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Structural response of CFRP materials subjected to simulated lightning strikes
No full textThe buckling response of Carbon Fibre Reinforced Polymer (CFRP) panels damaged by simulated lightning strikes is evaluated. Five panels constructed with carbon/epoxy material were subjected to an electrical current with 10/350µs waveform that simulates an idealized lightning strike. The buckling experiments were conducted in a newly designed modified buckling test, herein referred to as Compression After Lightning Strike (CALS), which is able to accommodate large composite panels and allows the full extent of the lightning damage to be included in the compression test. Stereo Digital Image Correlation (DIC) was used on both sides of the plate to obtain the strain and displacements that occur during the CALS test and hence determine how the lightning strike affects the mechanical performance of the structure. The experimental results were benchmarked and compared against a shell post-buckling finite element model (FEM). The results reported show good agreement between the experiment and the model predictions
Characterisation of lightning strike induced damage in CFRP laminates and components for wind turbine blades
No full textTo meet worldwide increases in energy demand Wind Turbine (WT) manufacturers are producing longer blades to generate more energy. These blades contain Carbon Fibre Reinforced Polymers (CFRP) in the load carrying structures to lightweight the blade. The introduction of the CFRP composites has presented new challenges in protecting the structure from lightning. The semiconductive nature of CFRP leads to an additional path to ground for the current in the structure and the anisotropic nature of the material’s thermal and electrical properties leads to large amounts of resistive heating especially in the through-thickness direction where the electrical conductivity is the lowest. The aim of this PhD is to devise a new means of assessing the damage and resulting structural behaviour caused by a lightning strike. A modelling framework is developed and validated against high fidelity experimental data that can be used by design engineers to understand the consequences of various lightning damage scenarios and the effectiveness of lightning protection methods. The framework is validated against a representative scale WT sparcap test component in the form of a large panel subjected to compression. The novel damage model is a thermal-electrical Joule heating model which simulates the resistive heating in a UD laminate with electric field dependent material properties to account for electric breakdown. The damage prediction is then exported into a structural Finite Element Model (FEM) by assuming the damaged elements have different material properties. The structural behaviour under compression loading is the main design driver for long slender WT blades. Therefore, the structural model simulates the behaviour of a damaged laminate in a non-linear post-buckling FEM. To validate and inform the damage model and the FEM two different types of tests were conducted. The first type of test simulated the lightning strikes and comprised of direct strike and conducted current tests. The effect of conducting current along the fibre direction showed a deleterious effect on the compressive and shear properties of the material. Initial direct strike tests were used to vary the typical lightning parameters to determine the largest influence on damage among peak current, specific energy, or charge. The last direct strike test is conducted on a representative WT sparcap panel. All damaged panels were evaluated using visual inspection, a new thermography technique, and X-ray computed tomography (CT). The newly developed damage model was validated using the experimental observations with the damage area predictions within 15% of the visual observation and the damage depth within 5% of the CT scans. Hence, the electric field dependency was successfully implemented in the model. The second test type was a structural test that incorporated the development of a new testing methodology named the compression after lightning strike (CALS) test. Large representative sparcap panel specimens, with and without lightning damage were tested to failure in the CALS rig and Digital Image Correlation (DIC) was used to determine the resulting surface displacements and strains. The structural model closely predicted the compressive behaviour and failure loads identified by the DIC. The resulting structural model calculated the first ply failure stresses from the LaRC failure criteria which were within 8% of experimental values, which provided a successful validation of the modelling framework
Lighting protection of carbon fibre reinforced plastic for wind turbines blades
No full textWind turbines are a widely used renewable energy source for the generation of electric power. The wind turbine industry is introducing longer blades to increase the power output a single turbine. To achieve this the blades have incorporated Carbon Fibre Reinforced Polymer (CFRP) materials. CFRP materials provide a strong, stiff and lightweight material which allows longer blades to be manufacture. However, longer blades have the higher potential for lightning strikes and the CFRP materials have a susceptibility to damage caused by lightning current. Wind turbine blades struck by lightning can incur damage which cost the industry in damages to the turbine, loss of revenue due to shut down of wind turbine and repair or replacement cost. The protection of these blades from lightning is of the utmost importance to reduce the overall operating cost of the wind turbine system. This project aims to develop lightning protection solutions to help mitigate damage and keep wind turbines operational. There are five goals to achieve this aim: 1. develop and validate a coupled thermo-electrical model of CFRP laminates subjected to lightning strikes; 2. develop, implement and validate models for damage and failure prediction of CFRP materials subjected to lightning strike; 3. validate the damage models by conducting experiments on CFRP coupons and sub-structural components subjected to lightning strike; 4. develop predictive models residual strength on CFRP materials post lightning strike; 5. validate the residual strength models by conducting panel compression experiments. The damage models are used to develop lightning protection concept and solutions. The structural models are used to predict remaining strength capability (residual strength, and stiffness) of the CFRP panels
Hybrid multiscale modelling to predict lightning damage on CFRP materials
No full textCarbon Fiber Reinforced Polymers (CFRPs) materials are increasingly being used in the wind turbine industry to reduce weight in turbine blades. However, CFRPs have a particular issue when dealing with lightning strikes because of their anisotropic material properties; in particular, their electrical and thermal conductivities. These issues cause significant damage when exposed to large electric currents. This paper presents a time dependent coupled thermal-electric joule heating model to predict the thermal damage of a CFRP panel when subjected to a lightning strike. The approach of this model uses volume fractions to determine the resulting fiber and resin damage separately. The damage prediction is calculated by a set of pyrolysis equations. The pyrolysis damage alters the materials properties of the elements. The electric current applied to the panel is the 10/350 standard waveform which corresponds to the waveform used to test wind turbines according to the IEC61400 section 24 Ed 1.0
Identification of lightning strike damage using Pulse Thermography through integration of thermal data
Get PDFPulse Thermography (PT) is based on the use of infrared imaging systems to detect thermal decay from a pulsed energy source as it passes through a component or sample. Carbon Fiber Reinforced Polymer (CFRP) composite materials damaged by simulated lightning strike are inspected using PT. A new damage detection approach is proposed which exploits the integral of the difference in temperature data over time between each pixel and a reference non-damaged pixel. The resulting integration provides a value for the thermal decay pixel-by-pixel relative to a non-damaged region, which provides a quantitative damage severity parameter. The proposed data processing method is evaluated using calibrated plate sample made from Glass Fiber Reinforced Polymer (GFRP) composite with known defects, and also to investigate CFRP samples damaged by lightning strikes. The calibrated GFRP plates are 4-ply stitched bi-axial E-glass fibers with a total laminate thickness of 2.4 mm. The known defects are 20 mm square PTFE inserts placed between plies to simulate a delamination. The resulting colormap correctly identifies the known defects and displays constant severity over the PTFE insert region. The same method is applied to the lightning damaged CFRP sample made of 5-ply stitched dry fabric. The resulting integration reveals the full extent of the damage, which cannot be identified by visual inspection
Damage in CFRP composites subjected to simulated lighting strike
No full textCarbon fibre composite materials are increasingly being used in the wind turbine, aerospace, and automotive industries to reduce the structural weight of components due to their high strength to weight and stiffness to weight ratios. However, the anisotropic material properties of Carbon Fibre Reinforced Polymers (CFRP), specifically their electrical and thermal conductivities, create challenges when protecting structures from lightning strike. Moreover, the exposure of CFRP structures/components to electric currents from lightning discharges can cause significant damage. This work investigates the damage inflicted and residual mechanical properties of a CFRP composite material that has been exposed to simulated lightning strike. Seven different CFRP laminate specimens were struck with simulated lightning strike using three different waveforms: the so-called 10/350 μs waveform, which simulates the first return stroke during a direct strike according to IEC 61400-24 Ed1.0, the second was a unipolar long stroke component, and the third was a combination of the first return stroke and the long stroke. Test specimens were prepared from CFRP panels that were damaged due to the lightning strike. The test specimens were subsequently subjected to compression and shear loading to determine the post-strike mechanical properties. The compression tests were conducted using uniaxial coupons and loaded in accordance with ASTM standard D6641. The shear tests were conducted using V-notch specimens utilizing an Iosipescu test rig in accordance with ASTM standard D5379. Digital Image Correlation was used to capture the strain fields on the specimens. The test results were compared against data obtained for pristine CFRP coupon samples that were not exposed to electrical current. The shear and compression strengths, compressive and shear stress-strain curves, compressive and shear moduli, and the maximum temperature captured in the specimens during the tests are presented and discussed. Key results include that the largest reduction of strength occurred in the specimens that were subjected to the largest current and specific energy, and further that damaged specimens respond with a higher degree of nonlinear behaviour than the pristine specimens
Damage prediction of CFRP materials subjected to lightning strike
No full textThis paper presents a coupled thermal-electric model to predict the thermal damage of a Carbon Fiber Reinforced Polymer (CFRP) material when subjected to a lightning strike. A Finite Element Model (FEM) is used to predict the heat response of the CFRP material by solving the Joule heating governing equations. Joule heating, also known as resistive heating, is the heating of the material when subjected to an electrical current. Solutions to the Joule heating model are developed using a time dependent simulation with the 10/350μs standard waveform used to test wind turbine blades in accordance to IEC61400 section 24 Ed 1.0. The time dependent model implements damage criteria and is able to identify damaged elements. The damage criteria are based on a combination of material decomposition by pyrolysis described by the Arrhenius equation. The COMSOL software engine was used to derive the results from the Joule heating model. An integrated MATLAB script was run during the simulation to determine the amount of damage that each element is subjected to during a lightning strike event. The final result is a damage map of the CFRP panel subjected to a lightning discharge. The damage model is validated through lightning discharge experiments. Two samples with unidirectional fibers were made by vacuum assisted liquid resin infusion to mimic the sparcaps of a wind turbine blade located near the wind blade tip region. The samples were tested using the arc entry test of IEC 61400-24 Ed 1.0 with simulated first return stroke electric current components (10/350μs) with magnitudes of 30 kA and 60 kA unipolar waveforms. The resulting damages were inspected by use of X-ray Computed Tomography (CT) to determine the total damaged volume. The CT scans used an imaging segmentation algorithm to systematically determine the location and type of the damage done to the CFRP. The resulting CT scans are compared to the damage model
Delamination prediction on CFRP materials subjected to a lightning strike
Get PDFThis paper presents a coupled thermal-electric-mechanical model to predict the delamination damage of a Carbon Fiber Reinforced Polymer (CFRP) material when subjected to a lightning strike. A Finite Element Model (FEM) is used to predict the heat response of the CFRP material by solving the Joule heating governing equations. The results of the heat response are coupled with thermal stresses to predicte interlaminar stresses. A bilinear traction law is used to predict the delamination of the laminate. Solutions to the model are developed using a time dependent simulation with the 10/350µs standard waveform to mimic a typical lightning strike on a wind turbine blade in accordance to IEC61400 section 24 Ed 1.0. The time dependent model implements damage criteria and is able to identify damaged elements. The COMSOL software engine was used to derive the results from the thermal-electrical-mechanical model. The final result is a delamination map of the CFRP panel subjected to a lightning discharge
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