Carbon Fiber Reinforced Plastic Laminates Research Articles

Optimization of Thermal-Wave Radar Thermography by Transverse Heat Flow Suppression Technique for Accurate Defect Detection of CFRP Laminates

In this study, transverse heat flow suppression (THFS) technique was proposed to enhance the ability of thermal-wave radar thermography (TWRT) to resist lateral thermal diffusion and uneven heating. The enhanced TWRT was used to detect subsurface defects of carbon fiber reinforced plastic (CFRP) laminates. The principle of THFS was described in detail by the optical flow analogy. The three-dimension (3-D) thermal-wave model stimulated by the uneven linear frequency modulation (LFM) thermal flow was introduced. The thermal-wave signal was processed by several different postprocessing characteristic extraction algorithms [Cross-correlation algorithm (CC), dual-orthogonal demodulation algorithm (DOD), fractional Fourier transform (FrFT), and principal component analysis (PCA)]. The comparison between normalized DOD amplitude/phase and normalized DOD-THFS amplitude/phase was carried out. The simulation results depicted THFS can significantly improve the difference between defect location and nondefect location. Nine CFRP specimens with artificial flat-bottom holes (FBHs) were prepared for nondestructive testing and evaluation (NDT&E) by enhanced TWRT. Seventy-two FBHs were prepared to test the probability of detection (PoD) of the enhanced TWRT. Hit/miss method was used to count defect information. The results demonstrated that the enhanced TWRT can realize the effective detection of defects (90% detection probability) with a diameter depth ratio of 5.06 under 95% confidence level. Compared with two state-of-the-art approaches, the proposed DOD-THFS phase has a better defect detection signal-to-noise ratio (SNR).

Effects of ply angle and blocking on open-hole tensile strength of composite laminates: A design and certification perspective

The failure strength of carbon-fibre reinforced plastic laminates under open-hole tension varies considerably with ply angle, ply blocking and loading direction. Here, laminates with various standard-angle and non-standard angle stacking sequences are subjected to both on- and off-axis loading in a comprehensive experimental and progressive damage finite element analysis testing campaign. It is found that interlaminar and intralaminar matrix damage can be beneficial when accumulated sub-critically in ply blocks aligned with loading direction, but can also lead to significant strength decreases owing to edge failure. In such cases, a numerical edge treatment is proposed for more accurate representation of open-hole tensile strength in large structures where holes are positioned away from free edges. The solution suppresses edge failure and results in up to 80% strength increases, challenging the validity of standard open-hole tension testing and current design rules for some applications.

Enhancing the fracture toughness of laminated composites through carbon nanotube belt stitching

The relatively weak interlaminar performance is one of the key factors that limit the light reducing efficiency of utilizing advanced fiber reinforced composites. In this study, unidirectional carbon fiber reinforced plastics laminates were stitched by using ultra-thin continuous carbon nanotube (CNT) belts in the thickness direction, aiming to enhance their interlaminar properties. A series of mechanical tests, including double cantilever beam testing, end notch flexure testing, tensile and bending testing, were implemented to reveal the contribution of CNT belts stitching. It has been found that after CNT belt stitching, the mode I and mode II interlaminar fracture toughness of the laminated composites are enhanced by 62.5% and 94.3%, respectively, which is mainly due to the bridge-connections between CNTs and CNT-resin interface. Meanwhile, CNT belt stitching results in a negligible variation of the tensile and bending performance of laminated composites.

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.

Damage Detection in Holed Carbon Fiber Composite Laminates Using Embedded Fiber Bragg Grating Sensors Based on Strain Information

Compared with metal materials, Carbon Fiber Reinforced Plastic (CFRP) is more excellent in performance, with high specific strength and high specific modulus. And among various CFRP structural elements, CFRP laminated plates have widespread applications. However, some indiscernible and hidden primary damage always appears on the CFRP laminated plate and will continuously evolve and expand after being loaded. This work starts with the analysis of the relationship between the changes of the strain distribution and damage expansion of CFRP laminated plates and proposes the damage recognition method of CFRP laminated plates based on strain information. Then, the CFRP laminate damage monitoring system based on Fiber Bragg Grating (FBG) sensor is established, and the CFRP laminate damage identification experiment based on FBG sensor is made to verify the accuracy of the CFRP laminate damage identification method. The results show that the maximum error between the load when the damage appears on the FBG sensor and that when the damage of CFRP laminated plates expands based on the simulated analysis does not exceed 16%. The accuracy of the damage recognition method of CFRP laminated plates is verified and the damage recognition of CFRP laminated plates and their expansion process is achieved.

Tensile, compressive, and fatigue strength of a quasi-isotropic carbon fiber reinforced plastic laminate with a punched hole

The mechanical properties of a quasi-isotropic carbon fiber reinforced plastic (CFRP) laminate with a punched hole were investigated. During the punching process, pressure was applied to the laminate through the upper and lower blank holders of a punching machine; the clearance between the punch and blank holders was set to be small to suppress damage in the CFRP laminates. Due to the dragging of plies encountered during punching, the surface of the punched hole was relatively uneven as compared to that of the drilled hole. However, the effect of the uneven surface created during the punching process was not as significant on the tensile and compressive strength of the open hole as compared to the manufacturing damages generated by drilling processes. The stress–number of cycles to failure curves for the open-hole tension–tension fatigue tests also showed comparable results between the punched- and drilled-hole specimens. These results indicate that there were no significant differences in the mechanical properties of CFRP laminates with a punched hole, and thus present the possibility of a highly productive hole-making process using the punching method.

An experimental study on impact behavior of quasi-isotropic CFRP laminates

Abstract This work was carried out to bring out the differences in drop weight low velocity impact behavior of two quasi-isotropic (QI) laminates. These laminates were made of AS4/914 carbon fiber reinforced plastic (CFRP). The layup sequence chosen was [0/45/-45/90]2S and [0//90/45/-45]2S. Two types of impactors were employed for the impact tests. One is a hemispherical-end and the other is of a conical-end. Coupon level specimens were supported on all four sides during the impact test. The impact energy was calculated for the laminates and the difference, as a comparison, in each case is presented. It has been observed that the layup sequence has a significant effect on the impact resistance of the laminates under hemispherical impactor. In the case of conical impactor, both the laminates were perforated while absorbing similar energy levels.

Factors influencing the tensile strength of carbon fiber reinforced plastic laminates for laser machining method and the underlined mechanisms

This paper presents an investigation into the influence of laser drilling on the tensile strength of carbon fiber reinforced plastic (CFRP) laminates of 1 and 2.5 mm thickness. The CFRP laminates were drilled using the microsecond laser, nanosecond laser, and picosecond laser, and the heat-affected zone (HAZ) was characterized and measured by optical microscopy. Through setting laser parameters, specimens with different HAZs were prepared, and then tensile strength tests were conducted. The results indicate that the tensile strength linearly depends on the width of HAZ. The tensile strengths of 1 mm specimens decrease by approximately 122 MPa/mm HAZ, and 2.5 mm specimens decrease by approximately 33.9 MPa/mm HAZ. Comparing the results of different thicknesses of CFRP, the effect of HAZ on tensile strength would be weakened with the increasing of laminate’s thickness. Actually, besides HAZ, cutting-induced geometric defects also seriously influence tensile strength, because of the stress concentration around these defects during the stretching process. Additionally, the mechanism why the HAZ influences tensile strength was revealed visually through FEM simulation: the resin matrix damage within HAZ causes the tensile loads not be transferred effectively between the fibers and then weakens the strength. It was the first time the underlined mechanism is revealed for HAZ-inducing strength reduction through FEM simulation. The laser machining method is more suitable for processing the thicker CFRP composites because the negative effect of HAZ is weakened for the thicker material.

Experimental observation and FEM analysis of initial fractures in quasi-isotropic CFRP laminates subjected to compressive loading

In this study, the mechanisms governing compressive strength in quasi-isotropic carbon-fiber reinforced-plastic laminates were clarified to gain an understanding of the results of non-hole compression tests. Synchronized observations of both sides of specimens provided insights into the locations of initial fractures in the width directions of the specimens. We also observed the areas surrounding the initial fractures in detail and found that the innermost 0° plies failed under the action of out-of-plane shear deformation. A finite element model was also used to elucidate the factors governing initial fracture. Analytical and experimental results suggest that the occurrence of initial fracture in quasi-isotropic laminates is dominated by out-of-plane shear stress. Based on the results of our analysis, a reasonable sequence of fractures was inferred.

Drilling performance of nano-diamond bonded SiC composite abrasives for CFRP laminates by dual-axis grinding wheel system

The design and fabrication of nano-diamond (ND) bonded silicon carbide (SiC) composite material were conducted as an attempt to obtain better drilling performance of SiC abrasives. The ND/SiC composite abrasives were fabricated by mechanical bonding of the NDs on the surface of SiC abrasives with the use of dry particle composing machine together with amorphous silica as a bond material. A comparative study on drilling performance for carbon fiber-reinforced plastic (CFRP) laminates by the SiC abrasives and ND/SiC composite abrasives was performed with dual-axis grinding wheel system. Hole-qualities such as roundness and roughness were improved by using the ND/SiC composite abrasives. Enhanced drilling performance of the ND/SiC composite abrasives was also obtained in terms of feed rate dependence of the thrust force. The ND/SiC composite abrasives could make holes with lower thrust force at higher feed rate, compared with the SiC abrasives due to the enhanced self-dressing effect together with the original drilling performance of the ND abrasive as synergetic effect caused by interfacial design between the ND/SiC composite abrasives and metal matrix in the grinding wheels. These results indicate the new controlling method of holding force of abrasive grains in metal bonded grinding wheel.

Formation of sub-wavelength laser induced periodic surface structure and wettability transformation of CFRP laminates using ultra-fast laser

Present work focuses on wettability transformation of carbon fiber reinforced plastic (CFRP) surface by generating micro/nano-structures using a femtosecond laser system. Laser induced periodic surface structures (LIPSS) were realized on carbon fiber (CF) surface at different defocus distances and pulse energies. The hydrophilic CFRP surface obtained just after the laser treatment progressively depicts hydrophobic behavior and further transforming to near superhydrophobic after cleaning followed by vacuum process for 4 h. Scanning electron microscopy (SEM) analysis was performed to capture the generated structure on the carbon fiber surface. Pulse fluence was effective parameters for transition of LSFL to HSFL. The lowest spatial period Λ ≈ 95 ± 6 nm and Λ ≈ 387 ± 15 nm was achieved for HSFL and LSFL, respectively, at 0.3 mJ.

Laser ultrasonic visualization technique using a fiber-optic Bragg grating ultrasonic sensor with an improved adhesion configuration

In this research, we attempt to establish a reliable structural health monitoring technique for composite materials by combining phase-shifted fiber-optic Bragg grating sensing with the laser ultrasonic visualization technology. In the first part of this article, a novel cross-adhesion configuration is designed to resolve the directionality problem of the phase-shifted fiber-optic Bragg grating ultrasonic sensing. In the adhesion configuration, Lamb waves are guided by an orthogonally bonded optical fiber from the adhesion point to the phase-shifted fiber-optic Bragg grating sensor. The analysis of the ultrasonic measurement results reveals that the proposed adhesion method enables us to use one sensor to receive Lamb waves in all in-plane directions with similar magnitude because two wave components propagating along with the two orthogonal directions are guided to the phase-shifted fiber-optic Bragg grating sensor and exhibit a linear superposition in the sensor. This simplified configuration gives our method an advantage over the existing approaches, such as the rosette configuration in which three or more phase-shifted fiber-optic Bragg grating sensors are required to relieve the sensing directionality. The phase-shifted fiber-optic Bragg grating ultrasonic sensor with the proposed adhesion configuration is then applied to visualize the propagation of ultrasonic waves in aluminum plates and carbon fiber–reinforced plastic laminates. Those verification experiments also show us that the new adhesion configuration is effective at protecting the phase-shifted fiber-optic Bragg grating ultrasonic measurement from the sensing directionality. Meanwhile, the broad bandwidth of the phase-shifted fiber-optic Bragg grating sensor enables us to visualize the propagation behavior of various Lamb wave modes over a broad frequency range. Finally, we also validate that the ultrasonic visualization technique merged with the phase-shifted fiber-optic Bragg grating ultrasonic sensing can be used to identify the hidden damage in the carbon fiber–reinforced plastic composite.

Internal Residual Strain Measurements in Carbon Fiber-Reinforced Polymer Laminates Curing Process Using Embedded Tilted Fiber Bragg Grating Sensor.

Carbon fiber reinforced plastics (CFRP) have many mechanical properties that are superior to those of conventional structural materials and are becoming more and more widely used. Monitoring the curing process used to produce such composite material is important to ensure the quality of the process, especially for the characterization of residual strains after the material has been manufactured. In this study, we present a tilted fiber Bragg grating (TFBG) sensor used to monitor the curing of CFRP composite materials. The TFBG sensor was embedded into the layers of CFRP laminates to study the curing residual strain of the laminates. The experimental results showed that the curing residual stress was about −22.25 MPa, the axial residual strain was −281.351 με, and lateral residual strain of 89.91 με. The TFBG sensor was found to be sensitive to the curing residual strain of the CFRP, meaning that it has potential for use in applications involving composite curing processes. Moreover, it is indeed possible to improve the properties of composite materials via the optimization and monitoring of their curing parameters.

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.

Combined loadings after medium velocity impact on large CFRP laminate plates: Tests and enhanced computation/testing dialogue

The development and certification of aeronautical composite structures is still largely based on the pyramid of tests. This approach is extremely costly in terms of number of tests, especially at the level of coupons. Moreover, these tests are highly conservative, under uniaxial loading, and do not represent the actual behavior at structure scale. To overcome these drawbacks, a new methodology has been developed at the Institut Clément Ader, which uses a complex loading test rig for technological specimens. This research focuses on the combined loading after impact of CFRP plates and highlights a specific behavior quite different from the usual CAI (Compression After Impact) response at the scale of coupons. In particular, compression, shear and combined shear/compression loadings were applied to large Carbon Fiber Reinforced Plastics (CFRP) laminated plates and the interaction of the impact damage with the post-buckling behavior has been investigated.

Investigation of shear failure load in ultrasonic additive manufacturing of 3D CFRP/Ti structures

Abstract Carbon fiber reinforced plastic (CFRP) is an extremely beneficial composite material in the aerospace and automobile industries owing to its high-strength-to-weight ratio, high stiffness, lightweight, and corrosion resistance. However, CFRP material alone is limited in its weight-bearing capabilities. A thin layer material such as Titanium (Ti) is often used along with CFRP laminates to address these issues. Traditional techniques used to join CFRP/Ti stacks include the use of adhesives, glues, or rivets, and bolts. These techniques have several limitations including weight addition, stress cracking, delamination, and limited operating temperatures. These limitations can be readily addressed by the use of solid-state welding techniques based on ultrasonic energy. One such technique is the Ultrasonic Additive Manufacturing (UAM) process, which is capable of fabricating 3D structures of CFRP/Ti laminar composites. Preliminary experimental studies proved the feasibility of using the UAM process to join CFRP/Ti stacks. Further development of this process needs a detailed investigation of the process parameters. This study aims to study the effect of critical process parameters including the ultrasonic energy and pre-surface roughness on the shear strength of the fabricated CFRP/Ti stacks using the UAM process. The study found that both ultrasonic energy and surface roughness have a positive impact on the resulting shear strengths of the UAM fabricated structures.

Improved Interlaminar Fracture Toughness and Electrical Conductivity of CFRPs with Non-Woven Carbon Tissue Interleaves Composed of Fibers with Different Lengths.

Non-woven carbon tissue (NWCT) with different fiber lengths was prepared with a simple surfactant-assistant dispersion and filtration method and used as interleaving to enhance both delamination resistance and electrical conductivity of carbon fiber reinforced plastics (CFRPs) laminates. The toughing effect of NWCT on both Mode I and Mode II interlaminar fracture of CFRPs laminate is dependent on length of fibers, where the shorter carbon fibers (0.8 mm) perform better on Mode I interlaminar fracture toughness improvement whereas longer carbon fibers (4.3 mm) give more contribution to the Mode II interlaminar fracture toughness increase, comparing with the baseline composites, and the toughness increase was achieved without compromising of flexural mechanical properties. More interestingly, comparing with the baseline composites, the electrical conductivity of the interleaved composites exhibited a significant enhancement with in-plane and through-the-thickness direction, respectively. Microscopy analysis of the carbon tissue interleaving area in the laminate indicated that carbon fibers with shorter length can form into a 3D network with more fibers aligned along through-the-thickness direction compared with longer ones. The shorter fibers thus potentially provide more effective fiber bridges, pull-out and matrix deformation during the crack propagation and improve the electric conductivity significantly in through-the-thickness direction.

Damage Investigation of Carbon-Fiber-Reinforced Plastic Laminates with Fasteners Subjected to Lightning Current Components C and D

The damage induced by lightning strikes in carbon-fiber-reinforced plastic (CFRP) laminates with fasteners is a complex multiphysics coupling process. To clarify the effects of different lightning current components on the induced damage, components C and D were used in simulated lightning strike tests. Ultrasonic C-scans and stereomicroscopy were used to evaluate the damage in the tested specimens. In addition, the electrothermal coupling theory was adopted to model the different effects of the arc and the current flowing through the laminate (hereinafter referred to as the conduction current) on CFRP laminates with fasteners under different lightning current components. Component C, which has a low current amplitude and a long duration, ablated and gasified the fastener and caused less damage to the CFRP laminate. Under component C, the heat produced by the arc played a leading role in damage generation. Component D, which has a high current amplitude and a short duration, caused serious surface and internal damage in the CFRP laminate and little damage to the fastener. Under component D, the damage was mainly caused by the Joule heat generated by the conduction current.

Comparison of different surface modifications on mechanical properties of HNTs incorporated CFRP for fine particle accumulation

This paper presents an investigation into the effect of particle accumulation on an amorphous halloysite nanotube (A-HNT)/Carbon fiber reinforced plastic (CFRP) laminate, fabricated through Vacuum-assisted Resin Transfer Molding (VaRTM). Resin blending and Electrophoretic deposition (EPD) methods were impeded to incorporate the A-HNTs. Bending, short beam shear (SBS), and end-notched flexure (ENF) tests were individually conducted on three sections, which were divided from an integral CFRP laminate. The anchoring effect of the carbon fibers and the scouring effect of the resin flow together lead to the different A-HNT accumulation patterns in both incorporation methods, and thus caused the observed distinctions in the properties of each separated section. Extensively incorporated A-HNTs showed a tendency to aggregate, resulting in the degradation of the material’s properties.

Effectiveness of Shear and Flexural Repair and Strengthening of Box section Steel Beams using CFRP Laminates

Steel structures are generally subjected to damages and defects due to different causes such as corrosion, fracture cracking, fire, buckling…etc. Damaged or defected parts may include girders, columns, welds, splices, base plates…etc. There is a wide range of techniques used for repair and strengthening starting with using protective coatings and ending up with full replacement of the damaged parts. Fiber Reinforced Plastic (FRP) laminates are widely used now in the field of repair and strengthening of different types of structures (reinforced concrete structures, steel structures, masonry structures, timber structures….etc.). The main goal of this research is to investigate the effectiveness of using Carbon Fiber Reinforced Plastic (CFRP) laminates in repair and strengthening of steel beams (in both flexural and shear). A total of five box-section steel beams were tested in three point load flexure test to determine the stiffness and ultimate load carrying capacity of the strengthened and repaired beams using CFRP laminates and compare the obtained results for these beams with those of the fifth beam which used as a control beam (without CFRP laminates). Test results showed that the effectiveness of using CFRP laminates for repair and strengthening of steel beams depends mainly on the obtained modes of failure. Highest effectiveness was obtained in tension failure modes while almost no effectiveness obtained in compression failure modes.