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External prestressed carbon fibre reinforced polymer (CFRP) straps can be used to strengthen shear deficient reinforced concrete (RC) structures. The strengthening system is associated with a number of parameters including the number of straps, strap locations, strap stiffness, and strap prestress. The initial goal of this research was to identify the optimum values for these parameters in order to design an efficient and effective shear retrofitting system. The shear friction theory (SFT) and modified compression field theory (MCFT) were identified as potential predictive theories to model the shear behaviour of RC beams retrofitted with CFRP straps. Possible modifications to the theories to reflect CFRP prestressed straps were investigated. Two popular optimisation algorithms namely the genetic algorithm (GA) and particle swarm optimisation (PSO) were coded and tested with six test functions. These algorithms were used to find the optimum shear retrofitting configurations and also to reduce the computational cost associated with the SFT and MCFT evaluations. An experimental investigation was carried out to validate the SFT and MCFT predictions for various CFRP strap configurations. The investigation consisted of an unstrengthened control beam and five CFRP strengthened beams. The shear behaviour of the beams was significantly influenced by the CFRP strap configurations. A critical load level where the beam stiffness started to deteriorate significantly was identified. It was found that there was a correlation between this load level and the yielding of the internal shear links and a rapid increase in crack openmg. The SFT and MCFT were validated using the experimental results. The peak shear capacities predicted using the SFT were more consistent with the stiffness deteriorating loads identified in the experimental investigation than with the ultimate loads of the beams. The reinforcement forces and crack opening values found from the SFT were consistent with the experimental results. The MCFT predicted the total shear response, ultimate shear capacity, crack opening, and internal and external reinforcement forces in the beams. The accuracy of the MCFT predictions reduced slightly when either the strap configuration was highly nonuniforrn or the initial prestress level in the straps was relatively low. The shear link yielding load levels predicted by the MCFT were found to be similar to the SFT predictions. By using the coded optimisation algorithms in combination with the SFT or MCFT, the optimum CFRP strap configurations were found for a selected case study. Both theories predicted an offset for the optimum strap locations from the locations associated with equal spacings along the shear span. A reasonable agreement between the SFT and MCFT predictions for the optimum shear strengths and strap locations was observed. A parametric study demonstrated that the concrete strength, internal shear link locations, beam depth, and shear span to depth ratio of the beam do not significantly influence the optimum strengthening configurations for the CFRP strap system. External prestressed carbon fibre reinforced polymer (CFRP) straps can be used to strengthen shear deficient reinforced concrete (RC) structures. The strengthening system is associated with a number of parameters including the number of straps, strap locations, strap stiffness, and strap prestress. The initial goal of this research was to identify the optimum values for these parameters in order to design an efficient and effective shear retrofitting system. The shear friction theory (SFT) and modified compression field theory (MCFT) were identified as potential predictive theories to model the shear behaviour ofRC beams retrofitted with CFRP straps. Possible modifications to the theories to reflect CFRP prestressed straps were investigated. Two popular optimisation algorithms namely the genetic algorithm (GA) and particle swarm optimisation (PSO) were coded and tested with six test functions. These algorithms were used to find the optimum shear retrofitting configurations and also to reduce the computational cost associated with the SFT and MCFT evaluations. An experimental investigation was ca1Tied out to validate the SFT and MCFT predictions for various CFRP strap configurations. The investigation consisted of an unstrengthened control beam and five CFRP strengthened beams. The shear behaviour of the beams was significantly influenced by the CFRP strap configurations. A critical load level where the beam stiffness started to deteriorate significantly was identified. It was found that there was a correlation between this load level and the yielding of the internal shear links and a rapid increase in crack opening. The SFT and MCFT were validated using the experimental results. The peak shear capacities predicted using the SFT were more consistent with the stiffness deteriorating loads identified in the experimental investigation than with the ultimate loads of the beams. The reinforcement forces and crack opening values found from the SFT were consistent with the experimental results. The MCFT predicted the total shear response, ultimate shear capacity, crack opening, and internal and external reinforcement forces in the beams. The accuracy of the MCFT predictions reduced slightly when either the strap configuration was highly nonuniform or the initial prestress level in the straps was relatively low. The shear link yielding load levels predicted by the MCFT were found to be similar to the SFT predictions. By using the coded optimisation algorithms in combination with the SFT or MCFT, the optimum CFRP strap configurations were found for a selected case study. Both theories predicted an offset for the optimum strap locations from the locations associated with equal spacings along the shear span. A reasonable agreement between the SFT and MCFT predictions for the optimum shear strengths and strap locations was observed. A parametric study demonstrated that the concrete strength, internal shear link locations, beam depth, and shear span to depth ratio of the beam do not significantly influence the optimum strengthening configurations for the CFRP strap system.

A variety of retrofitting methods are used to upgrade existing structures. For example, steel plates and Fiber Reinforced Polymer (FRP) jackets are externally bonded to members to increase their capacity in flexure and shear. However, due to the issue of corrosion these strengthening systems may lose their efficiency with time. FRP materials have been used to strengthen many structural components of different shapes and types. FRP jackets, FRP Strips, and FRP rods have commonly been used to rehabilitate existing structural components. The many advantages of using FRP as strengthening materials have made this material an attractive alternative: advantages such as lightweight, high strength, and ease of setting up. Among the many applications using FRP, Near Surface Mounted -- Fiber Reinforced polymer (NSM-FRP) is a promising technique used to strengthen concrete members. However, de-bonding issues have to be overcome to make this technique efficient and reliable. The NSM-FRP technique consists of making a groove along the surface of the concrete member to be retrofitted with depth less than the cover of the member. After cleaning the groove, epoxy paste is used to fill two-thirds of the groove's depth. The FRP element is then mounted in the groove. Finally, the groove is filled with epoxy and the excessive epoxy is leveled with surface of the concrete. This technique makes the FRP material completely covered by epoxy in the cover of the concrete. This method can be used for strengthening both the positive and negative moment regions of girders and slabs. Groove size, paste, concrete, and rods properties are the main variables that control the efficiency of the NSM-FRP rods. The main objective of this research project is to determine the behavior of reinforced concrete beams that are strengthened with NSM-CFRP reinforcement bars. In this research project, the bond characteristics of NSM-CFRP reinforcement bars are first determined from pullout tests. Then, NSM-CFRP rods are installed in reinforced concrete beams and the beams are tested. Loads, strains, and deflections are measured and theoretical and measured capacities are compared. Finally, the reliability and efficiency of using NSM-CFRP rods technique in retrofitting existing structures is observed.

The complexity of the fiber-matrix interphase in a composite is largely due to the myriad of variables (material, processing, and design) that affect its formation. The interphase, thus formed, has to be characterized at several levels (micro-structural, chemical, and mechanical) in order for one to fully understand the nature of the bond between the fiber and matrix and in order to perform a stress analysis of the fiber-interphase-matrix assemblage. A thorough thermo-mechanical characterization of the interphase is difficult, at present, due to the necessity of studying the interphase in situ, its small dimension (usually on the order of a micrometer), and its general complexity. However, a cursory glance at the literature shows that great progress has been made in all of the three levels of characterization mentioned above for various composite systems. Several recent attempts have focused on the physical characterization (evaluation of volume fraction, thickness, Young's modulus, shear modulus and coefficient of thermal expansion) of the interphase. Models of physical properties (thickness, Young's modulus, Poisson's ratio and coefficient of thermal expansion) of the interphase have been considered by several researchers in an effort to study the influence of the interphase on overall composite properties and behavior. Hypotheses on interphase formation and properties have been proposed and tested by some researchers. Both experimental characterization as well as modeling studies are necessary to achieve a more profound understanding of the interphase and its behavior. The interphase, in a composite, is usually modeled as a homogeneous region, despite the fact that it may have spatial property variations.However, it is important to the understanding of composite behavior to incorporate a realistic interphasial region into the analysis and testing of composite material systems. A new thermo-elastic model for the interphase properties in fiber-reinforced thermosets is proposed. The Young’s modulus and coefficient of thermal expansion of the interphase are assumed to be functions of distance from the fiber in this model. The Poisson’s ratio of the interphase is assumed to be the same as that of the matrix. The new model is used in a concentric cylinder assemblage analysis for the determination of the residual thermal stresses in unidirectional fiber-reinforced epoxies. The governing field equations in terms of displacements are solved in “closed form”. It is found that, although the solution is dilute, the property variations in the interphase have a distinct effect on the residual thermal stresses. This is significant, considering the fact that residual thermal stresses play an important role in controlling the structural performance of a composite. The new model is used in Mori-Tanaka analyses for the determination of non-dilute local stress fields in unidirectional fiber-reinforced epoxies under thermo-mechanical loading situations. The governing field equations in terms of displacements are solved in “closed form”. It is found that property variations in the interphase have a distinct effect on the local stresses. This is also significant, considering the fact that local stresses play an important role in controlling the structural performance of a composite. A model composite system consisting of a coated glass rod embedded in Epon 828 is considered; coatings are applied to the glass rod in succession to simulate two different interphase types. The model composite specimens are loaded in transverse compression and transverse shear, and the resulting in-plane displacements are measured by the use of the Moire interferometry technique. Differences in displacement fields between the various specimens, due to the presence of interphasial regions, are found to be minimal. More sensitive measurements are needed to measure pointwise displacements in the interphasial (coatings) region. / Ph. D.

In this work, a novel self-sensing technology is introduced for fiber-reinforced polymer (FRP) composites, offering an accurate and cost-effective solution for damage detection, localization, and fatigue life prediction. This dissertation implements an innovative approach, transforming structural carbon fiber tows into piezoresistive sensors that enable real-time structural health monitoring (SHM) without the need for additional sensor devices. The self-damage detection and memory (SDDM) hybrid composite material leverages the carbon fiber as a sensor network, with glass fiber providing electrical insulation. Damage detection capabilities are first introduced, demonstrated by tensile testing that revealed two distinct loading peaks and a sharp nonlinear increase in resistance at the point of carbon fiber failure, highlighting its capabilities for damage early warning. Progressive impact tests further confirmed the material's ability to permanently record microdamage, showcasing a self-memory function that could inform life-cycle predictions. Next, a practical sensor layout was developed, utilizing carbon fiber sensor tow branches connected in parallel each with varying resistances. This novel design can monitor large areas while minimizing the number of connections required to the DAQ circuit, significantly reducing manufacturing costs and complexities. Impact tests on carbon and glass fiber-reinforced composites validated the system’s ability to detect and precisely locate damage, with less than three percent error between the measured resistance and predicted damage location. These results highlight the effectiveness of the proposed damage localization framework, offering an efficient SHM solution for large-area composite structures. Lastly, this dissertation introduces a low-cost, real-time fatigue life prediction system that leverages the piezoresistive cumulative damage behavior of carbon fiber sensor tows. A Bidirectional Long Short-Term Memory (LSTM) neural network was implemented to predict fatigue life based solely on resistance time-history, with no need for explicit stress inputs. Fatigue tests conducted across various stress amplitudes were used to train and evaluate a LSTM model, with results indicating the model’s ability to accurately predict remaining life. Moreover, testing results showed a sharp increase in resistance before failure, demonstrating the carbon fiber sensor tow's damage early warning capabilities for both cyclic and quasistatic monotonic loading. This system presents a promising, cost-effective SHM method that not only ensures structural safety but also extends the service life of FRP composites through accurate fatigue life prediction.

The friction and wear of injection molded Poly(ether ether ketone) (PEEK) and Poly(ether imide) (PEI), PEEK/PEI blends with the weight compositions of 50/50 %, 70/30 %, and 85/15 %, with and without short carbon fibers were measured in a pin(52100 steel ball)-on-disk(polymer blend) configuration under dry friction. 50/50, 70/30, and 85/15 compositions were annealed to study the effect of crystallinity on wear test. The test variables were sliding speed and normal load. The wear mechanism of pure PEEK matrix was plowing and as the weight percentage of PEI in the blend was increased the wear mechanism changed to the generation of small particles. The wear rates of the unfilled PEEK/PEI blends were found to be a function of not only the blend composition, but also of the normal load, sliding speed and crystallinity in complex manner. However, the coefficient of friction of the unfilled blends did not seem to significantly depend on those testing parameters. When no wear debris was produced, it was below 0.15 otherwise it was ranged from 0.2 to 0.3. The 30 weight % carbon fiber reinforced (CFR) PEEK did produced wear particles but 70/30 and 100 % PEI composites showed reduced wear rates compared to those of unfilled blends. The coefficients of friction of CFR did not seem to be changed from those of the untreated blends except for 100% PEI. Presence of the incubation time before wear particles were produced indicated that the predominant wear mechanism was fatigue. An increase in friction correlated with the generation of wear particles and the formation of a wear groove. / Master of Science

No description available.

Thesis (D.Tech.: Mechanical Engineering)-Dept. of Mechanical Engineering, Durban University of Technology, 2007 xxx, 190 leaves, Annexures A-D / A composite material comprises two or more materials with properties that are superior to those of the individual constituents. Composites have become important engineering materials, especially in the fields of chemical plant, automotive, aerospace and marine engineering. The development of more advanced materials and manufacturing techniques in composites has grown from humble beginnings in the 1930s to a recognized and well-respected engineering discipline, providing solutions to conventional and challenging applications. At present, fibre-reinforced composites (FRCs) are amongst the most common types of composites used. They are produced in various forms with different structural properties, and designers, specifiers and end-users can choose from an almost endless list of these materials, providing design flexibility as well as low manufacturing and maintenance cost. Many suggest that composites have revolutionised the chemical and petro-chemical industries. Examples of applications include tanks and chemical reactor vessels that contains many hundreds of litres of hazardous chemicals, reinforced pipes measuring up to several meters in diameter conveying dangerous gases and so on. The South Africa Coal, Oil and Gas Corporation Limited (SASOL) was established in September 1950. From a small start-up, the company has grown to be a world leader in the commercial production of liquid fuels and chemicals from coal and crude oil. Sasol manufactures more than 200 fuel and chemical products at its main plants in Sasolburg and Secunda in South Africa as well as at several other plants abroad. Its products are exported to more than 90 countries around the world. The use of composites in general, and fibre reinforced composites in particular has received little support in Sasol through the years. Some sporadic use of these materials in the construction of process equipment, e.g. tanks, vessels and piping has taken place with varying degrees of success. While the use of equipment fabricated with fibre-reinforced composites has proven extremely successful in the chlorine producing facility in Sasolburg, catastrophic failures have taken place in Secunda in critical fire water systems made of these materials. The history of correct use and application of fibre-reinforced equipment has shown that the cost of ownership of such equipment is significantly lower than similar metallic equipment, therefore reducing costs and safety risks. However, even though this technology brings a company like Sasol closer to the realisation of the vast number of advantages and solutions offered by these materials, the reality is that most engineering personnel are still applying traditional (viz. steel and wood) technology as used by our predecessors. The work presented here attempts to indicate the relevance of fibre-reinforced composites for Sasol, and to detail efforts aimed at the raising of awareness amongst appropriate personnel at Sasol to increase the use of these materials in major capital projects and day-to-day maintenance contracts, therefore taking advantage of the superior performance of fibre-reinforced composites in demanding applications. In support of this drive, part of the work presented indicates the status as well as progress of the composites industry in the last few years. This project was therefore aimed at identifying the level of utilization of fibre-reinforced composites at Sasol, and the possible improvement in benefits of using these technologies. A methodology was developed, using engineering as well as marketing principles, to reach the engineering personnel in various divisions and seniority levels of Sasol to increase the awareness of the capabilities of composites materials, specifically regarding fibre-reinforced composites. Questionnaires were used to gauge the level of awareness while various methods, e.g. one-on-one meetings, seminars, conferences, electronic media, etc were used to upgrade the target groups’ knowledge. The results of the initial survey to determine the status of various dimensions in the company are indicated as well as the outcomes at the end of the research period. In support of the process in Sasol, the development, interaction and cross-pollination of international and national role-players in the fibre-reinforcement industry with respect to chemical containment and Sasol are indicated. The importance of this two-legged process is demonstrated: it ensures a professional national support framework for companies like Sasol. Results are indicated, compared and discussed to give future direction in this ongoing process. As important to this process was the development of appropriate technical resources (like design standards and codes) to enable their use within the group. It was recognised early on that raising the level of awareness of the target groups was not enough and that these resources had to be in-place down the line so that those who chose to could start to implement these material technologies with the aid of the resources. The development of the necessary resources is also discussed. Finally, it will be shown that significant growth has taken place regarding the awareness within the group over the course of implementation of this project. Specifically, about 20% of the target groups have moved from a stage of no knowledge to higher levels of confidence. In terms of use of these materials, significant growth has also taken place judging by the number of plant requests, activity on major capital projects and so on. In fact, from almost nothing in 1999, over the last 5 years in excess of R137 Million has been spent on capital equipment manufactured from composite materials, with the majority in the last 2 years.

Im Luftfahrtbereich haben sich kohlenstofffaserverstärkte Kunststoffe (CFK) wegen ihrer hohen spezifischen Festigkeiten und Steifigkeiten längst als Konstruktionswerkstoffe etabliert. In der Großserienfertigung von Automobilkarosserien kommt diese Materialgruppe jedoch nur zögerlich zum Einsatz. Offensichtlich sprechen noch viele Argumente für den Einsatz von metallischen Werkstoffen: Denn auch Leichtmetalle und pressgehärtete Stähle ermöglichen immer höhere Leichtbaugrade, ohne dabei signifikante Kostensteigerungen zu generieren. Zudem sind Fertigungs- und Montageabläufe für Metallkarosserien etabliert und weitgehend frei von Entwicklungsrisiken. Vor diesem Hintergrund erscheint es schwer, mit neuen Leichtbaumaterialien und den zugehörigen Bauweisen einen Durchbruch erzielen zu können. Dabei zeigt das Produktsegment der Supersportwagen schon deutlich, dass zusätzliche Leichtbaupotentiale durch beanspruchungsgerecht gestaltete und optimierte CFK-Strukturen für den Automobilbau eröffnet werden. Bislang lassen sich derartig optimierte CFK-Strukturen jedoch kaum wettbewerbsfähig in Großserie realisieren. An dieser Stelle ergeben sich Chancen und zugleich neue Herausforderungen für die Produktentwickler: Zum einen sind Faserverbundbauweisen zu erarbeiten, mit denen die Leichtbaupotentiale von CFK weitgehend ausgereizt werden. Zum anderen ist die automatisierte Fertigung bei hohen Taktraten zu ermöglichen. Die Lösung beider Teilaufgaben setzt den Einsatz geeigneter materialspezifischer Konstruktionsmethoden voraus.

In the Middle East region and many countries in the world, older reinforced concrete (RC) columns are deemed to be weak in seismic resistance because of their low amount of reinforcement, low grades of concrete, and large spacing between the transverse reinforcement. The capacity of older RC columns that are also slender is further reduced due to the secondary moments. Appropriate retrofit techniques can improve the capacity and behavior of concrete members. In this study, externally bonded Carbon Fiber Reinforced Polymer (CFRP) retrofit technique was implemented to improve the behavior of RC columns tested under constant axial load and cyclic lateral load. The study included physical testing of five half-scale slender RC columns, with shear span to depth ratio of 7. Three specimens represented columns in a 2-story, and two specimens represented columns in a 4-story building. All specimens had identical cross sections, reinforcement detail, and concrete strength. Two specimens were control, two specimens were retrofit with CFRP in the lateral direction, and one specimen retrofit in the longitudinal and lateral directions. A computer model was created to predict the lateral load-displacement relations. The experimental results show improvement in the retrofit specimens in strength, ductility, and energy dissipation. The effect of retrofitting technique applied to two full-scale prototype RC buildings, a 2-story and a 4-story building located in two cities in Iraq, Baghdad, and Erbil, was determined using SAP2000.