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Thermal methods for fatigue characterization of components
Full text linkIn recent years, the thermographic technique has been regarded as a key tool to investigate the fatigue behavior of materials instead of time and cost consuming traditional methods. One way to assess the fatigue as an irreversible process is studying the temperature signature due to the heat dissipation during the cyclic loading. Numerous studies have suggested that the shift from anelastic to inelastic strains results in a significant level of intrinsic dissipation, indicating the presence of the fatigue damage.
The primary objective of this study is to explore heat dissipation in fatigue tests through a combination of experimental methods and numerical models by utilizing the fundamental temperature component related to dissipation, the second amplitude harmonic (SAH) of the temperature. The proposed hybrid approach integrates experiments with a numerical model to pinpoint the specific volume generating heat during the fatigue test. The investigation also delves into the impact of loading frequency on the fundamental temperature component associated with dissipation.
In experimental part of the study, a comprehensive campaign was carried out to capture the material's response under both static and cyclic loading conditions. The experimental data are employed for comparing the accuracy of different thermal indices in fatigue limit estimation and establishing the numerical model, act as reference data for comparison.
In the numerical part of the study, models are established in COMSOL Multiphysics 5.6 to simulate the temperature distribution on the surface of the specimen. After the verification with analytical solution of 1-D heat equation, a model is proposed for heat source identification. One of the main controversial issues in fatigue characterization is addressed in this study, whether the entire gauge volume dissipates or not. Additionally, the effect of loading frequency on SAH of temperature reveals its limitation in thermographic measurement and fatigue assessment
Application Of Thermal Methods Based On Infrared Thermography For The Mechanical Characterisation Of Materials
Full text linkLa termografia è una tecnica a campo intero, senza contatto e non distruttiva che fornisce la mappa della temperatura da un corpo a temperatura superiore a 0 K, rilevandone la radiazione infrarossa. La caratteristica più importante che rende la termografia un'utile tecnica non distruttiva è la possibilità di ispezionare qualsiasi tipo di corpo senza alcuna interazione con esso, consentendo rilievi ad alta temperatura o ispezioni in ambienti con condizioni critiche senza contaminazioni di materiali e/o sensori.
A partire dalla prime osservazione ad opera di Sir William Herschel, nel 1800, la tecnica ha subito una notevole quantità di miglioramenti, infatti i recenti sensori IR beneficiano di un'elevata precisione (la risoluzione è all'incirca dell'ordine del centigrado) e di un'ampia gamma di temperature operative (da -20 Da ° C a 1000 ° C). Tuttavia, anche se le tecniche basate sulla termografia sono ampiamente diffuse in campo accademico, negli ultimi 20 anni si è osservato un ritardo significativo a causa dei maggiori costi delle apparecchiature a infrarossi e soprattutto a causa dell'assenza di standard che disciplinano tale tecnica. In questo senso, le difficoltà derivano dalla difficile collocazione della termografia in uno specifico campo di applicazione a causa della sua grande versatilità.
In medicina, il grande interesse della comunità scientifica per la prevenzione del cancro al seno e delle malattie neurologiche comporta la ricerca di metodi più accurati per la diagnosi precoce.
In questo senso, la termografia è utile per misurare le temperature superficiali del corpo in relazione al calore prodotto dall'organismo che passa attraverso la pelle e i tessuti. Negli ultimi anni, i ricercatori hanno sviluppato algoritmi accurati per l'analisi di immagini bidimensionali per rilevare la presenza di tumori, nonché per valutare i cambiamenti nella circolazione sanguigna al fine di localizzare la presenza di qualsiasi lesione ischemica.
D'altra parte, nel contesto industriale, le parole chiave che portano il ricercatore a trovare nuove tecniche introdotte nel processo di produzione sono il Time-to-Market e la Qualità.
Oggigiorno gli imprenditori sono interessati a ridurre il tempo tra il prodotto è fabbricato e venduto, in quanto determina i loro profitti. La riduzione dei tempi di commercializzazione è accompagnata dall'aumento della qualità dei prodotti. La continua richiesta della qualità nei prodotti industriali consente di integrare il controllo di qualità nel processo di produzione. Per fare questo, è necessaria una tecnica che fornisca test in-situ, non distruttivi, in linea durante del processo o a valle di esso, con prestazioni e elaborazione dati veloci.
Nell’ambito industriale, si trae vantaggio dalla possibilità di utilizzare una tecnica che non richiede tempi di fermo della produzione, che consente di eseguire manutenzione preventiva nonché azioni rapide e mirate sulle macchine e infine di ridurre il rischio di incidenti e catastrofi. Ad esempio, la termografia per quanto riguarda la tecnica degli ultrasuoni non ha bisogno di un accoppiamento acustico, e non è importante se i rilevatori sono posizionati lontano o vicino al campione da indagare, poiché il contatto non è richiesto.
Un altro aspetto interessante legato all'uso di tale tecnica è la possibilità di eseguire anche la caratterizzazione meccanica di materiali e componenti. Infatti, la termografia può essere adottata per valutare il limite di fatica dei campioni da laboratorio e componenti sottoposti a carico dinamico. Il metodo di prova standard per valutare le proprietà di fatica dei materiali richiede una procedura di prova estremamente duratura, sia per il numero di provini da testare sia per i tempi operativi delle macchine. Monitorando con una termocamera le prove a carico ciclico, ed seguendo una specifica procedura di test è possibile studiare la fatica e il danno del materiale, qualunque sia il materiale in un’ottica di riduzione del time-to-market del prodotto sia esso materiale o componente e di risparmio economico.
In questa tesi, verrà mostrata la capacità della Termografia alla caratterizzazione della fatica dei materiali (acciai inossidabili, compositi). In particolare, la prima parte (capitoli 1 e 2) si concentrerà sul fornire i concetti di base sulla fisica dei fenomeni di scambio termico e sulla comprensione della teoria dell'approccio TSA (Thermoelastic Stress Analysis). Quest'ultima, come verrà mostrato, è empirica e fornisce due metodi (fase termoelastica e ampiezza del segnale termoelastico) per studiare il comportamento del materiale sottoposto a carico ciclico. Nel capitolo 2, insieme all'approccio TSA, saranno presentati approcci energetici in grado di fornire maggiori informazioni sulle fonti di calore dissipative che producono danneggiamenti nei materiali. Questi approcci sono focalizzati sulla valutazione dei fenomeni irreversibili dai materiali misurando le variazioni di temperatura o valutando l'energia dissipata durante ciclo di isteresi.
Nel capitolo 3, il tentativo di unificare gli approcci sarà fornito presentando il modello adottato di variazioni di temperatura. Tali variazioni di temperatura saranno rappresentate secondo un modello che consente di separare tutte le componenti del segnale termico (reversibili ed irreversibili). In letteratura manca di una teoria unificata che valuti e utilizzi nello stesso tempo sia fonti di calore irreversibili termoelastiche reversibili sia dissipative. Inoltre, verrà fornita una spiegazione fisica per ogni componente del segnale termico al fine di migliorare la comprensione dei fenomeni che producono variazioni energetiche durante sollecitazioni cicliche. In effetti, la comprensione dei processi di fatica (fenomeni dissipativi viscosi e plastici) rappresenta la base per spiegare le variazioni di temperatura.
Il vantaggio di tale approccio è quello di ottenere con un singolo test / acquisizione / analisi termografica informazioni complementari sul comportamento del materiale in esame.
Dopo i capitoli iniziali sulla teoria e la fisica degli scambi di calore, verrà mostrato l'approccio 'energetico' adottato e verranno presentati accuratamente i metodi e gli algoritmi. Tale parte comprende tutti i dettagli delle diverse campagne sperimentali.
I risultati interesseranno tutti gli aspetti della fatica: dalla nucleazione della cricca alla meccanica della frattura.
Nel capitolo 4, tutti gli aspetti della fatica dei campioni non intagliati saranno discussi con particolare riferimento alla correlazione tra variazioni di temperatura e comportamento viscoso e plastico del materiale. Verrà discusso in dettaglio l’effetto del rapporto di sollecitazione o della microstruttura sulle variazioni di segnale termico.
Verrà presentata una procedura per la valutazione del limite di fatica che potrebbe fungere da base di sviluppo per una nuova generazione di prove di fatica mediante metodi termici.
Il capitolo 5 è incentrato sulla valutazione del comportamento a fatica dei materiali compositi Uno dei risultati più importanti si riferisce alla possibilità di separare il comportamento della fibra e della matrice al fine di studiare i fenomeni di danno on line in situ.
Nel capitolo 6 si troverà l'applicazione dell'approccio presentato alla meccanica della frattura. In questo caso, quando la cricca si è appena sviluppata, è interessante osservare che la termografia offrire supporto nel rilievo della crack driving force e dei fenomeni di crack opening. Inoltre, il modello di rappresentazione delle variazioni di temperature consente anche di ottenere dei parametri importanti per determinare la posizione della cricca e l'area di plastica.
L'intera dissertazione di questa tesi, è indirizzata a raccogliere tutte le teorie sullo studio della fatica utilizzando metodi termici, al fine di ottenere più informazioni sul danneggiamento da un singolo test.
Infatti, unificando la teoria relativa alle differenti componenti del segnale termico, che in letteratura sono trattati separatamente, è possibile:
• ridurre le tempistiche e i costi della campagna prove
• ottenere una maggiore comprensione del processo di danneggiamento, e dei legami tra l’energia dissipata e le variazioni di temperatura
• consentire il rilievo tempestivo ed il monitoraggio online / in-situ dei processi di danneggiamento mediante un’analisi rapida e non eccessivamente laboriosa.
Dal punto di vista del mercato, i metodi e le tecniche presentate, non solo ridurranno il time-to-market dei prodotti, ma contribuiranno ad aumentare la del processo di produzione.Thermography is a full-field, non-contact, non-destructive technique which provides the temperature map from a body at temperature higher than 0K, by detecting its infrared radiation. The most important characteristic making thermography a useful Non Destructive technique is the possibility to inspect any kind of body without any contact with it; this allows high temperature measurements or critical environment inspections without any material/sensor contaminations.
Since the age of its discovery by Sir William Herschel, 1800, the technique undergone a great deal of improvements, in fact recent IR detectors benefit of high accuracy (resolution is roughly in the order of centigrade) and wide operating temperature range (from -20°C to 1000 °C). However, even if the techniques based on thermography are widely diffused in academic field, in the last 20 years a significant delay is observed due to the higher costs of IR equipment and mostly due to the absence of Standards which discipline such the technique. In this sense, difficulties arises from the difficult collocation of thermography in a specific field of application due to its great versatility.
In fact, the feasibility of the Thermography extends from medical to many industrial applications.
In the field of Medicine, the great interest of scientific community for the prevention of breast cancer and neurological diseases involves the research of more accurate methods for early diagnosis.
In this sense, thermography is useful for measuring the surface temperatures from the body as related to the heat produced by the organism passing through the skin and tissues. In recent years, medical researchers developed accurate algorithms for 2-dimension imaging analysis to detect the presence of tumours, as well to evaluate changes in blood circulation in order to localise the presence of any ischemic lesion.
On the other hand, in the industrial framework, the keywords which lead the researcher to find new techniques being introduced in the manufacturing process were Time-to-Market and Quality.
Nowadays entrepreneurs are interested in reducing the time between the product is manufactured and sold, as it determines their profits. The reduction of the time to market is coupled with increasing the quality of products. The continuous demand of the quality in the industrial products, allows integrating the quality control in the manufacturing process. To do this, a technique providing in-situ, non-destructive tests during or at the end of the process with fast performing and data processing, is required.
In particular, by introducing Thermography in the manufacturing process, Industry takes advantage from the possibility of using a technique which does not require any production downtime, allows predicting maintenance as well as fast and focused actions on machines and finally to reduce the risk of accident and catastrophes. For example, Thermography with respect Ultrasound technique does not need of acoustic coupling, and it is not important if the detectors are located far away or near the sample being investigate, since the contact is not required.
Another interesting aspect related to the use of Infrared radiation by materials is the possibility to set up a technique that, somehow allows mechanical characterisation, as well. In fact, a part from the process controlling and non-destructive tests for assessing the quality of the products, thermography can be adopted for evaluating the fatigue limit, the yield strength of samples undergoing dynamic loading. The standard test method to evaluate fatigue properties of materials requires extremely long lasting test procedure, great number of material testes, and wide number of operating time of machines while by using an IR detector, by following a specific test procedure it is possible to study fatigue and the damage of the material, whatever the material is.
This novel way to perform fatigue tests is consistent with the spirit of ensuring a little time to market of products together with an economic saving.
In this thesis, it will be showed the capabilities of the Thermography to the fatigue characterisation of materials (stainless steels, composites). In particular, the first part (chapter 1 and 2) will be focused on providing the basic concepts on the physics of heat exchange phenomena as well as an understanding on the theory of Thermoelastic Stress Analysis (TSA) approach. This latter, as will be showed, is empirical and provides two methods (temperature variation of first order and thermoelastic phase shift) for studying the behaviour of material undergoing dynamic loading. In chapter 2, together with TSA approach, energetic approaches will be presented capable to provide more information on dissipative heat sources producing damage in the materials. These approaches are focused on the assessment of irreversible phenomena from materials by measuring temperature variations or by assessing the energy dissipated during the hysteresis loop.
In chapter 3, the attempt to unify the approaches is provided by presenting the adopted model of temperature variations. The model presents the measured temperature evolution, as separeted in its components of first and second order and relatively phase shifts.
In effect, literature lacks of a unified theory that assesses and uses in the same time both thermoelastic reversible and dissipative irreversible heat sources.
Due to the adopted model, a single temperature variation can be associated with relative physically explanation, with the advantage that by a single test/thermographic acquisition/analysis it is possible to assess complementary information on behaviour of the material being tested.
An appropriate study of energy dissipation determining the macroscopic physical phenomena related to the fatigue loading, has been setup before building all the models. In fact, the understanding of the fatigue processes (dissipative viscous and plastic phenomena) represents the basis for explaining the temperature variations.
After initial chapters on the theory and physics of heat exchanges, the adopted ‘energetic’ approach will be shown and the methods and algorithms will be accurately presented. Such the part incorporates all the details of experimental campaigns.
Results, will interest both the aspect of Fatigue: fatigue of smooth samples and fracture mechanics.
In chapter 4, all the aspect of fatigue of smoothed samples will be discussed in sight of viscous and plastic behaviour of material. It will showed the effect of loading ratio or the microstructure on the temperature variation by the sample undergoing cyclic loadings.
The result will focus on different types of materials: stainless steels and composites.
The procedure to estimate the fatigue limit will be presented together with the damage phenomena localisation by observing the parameter provided by the adopted model. This feature is very important in case of random loading of operating components for detecting the onset of damage.
Moreover, the procedure setup for evaluating the fatigue limit involves non-destructive fatigue tests, as it is possible to stop the test when a threshold value of temperature variation has been achieved. This could be used for a new generation of fatigue testing by means of thermal methods.
Chapter 5 will be focused on the assessment of fatigue behaviour of composites materials by using all the parameters presented for metals analysis. One of the most important achievements refers to the possibility of separating the behaviour of fiber and matrix in order to study damage phenomena o nline in situ.
Chapther 6, will present the application of presented approach on fracture mechanics. In this case, when the crack is just developed it is interesting to observe that the thermography can support in assessing otherwise the Paris’s law. So that, for real operating components, it is possible to make prediction live on the crack growth and prevent catastrophic failures.
Moreover, it will shown that by using the presented model, it becomes simple to determine the crack tip position and the plastic area, and distinguish also between static plastic area and the area of crack closure effect.
Apart from to present the strong impact of Thermography-based approaches in the field of mechanical characterisation, the whole dissertations of this Thesis, is addressed to gather all the theories on the study of fatigue by using thermal methods, in order to make more information on damage by a single test.
The results that will be presented, demonstrate the multistage characteristic of the adopted approach, from the data processing to the data analysis.
The purpose of presenting such the temperature variation model and the related analysis, is to unify all the components of temperature variations, which have been treated separately, in literature, in order to achieve:
• Experimental campaign testing time and costs reductions
• More understanding on specific damage process by assessing the temperature signals
• Detection and monitoring online/in-situ of damage processes by means of relatively simple setup.
On the point of view of the market, the presented methods and techniques, will not only reduce time-to-market of products, but also include and increase the quality in the manufacturing process
A quantitative investigation on the thermoelastic effect of CFRP laminates
No full texthe thermoelastic effect on CFRP laminates with various lay-ups is investigated. A thick low crimp unidirectional fabric reinforcement is adopted. The measured thermoelastic signal is compared with predictions from two analytical models based on the meso-mechanical bulk properties of the lamina and on assuming a strain witness behaviour of the surface resin rich layer
A Hydrogel Based on a polyaspartamide: Characterization and Evaluation of In-vivo Biocompatibility and Drug release in the Rat.
No full textAbstract
This paper deals with the characterization of a new microparticulate hydrogel obtained by gamma irradiation of α,β-poly[N-(2-hydroxyethyl)-dl-aspartamide] (PHEA).
When enzymatic digestion of PHEA hydrogel was evaluated using various concentrations of pepsin and α-chymotrypsin no degradation occurred within 24 h. In-vivo studies showed that this new material is biocompatible after oral administration to rats. PHEA hydrogel was also studied as a system for delivery of diflunisal, an anti-inflammatory drug. In-vitro release studies in simulated gastrointestinal juice (pH 1 or 6.8) showed that most of the drug was released at pH 6.8. In-vivo studies indicated that diflunisal-loaded PHEA microparticles significantly improved the gastric tolerance and oral bioavailability of the drug in comparison with free diflunisal.
These results suggest the potential application of PHEA hydrogel as a new delivery system for the oral administration of anti-inflammatory drugs.</jats:p
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
PRODUCTION VIA MICROFLUIDICS OF FIBRILLAR AND NANOMETRIC BIOMATERIALS FOR REGENERATIVE MEDICINE AND DRUG DELIVERY POURPOSES
No full textMicrofluidic technique offers mild operating conditions suitable for fabrication of fibers and
cell encapsulation and can be applied for controlled production of nanometric biomaterials1
.
Ionotropic polysaccharides are routinely employed for microfluidics. To be processed, nonionotropic polysaccharides, such as Hyaluronic Acid (HA), should be chemically crosslinked during
fabrication2
. In order to avoid chemical crosslinking, we designed an ionic strenght sensible HA
derivative suitable for microfluidics2
. Here we describe the use of such HA derivative for
fabrication of fibrillar and nanometric biomaterials for regenerative medicine and drug delivery
applications. For muscle tissue regeneration purposes, 3D microfibrillar scaffolds were produced
and characterized as guidance for Aortic Smooth Muscle cells (AoSMCs). Nanoparticles were
produced and characterized as drug delivery systhems for treatment of retinal angiogenic
pathologies
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
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