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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Electrical Impedance Characterization for Damage Detection in Carbon Fiber-Reinforced Polymer (CFRP) Laminated Composites

Almuhammadi, Khaled H. 10 1900 (has links)
The use of modern carbon fiber-reinforced polymer (CFRP) composite materials is becoming increasingly widespread recently. However, the failure modes of such composite structures are extremely complex and, unlike metals, they may suffer significant degradation with barely visible surface damage. Since the damage may cause serious decrease in material strength and lead to catastrophic failure, the development of reliable structural health monitoring techniques is indispensable and has a tremendous impact on the life-cycle cost spent for inspection and repair. Such techniques that are based on the change in the electrical properties of materials are promising and viable approach for maintaining the structural integrity. They are low-cost, fast, effective, and have high potential to be applicable on real structures where they can be monitored online and real-time. The topic of this PhD dissertation is mainly focused on a number of key developments and milestones towards monitoring damage in CFRP laminated composites and making electrical-based methods practical on real structures. One of the major components of these methods is the electrode, which is the interface between the external hardware and the monitored structure. We develop a novel method for surface preparation of composite laminates for better electrode quality using pulsed laser irradiation. Further, we provide a new insight on the anisotropic behavior of the contact impedance for the electrodes on CFRP laminated composites. Another major component for achieving reliable monitoring techniques is the in-depth understanding of impedance response of these materials when subjected to an alternating electrical excitation, information that is only partially available in the literature. For more efficient electrical signal-based inspections, we investigate the electrical impedance spectroscopy response at various frequencies of laminates chosen to be representative of classical layups employed in composite structures. Finally, we use different electrodes configurations on CFRP plates applied to one side mimicking the case of real structures that is undergoing a quasi-static indentation representative of the impact load. We investigate the coupling between the electrical measurements and the type of mechanical degradation using an in-house built electro-mechanical system that measures the change in impedance and phase angle in-situ and real-time.
2

Shear strengthening of reinforced concrete beams with carbon fiber reinforced polymer (CFRP) and improved anchor details

Quinn, Kevin Timothy 03 August 2010 (has links)
Fifteen tests were conducted to evaluate the shear performance of beams with carbon fiber reinforced polymer (CFRP) laminates and CFRP anchors. The specimens consisted of 24-in. deep T-beams. The specimens were strengthened in shear with CFRP laminates that were anchored using several different CFRP end anchorage details. Load was applied to the reinforced concrete members at three different shear span-to-depth ratios. Observations of the behavior and data from the tests were used to evaluate the performance of the CFRP laminates and CFRP anchors. Overall, a 30-40% increase in shear strength was observed when anchored CFRP laminates were installed on members loaded at a shear span-to-depth ratio greater than two. The CFRP strengthening system performed well when properly detailed CFRP anchors were installed. Design recommendations regarding the installation of the CFRP anchors were developed. The CFRP anchorage detail developed in this study provided additional CFRP material in critical locations to reinforce the anchor and prevent premature failures from occurring due to anchor rupture. Theoretical calculations predicting the shear strength of the retrofitted concrete members were carried out and compared to the measured strengths of the members. Based on this analysis, a design equation was developed that produced conservative results for all of the specimens tested. / text
3

Performance of concrete panels strengthened using carbon fiber reinforced polymers (CFRP)

Kim, Chang Hyuk 09 February 2015 (has links)
Many bridges are handling heavier loads than those expected at design, making it increasingly necessary to strengthen existing members or conduct repairs on damaged structural members. Carbon Fiber Reinforced Polymer (CFRP) materials have been broadly used to repair and strengthen reinforced concrete structures. Using CFRP materials as the strengthening material is an excellent solution because of their mechanical properties. CFRP has properties of high strength, corrosion resistance, and light weight. CFRP materials are being widely used for shear and flexural strengthening. Most studies have focused on uni-directional layout of CFRP strips in high shear regions of beams. Recent shear tests on full-scale I-girders have shown that the use of bi-directional CFRP layouts with CFRP anchors led to much higher shear strength increases than when using uni-directional layouts. The objective of the study is to determine the mechanism that governs shear strengthening of bridge girders using bi-directional CFRP and, in doing so, demonstrate the feasibility of using bi-directional CFRP for shear strengthening of large bridge I- and U-beams. Small-scale panel tests have been conducted to investigate parameters that influence the shear strength provided by bi-directional CFRP layouts. Panels were tested under compressive forces to simulate the compression struts that develop in the webs of I-beams. The applied loads generated bottle-shaped compressive struts. CFRP anchors were used to prevent early failure due to CFRP strip delamination from the panel surface. The panels, while not fully reproducing the boundary condition of girder webs, were tested ahead of full-scale girders to investigate a wide range of parameters in a cost-effective manner. The variables considered include the amount of CFRP and steel reinforcement, the inclination of CFRP fibers, and the layout and spacing of CFRP strips. The panel tests provide qualitative comparisons between the influence of the various parameters. The relative strength contributions of CFRP strips, steel stirrups, and concrete were evaluated. / text
4

Behavior of carbon fiber reinforced polymer (CFRP) anchors strengthening reinforced concrete structures

Sun, Wei, 1982- 09 February 2015 (has links)
Carbon Fiber Reinforcement Polymer (CFRP) materials are widely used to strengthen reinforced concrete structures because they are light weight, have high strength, and are relatively easy to install. In strengthening applications, CFRP strips are typically attached to the concrete surface using epoxy resin with fibers oriented in the direction needing additional tensile strength. However, if CFRP strips rely exclusively on bond strength with concrete, only 40% to 50% of the CFRP tensile strength can be developed before debonding occurs. In order to fully develop the tensile strength of CFRP strips, some form of anchorage is needed. CFRP anchors can be applied with relative ease and have recently been shown to provide effective anchorage of CFRP strips to concrete members. In many cases, however, current anchorage details may resulting in fracture or failure of CFRP anchors prior to developing the full strength of CFRP strips. Many design parameters, the effects of which are not well understood, can affect the behavior and strength of CFRP anchors. Moreover, previous studies have demonstrated that the quality of installation can influence anchor strength substantially. The objectives of the study presented are to: 1) provide engineers with design guidelines for CFRP anchors, and 2) deliver a reliable test for controlling the quality of installation and materials of CFRP anchorage systems. In all, 39 tests on 6”×6”×24” rectangular concrete beams were conducted to study the influence of five parameters on CFRP anchor strength and effectiveness: 1) the width of the CFRP strip being developed, 2) the material ratio of CFRP anchor to CFRP strip, 3) the concrete strength, 4) the length/angle of anchor fan, and 5) the bond between CFRP strip and concrete (bonded/unbonded). The same tests also served to develop the test methodology for quality control of the CFRP anchorage system. Based on experimental results, guidelines for designing CFRP anchors are proposed. A test specimen and methodology are also proposed for qualifying CFRP materials and anchorage-system installations. A Finite Element (FE) formulation was selected to provide a computational tool that is suited for simulating the behavior of CFRP strips and CFRP anchors. The ability of the selected FE formulation to reproduce the effects on behavior of varying the anchor-material ratio, concrete strength, length of anchor fan, and bond conditions was investigated. Six FE simulations were built by adjusting simulation parameters and comparing results with six experimental tests. Comparisons between experimental and numerical results indicate that the proposed FE formulation and parameter selections reproduced load-deflection and local strain behaviors with high fidelity. / text
5

Shear strengthening of reinforced concrete beams with bi-directional carbon fiber reinforced polymer (CFRP) strips and CFRP anchors

Alotaibi, Nawaf Khaled 23 September 2014 (has links)
The use of externally bounded Carbon Fiber Reinforced Polymer (CFRP) for strengthening existing RC structures has shown promising results. Although CFRP materials have high tensile strength, the ability to utilize that strength is limited by debonding of the CFRP laminates from the concrete surface. In order to prevent or delay debonding, CFRP anchors were used to provide an alternative means of transferring forces from CFRP strips to the concrete. Previous tests on prestressed I-girders strengthened with uni-directional and bi-directional CFRP strips showed that bi-directional CFRP application resulted in significant shear strength gain in comparison to a uni-directional application. The objective of this thesis is to evaluate the behavior of reinforced concrete beams strengthened in shear with bi-directional CFRP strips and CFRP anchors so that the findings from the previous work can be understood and implemented. Four 24 in. deep T-beams were fabricated at the Phil M. Ferguson Structural Engineering Laboratory at The University of Texas at Austin. Eight tests were conducted on these specimens to examine the effect of the bi-directional layout of CFRP on the shear strength. Specimens with 14-in. web width were selected as a part of the experimental program to allow for direct comparison with test results from the previous project. Additional beams with a web width of 8 in. were included to evaluate thinner webs similar to those in the I-girders. Test results indicate a significant increase in shear strength due to the bi-directional application of CFRP strips with CFRP anchors installed on beams with a shear span-to-depth ratio (a/d) of 3. Substantial shear strength gain up to 62% was achieved in beams with 14-in. webs. and up to 43% for beams with 8-in. webs. However, negligible shear strength gain was observed in beams with a/d of 1.5 (deep beams). Experimental test results demonstrate an interaction between the contribution of concrete, transverse steel and CFRP to the shear resistance of a reinforced concrete beam. The findings of this research contribute to a better understanding of the shear behavior of reinforced concrete members strengthened with externally bonded CFRP applied bi-directionally. Experimental results from this research project provide data needed in the field of CFRP shear strengthening since limited data are available on large-scale tests. / text
6

CFRP prestressed concrete exposed to moisture

Sivanendran, Shobana January 2017 (has links)
In environments of high moisture exposure, carbon fibre-reinforced polymer (CFRP) reinforcement is believed to be a good alternative to steel due to its non-corrosive properties. However the CFRP matrix experiences other types of changes, mechanical deterioration and transverse swelling, with exposure to moisture and sustained stresses. Specifically for structural applications, few studies have investigated the effects of moisture and stress-induced matrix-dominated changes to the CFRP-concrete bond when the CFRP rods are cast into concrete as prestressed reinforcement. Experiments were conducted to measure the transverse swelling and moisture uptake rates of stressed and unstressed CFRP rods immersed in water and concrete pore solution (CPS). CFRP rods were also embedded in concrete and immersed in water to observe whether the transverse swelling of the rods would cause the concrete to crack. Pull-out tests were conducted on stressed and unstressed CFRP rods cast in concrete cubes and immersed in water for up to 125 days. Six CFRP prestressed concrete prisms were fabricated, three were immersed in water for over 125 days and three were left in lab conditions before being tested in three-point bending. Moisture-induced transverse swelling did not appear to cause cracking of the concrete covering CFRP rods in water, contrary to the prediction of the thick-walled cylinder model, but possibly caused an increase in the CFRP-concrete radial contact pressure. The imposition of sustained bending stresses on wet CFRP rods was found to cause premature failure, believed to be a result of matrix softening and microcracking. These observations of swelling and matrix degradation were believed to affect the CFRP-concrete bond. The pull-out tests of unstressed CFRP rods indicated an increased likelihood for bond failure in the CFRP matrix with moisture exposure. This resulted in greater variations in the ultimate and residual bond stresses of the wet samples compared to the dry controls. For the prestressed pull-out samples, higher magnitudes of prestressing resulted in lower bond failure loads. Both outcomes are indicative of matrix weakening with moisture and stress exposure. However despite this, the three-point bending tests revealed no significant differences in behaviour between the wet and dry prestressed prisms. Although the wet prisms exhibited slightly greater variation in their post peak load behaviour compared to the dry prisms, in line with the observations from the pull-out tests. The work from this thesis reveals that despite observations of matrix deterioration and transverse swelling in CFRP rods exposed to moisture and stress, the effect of these changes to the overall behaviour of a CFRP prestressed concrete member are minimal. Any long-term effects are unlikely to cause significant changes to the behaviour of the member. However, the CFRP-concrete bond may be more variable in members exposed to moisture, which should be allowed for in the design process. Therefore CFRP is a promising alternative to steel reinforcement in high moisture environments, with good long-term durability.
7

Fatigue of Masonry Walls with CFRP Applied Externally for Out-of-Plane Loads

Williams, Joseph Louis 01 August 2009 (has links)
This master’s thesis presents an investigation on the effects of fatigue on fiber- reinforced polymers (FRP) when applied to masonry walls subjected to out-of-plane loading. The project aims to provide further research and add to the general testing database of FRP enhanced masonry. An introduction to the problems and solutions associated with unreinforced masonry is discussed along with a literature review on previous testing done in the field of FRP enhanced masonry. The investigation on the effects of fatigue on FRP when applied to masonry walls subjected to out-of-plane loading is performed through experimental testing. A total of four wall specimens (6 ft x 4 ft x 8 in) were constructed. One of the specimens was left unreinforced and used as a baseline for testing while the remaining three specimens were reinforced with carbon fiber reinforced polymer (CFRP) strips designed to take the out-of-plane loads capable of creating the cracking moment in the unreinforced wall. The material testing, construction of the test specimens, and CFRP application are all presented in this thesis. With the use of an oscillating shake table to generate the out-of-plane loading, the walls were fixed at the base and cantilevered from the shake table. By determining the frequency and amplitude that generated the cracking moment in the baseline unreinforced wall, the remaining three CFRP reinforced walls were tested at the same frequency and amplitude. The results from the testing of the three CFRP reinforced walls are presented along with time histories showing the shake table displacement and wall’s tip displacement versus time. In general, fatigue of masonry walls reinforced with CFRP strips can be managed as long as the out-of-plane reinforcement has sufficient strength and development length. Additional findings were made as a result from the testing. The development length of a vertical CFRP strip can be increased by adding a horizontal CFRP strips near the critical section and surface damage to CFRP may have severe consequences.
8

Monotonic and Fatigue Flexural Behaviour of RC Beams Strengthened with Prestressed NSM CFRP Rods

Badawi, Moataz Assad January 2007 (has links)
The use of near surface mounted (NSM) carbon fibre reinforced polymer (CFRP) reinforcement is a recent and a promising technique for increasing the flexural capacity and the fatigue life of reinforced concrete (RC) flexural members. Prestressing the NSM CFRP rod may be utilized for a further enhancement in the monotonic and fatigue flexural response of RC beams. The aim of this study is to investigate the effectiveness of strengthening RC beams with non-prestressed and prestressed CFRP rods to increase the monotonic and fatigue flexural strength of the beams. Twenty-two RC beams were fabricated. Five beams were not strengthened and acted as control to simulate an existing structural member. The other beams were divided into groups that were strengthened with non-prestressed CFRP rod (0% prestressed), and prestressed CFRP rod (40%, or 60% prestressed of the CFRP rod tensile strength). A beam from each group was tested under monotonic load and acted as a reference beam for those tested under cyclic loads. The test results showed that strengthening the RC beams with NSM CFRP rods increased both the monotonic flexural capacity and the fatigue strength. An increase in the yield and ultimate load of 26% and 50% was achieved, when the beams were strengthened with non-prestressed CFRP rod compared to the control beam. Also, the flexural stiffness of the strengthened beam was slightly enhanced by 16% over that of the control beam. When the beams were strengthened with prestressed CFRP rod (40% and 60%), considerable improvements in the cracking, yield, and ultimate loads were achieved as well as the flexural stiffness (serviceability). In a comparison to the control beam, an increase up to 91% in the yield load and 79% in the ultimate load were obtained, in addition to 52.6% improvement in the flexural stiffness (pre-yielding) when a prestressed NSM CFRP rod was applied. A model to predict the flexural behaviour of the beams (control, non-prestressed, 40%, and 60% prestressed strengthened beams) under monotonic loading using section analysis is presented. It includes a model for flexural crack spacing considering the effect of the CFRP reinforcement, and the transfer length model. For an easy use, the monotonic flexural behaviour model is adopted in a computer language (Visual Basic 6). A model based on strain-life approach is also utilized to predict the fatigue life of the beams at various load ranges for all tested RC beams. For a given load range, by obtaining the nominal maximum and minimum stresses using the monotonic flexural model, the fatigue life of a beam is estimated by accounting for the effect of notch (ribs of the reinforcing bars), and the effect of mean stress. In summary, this study presents the first North American experience by using prestressed NSM CFRP rod for strengthening RC beams. Using such high prestressing levels of 40% and 60% with NSM strengthening method is considered the original contribution for monotonic flexural behaviour. Under cyclic loading, investigating the fatigue behaviour and constructing the fatigue life curves for RC beams strengthened with non-prestressed NSM CFRP rod is a considerable contribution to the very limited information available in the literature. This study also includes the inventiveness of testing the fatigue response of the RC beams strengthened with prestressed NSM CFRP rod. A monotonic flexural model of strengthened RC beams with non-prestressed and prestressed NSM CFRP strengthened beams was developed to predict load versus deflection, strain in the concrete, strain in the tension and compression steel reinforcement, and strain in CFRP rod. The model is verified with the experimental results with excellent agreement. A model using strain-life approach was also developed to predict the fatigue life of non-prestressed and prestressed beams with a reasonable accuracy.
9

Monotonic and Fatigue Flexural Behaviour of RC Beams Strengthened with Prestressed NSM CFRP Rods

Badawi, Moataz Assad January 2007 (has links)
The use of near surface mounted (NSM) carbon fibre reinforced polymer (CFRP) reinforcement is a recent and a promising technique for increasing the flexural capacity and the fatigue life of reinforced concrete (RC) flexural members. Prestressing the NSM CFRP rod may be utilized for a further enhancement in the monotonic and fatigue flexural response of RC beams. The aim of this study is to investigate the effectiveness of strengthening RC beams with non-prestressed and prestressed CFRP rods to increase the monotonic and fatigue flexural strength of the beams. Twenty-two RC beams were fabricated. Five beams were not strengthened and acted as control to simulate an existing structural member. The other beams were divided into groups that were strengthened with non-prestressed CFRP rod (0% prestressed), and prestressed CFRP rod (40%, or 60% prestressed of the CFRP rod tensile strength). A beam from each group was tested under monotonic load and acted as a reference beam for those tested under cyclic loads. The test results showed that strengthening the RC beams with NSM CFRP rods increased both the monotonic flexural capacity and the fatigue strength. An increase in the yield and ultimate load of 26% and 50% was achieved, when the beams were strengthened with non-prestressed CFRP rod compared to the control beam. Also, the flexural stiffness of the strengthened beam was slightly enhanced by 16% over that of the control beam. When the beams were strengthened with prestressed CFRP rod (40% and 60%), considerable improvements in the cracking, yield, and ultimate loads were achieved as well as the flexural stiffness (serviceability). In a comparison to the control beam, an increase up to 91% in the yield load and 79% in the ultimate load were obtained, in addition to 52.6% improvement in the flexural stiffness (pre-yielding) when a prestressed NSM CFRP rod was applied. A model to predict the flexural behaviour of the beams (control, non-prestressed, 40%, and 60% prestressed strengthened beams) under monotonic loading using section analysis is presented. It includes a model for flexural crack spacing considering the effect of the CFRP reinforcement, and the transfer length model. For an easy use, the monotonic flexural behaviour model is adopted in a computer language (Visual Basic 6). A model based on strain-life approach is also utilized to predict the fatigue life of the beams at various load ranges for all tested RC beams. For a given load range, by obtaining the nominal maximum and minimum stresses using the monotonic flexural model, the fatigue life of a beam is estimated by accounting for the effect of notch (ribs of the reinforcing bars), and the effect of mean stress. In summary, this study presents the first North American experience by using prestressed NSM CFRP rod for strengthening RC beams. Using such high prestressing levels of 40% and 60% with NSM strengthening method is considered the original contribution for monotonic flexural behaviour. Under cyclic loading, investigating the fatigue behaviour and constructing the fatigue life curves for RC beams strengthened with non-prestressed NSM CFRP rod is a considerable contribution to the very limited information available in the literature. This study also includes the inventiveness of testing the fatigue response of the RC beams strengthened with prestressed NSM CFRP rod. A monotonic flexural model of strengthened RC beams with non-prestressed and prestressed NSM CFRP strengthened beams was developed to predict load versus deflection, strain in the concrete, strain in the tension and compression steel reinforcement, and strain in CFRP rod. The model is verified with the experimental results with excellent agreement. A model using strain-life approach was also developed to predict the fatigue life of non-prestressed and prestressed beams with a reasonable accuracy.
10

Design and Impact Effect Analysis of CFRP Golf Head

Hsieh, Wen-long 28 January 2008 (has links)
Abstract With the innovation of materials, many producers verify their golf heads. To improve the energy transmission and to keep stability while batting, the researcher tried to design golf head with a composite material and enlarge sweet spot zone. In addition, the researcher utilized the CAD software to construct and analyze data. He also applied the finite element of LS-DYNA software to simulate the impact question of a golf club head. Let¡¦s understand the change of geometry, impact effects and stresses of golf head by modifying the thickness of head. From the result of simulation, the CFRP impact face of a golf head has better velocity of a golf ball after impact than the Ti impact face. When the CFRP thickness of an impact face is 2.4~ 2.6 mm and the initial velocity is 45 m/s, the speed of the ball is 68.4~67.8 m/s and the angular velocity is about 2860~2570 rpm after impact. In a vertical direction, the zone of sweet spot is about ¡V30~25mm distance from the center of golf head face, in horizontal direction about -15~25mm. In the same basis of ball speed, when the face of golf head is CFRP, the zone of sweet spot will be wider than the Ti face. When the impact velocity is 55 m/s, the thickness of face about 2.6~ 2.8 mm is better, in 35 m/s, the thickness of face about 2.2~ 2.4mm is also better.

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