Carbon Fiber Reinforced Polymer Plates Research Articles

Experimental study on the bond behavior between CFRP plates and steel substrates under fatigue loading

This paper reports an experimental study on the fatigue behavior of bonded interfaces between carbon fiber reinforced polymer (CFRP) plates and steel plates. A total of 20 single-shear bonded joints were tested under monotonic and fatigue loading. The digital image correlation (DIC) method was employed. The effects of the load ratio, adhesive thickness and adhesive type on the fatigue life were investigated. The failure mode, fatigue life, relative slip, load-slip curve, CFRP strain and interfacial shear stress are presented. The results indicated that the cohesive failure was the dominated failure mode for all specimens. With the increase of the loading cycles, the debond initiated firstly at the loaded end and then propagated gradually toward the free end, which was obviously reflected by CFRP strain and shear stress distributions. The hysteresis was observed in the load-slip curve, indicating the energy dissipation occurred in the interfaces. The fatigue life decreased dramatically with the increase of the load ratio. When the load ratio was the same, the fatigue life decreased with an increasing adhesive thickness, and the fatigue life of the specimen with Sikadur-30 was longer than that with Araldite-2015. However, when the load range was similar, the fatigue life increased with an increase in the adhesive thickness, and the fatigue life of the specimen with Araldite-2015 was longer than that with Sikadur-30. The distribution shapes of the shear stress for two types of the specimens were approximately trapezoidal and triangular under monotonic and fatigue loading.

A comparative study between Fe-SMA and CFRP reinforcements for prestressed strengthening of metallic structures

In the last two decades, carbon fiber reinforced polymer (CFRP) composites have been used for the strengthening of various civil structures. Since recently, iron-based shape memory alloy (Fe-SMA) has been introduced as a promising material, and Fe-SMA reinforcements have been utilized for prestressed strengthening of existing structures. This study aims to conduct a comparative analysis between the structural performance and cost of using Fe-SMA vs. CFRP reinforcements for the prestressed strengthening of existing civil structures. First, a comprehensive study is presented that compares Fe-SMA and CFRP at the material level, with the main focus on the relevant structural aspects, such as the static tensile behavior, high-cycle fatigue (HCF) performance, creep and relaxation, corrosion resistance, thermal compatibility with common construction materials (i.e., steel and concrete), and the behavior under high temperatures and fire exposure. Second, the application complexity and the structural performance of the two strengthening techniques, by using mechanically anchored prestressed CFRPs or thermally activated Fe-SMAs, are compared at the system level by performing sets of static and HCF tests on CFRP and Fe-SMA strengthened steel plates and girders. Finally, a cost comparison is carried out for a specific case, where normal modulus (NM) CFRP plates and Fe-SMA strips were used for the prestressed strengthening of a real-scale steel I-girder. The cost analysis revealed that although the current price of Fe-SMA strips is higher than that of the NM CFRP plates, both of the strengthening systems are financially equivalent, when the achievable prestressing force and the cost of clamping systems are taken into consideration.

Damping of carbon fibre-reinforced plastics plates with holes

ABSTRACTThis paper mainly deals with the specific damping capacity (SDC) of carbon fibre-reinforced plastics (CFRP) plates with holes of different sizes, positions and quantity control. Through establishing a three-dimensional damping matrix and using the finite element analysis (FEA) and Matlab, numerical calculations were conducted on the SDC of CFRP plates with same layering scheme, and a vibration test as well as a damping analysis was implemented on the CFRP test pieces with middle holes. By comparing the calculation and experimental results, good accuracy was found in predicting the modal frequency as well as SDC of the CFRP vibrations through the three-dimensional damping matrix analysis model. By comparing the modal damping of plates with different hole sizes, it could be found that as the hole size increases, the frequency showed a slowly changing tendency. The SDC of the plates with holes could reach 170% of that of the plates without holes.

Tool wear mechanism in low-frequency vibration–assisted drilling of CFRP/Ti stacks and its individual layer

In this paper, a novel tool wear mechanism in low-frequency vibration–assisted drilling (LFVAD) of carbon fiber–reinforced plastic (CFRP)/Ti stacks and its individual layer has been proposed. Firstly, the difference of the effective rake angle, effective clearance angle, undeformed chip thickness, and tool-workpiece contact length between low-frequency vibration–assisted drilling and conventional drilling (CD) was analyzed based on the kinematic model. Then, the strength at different positions of the cutting edge was evaluated. Furthermore, the drilling experiments of CFRP/Ti stacks, CFRP plate, and Ti plate individually were taken under LFVAD and CD processes. Finally, the tool wear mechanism and the benefit of CFRP/Ti stacks and CFRP plate processed by LFVAD were expounded by kinematic analysis results, the analysis of drilling force, SEM images, and the profile of the cutting edge. The results show that the inner edge close to the chisel edge could be the weakest position of the drill in LFVAD. As the mean drilling force of LFVAD is lower than CD, the total wear volume of LFVAD is smaller than CD. Due to the change in the resultant cutting speed angle and the undeformed chip thickness, LFVAD produces the longer tool-workpiece contact length on the rake face and the flank face, which directly leads to a more uniform tool wear and a self-sharpening effect on the cutting edge. Thus, the reduction in the effective rake of the drill used in LFVAD is obviously smaller than the reduction in conventional drilling. The reduction in the effective clearance angle of the drill used in the LFVAD is also less than the reduction in conventional drilling, while this difference is attenuated as the cutting radius increases. The edge radius of the cutting edge is found to be much smaller than CD. In summary, the difference in the cutting edge geometry between LFVAD and CD is the root cause of the tool used in LFVAD to avoid severe edge chipping.

Experimental and analytical study on flexural strengthening of RC beams via prestressed EBROG CFRP plates

The present manuscript reports results obtained from both experimental and analytical studies on the behavior of reinforced concrete (RC) beams strengthened in flexure using prestressed carbon-fiber-reinforced polymer (CFRP) plates via the externally bonded reinforcement on grooves (EBROG) method. A major concern in strengthening beams with prestressed FRP plates is the proper transfer of prestressing force to the beam. The present study pioneers the use of EBROG technique in prestressed CFRP plates considering the proven superiority of the bond efficiency created by the EBROG technique over that by the EBR. For the purposes of this study, seven RC beams with cross-sections of 200 × 300 mm and lengths of 2600 mm were subjected to the four-point loading test in order to investigate the effects of different parameters including prestressing level, using anchorage system, and the strengthening technique employed. Experimental results indicated that the EBROG technique, in the absence of any end anchorage, was well capable of transferring the prestressing stresses to the beam, which obviously led to significant reductions in strengthening costs. Compared to the specimens strengthened through the EBR technique, those strengthened via the EBROG technique exhibited an enhanced flexural behavior as evidenced by their increased load carrying capacity, ductility, and CFRP debonding strain that were higher by 18, 53, and 36%, respectively. It was also observed that prestressing CFRP plates attached via EBROG method was able to postpone the debonding and increase the strength efficiency of the FRP plates. A maximum enhancement of 20% in ultimate load-carrying capacity was measured in prestressed beams strengthened via the EBROG technique relative to the non-prestressed ones.

Composite Strips with U-Shaped CFRP Wrap Anchor Systems for Strengthening Reinforced Concrete Beams

The contribution of carbon fiber reinforced polymer (CFRP) wrap as an anchorage system of CFRP plates, used for flexural strengthening, is investigated. Eighteen concrete beams (100 × 150 × 1400 mm3) were cast and cured for 28 days before strengthening with both CFRP plates (1.2 × 50 mm2) and anchorage-used, U-shaped CFRP wraps. These were adhered on the top of the CFRP plates at different longitudinal and transverse extensions. The mechanical response of the concrete beams was examined under four-point loading including measurements of load versus deflection, bond stress versus slippage and strain in CFRP plates. Crack formation and failure modes were also monitored and characterized. End anchorage regimes, using 300-mm-long U-shaped segmented CFRP wraps, impart the best improvement to load capacity, rigidity, and toughness (up to 80, 57, and 40%, respectively) without harming tangibly ductility in terms of deflection at failure. Anchorage cases involving attaching continuous single or double layers of U-shaped CFRP wrap on the top of CFRP plates impart lower but tangible overall improvements in mechanical performance of CFRP-strengthened beams. A simple empirical model is developed to predict ultimate strain in CFRP plate based on present results before examining using available literature data. Moreover, the efficiency of the present technique is compared to that present in literature.

Piezoelectric Based Lamb Waves Generation and Propagation in Orthotropic CFRP Plates: II. Influence of Interfacial Stress Distribution

This paper investigates the Lamb wave generation by the surface bonded circular piezoelectric (PZT) actuator and wave propagation within the orthotropic Carbon Fiber Reinforced Plastic (CFRP) plate considering the anisotropy of the elastic and damping properties of the materials; existence of the adhesive layer; and dependence of the interfacial stress distribution on the surface between host plate and actuator, on the anisotropy of the plate material, and on the excited frequency, wavelength and plate thickness. This part of our investigation includes FE based study of the shear stress distribution on the interface between circular PZT actuator and surface of orthotropic CFRP plate, and its dependence on the excited wavelength and plate thickness. The anisotropic elastic and damping properties of the plate material, which are used in the implemented finite element (FE) model, have been preliminary determined in the first part of our investigation. We compare the behavior of the wave generation, propagation and attenuation that are studied using this model with the similar dependencies obtained at the simulation of the non-dissipating plate excited by the periodical radially oriented force, which is distributed along the circumference bounding the actuator, i.e. 3D pin-force excitation case. The proposed results can be used at the design of SHM for the composite structures with the structural anisotropy and damping, and at making a reasonable choice of the frequency, type, dimensions and optimum placement of the actuators and sensors.

Experimental study on the bond behavior of the CFRP-steel interface under the freeze–thaw cycles

The bond–slip degradation relationship between carbon fiber-reinforced polymer and steel in a freeze–thaw environment is crucial to evaluate the long-term service performance of steel structures strengthened with carbon fiber-reinforced polymer plates. However, limited studies on the durability and long-term performance of the carbon fiber-reinforced polymer-steel-bonded interface are the major obstacle for the application of carbon fiber-reinforced polymer plates in strengthening steel structures. This paper reports an experimental study to investigate the effects of the carbon fiber-reinforced polymer bond length and the freeze–thaw cycles on the bond behavior of the carbon fiber-reinforced polymer-steel-bonded interface. The three-dimensional digital image correlation technique is applied to obtain displacements and strains on the surface of the single-shear specimen. The experimental results present herein include the failure mode, the ultimate load, the carbon fiber-reinforced polymer strain distribution, the displacement distribution, and the bond–slip relationship. The results show that the ultimate load increases with increasing bond length until a certain bond length value is reached, after which the ultimate load remained approximately constant, and the ultimate loads of carbon fiber-reinforced polymer-steel interface decrease gradually under freeze–thaw cycles. The bond–slip parameters degradation models are proposed, and the bond–slip degradation relationship under the freeze–thaw cycles is established. Finally, the bond–slip degradation relationship is confirmed through comparisons with the experimental results.

Study on Tensile Properties of CFRP Plates under Elevated Temperature Exposure.

Elevated temperature exposure has a negative effect on the performance of the matrix resin in Carbon Fiber Reinforced Plastics (CFRP) plates, whereas limited quantitative research focuses on the deteriorations. Therefore, 30 CFRP specimens were designed and tested under elevated temperatures (10, 30, 50, 70, and 90 °C) to explore the degradations in tensile properties. The effect of temperature on the failure mode, stress-strain curve, tensile strength, elastic modulus and elongation of CFRP plates were investigated. The results showed that elevated temperature exposure significantly changed the failure characteristics. When the exposed temperature increased from 10 °C to 90 °C, the failure mode changed from the global factures in the whole CFRP plate to the successive fractures in carbon fibers. Moreover, with temperatures increasing, tensile strength and elongation of CFRP plates decreases gradually while the elastic modulus shows negligible change. Finally, the results of One-Way Analysis of Variance (ANOVA) show that the degradation of the tensile strength of CFRP plates was due to the impact of elevated temperature exposure, rather than the test error.

Failure mechanisms governing anchoring force of friction-based wedge anchorage for prestressed CFRP plate

An efficient anchorage system is demanded for the Carbon Fiber Reinforced Polymer (CFRP) plates with various sizes in structure reinforcements. The short-term governing failure modes were studied experimentally and numerically in determining the anchoring force of friction-based wedge anchorages to fulfill for all failure criteria for different variables, including the barrel and wedge thickness, length of wedge corner cut, roughness of contact surface, and the pretension load. A series of anchorages were investigated using pull-off tests to obtain the maximum anchoring force and the sequence of failure modes. The anchorage behavior was then simulated by three-dimensional numerical models to attain the maximum anchoring force for each performance criterion under varying conditions. Both the sliding and interlaminar debonding, rather than the tensile rupture, were found to determine the maximum anchoring force of a compact anchorage in the specific range of the parameter values involved. The wide and thin CFRP plate in the anchorage zone was prone to interlaminar debonding and the narrow and thick one tended to sliding or rupture. The parametric studies demonstrated that the CFRP size, the friction coefficient, and the bolt pretension load have significant influences on the maximum anchoring force of the friction-based anchorages.

Mechanical behavior of CFRP on steel surface molded and bonded by vacuum-assisted resin transfer molding technology

This research is aimed to develop repair and reinforcement methods with excellent workability and usefulness for steel structural members that have been damaged through aging. Fiber-reinforced plastic, which has been used in various fields in recent years, is also used for building and civil engineering. Especially, it is assumed that carbon fiber-reinforced plastic (CFRP) has light weight and high-strength mechanical properties. Conventional methods of repair and reinforcement using CFRP are mainly hand-lay-up molding methods using carbon fiber sheet and adhesion methods using CFRP plate. However, both methods have some issues. Therefore, we apply the vacuum-assisted resin transfer molding to repair and reinforce steel structural members; the proposal of a construction procedure and a basic investigation of bond strength were carried out. In the basic investigation, a double-strap tensile shear test and a three-point bending test of steel/CFRP composite specimens were carried out. In the double-strap tensile shear test, in order to improve the adhesiveness of CFRP, the adhesive strength against shear force was compared between the specimen pre-coated with resin and a specimen not coated with resin after surface treatment. The results showed that the pre-coated specimen had high strength and stability. In the three-point bending test, bond strength and fracture mode against bending force were analyzed using adhesion length and number of laminated carbon fiber sheets as parameters. From the finite element method, the stress when peeling occurs due to the bending deformation was estimated and compared with the experimental result.

Debonding Damage Detection in CFRP Plate-Strengthened Steel Beam Using Electromechanical Impedance Technique.

Carbon fiber reinforced polymer materials are widely applied in structure strengthened engineering because of the many advantages of carbon fiber reinforced polymer. However, the debonding damage between the carbon fiber reinforced polymer and host structures occurs frequently, which might lead to the brittle failure of structure components, especially flexural ones. In this paper, an electromechanical impedance-based method, an important technique in structural health monitoring, was adopted to detect the debonding damage of carbon fiber reinforced polymer plate-strengthened steel beam by using lead zirconate titanate (PZT) transducers. A carbon fiber reinforced polymer plate-strengthened steel beam specimen was fabricated in the laboratory and two PZT sensors were attached at different locations on the carbon fiber reinforced polymer plate. The impedance signatures with variation of the different degrees of the debonding damage were measured by an impedance analyzer. The root-mean-square deviation method and the cross-correlation coefficient method were used to quantify the correlation between the electromechanical impedance and the debonding damage degree. The results reflect that an electromechanical impedance-based structural health monitoring technique can serve as a good method to detect the debonding damage of carbon fiber reinforced polymer plate-strengthened steel structures.

Measuring the Two-Dimensional Temperature Profile of Carbon Fiber Reinforced Polymers During Drilling Using Distributed Fiber Sensing

The spatio-temporal evolution of the temperature induced over carbon fiber reinforced polymer (CFRP) laminates during drilling is monitored in real-time using a distributed optical fiber sensor based on optical frequency-domain reflectometry (OFDR). The proposed distributed measurement technique enables the simultaneous monitoring of thousands independent points on a CFRP plate during machining, being of special interest to measure the internal temperature of a workpiece. Experimental results validate the use of distributed OFDR-based sensing for this novel application, demonstrating a precise reconstruction of the two-dimensional (2D) temperature profile around the drilled hole, with a 2 mm spatial resolution and a sampling rate of 23.8 Hz (corresponding to a measurement interval of 42 ms). The high spatial and temporal resolutions provided by OFDR sensing offer unique features for this application, allowing for an accurate identification of the temporal evolution and spatial distribution of the 2D temperature profile originated during drilling. By embedding the sensing optical fiber in the interface between CFRP laminates and metals, a full map of the internal (interlayer) temperature of the CFRP plate can be obtained, demonstrating a feature that cannot be obtained by any other sensing technology. The proposed method can constitute a relevant tool for the identification of potential high temperatures occurring during machining, which could affect the quality of the borehole, induce material defects, and compromise the safety of an entire composite structure in service.

Lamb wave inspection for composite laminates using a combined method of sparse reconstruction and delay-and-sum

A narrowband tone burst excitation is usually required in most Lamb wave inspection techniques in order to reduce the dispersion effect and maintain mode purity, especially in the case that dispersion compensation is hard to implement, such as the scenarios prior knowledge of accurate dispersion curves is not available. The duration of the excitation is relatively large because of its narrow bandwidth, which degrades the resolution for inspection. Following the emerging sparse representation theory, a combined method of sparse reconstruction and delay-and-sum (DAS) for high resolution Lamb wave inspection is proposed in this paper. The scattering signals are compressed in time domain without losing the information of the arrival time of each echo. Unlike most methods that signal decomposition and reconstruction use the same dictionary, the proposed approach utilizes two different dictionaries: one for signal decomposition and another for feature extraction. The DAS imaging method combined with the feature signals is applied to damage imaging for carbon fiber reinforced plastic (CFRP) laminates. The effectiveness of the proposed approach is validated on a quasi-isotropic laminated CFRP plate. The imaging results show that the proposed approach can achieve a higher localization resolution and lower noise floor without degrading the localization accuracy.

Influence of long-term outdoor exposure in a frigid zone on the CFRP-to-concrete bond behavior

The bond strength between a carbon-fiber-reinforced polymer (CFRP) plate or sheet and concrete is susceptible to harsh field environments and plays a key role in the strengthening function of CFRP in the externally bonded strengthening technology. In the present study, the influence of outdoor exposure in northeast China, a frigid zone, on the bond behavior between pultruded CFRP plate and concrete block was investigated using a single-lap shear test. The test results indicated that the exposure caused a change in the cohesive failure of concrete to debonding between the concrete and adhesive, similar to water immersion. The fracture energy, maximum shear bond stress and load capacity of the CFRP-concrete bond joint decreased with the exposure time. An environmental reduction factor was proposed to account for the degradation of the CFRP–concrete bond for the investigated exposure.

Locating Defects in Anisotropic CFRP Plates Using ToF-Based Probability Matrix and Neural Networks

This paper presents two algorithms, both based on the time of flight (ToF) of scattered waves, to locate defect in an anisotropic woven-fabric carbon fiber reinforced polymer (CFRP) plate. The first algorithm uses a probabilistic approach by constructing a probability matrix. Each element of this matrix is associated with a location and represents the probability of existing a defect at the corresponding spatial coordinates of the plate. For the probability matrix method, localization results are influenced by manually chosen parameters and by the anisotropy of the CFRP plate. The second algorithm, based on artificial neural networks (ANNs), enables us to improve the accuracy of locating defects. With this ANN method, the anisotropic feature can be “learned” by the neural network from training data. The ToF of scattered waves obtained from three sensor pairs were used directly as inputs of the neural network. The spatial coordinates of the defect are the ANN outputs. The difficulty of obtaining sufficient experimental data for ANN training was surpassed by using added blocks on the surface of the plate under test to simulate defects. This scheme was validated and proved to be effective by comparing the scattered waves from a delamination and from an added block. Conclusions have been drawn by comparing the two methods. The localization results obtained by the ANN algorithm are proved to be better.

CFRP Impact Damage Inspection Based on Manifold Learning Using Ultrasonic Induced Thermography

Impact damage, caused by low-energy impact, is inevitable during the whole life time of carbon fiber reinforced plastic (CFRP) material. However, the barely visible impact damage (BVID) is difficult to be detected by visual methods. Ultrasonic thermography (UT) is an emerging nondestructive testing technique that visualizes damage in thermal images captured by an infrared (IR) camera when the material is stimulated by ultrasound. However, noise and blurry edges around the high-temperature areas may cause confusion and lead to unreliable results in the thermal images of UT test. In this paper, an impact damage inspection method is proposed based on manifold learning for the CFRP material. Low-power ultrasonic excitation is used for this UT. The IR image sequences are processed as datasets in high-dimensional space. These datasets are reduced to lower dimensions by manifold learning to find the intrinsic structure in the two-dimensional manifold. Each dimension of the embedding manifold correlates highly with one degree of freedom underlying the original pixel: steady and random components. The steady component, which reflects the temperature rise caused by damage, is used for VID and BVID detection. The experimental system was set up, and CFRP plate specimens with different impact damage were tested. All the impact damage could be detected and shown in reconstructed static image with little noise. The proposed method using image sequences could provide a visualized, reliable, and effective impact damage inspection and localization means for CFRP material during manufacturing and in service.

Interfacial bond-slip degradation relationship between CFRP plate and steel plate under freeze-thaw cycles

The bond-slip degradation relationship of a carbon fiber reinforced polymer (CFRP)-steel bonded interface is crucial for predicting the long-term behavior of CFRP-strengthened steel structure under a freeze-thaw (F-T) environment. A total of 16 single-shear bonded joints were tested to investigate the effect of the bond length and the F-T cycles on the CFRP-steel bonded interfaces. The strains on the CFRP surface and the relative slips between the CFRP plate and steel plate were measured by using three-dimensional digital image correlation (3D-DIC) technology. The main properties of the bonded joints were examined, including the failure mode, the ultimate load, the load-slip curve, the CFRP strain distribution and the bond-slip relationship. The degradation models for the bond-slip parameters were formulated by introducing the damage factors, and the interfacial bond-slip degradation relationships were subsequently developed. Finally, the proposed degradation relationship was validated through comparison with the experimental bond-slip relationships and the predicted relationships under different F-T cycles.

Effects of gamma ray irradiation on penetration hole in and fragment size from carbon fiber reinforced composite plates in hypervelocity impacts

This study examines how gamma ray irradiation affects the penetration holes in and fragment sizes from carbon fiber reinforced plastic (CFRP) plates due to hypervelocity impacts. In order to do so, quasi-isotropic CFRP plates made of unidirectional pre-impregnated sheets and 1-mm-diameter spherical projectiles made of aluminum alloy (2017-T4) were used. Witness plates (200 mm × 200 mm × 2 mm) made of copper (C1100P-1/4H) and containing a 30-mm-diameter hole were placed 50 mm in front of and behind each target to examine the fragments based on ISO 11227. The fragments collected from the impact and rear sides of the target were compared. Gamma ray irradiation had a slight effect on the penetration hole, the fragment size distributions, and the craters on the witness plates. When the gamma ray irradiation was 10 MGy, the penetration holes at impact velocities of 2.4 km/s and 5.3 km/s became slightly smaller. At the impact velocity of 2.4 km/s, perforation of the 0.5 MGy, 6.5 MGy, and 10 MGy specimens prevented the projectile from fracturing.

The Effect of Thermal Shock Cycling on Low Velocity Impact Behavior of Carbon Fiber Reinforced Epoxy Composites

In this research, we investigated the effect of temperature variation on low velocity impact response of woven carbon fiber reinforced polymer (CFRP) composites. Carbon fibers were weaved in twill 2/2 type and they reinforced the composite performance. We applied epoxy as composite matrix and then fabricated CFRP plates by using vacuum assisted resin infusion molding (VARIM) method. We performed thermal cycling shock experiment between – 40 and + 120 °C for 20, 40, 60 and 80 cycles. Then, we exposed specimens to the low velocity impact test for various thermal cycling numbers. The results showed that thermal cycling shock can improve or degrade the impact behavior based on post curing and debonding processes. Debonding possibly occurred since there was a coefficient of thermal expansion (CTE) mismatch between composite components. We also observed that the (room temperature tested) RT samples and the samples exposed to 80 cycles between – 40 and + 120 °C had the best and weakest performance, respectively. Finally, the increase of cycle numbers in thermal shock cycling process degraded the composite structure and decreased the impact performance of CFRPs.