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Corrosion of steel reinforcements is the leading causes of malfunction or even failures of reinforced concrete (RC) structures nationwide and worldwide for many decades. This arises up to substantial economic burden on repairs and rehabilitations to maintain and extend their service life of those RC public projects. The inherent natures of glass fiber-reinforced polymers (GFRP) bars, from their superior corrosion resistance to high strength-to-weight ratio, have promoted their acceptance as a viable alternative for steel reinforcement in civil infrastructures. Comprehensive understanding of the bond between GFRP bars and concrete, in particular under in-service conditions or extremely severe events, enables scientists and engineers to provide their proper design, assessment and long-term predictions, and ultimately to implement them toward the corrosion-free concrete products. This research aims to develop a holistic framework through an experimental, analytical and numerical study to gain deep understanding of the bond mechanism, behavior, and its long-term durability under harsh environments. The bond behavior and failure modes of GFRP bar to concrete are investigated through the accelerated aging tests with various environmental conditions, including alkaline and/or saline solutions, freezing-thawing cycles. The damage evolution of the bond is formulated from Damage Mechanics, while detailed procedures using the Arrhenius law and time shift factor approach are developed to predict the long-term bond degradation over time. Besides, the machine learning techniques of the artificial neural network integrated with the genetic algorithm are used for bond strength prediction and anchorage reliability assessment. Clearly, test data allow further calibration and verification of the analytical models and the finite element simulation. Bond damage evolution using the secant modulus of the bond-slip curves could effectively evaluate the interface degradation against slip and further identify critical factors that affect the bond design and assessment under the limit states. Long-term prediction reveals that the moisture content and elevated temperature could impact the material degradation of GFRP bars, thereby affecting their service life. In addition, the new attempt of the Data-to-Information concept using the machine learning techniques could yield valuable insight into the bond strength prediction and anchorage reliability analysis for their applications in RC structures. / ND NASA EPCoR (FAR0023941) / ND NSF EPSCoR (FAR0022364) / US DOT (FAR0025913)

Delamination is currently a largely undetectable form of damage in composite laminate materials. This thesis will develop a method to more easily detect delamination damage within composite materials. Using finite element analysis modeling and lab testing, a new method from interpreting the results obtained from existing structural health monitoring techniques is developed. Lamb waves were introduced and recorded through an actuator and sensors made of piezoelectric material. The data was then analyzed through a novel data reduction method using the Fast Fourier Transform (FFT). Using the data from FFT, the idea of covariance of energy change was developed. By comparing the covariance of energy change in beams with differing delamination size, thickness and depth, correlations were able to be developed. With these correlations, the severity and of damage was able to be detected.

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The concept of employing engineering thermoplastics as toughness modifiers for Bismaleimide resins was utilized to improve the fracture toughness properties of these important materials, which have applications as matrix resins for high performance composites. Modifier molecular weight, end group functionality, backbone structure and weight percent incorporation were all studied with respect to their influence on K_lc, fracture toughness properties. Increases in fracture toughness were created with thermoplastic oligomers without sacrificing high temperature properties and desirable hot-melt processing conditions. Investigations were also made to study the morphological features that develop within these modified thermosets and their resistance to specific environments. In addition, unidirectional carbon fiber composites were prepared and their mode I and II strain energy release rates measured. Respectable increases in the interlaminar fracture toughness were obtained, 15 and 20 percent by weight loadings of maleimide terminated polysulfone modifiers yielded G_lc values of 489±25 and 734±10 J/m² respectively, a substantial improvement over the control value of 359±17 J/m². Laminates were prepared using carbon fibers that had been investigated in terms of their surface energies using Inverse Gas Chromatography. It was illustrated how this technique could distinguish between the acid-base properties of fibers possessing different degrees of proprietary surface treatments. Fiber composites containing both contrasting and subtle changes at the fiber-matrix interphase were prepared and their mechanical properties evaluated using a variety of test methods. Dramatic increases in laminate properties were measured for composites possessing contrasting interphases. Furthermore, the mode II fracture toughness test was sensitive to interphase differences; however, the mode I fracture toughness test was not. Specimens subjected to the new Continuous Ball Indentation test method (meso-indentation) were compared with single fiber micro-indentation test results. Differences were detected in composites prepared using untreated and surface treated fibers. The new method was also sensitive to changes in matrix ductility. Certain anomalies that were noted to be surprising from micro-indentation measurements were not present in the meso-indentation test results. These observations brought to light certain limitations found within the micro-indentation test, but further supported the new test method as a potential technique for fiber-matrix interphase evaluation. / Ph. D.

A detailed study of the feasibility of anchoring fiber reinforced plastic wires for civil engineering applications is presented. An experimental investigation using tensile testing machines is performed, testing anchorages of 1, 7 and 19 wires. Conventional strain gage and acoustic emission measurement techniques are used. The tests are essential in determining the failure load and failure mode. However, the experimental data alone do not provide enough information about the behavior of the anchorage to be used exclusively in the design process. The results are used to modify the design of the anchorage system. It is shown that for a successful anchorage system the choice of material for the load transfer medium is crucial. A solution is presented to overcome the high stress concentration at the load entry area of the wire into the anchor. A finite element analysis of the anchors for 1 and for 19 wires is used to assess the stress and strain fields inside the anchor, to validate the analytical model, and to determine locations of possible high stress concentrations. Three-dimensional and one-dimensional models, that utilize axisymmetry, are evaluated. The results of the numerical analysis are used to demonstrate the improvement as a result of a change in material choice or design of the anchor. It is shown that the modification of the load transfer medium results in a decrease of 30 % of the average stress level. In the analytical investigation, several common models are introduced that describe the fiber pullout behavior. Based on a recent treatment by C. H. Hsueh, a model is developed that describes the anchorage of a wire in a conical shaped socket using orthotropic materials. This model includes boundary conditions that are similar to the ones observed in the experiments. A parametric study is performed to obtain information on the ideal geometry of the anchor system. The results and predictions of the applied techniques, i. e. analytical description, finite element method and experimental investigation, are compared and contrasted. Based on the analytical, numerical and experimental results, recommendations for improving the design of the anchor system are made. Subsequently, a modified anchor system is proposed that utilizes the properties of a load transfer medium that has a variable stiffness. The inclusion of a pure resin collar and supporting wires is suggested. For a successful completion of this project, ideas are proposed and suggestions made for future work. / Ph. D.

Lewis acid/base interactions were shown to play an important role in the optimization of the interfacial adhesion of reinforcing carbon fibers to thermoplastic polymer matrices. Inverse gas chromatography (IGC) and x-ray photoelectron spectroscopy (XPS) were used to characterize the acid/base nature of the carbon fiber surfaces. Capillary column IGC (CIGC) was used to determine the acid/base nature of thermoplastic polymer surfaces. To quantify the non-dispersive (acid/base) interactions, the dispersive component had to be factored out by separate experiments. The carbon fibers (both surface pretreated and untreated) were found to be predominantly acidic while the polymer matrices (polysulfone, polycarbonate, and polyetherimide) exhibited basic properties. Single fiber fracture tests showed that increased acidity in the fiber surfaces (as a result of surface pretreatment) resulted in a significant improvement in the interfacial adhesion to the basic polymers. The acid/base interactions, however, were not solely responsible for the most favorable adhesion. The dispersive component and thus, the carbon fiber structure also played a role. / Master of Science

The need for better impact resistant composites has resulted in the development of many toughened resin systems. A combination of a tougher resin system along with higher strength fibers increases the impact resistance of the composite. The use of an adhesive layer between two plies of the improved prepreg system has been found to considerably increase the impact resistance. This concept is known as "Interleafing." This investigation studies the response of the interleaf materials to instrumented drop weight impact as compared with the response of non-interleaved materials. Two non-destructive quality evaluation techniques, namely, ultrasonics and eddy currents, are used to qualitatively evaluate the damage developed in the specimens. Several different energy levels of damage are studied. The interleaved laminate had significantly better impact response than the non-interleaved laminate for the same impact energy. The onset of delamination was delayed by the use of the interleaf. Whereas damage could be detected at an impact energy as low as 1.75 ft-lb in the baseline laminate; the interleaved laminate did not show any ultrasonic C-scan indication up to an impact of 2.45 ft-lb. The increase of delamination with increasing impact energy was slower in the interleaved specimen. The eddy current method is not effective in detecting damage in the interleaved laminate because of the shielding effect of the interleaf. Compression Strength After Impact (CSAI) could not be used for the test laminates in this project, but the Tensile Strength After Impact test provided useful results. The tensile strength after impact of the interleaved specimen was between 20%-80% more than the baseline laminate up to impact energy of 10 ft-lb. The advantage of the interleaved specimen reduced at higher energy levels of impact. / Master of Science