ADVANCED INDUSTRIAL X-RAY COMPUTED TOMOGRAPHY FOR DEFECT DETECTION AND CHARACTERISATION OF COMPOSITE STRUCTURES A thesis submitted to The University of Manchester for the degree of Doctor of Engineering in the Faculty of Engineering and Physical Sciences. 2010 MATHEW AMOS SCHOOL OF MATERIALS Contents Abbreviations 7 Abstract 8 Declaration and Copyright 9 Acknowledgements 10 Chapter 1 1 Introduction 11 1.1 Motivation of Research ……………………………………………….............. 11 1.2 Aims and Objectives ……………………………………………….................. 14 1.3 X-ray Computer Tomography……………………………………................... 14 1.3.1 Commercial Application of X-ray Computed Tomography………. 14 1.3.2 Current Limitations of Industrial X-ray Computed Tomography…………………………………………………………... 15 1.4 Organisation of Thesis…………………………………………………………. 17 Chapter 2 2 Literature Review 18 2.1 Non-Destructive-Testing………………………………………………............. 18 2.1.1 Radiography…………………………………………………………... 19 2.1.1.1 X-ray Physics……………………………………….......... 19 2.1.1.2 X-ray Detectors…………………………………………… 20 2.1.1.3 Application to Fibre-Reinforced-Plastic Composites……………………………………………….. 23 2.1.2 Ultrasonic Testing…………………………………………………….. 24 2.1.1.1 Ultrasonic Inspection Methods………………………….. 26 2.1.1.1 Application to Fibre-Reinforced-Plastic Composites……………………………………………….. 29 2.1.3 Infrared Thermography………………………………………............ 30 2.1.3.1 Application to Fibre-Reinforced-Plastic Composites……………………………………………….. 31 2.1.4 X-ray Computer Tomography……………………………………….. 32 2.1.4.1 Application to Fibre-Reinforced-Plastic Composites……………………………………………….. 34 2 Contents 2.1.5 Summary of NDT Capabilities with respect to FRP Composites……………………………………………………........... 34 2.2 X-ray Computer Tomography…………………………………………………. 36 2.2.1 A Brief History of X-ray CT………………………………………….. 36 2.2.2 X-ray CT Scanning Geometries…………………………………….. 37 2.2.3 CT Reconstruction……………………………………………........... 42 2.2.3.1 General Principles……………………………………….. 42 2.2.3.2 Fourier Slice Theorem…………………………………… 45 2.2.3.3 Filtered Back Projection…………………………………. 47 2.2.3.4 2D Fan Beam Reconstruction ………………………….. 50 2.2.3.5 3D Cone Beam Reconstruction………………………… 54 2.2.3.6 Iterative Reconstruction Techniques…………………… 58 2.2.4 Image Quality…………………………………………………………. 59 2.2.4.1 Contrast…………………………………………………… 60 2.2.4.2 Noise………………………………………………………. 61 2.2.4.3 Spatial Resolution………………………………….......... 64 2.2.4.4 Detection Sensitivity……...……………………………… 66 2.2.4.5 Artefacts…………………………………………………… 68 2.2.5 Truncated Data Problem…………………………………………….. 72 2.2.5.1 Region of Interest CT……………………………………. 73 2.2.5.2 Extending the CT Scan Field of View………………….. 77 2.2.5.3 Current Industrial Techniques for ROI CT and Extending the CT Field-of-View………………………… 80 2.2.6 Setup of the CT System……………………………………………... 81 2.2.6.1 CT instrumentation………………………………………. 81 2.2.6.2 Setup and Calibration……………….…………………… 83 2.3 Summary………………………………………………………………………… 84 Chapter 3 3 Development of Region-of-Interest CT techniques for Defect Characterisation in CFRP Laminates 86 3.1 Introduction……………………………………………………………………… 86 3.2 Objectives……………………………………………………………………….. 88 3.3 Fabrication of CFRP Laminated Samples……………………………………. 88 3.3.1 Sample Lay-up and Curing………………………………………….. 89 3.4 Region-of-Interest CT…………………………………………………………... 91 3.4.1 Errors from Missing Data…………………………………………….. 92 3 Contents 3.4.2 Truncation Artefacts………………………………………………….. 94 3.5 Experimental Approach………………………………………………………… 98 3.5.1 Processing in Matlab…………………………………………............ 99 3.5.2 Projection Datasets…………………………………………………... 100 3.5.3 Error Analysis…………………………………………………………. 101 3.6 Simple Data Completion By Edge Extension………………………………... 104 3.6.1 Cosine Tail Extensions………………………………………………. 105 3.6.1.1 Cosine Extension Length………………………….......... 106 3.6.2 Level Extensions……………………………………………………… 108 3.6.3 Results and Discussion………………………………………........... 109 3.7 ‘Estimation from Model’ Data Completion……………………………………. 112 3.7.1 Method…………………………………………………………........... 112 3.7.2 Results and Discussion………………………………………........... 116 3.7.3 Revised ‘Estimation from Model’ Method………………………….. 120 3.7.3.1 Individual Projection Magnitude Matching…………….. 121 3.7.3.2 Individual Projection Cosine Blending…………………. 122 3.7.3.3 Results and Discussion………………………………….. 123 3.8 Application of the Developed ROI CT Techniques to Cone Beam 126 Geometry………………………………………………………………………… 3.9 Conclusions……………………………………………………………………… 133 Chapter 4 4 Extending the CT Scan Field-of-View for Inspection of Glass-Fibre- Reinforced-Plastic Wind Turbine Blades 136 4.1 Introduction……………………………………………………………………… 136 4.2 Objectives……………………………………………………………………….. 138 4.3 Feasibility Study: NDT of Wind Turbine Blades using Radiographic and Ultrasonic Techniques…………………………………………….................... 138 4.3.1 Blade Sample…………………………………………………………. 139 4.3.2 Ultrasonic Testing Setup…………………………………………….. 140 4.3.3 Radiographic Setup………………………………………………….. 141 4.3.4 Experimental Results………………………………………………… 142 4.3.4.1 Main Spar – 81mm Defect………………………………. 142 4.3.4.2 Main Spar – Scanning Cross Section AA……………… 145 4.3.4.3 Trailing Edge – Scanning Cross Section BB………….. 147 4.3.5 Conclusions……………………………………………………........... 149 4.4 Offset CT – Acquisition of the Projection Data………………………………. 150 4 Contents 4.4.1 X-ray System Calibrations…………………………………………… 151 4.4.1.1 Manipulator……………………………………………….. 151 4.4.1.2 Source…………………………………………………….. 152 4.4.1.3 Detector…………………………………………………… 153 4.4.2 Acquisition Method……………………………………………........... 154 4.4.3 Results and Discussion………………………………………........... 157 4.5 Offset CT Reconstruction……………………………………………………… 159 4.5.1 Completion of the Projection Data………………………………….. 160 4.5.1.1 Offset Data Completion Method 1 – Tailing…………… 161 4.5.1.2 Offset Data Completion Method 2 – Estimation………. 165 4.5.2 Alteration of the Reconstruction Algorithm – Blanking……........... 174 4.5.3 Reconstruction of the New Projection Data……………………….. 174 4.5.4 Results and Discussion………………………………………........... 175 4.5.4.1 Offset CT Cylinder……………………………………….. 175 4.5.4.2 Offset CT Wind Turbine Blade………………………….. 177 4.5.5 Summary………………………………………………………………. 181 4.6 Simulation of Offset CT………………………………………………………… 181 4.6.1 Method…………………………………………………………............ 182 4.6.2 Results and Discussion………………………………………........... 183 4.6.2.1 Projection Completion Results………………………….. 183 4.6.2.2 Reconstruction Results………………………………….. 189 4.7 Conclusions……………………………………………………………………… 197 Chapter 5 5 Computer Tomography using a Dual Energy Approach for the Inspection of Highly Contrasting Materials 201 5.1 Introduction……………………………………………………………………… 201 5.2 Objectives……………………………………………………………………….. 205 5.3 ComeldTM Joints………………………………………………………………… 205 5.3.1 Joint Fabrication………………………………………………........... 206 5.4 Experimental Approach………………………………………………………… 209 5.4.1 CT Data Acquisition…………………………………………………... 210 5.4.1.1 Low Energy Scan………………………………………… 210 5.4.1.2 High Energy Scan………………………………….......... 210 5.4.2 Dual Energy Image Processing Technique………………………... 210 5.4.2.1 Image Segmentation by Histogram Thresholding……. 211 5.4.3 Grey Level Mapping (Scaling)………………………………………. 217 5 Contents 5.4.4 Merging of the Resultant High Energy and Low Energy Projection Stacks……………………………………………………... 221 5.5 Dual Energy Technique Validation……………………………………………. 223 5.5.1 CT Performance Measurement……………………………………... 223 5.5.2 Reference Phantom Fabrication…………………………………….. 224 5.5.3 Method…………………………………………………………............ 225 5.5.4 Results and Discussion………………………………………........... 228 5.5.4.1 Detail Signal-to-Noise-Ratio…………………………….. 228 5.5.4.2 Modulation Transfer Function…………………….......... 232 5.6 Dual Energy Technique Application…………………………………………... 233 5.6.1 Copper - Aluminium Magnetic Pulse Welded HVAC Component……………………………………………………………. 233 5.6.1.1 Method…………………………………………………….. 233 5.6.1.2 Results and Discussion………………………………….. 235 5.6.2 Carbon-Fibre-Reinforced-Plastic – Titanium ComeldTM Joint…………………………………………………………………….. 239 5.6.2.1 Method…………………………………………………….. 239 5.6.2.2 Results and Discussion………………………………….. 241 5.7 Conclusions………………………………………………………..................... 250 Chapter 6 6 Conclusions and Further Work 252 6.1 Conclusions……………………………………………………………………… 252 6.2 Further Work……………………………………………………………............. 254 Publications 256 References 257 Appendix A 269 Final word count 67,696 6 Abbreviations ART Algebraic Reconstruction Technique BPF Back Projection Filtration CCD Charge Coupled Device CFRP Carbon Fibre Reinforced Plastic CR Computed Radiography CT Computer Tomography DDA Digital Detector Array ESF Edge Spread Function FBP Filtered Back Projection FDK Feldkamp Davis Kress FOV Field of View FRP Fibre Reinforced Plastic FST Fourier Slice Theorem FWHM Full Width Half Maximum HE High Energy HVAC Heating Ventilation Air-Conditioning IQI Image Quality Indicator GFRP Glass Fibre Reinforced Plastic LE Low Energy MART Multiplicative Algebraic Reconstruction Technique MTF Modulation Transfer Function NDT Non Destructive Testing PSF Point Spread Function ROI Region of Interest SART Simultaneous Algebraic Reconstruction Technique SIRT Simultaneous Iterative Reconstruction Technique SNR Signal to Noise Ratio TFT Thin Film Transistor TOFD Time of Flight Diffraction UT Ultrasonic Testing WTB Wind Turbine Blade 7 Abstract X-ray Computer Tomography (CT) is well suited to the inspection of Fibre-Reinforced- Plastic (FRP) composite materials. However, a range of limitations currently restrict its uptake. The aim of the present research was to develop advanced inspection procedures