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Massive fatigue assessment of welded megastructures by advanced methods
Full text linkIn steel megastructures welding is a widely used and accepted joining technique. However, welds are geometrical discontinuities, resulting in severe local stress gradients, which strongly affect the fatigue strength of components. Advanced fatigue assessments are usually based on the local stress and strain state in the close neighborhood of such stress raisers. Despite this, current standards are lacking in giving a real guidance on how to perform a reliable fatigue assessment. Still, most of them do not refer to any local concept and lead to the nominal stress method. Even so, no recommendations exist on how to derive the nominal stress from a finite element (FE) model and it is left to the engineering assessment of a designer to establish which nominal stress is the right one.
Within this framework, the present Ph.D. thesis, focused on making the fatigue assessment of large steel structures possible, is divided into ten chapters. Purposes of this thesis are both giving scientific contributions to advanced local approaches for fatigue assessment and developing a method fully compliant with current standards, in order to be employed in the industrial context.
In the first chapter, a general introduction and the state of the art on fatigue design of welded structures are presented, with the aim to clarify the motivations of the present research work. In the second chapter, the adopted local approaches, namely the notch stress intensity factor (NSIF) based approach, the averaged strain energy density (SED) criterion and the peak stress method (PSM), are briefly introduced and described along with their theoretical frameworks. The third chapter deals with the fatigue behavior of large-scale welded cover plates, for which non uniform fatigue classification is highlighted at standards’ level. Through the application of the SED approach and adopting both bi-dimensional and three-dimensional FE models, parameters which mainly affect the fatigue strength are identified and alternatives to those provided by standards are proposed. Experimental data of four geometric variants are successfully summarized into a unique scatter band in terms of SED, regardless of the weld geometry. The suitability of the SED approach and of the related design curve to perform the fatigue assessment of welded cover plates is, therefore, not proven wrong. The fourth chapter is focused on the local SED numerical computation. Its principal drawback, consisting in the need of a specific control volume centered on a notch tip (i.e. at the weld toe and at the weld root in case of welded joints), within which the strain energy density has to be averaged, is overcome by using coarse meshes completely free-generated. The method and its limitations are formalized. For the sake of generality, some practical applications are given by using both Ansys® and Straus7® FE software. Robustness in terms of insensitivity to mesh pattern, mesh refinement and FE formulation are proven advantages of the method. The fifth chapter establishes a link between local stress fields, near weld toes and roots, and the nominal stress components evaluated at a proper distance from the weld. An analytical relationship between such distance and the loaded plate thickness is provided. A criterion to estimate SED values, both at the weld toe and at the weld root, as well as a posteriori the related NSIFs, as an explicit function of the nominal load components (membrane loads, shear loads and bending moments) is also presented. The proposed method is suitable for automation to perform the large number of fatigue assessments that a nowadays complex steel structure requires. However, the current lack of normative compliance of both SED and NSIF approaches can be a possible obstacle in industrial applications. That is why in the sixth chapter a method, scientifically and normatively compliant, to improve the classical nominal stress approach for welded structures, which is still the most widely accepted and recognized in standards, is proposed. The methodological problem of the nominal stress definition is overcome through an original, finite element based approach, which takes into account both membrane and bending effects. An experimental validation is presented and the implications in fatigue design of large steel structures are discussed. The seventh and eighth chapters present a finite element post-processor, developed to perform the almost automatic fatigue assessment of a large structure. The post-processor is compatible with Straus7® finite element solver and it is based on shell models to be suitable for large assemblies. Many of the findings of the present research work are automated: local SED and NSIF approaches are implemented, as well as the modified nominal stress and, finally, the classical nominal stress and hot spot stress approaches. A very good agreement between “manually” performed assessments, both through global and local approaches, and those rapidly performed by using the post-processor is found. Further good agreement is found between expected fatigue lives estimated through the local SED approach and those estimated through the modified nominal stress, in the presence of bending stresses. The ninth chapter deals with the rapid estimation of residual notch stress intensity factors (R-NSIFs), due to the welding process, by using the PSM and Sysweld® FE dedicated software. First, the calibration of the PSM in Sysweld® environment is presented; afterwards, practical applications of the PSM to evaluate the R-NSIFs are illustrated. Finally, the tenth chapter presents some overall concluding remarks, in order to discuss the main obtained results.Nelle megastrutture in acciaio la saldatura è una tecnica di giunzione ampiamente utilizzata. Tuttavia, le saldature rappresentano delle discontinuità geometriche e introducono elevati gradienti tensionali locali che influiscono negativamente sulla resistenza a fatica dei componenti. Secondo la letteratura scientifica recente, le analisi di resistenza a fatica più avanzate si basano sugli stati di tensione o deformazione locali calcolati in prossimità dei punti singolari. Ciononostante, le normative vigenti mancano di fornire una guida reale su come eseguire tali stime della resistenza fatica: la maggior parte di esse non fa riferimento agli approcci locali e prevede l'impiego del metodo della tensione nominale. Tuttavia, non esistono raccomandazioni su come ottenere la tensione nominale mediante un modello agli elementi finiti ed è demandato alla capacità ingegneristica del progettista stabilire quale sia quella adeguata.
In questo contesto, la presente tesi di dottorato, focalizzata sul rendere possibile la stima della resistenza a fatica delle grandi strutture in acciaio, è divisa in dieci capitoli. Scopo della tesi è sia fornire un contributo scientifico ad alcuni tra i più avanzati approcci locali per la stima della resistenza fatica, che sviluppare un metodo pienamente conforme alle normative vigenti, al fine di poter essere impiegato nel contesto industriale.
Il primo capitolo rappresenta l'introduzione generale sul tema trattato, lo stato dell'arte in materia di progettazione a fatica delle strutture saldate e le motivazioni della presente ricerca. Nel secondo capitolo vengono introdotti gli approcci locali adottati e le loro basi teoriche: l'approccio basato sui fattori di intensificazione delle tensioni (NSIF), il criterio della densità di energia di deformazione (SED) e il metodo della tensione di picco (PSM). Il terzo capitolo riguarda la caratterizzazione a fatica dei coprigiunti saldati, tipicamente impiegati come rinforzo nelle travi da ponte, per i quali è stata evidenziata una non uniforme classificazione a fatica a livello normativo. Mediante l'impiego dell'approccio locale SED e adottando modelli agli elementi finiti sia bidimensionali che tridimensionali, sono stati isolati i parametri che influiscono sensibilmente la resistenza e proposte soluzioni ottimizzate rispetto a quelle fornite dalle normative. I dati sperimentali ottenuti testando a fatica quattro differenti soluzioni geometriche sono sintetizzati con successo in un'unica banda di dispersione in termini di SED, indipendentemente dalla geometria della saldatura. L'approccio locale SED e la relativa curva di progettazione si sono dimostrati quindi adatti a stimare la resistenza a fatica dei coprigiunti saldati in acciaio. Il quarto capitolo è incentrato sul calcolo numerico del SED. La principale criticità dell'approccio, rappresentata dalla necessità di uno specifico il volume di controllo localizzato all'apice di un intaglio strutturale (al piede e alla radice dei cordoni di saldatura, nel caso dei giunti saldati), entro il quale l'energia di deformazione deve essere calcolata e mediata, è superata mediante l'utilizzo di maglie di calcolo (mesh) rade e generate in maniera completamente automatica da un generico algoritmo di meshatura. La soluzione proposta e le relative limitazioni di applicabilità sono stati formalizzati. La robustezza in termini di insensibilità alla tipologia di mesh, alla sua raffinatezza e alla formulazione degli elementi finiti sono alcuni dei vantaggi provati. La generalità del metodo è dimostrata anche mediante alcune applicazioni pratiche con l'impiego di differenti software agli elementi finiti. Il quinto capitolo stabilisce un collegamento tra i campi di tensione locali, in prossimità del piede e della radice dei cordoni di saldatura, e le componenti di tensione nominale valutate ad una adeguata distanza dalla saldatura stessa. Nel capitolo viene fornita una relazione analitica tra tale distanza e lo spessore della piastra caricata; inoltre, viene presentato un criterio per stimare il valore del SED, sia al piede che alla radice dei cordoni di saldatura, e, a posteriori, i relativi NSIFs, come esplicita funzione delle componenti di tensione nominale (sollecitazione membranale, flessionale e tagliante). Tale metodo si presta all'automatizzazione e, quindi, a condurre l'enorme quantità di verifiche a fatica richieste per una complessa struttura saldata in acciaio. Tuttavia, l'attuale mancanza di conformità normativa degli approcci locali SED e NSIF rappresenta un possibile ostacolo nelle applicazioni industriali. Per questo motivo nel sesto capitolo viene proposto un metodo, conforme alle normative vigenti e alla letteratura scientifica, per modificare il classico approccio nominale, che tuttora è il metodo di riferimento ampiamente accettato e riconosciuto. Il problema metodologico della definizione di tensione nominale in un modello agli elementi finiti viene superato attraverso un approccio originale che tiene conto sia degli effetti membranali che di quelli di flessionali. Viene presentata una validazione sperimentale e vengono discusse le implicazioni nella progettazione a fatica delle grandi strutture in acciaio. I capitoli settimo e ottavo presentano un post-processore ad elementi finiti, sviluppato per automatizzare l'analisi della resistenza a fatica di una struttura di grandi dimensioni. Il post-processore è basato sul solutore ad elementi finiti Straus7® ed è compatibile con modelli di tipo shell per essere adatto ai grandi assiemi strutturali. Molte delle proposte illustrate nella presente tesi sono state quindi automatizzate: gli approcci locali SED e NSIF, il metodo della tensione nominale modificato e, infine, gli approcci classici basati sulla tensione nominale e di hot spot. È inoltre mostrato un ottimo accordo tra le analisi condotte "manualmente", sia attraverso approcci globali che locali, e quelle eseguite rapidamente utilizzando il post-processore, riscontrando generalmente un ottimo accordo tra la vita a fatica stimata mediante l'approccio locale SED e quella stimata attraverso il metodo della tensione nominale modificata. Il nono capitolo tratta la stima rapida dei fattori di intensificazione delle tensioni residue (R-NSIFs), conseguenti al processo di saldatura, utilizzando il PSM e il software agli elementi finiti dedicato Sysweld®. Innanzitutto, viene presentata la calibrazione del PSM in ambiente Sysweld®; quindi vengono illustrate alcune applicazioni pratiche. Infine, il decimo capitolo riporta alcune osservazioni conclusive di carattere generale e la discussione dei principali risultati ottenuti
ON THE USE OF THE AVERAGE STRAIN ENERGY DENSITY FOR THE CALCULATION OF RESIDUAL NOTCH STRESS INTENSITY FACTORS
No full textThe present work deals with the numerical assessment of the residual stress fields induced by thermal loads in the proximity of sharp V-notches, as weld-like geometries. According to the notch stress intensity factor (NSIF) approach, the weld bead is modeled as a sharp (zero radius) V-notch. Residual NSIFs (R-NSIFs) are suitable to quantify the intensity of the as- ymptotic stress distributions, close to the weld toe, arising from the solidification of the fused zone during an arc welding process. The direct R-NSIFs determination from the local stress distributions requires meshes very refined in these regions. Here, it is shown how this limitation can be overcome if the R-NSIFs are calculated by means of the strain energy den- sity (SED)
Strain Energy Density Based Assessment of Cracked Terfenol-D Specimens Under Magnetic Field and Different Loading Rates
No full textThe purpose of this work is the characterization of the fracture behaviour of giant magnetostrictive materials subjected to a magnetic field. Both experimental and numerical investigations have been performed, focusing on iron and rare earth alloys, such as the commercially named Terfenol-D. Tests have been carried out on single-edge precracked specimens subjected to three-point bending in the presence and absence of a magnetic field and fracture loads have been measured at different loading rates. Recent studies on local stress fields in proximity of crack and notch tips have shown that Strain Energy Density (SED), averaged in a circular control volume which includes a crack tip, could be a robust parameter in the assessment of brittle fracture resistance of several materials. Coupled-field analyses have then been performed on both plane stress and plane strain finite element models and the effect of the magnetic field on fracture resistance of Terfenol-D alloy was predicted in terms of averaged SED. A relationship between the SED’s control volume size and the loading rate has also been proposed
Fracture Behavior of Cracked Giant Magnetostrictive Materials in Three-Point Bending under Magnetic Fields: Strain Energy Density Criterion
No full textIn this paper, the fracture behavior of cracked giant magnetostrictive materials has been investigated both numerically and experimentally. Works in literature have been revisited, focusing on iron and rare-earth alloys, such as Terfenol-D. Three-point bending tests have been carried out on precracked specimens and the fracture loads have been measured in the presence and absence of a magnetic field. Recent studies on local stress fields in proximity of crack tips have shown that the Strain Energy Density (SED) can be a robust parameter in the brittle fracture assessment. Coupled-field finite element analyses have then been performed and the effect of the magnetic field on Terfenol-D fracture resistance has been discussed in terms of Energy Release Rate and averaged SED
Fatigue assessment of welded joints in large steel structures: a modified nominal stress definition
No full textAccording to the notch stress intensity factor (N-SIF) approach to the fatigue assessment of welded joints, the weld toe and the weld root are modeled as sharp V-notches. The N-SIFs quantify the intensity of the elastic stress field near the points of singularity and capture not only the weld shape effect, but also the size and the loading condition (membrane and bending) effects. The computational effort required to evaluate the N-SIFs, due to the necessity of very refined meshes, is a strong limitation in megastructures application. This limit can be overcome estimating notch stress intensities through the strain energy density (SED). Here, a SED based modified nominal stress is presented and how to compute it by means of a finite element analysis is shown. The effect of loading condition (membrane and bending) is addressed by introducing a proper coefficient obtained linking local concepts to the classical nominal stress components. Then, the implications in fatigue design of large steel structures are discussed
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
Variations on the Author
Full text link“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
Full text linkWe 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
Full text linkWe 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
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