmonic analysis to investigate intrinsic dissipation of fatigue loaded Ti6Al4Va produced by laser powered bed fusion
No full textAuthor Correction:A 41,500 year-old decorated ivory pendant from Stajnia Cave (Poland)
No full textCorrection to: Scientific Reports https://doi.org/10.1038/s41598-021-01221-6, published online 25 November 2021The original version of this Article contained errors in the author list where Marjolein D. Bosch was omitted from the author list, and Mikołaj Urbanowski was incorrectly listed as an author of the original Article, and has subsequently been removed.The Author contributions section now reads:“S.T. W.N. and A.N. conceived the project; S.T., W.N., A.P., M.B., S.C., M.D., H.F., A.M., M.D. B., D.P., M.P.R., C.M.R., V.S-M., G.M.S., P.S., M.S., K.S., A.V., F.W., H.W., A.W., M.Z., S.B., A.N., J-J. H., performed research; S.T., A.P., W.N., M.B., M.D.B., S.C., M.D., H.F., A.M., D.P., M.P.R., C.M.R., V.S-M., G.M.S., P.S., M.S., K.S., A.V., F.W., H.W., A.W., M.Z., S.B., A.N., J-J. H. analysed all archaeological data; S.T. and A.P. wrote the paper with the collaboration of all the co-authors.”The original Article and its accompanying Supplementary Information file have been corrected
Transforming Power Relationships: Leadership, Risk, and Hope. IHS Political Science Series No. 135, May 2013
Get PDFChronic communal conflicts resemble the prisoner’s dilemma. Both communities prefer peace to war. But neither trusts the other, viewing the other’s gain as its own loss, so
potentially shared interests often go unrealized.
Achieving positive-sum outcomes from apparently zero-sum struggles requires a kind of riskembracing leadership. To succeed leaders must: a) see power relations as potentially
positive-sum; b) strengthen negotiating adversaries instead of weakening them; and c) demonstrate hope for a positive future and take great personal risks to achieve it.
Such leadership is exemplified by Nelson Mandela and F.W. de Klerk in the South African democratic transition. To illuminate the strategic dilemmas Mandela and de Klerk faced, we examine the work of Robert Axelrod, Thomas Schelling, and Josep Colomer, who highlight important dimensions of the problem but underplay the role of risk-embracing leadership. Finally we discuss leadership successes and failures in the Northern Ireland settlement and the Israeli-Palestinian conflict
Interactive visualisation techniques for large time-dependent data sets
No full textThe research described in this thesis was part of a larger research project about multi-phase flows. These flows are characterised by a sharp transition between the fluids, the so-called phase front. One of the goals of the project was to study the evolution of the phase fronts using CFD, i.e. to study the development of the surfaces over time and to understand how they change and interact with each other. In order to study the evolving fronts, methods were needed for detecting and extracting them in the first place, and subsequently for tracking the phase fronts over time, and finding a way to visualise them interactively. The focus of this research was directed towards efficient techniques for interactive isosurfacing from very large time-dependent data sets. Fast-access data structures that were designed to perform one particular visualisation task efficiently were examined first. These data structures make use of the properties of a particular visualisation algorithm and are made to fit the algorithm closely. The advantage of data structures like these is that they are designed to perform a particular visualisation task very fast. However, the drawback is that they are also limited to perform only that visualisation task. The second approach that was explored is the use of multi-resolution data structures. These structures enable the data to be accessed at several levels of resolution. A multi-resolution data structure provides the flexibility to switch between different visualisations and is designed to handle large data sets by trading off data resolution for speed. This multi-resolution approach was extended to time-dependent data sets. Techniques for region-of-interest selection and time-window management were added to provide interactive visualisation and space-time navigation of these large 4D data sets.Electrical Engineering, Mathematics and Computer Scienc
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