Impact Detection Techniques Using Fibre-Optic Sensors for Aerospace & Defence Ryan Nichols John Submitted for the degree of Engineering Doctorate Heriot-Watt University School of Engineering and Physical Sciences March 2015 The copyright in this thesis is owned by the author. Any quotation from the thesis or use of any of the information contained in it must acknowledge this thesis as the source of the quotation or information. i ABSTRACT Impact detection techniques are developed for application in the aerospace and defence industries. Optical fibre sensors hold great promise for structural health monitoring systems and methods of interrogating fibre Bragg gratings (FBG) are investigated given the need for dynamic strain capture and multiplexed sensors. An arrayed waveguide grating based interrogator is developed. The relationships between key performance indicators, such as strain range and linearity of response, and parameters such as the FBG length and spectral width are determined. It was found that the inclusion of a semiconductor optical amplifier could increase the signal-to-noise ratio by ~300% as the system moves to its least sensitive. An alternative interrogator is investigated utilising two wave mixing in erbium-doped fibre in order to create an adaptive system insensitive to quasistatic strain and temperature drifts. Dynamic strain sensing was demonstrated at 200 Hz which remained functional while undergoing a temperature shift of 8.5 °C. In addition, software techniques are investigated for locating impact events on a curved composite structure using both time-of-flight triangulation and neural networks. A feature characteristic of composite damage creation is identified in dynamic signals captured during impact. An algorithm is developed which successfully distinguishes between signals characteristic of a non-damaging impact with those from a damaging impact with a classification accuracy of 93 – 96%. Finally, a demonstrator system is produced to exhibit some of the techniques developed in this thesis. ii DEDICATION This work could not have been carried out without the help and support of my supervisors, Ian Read at BAE Systems Advanced Technology Centre and William MacPherson at Heriot-Watt University. Thank you for the many ideas, the helpful feedback and tireless reading of my work. I’d also like to thank Eoin Murphy and Peter Foote for their advice and help at different points over the last few years. I’d also like to thank my parents for their love and support during my life which has certainly helped me get to where I am today. Finally, I’d like to thank Lynsey for being there for me all the time and for the constant encouragement which kept me going until the end. iii iv TABLE OF CONTENTS 1. Introduction .............................................................................................................. 1 1.1. Structural Health Monitoring .......................................................................... 1 1.1.1. Commercial justification .......................................................................... 1 1.1.2. Composite materials and damage ............................................................. 2 1.1.3. Smart structures ....................................................................................... 3 1.1.4. Prognosis and diagnosis ........................................................................... 3 1.2. Optical Fibre Technology ............................................................................... 4 1.3. Aim of Thesis ................................................................................................. 5 1.4. Aerospace Operating Requirements ................................................................ 5 1.5. Outline of Thesis ............................................................................................ 7 1.6. References ...................................................................................................... 9 2. Background Theory and Literature Review ............................................................ 11 2.1. Composite Materials ..................................................................................... 11 2.1.1. Introduction ........................................................................................... 11 2.1.2. Comparison to other materials ............................................................... 13 2.1.3. Composite damage ................................................................................ 14 2.2. Acoustic Waves ............................................................................................ 15 2.3. Electrical Sensors ......................................................................................... 16 2.3.1. Strain gauges ......................................................................................... 16 2.3.2. Piezoelectric transducers ........................................................................ 17 2.4. Optical Fibres ............................................................................................... 18 2.5. Optical Fibre Sensors .................................................................................... 20 2.5.1. Intensity based sensors ........................................................................... 20 2.5.2. Fibre Bragg gratings .............................................................................. 22 2.5.3. Chirped grating sensors.......................................................................... 24 2.5.4. Long period gratings .............................................................................. 25 2.5.5. Fabry-Perot interferometric sensors ....................................................... 27 2.5.6. Distributed sensing ................................................................................ 29 2.5.7. Sensor choice ......................................................................................... 30 2.6. Comparison of Electrical and Optical Fibre Sensors ...................................... 32 2.7. Interrogation Systems ................................................................................... 33 2.7.1. Comparison of techniques ...................................................................... 33 2.7.2. Light sources ......................................................................................... 37 2.7.3. Light amplification ................................................................................ 38 2.8. Multiplexing ................................................................................................. 39 2.8.1. Wavelength division multiplexing.......................................................... 39 2.8.2. Time-division multiplexing .................................................................... 40 v 2.8.3. Spectrally coded multiplexing ................................................................ 41 2.8.4. Further multiplexing techniques ............................................................. 42 2.9. Measurements ............................................................................................... 42 2.9.1. Operational load monitoring .................................................................. 43 2.9.2. Temperature .......................................................................................... 45 2.9.3. Acoustic Emission ................................................................................. 46 2.9.4. Impact detection .................................................................................... 47 2.10. Conclusion .................................................................................................... 48 2.11. References................................................................................................. 50 3. Impact Location ...................................................................................................... 59 3.1. Introduction .................................................................................................. 59 3.1.1. Aim ....................................................................................................... 59 3.1.2. Impact detection .................................................................................... 59 3.2. System Development .................................................................................... 60 3.2.1. Test structure ......................................................................................... 60 3.2.2. Data acquisition ..................................................................................... 61 3.2.3. Impact energy determination .................................................................. 64 3.2.4. Integrated demonstration system ............................................................ 66 3.3. Time-Of-Flight Triangulation ....................................................................... 67 3.3.1. Location algorithm ................................................................................ 67 3.3.2. Speed of acoustic waves ........................................................................ 69 3.3.3. Results ................................................................................................... 69 3.4. Neural Network ............................................................................................ 73 3.4.1. Theory ................................................................................................... 73 3.4.2. Implementation ...................................................................................... 74 3.5. Comparison Of Location Techniques ............................................................ 75 3.6. FBG System ................................................................................................. 78 3.6.1. System design ........................................................................................ 78 3.6.2. Results ................................................................................................... 80 3.7. Conclusions .................................................................................................. 81 3.8. References……………………………………………………………………............82 4. Damage Classification ............................................................................................ 83 4.1. Introduction .................................................................................................. 83 4.2. Impact Tests ................................................................................................. 84 4.3. Experimental Results .................................................................................... 88 4.3.1. Impact signals ........................................................................................ 88 4.3.2. Frequency analysis of strain data ........................................................... 94 4.4. Impact Damage Classification ....................................................................... 96 vi 4.5. Conclusions .................................................................................................. 98 4.6. References .................................................................................................... 99 5. Arrayed Waveguide Grating Interrogator .............................................................. 101 5.1. Introduction ................................................................................................ 101 5.1.1. Arrayed Waveguide Grating ................................................................ 104 5.2. Principle of Operation ................................................................................. 105 5.2.1. Calibration of interrogation function .................................................... 107 5.2.2. Bonded Calibration Comparison .......................................................... 109 5.3. System Design Parameters .......................................................................... 112 5.3.1. Effect of FBG spectral width ............................................................... 112 5.3.2. Comparison of AWG Bandwidth ......................................................... 117 5.3.3. FBG Length Considerations ................................................................. 118 5.3.4. Effect of Grating Length on Grating FWHM........................................ 123 5.3.5. Design summary based upon model with experimental evidence.......... 124 5.4. System Performance and Stability ............................................................... 124 5.5. Conclusion .................................................................................................. 128 5.6. Discussion .................................................................................................. 129 5.7. References…………...…………………………………………………….…130 6. Two Wave Mixing Interrogator ............................................................................. 133 6.1. Introduction ................................................................................................ 133 6.2. Theory ........................................................................................................ 134 6.2.1. Two-wave mixing ................................................................................ 134 6.2.2. Erbium-doped fibre .............................................................................. 135 6.3. Experiment ................................................................................................. 135 6.3.1. Static testing ........................................................................................ 135 6.3.2. Dynamic testing ................................................................................... 139 6.3.3. Multiplexing ........................................................................................ 137 6.4. Conclusions ................................................................................................ 143 6.5. Discussion .................................................................................................. 143 6.6. Future Work ............................................................................................... 143 6.7. References .................................................................................................. 145 7. Demonstrator System ............................................................................................ 148 7.1. Introduction ................................................................................................ 148 7.2. System Implementation............................................................................... 148 7.3. System Results............................................................................................ 153 7.3.1. Signal Capture ..................................................................................... 153 7.3.2. Initial Neural Network Results ............................................................. 155 7.3.3. Neural Network Testing ....................................................................... 156 7.4 Conclusions and Discussion...............................................................................160 vii 8. Conclusions And Future Work .............................................................................. 161 8.1. Conclusions ................................................................................................ 161 8.2. Future Work…………………………………………………………………164 viii LIST OF PUBLICATIONS R. N. John, I. Read, and W. N. MacPherson, “Design considerations for a fibre Bragg grating interrogation system utilizing an arrayed waveguide grating for dynamic strain measurement”, Meas. Sci. Technol. 24 075203 (9pp) (2013). R. N. John, I. Read, and W. N. MacPherson, "Adaptive dynamic FBG interrogation utilising erbium-doped fibre" Proc. SPIE 8692, Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2013, 869220 (April 19, 2013). R. John, I. Read and W. N. MacPherson, “Discrimination Between Damaging and Non- damaging Impact Events on Composite Structure using SHM Sensor Signal Analysis,” SAE Aerotech Congress & Exhibition, Toulouse, France, (Oct. 18-21, 2011). R. John , I. Read and W. N. MacPherson, “Impact Damage Assessment by Sensor Signal Analysis” Proceedings of IWSHM Conf., Stanford University, USA, (Sept. 2011). Patent Application “Data Processing”, Application No GB1114418.5, Date Lodged, 22.08.2011, Applicant(s) BAE SYSTEMS plc. OTHER DISSEMINATION ACTIVITIES R. John, I. Read, and W. N. MacPherson, “Impact Damage Assessment by Sensor Signal Analysis”, IOP Instrument Science and Technology Group, Half-day meeting on Structural Health Monitoring , Institute of Physics, London (10th May 2012). ix LIST OF ABBREVIATIONS A/D Analogue to digital AE Acoustic emission ANN Artificial neural network ASE Amplified spontaneous emission AWG Arrayed waveguide grating BVID Barely visible impact damage CBM Condition based maintenance CCD Charge coupled device CFBG Chirped fibre Bragg grating DAQ Data acquisition device ECM Electronic counter measures EDF Erbium doped fibre EDFA Erbium doped fibre amplifier EFPI Extrinsic Fabry-Perot interferometric ELED Edge emitting light emitting diode FBG Fibre Bragg grating FFT Fast Fourier transform FP Fabry-Perot FSR Free spectral range FTS Fourier transform spectroscopy FWHM Full width at half maximum ICGI Identical chirped grating interrogation IFPI Intrinsic Fabry-Perot interferometric InGaAs Indium gallium arsenide InP:Fe Iron doped indium phosphide LED Light emitting diode LPG Long period grating NDE Non-destructive evaluation NDI Non-destructive inspection NDT Non-destructive testing OBR Optical backscatter reflectometry OC Optical coupler OLM Operational load monitoring OPD Optical path difference OSA Optical spectrum analyser PD Photodiode PRC Photorefractive crystal PZT Lead zirconate titanate SCM Spectrally coded multiplexing SFS Superfluorescent source SHM Structural health monitoring SLD Superluminescent diode SMF-28 Corning proprietary single mode fibre. SNR Signal to noise ratio SOA Semiconductor optical amplifier TDM Time division multiplexing x