Carbon Fiber Reinforced Plastics Structures Research Articles

Theoretical investigation on scattering of ultrasonic Lamb waves at damaged area in quasi-isotropic CFRP laminates based on Mindlin plate theory

ABSTRACT With the wide application of carbon fibre reinforced plastic (CFRP) structure, a structural health monitoring (SHM) system that can easily detect damages in CFRP structures is urgently needed. Since it is possible to detect damages effectively in a large area in CFRP by using the scattering of Lamb waves, in this research, we investigated the scattering of Lamb waves at an impact damaged area in quasi-isotropic (QI) CFRP laminate. By modelling the impact damage as a cylinder area with a uniform stiffness degradation, the scattering waves are calculated based on Mindlin plate theory, which is more precise than Kirchhoff plate theory. It is shown that in the low-frequency region, the calculation results based on Kirchhoff and Mindlin plate theory was similar, but in a higher frequency range, the results differed. The calculation results were compared with the simulation results, suggesting the validity of Mindlin plate theory within a practical frequency range. Moreover, we validated the analytical and simulation results experimentally using a QI CFRP laminate with a hole. We also conducted some parameter study based on the analytical calculation to clarify influential stiffness components. Since scattering behaviour of Lamb waves can be calculated in a fast and easy way using this method, this work is useful for the successful development of a scattering-based SHM system.

Study on Bearing Strength and Failure Modes of Single Bolted Joint Carbon/Epoxy Composite Materials.

The growth of the Urban Air Mobility (UAM) industry emphasizes the need for considerable study into assembly procedures and dependability to guarantee its effective integration into air transport networks. In this context, this study seeks to evaluate the mechanical characteristics of bolted joint Carbon Fiber Reinforced Plastic (CFRP), with a particular emphasis on bearing strength. By altering the w/D (specimen width to hole diameter) and e/D (distance between hole center and specimen end to hole diameter) ratios, the study investigates how edge and end distances affect material performance. The study discovered a shift from tension to bearing failure at w/D ratios of 4.0, with maximum bearing strength decreases of 90.50% and 69.96% compared to full bearing failure. Similarly, for e/D ratios of 1.5, 2.0, and 3.0, transitioning from shear to bearing failure at 2.0 resulted in maximum bearing strength losses of 94.90% and 75.96%, respectively. Maintaining a w/D ratio of at least 6.0 and an e/D ratio of at least 3.0 is critical for maintaining maximum performance and stability in CFRP structure design.

Few-shot meta transfer learning-based damage detection of composite structures

Damage detection and localization using data-driven approaches in carbon fiber reinforced plastics (CFRP) composite structures is becoming increasingly important. However, the performance of conventional data-driven methods degrades greatly under little amount of data. In addition, the scarcity of data corresponding to defect/damage conditions of CFRP structures lead to extreme data imbalance, which make this problem even more challenging. To address these challenges of few training data and the scarcity of damage samples, this paper proposes a few-shot meta transfer learning (FMTL)-based approach for damage detection in CFRP composite structures. This method leverages knowledge learnt from an unbalanced data domain generated from a single CFRP composite sample and adapts the knowledge to be applied for other data domains generated by CFRP samples with different structural properties. The contributions of this research include demonstrating the feasibility of harnessing knowledge from notably limited experiment data, designing an algorithm for configuring hyperparameters based on a specific FMTL task, and identifying the impacts of hyperparameters on learning performances. Results show that FMTL can improve the recall rate by at least 15% while preserving the ability to identify health conditions. This method can be extremely useful when we need to monitor health condition of critical CFRP structures, like airplanes, because they can rarely generate data under damage conditions for model training. FMTL enables us to build new models based on unbalanced source domain data with the cost of a minimal set of samples from the target domain.

Topology optimization for additive manufacturing of CFRP structures

Combining design optimization and additive manufacturing (AM) enables the full exploitation of the potential for heightening the performance of carbon fiber reinforced plastic (CFRP) structures. This study simultaneously conducts topology optimization and fiber path design by employing the radial basis function (RBF) based level set function (LSF). Fiber paths are determined instinctively for the inherent advantages of the LSF, and fiber orientations are parameterized accordingly. Manufacturing drawbacks such as gaps and overlaps can be avoided by introducing a fast-marching method. To verify the effectiveness of the optimization method, three groups of optimized and empirical designs are fabricated by the AM technique, respectively, and the experimental tests are further carried out. Finite element (FE) models are also reconstructed based on the printing schemes, and then the FE simulation is validated by the experimental tests. With the proposed optimization method, stiffnesses for all three groups of the optimal samples are significantly improved compared with the empirical counterparts. The FE modeling technique is capable of reproducing the experimental results. This study paves a new way to develop an integrated framework of optimization, additive manufacturing, experimentation, and validation to deliver high-performance CFRP structures.

Lamb waves evaluation in CFRP plates with laser shock wave technique

Lamb waves, which are guided waves propagating along the surface of a thin plate structure, have been investigated to remotely perform nondestructive testing for aircraft or infrastructures made of carbon fiber reinforced plastic (CFRP) plates. CFRP plates are prone to internal damage such as delamination, which does not occur in metallic materials. Although nondestructive methods such as X-ray, flash thermography, etc. have been investigated in several studies, they may have some disadvantages such as risks of radiation exposure or limitation in terms of size of the structures to be tested for the former, and heat diffusion for the latter. Laser thermoelasticity can generate Lamb waves remotely and nondestructively. However, the generated Lamb waves have a small amplitude; therefore, hundreds of measurements must be averaged to achieve a sufficient signal-to-noise ratio. For these reasons, it has not yet been demonstrated as possible to detect damage in large CFRP structures within a short time using nondestructive, remote-controlled methods. As a first step towards performing nondestructive tests in CFRP plates, this paper demonstrates the generation of A0 mode Lamb waves in the experimental plate samples using laser-induced plasma (LIP) shock waves. The validity of the proposed system is verified by comparing the phase velocities of Lamb waves in the CFRP plates with three different layups obtained with the proposed system, with the theoretical values obtained analytically. This system can generate vibrations in CFRP plates in frequency ranges of a few hundred kHz and allow to determine the phase velocity of the experimental Lamb waves up to approximately 100 kHz.

In-situ damage self-sensing and strength recovery activated by self-electricity in carbon fibrous composites embedded with multi-functional interleaves

It is challenging to simultaneously in-situ monitor and then repair the delamination crack of carbon fiber reinforced plastics (CFRPs) without sacrificing the initial mechanical properties. Herein, two sandwich interleaves are prepared by using healable poly (ethylene-co-methacrylic acid), reinforced and conductive carbon nanotubes (CNTs), and shear-resistant epoxy columns, to achieve self-sensing and self-healing functionalities. The results reveal that both interleaves show significant self-resistance changes to damage initiation and tensile strength of the interleaves themselves can be restored to 67.9–69.1%. Also, the interleaved CFRP exhibits obvious resistance change features during the progressive damage process after multiple healings. Moreover, a self-healing efficiency of more than 90% to interlaminar shear strength after two damage-healing cycles can be obtained. Furthermore, the interleaf with more CNTs possesses higher initial and healed strength. Therefore, the proposed interleaves can be used to prepare CFRP structure with self-sensing and self-healing functions, thereby with improved service reliability and service life.

A time-of-flight based weighted imaging method for carbon fiber reinforced plastics crack detection using ultrasound guided waves

Carbon fiber reinforced plastics (CFRP) are extensively used in aerospace, marine, and automotive fields. A variety of imaging methods for plate structures are proposed to find the damage as early as possible and ensure the safety of the structure. However, due to the anisotropy of CFRP, current methods still have some limitations for CFRP damage imaging, such as inaccurate localization, large spots and obvious artifacts. To improve the damage imaging performance of CFRP structures, a time-of-flight based weighted imaging method is proposed in this paper. First, a time-of-flight based weighted function is designed, which can be calculated according to the time-of-flight of potential scatterers and scattering signals. Then, the Hausdorff distance between the reference signal and the damage signal is implemented as damage influencing factor to characterize the impact of the damage on the sensing path. Finally, all time-of-flight based weighted functions are multiplied with damage influencing factors and fused for damage imaging and localization. The experimental results show that this method has less artifacts and lower positioning error, and can accurately locate crack damage with different states compared with other imaging methods.

Multiscale uncertainty propagation analysis and reliability optimization of the CFRP crossbeam of the twist beam axle

Carbon fiber reinforced plastics (CFRP) have the advantages of being light weight and having high strength, which means that they can be used to effectively realize the lightweight design of automobile components. In this paper, the original steel crossbeam of the twist beam rear axle of a passenger car is replaced by CFRP, and then the cross-section improvement design is implemented. The ply stacking sequence of the CFRP crossbeam is also optimized to enhance its structural performance. Considering the multiscale characteristics of CFRP, a multiscale analysis model of the CFRP crossbeam is established, and the effects of microscopic, mesoscopic and macroscopic structures and material parameters on the stiffness, maximum stress and vibration characteristics of the CFRP crossbeam are discussed. The multiscale propagation law of the random-interval mixed uncertain parameters of the CFRP crossbeam is studied by using the multiscale model. On this basis, the reliability optimization design model of CFRP crossbeam considering multiscale uncertainty is established, and the multiscale reliability optimization method of the CFRP crossbeam of twist beam axle based on approximate models, Monte Carlo simulation (MCS) and intelligent optimization algorithms is studied. The optimization method of the combination of the ply stacking sequence optimization and the multiscale reliability optimization of the CFRP structure is proposed, the mass is reduced by 46.96% while ensuring reliability, and other structural performances are improved. Meanwhile, a theoretical research basis and technical support for the design and development of CFRP suspension components are provided, and the application of CFRP in the automobile field is promoted.

Prognostics and health management of composite structures under multiple impacts through electromechanical behavior and a particle filter

Self-sensing techniques are restricted to monitoring the various types of damage caused during repeated impact testing, and only a few studies have investigated the prognostics of carbon fiber reinforced plastics (CFRPs); in these studies, the electrical resistance of CFRPs was gauged in real time during multiple-impact testing. Therefore, real-time prognostics and health management using electromechanical behavior data obtained from CFRP structures under repeated impact testing are proposed herein. The health condition of the CFRP is observed in real time during impact testing using mechanical and electromechanical behavior data. Further, the types of failure observed during impact testing are investigated using real-time self-sensing data. Moreover, a particle filter is used for predicting the electromechanical behavior and the remaining number of useful impacts during repeated impact testing conducted using a physics-based prognostics tool. The applicability of the proposed methodology was confirmed by monitoring and predicting impact damage growth on the wind-turbine blade within a 5% prediction error. An advanced-condition-based monitoring technique with the diagnostics and prognostics of the current health state was designed successfully, and an application of the introduced method was demonstrated for industrial use.

Strength evaluation of CFRP structure of high pressure hydrogen tank based on mesoscale analysis

For designing with rigorous cost reduction and safety, high pressure hydrogen tanks manufactured from carbon fiber reinforced plastic (CFRP) of fuel cell vehicles, a method that can precisely predict the burst pressures of the tank is urgently needed. CFRP layers of tanks are formed with complicated mesostructured consisting of carbon fiber bundles and matrix resin using a filament winding process. A methodology that approaches by using cross shaped specimens to present fiber bundle conditions in CFRP layer of the tank is scrutinized. From the analysis and experiment with the specimen, it was found that strain rate dependency and tensile compression asymmetry of the resin have a great influence in addition to the fiber strength in the fiber direction on the CFRP fracture process. Burst pressures were predicted by CFRP mesoscale model with the load and boundary condition from macroscale analysis of conventional axisymmetric continuum model of the tank and validated through an actual tank burst test. As the results, we found the burst pressure of tank could be accurately predicted based on carbon fiber ruptures in the mesoscale model.

From Grave to Cradle - Development of Weft Knitted Fabrics Based on Hybrid Yarns from Recycled Carbon Fibre Reclaimed by Solvolytic Process from of EOL-Components

Carbon fibre (CF) is widely used in CF reinforced plastic (CFRP) components. However, waste CF, CFRP and the end-of-life (EOL) CFRP structures will cause an even bigger problem in the next years because of strict environmental regulations. Currently, recycling is carried out almost entirely by the use of pyrolysis to regain CF as a valuable resource. This high temperature process is very energy consuming and the resulting fibres are brittle. Hence, not suitable for textile processing into yarns or fabrics. To enable a grave to cradle circle, a new approach based on a solvolytic recovery of CF and the subsequent spinning process to obtain a hybrid yarn suitable for weft knitting processing is the focus of the international research project IGF/CORNET 256EBR “3D-r-CFRP”.

In situ elastic constant determination of unidirectional CFRP composites via backwall reflected multi-mode ultrasonic bulk waves using a linear array probe

The determination of elastic constants of a transversely isotropic carbon fiber reinforced plastic (CFRP) composite is the prerequisite of mechanical strength calculation, material degradation evaluation, ultrasonic non-destructive testing for main load-bearing CFRP structures both after manufacturing and in service. Conventional ultrasonic methods request extra sample preparation, water immersion condition or special designed goniometric devices to rotate the test structures, making the elastic constants measurement expensive, inconvenient and not in situ. In this paper, a novel ultrasonic method that enables in situ determination of elastic constants via backwall reflection method (BRM) using a linear array probe is proposed. The BRM can achieve single sided measurement of ultrasonic travel times in various directions including the fiber direction using different pairs of transmitter and receiver array elements. Both quasi longitudinal and quasi shear waves are captured via the mode conversions from the multiple surface reflections. All elastic constants are determined through the particle swarm optimization by minimizing the sum of the squared deviations between the BRM measured and theoretically calculated multi-mode bulk wave travel times in the fiber orthogonal and parallel planes. This method is experimentally verified on a 4.45 mm thick unidirectional T700 carbon fiber/epoxy CFRP composite. The BRM measured Young's modulus in the fiber direction agrees well with that measured by tensile test, with a small deviation of −4.58%. This work proves that the proposed method is single sided, easy to operate, without the necessity of sample preparation, water immersion and extra rotation device, and can determine all elastic constants with high precision, which is therefore promising for in situ applications.

Statistical creep failure time of unidirectional carbon fiber reinforced plastic under bending load

Our developed accelerated testing methodology (ATM) based on the matrix resin viscoelasticity for the creep and fatigue failure life prediction of fiber reinforced polymers (FRP) was applied to the statistical prediction of long-term creep failure life for the longitudinal bending of unidirectional Carbon fiber reinforced plastic (CFRP) laminates which is an important basic item for the durability design of CFRP structures used for aircraft and others. As results, the statistical creep failure times measured under several constant bending loads at an arbitrary temperature for unidirectional CFRP laminates were agreed with the predicted results obtained by substituting the matrix resin viscoelasticity and the flexural static strengths of CFRP laminates statistically and easily measured at various temperatures into the formulation of ATM. The long-term creep strength under bending load at an arbitrary temperature predicted were compared with that under tension load obtained by our previous paper. It was clear that the creep strength under bending load degreases drastically with increase in time and temperature comparing with that under tension load; therefore, the effect of time and temperature on the creep failure life under bending load is larger than that under tension load.

Research progress on the quality and efficiency of carbon fiber reinforced thermoplastic polymer composites processed by laser

Carbon fiber reinforced plastics (CFRP) has been widely used in aerospace, military weapons, new energy and high-end vehicles due to its specific and superior strength, low density and long-lasting wear resistance. The processing of CFRP is very different from the traditional metal processing because of its heterogeneous structure, anisotropy and superior wear resistance of CFRP. Usually, CFRP has often various defects in mechanical processing and water jet processing, such as interlayer tearing, fiber pull-out, delamination, tool wear, and abrasive penetration. Through laser processing, the problems of CFRP materials existing in mechanical processing can be overcome. However, due to the huge differences in thermal physical properties such as thermal conductivity and vaporization temperature of carbon fiber and resin in CFRP, its laser processing also turns out to have some issues, such as heat-affected zone and fiber end expansion. This paper summarizes the research results of CFRP laser processing in China and internationally. Research progress was introduced in detail from various aspects including laser and material mechanism as well as laser processing parameters. It reports the experimental and theoretical studies covering the process accuracy in edge quality and the thermal characteristics in terms of heat-affected zone. Eliminating the heat-affected zone in the polymer matrix are considered the major obstacles of CFRP industrial applications. The methods of improving processing efficiency by increasing material removal rate and reducing processing time were reviewed. The influence of different thermal conductivity of carbon fiber and resin matrix in CFRP on the processing quality was discussed. Finally, the development trend and challenges of thermal conductivity of carbon fiber and resin in CFRP in theoretical modeling were proposed.

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.