Carbon Fiber Reinforced Polymer Laminates Research Articles

Experimental and analytical investigation on effect of openings in behavior of reinforced concrete deep beam and enhanced by CFRP laminates

This work studies the potential of producing web openings within the shear span for the reinforced concrete (RC) deep beams. Carbon fiber reinforced polymer (CFRP) laminates are used to improve the shear strength. 13 specimens with dimensions of 120, 750, and 1800 mm are prepared for the tests. Shapes of the web openings are square and circular with diameter of 230, 300, and 370 mm. Their locations are in the center, upper and lower of the shear span. Three angles for wrapping 0°, 45°, and 90° are used to strengthen the beams with circular openings. The shear strength of RC deep beams decreased in the range of 22–74 % depending on the diameter, location, and shape of the openings. While CFRP improved the shear strength, stiffness and toughness in about 17–48 %, 0.7–17 % and 4–20 % respectively. The previous percentages depended on size of openings and the orientation of CFRP laminates in RC deep beams with circular openings. The laminates of CFRP are able to repair and restore the bearing capacity of beams by approximately 100 % of the same beams before they failed. On the other hand, Strut and Tie method established to estimate the maximum strength of the RC deep beams and it results that the theoretical method succeeded in guessing the practical results.

Acoustic emission for identification of the dominant stress component in polymer composites at early loads

Carbon Fiber Reinforced Polymer (CFRP) composites are broadly used in engineering applications. Their inherent anisotropy due to different fiber orientations can be considered an advantage since the strength of the component can be designated in preferential loading directions. However, this anisotropy leads to multiaxial stress conditions, complicating their damage sequence and mechanical response. Identification of these multiaxial conditions at early loading stages is of paramount importance to predict the upcoming structural response of the material. Acoustic Emission (AE) is applied in this study to CFRP laminates with different stacking sequences, in which different multiaxial conditions are generated. Laminates consisting of 30o plies are characterized by dominant shear stresses, whereas in laminates with 60o layers transverse normal stresses govern the stress state. Through quasi-static and incremental loading, it is shown that certain AE features can be used to identify the dominant stress component rather than just the occurring damage mode even at early loads, before severe fracture influences the mechanical capacity. This is of great importance in cases that detrimental shear stresses are generated, leading to delaminations and mechanical deterioration. AE can be used in this direction to predict the upcoming damage and to take necessary measures to avoid final failure.

Vision-Based Defect Measurement of Drilled CFRP Composites Using Double-Light Imaging

Drilling-induced defects in carbon fiber-reinforced polymers (CFRP) composite laminate can lead to the risk of aircraft structural failure and even aviation disaster. Measuring these defects is, then, inevitable for aerospace vehicles assembly. Tear is a representative drilling-induced delamination defect. But the most widely used measurement method is still based on human vision and vernier calipers. With low measurement efficiency and large measurement error, this is no longer suitable for industrialized large-scale production. Machine vision provides a new technology to address this problem. However, the semi-specular and textured surface around the drilled holes in CFRP laminates makes machine vision-based tear measurement difficult. To address these issues, we propose a novel automatic tear measuring system to obtain the tear point with maximum radial length (TPMRL). The method consists of three parts: (1) a double-light imaging frame is designed to illuminate the object hole under two different lighting conditions and acquire images; (2) a series of image processing procedures are proposed to accurately obtain inner hole information and tear edge information; (3) combined processing of inner hole information and tear edge information is performed to obtain the maximum radial length of the tear area (MRLTA). The experimental results show that the proposed system is able to measure the MRLTA and is superior to some existing relative approaches.

Is Graphene Always Effective in Reinforcing Composites? The Case of Highly Graphene-Modified Thermoplastic Nanofibers and Their Unfortunate Application in CFRP Laminates.

Graphene (G) can effectively enhance polymers' and polymer composites' electric, thermal, and mechanical properties. Nanofibrous mats have been demonstrated to significantly increase the interlaminar fracture toughness of composite laminates, hindering delamination and, consequently, making such materials safer and more sustainable thanks to increased service life. In the present paper, poly(ethylene oxide) (PEO), polycaprolactone (PCL), and Nylon 66 nanofibers, plain or reinforced with G, were integrated into epoxy-matrix Carbon Fiber Reinforced Polymers (CFRPs) to evaluate the effect of polymers and polymers + G on the laminate mechanical properties. The main aim of this work is to compare the reinforcing action of the different nanofibers (polyether, polyester, and polyamide) and to disclose the effect of G addition. The polymers were chosen considering their thermal properties and, consequently, their mechanism of action against delamination. PEO and PCL, displaying a low melting temperature, melt, and mix during the curing cycle, act via matrix toughening; in this context, they are also used as tools to deploy G specifically in the interlaminar region when melting and mixing with epoxy resin. The high extent of modification stems from an attempt to deploy it in the interlaminar layer, thus diluting further in the resin. In contrast, Nylon 66 does not melt and maintain the nanostructure, allowing laminate toughening via nanofiber bridging. The flexural properties of the nanomodifed CFRPs were determined via a three-point bending (3PB) test, while delamination behavior in Mode I and Mode II was carried out using Double Cantilever Beam (DCB) and End-Notched Flexture (ENF) tests, respectively. The lack of a positive contribution of G in this context is an interesting point to raise in the field of nanoreinforced CFRP.

Effect of Using ECC Layer on the Flexural Performance of RC Beams Previously Strengthened with EB CFRP Laminates

This paper studies the efficiency of applying an engineered cementitious composite (ECC) layer to the tensile surface of (RC) beams that were previously strengthened using externally bonded (EB) carbon fiber-reinforced polymer (CFRP) laminates. One control and ten strengthened RC beams were produced and tested utilizing a four-point loading regime. For strengthened beams, two beams were kept strengthened using only CFRP, and additional ECC layers were added to the rest of the strengthened beams. The CFRP width and overlap length and position were among the test factors. Experimental results revealed that strengthening RC beams with CFRP laminates enhanced both the stiffness and flexural capacity of beams. Additional enhancements were obtained through the application of the additional ECC layers. The existence of the ECC layer alongside the CFRP laminate improved the flexural capacity by 102% and 125% when using CFRP widths of 50 mm and 100 mm, respectively, and the stiffness was improved by an average value of 318%. Three-dimensional (3D) finite element models (FEMs) were developed using ABAQUS software and verified against the experimental results to model the response of the tested beams. The verified model was used to conduct a parametric study to consider the effect of the ECC layer thickness and the reinforcement ratio on the strengthened beam behavior. The numerical results revealed that the effect of the reinforcement ratio was more significant than the ECC layer thickness in enhancing the load-displacement response, especially after the cracking stage.

A study of the fracture mechanisms of hybrid carbon fiber reinforced polymer laminates reinforced by thin‐ply

AbstractThe main stress component which creates delamination in bonded single lap joints with composite adherends is the transverse tensile stress. Therefore, the following study investigates the behavior of composite laminates (reference and hybrid laminates reinforced by thin‐ply) under transverse tensile loading. Texipreg HS 160T700 and NTPT‐TP415 were used as the conventional carbon fiber reinforced polymer (CFRP) and thin‐ply respectively. Hybrid composite laminates were studied using different amounts of thin‐ply, applied through the thickness. The manufactured laminates, of unidirectionally stacked construction, were tested under transverse tensile loading. Digital image correlation was performed to investigate the average peel strain distribution for the composite and to better understand the phenomena associated to the use of hybrid laminates. Experimental results show that the reinforced hybrid composite laminates, created using thin‐plies, present higher failure load compared to the reference conventional CFRP or thin‐ply laminates. This was found to be due to the higher ductility enabled by the presence of thin‐plies. Distributing a constant amount of thin‐ply through the thickness was found to increase the laminate transverse strength, as the thin‐ply laminates act as a barrier against crack propagation. A representative volume element was studied for each configuration since this numerical method brings the opportunity to investigate the studied configurations in microscale.

Effect of bolt size on the bearing strength of bolt-connected orthotropic CFRP laminate

Carbon fiber reinforced polymer (CFRP) has the advantages of lightweight, high strength and corrosion resistance. CFRP laminate is widely used in the reinforcement and rehabilitation of concrete and steel structures such as buildings, bridges, and tunnels. In this work, the structural arrangement of six kinds of orthotropic CFRP laminates that would suffer from bearing failure was determined with the help of a failure map. The bearing strength and failure mechanism of the specimens at the joint were studied through tensile bearing tests. Finally, a strength prediction model about the effect of bolt-hole size was established based on the Weibull distribution theory. The research results show that the bearing strength of the CFRP laminates is insignificantly affected by the thickness of the CFRP laminates, however, is greatly affected by the diameter of the bolt under the same thickness and lay-up form. The bearing strength of the CFRP laminates decreases with the increase in the diameter of the bolt hole. It is found that the specimen with a symmetrical layup shows a more obvious size effect compared to the specimen with an asymmetric layup. The model established based on the Weibull distribution theory can well predict the bearing strength of bolted-connected CFRP laminates with different diameters.

Effect of Normal and Rubberized Concrete Properties on the Behavior of RC Columns Strengthened with EB CFRP Laminates and Welded Wire Mesh under Static Axial Loading.

The huge amounts of old and damaged tires spread worldwide has caused many complex environmental risks. The old tires have been converted to crumb rubber (CR) and tire recycled steel fiber (RSF) to facilitate their use. This study used CR to partially replace natural sand in reinforced (RC) columns. Externally bonded (EB) carbon-fiber-reinforced polymer (CFRP) laminates, welded wire mesh (WWM), and RSF were used to enhance the axial behavior of the tested columns to overcome the concrete deficiencies resulting from the inclusion of the CR instead of natural sand. Eighteen columns were prepared and tested to discuss the effects of strengthening type, CR content, RSF, and strengthening area on the axial behavior of the RC columns. Certain columns were internally reinforced with WWM, while others were externally strengthened with EB CFRP laminates. Partially or fully EB CFRP laminates were used to strengthen the columns. Moreover, one column was cast with NC and 0.2% RSF to investigate the role of RSF in confining the column. The results demonstrated a concrete strength reduction for the rubberized concrete (CRC) as the CR content increased. Conversely, the strengthened columns experienced higher load capacities than the corresponding un-strengthened ones cast with the same concrete mix. Moreover, adding 2% RSF to the NC mix could enhance the column capacity, although it decreased the concrete strength. Furthermore, using two CFRP layers increased the load capacity and ductility of the strengthened columns. The strengthened column cast with 50% CR showed the highest load efficiency (334.3% compared to the un-strengthened one).

Experimental characterization and numerical study on the interlaminar fracture toughness of carbon fibre reinforced polymer laminates reinforced with carbon nanotubes

AbstractThe present study is focused on the nanoreinforcement of carbon fiber reinforced polymer laminates to evaluate its effect on delamination growth in double cantilever beam specimen. Multi‐walled carbon nanotubes were sprayed on the laminate interface using air‐brush spraying technique. Three nanofiller contents of 0.05 wt.%, 0.1 wt.%, and 0.5 wt.% were examined to evaluate the mode I fracture toughness (GIC) in a double cantilever beam specimen and the results were compared with the base laminate (0.0 % carbon nanotubes). The average mode I fracture toughness of composite laminates increased from 180.6 J/m2 to 270.92 J/m2 due to the addition of 0.05 wt.% multi‐walled carbon nanotubes, and to 268.27 J/m2 for 0.1 wt.% multi‐walled carbon nanotubes. Mechanisms for the increase in GIC of the nanofiller reinforced laminates are discussed based on the experimental results and microscopic study of fractured specimen surfaces from scanning electron microscopy. Energy dissipation from carbon nanotube bridging ahead of the crack front is the major toughening mechanism resulting in improved fracture toughness. Numerical simulations of delamination growth were performed as well by using virtual crack closure technique analysis in Abaqus. The R‐curve behaviour from the experiments is implemented in the finite element modeling of delamination growth to validate the effect of nanoreinforcements.

Engineering uniformity in mass production of MWCNTs/epoxy nanofibers using a lateral belt-driven multi-nozzle electrospinning technique to enhance the mechanical properties of CFRPs

Electrospinning is one of the most diverse, cost-effective, and ecologically renowned methods for generating continuous nanofibers, but electrospinning of thermosetting polymers like epoxy and their mass volume productions have been a major challenge in recent years. This study proposes a fabrication method by addressing electrospun MWCNTS/epoxy (CNT/epoxy) nanofiber's volume processing, reproducibility, and accuracy issues in an effective manner. Lateral belt-driven (LBD) multi-nozzle electrospinning is here newly adopted to enhance the mechanical properties of the carbon fiber-reinforced polymer (CFRP) laminates. With the LBD approach, the electric field can be uninterruptedly distributed to deposit a uniform layer of CNT/epoxy scaffolds over the entire width of CFRP prepreg. This study optimizes the distance between two nozzles, the motion of the lateral belt, and the electrospinning period. The laminates made of coated CFRP show enhanced mechanical properties when compared to pristine CFRPs. Under high-stress conditions, these CFRP laminates' interlaminar shear strength (ILSS) and fatigue performance demonstrated 29% and 27% improvements, respectively. This study, which demonstrated the success of using an LBD multi-nozzle system, has enormous opportunities to produce thinner and continuous fibers with more concentrated collections at a faster rate, which is critical for commercial applications.

Investigation of process parameters on flexural properties for CFRP composites

Advanced materials like composites are being increasingly used nowadays because of their excellent mechanical properties. Carbon fiber reinforced polymers (CFRP) are one such type of advanced composites that offer high strength to weight ratio, high modulus, high flexural strength, and very low coefficient of thermal expansion. The flexural properties are important mechanical characteristics for any advanced material. In this research paper, the effect of process parameters on the flexural behaviour of the CFRP sheet has been investigated experimentally and numerically. The thickness of laminate and ply stacking sequence or ply orientation have been considered as process parameters. The CFRP laminates have been fabricated using the wet layup compression method. The samples have been prepared according to ASTM D790 to measure flexure properties. The flexural strength and flexural modulus have been obtained for different laminate thicknesses with various stacking sequences. The finite element (FE) simulation has been performed for the same process parameters using ABAQUS. The assembly of the test setup has been modelled followed by assigning proper material properties and applying boundary conditions for simulation. The results from experimental and simulation techniques have been compared and it has been found that flexural strength increases with an increase in laminate thickness while there is no significant influence of thickness on flexural modulus. It has also been observed that flexural properties change dramatically for different ply-stacking sequences. The results of the present work would be helpful to designers and composite manufacturers in selecting the proper thickness and stacking sequence according to application.

Examining slow crack growth metrics and competing failure modes in IM7/5320-1 carbon fiber reinforced polymer laminates with pre-existing damage states

The growing use of composites in aircraft structures has necessitated a slow growth criteria to assess the life of these materials and an appropriate similitude parameter. Due to the simplified stress state and inherent interactions of intralaminar and interlaminar damage progression in multi-directional ply layups, there have been questions about the applicability of double cantilever beam specimens to aerospace applications. This study examines these interconnected damage growth mechanisms in [±453]s and [±452/902]s layups via compact tension specimen geometries, with and without pre-existing damage, in order to examine the efficacy of proposed similitude parameters in capturing slow crack growth. The change in stored potential strain energy per cycle provided the most unified results across the 18 tests under different loading and pre-existing damage conditions. Via microscopy of the specimens after damage progression, the similitude parameters were connected to overall trends in the damage mechanisms. The fatigue results, coupled with the microscopy of the interactions amongst the complex damage mechanisms, demonstrates the need to analyze proposed similitude parameters across various sample geometries and stress states, which will be more enable an understanding of composite material fatigue performance in aerospace applications.

Interlaminar fracture toughness of carbon fibre composites with electrospun nanofibrous interleaves of polystyrene and cellulose nanocrystals

Polystyrene nanofibres reinforced with cellulose nanocrystals have been produced by electrospinning. Two different orientations of the nanofibres were produced, namely aligned and random, and these nanofibrous interleaves were investigated both as-spun and after thermal treatment. Aligned nanofibres exhibited a 2500% increase in their Young’s modulus compared to the randomly orientated materials, while the ultimate tensile strength increased by up to 300%. It is also demonstrated that crazing occurs in the nanofibres, which is thought to enhance the fracture properties of the materials. Interleaves of both orientations were then included in a carbon fibre-reinforced polymer laminate. The fracture toughness of the laminate was determined under both mode I and mode II testing. No increases in mode I toughness were observed. It was, however, found that the aligned interleaves increased the mode II toughness of the composite laminate (from 1.9 ± 0.3 to 2.7 ± 0.2 kJ m−2). This increase is demonstrated to be due to a combination of nanofibre bridging and the formation of microcracks in the resin under applied tensile and shear load.

Machining inclination selection method for surface milling of CFRP workpieces with low cutting-induced damage

Carbon fiber reinforced polymers (CFRP) are being extensively employed in the manufacturing of complex curved components in the aerospace industry owing to their high specific stiffness and designable properties. Curved CFRP components need to be surface-milled using ball-end mill cutters to ensure the performance of the connection assembly between curved composite materials. Machining inclination is a key parameter in the formation of burr damage in CFRP parts milled using ball-end mill cutter, due to the particularity of the tool structure. In this study, the influence mechanism of machining inclination on the burr damage of CFRP components was analyzed theoretically. To machine a surface with low burr damage, an innovative method for machining inclination selection considering the anisotropy and cascade characteristics of CFRP was proposed. The influence of machining inclination on material damage was revealed, and the reliability of the method was verified by conducting experiments on flat and curved laminates. This paper puts forward reasonable suggestions for selecting the machining inclination in ball-end mill cutters employed for CFRP laminates.

Effect of EPD coated silanized graphene oxide on carbon fiber reinforced plastic: An emphasis on mechanical properties at cryogenic temperatures

The space industry is constantly exploring lightweight, high-strength materials, particularly carbon fiber reinforced polymer (CFRP) composites. Nonetheless, in some cryogenic space applications, neat CFRP fails miserably. Here, silane functionalized graphene oxide (SGO) was coated on the carbon fiber surface to modify its properties. SGO is deposited onto a PAN-based plain weave carbon fiber using the electrophoretic deposition (EPD) process. SGO coated carbon fiber, along with epoxy resin, was used to fabricate CFRP laminates. From mechanical studies of CFRP samples, tensile, flexural, and impact strength improved by 13.1 %, 26.6 %, and 3.9 %, respectively, at room temperature (RT). The improvements were 15.1 %, 35.3 %, and 13.5 %, respectively, at cryogenic temperatures (CT). FTIR studies confirmed the emergence of a flexible Si-O-Si bond, which is a key factor for improved strength. AFM and SEM analysis were conducted to investigate contact roughness improvement. Contact angle microscopy examined SGO-induced wettability of modified carbon fiber. Also, a fracture study showed that the failure mode of SGO-coated CFRP at CT had changed from interface failure to matrix failure.

Predicting Crack Width in CFRP-Strengthened RC One-Way Slabs Using Hybrid Grey Wolf Optimizer Neural Network Model

This study deploys a hybrid Grey Wolf Optimizer Neural Network Model for predicting the crack width in reinforced concrete slabs strengthened with carbon fiber-reinforced polymers (CFRP). Reinforced concrete (RC) one-way slabs (1800 × 400 × 120 mm in size) were strengthened with CFRP with various lengths (1800, 1100, and 700 mm) and subjected to four-point bending. The experimental results were compared to corresponding values for conventional RC slabs. The observed crack width results were recorded, and subsequently examined against the expression recommended by Eurocode 2. To estimate the crack width of CFRP-reinforced slabs, ANN combined with the Grey Wolf Optimizer algorithm was employed whereby the applied load, CFRP width/length, X/Y crack positions, and stress in steel reinforcement and concrete were defined as the input parameters. Experimental results showed that the larger the length and width of the carbon fiber, the smaller the maximum crack width in the tensile area of the slab at the final load step. On average, the crack width in slabs retrofitted with CFRP laminates increased by around 80% compared to a slab without CFRP. The results confirm that the equation provided by Eurocode 2 provides an unconservative estimation of crack widths for RC slabs strengthened with CFRP laminates. On the other hand, the results also confirm that the proposed informational model could be used as a reliable tool for estimating the crack width in RC slabs. The findings provide valuable insight into the design approaches for RC slabs and rehabilitation strategies for existing deficient RC slabs using CFRP.

Effects of Elastic Couplings in a Compressed Plate Element with Cut-Out.

Analytical calculations were performed on carbon fiber-reinforced polymer (CFRP) laminates in an asymmetrical configuration. The asymmetric configuration of composites was investigated, where extension-twisting and extension-bending couplings were used to obtain the elastic element. Analysis of the presence of elastic couplings was conducted according to Classical Laminate Theory (CLT). Components of matrices A, B, and D, as well as the parameters Dc and Bt, were obtained using the MATLAB software environment. The results show that couplings between the extension and bending, as well as between the extension and twisting, were strongly dependent on specimen plies' orientation. Moreover, additional analysis was performed on the influence of layer angle on the terms which are components of the Bt and Dc coefficients. The results indicate that the angle of laying fibers around 45-50° significantly amplifies the effects of elastic couplings.

Convolutional neural network method for damage detection of CFRP in electrical impedance tomography

Damage detection is vitally important for carbon fiber reinforced polymer (CFRP) laminates. When CFRP laminates are damaged, its impedance property is changed. Based on the electrical properties of CFRP laminates, the changed conductivity distribution can be reconstructed with the electrical impedance tomography (EIT) method. The detection method is attractive due to its simple equipment, low cost, and easy operation. However, image reconstruction of EIT faces a serious ill-conditioned nonlinear inverse problem. In order to solve this problem, a feature fusion convolutional neural network based on the dense connection (FF-D) method is applied in EIT to establish the mapping relationship between voltage measurement and conductivity distribution in this paper. The optimization can extract and utilize features to a greater degree and improve reconstruction accuracy and robustness. For the purpose of simulating the electrical properties of CFRP better, the conductivity values measured by an impedance analyzer are used as the data set. The correlation coefficient (CC) and root mean square error (RMSE) are used as indicators to evaluate the quality of image reconstruction. The simulation and experimental results suggest that the FF-D method can reconstruct images better than typical algorithms based on deep learning and conventional algorithms of EIT.

Recovery of carbon fiber-reinforced polymer waste using dimethylacetamide base on the resin swelling principle.

The mechanical recycling method of the carbon fiber-reinforced polymer (CFRP) has the advantages of simple process, less pollution and low cost, but only low utilization value of carbon fibers in powder or short fibers form can be obtained. To reduce the length and strength loss of the recycled carbon fibers, a novel and cost-effective dimethylacetamide (DMAC) swelling technique was developed to achieve rapid delamination of the CFRP laminates under mild conditions (120°C-160°C, 1h). The corresponding swelling ratios and mass-loss rates of cured epoxy resin (CEP) were about 121.39%-157.39% and 0-0.69%, respectively. Excessive swelling of CEP in DMAC resulted in the cracking of the resin matrix between the adjacent carbon fiber layers. Thus the CFRP laminates were delaminated into soft single carbon fiber layers, which showed excellent cutting performance and reinforcing properties. The delamination products were cut into thin strips of different sizes and vacuum bag molded into new CFRP laminates. The flexural strength and tensile strength of the newly produced CFRP laminates were about 76.38%-90.98% and 94.61%-98.54% of the original CFRP laminates, respectively. More importantly, the chemical compositions of DMAC and CEP were unchanged during the physical swelling process. No organic pollutants (caused by resin degradation) were generated. And the used DMAC can be easily recycled by filtration. Therefore, this study provides a strategy for low-cost and high-valued recycling of CFRP waste.

Experimental analysis of three essential types of hole making strategies in aircraft assembly technology concerning delamination factor

Large-diameter hole drilling for Carbon-Fibre-Reinforced-Polymer (CFRP) is challenging because of its non-homogeneity and anisotropic features. This research will illustrate the hand-laminated and unidirectional CFRP based on three-hole making strategies for helical milling, large hole and pilot hole drilling. In this article, CFRP analyzes and optimizes axial cutting force and Delamination Factor (DF) based on experimental data collected at different FR (Feed Rate). CFRP's large diameter hole quality analysis includes cutting speed (Rotational Tool Speed) and scanning electron microscopy (SEM) analysis of helical milling, pilot hole and large hole drilling. A comparative study between large hole diameter, pilot hole drilling, and helical milling of CFRP was also conducted to discover the better technology for hole calibration quality by analyzing Machining Force (MF) and DF. According to experimental tests of CFRP laminates, the MF is proportional to the FR. Compared to helical milling, pilot hole drilling with proper cutting parameters can create superior quality holes. As a result, it is recommended that pilot hole drilling be done at a low feed rate and a high rotational tool speed for optimum hole quality. The hole calibration was analyzed to distinguish the cutting parameters (Cutting Speed, FR, and MF) to reduce DF and improve hole quality. Low FR was necessary for CFRP high-speed drilling because delamination at the hole exit was a concern.