Carbon Fiber Reinforced Polymer Panel Research Articles

Restoration of Compressive Strength of CFRP Plates with Edge Delamination through CNT Crack-bridging Deposited by Resin Pre-coating Solution

Edge delamination, with only micro-scale crack openings, in carbon-fiber-reinforced-polymer (CFRP) composites due to light impact, machining and structural assembly is challenging to repair. This study presents a simple pressureless method to repair edge delamination in CFRP laminates. The compressive strength of CFRP panels with around 5–10 mm deep edge delamination cracks have been fully restored without utilizing any special equipment. Acetone-diluted resin pre-coating (RPC) solution, consisting of 95 m/m% acetone and 5 m/m% resin (without hardener), is utilized to penetrate deeply into sharp delamination cracks through capillary action, and 1 m/m% CNT dispersed into the RPC solution (RPC + CNT) is deposited inside the delamination cracks forming CNT-toughened adhesive joints. The compressive strengths of long cure time (3 month) are compared with 2-week results, showing 2-weeks are not sufficient for complete curing. With the 3-month curing period, the compressive strength of CFRP with edge delamination is 100 % restored by the RPC + CNT technique.

Silver-based conductive coatings as lightning strike protection for composite aircraft

Lightning strike protection of composite structures is an increasingly studied subject in the aerospace industry due to the need for finding novel protection solutions for low conductive composites. This work presents two silver-based lightning protective conductive coatings for potential replacement of expanded copper foil (ECF) as lightning strike protection (LSP). The first solution consists of an electroless method to produce silver-coated carbon fibres (SCCF). The SCCF are integrated as a sacrificial ply on a carbon fibre reinforced polymer (CFRP) panel with four different areal densities, using an epoxy-based primer or a PEDOT:PSS conductive ink as binder. The second solution is a silver-based commercial product consisting of a two-part epoxy with silver flakes, which acts as a sprayable thin coating beneath the paint layer.The protected CFRP panels were covered with aerospace-grade paint for realistic finishing and were subjected to high impulse current strikes. All four conductive coatings containing SCCF exhibited around 45%–50% retention of flexural strength, although largely inferior to painted expanded copper foil's resistance to emulated lightning strikes. The silver-based commercial product showed 83% retention of flexural strength and 97% retention of effective bending modulus, with a comparable performance to the painted expanded copper foil. The resulting retention of flexural strength, the absence of severe damage to the structure, its versatility for complex geometries and the easy manufacturability of the silver-based commercial coating makes this approach a good candidate for the replacement of ECF as a solution for LSP.

Fibre volume fraction screening of pultruded carbon fibre reinforced polymer panels based on analysis of anisotropic ultrasonic sound velocity

Composites have become the material of choice in a wide range of manufacturing applications. Whilst ultrasound inspection is a well-established non-destructive testing (NDT) technique, the application to composite imaging presents significant challenges stemming from the inherent anisotropy of the material. The fibre-volume fraction (FVF) of a composite plays a key role in determining the final strength and stiffness of a part as well as influencing the ultrasonic bulk velocity.In this work, a novel FVF determination technique, based on the angular dependence of the sound velocity with respect to the composite fibre direction, is presented. This method is introduced and validated by inspection of pultruded carbon fibre reinforced polymer (CFRP) panels commonly used in the manufacture of high-power wind turbine blades. Full matrix capture (FMC) data acquired from a phased array (PA) ultrasonic probe is used to generate calibration data for samples ranging in FVF from 60.5% to 69.9%. Sample velocity, as a function of propagation angle, is used to estimate the FVF of samples and ensure they fall within the desired range. Experimental results show values of 61.1, 66.1 and 68.3%, comparing favourably to the known values of 60.5, 66.3 and 69.9% respectively.The work offers significant potential in terms of factory implementation of NDT procedures to ensure final parts satisfy standards and certification by ensuring any FVF inconsistencies are identified as early in the manufacturing process as possible.

Experimental and Numerical Study of the Influence of Pre-Existing Impact Damage on the Low-Velocity Impact Response of CFRP Panels.

This paper presents an experimental and numerical investigation on the influence of pre-existing impact damage on the low-velocity impact response of Carbon Fiber Reinforced Polymer (CFRP). A continuum damage mechanics-based material model was developed by defining a user-defined material model in Abaqus/Explicit. The model employed the action plane strength of Puck for the damage initiation criterion together with a strain-based progressive damage model. Initial finite element simulations at the single-element level demonstrated the validity and capability of the damage model. More complex models were used to simulate tensile specimens, coupon specimens, and skin panels subjected to low-velocity impacts, being validated against experimental data at each stage. The effect of non-central impact location showed higher impact peak forces and bigger damage areas for impacts closer to panel boundaries. The presence of pre-existing damage close to the impact region leading to interfering delamination areas produced severe changes in the mechanical response, lowering the impact resistance on the panel for the second impact, while for non-interfering impacts, the results of the second impact were similar to the impact of a pristine specimen.

Modal properties investigation of a carbon/glass fiber-reinforced polymer composite sandwich structure with lozenge-like core: An experimental and finite element simulation

The vibration properties of a carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP) composite sandwich panels with lozenge-like core and free-free boundary conditions are experimentally determined in this research. The sandwich panels manufactured for this study are composed of CFRP, GFRP, and epoxy resin. A modal experiment is performed using a modal hammer with an acceleration sensor to obtain modal properties of the structure, including natural frequencies, damping ratio, mode shapes, and frequency responses. Eventually, the comparison of vibration characteristics of the CFRP sandwich panel with the GFRP sandwich panel with almost identical geometry is discussed in detail. Furthermore, a finite element simulation is developed to validate the precision of the experimental data. It is recognized that the finite element results are in remarkably good agreement with those obtained by modal experiments. Hence, the finite element analysis and experiment method could be utilized to predict the modal properties of the CFRP and GFRP composite sandwich panels.

Assessment of Conductive Sites on Carbon Fiber Reinforced Polymer Plastics' Surfaces Using Electrochemical Methods

The components of automobile bodies have been manufactured primarily from steel since cars were first made. With the goal of energy savings, light-weighting is being adopted by substituting steel with a variety of light metals and composite materials. A DOE sponsored project in collaboration with PPG and Ford studied the use of automotive closure panels comprising an inner carbon fiber reinforced polymer (CFRP) sheet and an outer aluminum alloy (AA) sheet. The ends of closure panels are sealed with hem flange joints. A major concern is the formation of a galvanic cell within the joint wherein CFRP is the cathode that will accelerate the corrosion of the AA sheet. Various combinations of AA6xxx alloys with CFRP having two different orientations of carbon fiber, i.e., random and twill, are studied in this work. CFRP can only exert a galvanic driving force on a coupled AA panel by electrical connection at cut edges where sheared carbon fibers are exposed, or if carbon fibers are exposed on the CFRP panel surface by lack of coverage of the insulating epoxy matrix. It is believed that processing and curing variables lead to varied epoxy cover on the surfaces of CFRP. Electrochemical measurements such as the measurement of oxygen reduction limiting current density during cathodic polarization show that as-fabricated surfaces of CFRP panels exhibit some extent of electrochemical activity, indicating some lack of coverage of C fibers. It is of interest to characterize the epoxy cover on the CFRP surfaces in terms of the nature of the active sites (e.g., whether C fibers are totally uncovered or covered by a very thin layer of epoxy) and the density and distribution of the sites. A technique was developed to identify the exact locations of active sites on both random and twill CFRP by electrodeposition of small amounts of Cu. If the amount of deposited copper is small, the deposits indicate the location and number of the active sites on the CFRP surface. These deposits were also analyzed by serial sectioning using Focused Ion Beam/Scanning Electron Microscope. High resolution images of the deposit/epoxy interfaces provide a description of the nature of the defective/conductive regions on CFRP surfaces. Rotating disk electrode experiments were also performed to determine the oxygen reduction reaction (ORR) limiting current densities of CFRP at different electrode rotation rates i.e., different diffusion boundary layer thicknesses. These aspects are related to the overall mass transport in the electrolyte, which provides a deeper understanding of the kinetics on CFRP and the nature of active sites behaving as microelectrodes. The observations from the copper electrodeposition study and rotating disk electrode experiments will be correlated and the role of CFRP as a cathode contributing to the corrosion of AA in the galvanic couple will be presented.This work was supported by the U.S. Department of Energy through award DE-EE0007760, in collaboration with PPG Industries and Ford Motor Company.

Hole Quality Assessment of Drilled Carbon Fiber Reinforced Polymer (CFRP) Panel Using Various Custom Twist Drill Geometries

Excellent hole quality is necessary for the aerospace industry’s highly abrasive carbon fiber reinforced polymer (CFRP) drilling process. This work considered three different twist drill designs, tapered web, burnishing, and subland drill reamer for the drilling process. The drill bits were made of tungsten carbide and machined with custom helix angle, primary clearance, point angle, and chisel angle. The primary objective of this research is to determine the thrust force signature for each custom drill bit design and the delamination factor for the hole drilling at 3000 rev/min and 0.05 mm/rev. The finding indicates that the tapered web gave the best design by improving the maximum thrust force by 14.6% in drilling CFRP panels. Additionally, the tapered web design led to a low delamination factor on both entrance (1.0186) and exit sides (1.0475). The thrust force is directly proportional to the delamination factor when drilling a CFRP material in a single shot operation. The subland drill reamer produces higher thrust force, and delamination proved that the combination of drill and reamer design was unsuitable for high-speed drilling.

A novel methodology to research the residual compressive mechanical properties of composite stiffened panel after low velocity impact based on quantification damage simulation

AbstractComposite stiffened panels have become one of the most usual components in the design process of the aircraft's structure due to their splendid mechanical properties in the last decades. The main purpose of this work is investigating and presenting a more precise method to forecast the residual compressive mechanical behavior of the composite stiffened panel. First of all, an axial compression after impact (CAI) test is performed to understand the failure mechanism of different type of L‐shaped stiffened carbon fiber reinforced polymer panels with barely visible impact damage, which is introduced by the low velocity impact test. Subsequently, a finite element model, which contains a quantification damage model, is established in ABAQUS software based on the CAI test results to predict the mechanical behavior of these composite stiffened panels during the CAI test process. Both of the experimental and predicted results demonstrate that the propagation of the delamination caused by LVI is going to exert a destructive influence on the total failure of the composite stiffened panel, and the prediction results of the residual compressive strength of these composite stiffened panels are well consistent to that measured in the CAI test.

MXene Reinforced Thermosetting Composite for Lightning Strike Protection of Carbon Fiber Reinforced Polymer (Adv. Mater. Interfaces 17/2021)

MXene-based Lightning Strike Protection Coating In article number 2100803, Vipin Kumar and co-workers investigate MXene as an effective filler to improve the electrical and mechanical properties of divinylbenzene (DVB) thermosetting resin. Its performance as a lightning strike protection coating for carbon fiber reinforced polymer (CFRP) is evaluated. MXenes-DVB coating reduces the lightning surface damage from 40.61 cm2 (reference CFRP panel) to 13.29 cm2 (CFRP coated with 72 g cm−2 weight of MXenes-DVB).

MXene Reinforced Thermosetting Composite for Lightning Strike Protection of Carbon Fiber Reinforced Polymer

AbstractTi3C2 – a member of the MXenes (2D transition metal carbides and nitrides) family, is investigated as an effective filler to improve the electrical, mechanical, and thermal properties of divinylbenzene (DVB) thermosetting resin. Consequently, its performance as a lightning strike protection (LSP) coating for carbon fiber reinforced polymer (CFRP) is evaluated. Polyaniline (PANI) – dodecylbenzene sulfonic acid (DBSA) complex is used to cure the DVB resin. The synergic effect of MXenes (with surface that is negatively charged) with polyaniline (positive charge) shows electrostatic bonding and improved electrical conductivity in the composite. The addition of MXenes at 2 wt% into the PANI‐DVB composite shows ≈139%, 10%, and 9% improvement in electrical conductivity, flexural strength, and flexural modulus, respectively, compared to the neat PANI‐DVB composite. The composites are investigated using various material characterization techniques including Fourier transforms infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and scanning electron microscopy. Furthermore, MXenes‐DVB is utilized to create a conductive thermosetting coating on top of a CFRP substrate and tested against a lightning strike of 100 kA. CFRP with MXenes‐DVB coating reduced the surface damage from 40.61 cm2 (reference CFRP panel) to 13.29 cm2 (CFRP coated with MXenes‐DVB).

Vibration Parameters for Impact Detection of Composite Panel: A Neural Network Based Approach

The need for reliable methodologies for structural monitoring is certainly a current line of research in many engineering sectors. The detection of the impact on composite materials is in fact a recent subject of study, aimed at safeguarding the mechanical integrity and improving the useful life of structural components. In such a context, the work deals with evaluation of the use of neural algorithms for localizing the position of the impacts on composite structures. Starting from FE (finite element) simulations, representative of the dynamic response of a CFRP (Carbon Fiber Reinforced Polymer) panel as a benchmark, the approach has been finally validated experimentally by modal parameters identification.

Planar capacitive imaging for composite delamination damage characterization

The objective of this study is to develop and validate a noncontact, nondestructive inspection and imaging method for rapidly assessing the subsurface condition of carbon fiber-reinforced polymer (CFRP) composite structures. The method works on the principle of planar electrical capacitance tomography (ECT). Unlike conventional ECT systems, electrodes are arranged on a rectangular planar surface in the form of a grid. The volume on one side of the electrode plane is interrogated with an electric field, and the mutual capacitances between the electrodes are measured. The volumetric electrical permittivity distribution of the interrogated region is then reconstructed from the measured capacitance responses. In this work, an ECT image reconstruction algorithm was implemented, and a customized planar capacitive imaging system was prototyped. First, different objects were placed near the electrode array, and the corresponding volumetric change in electrical permittivity was successfully captured. Second, the planar ECT system was employed for detecting artificially introduced subsurface defects in 3D-printed objects. Third, CFRP specimens with different sizes of single-layer delamination were fabricated and subjected to ECT interrogation. The results confirmed that the planar ECT system could detect the location and size of delamination in CFRP panels. The accuracy and resolution of the planar ECT prototype were also characterized.

A damage threshold prediction model of CFRP panel by hail impact based on delamination mechanism

Dynamic response mechanism of CFRP (Carbon Fiber Reinforced Polymer) panel under high-speed ice ball impact was challenging to elucidate due to inherent heterogeneity and anisotropy properties. This further leads to difficulties in theoretical prediction of the damage threshold. Simulation modelling of CFRP panel subjected to high-speed ice ball impact was conducted in this study. Through coupling SPH (Smoothed Particle Hydrodynamics) and FEM (Finite Element Method), the simulation has captured the deformation and damage of the CFRP panel and ice ball. A damage threshold prediction model of CFRP was then proposed based on the mechanism analysis of the dynamic deformation and delamination. The traditional prediction of CFRP impact damage threshold, that is, parametric studies on the damage threshold of CFRP panel influenced by different impact speed, ice ball diameter and panel thickness was performed. Finally, the results show that the CFRP damage threshold law obtained from parameterized simulation matches the previous experimental results, and a conservative damage threshold can be predicted from the proposed theoretical model. The proposed theoretical model can contribute to predicting the damage threshold of CFRP panels subject to ice ball impacts at low cost.

High-velocity impact response of CFRP panels reinforced with stiffeners

With a goal of reducing the weight of fan blade containment system for jet engine, the high-velocity impact response of carbon fiber reinforced polymer (CFRP) panels reinforced with stiffeners were tested and analyzed. Ballistic impact tests on stiffened panels have been carried out with cylindrical Ti-6Al-4V projectiles at different velocities (210 m/s, 70 m/s and 90 m/s). Results showed there was a significant influence observed due to different impact velocities on the response of the panel skin and the stiffener, and the middle stiffener played an important role in impact energy absorption. Numerical simulations were conducted to provide further insight into the target response by employing a user subroutine VUMAT with revised Hou criteria in commercial finite element (FE) software ABAQUS. The stiffened CFRP panel was found to possess a 14% higher critical penetrating velocity than the CFRP laminated plates with same mass by using validated FE models. Combining with detailed discussion of high-velocity penetration process, the energy absorption mechanism of the stiffened panel under high-velocity impact was analyzed, indicating that the improvement on high-velocity impact response is due indirectly to structural alteration induced better energy absorption by less punch-shear laminate breakage and more fiber tensile failure.

Eddy Current Array Inspection of Damaged CFRP Sandwich Panels

Abstract The use of eddy current (EC) arrays to detect damage in sandwich panels, such as disbonding of the carbon fiber reinforced polymer (CFRP) face-sheet to the core, is investigated. It is shown that the array is very sensitive to slight core crush and can readily find small dents and disbonds. At the same time, the eddy current array can look much deeper into the honeycomb to detect defects such as tears. The phase map of the EC signal can be used in some cases to distinguish between different types of damage. EC arrays offer the ability to rapidly scan large areas of CFRP panels.

Critical bending load of CFRP panel with shallow surface scratch determined by a tensile strength model

Carbon fibre reinforced polymer (CFRP) composite structures may experience light surface damage such as shallow surface scratch around 50 μm or less in applications. In this study, critical bending loads of 2D woven CFRP panels with and without shallow surface scratches were assessed under three-point-bending (3-p-b) conditions and then modelled by a simple strength theory for composite fracture. This closed-form tensile strength solution is a non- Linear Elastic Fracture Mechanics (LEFM) model derived specifically for heterogeneous composites. Direct tensile tests of the CFRP panels (without shallow surface scratch) were also performed, and measured tensile strength was compared with that from 3-p-b tests. The relative error between the two different tests was only around 6.7%. The ply thickness around 140 μm was selected as the controlling composite microstructure parameter or characteristic composite unit Cch in this non-LEFM model. This simple model shows that a suitable composite characteristic microstructure measurement such as Cch = ply thickness can significantly simplify composite fracture analysis. A simple statistical analysis using a 2D normal distribution methodology for strength measurements was also introduced so that the experimental scatters can be analysed. The 95% reliability band can be conveniently used in safe design of composite structures.

A smart multi-functional printed sensor for monitoring curing and damage of composite repair patch

A novel multifunctional diagnostic sensor is developed as a cost-effective, in-service structural health monitoring system for determining the initial quality of curing of a bonded composite repair patch and assessing its long-term durability on composite structure. The proposed multi-functional sensor technology involves the creation of a ‘tailor-to-order’ 2D conductive patterns onto step-sanded repair surface of composite repair patch using inkjet printing. In employing this methodology, bondline quality during curing and in service was successfully assessed via impedance spectroscopy and resistance change measurements, respectively. The ability of this technology to effectively monitor the integrity of the bondline and the extent of damage in real-time was investigated by subjecting the scarf-repaired carbon fiber reinforced polymer panels to 3-point bending fatigue and low-velocity impact tests. The obtained results were compared with those of transient infrared thermography and ultrasound inspection techniques, thus validating the proposed method.

Keyhole cutting of carbon fiber reinforced polymer using a long-duration nanosecond pulse laser

The machining performance of a high-energy nanosecond pulse laser with a near-infrared wavelength is investigated for carbon fiber-reinforced polymer (CFRP) with two different fiber arrangements. This research work demonstrates for the first time that a keyhole mode cutting can be achieved for CFRP materials using a high-energy nanosecond pulse laser of a Long Pulse mode (120 ns). Specifically, it is shown that the short-duration Q-Switch mode (8 ns) results in ineffective material removal for CFRP, despite much higher peak laser power intensity than the Long Pulse mode. In Long Pulse mode, multi-pass straight line and contour cutting experiments are further performed to investigate the effect of laser processing parameters and resultant machined surface integrity. Plasma absorption effects using both pulse modes are discussed. The results show that a 2.2 mm thick cross-ply CFRP panel can be cut through using as few as 6 laser passes, and a high-quality machined surface can be produced with a limited heat-affected zone and minimal fiber pull-out using Argon assist gas. The successful outcomes from this work provide the key to enable efficient CFRP laser machining using high-energy nanosecond pulse lasers, and offer insight into the unique energy absorption mechanisms for CFRP laser machining.

Tensile properties of CFRP manufactured by resin transfer molding considering stacking sequences at various strain rates

This paper is concerned with the improvement of a tensile test method and the material properties of carbon fiber reinforced polymer manufactured by resin transfer molding considering stacking sequences at various strain rates for auto-body. Auto-body structure experiences the strain rates up to several hundreds per second during car crash. In order to apply the carbon fiber reinforced polymer panel into auto-body structures, it is critical to acquire the material properties of carbon fiber reinforced polymer at various strain rates considering stacking sequences. The tensile test method is improved for acceptable test results. Test specimens are modified for high-speed tensile tests of carbon fiber reinforced polymer in order to achieve designated strain rates and eliminate the effect from unfavorable conditions of inhomogeneity of deformation. Various types of grip tab materials are employed for acceptable failure modes. Tensile tests have been carried out with non-crimp fabric made by 50K high strength carbon fiber (NCF) and woven fabric made by a 2/2 twill pattern of 3K high strength carbon fiber (TWILL) with different stacking sequences of 0° and 90° unidirectional cases as well as [0°/90°] and [45°/−45°] symmetric cases. Digital image correlation method and force equilibrium grid method are adopted for strain and stress measurement of a carbon fiber reinforced polymer specimen during a tensile test. The material properties acquired indicate that the carbon fiber has little rate dependency, while the epoxy matrix has remarkable rate hardening at strain rates from 0.001 s−1to 100 s−1.

Effect of machining parameters on surface quality during edge trimming of multi-directional CFRP material: Taguchi method

In recent years, carbon fiber-reinforced polymer (CFRP) materials are gaining tremendous attention by industries especially in aerospace industry for product structures due to the superior material properties such as high strength, low weight and corrosion. This study presents a research on the influence of machining parameters utilizing uncoated tungsten carbide router or burrs tool towards surface quality during edge trimming of a specific CFRP material. CFRP panel of 10.31 mm in thickness with the total of 38 number of plies has been chosen to be the main study material. A famous statistical method namely Taguchi Method (L4) has been deployed to plan the overall experimental design. Two main factors investigated in this study were spindle speed and feed rate. Surface roughness measurement was taken using Mitutoyo Surftest SJ-410. Moreover, optical microscope Nikon MM-800 is utilized to further observe the trimmed surfaces. The result reveals that the spindle speed was the most influential parameter towards the trimmed surface quality. Run no. 3 (R3) generated approximately 2.5 times better Ra value than Run no. 3 (R2) at constant feed rate (1500 mm/min) but vary in spindle speed by the difference of 3000 rpm. From further observations of trimmed surface through optical microscope, obvious matrix degradation and wrench areas were spotted on the R2 trimmed surface compared to the R3. From ANOVA analysis, spindle speed was also indicated as the significant factor. Details results elaborated and discussed further in this paper.