Carbon Fiber Reinforced Plastic Laminates Research Articles

Experimental and numerical evaluation of effects of kidney-shape carbon fiber on transverse cracking of carbon fiber reinforced plastics

This study aims to investigate the effects of fiber cross-sectional shape on the trans- verse crack extension of carbon fiber reinforced plastics (CFRP) both experimentally and numerically. Kidney-shape carbon fiber was compared with the laminates using conventional round-shape carbon fiber by monitoring the transverse crack accumu- lation behavior during static tensile tests. The experimental results showed that kidney-shape carbon fibers retard the initiation of transverse cracks and suppress their accumulation. Moreover, a micromechanical simulation based on finite ele- ment method was performed to consider the effects of fiber cross-sectional shape on crack propagation. In the case of the major damage process of the two kinds of CFRPs to be different in the simulations, the numerical results agreed well with the experimental ones. It was experimentally and numerically demonstrated that the kidney-shape carbon fiber suppresses the transverse crack accumulation and retards the crack initiation in CFRP laminates compared with round-shape carbon fiber.

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.

Energy Absorption Characteristics of a CFRP-Al Hybrid Thin-Walled Circular Tube under Axial Crushing

Thin-walled tubes have gained wide applications in aerospace, automobile and other engineering fields due to their excellent energy absorption and lightweight properties. In this study, a novel method of entropy-weighted TOPSIS was adopted to study the energy absorption characteristics of a thin-walled circular tube under axial crushing. Three types of thin-walled circular tubes, namely, aluminum (Al) tubes, carbon-fiber-reinforced plastics (CFRP) tubes and CFRP-Al hybrid thin-walled tubes, were fabricated. Quasi-static axial crushing tests were then carried out for these specimens, and their failure modes and energy absorption performance were analyzed. The CFRP material parameters were obtained through tensile, compression and in-plane shear tests of CFRP laminates. The finite element models for the quasi-static axial crushing of these three types of circular tubes were established. The accuracy of the finite element models was verified by comparing the simulation results with the test results. On this basis, the effects of the geometric dimension and ply parameters of a CFRP-Al hybrid thin-walled circular tube on the axial crushing energy absorption characteristics were studied based on an orthogonal design and entropy-weighted TOPSIS method. The results showed that Al tube thickness, CFRP ply thickness and orientation have great effect on the energy absorption performance of a CFRP-Al hybrid thin-walled circular tube, whereas the tube diameter and length have little effect. The energy absorption capability of a CFRP-Al hybrid tube can be improved by increasing the thickness of the Al tube and the CFRP tube as well as the number of ±45° plies.

Measurement of microscopic strain distributions of CFRP laminates with fiber discontinuities by sampling moiré method

Carbon fiber-reinforced plastics (CFRP) laminate structures manufactured using prepregs usually contain fiber discontinuities that result from the prepreg cutting for design and cost-efficiency. In this study, the sampling moiré method was used to measure the microscopic strain of unidirectional CFRP laminates with fiber discontinuities to investigate the deformation behavior that causes damage. In addition to the strain distributions of the laminate before and after cracking, the strain results due to thermal residual stress relaxation and residual plastic deformation during cracking were also obtained. The experimental results of this study were in good agreement with the finite element method results. Our results revealed that the sampling moiré method is an accurate method of measuring microscopic strain that can be used to evaluate the mechanical properties, damage behavior, residual strains in laminates with fiber discontinuities, and other different materials and structures.

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.

Demonstration of High-Resolution DFB Fiber Laser Acoustic Emission Sensing System for CFRP Laminates

This paper proposes an interrogation method to improve the resolution of DFB (distributed feedback) fiber laser acoustic emission (AE) sensing system. In this method, the coefficients of three ellipses formed by every two-channel interference fringe are estimated using bias-corrected ellipse-specific fitting (BCESF). The system parameters can be further corrected using the coefficients of the three ellipses. A resolution of 2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> pm/ √Hz at 100 kHz, which is approximately 10 dB higher than the traditional method, is achieved using the system parameters correction. The carbon fiber reinforced plastic (CFRP) laminates tensile test shows that the improved DFB fiber laser AE sensing system exhibits better performance than the PZT sensor. The damage initiation, damage spread, and instability stages of CFRP laminates' failure process can be accurately recognized.

Embedment Effect on Eliminating Damage of CFRP Pull-riveting Process by Simulation Study

The rivet joints have been widely applied in aerospace and vehicle fields. During the joining process of the carbon fiber reinforced plastic (CFRP) laminates, the pre-tightening force of pulling-rivet was the key factor to ensure the connection performance. To predict the impact of clamping loads on stress and failure of laminates, the value of stress and damage evolution of the wall of a hole under the pre-tightening force were simulated by the finite element method. The results of the simulation showed that excessive clamping force led to the damage and failure of CFRP in the hole edge. Connection performance together with progressive failure process and failure modes of CFRP laminates with various pre-tightening forces were investigated. A kind of metal embedded parts embedded in the laminates was designed to reduce the damage by the simulation study. Simulation results showed that embedment reduced the failure and damage efficiently. The embedment reduced about 64% of the maximum stress.

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.

Simulated lightning strike investigation of CFRP comprising a novel polyaniline/phenol based electrically conductive resin matrix

Traditional carbon fiber reinforced plastics (CFRPs) have been reported to exhibit lower electrical conductivity in the through-thickness direction, and have since paved the path for developing new resin systems to improve the through-thickness electrical conductivity and functionality of CFRP laminates. This study designs and investigates a novel Polyaniline (PANI)/Phenol resin, which possesses high electrical conductivity and mechanical strength. The CFRP manufactured by this PANI/Phenol resin with an addition of up to (~23.30%) phenol resulted in 0.6 S/cm through-thickness conductivity and demonstrated flexural strength and modulus values of 477 MPa and 59.2 GPa, respectively. We further evaluate the effectiveness of lightning strike protection of PANI/Phenol-based conductive CFRP laminates with a simulated lightning strike test. In essence, this research reports on PANI/Phenol-based CFRP's effectiveness as a material for lightning suppression without applying traditional metal-based lightning strike protection (LSP) systems.

Localization of low velocity impacts on CFRP laminates based on FBG sensors and BP neural networks

Carbon fiber reinforced plastic (CFRP) structures are vulnerable to low-speed impacts, which will lead to almost invisible impact damage. Therefore, the timely localization of impact is of great significance to damage detection and maintenance of the structure. In this article, a low velocity impact supervisory and testing system based on fiber Bragg grating (FBG) sensors was built up for CFRP laminates to obtain the low velocity impact strain sensitivity model. Meanwhile, genetic algorithm was applied to optimize the configuration of the FBG sensing network. The eigenvectors of the impact signals were extracted by applying fast Fourier transform (FFT) transform and principal component analysis (PCA) technology used as the input of the back propagation (BP) neural network model, while the corresponding impact coordinates were used as the output, to train the model. After training, the impact position prediction model based on BP neural network was obtained, thereby achieving the impact localization for CFRP laminates successfully with an average localization error of 2.1 cm.

Resonance Bond Testing: Theory and Application

Resonance bond testing is a nondestructive testing (NDT) technique that is used to detect disbonds, delaminations, and other voids in composite materials. The aerospace industry has seen an increase in the use of carbon fiber reinforced plastics (CFRP) for aircraft and spacecraft construction. Composite materials offer many advantages over traditional metallic structures, which include weight savings, increased strength, design for specific load paths, and the ability to easily construct geometrically complex structures. Resonance bond testing has many established uses for metallic structures as well, such as aluminum skin-to-skin and skin-to-core bonds. This bond testing technique has been around for many decades but is used by only a small portion of the NDT community. Ultrasonic testing (UT), specifically phased array ultrasonic testing (PAUT), using linear array techniques has proven to be a reliable method for the inspection of CFRP laminates. When composite structures do not permit the use of high-frequency sound waves due to rapid attenuation, resonance bond testing is a proven alternative. In this paper, the authors will discuss the theory behind resonance bond testing and how it has and continues to play an important role in the NDT industry.

Machining quality of high speed helical milling of carbon fiber reinforced plastics

Among advanced cutting methods, High Speed Milling (HSM) is often recommended to improve the productivity and to reduce the costs of machining parts. As every cutting process, HSM is characterized by some defects like surface roughness and delamination are the main defects generated in composite materials. The aim of this experimental work is the studying of the machining quality of woven Carbon fiber reinforced plastics (CFRP) using the HSM technology. Experiments were done using different machining parameters combinations to make opened holes in CFRP laminates. This study investigated the effect of cutting speed, orbital feed speed, hole diameter on the delamination defect and surface roughness responses generated in the drilled holes. The design of experimental tests was generated using the approach of Central Composite Design (CCD). The characterization of these responses was treated with the Analysis of variance (ANOVA) and Response surface methodology (RSM). Results showed that the surface roughness is highly affected by the orbital feed speed (F) with contribution of 22.45%. The delamination factor at entry and exit of holes is strongly influenced by the hole diameter D (25.97% and 57.43%) respectively. The developed model equations gave a good correlation between the empirical and predicted results. The optimization of the milling parameters was treated using desirability function to minimize the surface roughness (Ra) and the delamination factor simultaneously.

Drill-milling process and novel drill-milling tool for making a hole on carbon fiber–reinforced plastics

During carbon fiber–reinforced plastic (CFRP) hole-making, exit damages such as burrs and delamination frequently occur. Some hole-making technologies are invented to cater to the demands of the high hole quality for CFRP laminates. To adequately eliminate the process-induced damages, a novel dill-milling tool is designed, and a series of drill-milling experiments are conducted on CFRP by using this novel drill-milling tool. Theoretical and experimental analyses of the drill-milling process and the performance of the novel drill-milling have been conducted. The result indicates that compared with the drilling process, using the new drill-milling process, the burrs can be thoroughly removed and the delamination can be reduced significantly, even removed. The changes of the hole quality and the exit damages are different with the variations of the helical milling parameters. Therefore, by comprehensive consideration, proper helical milling parameters should be chosen adequately. Furthermore, in theory, the right-handed helix angles βy should be raised, which is favorable for improving the hole accuracy and reducing the exit damages.

Effects of ply thickness deviation and ply angle misalignment on the surface accuracy of CFRP laminates

This study examines two factors that affect the surface accuracy of symmetric carbon-fibre-reinforced plastic (CFRP) laminates: ply angle misalignment and ply thickness deviation. First, based on classic laminate theory, theoretical expressions for the effects of these factors are deduced. Then, COMSOL (FEM software) is used to analyse the relationship between these factors and the surface accuracy, and Zernike polynomials are used to fit the surface deformation of the CFRP laminates. It proves that ply angle misalignment and ply thickness deviation can cause astigmatic and power aberrations. Based on the integral constraint method in the form of Lagrangian multipliers, the rigid body displacement problem of the free thermal expansion simulation of CFRP laminates has been solved. Finally, three groups of experimental tests show that ply thickness deviation does occur. A ZYGO interferometer is used to observe the surface accuracy of the laminates, and it shows that astigmatism is the dominant aberration.

Dynamic fracture in CFRP laminates: Effect of projectile mass and dimension

The diverse mechanical properties and geometric configurations of the projectiles usually cause challenges to the accurate damage prediction of the carbon fiber reinforced plastic (CFRP) laminates. In this study, the mass and dimension of rigid projectiles are considered to explore their influence on the damage behaviors of CFRP, including dynamic deformation, fracture pattern, failure, etc. Experiments of different projectiles impacting on CFRP laminates are carried out, and a wide range of projectile kinetic energy (from 4.1 J to 548.4 J) are considered. The high-speed photography and non-invasive detection technique are used to capture dynamic fractures in CFRP laminates. Besides, a validated progressive damage numerical model is established for further investigation. The result indicates that the projectile dimension is associated with the penetration and delamination mode of CFRP, while the projectile mass affects dynamic deformation.

Experimental and numerical studies on free vibration of CFRP laminate with cutout

Carbon fiber reinforced plastic (CFRP) laminate with cutout is a lightweight structure in mechanical equipment design. In this paper, experiments and numerical analysis are conducted to study the free vibration of the CFRP laminate with cutout. The damping loss factor (DLF) of one-layer CFRP laminate is calculated reversely for providing the reliable material properties. The effect of the cutout shape on the free vibration of the CFRP laminate with cutout is studied. Meanwhile, by taking the circle cutout for instance, the effect of the cutout size, location, and number on the free vibration of the CFRP laminate with cutout are investigated. The software ABAQUS is used to perform the numerical modal analysis of the CFRP laminate with cutout, and the natural frequencies of mode I to mode III are obtained. According to the damping strain energy model, the stress and strain components are extracted for calculating the DLF by program written in MATLAB. Using B&K vibration testing system, experiments are carried out for the validation of the numerical modal analysis results by applying “single-input single-output” (SISO) approach. In addition, the stiffness of the CFRP laminate with cutout is studied by numerical analysis and experiments. How the affecting factors affect the free vibration of the CFRP laminate with cutout is discussed, and the relationship between the DLF and stiffness is explored. This study provides a reference for the application of the CFRP laminate with cutout.

A micromechanical model of carbon fiber-reinforced plastic and steel hybrid laminate composites

The fracture strain of carbon fiber-reinforced plastics (CFRPs) within CFRP/steel hybrid laminate composites is reportedly higher than that of CFRPs due to transverse compressive stress induced by the steel lamina. A micromechanical model was developed to explain this phenomenon and also to predict the mechanical behavior of CFRP/steel hybrid laminate composites. First, the shear lag theory was extended to calculate stress distributions on fibers and matrix material in a CFRP under multiaxial stress condition, considering three deformation states of matrix (elastic and plastic deformation and fracture) and the transverse compressive stress. Then, the deformation behavior of CFRP was predicted using average stress in the ineffective region and the Weibull distribution of carbon fibers. Finally, the mechanical properties of CFRP/steel hybrid laminate composites were predicted by considering the thermal residual stress generated during the manufacturing process. The micromechanical model revealed that increased transverse compressive stress decreases the ineffective lengths of partially broken fibers in the CFRP and results in increased fracture strain of the CFRP, demonstrating the validity of the current micromechanical model.

Cooling rate influence on the warp of a CFRP asymmetric laminated beam

This paper describes the age-related warp of carbon fiber reinforced plastic (CFRP) asymmetric laminated beams due to the viscoelasticity, anisotropy, and nonhomogeneity of CFRP laminates. Asymmetric orthogonally laminated CFRP beams held at a high curing temperature were cooled at various rates with and without constraints against warping during cooling. Furthermore, the warping of CFRP beams with the constraint lifted after cooling was measured during the aging processes. The age-related warping generated under various conditions was predicted using the viscoelastic and thermal properties of the CFRP laminates. The results were compared with measured data. The results were interpreted as follows. The age-related warping of CFRP beams depends strongly on the process conditions of the cooling rate and constraint, and the thermal and physical age-related shrinkage and viscoelasticity in the transverse direction of unidirectional CFRP affect the age-related warp of CFRP beams. Thermoviscoelastic analysis with physical age-related shrinkage is effective for this prediction.

Dispersion relation of Lamb waves in cross-ply composite laminates using multi-layered models

Lamb waves can be used in carbon fiber reinforced plastics (CFRP) for establishing both structural health monitoring systems and nondestructive inspection techniques. In this study, we investigated the dispersion relation of Lamb waves propagating in a cross-ply CFRP laminate. Using a formalism of the multi-layer Lamb wave model, we compared a homogeneous single-layer model and multi-layer models. The results suggest that the velocity of the S0 mode was the same for all the models in the low-frequency region. Thus, the use of homogeneous parameters is valid for the S0 mode. However, it was found that in high-frequency regions, there was a discrepancy between the velocities of Lamb wave modes for the homogeneous model and multi-layer models. This is because Lamb waves can be localized in a particular layer, and the velocity can be affected by the stiffness constants of the layer. This work elucidates the characteristics of Lamb wave propagation by showing that the stacking sequence of the laminate can have an effect on the wave velocities and wave structures, enabling researchers to improve Lamb wave-based defect detection.

Influence of extremely cold environmental conditions on interfacial fracture phenomenon of aerospace grade unidirectional composites

An investigation of interfacial fracture modelling of carbon fibre reinforced plastic (CFRP) laminates at the atmosphere similar to the cruise condition of an aircraft has been carried out in this article. The Mode I & Mode II fracture tests are carried out using specimen of aerospace grade composite (AS4/914) at−550C. Compliance based methods for estimation of crack growth have been adopted for monitoring during the fracture tests. A new set of thermally influenced interfacial fracture properties are evaluated for AS4/914 laminates. A modified traction – separation law has been derived for Mode-I loadings considering the influence of fibre bridging under influence of extremely cold environmental conditions. This law has been evaluated from the superposition of traditional bilinear law and derived bridging law. A significant reduction in interlaminar fracture toughness has been observed due to the fragile nature of the matrix under cold environment. Experimentally obtained interfacial fracture properties are implemented into the numerical formulation. Extended Isogeometric Analysis augmented with Cohesive Zone Modelling (XIGA-CZM) for unidirectional CFRP laminate has been enhanced and enriched with thermally influenced fracture toughness properties for further investigation. Results obtained from the present XIGA-CZM formulation shows a good agreement with experimentally obtained results. Scanning Electron Microscope (SEM) observation of fractured surfaces are carried out to investigate the nature of crack propagation in CFRP laminate at−550C.