Carbon Fiber Reinforced Polymer Structures Research Articles

Impact Localization of CFRP Structure Based on FBG Sensor Network

Low energy impact can induce invisible damage of carbon fiber reinforced polymer (CFRP). The damage can seriously affect the safety of the CFRP structure. Therefore, damage detection is crucial to the CFRP structure. Impact location information is the premise of damage detection. Hence, impact localization is the primary issue. In this paper, an impact localization system, based on the fiber Bragg grating (FBG) sensor network, is proposed for impact detection and localization. For the completed impact signal, the FBG sensor and narrow-band laser demodulation technology are applied. Wavelet packet decomposition is introduced to extract available frequency band signals and attenuate noise. According to the energy of the available frequency band signal, an impact localization model, based on the extreme learning machine (ELM), is established with the faster training speed and less parameters. The above system is verified on the 500 mm × 500 mm × 2 mm CFRP plate. The maximum localization error and the minimum localization error are 30.4 mm and 6.7 mm, respectively. The average localization error is 14.7 mm, and training time is 0.7 s. Compared with the other machine learning methods, the localization system, proposed in this paper, has higher accuracy and faster training speed. This paper provides a practical system for impact localization of the CFRP structure.

Experimental characterization of composite laminates under low-velocity multi-impact loading

The use of composite materials undergoes a significant growth in many industries including aeronautic, railway, and automotive. Laminated composites are often subjected to severe loading conditions such as hailstones on aircraft and road gravels on automobiles. The present paper deals with the damage tolerance in carbon fiber-reinforced polymer structures caused by low velocity multiple impacts. Low velocity multi-impact tests at different energy levels (10, 15, and 20 J) are performed in order to simulate the gravels' impact conditions corresponding to mass of 10–50 g for velocity of 100–150 km/h. These tests were carried out on composite laminates with three types of stacking sequences (AERO, QIQH45, and QIQH60). AERO, widely used in aviation industry, is symmetric stacking sequence, and the two others QIQH45 and QIQH60 are quasi-isotropic and quasi-homogeneous laminates based on 45° and 60° ply orientation, respectively. Two specific parameters D and d are deduced from force–displacement and force–time responses which make it possible to rank the three stacking sequences to their damage tolerances. The impact damage assessment within the composite material with non-destructive testing method using ultrasonic inspection gives a good correlation with parameter D. Better impact damage tolerance is achieved with QIQH45 than the QIQH60 and AERO laminates. The parameter D can be regarded as a failure criterion for dimensioning of composite structures subjected to repetitive impacts.

Sulfate Environment Influences on the Interfacial Bond Performance between CFRP and Concrete

The potential of Carbon Fiber Reinforced Polymer (CFRP) in the reinforcement of concrete structures has been shown in many studies and practical applications. Some concrete structures are in harsh environment so its reinforcement mechanical properties decrease due to time and environment effect. There are many region where plenty of saline soil, so much SO42- ions could permeate into CFRP and concrete structures and cause corrosion. Na2SO4 solution was adopted to simulate the harsh environment and through single shear test to study the interfacial bond properties between CFRP and concrete, and SEM detection technique was adopted to analyse the deterioration mechanism. Three parameters just like concrete surface roughness, concrete compressive strength and specimens corrosion time were studied. As the results indicate: With the increase of the roughness, the interface ultimate load also increases gradually. Bond stress occurred biggest value at 7-day, the interface ultimate load drops greatly at various roughness levels after 30 days at different concrete strength. At 60 days, some crystal structures connecting into bulky crystal and the concrete and the adhesive layer were damaged seriously; lacking of epoxy resin connection, the fiber is weak in integrity, which has great influence on bonding effect of concrete.

A Fast Forward Simulation Scheme for Eddy Current Testing of Crack in a Structure of Carbon Fiber Reinforced Polymer Laminate

High frequency Eddy Current Testing (ECT) is one of the key non-destructive testing techniques for ensuring integrity of a structure of Carbon Fiber Reinforced Polymer (CFRP) material. An efficient numerical simulator is indispensable to enhance the performance of quantitative ECT for CFRP structures from both point of view of probe optimization and defect sizing. In this paper, a fast forward simulation scheme based on the A-Φ formulation and databases approach is proposed, implemented and experimentally validated for the rapid and high precision simulation of ECT signals due to defects in a CFRP plate by updating an FEM-BEM hybrid code for ECT problem. Comparison of numerical results of the present method with those of the conventional full FEM-BEM code and the experimental results for artificial cracks in CFRP laminate plates indicates that the proposed novel fast forward scheme can predict ECT signals over 300 times faster but without worsening numerical accuracy, which enables it to be applied to efficient reconstruction of cracks in CFRP plates and for probe optimization.

Laser beam machining of carbon fiber reinforced composites: a review

Carbon fiber reinforced polymer (CFRP) composites gained wide acceptance in aerospace, automotive and marine industries due to their superior properties. It became the major structural material that substitutes metals in many weight-critical components such as the new A350 and the B787 aircrafts, with composite content to exceed the 50%. Although CFRP structures are manufactured to near-net-shape, edge trimming, drilling, sawing, milling, and grinding operations are unavoidable. Being anisotropic, inhomogeneous and highly abrasive, their conventional machining is normally associated with delamination, fibers pull-out, inadequate surface quality, and tool wear. Other nontraditional processes which include abrasive water jet machining (AWJM), ultrasonic machining (USM), and electrodischarge machining (EDM) offer substitute to the conventional methods. Laser beam machining (LBM) is an emerging technology offering an excellent alternative for machining CFRP composites. This paper reviews the research work carried out in the area of LBM of CFRP materials. It reports the experimental and theoretical studies covering the process accuracy in terms kerf width, kerf depth and edge quality, and the thermal characteristics in terms of heat-affected zone (HAZ). Minimizing the kerf taper, increasing kerf depth, and eliminating the HAZ in the polymer matrix are considered the major obstacles of CFRP industrial applications. Methods of improving the machining productivity by reducing the machining time and increasing the material removal rate (MRR) and kerf depth are reviewed. Several mathematical and statistical modeling and optimization techniques have been critically examined. The concept of specific energy and its impact on HAZ and kerf width is introduced. The relationship between laser type and HAZ is discussed. The current work furthermore outlines the possible trends for future research.

Modelling mechanical lightning loads in carbon fibre-reinforced polymers

This paper describes a new approach for the simulation of lightning-induced damage in carbon fibre-reinforced polymer (CFRP) composite materials. In the past, the lightning-induced damage was mainly described by thermal or thermoelectric approaches. However these approaches need to be supplemented by mechanical pressure/force concepts for describing the non-thermal mechanical damage caused by transient lightning current components, since thermal or thermoelectric simulations cannot describe and predict the global mechanical response and damage of CFRP structures. In this work, the elastic response and intralaminar damage of protected CFRP structures are computed using numerical methods. The finite volume (FV) approach is used to calculate the arc root expansion and the finite element (FE) approach is used to determine the effects of magnetic forces and of the shock waves due to supersonic lightning channel expansion. Furthermore, the near surface-explosions of the lightning protection layer are taken into account including an accurate arc root behaviour on the surface of the sample. This novel approach predicts the response of CFRP structures subjected to lightning strike loads without the need to fit the model to experimental data or recourse to experimentally determined parameters. The predictions agree well with experimental results and provide a new insight on the physics behind the mechanical behaviour of CFRP structures subjected to lightning strike.

Detection of Flat-Bottom Holes in Conductive Composites Using Active Microwave Thermography

Active microwave thermography (AMT) is an integrated nondestructive testing (NDT) technique that utilizes a microwave-based thermal excitation and subsequent thermal measurement. AMT has shown potential for applications in the transportation, infrastructure, and aerospace industries. This paper investigates the potential of AMT for detection of defects referred to as flat-bottom holes (FBHs) in composites with high electrical conductivity such as carbon fiber-based composites. Specifically, FBHs of different dimensions machined in a carbon fiber reinforced polymer (CFRP) composite sheet are considered. Simulation and measurement results illustrate the potential for AMT as a NDT tool for inspection of CFRP structures. In addition, a dimensional analysis of detectable defects is provided including a radius-to-depth ratio threshold for successful detection.

Hashin Failure Theory Based Damage Assessment Methodology of Composite Tidal Turbine Blades and Implications for the Blade Design

A damage assessment methodology based on the Hashin failure theory for glass fiber reinforced polymer (GFRP) composite blade is proposed. The typical failure mechanisms including the fiber tension/compression and matrix tension/compression are considered to describe the damage behaviors. To give the flapwise and edgewise loading along the blade span, the Blade Element Momentum Theory (BEMT) is adopted. In conjunction with the hydrodynamic analysis, the structural analysis of the composite blade is cooperatively performed with the Hashin damage model. The damage characteristics of the composite blade, under normal and extreme operational conditions, are comparatively analyzed. Numerical results demonstrate that the matrix tension damage is the most significant failure mode which occurs in the mid-span of the blade. The blade internal configurations including the box-beam, Ibeam, left-C beam and right-C beam are compared and analyzed. The GFRP and carbon fiber reinforced polymer (CFRP) are considered and combined. Numerical results show that the I-beam is the best structural type. The structural performance of composite tidal turbine blades could be improved by combining the GFRP and CFRP structure considering the damage and cost-effectiveness synthetically.

Influence of a fiberglass layer on the lightning strike damage response of CFRP laminates in the dry and hygrothermal environments

This paper experimentally investigates the lightning strike response of a fiberglass/carbon woven hybrid laminate ([(0/90)g(45/02/−452/902)c]s) under dry and hygrothermal conditions. Damage characteristics are evaluated using various inspection methods and possible damage mechanisms are discussed. Observations demonstrate that as the increase of lightning amplitude, notably enlarged carbon fiber breakage and fiberglass delamination areas as well as fiber bulging are presented in the in-plane and in-depth directions. Compared to a [452/02/−452/902]s carbon woven specimen, the carbon fiber damage area, internal delamination area and damage depth for the hybrid one are 4.94 times, 2.26 times, 3.21 times greater under 22 kA lightning strike and 3.34 times, 1.97 times, 1.67 times greater under 32 kA strike, respectively. Hygrothermal aging significantly intensifies lightning strike damage on the hybrid specimen with complete detachment of fiberglass layer and massive internal delamination. The possible mechanism for the present damage behavior is analyzed. These results suggest that the fiberglass layer is not expected to protect the integrity of the carbon fiber reinforced polymer structure from lightning strike damage.

Smart Compressed Sensing for Online Evaluation of CFRP Structure Integrity

This paper investigates a smart sensing technique for online and automatic evaluation for carbon fiber reinforced polymer (CFRP) structures’ integrity. A compressed sensing (CS)-based algorithm framework integrating sampling and defect evaluation together is proposed enabling intelligence of an open-ended waveguide imaging system for the first time. Compared to traditional raster scan designs which require complete sampling, this smart CS technique can generate whole damage pattern while the scanning is conducting, the sensor intelligence is achieved by the proposed algorithm framework rather than seeking a hardware update. The CS enables accumulated downsampling and sparse recovery based on the frequency sparsity of impact damages on CFRP structures. Damage pattern in the reconstructed image can be detected using the proposed histogram threshold edge detection (HTED) algorithm when the image is stabilized enough. Edge-preserving smoothing is used to improve the stability of the reconstructed image. The experimental results illustrate time efficiency of the framework and more accurate damage localization using HTED. The proposed smart CS technique is attractive in quality control of CFRPs production. This technique can also be applied to situations where the sampled data is partly lost. Dispensing with hardware updates incurs minimum disruption and also benefits cost control and improves the productivity.

Multiscale modeling for long-term life prediction of CFRP structures under cyclic loading

Long-term life prediction of carbon fiber reinforced polymers (CFRP) structures under cyclic loading is performed by multiscale modeling as the combination of fiber scale, layer scale, and structure scale. The concept of failure index for fiber and matrix based on the micromechanics of failure theory (MMF) developed by Sung-Kyu Ha is introduced for the determination of the failure corner in structure, failure layer in laminates, and failure mode of fiber or matrix. The accelerated testing methodology (ATM) developed by the authors is used for long-term life prediction of CFRP structures. An MMF/ATM method combined with MMF and ATM is applied to the long-term life prediction of open hole compressive CFRP laminates under cyclic loading as well as static loading. Then the predicted results are compared with the experimental ones. Results clarified that MMF/ATM method is useful for the long-term life prediction of CFRP structures under static and cyclic loading because the results mutually agree well.

Eddy Current Testing of Delamination in Carbon Fiber Reinforced Polymer (CFRP): A Finite Element Analysis

ABSTRACTCarbon fiber reinforced polymer (CFRP) is widely used thanks to its excellent material properties. However, damages, especially delamination, can cause significant reduction in strength of CFRP structures. This article attempts to study the nondestructive testing of delamination using eddy current (EC) method. To this end, we design a novel probe consisting of dual coplanar rectangular coils placed perpendicular to sample surface, and make use of the vertical sections of the coils to induce vertical EC in the test sample. Perturbation of the vertical EC by delamination gives us a chance to detect delamination. Simulation results by the finite element method demonstrate the feasibility of the proposed method.

Doublet analysis of changes in electric potential induced by delamination cracks in carbon-fiber-reinforced polymer laminates

The early detection of delamination cracks, which compromise the compressive strength of carbon-fiber-reinforced polymer (CFRP) laminates, is necessary to guarantee the integrity of CFRP structures. In this study, a simple analysis method for calculating the changes in the electric potential induced by delamination cracks in CFRP laminates is proposed for three-dimensional structures. First, a three-dimensional orthotropic doublet potential function is newly defined to simulate the delamination cracks. This function is then combined with a simple electric current analysis technique, specifically, the orthotropic electric potential function method. This novel method can be applied in a practical nondestructive detection technique to determine the location and size of delamination cracks. Comparison with results obtained using the commonly used finite-element method demonstrates the effectiveness of the proposed simple analysis method for calculating changes in electric potential.

Localization of impact on composite plates based on integrated wavelet transform and hybrid minimization algorithm

Carbon fiber reinforced polymer (CFRP) composite material has been widely used in engineering structures due to its outstanding physical properties. However, the susceptibility to internal damage caused by the low-velocity impact event is a critical problem and monitoring of impact is essential for the integrity and reliability of CFRP structures. An integrated impact localization scheme is proposed and validated in this paper. The scheme is based on the arrival time obtained by wavelet transform (WT) analysis of signals received by dynamic strain gauges, triangulation localization technique together with a proposed hybrid particle swarm optimization (PSO) and genetic algorithm (GA) algorithm. The proposed localization scheme is validated by experimental testing of two CFRP plates with cross-ply [0/90]4s and symmetrical stacking [0/45/90/−45/0/90/−45/0]s respectively. The calculated impact locations are generally in agreement with the testing impact locations with acceptable accuracy for engineering application.

Development of an FBG Sensor Array for Multi-Impact Source Localization on CFRP Structures.

We proposed and studied an impact detection system based on a fiber Bragg grating (FBG) sensor array and multiple signal classification (MUSIC) algorithm to determine the location and the number of low velocity impacts on a carbon fiber-reinforced polymer (CFRP) plate. A FBG linear array, consisting of seven FBG sensors, was used for detecting the ultrasonic signals from impacts. The edge-filter method was employed for signal demodulation. Shannon wavelet transform was used to extract narrow band signals from the impacts. The Gerschgorin disc theorem was used for estimating the number of impacts. We used the MUSIC algorithm to obtain the coordinates of multi-impacts. The impact detection system was tested on a 500 mm × 500 mm × 1.5 mm CFRP plate. The results show that the maximum error and average error of the multi-impacts’ localization are 9.2 mm and 7.4 mm, respectively.

Experimental investigation of fatigue destruction of CFRP using the electrical resistance change method

CFRP (Carbon Fiber Reinforced Polymer) composites are one of the most popular composites used as the structural material in high-loaded structures. Important disadvantage of this material is the difficulty in predicting the time of fatigue destruction. These changes decrease the stiffness of the structure, but in practice it is usually not possible to measure it. Therefore, it is necessary to find another method to determine the moment when destruction of the composite begins. In this paper, one of possible methods – measuring of the electrical resistance change was investigated. Bending beam-shape specimens were tested under static and periodically varied load. Bending moment, deflection in the center of the probe and 8 voltages were measured. Performed experiment shows that measuring of the electrical resistance change provides better information about destruction of CFRP structure than a measure of the bending moment.

Self-sensing time-domain reflectometry for detection of the bearing failure of a CFRP laminate fastener hole: effect of peeling of the insulator

Carbon-fiber-reinforced polymer (CFRP) composites are used in aerospace and automobile structures. For many CFRP structures, mechanical metallic fasteners are adopted. In internal structures such as a wing box, damage to CFRP structures around the fastener holes is difficult to see. A simple method of finding damage around fastener holes is thus required for CFRP structures. This study applies self-sensing time-domain reflectometry to detect bearing failure around fastener holes of CFRP structures. In a previous paper, the effect of fasteners was minimized when the micro-strip line (MSL) was placed apart from the fasteners. This study numerically and experimentally investigates the peeling of glass-fiber-reinforced polymer (GFRP) insulator. A bearing failure results in embossing at the edge of a fastener hole. The embossing produces a wedge that causes the peeling of GFRP, generating a gap between the GFRP and CFRP. The gap strongly affects the characteristic impedance of the MSL.

Non-Destructive Inspection of Adhesively Bonded Joints using Amplitude Modulated Thermography

The goal of this work is to design thermal waves specifically for pulsed-phase infrared thermography (PPT) of bonded joints and to test their effectiveness on adhesive lap joints in carbon-fiber reinforced polymers structures. The method was applied to both single and double lap joint specimens to test different defect depths and multiple lap joint interfaces. An optimal thermal wave was designed for each specimen type based on the blind frequency information obtained from square pulse thermal wave excitation of the lap joint specimens. Modifications to the optimal thermal wave were then implemented based on the physical limitations of the lamp systems and different threshold settings for the blind frequency calculation. Thermal input waves based on the linear combination of discrete frequency waves were also implemented. The discrete frequency waves improved the defect image contrast in both the single and double lap joint specimens, while the optimal thermal input wave had the opposite effect and instead highlighted surface features. The difference in performance between the two thermal input waves is presumed to be due to the ability to separate frequency components in the transformation of data and not due to a critical thermal energy input threshold.

Defect detection in CFRP structures using pulsed thermographic data enhanced by penalized least squares methods

Pulsed thermography (PT) is a widely used non-destructive testing (NDT) method for detecting defective regions in carbon fiber reinforced polymers (CFRP) structures. In order to improve the spatial and temporal resolution of thermographic data, thermographic signal reconstruction (TSR) is often adopted for data processing and analysis. However, TSR only performs data filtering along the time direction, while the spatial information is not exploited for noise reduction. In addition, TSR cannot handle the non-uniform backgrounds commonly existing in thermal images. To get around these problems, this paper extends the utilization of the penalized least squares methods to defect detection in CFRP structures. The experiment results show that, with the aid of penalized least squares, the defective regions in thermal images are characterized more clearly, while the signal-to-noise ratio (SNR) values are increased significantly.

Impact damage detection of a carbon-fibre-reinforced-polymer plate employing self-sensing time-domain reflectometry

A non-destructive inspection method of carbon fibre reinforced polymer (CFRP) is self-sensing technology that uses carbon fibres as sensors. The technology applies electric current to composites and measures the electrical resistance change to detect damage. Although the technique detects damage precisely, it requires many electrodes on the CFRP surface. Time-domain reflectometry (TDR) has previously been proposed to detect damage over a wide area with a small number of electrodes and applied to detect fibre breakage in unidirectional CFRP ply specimens. In the present paper, experiments are conducted for a cross-ply CFRP laminate subjected to impact damage. Three impact loads are applied and the damage is inspected employing self-sensing TDR and a micro-strip line. Results show that the self-sensing TDR can be applied to laminated CFRP structures that have several fibre directions and when damage such as a delamination crack extends under or close to the micro-strip line.