Graphite epoxy composites lack effective mechanisms for absorbing local impact energy often resulting in penetration and a structural strength reduction. The effect of adding small amounts of two types of high strain hybrid components on the impact resistance of graphite epoxy composites subjected to projectiles traveling at ballistic velocities (greater than 900 ft/sec) has been studied. The hybrid components tested include superelastic shape memory alloy (SMA), a material having an unusually high stra in to failure (15 - 20%), and a high performance extended chain polyethylene (ECPE) known as Spectra™, a polymer fiber traditionally used in soft and hard body armor applications. 1.2% volume fraction superelastic SMA fiber layer was embedded on the specimens front, middle, and backface to determine the best location for a hybrid component in the graphite composite. From visual observation and energy absorption values, it was concluded that the backface is the most suitable location for a high strain hybrid component. Unlike the front and middle locations, the hybrid component is not restricted from straining by surrounding graphite material. However, no significant increases in energy absorption were found when two perpendicular SMA layers and an SMA-aramid weave configuration were tested on the backface. In all cases, the embedded SMA fibers were pulled through the graphite without straining to their full potential. It is believed that this is due to high strain rate effects coupled with a strain mismatch between the tough SMA and the brittle epoxy resin. However, a significant increase in energy absorption was found by adding ECPE layers to the backface of the composite . With only a 12% increase in total composite mass, a 99% increase in energy absorption was observed. / Master of Science

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Carbon ber composites have become popular in aerospace structures and applications due to their light weight, high strength, and high performance. Recently, scientists have begun investigating hybrid composites that include fibers and particulate fillers, since they allow for advanced tailoring of mechanical properties, such as improved fatigue life. This project investigated a hybrid carbon ber reinforced polymer (HCFRP) that includes carbon fiber and additional alumina nanoparticle fillers, which act as embedded nano stress-sensors. Utilizing the piezospectroscopic e ect, the photo-luminescent spectral signal of the embedded nanoparticles has been monitored as it changes with stress, enabling non-contact stress detection of the material. The HCRFPs stress-sensitive properties have been investigated in-situ using a laser source and a tensile mechanical testing system. Hybrid composites with varying mass contents of alumina nanoparticles have been studied in order to determine the e ect of particle content on the overall stress sensing properties of the material. Additionally, high resolution photo-luminescent maps were conducted of the surfaces of each sample in order to determine the particulate dispersion of samples with varying alumina content. The dispersion maps also served as a method of quantifying particulate sedimentation, and can aid in the improvement of the manufacturing process. The results showed that the emitted photo-luminescent spectrum can indeed be captured from the embedded alumina nanoparticles, and exhibits a systematic trend in photo-luminescent peak shift with respect to stress. The stress maps showed a linear increase in peak shift up to a certain critical stress, and matched closely with the DIC strain results. Therefore, the non-contact stress sensing results shown in this work have strong implications for the future of structural health monitoring and nondestructive evaluation (NDE) of aerospace structures.

Fiber hybridization involves the blending of high and low performance fibers in a common matrix to yield a composite with a balance of properties that cannot be achieved by using either fiber alone. In this study, the random wetlay process was used as a compounding method to investigate the effects of fiber hybridization on the mechanical, viscoelastic, and sorption characteristics of steam-exploded wood (SEW) and lyocell (high performance regenerated cellulose) fiber-reinforced polypropylene (PP) composites. The two fiber types were blended in varying proportions within a fixed total fiber content of 50 wt. % and compared with non-hybrid lyocell- and SEW-PP controls. Using PP matrix as basis, it was observed that moduli of all composites generally increased with increasing lyocell concentration, ranging from a minimum 66 % for SP 50 (SEW/PP control) to a maximum 233 % for LP 50 (lyocell/PP control). Ultimate strengths on the other hand, declined for SP 50 but increased with the inclusion of lyocell fibers. Comparisons of hybrid (having 5 - 20 wt % lyocell) with non-hybrid (having 25 - 50 wt. % lyocell) composites revealed a surprisingly greater strength and modulus-building efficiency (by as much as 2.6 times) in the hybrid composites. This observation indicated possible synergism between lyocell and SEW. Analyses of composite property gains as a function of fiber cost also showed greater cost benefits (highest for tensile modulus) in favor of hybridization. The advantages of fiber hybridization on composite properties were again evident under dynamic mechanical analysis where no significant differences in the storage moduli were found between a hybrid composite with 20 wt. % lyocell and a non-hybrid composite with 50 wt. % lyocell loading. Application of the time-temperature superposition principle (TTSP) made it possible to predict storage moduli over extended frequencies for PP and its composites. Comparison of shift factor versus temperature plots revealed decreasing relaxation times of PP with increasing lyocell concentration, which indicated that PP interacted better with lyocell than with SEW fibers. Finally, it was observed from sorption tests that hybrid composites absorbed less moisture than non-hybrid counterparts of either fiber type. The reasons for this observation were not apparent. It is however possible that moisture transport mechanisms within the composites may have been modified as a result of hybridization. / Master of Science

As fiber reinforced polymer (FRP) structures find application in highway bridge structures, methodologies for describing their long-term performance under service loading will be a necessity for designers. The designer of FRP bridge structures is faced with out-of-plane damage and delamination at ply interfaces. The damage most often occurs between hybrid plys and dominates the life time response of a thick section FRP structure. The focus of this work is on the performance of the 20.3 cm (8 in) pultruded, hybrid double web I-beam structural shape. Experimental four-point bend fatigue results indicate that overall stiffness reduction of the structure is controlled by the degradation of the tensile flange. The loss of stiffness in the tensile flange results in the redistribution of the stresses and strains, until the initiation of failure by delamination in the compression flange. These observations become the basis of the assumptions used to develop an analytical life prediction model. In the model, the tensile flange stiffness is reduced based on coupon test data, and is used to determine the overall strength reduction of the beam in accordance the residual strength life prediction methodology. Delamination initiation is based on the out-of-plane stress sz at the free edge. The stresses are calculated using two different approximations, the Primitive Delamination Model and the Minimization of Complementary Energy. The model successfully describes the onset of delamination prior to fiber failure and suggests that out-of-plane failure controls the life of the structure. / Master of Science

The excellent in-plane strength and stiffness to-weight ratios, as well as the ease of manufacturing have made the carbon fiber reinforced polymer composites (CFRPs) suitable structural materials for variety of applications such as aerospace, automotive, civil, sporting goods, etc. Despite the outstanding performance of the CFRPs along their fibers direction (on-axis), they lack sufficient strength and performance in the out-of-plane and off-axis directions. Various chemical and mechanical methods were reported to enhance the CFRPs' out-of-plane performance. However, there are two major drawbacks for utilizing these approaches: first, most of these methods induce damage to the carbon fibers and, therefore, deteriorate the in-plane mechanical properties of the entire CFRP, and second, the methods with minimal deteriorating effects on the in-plane mechanical performance have their own limitations resulting in very confined mechanical performance improvements. These methods include integrating nano-sized reinforcements into the CFRPs' structure to form a hybrid or hierarchical CFRPs. In lieu to all the aforementioned approaches, a relatively novel method, referred to as graphitic structures by design (GSD), has been proposed. The GSD is capable of grafting carbon nanotubes (CNTs) onto the carbon fibers surfaces, providing high concentration of CNTs where they are most needed, i.e. the immediate fiber/matrix interface, and in-between the different laminae of a CFRP. This method shows promising improvements in the in-plane and out-of-plane performance of CFRPs. Zinc oxide (ZnO) nanorods are other nano-sized reinforcing structures which can hybridize the CFRPs via their radially growth on the surface of carbon fibers. Among all the reported methods for synthesizing ZnO nanorods, hydrothermal technique is the most straightforward and least destructive route to grow ZnO nanorods over carbon fibers. In this dissertation, the GSD-CNTs growth method and the hydrothermal growth of ZnO nanorods have been utilized to fabricate hybrid CFRPs. The effect of different ZnO nanorods growth morphologies, e.g. size distribution and alignment, on the in-plane tensile performance and vibration attenuation capabilities of the hybrid CFRPs are investigated via quasi-static tension and dynamical mechanical analysis (DMA) tests, respectively. As a result, the in-plane tensile strength of the hybrid CFRPs were improved by 18% for the composite based on randomly oriented ZnO nanorods over the carbon fibers. The loss tangent of the CFRPs, which indicates the damping capability, increased by 28% and 19% via radially and randomly grown ZnO nanorods, respectively. While there are several studies detailing the effects of dispersed nanofillers on the fracture toughness of FRPs, currently, there are no literature detailing the effect of surface GSD grown CNTs and ZnO nanowire -on carbon fiber- on the fracture toughness of these hybrid composites. This dissertation probes the effects of surface grown nano-sized reinforcements on the fracture toughness via double cantilever beam (DCB) tests on hybrid ZnO nanorod or CNT grafted CFRPs. Results show that the surface grown CNTs enhanced the Mode I interlaminar fracture toughness (GIc) of the CFRPs by 22% and 32%, via uniform and patterned growth morphologies, respectively, over the reference composite based on untreated carbon fiber fabrics. The dissertation also explains the basis of the improvements of the fracture toughness via finite element method (FEM). In particular, FEM was employed to simulate the interlaminar crack growth behavior of the hybrid CFRPs under Mode I crack opening loading conditions embodied by the DCB tests. These simulations revealed that the hybrid CFRP based on fibers with uniform surface grown MWCNTs exhibited 55% higher interlaminar strength compared to the reference CFRPs. Moreover, via patterned growth of MWCNTs, the ultimate crack opening resistance of the CFRPs improved by 20%. To mimic the experimental behavior of the various CFRPs, a new methodology has been utilized to accurately simulate the unstable crack growth nature of CFRPs. Several investigations reported the effects of adding nanomaterials-including CNTs- as a filler phase inside the matrix material, on the impact energy absorption of the hybrid FRPs. However, the impact mitigation performance of CFRPs based on ZnO nanorod grafted carbon fibers has not been reported. The dynamic out-of-plane energy dissipation capabilities of different hybrid composites were investigated utilizing high velocity (~90 m/s) impact tests. Comparing the results of the hybrid MWCNT/ZnO nanorod/CFRP with those of reference CFRP, 21% and 4% improvements were observed in impact energy absorption and tensile strain to failure of the CFRPs, respectively. In addition to elevated stiffness and strength, CFRPs should possess enough tolerance not only to monotonic loadings, but also to cyclic loadings to be qualified as alternatives to traditional structural metal alloys. Therefore, the fatigue life of CFRPs is of much interest. Despite the promising potential of incorporating nano-sized reinforcements into the CFRPs structure, not many studies reported on the fatigue behavior of hybrid CFRPs so far. In particular, there are no reported investigations to the effect of surface grown CNTs on the fatigue behavior of the hybrid CFRPs, due to fact that almost all the CNT growth techniques (except for the GSD method) deteriorated the in-plane performance of the hybrid CFRPs. The hybrid ZnO nanorod grafted CFRPs have not been investigated under fatigue loading as well. In this dissertation, different hybrid CFRPs were tested under tension-tension fatigue to reveal the effects of the different nano-reinforcements growth on the fatigue behavior of the CFRPs. A remarkable fatigue damage tolerance was observed for the CFRPs based on uniform and patterned grown CNT fibers. Almost two decades of fatigue life extension was achieved for CFRPs based on surface grown MWCNTs. / Ph. D. / Carbon fiber reinforced polymer composites (CFRPs) are light-weight materials with excellent strength and stiffness along the direction of the fibers. These great mechanical properties have made CFRPs suitable structural materials for variety of applications such as aerospace, automotive, civil, sporting goods, etc. Despite the outstanding performance of the CFRPs along their fibers direction (on-axis), they lack sufficient strength and performance in the out-of-plane and off-axis directions. Various chemical and mechanical methods were reported to enhance the CFRPs’ out-of-plane performance. However, there are two major drawbacks for utilizing these approaches: first, most of these methods induce damage to the carbon fibers and, therefore, deteriorate the in-plane mechanical properties of the entire CFRP, and second, the methods with minimal deteriorating effects on the in-plane mechanical performance have their own limitations resulting in very confined mechanical performance improvements. These methods include integrating nano-sized reinforcements into the CFRPs’ structure to form a hybrid or hierarchical CFRPs. In lieu to all the aforementioned approaches, a relatively novel method, referred to as graphitic structures by design (GSD), has been proposed. The GSD is capable of grafting carbon nanotubes (CNTs) onto the carbon fibers surfaces, providing high concentration of CNTs where they are most needed, i.e. the immediate fiber/matrix interface, and in-between the different layers of a CFRP. This method shows promising improvements in the in-plane and out-of-plane performance of CFRPs. Zinc oxide (ZnO) nanorods are other nano-sized reinforcing structures which can hybridize the CFRPs via their radially growth on the surface of carbon fibers. Among all the reported methods for synthesizing ZnO nanorods, hydrothermal technique is the most straightforward and least destructive route to grow ZnO nanorods over carbon fibers. In this dissertation, the GSD-CNTs growth method and the hydrothermal growth of ZnO nanorods have been utilized to fabricate hybrid CFRPs. The effect of different ZnO nanorods growth morphologies, e.g. size distribution and alignment, on the in-plane tensile performance and vibration damping capabilities of the hybrid CFRPs are investigated via tension and dynamical mechanical analysis (DMA) tests, respectively. As a result, the in-plane tensile strength of the hybrid CFRPs were improved by 18% for the composite based on randomly oriented ZnO nanorods over the carbon fibers. The loss tangent of the CFRPs, which indicates the damping capability, increased by 28% and 19% via radially and randomly grown ZnO nanorods, respectively. Fracture toughness is a measure for the capability of a material to withstand a load in the presence of damage (i.e. crack) in the material’s structure. While there are several studies detailing the effects of dispersed nanofillers on the fracture toughness of FRPs, currently, there are no literature detailing the effect of surface GSD grown CNTs and ZnO nanowire -on carbon fiber- on the fracture toughness of these hybrid composites. This dissertation probes the effects of surface grown nano-sized reinforcements on the fracture toughness via double cantilever beam (DCB) tests on hybrid ZnO nanorod or CNT grafted CFRPs. Results show that the surface grown CNTs enhanced the Mode I interlaminar fracture toughness (G_Ic) of the CFRPs by 22% and 32%, via uniform and patterned growth morphologies, respectively, over the reference composite based on untreated carbon fiber fabrics. The dissertation also explains the basis of the improvements of the fracture toughness via finite element method (FEM). In particular, FEM was employed to simulate the interlaminar crack growth behavior of the hybrid CFRPs under Mode I crack opening loading conditions embodied by the DCB tests. These simulations revealed that the hybrid CFRP based on fibers with uniform surface grown MWCNTs exhibited 55% higher interlaminar strength compared to the reference CFRPs. Moreover, via patterned growth of MWCNTs, the ultimate crack opening resistance of the CFRPs improved by 20%. To mimic the experimental behavior of the various CFRPs, a new methodology has been utilized to accurately simulate the unstable crack growth nature of CFRPs. Several investigations reported the effects of adding nanomaterials - including CNTs - as a filler phase inside the matrix material, on the impact energy absorption of the hybrid FRPs. However, the impact mitigation performance of CFRPs based on ZnO nanorod grafted carbon fibers has not been reported. The dynamic out-of-plane energy dissipation capabilities of different hybrid composites were investigated utilizing high velocity (~90 m/s) impact tests. Comparing the results of the hybrid MWCNT/ZnO nanorod/CFRP with those of reference CFRP, 21% and 4% improvements were observed in impact energy absorption and tensile strain to failure of the CFRPs, respectively. In addition to elevated stiffness and strength, CFRPs should possess enough tolerance not only to monotonic loadings, but also to cyclic loadings to be qualified as alternatives to traditional structural metal alloys. Therefore, the fatigue life (i.e. the number of loading cycles to failure) of CFRPs is of much interest. Despite the promising potential of incorporating nano-sized reinforcements into the CFRPs structure, not many studies reported on the fatigue behavior of hybrid CFRPs so far. In particular, there are no reported investigations to the effect of surface grown CNTs on the fatigue behavior of the hybrid CFRPs, due to fact that almost all the CNT growth techniques (except for the GSD method) deteriorated the in-plane performance of the hybrid CFRPs. The hybrid ZnO nanorod grafted CFRPs have not been investigated under fatigue loading as well. In this dissertation, different hybrid CFRPs were tested under tension-tension fatigue to reveal the effects of the different nano-reinforcements growth on the fatigue behavior of the CFRPs. A remarkable fatigue damage tolerance was observed for the CFRPs based on uniform and patterned grown CNT fibers. Almost two decades of fatigue life extension was achieved for CFRPs based on surface grown MWCNTs.

In order to reduce over-dependency on fossil fuels and to create an environment that is free of non-degradable plastics, and most importantly to reduce greenhouse gas emission, bio-based products are being developed from renewable resources through intense research to substitute conventional petrochemical-based plastics with renewable alternatives and to replace synthetic fibers with natural fibers. Many authors have done quite a lot of work on synthesizing polymers from renewable origin. Polylactic acid (PLA) has been developed and characterized, and it was found that it has enormous potential and can serve as an alternative to conventional thermoplastics in many applications. Modification of the plant oil triglycerides has been discussed by many authors, and research is still going on in this area. The challenge is how to make these renewable polymers more competitive in the market, and if possible to make them 100% bio-based. There is also a major disadvantage to using a bio-based polymer from plant oils because of the high viscosity, which makes impregnation of fibers difficult. Although natural fibers are hydrophilic in nature, the problem of compatibility with the hydrophobic matrix must be solved; however, the viscosity of the bio-based resin from plant oils will complicate the situation even more. This is why many authors have reported blending of the renewable thermoset resin with styrene. In the process of solving one problem, i.e reducing the viscosity of the renewable thermoset resin by blending with reactive diluents such as styrene, another problem which we intended to solve at the initial stage is invariably being created by using a volatile organic solvent like styrene. The solution to this cycle of problems is to synthesize a thermoset resin from plant oils which will have lower viscosity, and at the same time have higher levels of functionality. This will increase the crosslinking density, and they can be cured at room temperature or relatively low temperature. In view of the above considerations, the work included in this thesis has provided a reasonable solution to the compounded problems highlighted above. Three types of bio-based thermoset resins were synthesized and characterized using NMR, DSC, TGA, and FT-IR, and their processability was studied. The three resins were subsequently reinforced with natural fibers (woven and non-woven), glass fibers, and Lyocell fiber and the resulting natural fiber composites were characterized by mechanical, dynamic mechanical, impact, and SEM analyses. These composites can be used extensively in the automotive industry, particularly for the interior components, and also in the construction and furniture industries. Methacrylated soybean oil (MSO), methacrylic anhydride-modified soybean oil (MMSO), and acetic anhydride-modified soybean oil (AMSO) were found to be suitable for manufacture of composites because of their lower viscosity. The MMSO and MSO resins were found to be promising materials because composites manufactured by using them as a matrix showed very good mechanical properties. The MMSO resin can completely wet a fiber without the addition of styrene. It has the highest number of methacrylates per triglyceride and high crosslink density. / Akademisk avhandling för avläggande av teknologie doktorsexamen vid Chalmers Tekniska högskola försvaras vid offentlig disputation, den 6:e maj, Chalmers, KE-salen, Kemigården 4, Göteborg, kl. 10.00.

Los volantes de inercia superan a las baterías eléctricas por su capacidad de absorber y ceder energía en poco tiempo y, si se fabrican con materiales compuestos, también por su reducido peso. La tesis presenta un estudio sobre los rotores de materiales compuestos aplicados a los acumuladores cinéticos para hacerlos más asequibles a usos industriales baratos. Para ello se proponen dos objetivos: obtener un sistema analítico de cálculo, y mejorar el diseño de rotores de bajo coste.Se desarrolla un sistema analítico de cálculo muy completo, tanto en las cargas como en las tensiones. Se consideran todas las cargas necesarias para el diseño mecánico del rotor: la fuerza centrífuga, la fuerza de aceleración y las tensiones residuales, térmica y de hidratación; y se determinan todas las componentes, normales y cortantes, de la tensión para cada punto del rotor.El cálculo en condiciones de tensión plana, utilizado por la mayoría de autores, se amplía con el cálculo en deformación axial constante, que es una variante mejorada de la deformación plana. Se comprueba que sus resultados son mejores que los de tensión plana cuando se comparan con los obtenidos en modelos de elementos finitos. Paralelamente, como aportación nueva de la tesis, se deducen las funciones de la variación de la tensión axial y de la tensión cortante radial-axial a lo largo del eje longitudinal del rotor. A partir de estos resultados se desarrolla un sistema general de cálculo que, además de unificar los sistemas de tensión plana y deformación axial constante, permite determinar todas las tensiones en cualquier posición radial-axial del rotor.Este sistema unificado de cálculo se amplia con tres particularidades: una aplicación de cálculo para resolver rotores multicapa, las ecuaciones especiales para los materiales singulares no resolubles con las ecuaciones generales, y el cálculo de capas con fibras orientadas axialmente aplicadas para refuerzo en configuraciones especiales.Con el objeto de mejorar las prestaciones del rotor se estudian dos procedimientos para crear tensiones de pretensado: generando tensiones durante el bobinado y utilizando las tensiones residuales térmicas. En el primero se elabora un sistema analítico de cálculo para determinar las tensiones residuales de bobinado y se complementa con una simulación mediante elementos finitos basada en submodelos incrementales. Ambos cálculos son capaces de simular el material no curado aplicando las propiedades viscoelásticas de los ensayos experimentales de otros autores. En el segundo se presenta un sistema nuevo, denominado pretensado térmico, basado en el curado por etapas, que genera tensiones residuales parecidas a las de bobinado pero con menos problemas de fabricación.El diseño de volantes se aplica a tres configuraciones básicas: rotores híbridos multicapa con materiales de rigidez progresiva, rotores de un solo material con anillos de elastómero y rotores con pretensado térmico.Sus prestaciones se valoran con tres variables: la masa, el volumen y el coste del material; de las cuales el coste es la principal y se utiliza para la optimización de la geometría.En cada configuración se determina la energía máxima para distintas relaciones de radios del rotor y se compara con el rotor de un sólo material. Se utilizan los materiales básicos usados en la fabricación de rotores: la fibra de carbono con matriz epoxi, la fibra de vidrio con matriz epoxi, el aluminio y el acero. Los dos materiales compuestos ofrecen mejores resultados que los metales, pero disminuyen sensiblemente en rotores con espesor de pared grande. En estos casos, la energía por unidad de coste mejora aplicando los anillos elásticos y el pretensado térmico. / Flywheels are better than electric batteries in that they absorb and yield energy in shorter time and, if made out of composite materials, also in that they weight less. This thesis presents a study of composite material rotors applied to kinetic accumulators in order to make them usable for low cost general industrial uses. Two objectives are proposed: a) to develop an analytical system for computation and b) to design alternatives in order to improve the performance on low-cost rotors.The analytical system is intended to be very complete, considering all relevant types of external loads and stress components. For the former, centrifugal, acceleration forces and residual, thermal and moisture stresses are included. For the latter, five normal and shear components are computed at each point of the rotor.The usual plane stress condition is expanded with the consideration of constant axial strain, along the lines of the plane strain hypothesis but with greater accuracy. It is shown that the current theory results fit the ones from finite elements much better than those from plain stress. As a new contribution, the functions for the axial stress and the radial-axial stress along the axis of the rotor are developed. From these results, a general system that unifies the plane stress and constant axial strain can compute the stress state at any position.In addition, the unified system includes three novel aspects: an extension of computation for multi-layer rotors, special equations for some materials in which behaviour present singularities and the computation of layers with fibers along the axial direction, which can be useful as a reinforcement for some configurations.Two procedures that can create beneficial residual stresses are studied: generating stresses during the filament winding and using the thermal stresses. For the first, analytical expressions are developed and validated and complemented with especially developed finite elements based on incremental submodels. In both cases the material is characterized by viscoelastic properties taken from the literature. For the second, a new procedure called thermal prestress is based on the accumulation of partial curing processes (by stages), which is able to create residual stresses similar to those of winding but involving simpler manufacturing.Three basic configurations are studied for the design: hybrid rotors with progressive stiffness along the radius, single material rotors with elastomer thin rings and rotors manufactured with thermal prestress, evaluating the performance as a function of the mass, volume and cost of the material. The latter is defined as the most important, and it is used as a reference for the geometry optimization.The maximum energy stored on each of the configurations is compared with that of a single material rotor, using the most common ones: glass and carbon fiber both with epoxy matrix, aluminium and steel. Results show that glass/epoxy has the highest storing capability per unit cost, although the number is greatly reduced when the thickness increases. If this rotor has a thin layer of carbon/epoxy, the capability does not increase, although it does with distributed elastomeric layers. There is also an increase with fabrication based on the thermal prestress technique.

Ο θερμομηχανικός χαρακτηρισμός του κράματος NiTi και η ενσωμάτωση του σε πολυμερική μήτρα με στόχο τη γέννηση εσωτερικών μηχανικών τάσεων όταν υπάρξει θερμική διέγερση, είναι επιγραμματικά ο σκοπός της παρούσας εργασίας. Το ‘ευφυές’ σύστημα που μελετάται στην εργασία αυτή αποτελείται από εποξειδική ρητίνη, ενισχυμένη με ίνες Kevlar 29 και ενσωματωμένα σύρματα Νικελίου-Τιτανίου. Στο πρώτο πειραματικό μέρος, περιλαμβάνεται η μελέτη και ο θερμομηχανικός χαρακτηρισμός του υλικού. Χρησιμοποιήθηκαν σύρματα NiTi διαμέτρου 0.3mm, αλλά και ράβδοι για τη διεξαγωγή κάποιων συγκεκριμένων πειραμάτων. Οι πειραματικές τεχνικές περιλαμβάνουν μηχανικά πειράματα εφελκυσμού σε σερβοϋδραυλικό πλαίσιο δοκιμών, ηλεκτρονιακή μικροσκοπία σάρωσης (SEM), οπτική μικροσκοπία, χρήση θερμοκάμερας υπερύθρου ακτινοβολίας, διαφορική θερμιδομετρία σάρωσης (DSC), δυναμική μηχανική ανάλυση (DMA), μέτρηση ηλεκτρικών ιδιοτήτων καθώς και ένα πρωτοποριακό σύστημα χαρακτηρισμού υλικών (σύστημα THERMIS), που αναπτύχθηκε στο εργαστήριο. Το δεύτερο τμήμα, περιλαμβάνει τη μελέτη του υβριδικού σύνθετου υλικού. Η παρασκευή του πραγματοποιείται σε αυτόκλειστο φούρνο (autoclave) και για την ενεργοποίηση του και την καταγραφή των παραμέτρων κατά τη λειτουργία του, χρησιμοποιείται το σύστημα THERMIS. Για να επιτευχθεί η σύγκριση μεταξύ της συμπεριφοράς του κράματος όταν αυτό ενεργοποιείται με και χωρίς την πολυμερική μήτρα να το περιβάλει, έχουν επιλεγεί δύο τύποι πειραμάτων: συνεχής ενεργοποίηση για μεγάλο χρονικό διάστημα (χαλάρωση τάσης ενεργοποίησης - activation stress relaxation) και κυκλική ενεργοποίηση-απενεργοποίηση για μεγάλο αριθμό επαναλήψεων (λειτουργική κόπωση – transformation fatigue). Τα πειράματα της λειτουργικής κόπωσης στα σύρματα, έδειξαν ότι η αρχική αναπτυσσόμενη τάση, μειώνεται εκθετικά, συναρτήσει των κύκλων ενεργοποίησης. Σύμφωνα με τα αποτελέσματα, η κοπωτική συμπεριφορά του σύρματος, δεν εξαρτάται από τη διάρκεια του χρόνου θέρμανσης ανά κύκλο, αλλά από τον αριθμό των κρυσταλλογραφικών μετασχηματισμών μεταξύ οστενιτικής - μαρτενσιτικής φάσης και αντίστροφα. Ο ρυθμός υποβάθμισης της ικανότητας του κράματος να ασκεί μηχανική τάση, είναι πολύ έντονος κατά τη διάρκεια των πρώτων εκατοντάδων κύκλων και μειώνεται όσο το φαινόμενο εξελίσσεται. Στα σύνθετα υλικά, όπως και στην περίπτωση των συρμάτων SMA, η υποβάθμιση της λειτουργικής ικανότητας των υβριδικών συνθέτων, φαίνεται ότι δεν εξαρτάται από το χρόνο της θέρμανσης ανά κύκλο, αλλά μόνο από το πλήθος των κρυσταλλογραφικών εναλλαγών. Στα πειράματα χαλάρωσης τάσης, η αρχική τάση των συρμάτων ήταν γύρω στα 500MPa και σύμφωνα με τα πειραματικά αποτελέσματα, με την πάροδο του χρόνου και υπό την επίδραση της θερμοκρασίας, λαμβάνει χώρα εκθετική μείωση του μεγέθους της. Το επίπεδο της θερμοκρασίας λειτουργίας, επηρεάζει δραματικά την υποβάθμιση της μηχανικής αναπτυσσόμενης τάσης, της οποίας ο ρυθμός είναι ιδιαίτερα αυξημένος κατά τις πρώτες ώρες λειτουργιάς του υλικού. Ποιοτικά, το ίδιο φαινόμενο συμβαίνει και στην περίπτωση της χαλάρωσης τάσης ενεργοποίησης των υβριδικών συνθέτων, με τη διαφορά ότι η υποβάθμιση είναι σαφώς πιο έντονη. Προτείνεται τέλος, η μελέτη της υποβάθμισης της ικανότητας των ενεργοποιητών, με βάση στατιστικά εργαλεία και μεθόδους, κατά αναλογία με άλλα κοπωτικά φαινόμενα στη φύση (κυρίως στη μηχανική), εφόσον ουσιαστικά πρόκειται για ακολουθία δράσεων που τελικά οδηγούν στην απώλεια της ικανότητας των υλικών μας να ασκούν τάση. Συνηγορεί εξάλλου σε αυτό και η μορφή των πειραματικών καμπυλών, που παρουσιάζουν μεγάλη ομοιότητα με τις αντίστοιχες καμπύλες S/N, στη μηχανική κόπωση των υλικών. / The present work, aims to the thermo-mechanical characterization of the NiTi Shape Memory Alloy and the characterization of ‘smart’ hybrid composites with embedded SMAs, under thermal activation. The composite structure that is being investigated consists of an epoxy resin matrix, Kevlar 29 fibers and NiTi SMA wires. The first experimental section deals with the thermo mechanical characterization of the Shape Memory Alloy. 0.3 mm in diameter wires were used. The experimental techniques, include mechanical tests using a servo-hydraulic testing apparatus, scanning electron microscopy (SEM), optical microscopy, thermal IR camera imaging, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), electrical measurements and a novel thermo mechanical characterization system (Thermis), which was tailor made according to the needs of this project. The second experimental section deals with the characterization of the composite material, which was developed using a special purpose furnace (autoclave) and was tested on the Thermis system. In order to compare the functionality of the SMA either in free condition or embedded in a polymer matrix, two experiments were chosen: continuous activation for a long time (activation stress relaxation) and recurrent thermal activation (transformation fatigue). The transformation fatigue experiments showed that the initially developed mechanical stress, reduces exponentially, according to the number of the crystalline transformations. The functional fatigue life of the alloy, does not depend on the heating time per cycle, but is strongly correlated to the number of the recurrent transformation between martensite and austenite. The stress reduction rate is increased during the first cycles and reduces as the phenomenon develops. As it concerns the composite materials, like the SMA wire case, the reduction of the stress generation capability, is not affected by the heating time per cycle, and is only related to the number of the crystalline transformations. During the stress relaxation experiments, the initial developed stress of the wires was about 500 MPa and according to the experimental results, as time passes and under the influence of the thermal field, the stress reduces exponentially. The temperature level strongly affects the reduction phenomenon and the reduction rate is very high during the first hours of the experiment. The same behavior is observed not only at the SMA wires but also at the composites, as well, noting that reduction in the later case is much more intense. At the end, the study of the fatigue and relaxation phenomena, using a statistical approach, is suggested, like many other fatigue cases in nature (especially in mechanics). The fatigue curves presented here resemble to the S/N curves that can derived from the case of mechanical fatigue of other structural materials, like steel or CFR composites.