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New Generation of Electrochemical Sensors for Nitric Oxide: Ruthenium/Carbon-Based Nanostructures and Colloids as Electrocatalytic PlatformsPeiris, W. Pubudu M. January 2009 (has links)
No description available.
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Engineered carbon-based scaffolds for hard and soft tissue repair, reconstruction or regenerationCzarnecki, Jarema S. January 2013 (has links)
No description available.
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The Effects of Multi-Axial Loading on Adhesive JointsMcFall, Bruce Daniel 01 June 2018 (has links)
No description available.
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Role of sizing agent on the microstructure morphology and mechanical properties of mineral-impregnated carbon-fiber (MCF) reinforcement made with geopolymersZhao, Jitong, Liebscher, Marco, Tzounis, Lazaros, Mechtcherine, Viktor 24 August 2023 (has links)
This report deals with the influence of different sizing agents on carbon fiber (CF) heavy tows towards their impregnation with a geopolymer (GP) suspension and resulting properties of mineral-impregnated carbon-fiber composites (MCF) produced in an automated, continuous process. Three different commercial CFs were investigated after treatment with either thermoplastic, epoxy, or vinyl-ester sizing agents and then compared to unsized CFs. During impregnating, diverse yarn-spreading behavioral modes and degrees of wetness were observed, indicating different degrees of impregnation, both in quality and quantity. All sizing agents decreased the hydrophobic nature of unsized CFs significantly. Supported by microscopic investigation, water contact angle measurement, and thermal gravimetric and mercury intrusion porosity, the epoxy sizing showed the best fiber-matrix distribution over the cross-section and a dense microstructure. Moreover, in single-fiber pullout tests each sizing brought about a significant increase in maximum pullout force, indicating enhanced bond between fiber and matrix. This is attributed to more intense interaction in the interfacial region, fully corroborating the topological characteristics obtained in microscopic analysis. With higher interfacial bond strength, the thermoplastic sizing enabled the highest tensile properties of MCF despite slightly less impregnation quality in comparison with epoxy-sized samples.:Abstract
Keywords
1. Introduction
2. Materials and methods
2.1. Raw materials
2.2. Preparation of the mineral-impregnated carbon fiber composites (MCF)
2.3. Single-fiber pullout tests
2.4. Characterization of fresh state properties of MCF
2.5. Mechanical testing of MCF
2.6. Analytical and morphological characterization
3. Results and discussion
3.1. Characterization of CF yarns
3.2. Characterization of yarn processing
3.3. Morphological and analytical characterization of MCF
3.4. Bending behavior of MCF
3.5. Pullout behavior of carbon fibers
3.6. Uniaxial tensile behavior of MCF
3.7. Microscopic analysis of the fracture surfaces of MCF
4. Summary and conclusions
CRediT authorship contribution statement
Declaration of Competing Interest
Acknowledgment
References
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Atomic Layer Deposition onto Fibers / Atomlagenabscheidung auf FasernRoy, Amit Kumar 19 March 2012 (has links) (PDF)
The main goal of this dissertation was to show that the principle of atomic layer deposition (ALD) can be applied to “endless” fibers. A reactor of atomic layer deposition has been designed, especially for coating depositions onto meter long bundles of fibers. Aluminum oxide (alumina), titanium oxide (titania), double layers of alumina and titania, as well as aluminium phosphate have been deposited onto bundles of carbon fibers using the home-built reactor. Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) images indicate that the coatings were uniform and conformal onto fiber surface. There was a good adhesion of the coatings to the fibers.
Alumina has been deposited using two separate aluminum sources (aluminum trichloride and trimethylaluminum), and water as a source of oxygen. In case of alumina deposition using aluminum trichloride and water, initial deposition temperature was 500 °C. In these conditions, a part of the fiber bundle has been damaged. Thus, the deposition temperature was decreased to 300 °C and the fibers were unaffected. In addition, during this process hydrochloric acid is formed as a byproduct which is a corrosive substance and affects the reactor and there was a chloride impurity in the coatings. Thus, aluminum trichloride precursor was replaced by trimethylalumium.
Alumina deposition onto carbon fibers using trimethylaluminum and water was carried out at a temperature of 77 °C. SEM images revealed that the fibers were unaffected and the coatings were uniform and conformal. Oxidation resistance of the carbon fibers was improved slightly after alumina deposition. Oxidation onset temperature of the uncoated fibers was about 630 °C. The resistance was linearly increased with the coating thickness (up to 660 °C) and getting saturated over a thickness of 120 nm. Titania coatings have been deposited using titanium tetrachloride and water. The physical appearances of the titania coatings were similar to the alumina coatings. The oxidation onset temperature of the titania coated carbon fibers was similar to the uncoated fibers but the rate of oxidation was decreased than the uncoated fibers. Two double layer coatings were deposited, alumina followed by titania (alumina/titania), and titania followed by alumina (titania/alumina). If the fibers were coated with the double layer of alumina/titania, they had almost same oxidation onset as alumina coated fibers but the rate of oxidation was decreased significantly compared to alumina coated fibers. This feature is independent of the thickness of the titania layers, at least in the regime investigated (50 nm alumina followed by 13 nm and 40 nm titania). On the other hand, the oxidation onset temperature of fibers coated with titania/alumina (20 nm titania /30 nm alumina) was approximately 750 °C. The fibers were burned completely when temperature was further increased to 900 °C and held another 60 minutes at 900 °C. This is significantly better than any other coating used in this dissertation.
ALD of titania and alumina in principle was known beforehand, this dissertation here applies this knowledge for the first time to endless fibers. Furthermore, this dissertation shows for the first time that one can deposit aluminum phosphate via ALD (planar surface as well as fibers). Aluminum phosphate might be special interest in the fiber coating because it is a rather soft material and thus might be used to obtain a weak coupling between fiber and matrix in composites. Aluminum phosphate was deposited using trimethylaluminum and triethylphosphate as precursors. Energy dispersive X-ray spectroscopy and solid state nuclear magnetic resonance spectra confirmed that the coating comprises aluminum phosphate (orthophosphate as well as other stoichiometries). Scanning electron microscopic images revealed that coatings are uniform and conformal. In cases of alumina and titania, it was observed that the coatings were delaminated from the ends of cut fibers and thus formed of clear steps. On the other hand, for aluminum phosphate coating it was observed that the border between coating and underlying fiber often being smeared out and thus formed an irregular line. It seems in case aluminum phosphate cohesion is weaker than adhesion, thus it might be act a weak interface between fiber and matrix. Alumina, titania, and double layer microtubes have been obtained after selective removal of the underlying carbon fibers. The carbon fibers were selectively removed via thermal oxidation in air at temperatures exceeding 550 °C. SEM and TEM images indicate that the inner side of the tube wall has the same morphology like the fibers. In addition, it was observed that the individual microtubes were separated from their neighbors and they had almost uniform wall thicknesses. The longest tubes had a length of 30 cm. / Das Hauptziel dieser Dissertation bestand darin nachzuweisen, dass die Atomlagenabscheidung (engl. atomic layer deposition (ALD)) auf „endlose“ Fasern angewendet werden kann. Es wurde ein Reaktor zur Atomlagenabscheidung gestaltet, der speziell für die Beschichtung meterlanger Faserbündel geeignet ist. Aluminiumoxid, Titanoxid, Doppelschichten aus Aluminiumoxid und Titanoxid sowie Aluminiumphosphat wurden mit Hilfe des selbstgebauten Reaktors auf Kohlefaserbündel abgeschieden. Rasterelektronenmikroskopische (REM) und transmissionselektronenmikroskopische (TEM) Aufnahmen zeigten, dass die Beschichtung auf den Fasern einheitlich und oberflächentreu war. Des Weiteren wurde eine gute Adhäsion zwischen Beschichtung und Fasern beobachtet. Das Prinzip der Beschichtung mit Titanoxid und Aluminiumoxid mit Hilfe der ALD war bereits vorher bekannt und im Rahmen dieser Dissertation jedoch erstmals auf "endlose" Fasern angewendet. Des Weiteren wird in dieser Dissertation erstmals gezeigt, dass es möglich ist, Aluminiumphosphat mittels ALD abzuscheiden (sowohl auf planaren Oberflächen als auch auf Fasern). Aluminiumphosphat könnte von besonderem Interesse in der Faserbeschichtung sein, da es ein relativ weiches Material ist und könnte daher als eine Art „schwacher“ Verbindung zwischen Faser und Matrix in Kompositen dienen. Die Oxidationsbeständigkeit von beschichten Kohlefasern wurde im Vergleich zu unbeschichteten Fasern bis zu einem gewissen Grad erhöht. Monoschichten von Aluminiumoxid und Titanoxid waren dafür wenig effektiv. Aluminiumphosphatbeschichtete Fasern waren deutlich besser geeignet als die beiden anderen. Eine Doppelschicht aus Titanoxid gefolgt von Aluminiumoxid verbesserte die Oxidationsbeständigkeit nochmals deutlich gegenüber allen anderen Beschichtungen, die in dieser Dissertation verwendet wurden. Mikroröhren aus Aluminiumoxid, Titanoxid und Doppelschichten wurden durch die selektive Entfernung der zugrunde liegenden Kohlefasern erhalten. Einzelne Mikroröhren waren von benachbarten Röhren getrennt und sie weisen eine nahezu einheitliche Wanddicke auf.
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Ανάλυση της απόκρισης σύνθετων πολυμερών υλικών υπό συνθήκες φωτιάς. Εφαρμογή σε αεροπορικές κατασκευές / Fire response of composite aerostructuresΣικουτρής, Δημήτριος 01 February 2013 (has links)
The current dissertation, titled “Fire Response of Composite aerostructures” deals with a crucial subject of the aeronautics industry that is the fire response of composite aerostructures, more specifically the issue of interest in this work is the fuselage fire burnthrough from an external liquid jet-fuel pool fire. Other fire issues that “bother” the aeronautics industry are the fire spread inside the cabin, smoke generation and toxicity of the fumes, but these are not handled in the current dissertation.
Aircraft structures are designed to withstand various loading scenarios during their operational life. These loading scenarios are associated to a great extent with normal aircraft operation (flight manoeuvres, take-off and landing). However there are situations where the aircraft structures are required to assure the safety of the passengers and crew. In the case of an emergency crash landing, the threat of an external jet-fuel fire always exists. Considering that the aircraft structure survives the impact, the survivability of the passengers and crew onboard the aircraft depends solely on the fire resistance of the aircraft structure. A measure of the fire resistance of an aircraft structure is the time needed for the flames to penetrate the fuselage and spread inside the cabin, the so-called, burn-through time.
So far, the aircraft fire resistance has been extensively studied by conducting lab, medium and full scale tests. The early lab scale tests were performed by the Federal Aviation Administration (FAA) and involved the Bunsen-burner flammability test of coupons for developing fire safe interior materials. As the application of polymer materials on aircrafts kept increasing, the problem of fire burn-through due to external fire emerged. Marker was one of the first to perform full-scale fuselage burn-through tests to access the insulating performance of materials. Also a statistical analysis was performed by Cherry and Warren that accessed and analyzed data from past accidents and their work resulted in proving the importance of fuselage fire hardening and the passengers’ lives that could be saved using low-cost solutions. These works led the FAA to proposed new fire testing procedures for aircraft materials.
The scope of this dissertation was to assess the performance of various structural materials in a pool-fire scenario. A simplified approach is made, approximating the pool-fire conditions with a flat panel burn-through test in accordance to the ISO2685:1998(E) Standard.
The originality of the present work comes from the fact that it incorporates a multistage approach in order to investigate the behaviour and response of composite aircraft structures in the possibility of a fire event. The current approach goes down on material level in order to investigate and model the deterioration (decomposition) of the polymer composite. Thus, it investigates and proposes a methodology of how the thermophysical properties of the composite are deteriorated due to the fire event. It proceeds into developing a progressive-damage material model (material properties varying with the deterioration degree) and finally implementing this custom material model into a commercial FE package and solving the loading scenarios.
Being more specific the current work begins with a quick review of the literature where incidents and work done on the burnthrough event for the past 20-30 years are summarized. It progresses then to presenting the various types of polymers used in the aircraft industry and their basic decomposition mechanisms, from the unsaturated polyesters to the epoxies and phenolics and in the end reference to the thermoplastics is made. Every organic material, hence, polymers used in aerospace applications, present a set of response characteristics when subjected to fire, specifically the heat release rate, thermal stability index, limiting oxygen index, flammability index, time-to-ignition, surface flame spread, mass loss, smoke density and smoke toxicity.
Following is the backbone of this dissertation, the kinetics modelling. Two approaches are made, one simplified using single stage kinetics where the decomposition degree a is calculated based on the Arrhenius reaction theory and using the kinetic triplets (kinetic parameters) extracted from thermogravimetry, TGA, data using the Friedman multi-curve method. The second approach is more complicated and considers multi-stage decomposition of the polymer composite. Specifically a 3-stage reaction network is considered for every material, the LY-Ref, and the two modified batches, one with ammonium polyphosphate AP423 and the other both with AP423 and multi-wall carbon nanotubes MWCNT. Again the kinetic parameters, activation energy EA, frequency factor A, and reaction order n, are extracted for every step using the van Krevelen methodology. In the end using the reaction rates equations the reconstruction of the TGA curves is achieved with an error of less than 5% from the test data. Correlations that consider the material deterioration and affect the thermophysical properties of the materials are proposed. Those expressions are being developed for both of the two kinetic approaches, the single and multi stage.
Another crucial part of this work is the measurement and calibration of the applied fire load. Again two fire load approaches are used, one according to the ISO2685 Standard where a propane burner was manufactured and calibrated according to the Standard for medium scale samples testing and a lab scale butane burner for small samples. The ISO2685 burner was also CFD simulated and the models calibrated against analytical expressions, ISO requirements and real measurements. The CFD simulations were performed so the heat flux or heat transfer coefficient to be extracted and used as input for the later thermal FE burnthrough models. The heat flux distribution of the lab-scale AML burner on the specimen surface was measured via a water cooled Schmit-Boelter SBG01 heat flux sensor manufactured by Hukseflux.
Manufacturing and material details are presented concerning the samples used for every test campaign. Metallic (AL2024-T3) samples, CFRP neat and modified, and hybrid GLARE ones where manufactured. Also the experimental work performed is described. Cone calorimetry testing data are available, results from thermogravimetry tests, differential scanning calorimetry, and finally the burnthrough tests with both the testing apparatuses, the ISO2685 one and the AML lab-scale burner.
The modelling work in this dissertation involved thermal models that were developed into a commercial FE package. It was not part of this work to develop a thermal solver so a commercial one was selected and all the developed methodology was adapted to its requirements and specifications. The boundary conditions on the models are presented both for the ‘hot’ front surface and the rear ‘cooling’ one. For the ‘hot’ one the heat flux distribution is used and for the ‘cooling’ one an equivalent convection is applied that accounts for both convective and radiative cooling. The decomposing material model is implemented into to FE solver via user defined subroutines for the single stage kinetics and the multi-stage approach. Finally the simulations were run and the results and models were compared against the available experimental results.
Since so far the burnthrough response of aerostructures was limited to coupon, samples and medium size flat panels. A more realistic approach was performed by developing a mathematical model of a real size test. The certification tests conducted by the FAA are for full size fuselage sectors under the fire load of a burning jet-fuel pan pool-fire. A burning jet-fuel pool fire is a complex phenomenon on its own, combining it with a decomposing fuselage structure make the modeling approach even more difficult to simulate if not impossible. Required data for the pool-sizes under investigation were not available, so data for large external hydrocarbon pool fires from literature were used. Also, because the main characteristic of a jet-fuel (kerosene) pool fire is that the flames are not clear, on the contrary, great amount of shoot is produced making combustion modeling and radiative heat transfer to the fuselage even more of a challenge to model, it was decided to try and tackle this full-scale approach by a simplified the modeling approach. Instead of liquid fuel combustion, an equal hot air stream with mass flow, velocity and temperature properties extracted from literature correlation data was performed.
Conclusively, in terms of completeness the benefit analysis performed by Cherry and Warren is presented in brief. The objective of their analysis was to assess the potential benefits, in terms of reduction of fatalities and injuries, resulting from improvements in fuselage burnthrough resistance to ground pool fires. Fire hardening of fuselages will provide benefits in terms of enhanced occupant survival and may be found to be cost beneficial if low-cost solutions can be found. The maximum number of lives saved per year in worldwide transport aircraft accidents, over the period covered by the data, if hardening measures were applied, was assessed to be 12.5 for the aircraft in its actual configuration (when the accidents occurred) and 10.5 for the aircraft configured to later airworthiness requirements.
These figures are completely significant and give an extra confirmation that this work on investigating the fire response of composite aerostructures is on the right track. As the work of Cherry and Warren concluded, the fire hardening measures in order to be applicable need to be cost efficient. The concept under which this whole dissertation stepped on was to investigate the fire response of composite aerostructures and the possibility of hardening the structure itself without the use of extra protective layers that add cost and weight to the overall aircraft and its maintenance. In the end it was concluded that there is the possibility of hardening the fuselage structure by design and by material. Incorporating composites into the structure it is possible to prolong the burnthrough time at least for 4-5 minutes before auto ignition occurs on the inner side of the fuselage. Auto ignition of the inner side fuselage cabin materials is mentioned since in NONE of the burnthrough tests of the CFRP composites and the GLARE samples flame penetration was observed. / Στην παρούσα διατριβή με τίτλο «Ανάλυση της απόκρισης σύνθετων πολυμερών υλικών υπό συνθήκες φωτιάς. Εφαρμογή σε αεροπορικές κατασκευές» πραγματοποιείται εργασία στην αριθμητική προσομοίωση και πειραματική διερεύνηση της συμπεριφοράς αεροπορικών κατασκευών σε συνθήκες φωτιάς. Στην μέχρι τώρα βιβλιογραφία οι διάφοροι έλεγχοι για πιστοποίηση των αεροπορικών υλικών αλλά και των αεροσκαφών στο σύνολό τους αποτελούνταν από εκτενείς πειραματικές δοκιμές σε μεσαία κλίμακα καθώς και σε πλήρους κλίμακας κατασκευές. Οι προδιαγραφές των ελέγχων ορίζονται από την Ομοσπονδιακή Διεύθυνση Αεροπλοΐας των Ηνωμένων Πολιτειών της Αμερικής, Federal Aviation Administration FAA. Όπως γίνεται αντιληπτό πλήρους κλίμακας δοκιμές είναι χρονοβόρες αλλά και οικονομικά ασύμφορες, για τον λόγο αυτό τα τελευταία χρόνια πραγματοποιούνται προσπάθειες από την FAA για καθιέρωση Προτύπων ελέγχου μικρής κλίμακας τα οποία σε συνδυασμό με αριθμητικά μοντέλα θα είναι σε θέση να προβλέπουν την συμπεριφορά των αεροπορικών κατασκευών σε συνθήκες φωτιάς από την φάση του σχεδιασμού τους. Θα εξασφαλίζεται έτσι καλύτερη διαχείριση οικονομικών και υλικών πόρων. Στην βιβλιογραφία ο μεγαλύτερος όγκος αριθμητικής μοντελοποίησης έχει πραγματοποιηθεί στους τομείς της ναυπηγικής και των θαλάσσιων κατασκευών καθώς επίσης και τα τελευταία χρόνια στον τομέα της αστικής δόμησης. Αριθμητική δουλεία πάνω στην συμπεριφορά των αεροπορικών κατασκευών είναι υπερβολικά περιορισμένη και εκεί στοχεύει να συμβάλει η παρούσα διατριβή. Οι αεροπορικές κατασκευές εκτός των περιορισμών και προδιαγραφών που θέτουν οι άλλες εφαρμογές απαιτούν την ελαχιστοποίηση του προστιθέμενου βάρους στην κατασκευή.
Διάφοροι τύποι πολυμερών συνθέτων υλικών χρησιμοποιούνται στην βιομηχανία, διακρινόμενα σε θερμοσκληρυνόμενα και θερμοπλαστικά. Αρχικά παρουσιάζονται τα θερμοσκληρυνόμενα ξεκινώντας από τους ευρέως χρησιμοποιούμενους πολυεστέρες και βινυλεστέρες, στις φαινολικές και εποξικές ρητίνες καταλήγοντας στους υψηλής θερμοκρασίας κυανεστέρες. Εν συνεχεία γίνεται αναφορά στα συνήθη χρησιμοποιούμενα θερμοπλαστικά, πολυπροπυλένιο PP, Poly-ether ether-ketone PEEK και polyphenylene Sulphide PPS. Φυσικά δεν παραλείπεται να γίνει σύντομη αναφορά και στις τυπικές διεργασίες θερμικής αποσύνθεσης των προαναφερθέντων πολυμερών.
Η συμπεριφορά των σύνθετων πολυμερών υλικών σε συνθήκες φωτιάς περιγράφεται από κάποια χαρακτηριστικά μεγέθη τα οποία χρησιμοποιούνται για την ποιοτική και ποσοτική σύγκριση των διαφόρων υποψήφιων αεροπορικών υλικών. Συγκεκριμένα τα μεγέθη αυτά είναι: Heat Release Rate HRR, Thermal Stability Index TSI, Limited Oxygen Index LOI, Extinction Flammability Index ESI, Time-to-Ignition, Surface Flame Spread, Mass Loss, Smoke Density, Smoke Toxicity. Οι διαδικασίες ελέγχου και τα υπολογιζόμενα μεγέθη γίνονται βάσει διεθνών Προτύπων που κυρίως για τον τομέα της αεροναυπηγικής ορίζονται από την Ομοσπονδιακή Διεύθυνση Αεροπλοΐας FAA.
Η αριθμητική προσομοίωση προυποθέτει γνώση της συμπεριφοράς των πολυμερών υλικών σε συνθήκες υψηλής θερμοκρασίας, για τον σκοπό αυτό πραγματοποιήθηκαν πειράματα απώλειας μάζας με χρήση θερμογραβιμετρίας TGA κατά την διάρκεια της οποίας η απώλεια μάζας καθώς και ο ρυθμός αυτής παρακολουθούνται και καταγράφονται σαν συνάρτηση του ρυθμού θέρμανσης. Μέσα από αυτά τα δεδομένα μπορεί να πραγματοποιηθεί εκτίμηση του τρόπου αποσύνθεσης του πολυμερούς. Αρχικά πραγματοποιήθηκε η θεώρηση της μονοβάθμιας αντίδρασης (single-stage reaction) που αποτελεί και την πλέον απλουστευμένη προσέγγιση. Στην θεώρηση αυτή θεωρείται πως η πολυμερής μήτρα περνάει από την «παρθένα» κατάσταση στην απανθρακομένη μέσα σε ένα βήμα. Η περιγραφή της αντίδρασης αυτής γίνεται με μια μονοβάθμια αντίδραση τύπου Arrhenius.
Σε δεύτερο βήμα χρησιμοποιήθηκε κινητική θεωρία πολλαπλών σταδίων (multi-stage kinetics) σύμφωνα με την οποία πραγματοποιήθηκε ακριβέστερη προσέγγιση της απόσύνθεσης της πολυμερούς μήτρας των συνθέτων υλικών με απόκλιση μικρότερη του 5% από τα πειραματικά δεδομένα της θερμογραβιμετρείας (thermogravimetry). Και στις δύο προσεγγίσεις της αποσύνθεσης υπολογίσθηκαν οι κινηματικές παράμετροι: συντελεστής συχνότητας A (frequency factor), ενέργεια ενεργοποίησης ΕΑ (activation energy), τάξη αντίδρασης n (reaction order) για κάθε στάδιο. Με την ολοκλήρωση αυτού του σταδίου υπήρχε μια αξιόπιστη δυνατότητα αναπαράστασης της διαδικασίας αποσύνθεσης στο πείραμα της θερμογραβιμετρίας.
Είναι γνωστό ότι οι διακυμάνσεις της θερμοκρασίας επηρεάζουν της τιμές των θερμοφυσικών ιδιοτήτων των υλικών. Αναλογιζόμενοι ότι στην διαρκεία της επιβολής της φλόγας στα σύνθετα υλικά όχι μόνο η θερμοκρασία αλλά και η σύσταση μεταβάλλεται συνεχώς λόγω της αποσύνθεσης κρίθηκε αναγκαία η ανάπτυξη μιας μεθοδολογίας που θα συμπεριλαμβάνει την επίδραση της αποσύνθεσης στην μεταβολή των θερμοφυσικικών ιδιοτήτων (θερμική αγωγιμότητα, ειδική θερμοχωρητικότητα και πυκνότητα) της πολυμερούς μήτρας και κατά συνέπεια του συνθέτου υλικού. Οι εξαγόμενες μαθηματικές σχέσεις χρησιμοποιήθηκαν στην αριθμητική προσομοίωση που ακολούθησε.
Με σκοπό την ορθή αριθμητική μοντελοποίηση κρίνεται αναγκαία η μέτρηση και βαθμονόμηση του θερμικού φορτίου τον πειραματικών δοκιμών. Το μετρούμενο θερμικό φορτίου χρησιμοποιήθηκε εν συνεχεία ως φόρτιση στα αναπτυχθέντα μοντέλα. Χρησιμοποιήθηκαν δύο πειραματικές διατάξεις εφαρμογής φλόγας, μία μεσαίας κλίμακας σύμφωνα με τις διατάξεις του FAA Standard, που περιγράφεται στο ISO2685:1998(E) “Aircraft – Environmental test procedure for airborne equipment – Resistance to fire in designated fire zones” και μίας εργαστηριακής κλίμακος. Πραγματοποιήθηκε μέτρηση με θερμοζεύγη και καλορίμετρο νερού καθώς και αριθμητική μοντελοποίηση με χρήση CFD για την πρώτη διάταξη. Ενώ για την εργαστηριακής κλίμακας έγινε μέτρηση με θερμοζεύγη και ενός αισθητήρα θερμικού φορτίου «water-cooled Hukseflux Schmit-Boelter SBG01 sensor».
Εν συνεχεία πραγματοποιήθηκε η κατασκευή των δοκιμίων των υποψήφιων υλικών καθώς και οι πειραματικές δοκιμές και έλεγχοι τους. Συγκεκριμένα πραγματοποιήθηκε: Θερμιδομετρία κώνου (cone calorimetry), Θερμογραβιμετρία (thermogravimetry), Θερμιδομετρία Διαφορικής Ανίχνευσης (Differencial Scanning Calorimetry, DSC), Μέτρηση Θερμικής αγωγιμώτητας, Δοκιμή διείσδυσης φλόγας (Fire burnthrough penetration).
Καθώς ο χαρακτηρισμός της αποσύνθεσης των πολυμερών υλικών, η μεταβολή των θερμοφυσικών ιδιοτήτων, η μέτρηση και βαθμονόμηση του επιβαλλόμενου θερμικού φορτίου καθώς και οι πειραματικές δοκιμές έχουν ολοκληρωθεί ακολουθεί η αριθμητική προσομοίωση. Οι συνοριακές συνθήκες θερμικού φορτίου και ψύξης επιλέχθησαν ως εξής. Ως φόρτιση θεωρήθηκε η κατανομή του θερμικού φορτίου (σε kW/m2) στην εμπρός επιφάνεια του πάνελ. Στην ψύξη της πίσω επιφάνειας λήφθηκε υπόψη τόσο η ελεύθερη μεταφορά θερμότητας με επαφή όσο και η ακτινοβολία. Το μοντέλο της συμπεριφοράς του υλικού διαμορφώθηκε κατάλληλα ώστε να γίνει κατανοητό από τις απαιτήσεις ενός εμπορικού κώδικα Πεπερασμένων Στοιχείων επίλυσης θερμικών προβλημάτων και προσομοιώθηκαν οι πειραματικές δοκιμές διείσδυσης φλόγας των δύο πειραματικών διατάξεων, μεσαίας και εργαστηριακής κλίμακος.
Πλέον της αριθμητικής προσομοίωσης της συμπεριφοράς σε φωτιά επίπεδων δοκιμίων αεροπορικών κατασκευών, πραγματοποιήθηκε προσπάθεια απλουστευμένης μοντελοποίησης των συνθηκών φλόγας ενός λιμνάζοντος όγκου καυσίμου αεροσκαφών στο εξωτερικό μιας ατράκτου. Δημιουργήθηκε ένα τρισδιάστατο ρευστομηχανικό μοντέλο πρόβλεψης του θερμικού φορτίου στην επιφάνεια μιας τυπικής ατράκτου σύμφωνα με τις προδιαγραφές γεωμετρίας του Προτύπου “Full-scale test evaluation of Aircraft fuel fire burnthrough resistance improvements” DOT/FAA/AR-98/52,1999. Τα ρευστομηχανικά αποτελέσματα συγκρίθηκαν με δεδομένα βιβλιογραφίας για μεγάλες φλεγόμενες δεξαμενές λιμνάζοντος καυσίμου.
Εκτός από την μελέτη της απόκρισης των αεροπορικών κατασκευών σε συνθήκες φλόγας σκοπός της παρούσας εργασίας είναι και η παρουσίαση λύσεων οι οποίες θα έχουν την δυνατότητα της βελτίωσης της συμπεριφοράς των υπαρχουσών δομών καθώς και των μελλοντικών σύνθετων δομών. Ενδεικτικά αναφέρεται η δυνατότητα χρήσης νανοεγκλεισμάτων, και βελτιωμένων μονωτικών υλικών, π.χ. aerogels. Όπως έχει ήδη αναφερθεί οι αεροπορικές κατασκευές θέτουν τον περιορισμό της ελαχιστοποίησης του προστιθέμενου βάρους, για τον λόγο αυτό η ενίσχυση των συνθέτων υλικών θα πρέπει να πραγματοποιηθεί σε επίπεδο υλικού και σχεδιασμού. Πρέπει δηλαδή η ίδια η κατασκευή που είναι ικανή να φέρει τα μηχανικά φορτία να εξασφαλίζει και την πιστοποίηση της FAA για συνθήκες φωτιάς.
Συνοψίζοντας, η παρούσα διατριβή πραγματοποιεί μια καινοτόμο, γρήγορη και αρκετά ακριβή προσέγγιση του σημαντικότατου ζητήματος της συμπεριφοράς των πολυμερικών σύνθετων αεροπορικών δομών σε συνθήκες φωτιάς Η πολυπλοκότητα του όλου φαινομένου επέβαλε την πραγματοποίηση παραδοχών και απλουστεύσεων. Καθώς όμως με την αυξανόμενη χρήση των συνθέτων υλικών στις αεροπορικές κατασκευές, ο τομέας της ασφάλειας σε συνθήκες φλόγας είναι συνεχώς αυξανόμενος και απαιτητικός. Για αυτό οι παραδοχές και θεωρήσεις της παρούσας διατριβής μπορούν να βελτιωθούν με χρήση νέων υπολογιστικών μεθόδων και πειραματικών δεδομένων με στόχο την ακόμα ακριβέστερη πρόβλεψη της συμπεριφοράς τον αεροπορικών δομών σε συνθήκες φλόγας.
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Repair and Retrofit Strategies for Structural Concrete against Thermo-Mechanical LoadingsGuruprasad, Y K January 2014 (has links) (PDF)
Reinforced cement concrete (RCC) structures have become an important aspect in most of the buildings in our society around the world. Most of the multistoried reinforced concrete buildings house important institutions such as hospitals, schools, government establishments, defense establishments, business centers, sports stadiums, super markets and nuclear power plants. The cost of construction of such multistoried RCC structures is very high, and these structures need to be maintained and restored based on their functionality and importance using repair and retrofit strategies when these structures undergo damage. The steps involved in restoring RCC structures that have damages using repair / retrofit measures consists of identifying the source or cause of damage, assessment of the degree or extent of damage that has taken place using nondestructive techniques. Based on the assessment of degree of damage suitable repair / retrofit strategy using the appropriate repair material is applied to achieve the required load carrying capacity or strength. The present work involves assessing the efficacy of carbon fibre reinforced polymer (CFRP) based system applied on pre-damaged structural members to restore the member’s strength and stiffness through experimental investigations and finite element predictions. To validate the macrolevel properties of predamaged concrete micromechanical analysis, microscale studies and analytical investigations have been conducted. Plain and reinforced concrete test specimens: cylinders, square prisms and rectangular prisms having 25MPa and 35MPa cylinder compressive strengths pre-damaged due to mechanical (monotonic and cyclic loading) and thermal loading (exposure to different temperature and time durations) with applications of CFRP repair subjected to compression is investigated to study the behavior and enhancement in the compressive strength and stiffness after application of repair. Non destructive testing of thermally damaged concrete (exposed to different temperature and exposure time) is conducted using ultrasonic pulse velocity and tomography methods to understand the degradation in the strength and stiffness of thermally damaged concrete. The results of the non destructive testing helps in assessing the amount of repair that can be applied. To validate the macro scale behavior of thermally damaged concrete micro scale studies was performed adopting micro indentation, petrography, Raman spectroscopy, scanning electron miscroscopy (SEM) and Electron probe micro analysis (EPMA).
During the event of a fire in RC structures which have been retrofitted. The high temperature caused due to fire tends to make the concrete to deteriorate and the repair material to delaminate. Loss of strength/ stiffness in concrete and delamination of the repair material in a retrofitted structural component in a structure causes instability which results in partial collapse or complete collapse of the structure. Thermal insulation of concrete and the repair material (CFRP) using geo-polymer mortar and simwool thermal fibre blanket exposed to high temperature and different exposure time are experimentally investigated. This is to evaluate the effectiveness of the thermal insulation in protecting epoxy based structural repair material(CFRP) from thermal damage and to minimize the delamination of the repair material when exposed to high temperatures.
Slender columns when loaded eccentrically fail at a load much lesser than their actual load carrying capacity. In RC buildings where additional floors need to be added, in those situations slender columns which are already eccentrically loaded tend to get damaged or fail due to additional load which act on them. Therefore to restore such columns experimental and finite element investigations on reinforced concrete slender columns having 25MPa cylinder compressive strength subjected to eccentric monotonic compressive loading with applications of CFRP repair is studied to understand the behavior and the enhancement in load carrying capacity after application of repair.
Experimental investigations are conducted to study fracture and fatigue properties of thermally damaged concrete geometrically similar notched plain and reinforced concrete beams having 25MPa cylinder compressive strength exposed to different combinations of temperature and durations with application of repair (CFRP). Nonlinear fracture parameters of thermally damaged concrete is computed which help in understanding the fracture behavior of thermally damaged concrete and application of repair. Effectiveness of CFRP repair and failure behaviour of these beams are studied when these thermally damaged notch concrete beams are subjected to monotonic and cyclic (fatigue) loading.
Reinforced concrete slender beams when subjected to unexpected loads such as earthquakes get damaged. The increase in load carrying capacity and fatigue life of reinforced concrete slender beams having 25MPa cylinder compressive strength in flexure subjected to monotonic and cyclic loading with applications of CFRP repair is investigated using experimental and finite element investigations.
Finite element analysis of concrete specimens pre-damaged due to mechanical (monotonic and cyclic loading) / thermal loading (exposure to different temperature and time durations) with applications of CFRP repair and assessment of amount of repair required is investigated.
Analytical (empirical) models are developed to assess the mechanical properties of concrete (elastic moduli, compressive strength and split tensile strength) exposed to different temperatures
and time durations. Nonlinear fracture parameters of geometrically similar plain concrete notch beams exposed to different temperature and time durations are determined. Fracture parameters (stress intensity factor) of thermally damaged plain and reinforced concrete notched beams with application of CFRP have been determined. Effect of size and shape of thermally damaged plain concrete compression members with application of CFRP wrap have been studied. Crack mouth opening displacements (CMOD), strains and crack lengths of thermally damaged plain concrete (PC) notched beams using digital image correlation has also been determined.
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Intégrité des tours aéroréfrigérantes en béton armé sous sollicitations extrêmes : Vent et séisme / Integrity of reinforced concrete cooling towers under extreme loads : Wind and EarthquakeLouhi, Amine 30 November 2015 (has links)
Il est prévu d’augmenter la durée de vie des centrales nucléaires actuellement opérationnelles. Le vieillissement des structures en béton armé telles que les tours aéroréfrigérantes doit être évalué, son incidence sur la capacité portante calculée. Dans le cas de fortes dégradations, le renforcement doit être envisagé, afin d’assurer la pérennité de ces tours face aux sollicitations extrêmes telles que les tempêtes de vent et les séismes. Ce travail vise à quantifier les effets néfastes que peut générer la réduction de section des aciers induite par la corrosion, en particulier sur la capacité portante des tours dans des conditions de sollicitations extrêmes monotones ou cycliques de types vent et séisme. Ces sollicitations sont certainement les plus sévères, entrainant la structure dans le domaine non linéaire, elles sont susceptibles d’induire des endommagements de type fissuration qui dans le cas de sollicitation cycliques peuvent s’avérer néfastes. Des modélisations numériques sont proposées pour déterminer la réponse quasi-statique ou dynamique de la structure, en tenant compte des apparitions de fissures dans le béton et de leur évolution via des lois de comportement appropriées du matériau béton, ainsi que la plastification des aciers. Dans le cas d’une sollicitation sismique, dans le but de comparer les approches de modélisation du séisme et d’évaluer la robustesse des résultats, les réponses dynamiques sont évaluées par trois méthodes différentes de calcul : l’approche dynamique temporelle non linéaire, la méthode spectrale et la méthode modale temporelle. Des études paramétriques portant sur l’amortissement, les combinaisons de charges et les configurations structurales, sont aussi menées. Dans le cas d’une sollicitation de type vent, la technique de renforcement à l’aide de matériaux composite, tel que le tissu de fibres de carbone (TFC) est modélisée. Le comportement de la structure endommagée présentant un taux de corrosion avancée, est évalué dans le régime pré- et post-fissuration, comparativement à la structure intègre. La perte de capacité portante est quantifiée, un renforcement permettant de restaurer l'intégrité et donc d’augmenter la durée de vie de la structure est proposé. / The authorities have planned to increase the lifetime of currently operating nuclear power plants. The ageing of reinforced concrete structures such as cooling towers should be evaluated and its impact on the bearing capacity calculated. In the case of significant damage, the strengthening must be considered to ensure the sustainability of these towers facing the risk of storms and earthquakes becoming more and more frequent. This work aims to quantify the adverse effects that can generate concrete cracks and rebar section loss induced by corrosion, especially on the bearing capacity of nuclear power plant cooling towers under monotonic or cyclic extreme load conditions (wind and earthquake). These loads are certainly the most severe, since they take the structure into the nonlinear domain and can induce or amplify cracking damage. Numerical simulations are proposed to determine the quasi-static or dynamic response of the structure, taking into account appearance of concrete cracks and their evolution via an appropriate material concrete law and rebar's yielding. In the case of a seismic load, the responses are evaluated by three different methods; the nonlinear response history analysis (NLRHA), the response spectrum analysis and the modal response history analysis (MRHA) in order to compare the earthquake modeling approaches and to evaluate the robustness of the results. Parametric studies on damping, load combinations and structural configurations, are also performed. In the case of a wind load, the strengthening technique using composite materials, such as carbon fiber reinforced plastic (CFRP) is modeled. The behavior of the damaged structure with an advanced corrosion rate is estimated in the pre- and post-cracking regime, compared to the undamaged structure. The drop of bearing capacity is quantified, a reinforcement designed is proposed to restore the integrity and thus increase the lifetime of the structure.
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The Repair of Laterally Damaged Concrete Bridge Girders Using Carbon Fiber Reinforcing Polymers (CFRP)Graeff, Matthew Kent 01 January 2012 (has links)
In recent years the use of carbon fiber reinforcing polymers (CFRP) to repair damaged structural components has become more accepted and practiced. However, the current reference for designing FRP systems to repair and strengthen reinforced concrete (RC) and prestressed concrete (PSC) girders has limitations. Similarly, very few resources address solutions for the debonding problem associated with CFRP laminates or the use of CFRP laminates to repair structural members with pre-existing damage. The included experimental program consists of testing both RC and PSC girders with simulated lateral damage and CFRP repairs. A total of 34 RC beams were statically tested under a 4-point loading until failure and had cross-section dimensions of 5” x 10” (14cm x 25.4cm), were 8’ long (2.44m), and were reinforced with either #3 or #4 mild steel rebar. 13 PSC girders having cross-section dimensions representing a half-scaled AASHTO type II shape, were 20’ long (6.1m), and were prestressed with five 7/16” (11.1mm) diameter low-lax 7-wire strands. Ten of the PSC girders were statically loaded until failure under a 4-point testing setup, but 3 PSC girders were dynamically tested under fatigue loading using a 3-point arrangement. Different configurations of CFRP laminates, number and spacing of CFRP transverse U-wraps, and amount of longitudinal CFRP layers are studied. The results present the flexural behavior of all specimen including load-deflection characteristics, strain characteristics, and modes of failure. Ultimately, results are used to recommend important considerations, needed criteria, and proper design procedures for a safe and optimized CFRP repair configuration.
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Návrh kompozitní objímky rotoru vysokootáčkového rotačního stroje / Design of the composite rotor sleeve of a high speed rotary machinePavlík, Ondřej January 2018 (has links)
The diploma thesis focuses on manufacturing carbon fiber and epoxy composite material using filament winding method. Material properties of manufactured composite are ap-proximated using analytical and numerical homogenization models. Calculated material properties are applied to design and evaluate reserve factor of retaining sleeve for high speed brushless permanent magnet synchronous motor. Margin of safety of designed rotor is evaluated using composite failure criteria. Test stand for both static and dynamic testing is designed, static test stand is manufactured and assembled. Static strength test is carried out.
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