Composite Structures: Effects of Defects

Preface xi 1 Characteristics of Composites 1 1.1 Introduction to Behavior 1 1.2 Introduction to Composite Analysis 3 1.3 Failure and Strength Methodologies 5 1.3.1 Lamina Failure Modes and their Influence upon Catastrophic Failure of Multidirectional Laminates 5 1.3.2 Design Values and Environmental Sensitivity 8 1.3.3 Design Values for Unnotched Multidirectional Laminates 11 1.3.4 Design Values for Notched Multidirectional Laminates 12 1.3.5 Material Variability 14 1.3.6 Strain-Based Failure Methodology 15 1.3.7 Composite Fatigue Behavior 15 References 16 2 DesignMethodology and Regulatory Requirements 19 2.1 Regulatory Requirements 19 2.2 Material and Process Specifications 22 2.3 Design Methodology 23 2.4 Design Values for Notched Multidirectional Laminates 24 2.5 Design Values for Bolted Joints 28 2.5.1 Determination of Fastener Loading 29 2.5.2 Failure Analysis at a Loaded and Unloaded Hole 29 2.6 Design Values for Bonded Joints and Bondlines 33 2.7 Design Values for Sandwich Structure 36 2.7.1 Facesheet Tension Design Values 36 2.7.2 Facesheet Compression Design Values 36 2.7.3 Sandwich Flexural Design Values 37 2.7.4 Out-of-Plane Loading 37 2.8 Statistical Allowables 37 2.9 Simulation of Temperature and Moisture Content 39 References 40 3 Material, Manufacturing, and Service Defects 43 3.1 Introduction 43 3.1.1 Differentiating Cosmetic from Structural Defects 46 3.2 Defects by Stage of Occurrence 46 3.2.1 Material Precure Defects 46 3.2.1.1 Fiber Damage and Defects 50 3.2.2 Manufacturing Defects 51 3.2.3 Service Defects 54 3.3 Defects by Location: Matrix-Dominated Defects 60 3.3.1 Matrix Degradation Due to Porosity and Voids 60 3.3.2 Matrix Degradation Due to Aged Material 63 3.3.3 Matrix Degradation Due to Errors in Curing (Pressure and Temperature) 64 3.3.4 Matrix Damage with No Fiber Breakage from Impact 67 3.3.5 Matrix Cracking and Crazing 67 3.3.6 Matrix Degradation Due to Anomalous Moisture Absorption 68 3.3.7 Matrix Degradation Due to UV Radiation or Surface Contamination 69 3.3.8 Matrix Degradation Due to High Temperature Exposure 70 3.3.9 Blisters 72 3.3.10 Matrix Degradation Due to Resin Mixture Error 73 3.4 Defects by Location: Fiber-Dominated Process Defects 73 3.4.1 Fiber Misalignment orWrinkles 73 3.4.1.1 In-PlaneWaviness 73 3.4.1.2 Out-of-PlaneWaviness 74 3.4.2 Excessive Ply Drops and Gaps 76 3.4.3 Fiber Damage 78 3.5 Defects by Location: Sandwich Composite Defects 78 3.5.1 Core Defect: Over-Expanded or Blown Core 79 3.5.2 Core Defect: Core Crushing or Movement 79 3.5.3 Core Defect: Core-Splice Spacing Exceeding Limits 80 3.5.4 Core Defect: Incorrect or Variable CoreThickness 80 3.5.5 Core Defect: Core Degradation Due to Core Defect -Water Entrapment in Core 81 3.5.6 Core Defect: Incorrect Core Density 81 3.5.7 Core Defect: Misaligned Nodes or Unbonded Nodes in Core Cell 81 3.5.8 Core Defect: Mismatched Nodes or Corrugations 82 3.5.9 Core Defect: Corrosion 82 3.5.10 Facesheet Defect: Pillowing,Wrinkling, or Orange Peel 82 3.5.11 Facesheet Defect: Dents in Facesheet 83 3.5.12 Facesheet/Core Disbond 83 3.5.13 Defects in Adhesive Fillets 84 3.5.14 Edge-Closeout Defects 84 3.6 Defects by Location: Mixed-Mode Fiber and Matrix Defects 86 3.6.1 Impact Damage with Fiber Breakage 86 3.6.2 Bearing Damage 87 3.6.3 Edge Cracking and Crushing 88 3.6.4 Cuts, Scratches, and Gouges 89 3.6.5 Composite Damage from Lightning Strikes 89 3.6.6 Misdrilled Holes 90 3.6.7 Mismatched Parts 90 3.6.8 Incorrect Fiber Orientation or Missing Plies 91 3.6.9 Galvanic Corrosion 91 3.6.10 Resin Migration and Uneven Fiber Volume Fraction 92 3.6.11 Residual Stresses and Dimensional Conformance 94 3.6.12 Delaminations 96 3.6.13 Composite Degradation Due to Excessive Temperature and Chemical Exposure 98 3.6.13.1 Fibers 98 3.6.13.2 Matrix 98 3.7 Defects by Location: Fastened and Bonded Joint Defects 98 3.7.1 Fastened Joints 98 3.7.1.1 Bearing Damage 99 3.7.1.2 Hole Delamination/Fraying 99 3.7.1.3 Hole Elongation or Out-of-Round Holes 99 3.7.1.4 Fastener Seating 99 3.7.1.5 Fastener Over-Torque 99 3.7.1.6 Fastener Under-Torque 100 3.7.1.7 Missing Fastener 100 3.7.1.8 Porosity Near the Fastener 100 3.7.1.9 Resin-Starved Bearing Surface 100 3.7.1.10 Insufficient Edge Margins 100 3.7.1.11 Tilted Hole 100 3.7.2 Bonded Joints 100 3.7.2.1 Poor Cure Due to Improper Material Chemistry: Mixing of Two-Part Resins or Material Past Shelf- or Out Life 103 3.7.2.2 Incorrect Bondline Thickness, Scarf, or Overlap Length 103 3.7.2.3 Zero-Volume Disbond or Long-Term Bond Failure Due to Contamination or Incorrect Surface Preparation 103 3.7.2.4 Bondline Degradation Due to Moisture or Incorrect Pressure During Processing 103 3.7.2.5 Bondline Degradation Due to Incorrect Heating Procedures 104 3.8 Future Directions 104 References 104 4 InspectionMethods 111 4.1 Introduction 111 4.2 Mechanical Vibration NDT Methods 117 4.2.1 Low Frequency Methods 117 4.2.1.1 Tap Testing 117 4.2.1.2 Mechanical Impedance 117 4.2.1.3 Membrane Resonance 118 4.2.2 High Frequency Ultrasonic Methods 118 4.2.2.1 Pulse Echo andThru-Transmission 118 4.2.2.2 Pulse Echo Ultrasonics 119 4.2.2.3 Through-Transmission Ultrasonics 119 4.2.2.4 Phased Array Ultrasonics 120 4.2.2.5 Ultrasonic Spectroscopy for Zero-Volume Disbonds 120 4.2.2.6 Ultrasonic Methods for Fiber Distortion 120 4.2.2.7 Guided LambWaves 122 4.2.2.8 Air-Coupled Ultrasonics 122 4.2.3 Acoustic Emission 123 4.3 Visual and Enhanced Visual Methods 125 4.3.1 Visual Inspections 125 4.3.2 Verification or Pressure Film 125 4.3.3 Leak Testing 126 4.4 Electromagnetic Radiation (X-Ray, Gamma, and Neutron) 126 4.4.1 Radiography 126 4.4.2 Computed Tomography 127 4.4.3 Compton Scattering 130 4.4.4 Neutron Tomography 131 4.5 Optical Methods 131 4.5.1 Shearography 131 4.5.2 Digital Image Correlation (DIC) 133 4.5.3 Hyperspectral Near Infrared Method for Resin Migration and Fiber Distortion 135 4.5.4 Laser Profilometers and Image Processing 136 4.6 Strain Measurement 137 4.7 Thermography 138 4.7.1 ActiveThermography 139 4.7.1.1 Thermoelastic Stress Analysis 139 4.7.2 Passive Thermography 139 4.8 Destructive Methods 142 4.8.1 Physical Tests 142 4.8.2 Density and Porosity Measurements 144 4.8.3 Microscopy 145 4.9 NDT Standards 147 References 147 5 Effects of Defects - Design Values and Statistical Considerations 159 5.1 Introduction 159 5.2 Effects on Laminate Properties 162 5.2.1 Cure Cycle Anomalies 162 5.2.1.1 Porosity 162 5.2.1.2 Nonuniformly Distributed Voids (Stratified Porosity) 163 5.2.1.3 Stratified Porosity or Delamination in L-Shaped Details 163 5.2.1.4 In-Plane Fiber Misalignment 164 5.2.1.5 FiberWaviness (Out-of-Plane) 164 5.2.1.6 Waviness in Curved Parts 168 5.2.1.7 Ply Gaps and Overlaps 170 5.2.2 Cuts, Scratches, and Gouges 171 5.2.3 Edge Delaminations 172 5.2.4 Foreign Object Impact 172 5.3 Effects on Sandwich Composites Properties 174 5.3.1 Facesheet to Core Disbonding 174 5.3.2 Facesheet Pillowing 175 5.3.3 Node Disbonds 176 5.3.4 Core Splicing 176 5.4 Effects on Bolted Joint Properties 179 5.4.1 Delaminations at the Holes 180 5.4.2 Oversize Holes 182 5.4.3 Over-Torqued Fasteners 182 5.4.4 Porosity Near Fasteners 184 5.5 Effects on Bonded Joint Properties 184 5.5.1 Assessment of Defects in Design of Bonded Joints 185 5.6 Statistical Considerations 185 5.6.1 Mean versus Design Values 189 5.6.2 Simpson's Paradox 189 5.6.3 Design of Experiments 189 5.7 Suggested Approach for Evaluation of Defects 191 5.8 Evaluation of Scaling and Multiple Defects 192 References 194 6 Selected Case Studies in Effects of Defects 201 6.1 Introduction 201 6.2 Case Study 1: The Ohio Timber Road IIWind Turbine Failure Due to Wrinkles 201 6.2.1 Event 201 6.2.2 Background 201 6.2.3 Investigation 202 6.2.4 Lessons Learned 202 6.3 Case Study 2: Faulty Repairs of Sandwich Core Structure 203 6.3.1 Event 203 6.3.2 Lessons Learned 203 6.4 Case Study 3: Bonded Repair Failure 205 6.4.1 Event 205 6.4.2 Investigation 205 6.4.3 Lessons Learned 205 6.5 Case Study 4: Air Transat 961 Sandwich-Composite Failure 206 6.5.1 Event 206 6.5.2 Investigation 206 6.5.3 Lessons Learned 207 6.6 Case Study 5: Debonding Failure of a Sandwich-Composite Cryogenic Fuel Tank 208 6.6.1 Event 208 6.6.2 Investigation 208 6.6.3 Lessons Learned 208 References 209 Glossary 211 Index 215

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