Strength Of Carbon Fiber Reinforced Polymer Research Articles

Effect of ply misalignment on the notched strength of composite laminates

Predicting the notched strength of carbon fiber-reinforced polymers is a crucial aspect of composite structure design, particularly when considering the uncertainties stemming from geometric features, material variability and defects. This study focuses on the influence of ply misalignment at the meso-scale level. The research employs a comprehensive methodology to establish notched strength allowables, integrating analytical (low fidelity), which have limitations in the representation of stacking sequence effects and in the generation of ply misalignments, and computational tools (high fidelity), employing a finite element model (FEM). The investigation emphasizes the need for a holistic understanding of these factors to enhance the accuracy of predictions. The results predicted by the analytical and, specially, the numerical models are in good agreement with experimental results. Both models underscore the decrease of the notched strength due to ply misalignment. Finally, a hybrid approach is proposed given that FEM predictions offer more accuracy and a detailed comprehension of the failure mechanisms, while fast analytical models can be used to propagate the uncertainties and to determine the allowables.

Outstanding interlaminar strength of carbon fiber reinforced epoxy resin via graphene oxide chemical bridge bonding

Carbon fiber reinforced polymers (CFRPs) are susceptible to interlaminar failure under high shear stress, which reduces their service life. Herein, we report a strategy to address the interlaminar damage problem of CFRPs by utilizing graphene oxide (GO) as a bridge to chemically bond carbon fibers and epoxy resin. Amino groups were grafted on the surface of carbon fibers to react with GO, then active amino functional groups were introduced onto GO, enabling crosslink with the epoxy resin. The chemical bonding force and mechanical interlocking effect of GO tightly integrated the carbon fibers with the epoxy resin, greatly reducing relative slip under high shear stress. This flexible and robust connection resulted in a significant improvement in the interfacial adhesion strength of CFRPs. The GO content and type of amino reactants were optimized to fabricate CFRPs, and the interlaminar shear strength and transverse fiber bundle tensile strength were increased to 76.36 MPa and 34.2 MPa, which were 46.2 % and 77.2 % higher than those of pre-modification materials, respectively. This research demonstrates that the chemical bonding provided by grafting graphene oxide significantly enhances interlaminar bond strength, thereby extending the service life of CFRPs and offering a promising path for manufacturing high-strength composites.

Modeling study on the mechanical performance of CFRP/Al single-lap rivet joints under hygrothermal environmental conditions

Carbon Fiber Reinforced Polymers (CFRP) and aluminum alloys, owing to their exceptional mechanical properties and lightweight attributes, are extensively used in aircraft manufacturing. However, prolonged exposure to a hygrothermal environment can compromise the mechanical integrity of composite joint structures. In this paper, accelerated aging experiments were designed to test the mechanical properties of CFRP after hygrothermal aging, a novel mechanical property prediction model tailored for the hygrothermal coupled environment is presented. The model accurately predicts the modulus and strength of CFRP after hydrothermal aging. A three-dimensional finite element model for the CFRP interference riveted structure, considering a tri-coupled state of moisture, temperature, and force was established by the application of subroutine and field superposition. The precision of this finite element model has been affirmed through accelerated aging tests. By integrating finite element analysis with experimental methods, this research delves into the failure modes and mechanisms of CFRP and its joints under hygrothermal conditions. It was discerned that the hygrothermal environment undermines the bond between fibers and the matrix, resulting in pronounced interlaminar delamination and shear failure in CFRP. The failure forms gradually change from “flaky” at lower levels of aging to “filamentary” at higher levels of aging. For CFRP riveted structures, the hygrothermal conditions influence their loadbearing capability and shift the primary positions of failure.

Constructing a new multiscale “soft-rigid-soft” interfacial structure at the interphase to improve the interfacial performance of carbon fiber reinforced polymer composites

The high strength and toughness of Carbon Fiber-Reinforced Polymers (CFRPs) are the main requirements for their properties in various fields. Herein, the “soft-rigid-soft” structure has been designed by us to achieve the aim of the improvement for the CFRP strength and toughness simultaneously. The soft-rigid-soft structural interface layer of CF was firstly constructed by carbon nanotubes (CNT) and Tannin/Aminopropyl isobutyl POSS (TA/NH2-POSS). The interfacial shear strength (IFSS) of the modified carbon fiber composite was 111.94 MPa, which was 84.08 % higher than that of the untreated fiber (60.81 MPa). Moreover the interlaminar shear strength(ILSS) and impact strength were increased by 50.43 % and 80.37 %. Enhancement of interface performance is mainly ascribed to CNT and TA/NH2-POSS, because they can effectively which is more effective to consume energy and decentralize stress. In this paper, the strength and toughness of CFRPs ware effectively improved by the in-depth and detailed design of the “soft-rigid-soft” multi-scale structure.

Research on joint strength and energy absorption of CFRP and Al sheets bonded-FDS hybrid joining

The application of a composite-metal alloy lightweight body structure is one of the ways to achieve lightweight in an automobile. Materials such as carbon fiber reinforced polymer (CFRP) and aluminum alloys are widely used in the automotive industry due to their excellent performance. However, the significant differences in performance parameters between CFRP and aluminum alloy materials make joining heterogeneous materials challenging. Therefore, a rational joining process is crucial for the development of CFRP–aluminum alloy structures. This study proposes a hybrid joining method that combines flow drill screw (FDS) and adhesive bonding to join commonly used aluminum alloy sheets with CFRP sheets in automotive body manufacturing. Finite element analysis simulations were used to compare and characterize the failure modes of the bonded-FDS hybrid joints in tensile tests. The results were verified for accuracy. Tensile tests were conducted to compare the mechanical performance and failure modes of bonded joints, FDS joints, and hybrid joints. By analyzing the load–displacement curve, it was found that the FDS process in the hybrid joining damages the internal structure of the adhesive layer. However, the joint strength is still higher than that of the single joint strength, and the energy absorption value is about 4.7 times that of the adhesive and 1.6 times that of the FDS. This study provides a new industrial solution and serves as a reference for failure performance in composite material structures in the automotive industry, especially in cases involving one-sided access joining points.

Improved mechanical properties of carbon fiber/epoxy composites via fiber surface grafting of rigid-flexible chain structure

The surface topography and interface characteristics play a significant role in determining the mechanical properties of carbon fiber reinforced polymer (CFRP) composites. These features are primarily influenced by the surface chemical structures and their interactions with the matrix resin. Here, we developed a rigid-flexible coating consisting of polyetheramine (PEA) and polydopamine (PDA) on the fiber surface to control the interface and improve the mechanical properties of CFRPs. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectrum (FT-IR) results validated the successful introduction of polymeric chains and fiber surface characteristics including rough surfaces, slim polymer layers and uniform dots were examined by scanning electron microscope (SEM) and transmission electron microscopy (TEM). Contact angle tests indicate that the functionalized coating noticeably enhances the interaction force between carbon fibers, promoting improved wettability by the matrix resin. The bending and interlaminar shear strength of CFRP prepared by functional carbon fiber are increased by 45.76 % and 47.63 %, respectively. The drop weight impact (DWI) test confirms the positive effect of improved interfacial properties on the out-of-plane impact resistance of CFRPs.

Highly conductive and mechanically robust MXene@CF core-shell composites for in-situ damage sensing and electromagnetic interference shielding

In this work, a new type of carbon fiber reinforced polymer (CFRP) composite was fabricated by introducing MXene nanoparticles onto the surface of carbon fibers (CF) via electrophoretic deposition (EPD) followed by thermal annealing. The MXene-reinforced CF/epoxy composites displayed enhanced mechanical properties and electrical conductivity as well as in-situ damage sensing capability. The uniformly deposited MXene nanoparticles contributed to a considerable enhancement of the flexural strength of CFRPs through hydrogen bonding and mechanical interlocking. The thermal annealing treatment reduced the amount of oxygen groups on the surface of MXene nanoparticles and enabled a 66 % increase of the out-of-plane electrical conductivity and a 20 % improvement of the electromagnetic interference (EMI) shielding effectiveness. The exceptional EMI performance of the core-shell hierarchical microstructure can be ascribed to the polarization of the inhomogeneous interfaces, the dipole polarization, and the conductive loss effect as a result of the presence of annealed MXenes on the surface of CFs.

Restoration of Compressive Strength of CFRP Plates with Edge Delamination through CNT Crack-bridging Deposited by Resin Pre-coating Solution

Edge delamination, with only micro-scale crack openings, in carbon-fiber-reinforced-polymer (CFRP) composites due to light impact, machining and structural assembly is challenging to repair. This study presents a simple pressureless method to repair edge delamination in CFRP laminates. The compressive strength of CFRP panels with around 5–10 mm deep edge delamination cracks have been fully restored without utilizing any special equipment. Acetone-diluted resin pre-coating (RPC) solution, consisting of 95 m/m% acetone and 5 m/m% resin (without hardener), is utilized to penetrate deeply into sharp delamination cracks through capillary action, and 1 m/m% CNT dispersed into the RPC solution (RPC + CNT) is deposited inside the delamination cracks forming CNT-toughened adhesive joints. The compressive strengths of long cure time (3 month) are compared with 2-week results, showing 2-weeks are not sufficient for complete curing. With the 3-month curing period, the compressive strength of CFRP with edge delamination is 100 % restored by the RPC + CNT technique.

Hygrothermal resistance of large tow carbon fiber and its reinforced polymer composites

AbstractLarge tow carbon fiber reinforced polymer (CFRP) has been used to reinforce structures serving in hygrothermal conditions, which can be greatly affected by hygrothermal conditions. In this study, the hygrothermal resistance of 48K and 12K carbon fibers and their CFRPs immersed in deionized water at 25, 40, and 60°C were analyzed and compared. The sizing agent on the carbon fiber surface was deboned or hydrolyzed after immersion, resulting in a reduction in the tensile strength, an increase in dispersion, and a decrease in surface activity for carbon fibers. The retention rate of tensile strength of 48K CFRP was higher than 89.7% but was less than that of 12K CFRP due to the higher dispersion of 48K fiber filaments. The interlaminar shear strength (ILSS) and Mode I interlaminar fracture toughness of 48K CFRP was greater than 12K CFRP due to the higher fiber surface activity of 48K carbon fibers and the stronger obstructing effect of 48K fiber tows on the diffusion of water. The degrees of post‐curing and hydrolysis of CFRP were less than 10%. The degradation of the fiber‐matrix interface and a decrease in the strength and elastic modulus of the matrix led to the degradation of the tensile properties, ILSS, and Mode I interlaminar fracture toughness of CFRP.Highlights Debonding of sizing agent reduced the strength of the carbon fiber filaments. Degrees of hydrolysis and post‐curing of the matrix caused by immersion were low. Retentions of ILSS and fracture toughness of 48K CFRP were higher than 12K CFRP. 48K fiber tow possessed a stronger obstructing effect on water diffusion.

Bio-inspired self-stitching for enhancing ductility and impact resistance of unidirectional laminated composites

This paper innovatively proposed sandwich-type hybrid carbon fiber reinforced polymer (CFRP) laminates with continuous layers and discontinuous self-stitching layers. The self-stitching structure, characterized by nesting of adjacent layers, was inspired by the unique overlapping feature of ironclad beetle and brick-and-mortar feature of nacre. Three-point bending according to ASTM D7264 were performed using an universal testing machine and Charpy impact according to ISO 179-2 were performed using a Digital IZOD Impact tester to evaluate the mechanical performance. The self-stitching microstructure can significantly enhance the toughness and pseudo-ductility of CFRP laminates by promoting delamination and activating large scale fiber bridgings additionally. Compared with unidirectional CFRP, a well-designed hybrid laminates with self-stitching layers can simultaneously improve the flexural stiffness, energy absorption and failure strain by 9.1%, 81.5% and more than 228% under quasi-static bending conditions, respectively. Meanwhile, the dynamic energy absorption of the well-designed hybrid laminate outperformed that of unidirectional CFRP by 66.5%. This work achieved a good trade-off between strength and toughness of CFRPs and provided a new way for the design of high-performance materials.

Review of the surface treatment process for the adhesive matrix of composite materials

To deepen the research on the surface treatment technology of bonding matrix of composite materials, carbon fiber reinforced polymer (CFRP) was selected as the research object, and the research status at home and abroad were reviewed from three aspects: matrix characteristics, durability and bonding and stripping effect before and after surface treatment. Because of the effects of existing surface treatments on the surface roughness, surface wettability, surface morphology, fatigue characteristics, shear strength, and tensile section morphology of CFRP, the influence mechanism of surface treatments on the bonding properties of CFRP was summarized, and the future research focus and direction prospected. The results show that the bonding effect depends on the good mechanical interlocking and adsorption/chemical bonding between the adhesive and CFRP surface. The surface treatment can remove the surface pollutants of CFRP, optimize the surface topography characteristics, and enhance the spreading ability of the adhesive on the substrate surface, thereby enhancing the bonding ability between CFRP and the adhesive. The combination of different surface treatments can significantly improve the preparation quality of the CFRP surface, and avoid the defects of the single surface treatment itself to a certain extent. The surface treatment technology of the bonding matrix is developing in the direction of precision, refinement, and accuracy.

Short-Term Performance of New Composite Anchorage with Multiple CFRP Tendons

The relatively low shear strength of carbon fiber-reinforced polymers (CFRPs) makes it difficult to anchor CFRP tendons. Although many anchorages with CFRP tendons have been developed, few studies have been conducted on composite anchorages with multi-CFRP tendons. In this study, four groups of new mechanical clamping–bonding composite anchorages with multi-CFRP tendons were designed, and the stress of each component of the anchorage was analyzed to reveal the force transmission mechanism of the anchorage. Moreover, the short-term performance of the anchorages was experimentally investigated. The key parameters, including the elastic modulus of the bonding medium, preloading force, cone angle, and bond length, were further analyzed using the finite-element method. The results showed that the composite anchorage reliably anchored multiple CFRP tendons, and the efficiency coefficients of the four anchorages were greater than 0.95. The suggested minimum bond length was approximately 35 times the diameter of the CFRP tendon when anchoring fewer than five tendons. Finally, a sufficient preloading force could effectively reduce the initial slip, whereas increasing the cone angle and elastic modulus could reduce the final slip.

Experimental and theoretical study on flexural behavior of steel–concrete composite beams strengthened by CFRP plates with unbonded retrofit systems

In this paper, the flexural behaviors of steel-concrete composite beams strengthened by prestressed carbon fiber-reinforced polymer (CFRP) plate with unbonded retrofit systems were investigated via four-point bending experiments. Three composite beams were strengthened by two groups of CFRP plate with thickness of 2mm or 3mm and prestress level of 10% or 15%, while one un-strengthened composite beam played a role as reference specimen. CFRP plate was transversely stretched by two scissors-type jacks and fixed on the tension flange of steel girder by an independently designed mechanical anchorage system. The experimental results reveal that the application of prestressed CFRP plate efficiently increased the yielding load and ultimate load of steel-concrete composite beams, but the ultimate deflection of mid-span was little affected. The increase of cross-sectional area of CFRP plates contributes more to stiffness of beams than that of prestress level. The CFRP strength utilization ratios of strengthened beams BS-1 and BS-3 at failure are 80%, under the condition of proper anchorage. Moreover, the prestress loss of CFRP plate was monitored for about 100h before test, which was less than 3%. Furthermore, a theoretical model was proposed to calculate the ultimate flexural resistance of strengthened steel-concrete composite beams with unbonded retrofit systems, which was validated with the experimental results.

On the role of dowel action in shear transfer of CFRP textile-reinforced concrete slabs

Carbon-fiber-reinforced polymer (CFRP) grids or textiles for concrete have high application potential in thin planar members such as bridge deck slabs or façade plates. With the high tensile strength of CFRP, shear failure governs design in wider regions, which calls for optimized shear models incorporating all relevant load-transfer mechanisms. The contribution of dowel action to shear resistance is often neglected in shear models derived specifically for FRP reinforcement. As reasons, the low transverse modulus of the anisotropic material and the small cross-sectional areas per reinforcement element and thus small cross-sectional stiffness are mentioned. Yet fundamental research on this mechanism for CFRP grids is amiss. This paper presents a new test setup for characterizing the dowel action of CFRP grids that are pre-strained, similar as in real applications where the reinforcement in the shear crack is stressed in flexural tension. The influence of (pre-) crack width, pre-strain and concrete cover is discussed. By comparison to results from flexural shear-tests it can be shown that, depending on the effective depth, the contribution to shear resistance can be significant. Furthermore, the important role of dowel cracking in the shear-compression failure mechanism could be analyzed using the results of the new dowel test.

Exit delamination at the material interface in drilling of CFRP/metal stack

Carbon fiber reinforced polymer (CFRP)/metal stacks have the merit of a high tensile strength-to-weight ratio with the CFRP and high compressive strength with the metal. To use CFRP as a material of structural component in industries, drilling is required to make holes for joining with other parts. In drilling, compared to the component built with CFRP only, the CFRP/metal stacks are less vulnerable to the exit-ply delamination caused by the thrust force because the metal part of the stack plays a role of a back-up plate. However, the delamination frequently occurs at the CFRP-metal interface, even though the thrust force is too small to generate delamination. Existing experimental studies reported that cutting torque as well as thrust force results in exit-ply delamination in CFRP/metal stack drilling, but the mechanism of delamination behavior at the CFRP-metal interface is not established. This study proposes a theoretical model to investigate the exit-ply delamination at the interface between CFRP and metal caused by the cutting torque. Using the model, we calculated the allowable feed limit to prevent cutting torque-induced exit-ply delamination. Through the drilling experiments, the proposed model was validated.

Effect of pre-existing microstructural damage and residual stresses on the failure response of carbon fiber reinforced polymers

A comprehensive numerical study, relying on high-fidelity finite element (FE) failure simulations, is presented to quantitatively understand the effect of pre-existing microstructural damage and residual stresses caused by instantaneous load/unload cycles on the failure response of a carbon fiber reinforced polymer (CFRP). A representative volume element (RVE) of this composite is virtually reconstructed/meshed and realistic pre-existing damage/stresses are induced into its microstructure via loading/unloading simulations with different types (tensile or compressive), directions, intensities, and sequences. After applying a final load causing failure, we study the impact of these parameters on the CFRP strength calculated via computational homogenization relying on FE simulation results. The study shows that the strength of a pre-damaged RVE is highly unpredictable, meaning despite causing notable microstructural damage, in some cases the effects of initial load/unload cycles on strength are either negligible and may even slightly increase that, while in other cases it could lead to a massive drop in strength. The lack of a meaningful trend and in particular no direct correlation between strength and the pre-loading parameters (intensity, type, direction, and number/sequence of cycles) and the extent of pre-existing microstructural damage could be attributed to the effect of residual stresses, causing a high level of uncertainty in the failure response of CFRP.

Concurrent topology and fiber orientation optimization of CFRP structures in space-borne optical remote sensor

Lightweight design is one of the most important concepts in the design of space-borne optical remote sensors. Carbon fiber reinforced polymer (CFRP), as one material with high ratio of modulus to density, is widely employed to realize the lightweight design. To maximize the stiffness and strength of CFRP with prescribed mass, fiber orientation and topology optimization are accomplished and optimum design result is acquired in this paper. First, the optimization algorithm of fiber orientation is put forward to find the optimal fiber direction and verified by theoretical analysis and simulations, illustrating that [90°/+ 45°/− 45°/0°] fiber orientation and symmetrical laminas are indispensable. Furthermore, the topology optimization algorithm including the density interpolation method and sensitivity analysis method is brought about to find the optimal material distribution. The algorithm is validated to be convergent and can provide a conceptual model which offers a reference for the critical design. Finally, acquired in the mechanical test is the performance of the optical remote sensor whose first three characteristic frequencies are respectively 66.95 Hz, 70.37 Hz, and 98.84 Hz. The mechanical amplification factor is 5.29, which meet the performance requirement of the stiffness and strength for the optical remote sensor.

Enhancing buckling capacity of angle steel using unbonded CFRP laminates processed by Vacuum-assisted Resin Transfer Molding (VaRTM)

The carbon fiber reinforced polymer (CFRP) adhesive-bonding technique has been increasingly developed for strengthening steel structures. However, the application of this strengthening method is still considered ineffective since it requires complicated and burdensome steel surface treatments. The bonding strength of CFRP on steel, on which this method highly depends, is also prone to performance decreases due to inevitable environmental exposures. Alternatively, unbonded CFRP strengthening is proposed in this study. A total of twelve specimens are prepared and tested to evaluate the possibility of application of this strengthening method in enhancing buckling performance of angle steels. All CFRPs on the strengthened specimens are created through a process of Vacuum-assisted Resin Transfer Molding (VaRTM). It is concluded from the investigation results that the unbonded CFRP is effective in strengthening angle steels, where the buckling strength enhancements by 8.5%-54.3%. The strength increase is clearly affected by CFRP length, number of CFRP layers, and steel’s slenderness ratio. A change in the curvature of the specimens due to the presence of unbonded CFRPs can also be observed. The VaRTM process produces higher fiber content of laminates. Analytical studies carried out at the end of this paper provide a reasonable solution for strength design and a good correspond for post-buckling prediction.

Shear strength investigation of the carbon fiber reinforced polymer-wrapped concrete beams using gene expression programming and finite element analysis

ABSTRACT Existing analytical and numerical models available in the literature can predict the shear strength of CFRP in RC beams based on the FRP strains and this requires measuring the FRP strain values. This research aims to predict the shear strength of CFRP in RC beams without the need to calculate the strains in FRP. Therefore, Gene Expression Programming (GEP) is used in this research to forecast the contribution of the CFRP to shear strength in the RC beams without the need for calculating the strain of the CFRP. A comparison was later performed amongst the empirical findings of the developed GEP model and other models available in published research. The adequate accuracy and highest predictive ability were noticed for the developed GEP model relative to the models published previously. Strengthened reinforced concrete (RC) beams with epoxy-bonded carbon fiber reinforced polymer (CFRP) sheets at various angles of orientation were examined to investigate their shear behavior. The experimental outcomes of the tested specimens were then applied to build a finite element (FE) model using ABAQUS. The proposed FE model was able to forecast the experimental behavior of the samples examined with sufficient precision.

Performance of RC beams externally strengthened with hybrid CFRP and PET-FRP laminates

The use of fiber-reinforced polymers (FRP) composite materials in strengthening applications of reinforced concrete (RC) structures has been gaining wide popularity in recent decades. This is due to its superior properties such as high strength to weight ratio, durability, and versatility. In fact, it is well established that bonding FRP materials to the soffit of RC beams enhances the flexural capacity of such beams. However, the non-yielding characteristic of FRP materials is a major concern, and often results in sudden and brittle failure mode of the strengthened member. To encounter this issue, a new type of FRP materials composed from polyethylene terephthalate (PET) fibers have been developed. Compared to conventional FRPs, PET-FRP have large deformability and possess a nonlinear stress-strain relationship. Employing PET-FRP in the retrofitting industry reduces construction waste, enhances the capacity of structures and provides a solution that encourages the concept of sustainability. However, these types of large rupture strain (LRS) FRPs have lower stiffness and tensile strengths than conventional FRPs. Therefore, the main aim of this study is to combine the lower stiffness and large rupture strain of PET-FRP sheets with that of the higher stiffness and strength of carbon FRP (CFRP) sheets resulting in a new hybrid composite system. The research program consists of four RC beams externally strengthened with CFRP, PET-FRP, and their hybrid combinations, in addition to a control unstrengthened beam specimen. The beams are tested under four-point bending and load-displacement curves along with the failure modes, strength, strain in the FRP, and ductility of the beam specimens are examined. Test results indicate that strengthening with PET-FRP laminates significantly enhances the deformation capacity of the strengthened specimens compared to that with CFRP. In addition, the hybrid mix between CFRP and PET-FRPs resulted in 46-48% strength improvement compared to the unstrengthened control beam. However, the effectiveness of the hybrid system was not pronounced in terms of ductility due to the premature debonding of the concrete cover that occurred before utilizing the full strain of the hybrid system. Hence, it is advised for future research studies to anchor the hybrid sheets by means of end U-wraps or FRP spike anchors to delay the debonding failure and to exploit the benefits of the proposed hybrid system.