| Περίληψη: | Over the past 20 centuries the properties of structural materials have improved tremen-dously in every aspect and further improvements are continue due to a slow trial and error process. One recurring goal of material development has been to emulate the ma-terials of nature. Among these, the most illusive is that of self-repair.
Inspired by the biological systems, scientists designed structural materials with a recov-ery mechanism triggered by the damage itself, called self-healing materials. Extrinsic and intrinsic are the main approaches used in order to impart self-healing functionalities to materials or structures. Extrinsic approach relies on capsules or vascules which act as reservoirs of the healing system while the intrinsic self-healing polymers are based on the inherent reversibility of bonding of the matrix polymer. The current dissertation aims at providing insights regarding polymers and composites implemented with either microcapsules or vascules.
In the direction of capsules, in the current thesis, the self-healing functionality is deliv-ered through a variety of different microcapsules consisted of the environmentally be-nign nature of poly (methyl methacrylate) (PMMA) or the widely used Urea Formalde-hyde (UF) shell material. A Bisphenol A diglycidyl ether (DGEBA, DER321, Dow) has been used as healing agent for PMMA capsules, EPON 828 for UF and Scandium (III) (Sc(SO3CF3)3) Triflate has been chosen as the catalyst. The employed methodology is to encapsulate the healing agent and to disperse the Scandium triflate catalyst in the host matrix, so that the healing occurs when the encapsulated material is released during the fracture and react with the Scandium Triflate. Towards a different approach, PMMA microcapsules with the catalyst on their shell (“all in one microcapsule”) are also pro-posed and compared to PMMA and Scandium Triflate separately.
A variety of different microcapsules, supplied by either Fundacion Tecnalia Research and Innovation (Spain) or University of Ioannina, Materials Science and Engineering Department (Greece) are evaluated regarding specific characteristics such as consistency in diameter, surface quality, accumulation tendency and filling percentage. The tools used for the first level of capsules observation are optical microscopy, scanning electron microscopy and transmission electron microscopy. The most promising capsules materi-al according to the evaluation criteria, which proceeded to the experimental campaign, are the following:
- PMMA (filled with 30 wt% DGEBA) combined with the addition of Scandium Triflate (ScT) as the catayst, two parts healing system
- PMMA (filled with 30 wt% DGEBA and 48 wt% ScT on the shell, “all in one capsule”), one-part healing system
- UF (filled with 60 wt% EPON828) combined with the addition of Scandium Triflate (ScT) the catayst, two parts healing system
An initial proof of concept includes compact tension experiments in polymers imple-mented with various contents of capsules (5 wt%, 10 wt%, 20 wt%, 30 wt%). An alterna-tive compact tension specimen is used, with a drilled hole, suitable for self-healing appli-cations that is crucial to avoid the complete separation of the crack surfaces. Compara-tive results are presented regarding the fracture toughness recovery after healing condi-tions. Moreover, the knock down effect on the mechanical performance resulting from capsules implementation is evaluated. It is conceivable that higher contents provide higher properties recovery but also higher knock down effect. Moreover, it is confirmed that the PMMA “all in one capsule” concept performs better regarding the knock down effect, as well as healing efficiency compared to the separate healing system with corre-sponding contents. UF microcapsules prove also to be promising at lower contents than PMMA as they achieve high healing efficiencies. The fracture surfaces of compact ten-sion samples are observed through scanning electron microscopy and differential scan-ning calorimetry tests are conducted for the determination of the polymer degree of cure.
The use of polymer matrix fiber-reinforced composites in structural applications is growing due to the excellent specific strength and stiffness of these materials. For that reason, capsules are also implemented within carbon fiber composites in order to assess the knock down effect and the healing efficiency under Mode II and three-point bending loading conditions. Two case studies are investigated, that of prepreg composites, im-plemented with either PMMA of UF capsules consisted of one or two-parts healing sys-tem. The experimental results are in line with the indications provided by the preliminary polymers study. PMMA “all in one capsule” concept is performing better than two-parts healing system (PMMA or UF) by reporting lower knock down effect and higher healing efficiency. An alternative concept is also applied in the case of three-point bending ex-periments by using the Acoustic Emission non-destructive technique in order to termi-nate the loading and avoid fiber breakage. The damage index induced by the loading and reloading procedure is also calculated. Moreover, the fracture surfaces indicating the capsule breakage are observed and differential scanning calorimetry tests determine whether a post curing effect due to the healing cycle is occur. Finally, a CFRP small-scale demonstrator (Skin-Stringer component) is manufactured, implemented with PMMA capsules (one-part healing system) within the adhesion of the bond line, and the healing efficiency is assessed under compression after impact loading conditions. The healing system activation is confirmed the damaged area reduction before and after heal-ing as observed through C-Scan.
In the direction of vascules, several raw materials are evaluated by their implementation within the composites. Copper, steel, wax, Teflon wires and 3D printed nets are consid-ered as potential vascule materials. Glass fiber reinforced composites (GFRP) are manu-factured and vascules are implemented from the different types of material at various diameters (0.2 mm, 0.6 mm, 0.9 mm, 1.2 mm, 1.4 mm, 1.8 mm). Optical microscopy is applied at specimens’ cross sections in order to assess the final diameter formation, the ply waviness and the resin rich zone around the vascules. Teflon wire, at 0.9mm diame-ter, is selected for further processing as it presents the minimum disruption to the com-posite, according to the evaluation criteria.
Infrared (IR) thermography is also applied (through a thermal camera) at composites in order to assess the final formation of the vascules and monitor the healing agent injec-tion process which is performed through a syringe. Three different heating techniques are applied, namely, heat plate with resistors, heating via copper wires and Ultraviolet (UV) lamps. Heat plate and copper wire heating are proposed as the techniques which can provide the most distinct IR images with respect to vascules formation. Post pro-cessing algorithms are developed in order to establish a procedure to further evaluate the images. Two different methods are applied (Sobel Filter, Central Difference Gradient) in an effort to enhance the appearance of vascules formation according to the temperature variations. Finally, the developed algorithm can be applied to all IR images in order to depict more accurately the actual path of the vascule within the composite (glass and carbon fiber composites (GFRP, GFRP)).
Three-point bending mechanical experiments are conducted in CFRP samples with tef-lon wire vascules. Following the same principle as in capsule composites, the experiment is terminated once the first acoustic emission hits occur. Two case studies are investigat-ed. In the first case study the healing system (resin and hardener) is injected premixed at each vascule after the damage occurs. In the second case study the healing agent and catalyst are separately injected at each vascule prior to damage. The knock down effect, compared to the reference material, is calculated. The experimental results indicate that the premixed healing system (case study 1) performs better compared to the separate injection (case study 2), regarding the calculated healing efficiency. The material stoichi-ometry and diffusion issues are eliminated due to premixing in case study 1. However, case study 2 is autonomous in terms of healing system activation, which makes it a proper candidate in case of upscaling the concept.
Vascular CFRP and GFRP open hole specimens are manufactured and tested under tension tension fatigue loading conditions. Two case studies are evaluated, that of com-posites with two vascules at mid plane and four vascules above and below midplane. The healing system (resin and hardener) is injected premixed once the damage area reaches the vascules position. The C-Scan technique is applied to monitor the damage progression and IR thermography for the injection process. The damaged area around the hole and its reduction after the healing system injection are calculated. It is evident that a trade-off is desired between the damaged area (which is lower in case with two vascules, at the respective loading cycles with that of four vascules) and the healing effi-ciency (which is higher at the case of four vascules).
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