Electrical Characterisation of Aerospace Grade Carbon-Fibre- Reinforced Polymers PhD Thesis Rabia Abid December 2015 CARDIFF SCHOOL OF ENGINEERING ABSTRACT This thesis is primarily concerned with experimental tests and computer simulations to determine electrical characteristics of carbon fibre composites used in aerospace. The work has involved an extensive review of published literature, numerical modelling and experimental findings on electrical characterization of carbon fibre composites. The numerical modelling on carbon composites was carried out using finite element package COMSOL, using preliminary conductivity values from literature. Lightning impulse waveform D was used, on a surface electrode to study the effect of lightning strike on carbon composites. Existence of shared equi-potentials was found. Low current testing using variable ply lay ups of carbon composites was performed, the resistance measurements were taken using two probe method for volume conductivity, these values were compared and complied well with the values in literature. Surface resistance was also measured using new techniques, which introduced material and contact resistance into the parasitic resistance, the measurements of parasitic resistance gave an accurate measure of current distortion in variable ply layups. The current distortion was more transverse to the fibre direction and less in line to the fibre direction. High current testing was performed using high voltage system in Cardiff high voltage lab, this was done for two current ranges Amperes and kA range. For both current ranges the samples were tested for mechanical strength requirements keeping in mind the mechanical design requirements for aircraft designers, it was found that the mechanical strength for samples impact with lightning impulse was not effected in Amperes range however it starts to effect in kA range at 11 kA. Thermal damage in carbon composites was viewed under the scanned electron microscope using continuous current injection into a carbon composite sample obtained from a T joint. The sample microstructure showed that the fibres have splits and cracks along their diameters which shows that the direct current will be quite damaging to carbon fibres and will disrupt their electrical network. ii PUBLICATIONS 1. R. Abid, A. Haddad, H. Griffiths, D. Clark, M. Cole, S. Evans , “Electrical Characterization Of Aerospace Graded Carbon Fiber Reinforced Plastic Composites Under Low Current DC And Impulse Energizations”, International Conference on Lightning and Static Electricity, USA, 2013 2. R. Abid, A. Haddad, H. Griffiths, D. Clark, M. Cole, S. Evans, “Mechanisms of Damage Propagation in Carbon Composites under High Impulse Current Magnitudes”, 50th International Universities Power Engineering Conference (UPEC), Staffordshire University, UK, 2015 AWARDS Best Paper Award Certificate, 50th International Universities Power Engineering Conference (UPEC 2015), Staffordshire University UK for the paper entitled “Mechanisms of Damage Propagation in Carbon Composites under High Impulse Current Magnitudes”. iii DECLARATION This work has not previously been accepted in substance for any degree and is not currently submitted in candidature for any degree. Signed…………………………………….. (candidate) Date…………………… Statement 1 This thesis is being submitted in partial fulfilment of the requirements for the degree of PhD Signed…………………………………….. (candidate) Date…………………… Statement 2 This thesis is the result of my own independent work/investigation, except where otherwise stated. Other sources are acknowledged by explicit references. Signed…………………………………….. (candidate) Date…………………… Statement 3 I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter-library loan, and for the title and summary to be made available to outside organisations. Signed…………………………………….. (candidate) Date…………………… Statement 4: PREVIOUSLY APPROVED BAR ON ACCESS I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter-library loans after expiry of a bar on access previously approved by the Graduate Development Committee. Signed…………………………………….. (candidate) Date…………………… iv ACKNOWLEDGEMENTS In the name of Allah Almighty, the most beneficent most gracious and most merciful, who blessed me with the wisdom, knowledge and strength to accomplish my goal. I want to thank my supervisors and mentors, Prof. Manu Haddad and Dr. Huw Griffiths, for their help and support all through the project. I am grateful for their patience during my studentship and for correcting my mistakes that always led to my betterment. I would also like to thank Dr. David Clark for designing experiments and devising measurement techniques for my work, and Mr. Chris Stones at MBLL and Airbus Group Innovations for sponsoring this studentship. I also thank my parents, Abid and Tasneem, for their great support. v NOMENCLATURE a Radius of the metal fastener A Total damage area AC Alternating current A Area of composite c A Cross-sectional area of fibre f b Distance between the metal fastener and the metal ring C Capacitance D1 Diameter of the central electrode on the top surface DC Direct current g Gap between top electrode and sample GF Gauge factor I Current I Current passing through the circuit circuit I Peak current p I Injected current t J Joule heating L Length of the sample l Electrode spacing N Number of fibres N Number of carbon fibres b N Number of diagonal plies d N Number of non-diagonal plies o vi P Electrical power dissipated via current flow into the conductor ec p Impact threat j Q Net electrical charge r Distance between the end of the fastener and the end of the metal ring R Resistance R Resistance at zero strain 0 R Apparent resistance a R Contact resistance c R Resistance of the composite c R Dispersion resistance d R Electrode resistance e R Total resistance of fibre t R Material resistance m R Resistance at peak current intensity v S Cross-sectional area of carbon fibres c S Total cross-sectional area of the sample t t Thickness of the composite t Thickness of diagonal plies d t Thickness of non-diagonal plies o t Thickness of a ply ply V Voltage V Fibre volume fraction f V Peak voltage Ip W Width of the sample x Spatial coordinate in x direction vii y Spatial coordinate in y direction Z Impulse resistance p α Euler angle δ Electrical ineffective length e ΔR Change in resistance ε Applied strain L ε Relative permittivity r ϑ Critical volume fraction ϑ Critical volume fraction of the conducting constituent c θ Ply orientation ξ Damage parameter ρ Apparent resistivity ρ Resistivity of the carbon-fibre-reinforced plastics c ρ Longitudinal resistivity long σ Conductivity σ Conductivity of the conducting phase 0 σ Cross-ply conductivity cp σ Longitudinal conductivity L σ Transverse conductivity T viii TABLE OF CONTENTS ABSTRACT ....................................................................................................................................................... II PUBLICATIONS ............................................................................................................................................... III AWARDS ............................................................................................................................................. III DECLARATION .............................................................................................................................................. IV ACKNOWLEDGEMENTS ............................................................................................................................... V NOMENCLATURE .......................................................................................................................................... VI TABLE OF CONTENTS ................................................................................................................................... IX INTRODUCTION .................................................................................................................................. 1 1.1. BACKGROUND........................................................................................................................................ 1 1.2. AIMS AND OBJECTIVES ........................................................................................................................... 2 1.3. CONTRIBUTIONS OF THIS THESIS ............................................................................................................ 3 1.4. CHARACTERISATION OF THE BEHAVIOUR OF DIFFERENT CARBON COMPOSITES ..................................... 4 1.5. MATERIAL AND PLY LAY-UPS ................................................................................................................ 5 1.6. THESIS LAYOUT ..................................................................................................................................... 5 STRUCTURE AND ELECTRICAL RESISTANCE OF CARBON-FIBRE-REINFORCED PLASTICS (CFRPS) - A REVIEW ........................................................................................................ 8 2.1 INTRODUCTION: MATERIAL CONSTRUCTION .......................................................................................... 8 2.2 STRUCTURAL AND ELECTRICAL PROPERTIES OF CARBON FIBRE COMPOSITES ....................................... 11 2.3 FACTORS AFFECTING THE STRENGTH OF CARBON FIBRE COMPOSITES .................................................. 13 2.4 FASTENERS .......................................................................................................................................... 14 2.5 STRUCTURAL PROCESSING OF CARBON FIBRES .................................................................................... 14 2.6 ELECTRICAL PROPERTIES OF RESIN ..................................................................................................... 18 2.7 LONGITUDINAL RESISTANCE ................................................................................................................ 19 2.8 TRANSVERSE RESISTANCE.................................................................................................................... 22 2.9 MEASURING THE ELECTRICAL CONDUCTIVITY OF CARBON-FIBRE-REINFORCED PLASTICS ................... 24 2.9.1 Four-probe resistance measurements ............................................................................................. 25 2.9.2 Four-probe measurements of potential change .............................................................................. 26 2.9.3 Advantages of four-probe resistance measurements ....................................................................... 26 2.9.4 Multi-probe method ........................................................................................................................ 27 2.10 ELECTRICAL RESPONSE OF CARBON-FIBRE-REINFORCED PLASTICS UNDER MECHANICAL LOADING ..... 27 2.11 ELECTRICAL RESISTANCE TO MEMORIZE MAXIMUM LOAD ................................................................... 32 2.12 MAPPING ELECTRICAL CONDUCTIVITY USING ELECTRICAL RESISTANCE ............................................ 34 2.1 ELECTRICAL RESISTANCE AS A DAMAGE MONITORING TOOL ............................................................... 38 2.2 CONCLUSIONS ...................................................................................................................................... 43 POTENTIAL AND CURRENT DISTRIBUTION IN CARBON COMPOSITE PANELS: NUMERICAL MODELLING AND LABORATORY TESTS ........................................................... 44 3.1 BACKGROUND...................................................................................................................................... 44 3.2 LIGHTNING ATTACHMENT ZONES ......................................................................................................... 46 3.3 DIRECT EFFECTS OF LIGHTNING ON AIRCRAFT ..................................................................................... 49 3.4 DEFINITION OF VOLTAGE WAVEFORMS ............................................................................................... 50 3.5 SINGLE-LAYERED PANEL—NUMERICAL MODELLING ........................................................................... 52 3.6 MODELLING A TWO-LAYERED PANEL ................................................................................................... 55 3.7 MODELLING THE EXPERIMENTAL PANEL STRUCTURE .......................................................................... 58 3.8 EXPERIMENTAL STUDIES OF FLAT PANELS ........................................................................................... 60 3.9 SIMULATIONS OF LIGHTNING STRIKES .................................................................................................. 61 3.10 CURRENT AND POTENTIAL DISTRIBUTION IN TWO LAYERED PANELS ................................................. 64 3.11 POTENTIAL PROFILES ........................................................................................................................... 69 3.12 THROUGH-THICKNESS POTENTIAL PROFILES ........................................................................................ 70 3.13 EMBEDDED ELECTRODE DESIGN SIMULATION ...................................................................................... 74 3.14 CONCLUSIONS ...................................................................................................................................... 77 ELECTRICAL CHARACTERISATION: SURFACE AND VOLUME RESISTANCE STUDY ...... 78 4.1 INTRODUCTION: TWO- AND FOUR-PROBE METHODS ............................................................................. 78 4.2 ABRASION TECHNIQUES ....................................................................................................................... 79 4.3 ELECTRODE FORMATION TECHNIQUES ` ............................................................................................... 79 4.3.1 Silk screening using conductive silver paint ................................................................................... 80 4.3.2 Open contacts with a spring-loaded configuration ......................................................................... 80 4.3.3 Sealed contacts with soldered terminals ......................................................................................... 82 4.3.4 Photo etching .................................................................................................................................. 83 4.3.5 Wire/braid setting with paint .......................................................................................................... 86 4.3.6 Thermal vacuum evaporation ......................................................................................................... 87 4.4 CFRP RESISTANCE MEASUREMENT TECHNIQUES AND RESULTS .......................................................... 89 4.4.1 Proposed measurement approach for carbon fibre samples .......................................................... 89 4.4.2 Test configurations and procedure for surface characterisation .................................................... 90 4.4.3 Resistance Calculation Procedure .................................................................................................. 92 4.4.4 Cross-ply sample ............................................................................................................................ 97 4.4.5 Resistance profiles on composite surfaces ...................................................................................... 97 4.5 VOLUME CHARACTERIZATION ........................................................................................................... 102 4.5.1 Test configurations for volume characterisation .......................................................................... 102 4.5.2 Impulse circuit .............................................................................................................................. 103 4.6 VOLUME CHARACTERISATION RESULTS ............................................................................................. 105 4.7 TWO-WIRE MEASUREMENT OF QUASI-ISOTROPIC PLY LAY-UPS .......................................................... 109 4.8 VARIABLE ALTERNATING CURRENT CIRCUIT ..................................................................................... 110 4.9 TWO-WIRE RESISTIVITY MEASUREMENTS FOR CROSS-PLY LAMINATES .............................................. 113 4.10 NON-DESTRUCTIVE ELECTRICAL CHARACTERISATION UNDER LIGHTNING IMPULSE CURRENTS OF LESS 100A 114 4.10.1 Concentric ring electrode tests ................................................................................................. 114 4.11 CONCLUSIONS .................................................................................................................................... 121 MECHANISMS OF DAMAGE PROPAGATION IN CARBON COMPOSITES UNDER HIGH IMPULSE CURRENT MAGNITUDES AND DC CURRENT ......................................................... 123 5.1 INTRODUCTION: DAMAGE TO CARBON FIBRE COMPOSITE PARTS/STRUCTURES DUE TO LIGHTNING STRIKES 123 5.2 PROTECTING CARBON FIBRE COMPOSITES AGAINST LIGHTNING STRIKES ........................................... 127 5.3 EXPERIMENTAL PROCEDURE FOR HIGH CURRENT TESTS..................................................................... 129 5.4 IMPULSE CURRENT TESTS (UP TO 350 A) ............................................................................................ 131 5.5 METAL RING SAMPLES SUBJECTED TO 350 A IMPULSES ..................................................................... 132 5.6 CHARACTERISATION OF CFRP PANELS UNDER WAVEFORM D LIGHTNING STRIKE ............................. 136 5.7 MECHANICAL TESTS ON WAVEFORM-D-IMPACTED SAMPLES ............................................................. 142 5.8 MICROSCOPIC DAMAGE ANALYSIS ..................................................................................................... 145 5.9 SIMULATIONS OF MULTI-LAYERED CARBON FIBRE COMPOSITES ........................................................ 149 5.9.1 Simulations without aluminium fasteners ..................................................................................... 149 x