Carbon Fiber Reinforced Plastics Structures Research Articles

Flexible ECT probe with front-end differential setting for inspection of curved CFRP structures

Carbon-fiber-reinforced plastic (CFRP) structures with curved surfaces are widely used. However, an efficient tool for curved CFRP inspection is still lacking. This paper proposes a flexible eddy current testing (ECT) probe with front-end differential setting designed specifically for curved CFRP structures inspection. The probe contains a pair of coils that are fabricated on a flexible printed circuit board . It can be bent to fit a curved surface without the need to redesign the probe. The two coils of the probe work simultaneously and highlight a defect's signal by the front-end differential configuration. It effectively suppresses common mode interference , which is generally evitable in the experiment of CFRP structures inspection using a high frequency ECT system. A finite element method (FEM) model is established to study the inspection principle in viewing of the anisotropic electrical conductivity of CFRP. A prototype probe is fabricated and tested, with which a flat and three curved CFRP samples with machined and natural impact cracks are imaged. A signal processing algorithm including detrending, gradient calculation and nonlinear transformation is developed to improve the probe's defect detectability . Experiment results show that a small defect located on the curved surface of a CFRP sample with dimensions: length 2 mm, width 1 mm and depth 0.4 mm can be detected by the probe. In addition, the effects of the operating frequency and orientation of the defect are discussed.

Modal Parameter Tracking in a Carbon Fiber-Reinforced Structure over Different Carbon Fiber Angles

The dynamics of carbon fiber-reinforced plastic (CFRP) change according to the carbon fiber angle, and a mode order shift may occur in CFRP specimens. The variation trends in modal parameters differ in each mode; thus, an efficient mode-tracking method is needed to identify the reliable dynamic behavior of the CFRP structure. The mode-tracking method was assumed to be applicable for the same configuration of the tested specimen except for the differences in carbon fiber angle of the CFRP specimen. Simple rectangular specimens were prepared for one isotropic material, SS275, and five anisotropic CFRP specimens with five carbon fiber angles ranging from 0° to 90°. An experimental impact test was conducted to obtain all the modal parameters. The proposed mode-tracking method was applied using three indicators: the modal assurance criterion (MAC) and two modal parameters (resonance frequency and modal damping ratio). The MAC value was valid for the three bending modes at 0°, 30°, and 90°, but not for the two torsional modes. However, the variation in the resonance frequencies was a more efficient indicator with which to track all the modes of interest, except for the second torsional mode. The variation in the modal damping ratio was also a valid indicator for the two torsional modes. Therefore, the proposed three indicators were all required to derive reliable mode tracking for the CFRP specimens considering the mode order shift.

Design Method Using Response Surface Model for CFRP Corrugated Structure under Quasistatic Crushing

The development of a carbon-fiber-reinforced plastic (CFRP) part is carried out by utilizing many experimental results in deciding the design. For this reason, the development period of a CFRP structure is long and an obstacle for commercialization. In this paper, multiple regression analysis is used to derive a response surface that estimates the generated load using the shape parameters of a corrugated collision energy absorbing structure to shorten the development period. To obtain the response surface, we conducted a quasistatic crushing experiment by using the length of linear portions (pitch) and the number of stacks (thickness) of a corrugated shape as parameters. When progressive crushing mode is observed, energy absorption efficiency decreases with the increase in pitch, and increases with the increase in the number of stacks. To discuss how energy absorption efficiency changes, a comparison examination is conducted using the derived response surfaces. Results indicate that specifications with high energy absorption efficiency can be accurately selected using the response surface of primary expression. In addition, differences in deformation mode were due to the influence of the stress at the corner portion of a part.

Analysis on ultrasonic waves generated in anisotropic carbon fiber reinforced plastic laminate by laser incidence from various directions

Laser-ultrasonic waves are used in nondestructive inspections because they can be excited on arbitrarily shaped surfaces by irradiation of a pulsed laser from a distance. During the non-contact scanning of a laser, the waveform of excited ultrasonic wave is changed because the incident angle varies substantially. The change of the waveform will be more obvious for carbon fiber reinforced plastics (CFRP) laminates that have anisotropic properties. In order to guarantee the inspection, in this paper, we investigated the generation of ultrasonic waves in CFRP laminates by a laser irradiation with various incident angles and azimuth angles. Firstly, we constructed a simulation model: the thermal force field caused by the laser absorption is derived from a heat conduction equation, and it is introduced into an FEM software for analyzing the propagation behavior of generated ultrasonic waves. Then, the simulation result is compared with an experiment result. Through the FEM analysis, we found that the amplitude of the excited waves has an obvious directivity because of the strong anisotropy in the thermo-elastic property of the CFRP. In addition, the change in propagation behavior in the case of a small incident angle of 30° was found to be inconspicuous. However, when the incident angle of the laser beam is larger than 60°, the directivity pattern changes depending on the azimuth angle. Thus, it is important to control the incident angle and direction when a laser ultrasonic inspection is applied to CFRP structures.

Relationship between Structural Stiffness and Viscous Damping Coefficient in Reinforced Carbon Structure under Varying Carbon Fiber Angles

A linearized dynamic model of a carbon-fiber-reinforced plastic (CFRP) structure can be formulated using the structural stiffness and viscous damping coefficient. The carbon fiber angle is an influential factor in determining the structural stiffness of CFRP structures by serially combining the stiffness of a binding matrix and that of a carbon fiber. The viscous damping coefficient of the CFRP structure is also highly sensitive to the carbon fiber angle; that is, it assumes a parallel series between the damping coefficient of the binding matrix and that of the carbon fiber. In this study, a sensitivity formula was derived to obtain the ratio of two parameters—the structural stiffness, and the viscous damping coefficient—by dividing all parameters by the value of the reference angle. The CFRP structure was chosen for a simple rectangular specimen with five carbon fiber angles, ranging from 0° (reference) to 90°. The identified modal parameters were used from the impact modal test conducted in a previous study. Sensitivity analysis was conducted for both the structural stiffness and the viscous damping coefficient. The sensitivity results revealed that the sensitivity index of the viscous damping coefficient was proportional to that of the structural stiffness. Even a small value of the viscous damping coefficient of the carbon fiber was sensitive to the CFRP structure because the carbon-fiber damping coefficient was parallel to the large damping coefficient of the binding matrix.

Numerical analysis of Lamb waves propagating through impact damage in a skin-stringer structure composed of interlaminar-toughened CFRP

Many structural health monitoring systems are being developed to detect the impact damages in carbon fiber reinforced plastic (CFRP) structures, which are widely adopted in the aviation industry. In this study, a refined finite element model is established to simulate a Lamb wave propagating through impact damage in a CFRP skin-stringer structure. The accurate stiffness constants are determined for toughened CFRP laminates, which include the resin layers. The impact damage of toughened CFRP laminates is modeled as a truncated-cone-shaped region in which the rigidity is homogeneously reduced. Furthermore, the directive generation of the Lamb wave through a macro fiber composite (MFC) actuator is incorporated in the model. Simulations are performed for several configurations of the MFC and impact damage. The observed tendency of the delay in the arrival time of the lowest antisymmetric Lamb wave mode is noted to be consistent with the experimental results. The proposed model can facilitate the determination of the optimized configuration of MFCs in CFRP structures.

Interlaminar damage assessment method of CFRP laminate based on Synchrosqueezed Wavelet Transform and ensemble Principal Component Analysis

The damage assessment of Carbon Fiber Reinforced Plastics (CFRP) laminate structure is very important to ensure the safety of the structure in service. Machine learning is a research hotspot in damage assessment of CFRP structures. However, there are problems such as noise interference, small number of training samples, and few damage types of training samples. In order to solve the above problems, the damage assessment method of CFRP laminate based on Synchrosqueezed Wavelet Transform and ensemble Principal Component Analysis was proposed. First, the damage feature extraction method based on Synchrosqueezed Wavelet Transform (SWT) and Singular Spectrum Analysis (SSA) was proposed to reduce the feature dimension and achieve effective extraction of damage features. Then, the damage assessment method based on ensemble Principal Component Analysis (PCA) was proposed to realize the damage assessment of CFRP structure. Finally, the simulation experiment was designed to verify the damage assessment results in the case of noise interference, small number of training samples, and few damage types of training samples. The results showed that the damage assessment method of CFRP laminate structure based on SWT, SSA and ensemble PCA was feasible. This paper provided a feasible method for evaluating the damage degree of CFRP laminate structure.

Novel Repair Procedure for CFRP Components Instead of EOL.

Today, numerous carbon fiber (CF) reinforced plastic (CFRP) components are in continuous usage under harsh environmental conditions. New components often replace damaged structural parts in safety-critical applications. In addition to this, there is also no effective repair method to initially restore the mechanics of these structures using dry fiber material. The high costs of CFRP components are not in proportion to their lifetime. The research project IGF-19946 BR “CFRP-Repair” addresses this specific challenge. By using an oxide semiconductor that is activated by ultraviolet (UV) irradiation, the thermoset matrix can be depolymerized and thus locally removed from the damaged CFRP component. Afterward, the harmed fibers can be physically removed from the laminate in this certain area. A load-adjusted tailored fiber reinforcement patch is subsequently applied and consolidated by local thermoset re-infiltrating. Using this procedure, the structure can be locally repaired with new CF. As a result, repaired CFRP structures can be obtained with reduced mechanics and an approximately original surface. This article gives an insight into the developed repair procedure of CFRP components in an innovative and more efficient way than the state-of-the-art.

On the effects of temperature on tensile behavior of carbon fiber reinforced epoxy laminates

As a highly promising lightweight material, carbon fiber reinforced plastics (CFRP) composites have been widely used in aerospace, marine and automotive industries, which could expose to low or high thermal environments with varying material properties. This study investigated the effects of temperature variation ranging from −30 °C to 160 °C on tensile properties of CFRP experimentally and theoretically. The specimens were prepared using thermo-forming technology; and the glass transition temperatures of epoxy resins were measured by dynamic mechanical analysis (DMA). Digital image correlation (DIC) technique was used to capture the variation in strain and damage fields. By means of scanning electron microscopy (SEM), the failure mechanisms, delamination and resin softening, were identified by scrutinizing the images of fracture areas subject to different temperatures. The two-parameter Weibull analysis was conducted to evaluate the theoretical tensile strengths from relatively discrete data. The obtained Weibull parameters are considered useful for the reliability analysis of CFRP structures. It is found that the tensile strength decreases with the increase of temperature in a nonlinear fashion, which can be modeled in terms of a tanh function by correlating with the temperature-dependent experimental data. The study is anticipated to provide fundamental understanding and a predictive tool for mechanical strength of composite structures operating under different thermal conditions.

Correlation of CFRP and AA7075 Aircraft Wing box structure using FEM

Composite materials are becoming more important in the construction of Mechanical structures. The primary objective of this paper is to analyse the Carbon Fibre Reinforced Plastic (CFRP) and Aluminium alloy (AA 7075) aircraft wing box using finite element method. Both results has been correlated. Both CFRP and AA7075 structures modelling and numerical simulations were executed at various loading conditions. The deformed configuration of the CFRP and AA7075 aircraft wing box along with stress plot have been computed using analysis. Based on these studies, the CFRP and AA7075 central wing box, representing a weight saving of up to one and a half tonnes compared to the most advanced aluminium alloys. The results also expose the increasing strength with reduced weight.

Low energy impact damage identification method of CFRP structure based on wavelet transform and probabilistic neural network

Low energy impact damage identification of carbon fiber reinforced plastics (CFRP) structure is of great significance to ensure the safe service of the structure. In this paper, the low energy impact damage identification method of CFRP structure by using fiber Bragg grating (FBG) sensor, continuous wavelet transform and probabilistic neural network (PNN) was studied. Firstly, the CFRP structure with low energy impact damage was actively excited, and the structural dynamic response signals were accurately detected by FBG sensors. Then, the dynamic response signals were subjected to continuous wavelet transform, and their wavelet time-frequency diagrams were extracted as the structural damage feature. On this basis, with damage feature as input and low energy impact damage as output, the damage identification model based on PNN was established. Finally, the experimental system was constructed to verify the feasibility of the method proposed in this paper. The results showed that for 12 groups of test samples, the damage identification model based on PNN can correctly identified 11 groups. This paper provided a feasible method for the low energy impact damage identification of CFRP structure.

On design of carbon fiber reinforced plastic (CFRP) laminated structure with different failure criteria

This study develops a topology optimization approach for design of carbon fiber reinforced plastic (CFRP) laminated components with different failure criteria to reduce the risk of structural failure. The discrete material and thickness optimization (DMTO) method is adopted to parameterize the design variables of thickness and orientation of CFRP composites, which is driven by the Method of Moving Asymptote (MMA) algorithm. A large number of local constraints associated with the failure criteria are aggregated in terms of a p-norm function. Analytical sensitivities are derived with respect to the design variables. In this study, a battery hanging structure in electrical vehicle (EV) is exemplified; and a DMTO based design is prototyped and validated through the in-house experimental tests first. To prevent different failure modes, the Hashin, Hoffman and Tsai-Wu failure criteria are then imposed as the design constraints together with manufacturing requirements in the optimization. A comparative study is performed for the design with and without such failure criteria. The results demonstrate that the maximum failure index of the optimized structure with the Tsai-Wu failure criterion decreases the most by 40%; and follows by the Hashin and Hoffman criteria by 24% and 33%, respectively. Finally, the CFRP structure is also optimized to design a so-called double-double laminates for demonstrating the generality of the proposed method. The study is anticipated to gain in-depth understanding of how the failure criteria would affect the design of fiber reinforced composite structures to ensure structural integrity.

Influence of Humidity on Fatigue Performance of CFRP: A Molecular Simulation.

The study on durability of carbon fiber reinforced plastics (CFRP) in complex environments is critical because of its wide applications. Herein, mechanical behavior of carbon fiber reinforced epoxy composites in the fatigue process were investigated under different humidity via molecular dynamics (MD) simulation method. Transversely isotropic atom based models were established to simulate the structure of CFRP at the atomistic level. Owing to the weak performance in vertical fiber direction, mechanical behavior in a 90° orientation was investigated. Mean stress and energy were both employed to describe the evolution of mechanical performance while mean squared displacement (MSD), radius of gyration (Rg), and free volume were performed to describe the evolution of structural change during the fatigue process. The results show that the humidity led to a weakened interfacial adhesive performance. Free volume became larger under cyclic load, which caused the water molecules to diffuse into the inside of epoxy resin. The distance between the matrix and fiber became larger in the dry system while it reduced because of the diffusion of water molecules in wet system. The rate of performance degradation decreased with the increase in humidity because of poor initial performance at high humidity.

Measurement of directionality in carbon fiber reinforced plastic composite with eddy current testing

In carbon fiber reinforced plastic (CFRP) composite, the alignment of continuous carbon fibers guides the directional flow of eddy currents, which is beneficial to the structural and damage detection. In this study, for the purpose of impact damage repair, the transmitter-receiver (T-R) and the flat-tangent eddy current probes are used to determine the fiber orientations and stacking sequence in the CFRP laminate by surface scanning. Theoretical analysis shows that the T-R probe can flexibly pick up the magnetic field generated by the stretched eddy current in CFRP layers. In the meanwhile, the flat-tangent probe possesses layer selective characteristics. By calculating the fiber distribution images of individual directions based on two-dimensional fast Fourier transform (2D-FFT) and comparing the order of pixel intensity values of these images, the fiber orientation and the stacking sequence in the laminate plates can be obtained simultaneously, which provides guidance for damage detection and repair of the CFRP structures.

Experimental and numerical investigation on the impact response of CFRP under 3-point-bending

The strain rate-dependent material characteristic of carbon fiber reinforced plastics (CFRP) is widely known and has been investigated in detail at coupon level. In this study, for the first time the strain rate dependent characteristic of a three-dimensional CFRP structure was investigated. The evolution of the determined strain rate dependency was correlated with the results at coupon level. For this purpose two special 3-point-bending fixtures were developed to obtain the flexural impact response of the investigated T700S DT120 prepreg system at coupon and component (hat profile) level. The rectangular coupon specimens were loaded with quasi-static to intermediate impact velocities from 3.3×10−5 to 10m s−1, while the structural sub components were tested using impact velocities from 3.3×10−5 to 1m s−1. With increasing impact velocities, the experimental tests showed a significant increase in force at first failure and maximum deflection at coupon level. The increases in force were of 52% and 120%, respectively. However, the increase for structural hat profile components was just 12.4% due to a different failure mode. The observed initial failure modes were compressive failure provoked by fiber kinking for the coupon and interlaminar shear failure for the structural component. Regardless of the different failure modes this work proves the necessity of considering the strain rate dependency of a composite material to accurately predict the maximum load capacity of a CFRP structure during a dynamic load event. Additionally, the comparison of the experimental results restults to numerical results revealed weaknesses in the prediction accuracy of the currently used models.

Temperature-Dependent Dynamic Characteristics of Carbon-Fiber-Reinforced Plastic for Different Spectral Loading Patterns.

The dynamic properties of carbon-fiber-reinforced plastic (CFRP) can be efficiently estimated through a modal damping coefficient and a resonance frequency, and the modal parameters can be calculated using a frequency response function (FRF). The modal parameters used in an CFRP FRF are influenced by the carbon fiber direction, temperature, and spectral loading pattern, as well as the operating conditions. In this study, three parameters—temperature, spectral loading pattern, and carbon fiber direction—were selected as the influential factors for CFRP dynamics, and the sensitivity index formulation was derived from the parameter-dependent FRF of the CFRP structure. The derivatives of the parameter-dependent FRF over the three considered parameters were calculated from the measured modal parameters, and the dynamic sensitivity of the CFRP specimens was explored from the sensitivity index results for five different directional CFRP specimens. The acceleration response of a simple CFRP specimen was obtained via a uniaxial excitation test at temperatures ranging from −8 to 105 °C for the following two spectral loading cases: harmonic and random.

Determination of Impact Damage in CFRP via PVDF Signal Analysis with Support Vector Machine.

Carbon fiber reinforced plastics (CFRPs) have high specific stiffness and strength, but they are vulnerable to transverse loading, especially low-velocity impact loadings. The impact damage may cause serious strength reduction in CFRP structure, but the damage in a CFRP is mainly internal and microscopic, that it is barely visible. Therefore, this study proposes a method of determining impact damage in CFRP via poly(vinylidene fluoride) (PVDF) sensor, which is convenient and has high mechanical and electrical performance. In total, 114 drop impact tests were performed to investigate on impact responses and PVDF signals due to impacts. The test results were analyzed to determine the damage of specimens and signal features, which are relevant to failure mechanisms were extracted from PVDF signals by means of discrete wavelet transform (DWT). Support vector machine (SVM) was used for optimal classification of damage state, and the model using radial basis function (RBF) kernel showed the best performance. The model was validated through a 4-fold cross-validation, and the accuracy was reported to be 92.30%. In conclusion, impact damage in CFRP structures can be effectively determined using the spectral analysis and the machine learning-based classification on PVDF signals.

Topology Optimization of Metal and Carbon Fiber Reinforced Plastic (CFRP) Laminated Battery-Hanging Structure.

This study addressed the topology optimization of a carbon fiber reinforced plastic (CFRP) laminated battery-hanging structure of an electric vehicle. To accommodate parameterization for thickness and orientation of CFRP materials, the discrete material and thickness optimization (DMTO) technique was adopted. To include metal material as a reinforcement structure into the optimization simultaneously, the DMTO technique was extended here to achieve concurrent optimization of CFRP thickness topology, CFRP orientation selection and the topology of the metal reinforcement plate. Manufacturing constraints were applied, including suppressing intermediate void across the thickness direction of the laminate, contiguity constraint and the symmetrical layers. Sensitivities of the objective function were derived with respect to design variables. To calculate analytical sensitivities, finite element analysis was conducted and strain vectors were exported from a commercial software (ABAQUS) into a mathematical analysis tool (MATLAB). The design objective was to minimize the local displacement subject to the constraints of manufacturing and mass fraction. The mechanical performance of the optimized CFRP structure was compared with the original steel structure. To validate the optimization results, a prototype of the CFRP battery-hanging structure was fabricated and experimental testing was conducted. The results show that the total mass of the CFRP battery-hanging structure was reduced by 34.3% when compared with the steel one, while the mechanical property was improved by 25.3%.

Crushing responses and energy absorption behaviors of multi-cell CFRP tubes

Design of multi-cell sectional configuration has been considered an effective way to enhance the crashworthiness performance of metallic thin-walled structure. Nevertheless, it remains under-studied whether this approach can achieve similar effect for fiber reinforced composite structures. Therefore, this paper aims to study the crushing responses and energy absorption behaviors of multi-cell squared carbon fiber reinforced plastic (CFRP) tubes fabricated by using hot compression molding process. The effects of CFRP cell number, wall thickness and trigger mechanism on crashworthiness characteristics were investigated. The experimental results showed that under the same weight, the energy absorption of double-cell tube was lower than that of single-cell tube. This was due to the fact that there was insufficient deformation in the T-shaped region of double-cell CFRP tube. The inner fronds generated in the middle ribs caused part of tubal wall near T-shaped region to bend completely outwards rather than to crush progressively under axial compression, thus losing certain capacity of energy absorption. In addition, a double-shell finite element model was constructed to gain further insights into the crushing behavior of the double-cell CFRP tube. The numerical model effectively replicated the collapse mode; and good agreement was obtained between the experimental and numerical load-displacement curves. The study exhibits considerable potential of developing crashworthy multi-cell CFRP structures.

Electrically conductive carbon fiber layers as lightning strike protection for non-conductive epoxy-based CFRP substrate

A large amount of electrically conductive fillers is needed to enhance a Carbon Fiber Reinforced Plastics (CFRP) electrical conductivity enough to withstand lightning strikes of peak currents. However, such large alien constituents hamper the inherent good mechanical properties of CFRP structures. In this work, a solution has been proposed to retain both desired properties in a CFRP laminate. Layer-wise hybrid laminate has been demonstrated as a solution for lightning strike protection of Carbon Fiber Reinforced Plastics (CFRP). Top few layers of a hybrid laminate are prepared using electrically conductive polymer-based resin (CF/C-POLY) to provide effective dissipation of lightning current while epoxy-based CFRP substrate (CF/Epoxy) provides the main structural strength. An insulating adhesive layer is used to bond CF/C-POLY and CF/Epoxy to prepare the laminate. The hybrid laminates were tested for their effectiveness against lightning strikes. Laminates were struck by modified lightning waveform of component A with peak current of -14 kA and -40 kA. The performance of the laminates against lightning strike were evaluated using high speed camera, high-speed and thermal camera. It is found that CF/C-POLY layer successfully defended the main structural component i.e. CF/Epoxy from lightning direct damage.