The application of carbon reinforced-fiber polymers (CFRP) to structures is a new development that is still under intense research. However, the rehabilitation or retrofit of damage reinforced concrete members by the external bonding of CFRP is becoming increasingly popular in the construction industry. The objective of the tests presented in this thesis is to study different CFRP designs on the reinforced concrete beams and compare their failure modes. The main goal is to determine the CFRP design on the reinforced concrete beams that result in a progressive and gradual failure mode with enough warning before final failure. Different CFRP designs are investigated and compared with theoretical predictions. A retrofitting concept is also employed in this research. The retrofitting concept is the idea of strengthening cracked structures. The strengthening of the beams performed in the lab is carried out under sustained loads and on previously cracking the beams to simulate the realistic case that is usually faced in practice on the field. The RC beams are strengthened in flexure to double their flexural capacity by applying the adequate amounts of CFRP to the tension face of the beams. Due to the CFRP strengthening and increasing the strength capacity of the beams, different CFRP anchorage methods are employed to the beams for additional shear reinforcement to ensure flexural failure. The different CFRP anchorage methods will also be observed for their effectiveness during the debonding and propagation mechanism as well as evaluated for their progressive failure mode.

About 77,600 bridges throughout the United States in the Federal Highway Association (FHWA) bridge database are listed as structurally deficient. This has created a need to either replace or strengthen bridges quickly and efficiently. Due to high costs for total replacement of deficient bridges, strengthening of existing bridges is a more economical alternative. A technique that has been developing over the past two decades is the strengthening of bridges using carbon fiber reinforced polymer (CFRP) sheets. The CFRP sheets are attached to the bottom of the bridge girders using structural adhesives so that the CFRP becomes an integral part of the bridge and carries a portion of the flexural loading. The CFRP sheets allow for an increase in the capacity of the bridge with minimal increase in the weight of the structure due to CFRP having a low density. Because the CFRP is expected to be an integral component and carry some of the long-term loading it is important to understand the long-term durability of the composite section. This thesis is part of a larger project, in which the long-term durability of the CFRP composite on concrete beams is investigated experimentally. The CFRP strengthened beams are exposed to fatigue testing and thermal-humidity cycling followed by failure testing. The testing scheme for this experiment allows for the investigation of the individual effects of fatigue and thermal-humidity loading as well as to explore the effects from combined fatigue and thermal-humidity loading. The investigation of the combined effects is a unique aspect of this experiment that has not been performed in prior studies. Results indicate that a polyurethane-based adhesive could provide a more durable bond for the CFRP-concrete interface than possible with epoxy-based adhesives.

Composite sandwich structures are prevalent in engineering applications where high strength to weight ratios are critical. A composite sandwich includes the addition of a core material between two composite face sheets to increase ultimate stress in compression and bending loading cases. The performance of many composite sandwich structure configurations is well understood in the undamaged case. This analysis examines a type of damage, low velocity indentation, and determines the effect on mechanical behavior. The scope of the analysis includes manufacturing sandwich composite structures, creating indentation in the composite, and testing the sandwich composite structure. The mechanical behavior of the composite sandwich structures is characterized through ASTM C364 test standard for compressive strength and ASTM C393 standard test standard for flexural properties of sandwich constructions. The experiment is conducted with varied indentation depth, core materials, composite sandwich thickness, and composite face sheet thickness. The findings are compared to control specimens and used to determine the effect of indentation depth and create a relationship for the mechanical performance of indented sandwich composites.

yes / This paper presents test results of six concrete beams reinforced with longitudinal carbon fiber reinforced polymer (CFRP) bars and without vertical shear reinforcement. All beams were tested under a two-point loading system to investigate shear behavior of CFRP reinforced concrete beams. Beam depth and amount of CFRP reinforcement were the main parameters investigated. All beams failed due to a sudden diagonal shear crack at almost 45°. A simplified, empirical expression for the shear capacity of FRP reinforced concrete members accounting for most influential parameters is developed based on the design-by-testing approach using a large database of 134 specimens collected from the literature including the beams tested in this study. The equations of six existing design standards for shear capacity of FRP reinforced concrete beams have also been evaluated using the large database collected. The existing shear design methods for FRP reinforced concrete beams give either conservative or unsafe predictions for many specimens in the database and their accuracy are mostly dependent on the effective depth and type of FRP reinforcement. On the other hand, the proposed equation provides reasonably accurate shear capacity predictions for a wide range of FRP reinforced concrete beams.

There has been a prevalence of long-distance running footwear incorporating carbon-fiber plates within their midsoles, effectively increasing their longitudinal bending stiffness (LBS). This modification of modern racing footwear has occurred concurrently with large improvements in running times (Bermon et al., 2021), putting into question how these footwear components affect performance (Muniz-Pardos et al., 2021). The current literature has investigated this at level running, but with the increasing popularity of trail running, it is of interest to investigate whether the benefits found during level running translate to graded running. Therefore, the overall aim of this study was to investigate the effects of increased footwear midsole longitudinal bending stiffness (i.e. carbon-fiber plates) on running energetics and biomechanics at various inclines. The effects of high LBS (Nike Vaporfly 4% with midsole intact) and low LBS (Nike Vaporfly 4% with mediolateral cuts made at the forefoot of the midsole through the carbon-fiber plate) footwear conditions were compared for running at 0°, 6°, and 12° inclines. Running energetics and biomechanics data were quantified by measuring metabolic rate and lower leg joint mechanics (from motion capture and ground reaction force measurements). Results from this study suggest that increasing longitudinal bending stiffness within the footwear midsoles has limited influence on running energetics (small non-significant improvements of metabolic power at all inclines), but has considerable effects on the biomechanics of the ankle and MTP joints. However, the most important between shoe differences were independent of grade, suggesting that the benefits of modern racing shoe observed for level running can be expected to translate to steep uphill running. Nevertheless, it should be noted that this study was only able to collect and use data for analysis from a limited number of participants (n=7), and therefore is underpowered, so there may be significant differences that go undetected

The research field of composite materials is particularly fascinating due to the design freedom they offer and the infinite number of constituent combinations, including those that are already explored, and many more that are yet to be tried. One composite material that holds great potential contains carbon in its fiber shape. Carbon fibers possess unique properties that excel in mechanical aspects, as well as interesting electrical and thermal properties that are yet to be fully explored. These fibers are readily available on the market and can be introduced as reinforcement in various lengths and orientations, yielding diverse results depending on the intended effect. Although carbon fiber reinforced polymer composites (CFRP) are present on the market for quite some time, specifically in high-performance applications, they are predominantly used when their performance outweighs their cost. Meanwhile, carbon fiber composite waste is starting to cumulate in noticeable amounts. This waste originates from both, production scrap and end-of-life scenarios, as components introduced in service life in the past 30 years are being decommissioned and discarded. Unfortunately, the prevalent solution for handling this waste is landfilling, due to its ease, affordability, and accessibility. Consequently, substantial amounts of composite waste are accumulating worldwide. Furthermore, it has finally come to our attention that our planet's resources are finite. Our exploitation of these resources has been largely devoid of consideration for the needs of future generations. As a result, recently, sustainability has emerged as a key enabler for a circular economy, driven by increasing environmental concerns and demands from customers and users for market transformation. The implementation of sustainable practices is now underway, albeit at a gradual pace.   In summary, we find ourselves facing a trifold predicament: a splendid material being underutilized due to production costs, the cumulative generation of CFRP waste resulting from a lack of foresight and suitable alternatives, and the urgent need to transition towards a circular economy due to resource depletion. This research work aims to address all three challenges by developing an integrated solution.   The current work demonstrates that it is possible to recycle carbon fiber model composites through a two-step pyrolysis treatment, a fully mature recycling technology. The study has been done in two stages which are presented in two journal papers included in the thesis. The primary objective of the first paper is to identify and optimize process parameters that maximize the retention of mechanical properties in the recovered fibers. The overall results achieved show good retention value; with over 90% retention on stiffness and 90% on strength. Encouraging results from initial experimental work, have spurred the research focus towards further investigation. Thus, the second paper reports on repetitive manufacturing and recycling cycles of two sets of identical model composites by using the two most effective recycling treatments identified through the parameter optimization. The mechanical performance and structural changes of the recycled fibers are characterized and analyzed. Although further analysis is required, current mechanical behavior shows recovered fibers suitable for secondary applications after two recycling cycles, with an abrupt decay in fiber properties after the third cycle.   With the waste challenge under control, through successful recycling of composite waste, it is time to find concrete applications for this research. Having recycled carbon fibers (rCF) with comparable performance to virgin carbon fibers (vCF) opens up opportunities for rCF mats and other intermediate products to compete in previously inaccessible markets.

The rapidly growing demand for carbon fiber reinforced plastics in high-tech industries, such as aerospace, defense, automotive, wind turbine engineering, building and sports, resulted in a high amount of waste in the form of dry waste (e.g., production off-cuts), wet waste (e.g., out-of-date prepreg) and end-of-life components waste (e.g., aircraft components). Furthermore, the production of carbon fibers is cost and energy-intensive. Therefore, technological developments for the gentle processing of recycled carbon fiber and its integration into high-performance composites with promising tensile properties have gained considerable attention. Consequently, injection molding, nonwovens and hybrid yarn technologies were developed in recent years to integrate recycled carbon fiber into the high-performance thermoplastic composite. It is unfortunate that these technologies develop composites with a lack of unidirectional fiber orientation; therefore, the potential of recycled carbon fiber in high-performance composites is not thoroughly exhausted. This thesis primarily addresses the development of an innovative structure with a unidirectional fiber orientation termed “unidirectional recycled carbon fiber tape structure” for high-performance thermoplastics composites. The technological concept of the unidirectional structure comprises fiber opening, carding, drawing and a novel tape-forming process. In this concept, fiber opening, carding, and drawing processes were utilized to develop homogeneous, uniform, and highly oriented hybrid slivers. In the next step, these hybrid slivers were converted into a unidirectional recycled carbon fiber tape structure through a novel tape-forming process. To implement this concept, technological developments (investigations, modifications, optimization and further developments), were carried out in fiber opening, carding and drawing processes to develop a hybrid sliver with improved uniformity, homogeneity and unidirectional orientation. In the second phase, conception, design, technological developments, construction and prototype development were implemented to develop a novel tape-forming process. The result confirms that tape development technology comprising fiber opening, carding, drawing and prototype tape forming processes is an innovative, eco-friendly and sustainable technology compared to existing technologies. Furthermore, the consolidation process transformed the unidirectional tape structure into high-performance thermoplastic composites. Subsequently, technology-structure-property relationships were established to develop composites with tailor-made properties. The analysis reveals that selecting optimum technological, consolidation and structural parameters develop tape and composite structures with unidirectional fiber orientation. As a result, experimental results of a high-performance composite developed from a unidirectional recycled carbon fiber tape structure show a very high tensile strength of 1350 ± 28 MPa and an E-module of 84.7 ± 2.3 GPa. This analysis confirms that unidirectional fibers configuration in composites brings a revolution toward developing cost-efficient, high-performance composites for load-bearing structural applications. Finally, theoretical and finite element modeling of tensile properties of high-performance composites reveals that modified models show good agreement with composite tensile properties.

Hybrid carbon fiber reinforced composites are a new breed of materials that are currently being explored and characterized for next generation aerospace applications. Through the introduction of secondary reinforcements, such as alumina nanoparticles, hybrid properties including improved mechanical properties and stress sensing capabilities can be achieved. In order to maximize these properties, it is necessary to achieve a homogeneous dispersion of particulate filler. Utilizing the photoluminescent properties of alumina, it is possible to compare local levels of particle concentration through emission intensities as a way to determine the effectiveness of the tested manufacturing parameters in increasing material homogeneity. Parameters of these photoluminescence emissions have been established to be stress dependent, which allows for in situ residual stress measurements. It is shown here that the application of silane coupling agents as particle surface treatments improves particle dispersion when compared to untreated samples. Reactive silane coupling agent (RSCA) treatments were found to provide for greater dispersion improvements when compared to non-reactive silane coupling agents (NRSCA). Higher resolution investigations into these samples found that treatment with a reactive coupling agent altered the stress state of particles concentrated around the fiber from a tensile stress state to a compressive stress state. This is proposed to result from bonding of the reactive groups on the coupling agent to the organic groups on the carbon fibers which adjusts the stress state of the particle. Future mechanical tests will verify the effects of the particle surface functionalization treatments on mechanical properties of the composites.

This work is devoted to the computational modeling of a lightning strike electric arc discharge induced air plasma and the material response under the lightning strike impact. The simulation of the lightning arc plasma has been performed with Finite element analysis in COMSOL Multiphysics. The plasma is regarded as a continuous medium of a thermally and electrically conductive fluid. The electrode mediums, namely the cathode and anode, have also been included in the simulation in a unified manner, meaning that the plasma and electrode domains are simulated concurrently in one numerical model. The aim is to predict the lightning current density, and the heat flux impinged into the anode's material surface, as well as the lightning arc expansion and pressure and velocity of the plasma flow. Our predictions have been validated by the existing experimental data and other numerical predictions reported by former authors.

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