Carbon Fiber Reinforced Polymer Plates Research Articles

Case Study on Prestressed CFRP Plates Applied for Strengthening Hollow-Section Beam Removed from an Old Bridge.

With the wide application of carbon fiber reinforced polymer (CFRP) plate, used for strengthening existed concrete structures, the prestressing technology of CFRP plate is becoming a hot topic, in order to sufficiently develop its high-strength peculiarity. In this paper, a full-scale hollow-section beam with length of 16 m taken from an old bridge which was in service for about 20 years was first examined for existed cracks and repaired by filling epoxy adhesive, and then the beam was strengthened with prestressed CFRP plates. The CFRP plates were tensioned and fixed with flat-plate anchorages at ends and bonded with adhesive on the bottom surface of the beam. The strengthened beam was experimentally studied using a four-point test to measure the concrete strain along the height of the mid-span section and the mid-span deflection. The finite element model of the strengthened beam was verified by the comparison of test results and used for an extending study of parametric analysis considering the effect of the length and amount of CFRP plates. Results indicated that with an increase in the length and amount of CFRP plates, the mid-span deflection of the beam decreases with the increased cracking resistance and bearing capacity, while the ultimate failure mode transfers from the under-reinforcement to the over-reinforcement.

Influence of elevated temperature on the microhardness and microstructure of carbon fiber reinforced polymers

Carbon fiber-reinforced polymer (CFRP) has attracted attention for the reinforcement and retrofitting of concrete structures. Thus, their degradation in elevated temperatures is important feature to investigate. Herein, influence of elevated temperatures on microstructure and microhardness properties of different types of CFRP was investigated. A series of CFRP plate-, sheet-, and bar-type samples were fired at 150, 250, 350, and 450°C. Crystal structural properties of epoxy resin matrices and carbon fibers in the CFRP composites were examined using X-ray diffraction analysis. An increase in surface defects was observed for the fired CFRP plates and bars; however, there was no significant difference between the microstructures of the fired CFRP sheets. Hardness of CFRP plates fired at 150 and 250°C substantially decreased from 0.67 to 0.62 and 0.47 GPa, respectively, compared to that of unfired CFRP plate. Similarly, hardness of CFRP bars fired at 150, 250, and 350°C decreased from 2.09 to 1.73, 1.63, and 0.62 GPa, which is reduced by 17, 22, and 70%, respectively, compared to that of unfired CFRP bar. Decrease in hardness is attributed to several factors, such as mechanical degradation in the resin matrix, fiber/matrix debonding, and an increase in microcracks on the surface.

Method for Reducing Warping Stresses in Torsionally Loaded I-Section Members using CFRP Plates

This paper presents a method for reducing warping normal stresses in torsionally loaded I-section members by utilizing carbon fiber-reinforced polymer (CFRP) plates. The CFRP plates are bonded to the outer surfaces of the flanges with a view to reducing warping normal stresses. The maximum warping normal stress in the flanges of a steel I-shaped member without the CFRP plates is compared to those obtained with the CFRP plates having various thicknesses. It has been found that the use of CFRP plates bonded to both flanges of the I-section result in a substantial reduction of the warping stresses. For the study presented, the maximum warping normal stress without CFRP plates drops down to merely one-third of its value when 0.625-in. thick CFRP plates are mounted on the outer surfaces of the I-section flanges.

Electrochemical impedance based interfacial monitoring for concrete beams strengthened with CFRP subjected to wetting–drying cycling and sustained loading

Environmental exposure and mechanical loads would cause interfacial degradation between the concrete and carbon fiber reinforced polymers (CFRP) and thus premature failure for concrete structures strengthened with externally bonded CFRP, while reliable monitoring techniques are still in demand. This paper presents an investigation into electromechanical impedance (EMI) based monitoring for concrete beams strengthened with CFRP subjected to wetting–drying (WD) cycling and sustained loading. Concrete beams were fabricated and strengthened in flexure with externally bonded CFRP plates. Strengthened beams were subjected to aging tests (WD cycling and sustained loading) and four-point bending tests, whilst the EMI method was applied to monitor the interfacial performance between CFRP and concrete. WD cycling and sustained loading degrade the interfacial performance and reduce the transferrable force via the CFRP-concrete interface. The EMI signals can reflect the changes in the interfacial performance and show good sensitivity to debonding of CFRP plates under sustained loading and monotonic bending. The main characteristics for estimating whether interfacial damage occurs are proposed based on the EMI response. More importantly, the EMI technique is practical and promising for monitoring the existing strengthened structures even when no EMI history data are available.

A two-step localization method using wavelet packet energy characteristics for low-velocity impacts on composite plate structures

Composite plate structures are prone to damages caused by low-velocity impacts. To locate these impacts, many impact localization methods using fiber Bragg grating (FBG) sensors have been developed. However, it is difficult to achieve the high localization accuracy when impact response signals are acquired by a FBG sensing system with a low sampling frequency in previous studies. In this paper, a two-step localization method using wavelet packet energy characteristics is proposed to address this problem. The proposed impact localization method consists of the collection of impact samples, feature extraction, and two-step localization. To collect impact samples, a FBG sensing system with a sampling frequency of 5 kHz and four FBG sensors is utilized to acquire the response signals induced by the low-velocity impacts on a carbon fiber reinforced plastic (CFRP) plate. Then, a new impact feature, which is sensitive to the low-velocity impacts and can eliminate the influence of the impact height, is defined according to the wavelet packet energy characteristics. Finally, a two-step localization method is designed to achieve the accurate localization for the impacts, in which the first step is the impact area identification based on a feature database and the second step is the impact localization based on the stochastic fractal search (SFS) algorithm. The results of four sets of experiments prove the effectiveness and satisfactory performance of the proposed impact localization method for the low-velocity impact localization on the CFRP plate.

Investigation of the Flexural Behavior of Preloaded and Pre-Cracked Reinforced Concrete Beams Strengthened with CFRP Plates

This paper investigates the flexural behavior of preloaded reinforced concrete (RC) beams, strengthened with Carbon Fiber Reinforced Polymer (CFRP) plates using an experimental program, analytical procedure, and Finite Element Method (FEM) simulation. The RC beams were subjected to preloads of 30%, 50% and 70% of the yielding load, prior to installation of the strengthening system. The eight RC-strengthened beams with a reinforcement configuration of 3Ø12 and two CarboDur S512 plates have been evaluated using bending tests. The failure modes of all the RC-strengthened beams were governed by the widening of flexural cracks within a constant bending zone, followed by debonding of the CFRP plates. The plates were debonding simultaneously or one plate prior to the other plate. The ultimate moment capacity is not significantly reduced while increasing preload levels from 0% to 70%. The moment capacity is increased by 70% to 80% in the CFRP strengthened beams, compared with un-strengthened beams indicating the potential of capacity enhancement that can be attained by externally bonded CFRP.

Reliability Analysis of Steel Bridge Girders Strengthened with CFRP Considering the Debonding of Adhesive Layer

One issue to consider while designing and constructing steel girders reinforced with carbon fiber-reinforced polymer (CFRP) plates in bridges is debonding failure. Previous studies showed that the parameters such as characteristics of material, load, adhesive, and CFRP plates have an effect on the failure probability of steel girder, which is represented by the reliability index. Therefore, this study analyzes the reliability indices of steel girders in bridges strengthened with CFRP plates to clarify the effects of debonding failure. Debonding and strength limit states are used to compare differences in reliability indices of different design scenarios. Strength and debonding margin the functions for the strength limit state and debonding limit state will be established in this study. The probability of failure is determined by a Monte Carlo simulation (MCS). It is found that the reliability index of debonding limit state is much lower than that of the strength limit state. This shows that the debonding failure should be considered significant in the reliability analysis of steel bridge girders strengthened with CFRP plate.

Flexural Strengthening of Large-Scale RC Beams with Nonprestressed and Prestressed CFRP Using Different Anchorages.

Externally bonded carbon-fiber-reinforced polymer (CFRP) technology can be used by different methods based on the anchorage device, CFRP type, and prestressing/nonprestressing. However, a direct comparison between the strengthening efficacies of different methods is still lacking. Seven large-scale RC beams were tested in this study to investigate the influences of the anchorage method, CFRP type, prestress, and prestressing system on the flexural strengthening efficacy of RC beams. The test results showed that the ultimate load increased by 38.3%, whereas the cracking and yielding loads were slightly affected when the anchorage method was enhanced from CFRP U-wraps to wedge-clamp anchors. The CFRP plate and CFRP sheet could provide a rather close flexural strengthening efficacy under the same CFRP strengthening amount. Compared to the nonprestressed CFRP plate, the prestressed CFRP plate was highly superior in improving the flexural behavior of RC beams. The cracking, yielding, and ultimate loads of the prestressed CFRP-strengthened specimens were 57.1%, 22.9%, and 5.9%, respectively, higher than those of the nonprestressed CFRP-strengthened specimen with an effective anchorage. The two types of prestressing systems based on the adhesive-friction anchor and wedge-clamp anchor were proven to be effective for flexural strengthening of RC beams with prestressed CFRP plates, and they could provide almost the same strengthening effect.

Experimental Study on the Flexural Behavior of Large-Scale Reinforced Concrete Beams Strengthened with Prestressed CFRP Plates

Carbon fiber–reinforced polymer (CFRP) systems have been widely used to strengthen concrete structures. To further study the flexural strengthening of concrete beams with prestressed CFRP plates, flexural tests of large-scale reinforced concrete beams were performed, and a tensioning and anchorage system suitable for engineering applications was used in this study. The effects of the prestressing level, strengthening amount, type, and bonding position of the prestressed CFRP plate on the flexural behavior were investigated. The test results showed that the cracking load, yielding load, and ultimate load of all prestressed strengthened beams could reach 2.25–3.38 times, 1.27–1.53 times, and 1.24–1.61 times those of the unstrengthened beam, respectively. The postcracking stiffness, cracking load, and yielding load could be increased by increasing the prestressing level and strengthening the amount of the prestressed CFRP plate. However, a high prestressing level of 50% could lead to a premature intermediate debonding of the CFRP plate before yielding of the steel reinforcements and a reduction in the ultimate load. The use of a low-strength prestressed CFRP plate could achieve a strengthening efficacy similar to that of a high-strength CFRP plate before yielding of the steel reinforcements, whereas the low-strength CFRP plate reduced the ultimate load by 25 kN compared with the high-strength one. The prestressed CFRP plate bonded on the two sides provided a lower flexural strengthening efficacy than that bonded on the bottom of the beam owing to the shorter force arm and the nonuniform stress of the side-bonded CFRP plate.

Reinforcing strategies of CFRP plate for coped steel beams against local buckling

Coped steel beams have been commonly adopted in steel frame structure construction, but the existence of coped regions severely limits the ultimate load carrying capacity and will even lead to unfavorable failure modes. This paper aims to study the reinforcing strategies of carbon fiber reinforced polymer (CFRP) plates for coped steel beams against local web buckling. A total of four single coped steel beams with and without reinforced CFRP plates were fabricated and experimentally tested under different reinforcing strategies. Typical failure modes, the influence of CFRP plate layers, and pasting inclination angles were investigated and discussed. It has been found that the CFRP plate was efficient in improving ultimate load carrying capacities and prohibiting local web buckling of coped beams. The reinforcing effect was more prominent with more layers of CFRP plates, and CFRP plates with inclined pasting angles could also provide higher post-peak ductility compared to horizontal pasting. A finite element model was also established and validated against the experimental results. A subsequent parametric analysis was conducted to quantitively characterize the influence of various reinforcing strategies. The simplified calculation model was proposed based on the experimental and simulation results, which is hoped to provide further insights into the design of singled coped beams in practical engineering.

Fatigue Crack Propagation Prediction of Corroded Steel Plate Strengthened with Carbon Fiber Reinforced Polymer (CFRP) Plates.

The purpose of this study is to investigate the mechanism of improving fatigue performance and the estimation model of fatigue life for corroded steel plate strengthened with CFRP plates. A new two-stage fatigue crack propagation prediction model for the corroded steel plate strengthened with CFRP plates was proposed; moreover, the identification of critical rust pits and the equivalent method of initial cracks, and the calculation method of stress intensity factor (SIF) values at the crack tip were established. The accuracy of the proposed model was verified by comparing the predicted and tested fatigue life of the corroded steel plate strengthened with CFRP plates. Finally, the proposed two-stage crack propagation model was applied to carry out a parameter analysis to investigate the effect of weight loss rate, equivalent initial crack size, adhesive thickness, CFRP stiffness and CFRP prestress level on the fatigue crack propagation of the corroded steel plate strengthened with CFRP plates. Results showed that the maximum depth and the average width of the rust pits were suggested to be taken as the equivalent dimensions of the initial semi-elliptical surface crack for the fatigue crack propagation prediction of corroded steel plate strengthened with CFRP plates. Increasing the weight loss rate of the corroded steel plate, the initial crack size or the adhesive thickness would accelerate the crack growth and reduce the fatigue life, whereas increasing the stiffness or prestress level of the CFRP plate would significantly reduce the crack growth rate and increase the fatigue life. The smaller the initial crack size, the more sensitive the crack propagation life was to the variation of equivalent initial crack size. The influence of adhesive thickness on the fatigue life was limited and convergent, and the application of prestressing could significantly improve the utilization rate of CFRP materials and the fatigue strengthening effect of the corroded steel plate.

A Point Crack Source Location Method without Velocity Information in Anisotropic Plates

Locating cracks in a solid object using acoustic emission (AE) is useful both for detecting defects during safety monitoring and for basic laboratory studies of fractures. We developed an acoustic source location (ASL) method without the use of velocity information with AE in anisotropics plates, such as carbon fiber-reinforced polymers. Assuming that the propagation velocity of an unknown elastic wave is constant in anisotropic materials, the objective function to be minimized is defined based on the elliptic wavefront shape-based technique. The objective function is minimized using an iterative method, such as the gradient descent method. As a result of the numerical experiments and PLB testing on a carbon fiber-reinforced polymer plate, the method is accurate within 5% and is stable against noise.

Improving shear strength of carbon fiber composite honeycombs with the surface microprinting technology

Carbon fiber reinforced plastics (CFRP) honeycomb structures are widespreadly applied in aerospace and aeronautical sectors due to their high specific strength and stiffness. However, the low shear strength of CFRP honeycomb structures limits its further application. In this paper, surface treatment methods for improving surface bonding strength of CFRP plates are studied. The results show that the surface microprinting technology (SMT) is a recommended surface treatment method for CFRP panels and can be used to improve the bonding strength among facesheet-core (between facesheets and cores) and core-core (among the honeycomb walls) of honeycomb sandwich structure. Meanwhile, the mechanical model of CFRP honeycomb structure is established, the theoretical prediction formula of shear strength of CFRP honeycomb structure is explicitly deduced. And further analysis and verification were conducted compared to shear tests. The strengthening mechanism of the surface treatment method is then revealed. The experimental results show that the treatment method of SMT can greatly improve the shear strength of CFRP honeycomb sandwich structure and provide technical database and guidelines for its application in the aerospace and aeronautical fields.

In-plane cyclic behavior of brick walls strengthened with CFRP plates embedded in the horizontal mortar joint

Unreinforced masonry (URM) buildings exhibit significant vulnerability under in-plane seismic loads due to poor tensile strength and integrity. An experiment was carried out to investigate the effectiveness of embedding carbon fiber reinforced polymer (CFRP) plates in the horizontal mortar joint on improving the in-plane seismic performance of URM wall that was prone to fail by diagonal shear failure mode. A total of five half-scale specimens, each measuring 2250 mm × 1500 mm × 240 mm, including two un-strengthened brick walls and three strengthened walls, were tested undergoing pre-compression stress combined with incremental lateral displacement. The CFRP plates were only embedded in the wall on one side in order to reduce the impact on building appearance and different reinforcement ratios were adopted. The failure mode, hysteretic behavior, shear strength, ductility, stiffness degradation and energy dissipation capacity of the specimens were discussed. The results showed that reinforcement can control the initiation and rapid growth of diagonal shear cracks. The strengthened masonry walls exhibited flexural rather than brittle shear failure with good ductility and energy dissipation. The ductility of the strengthened walls increased significantly with the increased reinforcement ratio and there was an optimal reinforcement ratio that maximized the ductility. The strengthened wall with medium reinforcement ratio showed the highest increase in ductility factor of 45.71% compared with the control specimen. The strengthened specimens displayed greater bearing capacity compared with the un-strengthened specimen. The largest increase in the shear strength of the strengthened walls was 10.16%, which indicates the effectiveness of the reinforcement method of used to embed CFRP in the horizontal joint to improve the in-plane seismic performance of URM walls. In addition, the abilities of several analytical models used to predict the shear capacity of the URM and strengthened walls were analyzed.

Fatigue behavior and digital image correlation monitoring of steel plates with mixed-mode edge cracks repaired with CFRP materials

In recent years, carbon fiber reinforced polymer (CFRP) has been widely used to improve the fatigue behavior of steel structures, while most relevant studies were focused on steel components with mode I cracks. This study aims to investigate the fatigue behavior of steel plates with mixed-mode I/II edge cracks patched with CFRP sheets or plates. A total of eleven specimens were fabricated and tested. A digital image correlation (DIC) system was employed as a non-contact measurement to monitor the fatigue crack propagation. Post-processed displacement fields were used to calculate stress intensity factors (SIFs) near the crack tips. Experimental results showed that double-sided CFRP repairing could significantly improve the fatigue life of edge-cracked specimens, especially those with mode I crack rather than mixed-mode I/II crack. Corresponding finite element (FE) analyses were conducted, and good consistency was achieved with test results of crack growth trajectories and fatigue lives. Further FE analysis considered three parameters: load application angle, CFRP layer number, and initial damage level.

A theoretical and experimental study on the pre-tensioning and anchorage of composite materials based on shape memory alloys

Prestressing technology can not only improve the bearing capacity of beam but also improve the utilization rate of carbon fiber-reinforced polymers (CFRPs). However, the existing prestressing and anchorage devices are complicated and have many components, which makes the construction process time-consuming and labor-intensive. In this study, a new tensioning technique based on the shape memory effect of shape memory alloy (SMAs) is proposed to simplify the prestressing process. A theoretical model of SMA-controlled recovery for prestressing is established, and the iterative solution of the theoretical model with multi-variable coupling is obtained. The tension anchorage device used in the new technology is designed based on the proposed theoretical model. Finally, the proposed theoretical model is used to simulate the prestressed CFRP reinforcement of a steel beam, and the SMA actuator manufactured from Ni55.2Ti44.7 is used for pre-tensioning and anchorage of CFRP plate on a steel beam. Further, the size of the prestressed CFRP plate is measured during the tensioning and anchorage process. The results verify that the proposed pre-tensioning and anchorage technique can exert the expected prestress force on the carbon fiber plate, and the accuracy of the established theoretical is also validated by comparing the experimental and theoretical results.

Intermediate Crack Debonding of Externally Bonded FRP Reinforcement-Comparison of Methods.

Many researchers around the world have made extensive efforts to study the phenomenon of fiber-reinforced polymer (FRP) debonding. Based on these efforts, code provisions and various models have been proposed for predicting intermediate crack (IC) debonding failure. The paper presents a comparison of seven selected models: fib bulletin 14 approach, Teng et al. model, Lu model, Seracino et al. model, Said and Wu model, Elsanadedy et al. model and ACI 440. The accuracy of each model was evaluated based on the test results of 58 flexural specimens with IC debonding failures of externally bonded (EB), carbon FRP plates or sheets found in the existing literature. The experimental database was prepared to include a wide range of parameters affecting the issue under consideration. A comparison of the measured and predicted load capacity values was made to evaluate the prediction accuracy of the considered models. The analysis included the limitation of the load capacity estimated based on IC debonding models as well as concrete crushing and FRP rupture types of failure. The results indicate that the latest models proposed for direct implementation in design guidelines—the Said and Wu model and the Elsanadedy et al. model—offer the best accuracy in predicting the load capacity. In contrast, the fib bulletin 14 approach shows a wide dispersion of predictions and a large proportion of highly overestimated results.

Jointing of CFRP/5083 Aluminum Alloy by Induction Brazing: Processing, Connecting Mechanism, and Fatigue Performance

Carbon fiber reinforced polymer (CFRP) is widely used in the lightweight design of high-speed trains due to its high specific strength. In order to further reduce the weight of the high-speed train body, it is necessary to study the joining process and fatigue properties of CFRP/aluminum alloys (CFRP/Al) structure. In this work, the CFRP plate and 5083P-O aluminum plate were successfully connected by an induction brazing method. The optimum parameters of induction brazing were determined to be an induction temperature of 290 °C, a normal pressure of 200 kPa, and a holding time of 5 s. After the 5083 plate was pre-anodized, the tensile strength of the CFRP/5083 joint reached a maximum value of 176.5 MPa. The anodization process introduced more surface micro-structures on the 5083 plate, leading to a better wetting behavior between CFRP and oxide film. Meanwhile, a new chemical bond, Al-O-C, was also formed at the interface of the CFRP/5083 joint. The fatigue limit of the CFRP/5083 joint was calculated to be 71.68 MPa through high-cycle fatigue (HCF) testing. The fatigue cracks initiated from the interface of CFRP/oxide film, and then propagated to base metal. Finally, the oxide film was peeled off from the base metal under shear stress, which contributed to the fracture of the CFRP/5083 joint. The bonding strength between CFRP and 5083 aluminum alloy is far from the conventional welded joints. Therefore, feasible approaches should be proposed to obtain a more robust bonding between CFRP and aluminum alloy in the future.

An experimental study on fatigue debonding growth of RC beams strengthened with prestressed CFRP plates

Externally bonded Fiber Reinforced Polymer (FRP) laminates are increasingly used to strengthen Reinforced Concrete (RC) members. However, FRP debonding remains a major drawback of this strengthening method. To better understand the mechanisms of FRP debonding, six RC beams strengthened with prestressed or non-prestressed Carbon Fiber Reinforced Polymer (CFRP) plates were subjected to static and fatigue loading. CFRP plate debonding was observed in both cases. However, the mechanism of debonding differed: under cyclical fatigue loading, debonding was initiated under both loading points simultaneously and propagated synchronously towards the nearest support whereas in static tests debonding began under a single loading point and progressed suddenly towards its adjacent support. The results also showed that stress redistribution induced coupling between accumulated fatigue damage in the steel reinforcement and fatigue debonding of the CFRP plate, accelerating the fatigue failure of the specimens.

Strengthening RC beams with mid-span supporting prestressed CFRP plates: An experimental investigation

This paper proposes a novel strengthening method for reinforced concrete (RC) beams using prestressed carbon fibre-reinforced polymer (CFRP) plates. In the proposed method, the CFRP plate is stressed by adjusting the position of the mid-span deviators. The strengthening process is fast and easy to apply because there is no need for hydraulic jacks or adhesives. To investigate the behaviour of the strengthened RC beams, a series of tests were conducted on both intact and damaged beams with different strengthening configurations, and the full range behaviours of the beams are reported in detail. According to the test results, the maximum stress of the CFRP plate reached 1500 MPa, and the beam strength was enhanced up to 43%. Some of the strengthening schemes achieved good ductility. As a result, the proposed strengthening method is found to be effective and can be further developed into a pseudoplastic CFRP strengthening system.