Καινοτόμες τεχνολογίες για τη βελτιστοποίηση της αυτο-ίασης σε κατασκευές προηγμένων σύνθετων υλικών

Fiber Reinforced Polymers (FRPs) are among the essential technological materials in the industrial and research communities due to their excellent specific properties. The low fracture toughness of the FPRs can consequence in detrimental damage phenomena that may reduce their structural integrity. The repair of damage is both time and money-consuming. Besides the traditional repair techniques, the approach of self–healing materials has attracted the attention of the research community. Inspired by the biological organisms, self-healing materials can heal (partially or fully) defects induced during service life by utilizing one or more of the existed methods. The extrinsic approaches are the capsule-based self-healing materials and the vascular. The intrinsic approach is based on the reversible dynamic bonds inside the polymer.The current thesis is based on the manufacturing of smart, multi-functional polymer composite materials with self-healing properties. More analytically, the optimization of the self-healing materials was accomplished via the nanomodification of capsule-based composites that can restore both the mechanical and electrical properties simultaneously. This achievement can lead to a composite that has both self-healing and self-sensing properties- as well as integrating the structural health monitoring functionality. It should be noted that the object of this thesis is only the study of the optimization of self-healing properties. In the intrinsic approach, intrinsically self-healing polymeric films were manufactured for easy application in composites. The reversible cross-links were introduced using three bis-maleimide oligomers with different molecular masses.Τα ινώδη σύνθετα υλικά πολυμερικής μήτρας αποτελούν μια από τις σημαντικότερες κατηγορίες υλικών καθότι παρουσιάζουν μια πληθώρα ιδιοτήτων οι οποίες διεγείρουν το ενδιαφέρον της επιστημονικής κοινότητας αλλά και της βιομηχανίας. Η χαμηλή τους δυσθραυστότητα οδηγεί σε σημαντικούς και «επικίνδυνους» για την δομική ακεραιότητα του συνθέτου, τύπους αστοχίας κατά την διάρκεια λειτουργίας του ενώ η επισκευή των ατελειών/βλαβών στο εσωτερικό του συνθέτου, χαρακτηρίζεται από υψηλή δυσκολία καθώς και αυξημένο κόστος. Πέραν των συμβατικών προσεγγίσεων επισκευής, η τεχνολογία της αυτο-ίασης (ΑΙ) έχει προκαλέσει μεγάλο ενδιαφέρον στην επιστημονική κοινότητα.Εμπνευσμένα από τους βιολογικούς οργανισμούς, τα αυτο-ιάσιμα υλικά επιτρέπουν την σχεδόν αυτόνομη διάγνωση και επούλωση των ατελειών που δύναται να σχηματιστούν στο εσωτερικό των υλικών χρησιμοποιώντας μία ή και περισσότερες από τις μεθόδους αυτο-ίασης. Η πρώτη βασίζεται στην ενσωμάτωση καψουλών στο υλικό, η δεύτερη στην ενσωμάτωση αγγείων ή δικτύων και η τρίτη σε εγγενώς αυτο-ιάσιμα πολυμερή.Σκοπός της παρούσας διατριβής είναι η παρασκευή προηγμένων έξυπνων πολυ-λειτουργικών σύνθετων υλικών πολυμερικής μήτρας με ικανότητα αυτο-ίασης της βλάβης. Συγκεκριμένα, πραγματοποιήθηκε η βελτιστοποίηση μέσω νανο-τροποποίησης της μεθόδου αυτο-ίασης με κάψουλες για την εισαγωγή τους σε σύνθετα υλικά όπου θα μπορούν να ανακτούν τις μηχανικές αλλά και τις ηλεκτρικές τους ιδιότητες ταυτόχρονα. Ο συνδυασμός της ανάκτησης των παραπάνω ιδιοτήτων μπορεί να οδηγήσει σε ένα σύνθετο το οποίο θα έχει την δυνατότητα αυτο-διάγνωσης της βλάβης και κατ’ επέκταση παρακολούθησης της δομικής του ακεραιότητας μέσω ηλεκτρικών μεθόδων. Στην διατριβή αυτή περιλαμβάνεται μόνο η μελέτη της αυτο-ίασης των μηχανικών και ηλεκτρικών ιδιοτήτων ταυτόχρονα και όχι της αυτο-διάγνωσης της βλάβης. Για την μέθοδο της εγγενούς αυτο-ίασης πραγματοποιήθηκε η κατασκευή πολυμερικών φιλμ ώστε να δίνεται η δυνατότητα στον κατασκευαστή να εισάγει την τεχνολογία με εύκολο τρόπο στο σύνθετο

Capsule-Based Self-Healing and Self-Sensing Composites with Enhanced Mechanical and Electrical Restoration

In this work, we report for the first time the manufacturing and characterization of smart multifunctional, capsule-based self-healing and self-sensing composites. In detail, neat and nanomodified UF microcapsules were synthesized and incorporated into composites with a nanomodified epoxy matrix for the restoration of the mechanical and electrical properties. The electrical properties were evaluated with the use of the impedance spectroscopy method. The self-healing composites were subjected to mode-II fracture toughness tests. Additionally, the lap strap geometry that can simulate the mechanical behavior of a stiffened panel was used. The introduction of the nanomodified self-healing system improved the initial mechanical properties in the mode-II fracture toughness by +29%, while the values after the healing process exceeded the initial one. At lap strap geometry, the incorporation of the self-healing system did not affect the initial mechanical properties that were fully recovered after the healing process

Mechanical Properties Assessment of Low-Content Capsule-Based Self-Healing Structural Composites

Microcapsule-based carbon fiber reinforced composites were manufactured by wet layup, in order to assess their mechanical properties and determine their healing efficiency. Microcapsules at 10%wt. containing bisphenol-A epoxy, encapsulated in a urea formaldehyde (UF) shell, were employed with Scandium (III) Triflate (Sc (OTf)3) as the catalyst. The investigation was deployed with two main directions. The first monitored changes to the mechanical performance due to the presence of the healing agent within the composite. More precisely, a minor decrease in interlaminar fracture toughness (GIIC) (−14%), flexural strength (−12%) and modulus (−4%) compared to the reference material was reported. The second direction evaluated the healing efficiency. The experimental results showed significant recovery in fracture toughness up to 84% after the healing process, while flexural strength and modulus healing rates reached up to 14% and 23%, respectively. The Acoustic Emission technique was used to support the experimental results by the onsite monitoring

Development of self-contained microcapsules for optimised catalyst position in self-healing materials

Self-contained microcapsules for use in self-healing epoxy resin are successfully synthesized by suspension polymerization process. The microencapsulation of an epoxy resin using Polymethylmethacrylate (PMMA) as a shell material and the location of scandium triflate (Sc(OTf)3) as the catalyst into microcapsules shell during the microencapsulation processes is presented (PMMA/Sc(OTf)3-walled microcapsules). Spherical microcapsules of 80 μm in diameter with a liquid core content of 30 wt% (determined by HPLC) are produced. Catalyst location on microcapsules are assessed qualitatively by SEM-EADS and quantitatively by TGA showing high yields (⁓70 wt%). The evaluation of the healing efficiency was assessed in terms of fracture toughness recovery. PMMA/Sc(OTf)3-walled microcapsules showed an increased healing efficiency than that of conventional PMMA-walled capsule. The healing efficiency of the PMMA-walled capsules was 46.7 and 55.1% when the system healed at 80 and 120 °C, respectively. However, in the case of PMMA/Sc(OTf)3-walled microcapsules healing efficiency increased to 57.5 and 79.1% for the same healing temperatures.European Commission, FP7, 605412, HIPOCRATE

3R Composites: Knockdown Effect Assessment and Repair Efficiency via Mechanical and NDE Testing

In this study, the mechanical properties of purposefully synthesized vitrimer repairable epoxy composites were investigated and compared to conventional, commercial systems. The purpose was to assess the knockdown effect, or the relative property deterioration, from the use of the vitrimer in several testing configurations. Mechanical tests were performed using ILSS, low-velocity impact, and compression after impact configurations. At modeled structure level, the lap strap geometry that can simulate the stiffening of a composite panel was tested. Several non-destructive evaluation techniques were utilized simultaneously with the mechanical testing in order to evaluate (i) the production quality, (ii) the damage during or after mechanical testing, and (iii) the repair efficiency. Results indicated that the new repairable composites had the same mechanical properties as the conventional aerospace-grade RTM6 composites. The electrical resistance change method proved to be a valuable technique for monitoring deformations before the initiation of the debonding and the progress of the damage with consistency and high sensitivity in real time. In terms of repair efficiency, the values ranged from 70% to 100%