Impact Detection Techniques Using Fibre-Optic Sensors for Aerospace & Defence

175 Pages·2015·5.81 MB·English

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.
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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.

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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.
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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
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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
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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
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8. Conclusions And Future Work .............................................................................. 161
8.1. Conclusions ................................................................................................ 161
8.2. Future Work…………………………………………………………………164

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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).
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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
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