Unidirectional CFRP Research Articles

Residual Strain Monitoring and Dynamic Characteristics of Hybrid Hollow Square Tube with Metal–FRP–Metal Sandwich Walls

In order to design and fabricate a novel hollow square tube (HST) for ram structure in machine tools, the hybrid HST with sandwich walls based on steel skins and unidirectional carbon fiber-reinforced polymer (CFRP) composite core is proposed. A detailed co-cured fabrication method with embedded fiber Bragg grating (FBG) sensors for residual strains/stresses determination in a hybrid metal–composite structure is presented. Results reveal that the hybrid HST has undergone complex residual strain history, and the strain rate is about 10 times the cooling rate. The tensile strains in the dwell stage transform into compressive strains in the cooling stage due to the mismatch of the coefficients of thermal expansion of the steel plates and the CFRP composite. A comparison of the residual strains in the cooling phase obtained by FBG sensors with those obtained by theoretical calculation is carried out. Furthermore, the dynamic characteristics of the hybrid HST and the steel HST are tested. The results showed that the damping of the hybrid HST is 586% higher than that of the steel HST, while the hybrid HST has a lower first natural frequency (4.6% reduction) and mass (15.9% weight reduction). The influence of co-cure temperature and cooling rate on the size and state of the residual strains is analyzed, which might be helpful to guide the manufacturing of sandwich structures in machine tools. This novel hybrid HST may be used for online health monitoring and safety evaluation to build intelligent machine tools structures.

An innovative frictional anchorage system for flat CFRP ribbon strips

Unidirectional carbon fibre-reinforced polymer (CFRP) materials represent a promising alternative to steel because of their lightweight, high tensile strength, and excellent corrosion and fatigue resistance. Stress-ribbon structural systems define the potential application object of unidirectional flat CFRP strips. However, anchorage difficulties make this idea problematic—the typical gripping systems are inefficient for anchoring the CFRP ribbons because of the tremendous thrust forces-induced stress concentration. Thus, the innovative frictional spiral anchorage system of the flat CFRP strips is the object of this study, which formulates the design principles for the frictional joins. Furthermore, the proposed theoretical model minimises the gripping system size, satisfying the CFRP strength limitation when the developed modified Archimedean spiral determines the shape of the contact surface. This work experimentally demonstrates the theoretical concept’s adequacy—the mechanical resistance prediction error does not exceed 7% in the loading range covering the service load conditions. In addition, the manuscript presents several examples of frictional gripping systems, including the stress-ribbon pedestrian bridge prototype, and provides further insights into the frictional anchorage systems.

Improving adhesive behavior of fiber reinforced composites by incorporating electrospun Polyamide-6,6 nanofibers in joining region

Adhesive joining of fiber reinforced polymer (CFRP) composite components is demanded in various industrial applications. However, the joining locations frequently suffer from adhesive bond failure between adhesive and adherent. The aim of the present study is improving bonding behavior of adhesive joints by electrospun nanofiber coatings on the prepreg surfaces that have been used for composite manufacturing. Secondary bonding of woven and unidirectional CFRP parts was selected since this configuration is preferred commonly in aerospace practices. The optimum nanofiber coating with a low average fiber diameter and areal weight density is succeed by studying various solution concentrations and spinning durations of the polyamide-6.6 (PA 66) electrospinning. We obtained homogeneous and beadles nanofiber productions. As a result, an average diameter of 36.50 ± 12 nm electrospun nanofibers were obtained and coated onto the prepreg surfaces. Prepreg systems with/without PA 66 nanofibers were hot pressed to fabricate the CFRP composite laminates. The single-lap shear test coupons were prepared from the fabricated laminates to examine the effects of PA 66 nanofibers on the mechanical properties of the joint region of the composites. The single-lap shear test results showed that the bonding strength is improved by about 40% with minimal adhesive use due to the presence of the electrospun nanofibers within the joint region. The optical and SEM images of fractured surfaces showed that nanofiber-coated joints exhibited a coherent failure while the bare surfaces underwent adhesive failure. The PA66 nanofibers created better coupling between the adhesive and the composite surface by increasing the surface area and roughness. As a result, electrospun nanofibers turned adhesive failure into cohesive and enhanced the adhesion performance composite joints substantially.

Synergistically Improving Mechanical and Interfacial Properties of Epoxy Resin and CFRP Composites by Introducing Graphene Oxide

In this study, graphene oxide (GO) was employed as nanoscale reinforcement for the development of high-performance carbon fiber reinforced plastic (CFRP) composites. Initially, epoxy resin was modified by incorporating GO with different weight proportion from 0.05 to 0.6 wt.%. Then the unidirectional CFRP composites were prepared with the modified epoxy resin by winding and compression molding technique. The optimized GO-CFRP composites with GO content of 0.1 wt.% present tensile strength of 2756 MPa and monofilament interfacial shear strength of 29.06 MPa, respectively, which are 14.4% and 12.5% higher than the corresponding values of the pristine CFRP composites. To intuitively observe the fracture process of the CFRP composites, the digital image correlation system was employed. It is verified that the moderate addition of GO can improve the stress concentration of the CFRP composites during the deformation process. In addition, the reinforcing mechanism is investigated by analyzing the fracture surface of the modified epoxy resin and the CFRP composites. The results indicate that GO can make the cracks deflect or bifurcate along with the epoxy resin which closes to graphene, resulting in synergistically improved mechanical and interfacial properties of the GO-modified CFRP composites.

Tailoring thermal expansion of mechanically interlocked carbon fiber composite reinforced metal

AbstractA method of controlling the thermal expansion of lightweight metals through the insightful incorporation of unidirectional carbon fiber reinforced polymer (CFRP) is demonstrated. The local and directional coefficients of linear thermal expansion (CLTE) of a carbon fiber composite reinforced aluminum (CFRP/Al) specimen can be controlled by tailoring the CFRP volume fraction and CFRP layup orientation, with a 36% reduction in CLTE achieved as compared to the baseline 7075 aluminum at a CFRP volume fraction of 0.26. When a unidirectional CFRP reinforcement is considered, the CLTE as predicted by the modulus modified rule of mixtures is within 1.3% of experimentally obtained values for CFRP volume fractions of 0.10, 0.16, and 0.26. Thermal digital image correlation (T‐DIC) was performed to obtain local and directional CLTE values for the anisotropic CFRP/Al specimens which indicated CLTE is heterogeneous across the specimen length and width, which is otherwise undetected using standard quartz rod dilatometry to measure the CLTE of the macroscopic sample. Evidence for a minimum critical CFRP reinforcement length is also considered based upon the local thermal strain data collected by T‐DIC. Using the mixed material approach presented here, it is possible to minimize or eliminate the thermal expansion mismatch effects that occur in mixed material structures, such as an automotive body structure (i.e., body in white).

Statistical fatigue failure life of unidirectional CFRP under bending load

Our developed accelerated testing methodology (ATM) based on the matrix resin viscoelasticity for the failure life prediction of carbon-fiber reinforced polymers (CFRP) was applied for statistical prediction of long-term fatigue failure life under bending load for the longitudinal direction of unidirectional CFRP laminates, which is an important basic aspect of design for the durability of CFRP structures used for aircraft and other applications. First, the formulations for fatigue life prediction of FRP under cyclic loading are introduced. Second, the time-dependent and temperature-dependent viscoelasticity of matrix resin and the statistical flexural static strengths of CFRP laminates at various temperatures are introduced as the basic data of failure life prediction of CFRP laminates. Third, fatigue bending tests for CFRP laminates are performed at various load levels with various frequencies and temperatures. A master curve of non-dimensional fatigue strength versus number of cycles to failure (S-N master curve) is constructed by substituting data of the second and third steps into the formulations. Finally, the long-term fatigue strength data obtained under cyclic bending load for unidirectional CFRP laminates are compared with the long-term creep strength obtained in the previous paper.

Lightning strike damage resistance of carbon‐fiber composites with nanocarbon‐modified epoxy matrices

AbstractCarbon‐fiber reinforced polymer (CFRP) composites are replacing metal alloys in aerospace structures, but they can be vulnerable to lightning strike damage if not adequately protected due to the poor electrical conductivity of the polymeric matrix. In the present work, to improve the conductivity of the CFRP, two electrically conductive epoxy formulations were developed via the addition of 0.5 wt% of graphene nanoplatelets (GNPs) and a hybrid of 0.5 wt% of GNPs/carbon nanotubes (CNTs) at an 8:2 mass ratio. Unidirectional CFRP laminates were manufactured using resin‐infusion under flexible tooling (RIFT) and wet lay‐up (WL) processes, and subjected to simulated lightning strike tests. The electrical performance of the RIFT plates was far superior to that of the WL plates, independent of matrix modification, due to their greater carbon‐fiber volume fraction. The GNP‐modified panel made using RIFT demonstrated an electrical conductivity value of 8 S/cm. After the lightning strike test, the CFRP panel remains largely unaffected as no perforation occurs. Damage is limited to matrix degradation within the top ply at the point of impact and localized charring of the surface. The GNP‐modified panel showed a comparable level of resistance against lightning damage with the existing copper mesh technology, offering at the same time a 20% reduction in the structural weight. This indicates a feasible route to improve the lightning strike damage resistance of carbon‐fiber composites without the addition of extra weight, hence reducing fuel consumption but not safety.

Meso/macroscale study on the shear failure of CFRP laminates considering fibre rotation

The shear failure mechanisms of ±45° braided and unidirectional carbon fibre reinforced polymer (CFRP) laminates are investigated. Off-axis tensile tests were carried out to determine the performance and failure modes of laminates with different numbers of layers. A mesoscale unit cell finite element model was established, and the simulation was validated. The results show that braided laminates and unidirectional laminates have different failure cracks. With the increase in the number of layers, the failure mode of laminates changes. For laminates with 8 or more layers, the necking effect appears at the fracture, and there is a fibre rotation strengthening stage in the stress–strain curve. The stress–strain response functions of laminates in the fibre rotation strengthening stage fitted according to test data can effectively reflect the properties of ±45° braided and unidirectional CFRP laminates.

Axial behavior of square and circular concrete columns confined with CFRP sheets under elevated temperatures: Comparison with welded-wire mesh steel confinement

The main goal of this study was to carry out an experimental investigation of the effectiveness and the efficiency of using carbon fiber-reinforced polymer (CFRP) strips and welded wire mesh jackets for retrofitting square and circular columns exposed to highly elevated temperatures. The investigated variables included the geometry of the column, strengthening scheme, and heating temperature. The samples were divided into three groups. The first group contains column samples not exposed to elevated temperatures and had no strengthening applied. The second category contains un-strengthened column samples subjected to elevated temperatures; the third category comprises strengthened column samples exposed to elevated temperatures. The elevated temperature exposure consisted of heating the specimens to 600 °C for 3 h. The column samples were subjected to axial compression testing till failure. The test results showed that both the circular and square columns exhibited reductions in axial capacity and initial stiffness when exposed to elevated temperatures. The use of welded wire mesh confinement around the circular and square columns exposed to elevated temperatures provided a more significant increase in strength as well as stiffness compared to strengthening the columns with unidirectional CFRP strips. In contrast, the CFRP repaired heated columns exhibited a significant increase in ductility and deformation capability without any reduction in strength than the WWM jacketed heated columns. Furthermore, the confinement effectiveness of CFRP jacketing provides considerable confinement to micro-cracked concrete compared to the WWM jacketed RC circular columns. However, in square columns the confining effectiveness for WWM was better compared to CFRP strips. Moreover, the use of CFRP strips were ineffective for recovering the stiffness of columns exposed to elevated temperatures. In addition, the results demonstrated that the use of welded wire mesh confinement and CFRP strips are cost-effective retrofitting techniques that could be used for circular and square RC columns.

Feasibility Study of EDM-Assisted Combined Turning for Unidirectional CFRP

Carbon fiber-reinforced plastics (CFRP), which are classified as functional resins, are rapidly replacing conventional materials because of their excellent properties. Typically, they have been used to fabricate components of airplanes or cars. In the field of medicine, the demand for micro-machined products manufactured with lathes is also increasing. However, owing to the significant tool wear caused by the carbon fiber, CFRP machining can result in burrs and inaccuracies in the finished product. The tool wear caused by carbon fiber must be reduced to ensure high dimensional accuracy. In this study, the possibility of combining conventional turning with electric current or electrical discharge machining was explored.

Charpy impact properties of uni-directional carbon fiber-reinforced polymer tendons with protective layers

The present study explored potential forms of impact protection for carbon fiber-reinforced polymer (CFRP) tendons. Charpy impact tests were performed on CFRP rods and strands under protections of polyvinyl chloride (PVC) and poly-ethylen (PE). In the experimental program, the impact damage pattern and absorbed energy of CFRP tendons with and without external protection were obtained. The results show that the energy absorption capacity of CFRP tendons was improved under the protection of surface layers; and PE layers exhibited the highest protection effectiveness among all protection forms in terms of the increased absorbed energy and caused additional structural weight. Besides, two empirical expressions were established to quantify the relationship between the thickness of PE layers and the energy absorption capacity of the protected CFRP tendons, which may provide a simple tool to estimate the thickness of the layer required for the protection of a specific CFRP tendon in practical applications.

Tool wear prediction in edge trimming of carbon fiber reinforced polymer using machine learning with instantaneous parameters

Rapid tool wear occurs during the edge trimming of CFRP components due to the high strength and abrasive nature of the carbon fibers, leading to potential surface damage and delamination. This paper presents a hybrid approach by combining the machine learning technique and the physics-based kinematics model for tool wear prediction in edge trimming of CFRP. For edge trimming of unidirectional (UD) CFRP, the feature parameters of the machine learning models consist of the instantaneous fiber cutting angle, uncut chip thickness, and instantaneous radial force instead of the average force of a certain cutting period. The training data from UD CFRP is used to predict the overall radial force in edge trimming of multidirectional (MD) CFRP considering the nonuniform wear length distribution along the tool edge due to different fiber orientation angles of the UD layers. The machined surface, tool edge profile, and chip morphology were examined to demonstrate the nonuniform tool wear length and its effect on surface quality in edge trimming of MD CFRP. The tool wear monitoring in edge trimming of MD CFRPs with various lay-up configurations is achieved without the need for data training experiments on MD CFRP. The predictions from the machine learning methods are compared with experimental results in edge trimming of UD and MD CFRPs. As a result, the proposed method is able to reduce the experimental cost, considering the increased dimensionality if experimental data training of various fiber orientations of UD CFRP and various fiber lay-up configurations of MD CFRP is required.

Prediction model of chisel edge thrust force and material damage mechanism for interlaminar-direction drilling of UD-CFRP composite laminates

Interlaminar-direction drilling of carbon fiber reinforced plastic (CFRP) is a new machining mode different from the conventional drilling. The thrust force on chisel edge of interlaminar-direction drilling is a key factor for the machining quality of CFRP. Therefore, it is necessary to accurately predict the thrust force on chisel edge, and reveal the material damage mechanism for the study of CFRP interlaminar-direction drilling. In this work, the chisel edge of the conventional twist drill is divided into two parts: the cutting edge and the rake face. By respectively establishing the thrust force models of the cutting edge and the rake face, the prediction model of the thrust force of chisel edge for interlaminar-direction drilling of unidirectional CFRP (UD-CFRP) is established. The reliability of the prediction model is verified by the designed experiments. Moreover, the response law of the thrust force of chisel edge to the machining parameters is discussed in detail, and the material damage mechanisms under the action of chisel edge and main cutting edge are revealed by micro characterization of materials. This study is expected to enrich the theory of interlaminar-direction drilling of CFRP, and provide a theoretical basis for guiding the strategy about interlaminar-direction drilling of CFRP.

Bio-inspired self-stitching discontinuous fiber reinforced composites with enhanced ductility and energy absorption

This paper innovatively proposed a biomimetic self-stitching carbon fiber reinforced polymer (CFRP) with a unique microstructure of inter-laminar overlapping region inspired by the overlapping feature of ironclad beetle and brick-and-mortar feature of nacre. To investigate the effectiveness of self-stitching microstructure, two baseline configurations of continuous unidirectional CFRP and discontinuous staggered CFRP were designed. Three-point bending tests showed that the self-stitching microstructure can significantly enhance the pseudo-ductility of CFRP composite by inhibiting crack propagation through additional fiber bridging. In addition, varying the overlap length can also result in different mechanical responses and failure modes. Compared to unidirectional CFRP, the self-stitching CFRP with a well-designed discontinuity at the ply level can improve failure strain and energy absorption by more than 228% and 85%, respectively.

I-V characteristics and temperature response of unidirectional CFRP exposed to simulated lightning current in the fiber direction

This study examined the I-V characteristics and temperature response of unidirectional CFRP exposed to simulated lightning current by performing direct electrical conduction tests that could minimize the effects of thermal flux from the arc root, complex arc behavior, and shock wave. A numerical analysis was carried out to clarify the heating mechanisms. The results show that the electrical conductivity of unidirectional CFRP in the fiber direction under impulse current tests is almost identical to that measured in the steady-state and becomes constant during lightning current increasing. Cross-sectional observation of the specimen revealed that matrix cracks and delamination occurred in the temperature range where the CFRP thermal decomposition occurred. In addition, it was necessary to consider the formation energy of the pyrolysis gas to estimate the temperature response, which was estimated to be approximately 2.163 × 107 J/kg for the toughened epoxy CFRP.

A Novel Micromechanics-Model-Based Probabilistic Analysis Method for the Elastic Properties of Unidirectional CFRP Composites

Considerable uncertainties in the mechanical properties of composites not only prevent them from having efficient applications but also threaten the safety and reliability of structures. In order to determine the uncertainty in the elastic properties of unidirectional CFRP composites, this paper develops a probabilistic analysis method based on a micromechanics theoretical model and the Monte Carlo simulation. Firstly, four commonly used theoretical models are investigated by calculating the deterministic elastic parameters of three unidirectional CFRP composites, which are compared with experimental outcomes. According to error analyses, the bridging model is the most brilliant one, with errors lower than 6%, which suggests that it can be used in probabilistic analyses. Furthermore, constituent parameters are regarded as normally distributed random variables, and the Monte Carlo simulation was used to obtain samplings based on the statistics of constituent parameters. The predicted probabilistic elastic parameters of the T800/X850 composite coincide with those from experiments, which verified the effectiveness of the developed probabilistic analysis method. According to the probabilistic analysis results, the statistics of the elastic parameters, the correlations between the elastic parameters, and their sensitivity to the constituent’s properties are determined. The moduli E11, E22, and G12 of the T800/X850 composite follow the lognormal distribution, namely, ln(E11)~N[5.15, 0.0282], ln(E22)~N[2.15, 0.0242], and ln(G12)~N[1.48, 0.0382], whereas its Poisson’s ratio, v12, obeys the normal distribution, namely, v12~N(0.33, 0.0122). Additionally, the correlation coefficients between v12 and E11/E22/G12 are small and thus can be ignored, whereas the correlation coefficients between any two of E11, E22, and G12 are larger than 0.5 and should be considered in the reliability analyses of composite structures. The developed probabilistic analysis method based on the bridging model and the Monte Carlo simulation is fast and reliable and can be used to efficiently evaluate the probabilistic properties of the elastic parameters of any unidirectional composite in the reliability design of structures in engineering practice.

Numerical Analysis of Micro-Residual Stresses in a Carbon/Epoxy Polymer Matrix Composite during Curing Process.

The manufacturing process in thermoset-based carbon fiber-reinforced polymers (CFRPs) usually requires a curing stage where the material is transformed from a gel state to a monolithic solid state. During the curing process, micro-residual stresses are developed in the material due to the different chemical–thermal–mechanical properties of the fiber and of the polymer, reducing the mechanical performance of the composite material compared to the nominal performance. In this study, computational micromechanics is used to analyze the micro-residual stresses development and to predict its influence on the mechanical performance of a pre-impregnated unidirectional CFRP made of T700-fibers and an aeronautical grade epoxy. The numerical model of a representative volume element (RVE) was developed in the commercial software Abaqus® and user-subroutines are used to simulate the thermo-curing process coupled with the mechanical constitutive model. Experimental characterization of the bulk resin properties and curing behavior was made to setup the models. The higher micro-residual stresses occur at the thinner fiber gaps, acting as triggers to failure propagation during mechanical loading. These micro-residual stresses achieve peak values above the yield stress of the resin 55 MPa, but without achieving damage. These micro-residual stresses reduce the transverse strength by at least 10%, while the elastic properties remain almost unaffected. The numerical results of the effective properties show a good agreement with the macro-scale experimentally measured properties at coupon level, including transverse tensile, longitudinal shear and transverse shear moduli and strengths, and minor in-plane and transverse Poisson’s ratios. A sensitivity analysis was performed on the thermal expansion coefficient, chemical shrinkage, resin elastic modulus and cure temperature. All these parameters change the micro-residual stress levels and reduce the strength properties.

Laser ablation of CFRP surfaces for improving the strength of bonded scarf composite joints

Repairing damaged composite parts using scarf technique requires a careful selection of treatment methods for composite surface. Laser treatment is one of the emerging techniques to treat the milled composite surface by unlocking various levels of morphological changes and, thus, optimizing joint strength. However, laser parameters, e.g., energy density (fluence), should be carefully determined to ensure the acceptable structural recovery. Here, the influence of CO2 laser with relatively high fluence (ablation effect) on the surface characteristics (roughness, morphology, wettability) and scarf joint strength with associated failure modes of unidirectional (UD) and quasi-isotropic (QI) carbon fiber-reinforced plastic (CFRP) laminates is studied. We found that the ablation effect using CO2 laser at 3.6 J/m2 was considered safe for UD laminates as their joint strength was comparable with that treated by manual sanding. The ablation at higher fluence (8.4 J/m2) reduced the joint strength in UD laminates due to severe damage occurred in 0∘ fibers that triggered adhesive failure. In QI laminates, 3.6 J/m2 laser fluence could improve joint strength since the cohesive failure was activated in off-axis plies (90∘, +45∘, −45∘).

Tailoring fiber arrangement in subsurface damage layer of unidirectional CFRP composites by reverse multi-pass cutting

While the directionality and integrity of microscopic fiber arrangement play an important role in governing the mechanical properties and deformation behavior of unidirectional carbon fiber reinforced polymer (UD-CFRP) composites, minimizing or eliminating the deformation-induced evolution of fiber arrangement is crucial for maintaining the high performance of the advanced composite materials. In the present work, we elucidate the depth-sensing deformation mechanisms of UD-CFRP under different cutting strategies of conventional single-pass and multi-pass by experiments and corresponding micromechanical finite element simulations. Experimental and simulation results reveal diversiform maps of fiber arrangement evolution in subsurface damage layer under different cutting strategies, as well as their correlations with machined surface quality in terms of surface finish, residual stress and mechanical properties. Subsequently, a novel cutting strategy of reverse multi-pass is proposed to tailor the directionality and integrity of microscopic fiber arrangement in subsurface damage layer, which is accompanied with reduced subsurface damage layer, lowered surface roughness and enhanced hardness and elastic modulus, as compared to the cutting strategy of conventional multi-pass. Current findings provide a theoretical basis for the understanding of formation mechanisms of machined surface of CFRP composites, as well as the rational selection of cutting strategies for improving the machinability of CFRP composites.

A data-driven approach to full-field nonlinear stress distribution and failure pattern prediction in composites using deep learning

An image-based deep learning framework is developed to predict nonlinear stress distribution and failure pattern in microstructural representations of composite materials in this paper. The work is motivated by the complexity and computational cost of high-fidelity simulations of such materials. The proposed deep learning framework predicts the post-failure full-field stress distribution and the crack pattern in two-dimensional representations of the composites based on their microstructures. The deep learning framework contains two stacked fully-convolutional networks, namely, Generator 1 and Generator 2, trained sequentially. First, Generator 1 learns to translate the microstructural geometry to the full-field post-failure stress distribution. Then, Generator 2 learns to translate the output of Generator 1 to the failure pattern. A physics-informed loss function is also designed and incorporated to further improve the performance of the proposed framework and facilitate the validation process. The material of interest is selected to be a unidirectional carbon fiber-reinforced polymer composite. 4500 microstructural representations are synthetically generated and simulated using an efficient finite element framework to provide a sufficiently large data set for training and validating the deep learning framework. It is shown that the proposed deep learning approach can predict the composites’ post-failure full-field stress distribution and failure pattern, two of the most complex phenomena to simulate in computational solid mechanics, with an impressive accuracy of 90%.