Carbon Fiber Reinforced Polymer Plates Research Articles

Comparative investigations on machinability and surface integrity of CFRP plate by picosecond laser vs laser induced plasma micro-drilling

Carbon fiber reinforced plastic (CFRP) is being widely used due to its excellent mechanical properties and corrosion resistance. However, CFRP with the characteristics of anisotropy and inhomogeneous result in difficulty to drill micro-holes. Traditional methods are used to drill micro-holes resulting in poor performance, such as fiber pull-out, tool wear, and bad processed surface quality. Laser machining has great potential on drilling CFRP micro-holes without tool wear and fiber pull-out, but leads to large taper and recast layer. In this study, laser induced plasma micro-drilling (LIPMD) was proposed to reduce the taper as well as improve the processed surface integrity. A comprehensive analysis processed sidewall surface quality, taper, roundness and surface integrity were conducted, and LIPMD process showed more advantages over laser in air (LIA). It is concluded that the processed sidewall machined by the LIPMD has less epoxy recast layer, and the surface of fiber is smoother than that of in the LIA process, diameter difference between the top and bottom of micro-holes is reduced by about 24.83%-32.02%, and the taper is reduced by 32.02%-47.44% compared with the LIA process. This investigation demonstrates the applicability of the LIPMD as a potential process for improved machinability and surface integrity of CFRP material.

K‐Means Clustering for Prediction of Tensile Properties in Carbon Fiber‐Reinforced Polymer Composites

The application of computer algorithms to identify patterns in data is referred to as machine learning. The algorithms are used to learn complex relationships and build models for various predictions. Herein, the k‐means method is used, one of the unsupervised learning methods in machine learning, to predict Young's modulus and ultimate tensile strength (UTS) of carbon‐fiber‐reinforced polymers (CFRPs), and their experimental Young's modulus and UTS values are compared. The k‐means method categorizes CFRP into four colors: carbon fiber, epoxy resin matrix, defects, and contamination. The prediction of Young's modulus and UTS of CFRP with different porosities and carbon fiber orientation demonstrates the effectiveness of the k‐means method. Furthermore, the experimental values of Young's modulus and UTS of commercial CFRP plate are closer to the predicted values than the catalog values. These results suggest that the k‐means method can predict Young's modulus and UTS of CFRP accurately, instantly, and automatically. The k‐means method is promising as a new technique to accurately and instantly understand the mechanical and physical properties of CFRPs without any material test.

Long-term behavior of CFRP plates under sustained loads

This paper studies the long-term behavior of carbon fiber–reinforced polymer (CFRP) plates under sustained loads. Two typical loading conditions, CFRP plate loading controlled by displacement constraints and force constraints, are studied. The first section investigates the prestress loss of the CFRP plate when it is loaded via displacement constraints, which serves as a prestressed system applied for prestressed structures. Short- and long-term measurements show that the prestress loss of CFRP is caused by material creep, temperature change, and CFRP and anchorage slippage. When the CFRP prestress level is 5.0%–12.3%, the average prestress loss after 42 days is 5.1%. The second section investigates the permanent deformation of the CFRP plate with curved anchorages caused by creep under a certain sustained load (i.e., controlled by force constraints), which serves as a suspension bar or cable applied for determinate structures. The creep strain-to-time relationship, creep rate, and residual strength were obtained. The creep strain-to-time relationship strongly depends on the stress level applied; the creep coefficient is less than 10.94% and the residual strength is greater than 80% of the initial tensile strength after 1000 h. The findings show the effectiveness and reliability of these two typical cases of CFRP plates under long-term sustained loads.

Edge illumination X-ray phase contrast imaging for impact damage detection in CFRP

Carbon fibre reinforced polymer (CFRP) structures are widely used in the aerospace industry for their low weight and strong mechanical properties. However, they are prone to impact damage due to their brittle nature. Edge Illumination X-ray Phase Contrast imaging (EI XPCi), a novel, phase-based X-ray imaging technique, was compared against two established non-destructive evaluation methods for impact damage detection in CFRP plates. Planar EI XPCi was shown to allow the detection of a larger extent and more details of damage than both conventional X-ray imaging using contrast agents and ultrasonic immersion C-scans. However, both X-ray methods were unable to detect a delamination present in the plate, which was only observed from the ultrasonic C-scan. Planar EI XPCI has potential for rapid in-process scanning, where only 2D projection imaging is possible.

Fatigue behavior of notched steel beams strengthened by a prestressed CFRP plate subjected to wetting/drying cycles

Prestressed carbon fiber reinforced polymer (CFRP) strengthening is effective in suppressing crack propagation of steel beams. However, the fatigue behavior of notched steel beams with prestressed CFRP under a hygrothermal environment is not fully understood. Generally, notch machining on the steel beam is an ordinary method to simulate a fatigue defect for laboratory tests. In this study, the interfacial energy release rate at the notch location was first derived for strengthened steel beams. The theoretical results show that the interfacial energy release rate can be significantly reduced by prestressed CFRP strengthening, contributing to improving the interfacial fatigue performance. In addition, notched steel beams strengthened by prestressed CFRP (with a prestress level of 25% of tensile strength of CFRP plate) were exposed to wetting/drying cycles (WDCs) and then tested under fatigue loading. The experimental results show that prestressed CFRP strengthening increases the fatigue resistance of interfacial debonding initiation and failure 3.47 times and 7.83 times, respectively, which agrees with the tendency revealed by the theoretical results. In contrast, WDC exposure reduced the fatigue lives of interfacial debonding initiation and failure by 34.6% and 27.2%, respectively. An S – N curve based on the maximum principal interfacial stress at the notch location was proposed to predict the tendency of fatigue life of debonding initiation for strengthened steel beams. Moreover, the tendency prediction for fatigue life of interfacial debonding development for the strengthened steel beams with/without WDC exposure was developed based on Paris’ law. • Theoretically reveals that the prestressed CFRP reduces the interfacial energy release rate. • Prestressed CFRP strengthening significantly improves interfacial fatigue life. • Hygrothermal exposure leads to a significant reduction of interfacial fatigue life.

Impact Damage Detection Using Chirp Ultrasonic Guided Waves for Development of Health Monitoring System for CFRP Mobility Structures.

When impact damage occurs in carbon fiber-reinforced plastic (CFRP) structures, it is barely visible but may cause significant degradation in the mechanical properties of the structure. Hence, a structural health monitoring (SHM) system that can be installed in CFRP mobility structures and is sensitive to impact damage is needed. In this study, we attempted to establish an SHM system based on ultrasonic guided waves, which are generated by inputting a broadband chirp signal into a film-like piezoelectric actuator. The relationship between impact damage size and maximum time-of-flight (ToF) delay was investigated for three types of CFRP plates: woven, non-woven, and hybrid laminates. As a result, it was found that the maximum ToF delay increased linearly with an increase in the damage size for all CFRP laminates. Moreover, the amplitude of the A0 mode was found to be significantly affected by the damage length in the wave propagation direction. Thus, this SHM method using chirp ultrasonic waves can quantitatively evaluate the size and extent of the impact damage in CFRP laminates.

Semi-Analytical Finite Element Method for calculating dispersion curves of a CFRP plate

Composite Materials are widely used thanks to their mechanical characteristics. Non destructive evaluation requires knowing the exact dispersion curves to determine the propagative waves and to resolve the phase velocity of symmetrical and antisymmetrical modes. The aim of this paper is to plot the dispersion curves of a Carbon Fibre Reinforced Polymer plate using Semi Analytical Finite Element Method algorithm. This method combines the analytical expression of the displacement and the finite element method procedure. The resultant advantage is both simplicity and rapidity. The obtained results showed that the method accuracy depends on the elements number of meshing. To ensure good precision and fast computation of the method, the elements number and the order of the interpolation functions must be optimized.

Fatigue Performance of CFRP‐Strengthened Rib‐to‐Diaphragm Welded Details of Orthotropic Steel Decks: Experimental and Numerical Evaluation

Rib‐to‐diaphragm welded details of orthotropic steel decks (OSDs) are susceptible to fatigue cracking. To deal with such cracking problem, a strengthening method using externally bonded carbon fiber‐reinforced polymer (CFRP) plates was investigated by fatigue experiment and numerical evaluation. A full‐scale OSD specimen with two ribs and two diaphragms was tested to evaluate fatigue cracking behavior, as well as the stress before and after strengthening. An experimentally validated numerical model was established to confirm the optimal opportunity for CFRP strengthening. The influence of different CFRP strengthening measures were then discussed on the stress intensity factors at crack tips. It was shown that bonding of CFRP plates can significantly reduce stress of rib‐to‐diaphragm welded details. The best opportunity for CFRP strengthening is selected as the length of crack reaches at 25.00 mm where the amplitude of stress intensity factor is the smallest. The attachment of CFRP plates by bonding on the surface of cracked plates can reduce stress response at cracked region by more than 30%. The length of crack is of no flat growth until the total number of loading cycle after strengthening arrived at 0.8 million. Results of SIFs calculated by numerical model dropes after strengthening with reduction rate more than 35% and 18% at two crack tips and crack midpoint, respectively. Compared to the geometric dimensions of CFRP, the elastic modulus of CFRP and glue layer shows more significant influence on the fatigue performance of Rib‐to‐diaphragm welded details due to their stronger restraint to the propagation of crack peaks. This study provided a technical reference to the design and actual application of CFRP in the strengthening of welded details in OSDs.

CRACK ANALYSIS OF CFRP REINFORCED CONCRETE BEAMS UNDER SECONDARY LOADING

The paper established the calculation formulas on the average crack spacing and the maximum crack width of CFRP（Carbon Fiber Reinforced Polymer）reinforced concrete beam under the secondary loading. Conversion of CFRP plate area into the reinforcement ratio of the reinforced beam, the calculation formula on the average crack spacing of CFRP reinforced concrete beam under the secondary loading was established. On basis of the calculation formula on the maximum crack width of concrete beam, the calculation formula on the maximum crack width of CFRP reinforced concrete beam under the secondary loading was established. The average crack spacing and the maximum crack width calculated by the formulas in the paper were compared with the test data, it was verified that the formula is correct.

Welding and Riveting Hybrid Bonding of 6061 Al and Carbon Fiber Reinforced Composites.

Welding and riveting hybrid bonding technology was applied to join 6061 aluminum alloy and carbon fiber reinforced plastics (CFRP). The laser-arc hybrid welding process and stepped rivets were used in the experiments to reduce the impact of the poor heat resistance of composites. The effect of hybrid welding arc current on the formation and mechanical properties of 6061 Al/CFRP joints was studied. Tensile shear load up to 4.65 kN was achieved by adjusting process parameters. The welding process and mode of the fracture were analyzed. The hybrid bonded joint obtained consisted of two parts: a welded joint of Al plate and Al rivet, and a bonded interface between Al plate and CFRP plate. The mechanical properties of the hybrid joint were mainly determined by the Al plate/Al rivet welded joint. The results of the study show that there are three interfacial bonding mechanisms between aluminum and CFRP. In addition to mechanical bonding between the Al plate and CFRP plate, there were also metallurgical bonding of Al-Mg intermetallic compounds with resin matrix and chemical reactions of aluminum with resin and carbon fibers at the interface, which could improve the mechanical properties of the joints.

The anti-arch inhibition effect of multispan continuous girder bridge strengthened with prestressed CFRP plates

To study the interactions of longitudinal deformations on multispan continuous girder bridges strengthened with the prestressed Carbon fiber reinforced polymer (CFRP) plates, the reinforcement mechanism, and the stress distribution of bridges are analyzed. Then, a refined three-dimensional numerical model of continuous girder bridge strengthened with prestressed CFRP plates is established to study the influence of the number of adjacent spans on the reinforcement effect. The study shows that there is an “anti-arch inhibition effect” when multispan continuous girder bridges are strengthened with prestressed CFRP plates. Finally, to assess the “anti-arch inhibition effect”, the tension prestress of a continuous girder bridge strengthened with prestressed CFRP plates is optimized so that each span of a multispan continuous girder bridge can achieve a balanced reinforcement effect. The results show that the interaction between adjacent spans should be considered, and the ultimate reinforcement effect of the full-bridge is not optimal when the tensile prestress of the CFRP plate is the same for each span. By optimizing the prestressed ratio of the CFRP plates, the reinforcement effect can be optimized, the adverse effects of adjacent spans can be reduced, and the utilization rate of CFRP reinforcement can be improved.

Numerical investigation for the flexural load behaviour of profiled composite slab strengthened with CFRP

The profiled composite slab (PCS) is a composite structural system consisting of profiled steel sheet in-filled with concrete in the troughs and connected using studs. This system is a structural load bearing element. Lately, this system has been increasingly used as flooring or roofing structural element. The PCS has become as an extension or development to the most popular now a day's composite slab called profiled steel sheet dry board which was brought to the United Kingdom as an alternative to timber joist flooring for small span domestically buildings. Similarly, to other structural members, PCS demands strengthening and continuous upgrading due to many reasons like exposing to accidents or to improve its ability to bear an extra load. There are a quite limited studies related to the improvement of the PCS strength thus, this study suggests investigating the effect of using carbon fiber reinforced polymer (CFRP) plates to strength the PCS and enhance its ability to carry an extra load. Three parametric studies were considered in this study, the effect of thickness, length and location of the CFRP plates on the flexural load behaviour. The FE analysis results, proved that using CFRP as a strengthen technique for the PCS system has a positive effect on the ultimate flexural bearing capacity which increased by 16.5%, 29.5% and 41.0% using different CFRP plate thickness (1.5, 3.0, and 4.5 mm, respectively). Moreover, when different CFRP plates length (1000, 1500 and 2000 mm) were used the ultimate flexural bearing capacity raised up to 4.0%, 10.0% and 16.5% respectively while the amount of increment were 16.5% and 39.0% when using different CFRP number of sheets (2 and 4 sheets respectively). Lastly, this study strongly recommends using CFRP sheets as a strengthen technique for the PCS system.

High-Temperature Effect on the Tensile Mechanical Properties of Unidirectional Carbon Fiber-Reinforced Polymer Plates.

Carbon fiber-reinforced polymer (CFRP) has the advantages of a high strength-weight ratio and excellent fatigue resistance and has been widely used in aerospace, automotive, civil infrastructure, and other fields. The properties of CFRP materials under high temperatures are a key design issue. This paper presents the quasi-static tensile mechanical properties of unidirectional CFRP plates at temperatures ranging from 20 to 600 °C experimentally. The laser displacement transducer was adopted to capture the in situ displacement of the tested specimen. The results showed that the tensile strength of the CFRP specimen was affected by the high-temperature effect significantly, which dropped 68% and 16% for the 200 and 600 °C, respectively, compared with that of the room temperature. The degradation measured by the laser transducer system was more intensive compared with previous studies. The elastic modulus decreased to about 29% of the room temperature value at 200 °C. With the evaporation of the resin, the failure modes of the CFRP experienced brittle fracture to pullout of the fiber tow. The study provides accurate tensile performance of the CFRP plate under high-temperature exposure, which is helpful for the engineering application.

Fatigue behavior of corroded steel plates strengthened with CFRP plates

This paper presents an experimental study performed to investigate the effectiveness of carbon fiber reinforced polymer (CFRP) plates in extending fatigue life of corroded steel plates. Specimens with five kinds of corrosion damage level, four kinds of strengthening configuration and four kinds of applied fatigue stress amplitude were tested and the primary fatigue behavior indications consist of failure mode, fatigue fractography, fatigue life and crack propagation were analyzed. Results showed that externally patching CFRP plates significantly strengthened the fatigue behavior of corroded steel plates, and even made the fatigue life of corroded steel plates recovered or exceeded that of the un-corroded ones, the fatigue life of the patched specimen which presented the most significant strengthening effectiveness even showed a fatigue life extension of more than 85.3 times with respect to the un-patched corroded steel plate and approximately 2 times over the un-corroded steel plate. Results also indicated that different strengthening configurations appeared to have diversity effect on the fatigue life extension and crack growth retardation of corroded steel plates, end anchorage measures effectively delayed the failure process of CFRP bonding interface and improved the fatigue strengthening effect, increasing CFRP strengthening stiffness or adopting prestressed CFRP plates significantly reduced the crack growth rate and extended the fatigue life of corroded steel plates. The outcome of the tests provided meaningful references and essential data for the reliable application of CFRP strengthening system in the fatigue performance improvement of corroded steel structures.

Flexural behaviour of notched steel beams strengthened with a prestressed CFRP plate subjected to fatigue damage and wetting/drying cycles

Applying a prestressed Carbon Fibre Reinforced Polymer (CFRP) to strengthen deficient steel structures is a favourable approach. However, fatigue loading and hygrothermal exposure can weaken adhesive bonding. A theoretical analysis of the interfacial stress distribution at the notch location was first carried out to explain the mechanism of delay of the interfacial debonding of notched steel beams after prestressed CFRP strengthening. Then, the prestress loss and flexural behaviour of specimens (with a prestress level of 25%) subjected to fatigue damage (FD) and/or wetting/drying cycles (WDCs) with 3.5 wt% NaCl solution were further experimentally investigated. Theoretical results showed that prestressed CFRP strengthening can significantly reduce the interfacial stress at the notch site and delay interfacial debonding. The CFRP prestress loss mainly occurs in the hardening stage of the adhesive layer, and the total prestress loss is less than 2.7% for specimens subjected to WDC exposure, indicating the effectiveness of the end anchorage system. The adhesive microcracks at the notch induced by FD and adhesive bonding degradation caused by WDCs significantly affect the interfacial debonding. In addition, NaCl solution penetration accelerated by microcracks causes further degradation of interfacial bonding. Nevertheless, the end anchorage system showed promising stiffness and load-carrying capacity behaviour for the specimens subjected to FD, WDCs, and the combined effects. The mixed failure mode of CFRP fracture and CFRP extraction from the end anchorage demonstrates that the strength of the CFRP plate was thoroughly utilized for the specimens with prestressed CFRP.

Influences of the joint and epoxy adhesive type on the CFRP-steel interfacial behavior

The bond behavior of the interface between carbon fiber reinforced polymer (CFRP) and steel is an important factor affecting the strengthening effect. In this study, the double-shear and single-shear bonded joints were used to research the influences of test methods and epoxy adhesives on the CFRP-steel interfacial behavior. The test parameters included the joint type, adhesive type and adhesive thickness. The failure mode, ultimate load, strain on the CFRP, interfacial shear stress and bond-slip curve were compared. Test results show that the joint type affects the ultimate load, CFRP strain and interfacial shear stress but has no influence on the bond-slip relationship and failure mode. The ultimate loads of the interfaces tested by the double-shear bonded joints are higher than those of the single-shear bonded joints, and the ratio of the ultimate load is determined by the stiffness ratio between the CFRP plate and steel substrate. The adhesive type can affect the failure mode, ultimate load, shapes of the shear stress distribution and bond-slip curve. The ultimate load of the bonded joints with the adhesive A can reach two times that of the joints with the adhesive S. Compared to the specimen with the adhesive S, the specimen with the adhesive A has a lower peak shear stress but much longer stress transfer length. The bond-slip relationships are approximately bilinear and trapezoidal shape, respectively, for the specimens with the adhesives S and A.

The curve cluster analyses for the characterizations of material defects by long-pulsed laser thermography

Laser thermography is a novel non-destructive testing method enabling the detections of defects on vertical surfaces (e.g., cracks) and those on parallel surfaces (e.g., delaminations). The thermal response of defects near the surface of the specimen in the temperature field is the basis for defect detection work; however, the thermal response characteristics of defects under direct laser irradiation have not been systematically investigated. In this paper, we investigated the thermal response characteristics of five materials under long-pulsed laser excitations: copper, 45 steel, aluminum alloy, carbon fiber reinforced polymer plate, and ceramic resin composite coating. These specimens exhibited significantly different thermal conductivities. An important characterization parameter, characteristic temperature, was found, which can characterize the difference in thermal conductivity of different specimens. The surface of the 45 steel was coated with 3Cr13 coating, and cracks on the coating surfaces generated significantly different thermal response characteristics. The findings of this study illustrated the feasibility of long-pulsed laser thermography for demonstrating the heat entrapment effect due to cracks and provide a method for defect detection.

Method of Designing a Friction-Based Wedge Anchorage System for High-Strength CFRP Plates.

The cables of high-strength carbon fiber reinforced polymer (CFRP) plates are starting to be applied to large spatial structures. However, their main anchorage systems rely on the adhesive force, which entails risks to their integrity resulting from aging of the binding agent. In this study, a friction-based wedge anchorage system was designed for CFRP plates. The working mechanism of the proposed anchorage system was explored both theoretically and experimentally. The anti-slip mechanism and condition of CFRP plates were formulated so that the equivalent frictional angle of the contact surface between a CFRP plate and wedges must not be smaller than the sum of the dip angle of the wedge external conical surface and the frictional angle between the wedges and barrel. An analysis of the stress distribution in the anchorage zone of the CFRP plate was conducted using the Tsai-Wu failure criterion, which concluded that the compressive stresses should be reduced on the section closer to the load-bearing end of the anchorage system. Furthermore, the anchorage efficiency coefficient was proposed, which depends on stress concentration coefficients, plate thickness, length of anchorage zone, dip angle of wedge external conical surface, and its frictional angle. Then, it was determined that the minimum length of an anchorage zone for the CFRP plates with various specifications should be at least 49 times larger than the CFRP thickness. A finite element analysis and static tensile tests on six specimens were carried out. The experimental results revealed that the anchorage efficiency coefficient of the optimized anchor reached 97.9%.

Analysis of Air-Coupled Transducer-Based Elastic Waves Generation in CFRP Plates.

In this paper, the analysis of non-contact elastic waves generation in carbon fiber reinforced-polymer (CFRP) plate was conducted. Full non-contact elastic waves generation and sensing methods were also analyzed. Elastic waves generation was based on an air-coupled transducer (ACT) while waves sensing was based on a laser Doppler vibrometer. The excitation frequency was equal to 40 kHz. An optimal ACT slope angle for the generation of elastic waves mode was determined with the aid of dispersion curves calculated by using a semi-analytical model. Due to the stack sequence in the composite plate (unidirectional composite), ACT slope angles were different for waves generation in the direction along and across reinforcing fibers direction. Moreover, experimental verification of the optimal ACT slope angles was conducted. It was possible to generate A0 wave mode in the direction along and across the reinforcing fibers. Optimal angles determined using ACT were equal to 16° (along fibers) and 34° (across fibers). In the case of optimal angles, elastic waves amplitudes are almost two times higher than for the case of ACT oriented perpendicularly to the plate surface. Moreover, experimental results based on ACT showed that it was possible to generate the SH0 mode in the direction across the fiber for optimal angles equal to 10°. Finally, based on the A0 wave mode propagation, the process for localization of discontinuities was performed. Discontinuities in the form of additional mass simulating damage were investigated. A simple signal processing algorithm based on elastic wave energy was used for creating damage maps. Authors compared discontinuity localization for ACT oriented perpendicularly to the plate and at the optimal slope angle. The utilization of non-contact waves excitation at optimal ACT slope angles helped to focus the wave energy in the desired direction. Moreover, in this case, elastic waves with the highest amplitudes were generated.

Weighted adaptive Kalman filtering-based diverse information fusion for hole edge crack monitoring

The information fusion of different sensors is an effective means of improving the damage quantification of aircraft structures operating in a complex environment. A weighted adaptive Kalman filtering-based information fusion method (WAKFIFM) is proposed to achieve the feature-level fusion of diverse signals of the hybrid piezoelectric-fiber optic sensor network. The damage identification results obtained from guided wave signals and strain signals are described by the quantification curves of damage index (DI) and crack length. A new fusion curve is obtained by fitting the above two quantification curves using the quadratic polynomial method. The mean square error and the root mean square error of the fusion curve and the actual curve are used as the error and the covariance of the proposed WAKFIFM, respectively. The weighted value in WAKFIFM is the reciprocal of the square of the actual measured maximum DI. The hole edge crack propagation experiments are conducted on both an aluminum plate and a carbon fiber reinforced polymer plate in a load-temperature changing environment.A new damage location method integrated with DI information is developed to obtain a more accurate result. The feature-level fusion results show that the proposed WAKFIFM can improve the accuracy of damage quantification, compared against that obtained from the single guided wave signals or fiber optic sensor signals.