This research program investigates the performance of a steel truss bridge when subjected to both localized web damage and a subsequent post-tensioned strengthening approach. The investigation utilizes a combined approach involving an experimental scale model bridge and a numerical computer model generated using the commercial finite element software RISA 3-D. The numerical model is validated using test data and further extended to parametric studies in order to investigate the theoretical load rating, strain energy, load redistribution, mode shapes and frequency of the bridge for control, damaged and strengthened states. The presence and severity of damage are found to significantly influence the global safety and reliability of the bridge. Also, higher order modes are more susceptible to changes in shape and frequency in the presence of damage. A recovery of truss deflection and a reduction of member forces are achieved by the proposed strengthening method.

The role of the interphase in fiberglass reinforced composites was studied by a combination of theoretical analysis, mechanical tests, and several high-resolution analytical techniques. The interphase was varied in composition by using epoxy and polyester matrix polymers with and without added coupling agents, as well as four fiber surface modifications. Different coupling agents on the fibers were shown to change the fiber tensile strength markedly. Filament wound unidirectional composites were tested in short beam "shear." Corresponding samples were fabricated by embedding one to seven fibers in the center of polymer dogbone specimens that were tested in tension to determine critical fiber lengths. Those values were used in a new theoretical treatment (that combines stress gradient shear-lag theory with Weibull statistics) to evaluate "interfacial shear strengths". The fact that results did not correlate with the short beam data was examined in detail via a combination of polarized light microscopy, electron microscopy (SEM) and spectroscopy (XPS or ESCA) and mass spectroscopy (SIMS). When the single fiber specimens were unloaded, a residual birefringent zone was measured and correlated with composite properties, as well as with SIMS and SEM analysis that identified changes in the locus of interphase failure. Variations in the interphase had dramatic effects upon composite properties, but it appears ·that there may be an optimum level of fiber-matrix adhesion depending upon the properties of both fiber and matrix. Fiber-fiber interactions were elucidated by combining tensile tests on multiple fiber dogbone specimens with high-resolution analytical techniques. In general, this work exemplifies a multidisciplinary approach that promises to help understand and characterize the structure and properties of the fiber-matrix interphase, and to optimize the properties of composite materials. / Master of Science

A lightweight carbon fiber reinforced polymer (CFRP) c-channel beam was previously designed using analytical theory and finite element analysis and subsequently manufactured through a pultrusion process. Physical testing revealed the prototype did not meet the bending and torsional stiffness of the beam model. An investigation revealed that the manufactured prototype had lower fiber content than designed, compacted geometry, an altered ply layup, missing plies, and ply folds. Incorporating these changes into the beam model significantly improved model-experiment agreement. Using what was learned from the initial prototype, several new beam designs were modeled that compare the cost per weight-savings of different composite materials. The results of these models show that fiberglass is not a viable alternative to CFRP when designing for equivalent stiffness. Standard modulus carbon was shown to have slightly lower cost per-weight savings than intermediate modulus carbon, although intermediate modulus carbon saves more weight overall. Core materials, despite potential weight savings, were ruled out as they do not have the crush resistance to handle the likely clamp loads of any attaching bolts. Despite determining the ideal materials, the manufactured cost per weight-savings of the best CFRP beam design was about double the desired target. / Graduation date: 2013 / Access restricted to the OSU Community at author's request from Oct. 5, 2012 - Oct. 5, 2014

When using Finite Element Analysis (FEA) to model notched composite panels, the values of certain material properties can have a great effect on the outcome of the simulation. Progressive damage modeling is used to model how a composite structure will fail, and how that failure will affect the response of the structure. Many different progressive damage models exist, but the formulation known as Hashin damage is used to model failure in tension and out-of-plane bending in this study. This model has ten different material properties that are used to define the damage response of the material. Each of these material properties must be calculated experimentally in a time consuming and expensive process. A method of determining which properties will have the greatest effect on the model, and therefore, which to spend the most money on accurate tests, is a factorial analysis sensitivity study. Studies of this nature have been used in many different situations regarding material properties testing and optimization. The work presented in this study uses several factorial analysis designs to perform a sensitivity study on the ten Hashin damage parameters in a variety of situations. Five different ply layups are used in modeling specimens that are loaded in tension and out-of-plane bending. The results of this study show that the significant factors depend on the ply layup and loading scenario, but there are generally less than three factors that play a significant role in modeling the failure of the panels. This means that in most cases, rather than spending substantial money on finding ten different material properties, the time and money can be focused on a small subset of the properties, and an accurate model can still be achieved. While the results of the scenarios presented may not apply to all scenarios, the methods presented can be used to perform a similar study in other specific scenarios to find the significant factors for that case. / Graduation date: 2013

Submitted in fulfillment of the requirements of the Degree of M.Tech.: Mechanical Engineering, Durban University of Technology, 2017. / Fatigue crack initiation and propagation behaviour of CFRP have been of great importance because such composites are often used in engineering components that are subjected to continuous cyclic loading. The objective of this thesis work was to investigate the damage characteristics of the fatigue properties of CFRP composites by the modification of the polymer matrix with nanoclay addition. Carbon fibre reinforced epoxy was produced via vacuum assisted resin infusion moulding method (VARIM) with nanoclay concentrations of 0wt%, 1wt%, 3wt% and 5wt%. Tension-tension fatigue tests were conducted at loading levels of 90%, 75% and 60%. The frequency that was used was 3Hz with R value of 0.1. The results showed that at nanoclay percentages of 0wt%, 1wt% and 3wt% there was a consistent trend, where the number of cycles increased in fatigue loading percentages of 90%, 75% and 60%. At 5wt% nanoclay percentage the number of fatigue cycles dropped significantly at the 90% fatigue loading. The brittle nature of the 5wt% laminate became dominate and the sample fractured early at low fatigue cycle numbers. At the 75% fatigue loading, the number of cycles increased and at 60% fatigue loading the 5wt% nanoclay sample exceeded the number of cycles of all the nanoclay percentages by 194%. This was due to the intercalated arrangement of the nanoclays favouring the slow rate of surface temperature increase, during fatigue testing, at low fatigue cycle loading. The Crack Density analysis was performed and showed that at the same time in the fatigue cycle life, the 1wt% had 55 cracks, 3wt% had 52 cracks and the 5wt% had 50 cracks, for the 60% fatigue loading. This proved that it took longer for the cracks to initiate and propagate through the sample as the nanoclay percentage increased. Impact and hardness testing showed that the 5wt% exhibited brittle behaviour, which contributed to the results above. Scanning electron microscopy examination highlighted that the agglomeration of nanoclays delayed the crack initiation and propagation through the specimen and that the extent of fatigue damage decreased as the nanoclay percentage increased. A fatigue failure matrix was developed and showed that delamination, fibre breakage and matrix failure were the predominate causes for the fatigue failure. / M

Use of fiber reinforced polymer (FRP) material has been a good solution for many problems in many fields. FRP is available in different types (carbon and glass) and shapes (sheets, rods, and laminates). Civil engineers have used this material to overcome the weakness of concrete members that may have been caused by substandard design or due to changes in the load distribution or to correct the weakness of concrete structures over time specially those subjected to hostile weather conditions. The attachment of FRP material to concrete surfaces to promote the function of the concrete members within the frame system is called Externally Bonded Fiber Reinforced Polymer Systems. Another common way to use the FRP is called Near Surface Mounted (NSM) whereby the material is inserted into the concrete members through grooves within the concrete cover. Concrete beam-column joints designed and constructed before 1970s were characterized by weak column-strong beam. Lack of transverse reinforcement within the joint reign, hence lack of ductility in the joints, and weak concrete could be one of the main reasons that many concrete buildings failed during earthquakes around the world. A technique was used in the present work to compensate for the lack of transverse reinforcement in the beam-column joint by using the carbon fiber reinforced polymer (CFRP) sheets as an Externally Bonded Fiber Reinforced Polymer System in order to retrofit the joint region, and to transfer the failure to the concrete beams. Six specimens in one third scale were designed, constructed, and tested. The proposed retrofitting technique proved to be very effective in improving the behavior of non-ductile beam-column joints, and to change the final mode of failure. The comparison between beam-column joints before and after retrofitting is presented in this study as exhibited by load versus deflection, load versus CFRP strain, energy dissipation, and ductility.

The demand for materials with high mechanical performances such as Carbon Fiber Reinforced Plastics (CFRP) is increasing. However, there are major challenges in machining CFRP as it involves delamination, fiber pullouts, and extreme cutting tool wear. Analysis of chip formation mechanisms and prediction of associated cutting forces in CFRP machining enables one to address these challenges. This study proposes a mechanistic cutting force model for milling operations of the CFRP workpiece, considering its non-homogeneity and anisotropy, by taking into account variations of fiber cutting angle during machining. A mechanistic model of cutting force constants is obtained from a number of experimentally measured unidirectional CFRP milling forces. The obtained mechanistic force model predictions are verified against experimentally measured milling forces with arbitrary tool path indicating the accuracy of the proposed mechanistic model in predicting cutting forces. The proposed mechanistic cutting force model is capable of being integrated into the manufacturing process to allow optimized machining of quality certified CFRP work-pieces. / Graduate

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Vita. / Includes bibliographical references. / by Kwai-Yung Benjamin Su. / Sc.D.

Thesis (Ph. D.)--University of Nevada, Reno, 2006. / "May, 2006." Includes bibliographical references (leaves 105-107). Online version available on the World Wide Web.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2006. / Includes bibliographical references (leaves 126-133). Also available in electronic version.