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Towards Material Testing 2.0: thermomechanical characterization of steels through inverse methods
No full textIl Material Testing 2.0 (MT2.0) rappresenta la nuova frontiera nella caratterizzazione delle proprietà termomeccaniche dei materiali, specialmente degli acciai. Nel MT2.0, i metodi inversi, come ad esempio il metodo dell’aggiornamento del modello agli elementi finiti e il metodo dei campi virtuali, vengono applicati a prove sui materiali con provini specificamente studiati per generare, in maniera simultanea, stati eterogenei di deformazione, tensione e temperatura. Questo approccio massimizza le informazioni ottenibili da una singola prova e permette di calibrare in modo efficace i parametri di modelli costitutivi anche avanzati in grado di descrivere il comportamento del materiale testato. Durante la prova, i campi di deformazione e temperatura eterogenei sul provino vengono monitorati utilizzando tecniche di misura a campo intero come la correlazione digitale di immagini e la termografia ad infrarossi. La conoscenza dei campi di deformazione e temperatura riveste infatti un ruolo fondamentale nella procedura d’identificazione dei parametri costitutivi del materiale. Il MT2.0 grazie alle sue potenzialità è quindi ragionevolmente candidato a sostituire in futuro le prove standard basate su stati omogenei di tensione, deformazione e temperatura sul provino.
L’attività di ricerca presentata in questa tesi di dottorato lavora nella direzione di estendere l’applicazione del MT2.0 alla caratterizzazione avanzata degli acciai ed è divisa in tre parti che corrispondono ad altrettanti campi di applicazione: (i) prove di rigonfiamento per studiare, con stati di tensione e deformazione eterogenei, le proprietà meccaniche delle lamiere, (ii) prove di trazione eterogenee ad alta temperatura per caratterizzare il comportamento termomeccanico degli acciai e (iii) prove combinate di trazione-torsione per studiare il comportamento plastico degli acciai prodotti mediante manifattura additiva. Nella prima parte, è stata sviluppata una macchina per realizzare prove di rigonfiamento e prove Nakajima. A differenza di soluzioni commerciali rigide e costose, la macchina progettata offre elevata flessibilità all’utilizzo con diverse geometrie di premilamiera e punzone. Inoltre, è pensata per essere integrata facilmente con la correlazione digitale di immagini stereo come sistema di misura a campo intero della deformazione. Nella seconda parte, è stata disegnata una nuova geometria di provino per prove di trazione ad alta temperatura da eseguire con il sistema di test Gleeble. La geometria è stata studiata attraverso simulazioni multifisiche agli elementi finiti elettro-termo-meccaniche realizzate con il software commerciale ABAQUS/Standard®. L’obiettivo era generare sul provino stati eterogenei di deformazione, tensione e temperatura tali da consentire di caratterizzare, mediante l’applicazione di metodi inversi, il comportamento termomeccanico di acciai in un ampio intervallo di temperatura con una singola prova. La geometria “eterogenea” di provino così definita è stata successivamente validata con prove sperimentali sull’acciaio laminato a freddo DC05EK per calibrare i parametri della legge di incrudimento di Johnson-Cook accoppiata al criterio di snervamento di Hill48. Il metodo dei campi virtuali è stato utilizzato come strategia di identificazione inversa. Infine, nella terza parte, è stato condotto uno studio approfondito per analizzare l’effetto combinato di diverse strategie di scansione laser e orientamenti rispetto alla piattaforma di stampa sul comportamento plastico dell’acciaio inossidabile 316L prodotto mediante manifattura additiva con la tecnologia di fusione laser selettiva a letto di polvere. La forte influenza di queste variabili di stampa sulle proprietà meccaniche del materiale è stata rilevata conducendo prove standard di trazione e indagata attraverso la microscopia ottica e la microscopia elettronica a scansione. In seguito, sono stati condotti test di torsione pura e trazione su provini cilindrici dello stesso materiale al fine di calibrare un modello di plasticità per diversi orientamenti rispetto alla piattaforma di stampa. Il metodo dell’aggiornamento del modello agli elementi finiti è stato impiegato per l’identificazione inversa dei parametri di diversi modelli di plasticità. Lo studio dimostra i limiti dei modelli di plasticità tradizionali nel descrivere il comportamento torsionale osservato durante le prove.Material Testing 2.0 (MT2.0) represents the new frontier in characterizing the thermomechanical properties of materials, especially steels. Inverse methods, e.g. Finite Element Model Updating (FEMU) and Virtual Fields Method (VFM), can be applied to specimens specifically designed to generate heterogeneous strain, stress, and temperature states simultaneously. Such approach maximizes the information that can be obtained from a single test and allows to calibrate efficaciously the parameters of advanced constitutive models. The heterogeneous strain and temperature fields are measured using suitable full-field techniques, e.g. Digital Image Correlation (DIC) and InfraRed Thermography (IRT), which play a key role for the success of the identification procedure. Thanks to their flexibility and potentialities, heterogeneous tests coupled with inverse methods (i.e. MT2.0) are reasonable candidates in the future to replace standard tests based on homogeneous stress, strain, and temperature states on the specimen.
The research activity presented in this doctoral thesis works in the direction of extending the application of MT2.0 to advanced characterization of steel and it is divided in three parts that correspond to different experimental procedures: (i) heterogeneous bulge tests to study the mechanical properties of sheet metals, (ii) heterogeneous hot tensile test for characterizing the thermomechanical behavior of steels and (iii) combined tension-torsion tests to study the plastic behavior of steels produced by Additive Manufacturing (AM). In the first part, a machine was developed to conduct bulge and Nakajima tests. Unlike rigid and expensive commercial solutions, this machine offers flexibility in using various die and punch geometries. Additionally, it is specifically designed for an easy integration with stereo-DIC as full-field strain measurement system. In the second part, a novel specimen geometry for hot tensile tests was studied in ABAQUS/Standard® through multiphysics electro-thermo-mechanical simulations to be tested using the Gleeble system. The heterogeneous specimen geometry was then validated with a real test on the DC05EK cold-rolled steel to calibrate the parameters of the Johnson-Cook hardening law coupled with Hill48 anisotropy yield criterion. The non-linear Virtual Fields Method (VFM) was used as inverse identification strategy. Finally, in the third part, a detailed study was conducted to analyze the combined effect of different laser scan strategies and orientations with respect to the build platform on the plastic behavior of the 316L stainless steel printed by Laser Powder Bed Fusion (L-PBF) additive technology. The strong influence of these variables on the mechanical properties of the material was detected through standard tensile tests and investigated through optical microscopy (OM) and scanning electron microscopy (SEM). Subsequently, pure torsion and tensile tests were conducted on cylindrical specimens of the same material in order to calibrate a plasticity model for different orientations with respect to the build platform. The Finite Element Model Updating method (FEMU) was employed as strategy for the inverse identification of the parameters of different plasticity models. The study demonstrates the limits of the classical plasticity models in describing the torsional behavior observed during the tests
Combined effect of process variables on the plastic behaviour of 316L stainless steel printed by L‐PBF
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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