Carbon Fiber Reinforced Polymer Components Research Articles

A novel energy‐based framework for characterizing the strain‐softening behavior of CFRP composites using cyclic loading

AbstractComplex failure modes of carbon fiber reinforced polymer (CFRP) composites create significant challenges for CFRP components' progressive damage analysis (PDA). Though important in PDA, the phenomenological strain‐softening model (SSM) for characterizing the degradation behavior in the fiber‐parallel direction (dominated by fiber breakage) of CFRP is difficult to be constructed by current approaches due to the poor toughness of carbon fibers. The work aims to obtain a refined SSM of plain woven CFRP composites through experimental methods and apply it to the PDA of corresponding components. A novel phenomenological cyclic‐loading‐based strain‐softening model (CLSSM) was proposed based on the theory of equivalent dissipation energy during static and cyclic loading. Firstly, the release behavior of dissipation energy during the cyclic loading process was researched. Secondly, a newly designed search algorithm taking degraded mechanical properties during cyclic loading as input and taking the static strain‐softening curve as output was utilized to build up CLSSM. Thirdly, a three‐point bending test was conducted and a corresponding numerical model was constructed for comparison to verify the proposed model. The entire framework is highly programmatic, and results show that CLSSM is fast for characterizing the strain‐softening behavior, and effectively simulates the progressive failure process of CFRP components.

Improvement of Heating Uniformity by Limiting the Absorption of Hot Areas in Microwave Processing of CFRP Composites.

Carbon fiber reinforced polymer (CFRP) composites are integral to today’s industries. Curing or consolidation are vital processes for manufacturing CFRP components. Microwave processing has many advantages compared with conventional processing technologies using ovens or autoclaves; however, the uneven temperature distribution caused by the non-uniform microwave field has a significant influence on the quality of the cured products. In this study, we propose a new idea to solve this problem, i.e., limiting the absorption of hot areas. Under such circumstances, cold ones can catch up with them more easily. To adjust the absorbing capability of the CFRP laminate, periodically arranged metallic resonance structures supported by a dielectric spacer are introduced on its surface. The dielectric spacer, made of epoxy matrix and strontium titanate particles, is designed to possess a dielectric constant positively related to temperatures. In this situation, the microwave absorption (2.45 GHz) of the metal-dielectric-CFRP configuration is changed from 97.6% at room temperature to 55.9% at 150 °C continuously. As a result, a reduction of 43.1% in maximum temperature difference and 89% in standard deviation has been achieved.

Zone-regulated microwave heating of CFRP laminates via ultrathin and flexible resonance structures with different working frequencies

Carbon fiber reinforced polymer (CFRP) composites are integral to today's industries. Curing is a vital process for manufacturing CFRP components. Recently, great attention has been devoted to curing CFRP components via controllable temperature gradients, taking their geometries and structures into consideration. Here, for the first time, we proposed an idea to regionally manipulate the temperature distribution of the CFRP laminate through frequency-selective absorption. Specifically, two kinds of periodically arranged metallic resonance structures (18 μm thick) working at 915 MHz and 2.45 GHz were designed and fabricated. Frequency-selective absorption and thus zone-regulated microwave heating have been realized by introducing these ultrathin and flexible resonance structures on the CFRP laminate. On this basis, two applications have also been demonstrated, including a dynamically patterned thermal display and a uniform heating process on laminates with non-uniform thickness.

Standardised quantification of structural efficiency of hybrid reinforcement systems for developing concrete composites

Various types of materials and technologies have been developed either for reinforcing or strengthening concrete structures. However, there is no uniform methodology to quantify and compare the mechanical characteristics of different reinforcement systems. Residual stiffness of flexural elements is the suggested measure of the reinforcement’s structural effectiveness, involving concrete into composite action. This manuscript proposes a new testing layout designed to form multiple cracks in a small laboratory specimen and a simplified analytical approach to quantifying the flexural stiffness of the standardised test samples. The achieved analytical solution to the stiffness problem requires neither iterative calculations nor the loading history description. That makes it acceptable for quantifying and comparing the residual stiffness of elements with various combinations of reinforcement materials. The proposed analysis procedure also enables determining the mechanical performance decay under repeated and long-term loading conditions without a loss of the calculation adequacy. Flexural tests illustrate the application of the proposed technique—the 38 specimens were mechanically loaded until failure. Several composite reinforcement schemes, including steel and glass fibre reinforced polymer (GFRP) bars, externally bonded carbon fibre (CF) sheets, and near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) strips in various combinations, are considered. The analysis reveals an exceptional efficiency of hybrid reinforcement systems, combining steel and CFRP components.

Broadband nonlinear elastic wave modulation spectroscopy for damage detection in composites

A non-destructive testing procedure is proposed for damage detection in composites using full wavefield measurement obtained using a scanning laser Doppler vibrometer. Vibrations are excited using two low-power piezoelectric actuators leading to nonlinear elastic wave modulation at the defect. One actuator is supplied with a broadband chirp signal and the other actuator is supplied with a single-frequency sine signal. First, a time–frequency filtering method is proposed to extract specific nonlinear components of interest (e.g. second higher harmonic and first modulation sideband) without the need for multiple excitation sequences. Next, damage maps are constructed using broadband bandpower calculation of the filtered nonlinear components. It is demonstrated that the modulation sidebands provide an exclusive imaging of defect nonlinearity, and are not affected by potential source nonlinearity. The proposed damage map construction procedure is applied for various carbon fiber reinforced polymer test specimens with different damage features: (1) coupon with quasi-static indentation damage, (2) coupon with artificial delaminations, (3) bicycle frame with impact damage, and (4) stiffened aircraft panel with partially debonded stiffener. The obtained results indicate the high performance of the developed procedure for detection of various defect types in curved and/or stiffened carbon fiber reinforced polymer components.

Multifunctionality of polymer composites based on recycled carbon fibers: A review

Carbon fiber reinforced polymers (CFRP) offer outstanding lightweight potential and can play a key role for modern energy and mobility concepts. However, production of carbon fibers is energy- and cost-intensive, while at the same time waste rates of common manufacturing technologies are quite high and repair possibilities for damaged parts still limited. Therefore, holistic recycling approaches are urgently required in order to reach acceptable cost-efficiency and sustainability. What makes the recycling so challenging, is the fact that true recycling, i.e. re-usage of fibers in high-performance composites, requires preservation of a high fiber length and enabling of accurate fiber orientation. This generates a trade-off between the best possible exploitation of the fiber properties and the effort to minimize the recycling costs. Hence, this paper does not only give a brief overview of technologies to recover carbon fibers from waste and to process them to new CFRP components. In addition, different approaches are presented, that exploit the specific characteristics of semi-finished products based on recycled carbon fibers, in order to achieve process- or material-related multifunctionality. This includes quasi-plastic deformation behavior (enables deep-drawing or curved tow placement), improved surface quality through reduced fiber print-through, robust resin impregnation through supersaturated nonwovens, and high energy absorption.

Estimation of Contact Resistivity in Lightning Protection Equipotential Bonding Joints of Wind Turbine Blades

Modern lightning protection systems for wind turbine blades with conducting structural elements, e.g., carbon fiber reinforced polymer (CFRP) spar caps, contain equipotential bonding joints to prevent sparking during strikes. Significant current levels are experienced through the joints and the characterization of the electrical contact at the bonding regions is essential for reliable protection. Therefore, this article aims to characterize the contact resistivity of several equipotential bonding joints. The proposed methodology first measures the total resistance of the samples, and then the bulk resistance of the conductive elements is computed using the finite-element method. The latter is required to predict the spreading effects in CFRP components due to the strong anisotropic nature of such materials. After that, the contact resistance is calculated by subtracting the predicted bulk resistances from the measured total resistances. The developed procedure was applied to three typical equipotential bonding materials: expanded copper foil (ECF), biaxial (BIAX) CFRP, and unidirectional (UD) CFRP. Both ECF and BIAX CFRP showed superior contact quality than the UD CFRP, with one to two orders of magnitude smaller contact resistivity.

Experimental Characterization of a High-Damping Viscoelastic Material Enclosed in Carbon Fiber Reinforced Polymer Components

This work aims to identify the damping properties of a commercial viscoelastic material that can be embedded and cured between the layers of composite laminates. The material may be adopted for reducing the vibration response of composite panels, typically used in automotive and aerospace applications, e.g., as vehicle body shell components. In order to objectively estimate the actual potential to enhance the noise vibration and harshness aspects, the effects of the viscoelastic material on the modal parameters of carbon/epoxy thin panels are quantitatively assessed through experimental modal analysis. Two different experiments are conducted, namely impact hammer tests and shaker excitation measurements. Based on the results of the experimental campaign, the investigated material is confirmed as a promising solution for possibly reducing the severity of vibrations in composite panels, thanks to its high damping properties. Indeed, the presence of just one layer proves to triple the damping properties of a thin panel. An approximate damping model is derived from the measured data in order to effectively simulate the dynamic response of new design solutions, including thin composite panels featuring the viscoelastic material.

Experimental and numerical study of hat shaped CFRP structures under quasi-static axial crushing

Carbon fiber reinforced polymers (CFRP) has been increasingly applied in automobile industry for vehicle body lightweight and safety performance improvement. However, design of CFRP components especially for crushing structures is still highly ambiguous. The present study aims to study the deformation behaviour and energy absorption of the hat shaped CFRP structures and optimize the section shape. Two types of hat shaped CFRP structures with various thicknesses and ply orientation were tested under axial quasi-static crushing. The results show that the Type II hat shaped structure presents a stable progressive crushing mode and better energy absorbing ability as compared with the Type I hat shaped structure. Then, a finite element model was developed using the multi-layer shell element method, and was validated by the axial crushing test results. Finally, the section shape of the Type II CFRP structure was optimized through the surrogate model of radial basis function and global response surface method, and the influences of the section shape on crushing behaviours and energy absorbing abilities were analysed.

High resolution X-ray computed tomography: A versatile non-destructive tool to characterize CFRP-based aircraft composite elements

The aim of the current study is to propose a versatile, non-destructive inspection strategy to evaluate the structure of two different aircraft carbon fibre reinforced polymer (CFRP) -based composite configurations, which are widely used for structural elements, respectively layered composite and sandwich structure. X-ray computed tomography (CT) has been used as a flexible method for assessment of porosity levels in CFRP components in both types of configuration, permitting to investigate the volume distribution of the individuated pores/voids, giving an interactive 3D exploration and quantitative analysis of porosity by the complete 3D volume rendering of the analysed composites. Moreover, specific data analysis could supply detailed information on size, shape and position of pores or defect or misalignment which could be a valuable indication for damage tolerant design methodology or local stress analysis. This presented non-destructive approach based on CT can be a good alternative to standard destructive methods based on the acid digestion or 2D microscopy for porosity evaluation in composite, especially for severe stressed element in aerospace structure. Of course, the high resolution X-ray CT cannot substitute the ultrasounds test for analysing full scale components, but it can be complementary to it to provide accurate quantitative results on small samples used as references.

Fabrication of the silver modified carbon nanotube film/carbon fiber reinforced polymer composite for the lightning strike protection application

Carbon fiber reinforced polymer (CFRP) composites with low density, corrosion resistance and excellent mechanical properties are widely applied in the aircraft industry, instead of the traditional metallic material. Their poor electrical conductivity leads to the vulnerability to the lightning strike (LS). In this paper, highly conductive silver modified carbon nanotube film (SMCNF) was developed via the electrophoretic deposition (EPD) method for protecting CFRP structures and components of the aircraft. Lightning strike protection (LSP) efficiency of the SMCNF/CFRP composite was characterized from the micro-scale to the macro-scale, and its possible protective mechanism was discussed. The compressive strength of the SMCNF/CFRP composite after the simulated LS test maintains 91.05% and is higher than that of the Cu mesh/CFRP composite (83.28%). Compared to commercial copper mesh LSP material, it can reduce the weight by 27.4% with the better residual compressive strength ratio. Therefore, highly conductive and lightweight SMCNF as a LSP layer can effectively reduce the damage of the CFRP matrix caused by the LS.

Development of a numerical material model for axial crushing mechanical characterization of woven CFRP composites

Nowadays, the significant potential of carbon fiber reinforced polymer (CFRP) composites in terms of weight to performance has contributed to their wide application in industrial fields. However, it is widely accepted that design of CFRP components remains rather challenging as laborious experimental work is usually required. Therefore, to reduce the cost and improve the CFRP product development efficiency, the present work aims to develop a numerical material model for numerical prediction of mechanical responses of composite structures under external loading. Both intralaminar and interlaminar failure mechanisms are taken into account with the aid of user material subroutine VUMAT and cohesive surface capability in Abaqus/Explicit. Particularly, a plasticity formulation, post-failure definitions including both continuum damage mechanics and progressive damage mechanics models, and smeared formulation accounting for mesh independence are all incorporated into the intralaminar material model. Finally, the model has been assessed by using two off-angle tension cases and axial crushing tests of corrugated specimen, and good agreement between experiment and simulation demonstrates that the proposed methodology can be used for mechanical characterization of woven composites under these loading cases.

Experimental Study on Spring-in Deformation for L-shape CFRP Components

The carbon fiber reinforced polymer (CFRP) is one of the modern composite material which is widely used in space applications, automobile industries and many more because of its high specific properties over conventional metals and alloys. Various manufacturing processes are available for preparation of CFRP components. Hand layup process is a versatile process for manufacturing a composite material because of its simplicity and cost-effectiveness. In this research paper, L-shape components from CFRP have been manufactured using the hand layup process. The mixture of different epoxy resin systems along with proper hardener is applied to dry carbon fabric to prepare composite parts using hand layup process. The spring-in deformation has been measured for CFRP L-shaped components fabricated from four and eight layer laminates with different epoxy resin systems. The spring-in deformation is minimum for eight layer components made using Hinpoxy VB resin system.

A Novel Methodology for Macroscale, Thermal Characterization of Carbon Fiber-Reinforced Polymer for Integrated Aircraft Electrical Power Systems

Carbon fiber-reinforced polymer (CFRP) is increasingly used for aero-structure applications due to their high strength-to-weight ratio. The integration of the on-board electrical power system (EPS) with CFRP is challenging due to the requirement to thermally and electrically isolate these systems to meet existing safety standards. By capturing the thermal characteristics of CFRP at a macro (component) scale for CFRP components, it is possible to understand, and design for, the increased integration of the EPS into CFRP aero-components. A significant challenge is to develop a macroscale characterization of CFRP, which is not only of an appropriate fidelity for compatibility with systems-level models of an EPS but also can be used to represent different geometries of CFRP components. This paper presents a novel methodology for capturing a transient, macroscale thermal characterization of CFRP with regard to component layup and geometry (thickness). The methodology uses experimentally derived thermal responses of specific resin and ply orientation CFRP samples to create a generalized relationship for the prediction of thermal transfer in other sample thicknesses of the same material type. This methodology can be used to characterize thermal gradients across CFRP components in aircraft EPS integration applications, ultimately informing the optimized integration of the EPS with CFRP.

Computed Laminography of CFRP Using an X-Ray Cone-Beam and Robotic Sample Manipulator Systems

Carbon fiber-reinforced polymers (CFRPs) are of interest to the aerospace sector for meeting future CO2 emission targets due to their weight reduction potential. However, the detection of structural and matrix defects is crucial for determining the performance and suitability of CFRPs in current and future generations of aircraft. Computed laminography (CL), a well-established nondestructive testing method, is well suited to the scanning of CFRP components with large aspect ratios, for which the conventional computed tomography (CT) is less suitable. Utilizing an existing Nikon Metrology custom build X-ray CT scanner, two lift-in lift-out robotic sample manipulator systems are used to extend the capability of the system and allow the exploration of atypical scanning geometries. Implementing raster and limited angle trajectories, reconstructions using the ASTRA Tomography Toolbox and the Simultaneous Iterative Reconstruction Technique algorithm are able to show structural defects in CFRPs, despite the reduced information inherent with CL systems. This paper reports on the system design and initial experiments that demonstrate benefits and drawbacks of different design options and scanning trajectory choices.

Integration of reverse engineering and ultrasonic non-contact testing procedures for quality assessment of CFRP aeronautical components

Nowadays, the quality assurance of aeronautical components is a very crucial issue. Diverse defects can be generated during composite material components manufacturing such as voids, delamination, cracks, etc. The identification of these defects requires the use of different types of inspection methods. In this paper, two diverse non-contact inspection techniques, i.e. a laser-based reverse engineering method and an ultrasonic testing procedure, are integrated to provide a complete quality assessment of carbon fibre reinforced polymer components for applications in the aeronautical field. A custom made software code was developed in order to create a user interface allowing for the visualization and analysis of the reverse engineering and ultrasonic information for the detection of geometrical and internal flaws of the component under inspection.

Monitoring stress changes in carbon fiber reinforced polymer composites with GHz radiation.

We performed proof of concept experiments to demonstrate that the reflected power of GHz illumination from the surface of carbon fiber reinforced polymer (CFRP) composites is linearly related to the stress in the material. We introduce a stress coefficient to describe the change in normalized power with applied stress, analogous to the stress-optic coefficient, because the effect is attributed to changes in the refractive index of the effective medium comprising the polymer matrix and carbon fibers. Stress coefficients of -0.549±0.134/GPa and -0.154±0.024/GPa were measured for two different composite materials, both linear in the measurement range of 40MPa and 100Mpa, respectively. This technique opens up the possibility of non-destructive evaluation of stresses in CFRP components for quality assurance in manufacturing and in structural health monitoring of in-service aerospace and automotive parts.

Pull-out behavior of CFRP ground anchors with two-strap ends

Pull-out experiments were performed on three carbon fiber-reinforced polymer (CFRP) ground anchors simulating their applications in different rock types which provided different confinement levels. The CFRP tendons comprised, on the ground side, a prefabricated conical anchor body of high-strength grout in which two CFRP straps were embedded. The anchors reached an average load-bearing capacity of 1384kN with final failure occurring in the CFRP straps. Grout failure was successfully prevented by an adequate grout selection and the installation of CFRP confinement rings to balance spreading forces at the strap ends. The confinement level provided by the surrounding media influenced the activation of the CFRP components in the anchor body by influencing the friction at the CFRP/grout interfaces.

Long-term moisture effects on the interfacial shear strength between surface treated carbon fiber and epoxy matrix

In recent years, carbon fiber reinforced polymer (CFRP) composites have found increasing applications in marine and offshore area, where the CFRP components are subjected to a persistent attack of moisture. The performance degradation of composites under those critical service conditions becomes a key issue. In this work, silane coating and multiwalled carbon nanotubes were applied on carbon fibers to enhance the fiber/matrix interfacial bonding strength. The long-term effects of moisture on the interfacial shear strength (IFSS) of the composites in underwater environments, such as de-ionized water and simulated seawater, have been studied using single fiber microbond method. The silane coating and carbon nanotube-modified silane coating are found to contribute 14.5% and 26.3% increase in IFSS of the CFRP in dry air, and well maintain this improvement during a 120-day immersion test in de-ionized water and simulated seawater.

Influence of Special Tool Geometry in Drilling Woven CFRPs Materials

Machining processes of Carbon Fiber Reinforced Polymer (CFRP) are commonly required in order to achieve final assembly specifications. Despite the good mechanical properties of this kind of materials, they are hard to be machined due to the presence of hard particles; delamination, fiber pull-out and matrix thermal degradation are usually observed during its machining. Drilling operations are required before mechanical joining of the CFRP components. The actual interest in reducing delamination rests in the fact that it is the most serious damage found during drilling. In this work, a comparative study of three special geometries under different cutting conditions is presented. Thrust force and torque were monitoring during drilling tests and delamination extension was quantified. Results showed that a good drill tip geometry and feed rate selection is fundamental to reduce delamination damages.