Composite Structures

RANI ELHAJJAR is Fulbright scholar and tenured associate professor at the College of Engineering and Applied Science at the University of Wisconsin in Milwaukee. His experiences include senior positions in quality, certification, and build reliability engineering at Intel, Boeing and SpaceX. PETER GRANT has over 35 years' experience in Aerospace specializing in analysis and design of composite structures, with a background also in metallic airframe primary structure. Primary positions held have been leads at both British Aerospace and the Boeing company. CINDY ASHFORTH has over 25 years' experience in aircraft certification as both a certification manager and composite specialist. She is the FAA's Senior Technical Specialist for Advanced Composite Materials, where she manages the FAA's Composite Plan and supports composite continued operational safety, certification, and education initiatives.

• Preface xi1 Characteristics of Composites 11.1 Introduction to Behavior 11.2 Introduction to Composite Analysis 31.3 Failure and Strength Methodologies 51.3.1 Lamina Failure Modes and their Influence upon Catastrophic Failure of Multidirectional Laminates 51.3.2 Design Values and Environmental Sensitivity 81.3.3 Design Values for Unnotched Multidirectional Laminates 111.3.4 Design Values for Notched Multidirectional Laminates 121.3.5 Material Variability 141.3.6 Strain-Based Failure Methodology 151.3.7 Composite Fatigue Behavior 15References 162 DesignMethodology and Regulatory Requirements 192.1 Regulatory Requirements 192.2 Material and Process Specifications 222.3 Design Methodology 232.4 Design Values for Notched Multidirectional Laminates 242.5 Design Values for Bolted Joints 282.5.1 Determination of Fastener Loading 292.5.2 Failure Analysis at a Loaded and Unloaded Hole 292.6 Design Values for Bonded Joints and Bondlines 332.7 Design Values for Sandwich Structure 362.7.1 Facesheet Tension Design Values 362.7.2 Facesheet Compression Design Values 362.7.3 Sandwich Flexural Design Values 372.7.4 Out-of-Plane Loading 372.8 Statistical Allowables 372.9 Simulation of Temperature and Moisture Content 39References 403 Material, Manufacturing, and Service Defects 433.1 Introduction 433.1.1 Differentiating Cosmetic from Structural Defects 463.2 Defects by Stage of Occurrence 463.2.1 Material Precure Defects 463.2.1.1 Fiber Damage and Defects 503.2.2 Manufacturing Defects 513.2.3 Service Defects 543.3 Defects by Location: Matrix-Dominated Defects 603.3.1 Matrix Degradation Due to Porosity and Voids 603.3.2 Matrix Degradation Due to Aged Material 633.3.3 Matrix Degradation Due to Errors in Curing (Pressure and Temperature) 643.3.4 Matrix Damage with No Fiber Breakage from Impact 673.3.5 Matrix Cracking and Crazing 673.3.6 Matrix Degradation Due to Anomalous Moisture Absorption 683.3.7 Matrix Degradation Due to UV Radiation or Surface Contamination 693.3.8 Matrix Degradation Due to High Temperature Exposure 703.3.9 Blisters 723.3.10 Matrix Degradation Due to Resin Mixture Error 733.4 Defects by Location: Fiber-Dominated Process Defects 733.4.1 Fiber Misalignment orWrinkles 733.4.1.1 In-PlaneWaviness 733.4.1.2 Out-of-PlaneWaviness 743.4.2 Excessive Ply Drops and Gaps 763.4.3 Fiber Damage 783.5 Defects by Location: Sandwich Composite Defects 783.5.1 Core Defect: Over-Expanded or Blown Core 793.5.2 Core Defect: Core Crushing or Movement 793.5.3 Core Defect: Core-Splice Spacing Exceeding Limits 803.5.4 Core Defect: Incorrect or Variable CoreThickness 803.5.5 Core Defect: Core Degradation Due to Core Defect –Water Entrapment in Core 813.5.6 Core Defect: Incorrect Core Density 813.5.7 Core Defect: Misaligned Nodes or Unbonded Nodes in Core Cell 813.5.8 Core Defect: Mismatched Nodes or Corrugations 823.5.9 Core Defect: Corrosion 823.5.10 Facesheet Defect: Pillowing,Wrinkling, or Orange Peel 823.5.11 Facesheet Defect: Dents in Facesheet 833.5.12 Facesheet/Core Disbond 833.5.13 Defects in Adhesive Fillets 843.5.14 Edge-Closeout Defects 843.6 Defects by Location: Mixed-Mode Fiber and Matrix Defects 863.6.1 Impact Damage with Fiber Breakage 863.6.2 Bearing Damage 873.6.3 Edge Cracking and Crushing 883.6.4 Cuts, Scratches, and Gouges 893.6.5 Composite Damage from Lightning Strikes 893.6.6 Misdrilled Holes 903.6.7 Mismatched Parts 903.6.8 Incorrect Fiber Orientation or Missing Plies 913.6.9 Galvanic Corrosion 913.6.10 Resin Migration and Uneven Fiber Volume Fraction 923.6.11 Residual Stresses and Dimensional Conformance 943.6.12 Delaminations 963.6.13 Composite Degradation Due to Excessive Temperature and Chemical Exposure 983.6.13.1 Fibers 983.6.13.2 Matrix 983.7 Defects by Location: Fastened and Bonded Joint Defects 983.7.1 Fastened Joints 983.7.1.1 Bearing Damage 993.7.1.2 Hole Delamination/Fraying 993.7.1.3 Hole Elongation or Out-of-Round Holes 993.7.1.4 Fastener Seating 993.7.1.5 Fastener Over-Torque 993.7.1.6 Fastener Under-Torque 1003.7.1.7 Missing Fastener 1003.7.1.8 Porosity Near the Fastener 1003.7.1.9 Resin-Starved Bearing Surface 1003.7.1.10 Insufficient Edge Margins 1003.7.1.11 Tilted Hole 1003.7.2 Bonded Joints 1003.7.2.1 Poor Cure Due to Improper Material Chemistry: Mixing of Two-Part Resins or Material Past Shelf- or Out Life 1033.7.2.2 Incorrect Bondline Thickness, Scarf, or Overlap Length 1033.7.2.3 Zero-Volume Disbond or Long-Term Bond Failure Due to Contamination or Incorrect Surface Preparation 1033.7.2.4 Bondline Degradation Due to Moisture or Incorrect Pressure During Processing 1033.7.2.5 Bondline Degradation Due to Incorrect Heating Procedures 1043.8 Future Directions 104References 1044 InspectionMethods 1114.1 Introduction 1114.2 Mechanical Vibration NDT Methods 1174.2.1 Low Frequency Methods 1174.2.1.1 Tap Testing 1174.2.1.2 Mechanical Impedance 1174.2.1.3 Membrane Resonance 1184.2.2 High Frequency Ultrasonic Methods 1184.2.2.1 Pulse Echo andThru-Transmission 1184.2.2.2 Pulse Echo Ultrasonics 1194.2.2.3 Through-Transmission Ultrasonics 1194.2.2.4 Phased Array Ultrasonics 1204.2.2.5 Ultrasonic Spectroscopy for Zero-Volume Disbonds 1204.2.2.6 Ultrasonic Methods for Fiber Distortion 1204.2.2.7 Guided LambWaves 1224.2.2.8 Air-Coupled Ultrasonics 1224.2.3 Acoustic Emission 1234.3 Visual and Enhanced Visual Methods 1254.3.1 Visual Inspections 1254.3.2 Verification or Pressure Film 1254.3.3 Leak Testing 1264.4 Electromagnetic Radiation (X-Ray, Gamma, and Neutron) 1264.4.1 Radiography 1264.4.2 Computed Tomography 1274.4.3 Compton Scattering 1304.4.4 Neutron Tomography 1314.5 Optical Methods 1314.5.1 Shearography 1314.5.2 Digital Image Correlation (DIC) 1334.5.3 Hyperspectral Near Infrared Method for Resin Migration and Fiber Distortion 1354.5.4 Laser Profilometers and Image Processing 1364.6 Strain Measurement 1374.7 Thermography 1384.7.1 ActiveThermography 1394.7.1.1 Thermoelastic Stress Analysis 1394.7.2 Passive Thermography 1394.8 Destructive Methods 1424.8.1 Physical Tests 1424.8.2 Density and Porosity Measurements 1444.8.3 Microscopy 1454.9 NDT Standards 147References 1475 Effects of Defects – Design Values and Statistical Considerations 1595.1 Introduction 1595.2 Effects on Laminate Properties 1625.2.1 Cure Cycle Anomalies 1625.2.1.1 Porosity 1625.2.1.2 Nonuniformly Distributed Voids (Stratified Porosity) 1635.2.1.3 Stratified Porosity or Delamination in L-Shaped Details 1635.2.1.4 In-Plane Fiber Misalignment 1645.2.1.5 FiberWaviness (Out-of-Plane) 1645.2.1.6 Waviness in Curved Parts 1685.2.1.7 Ply Gaps and Overlaps 1705.2.2 Cuts, Scratches, and Gouges 1715.2.3 Edge Delaminations 1725.2.4 Foreign Object Impact 1725.3 Effects on Sandwich Composites Properties 1745.3.1 Facesheet to Core Disbonding 1745.3.2 Facesheet Pillowing 1755.3.3 Node Disbonds 1765.3.4 Core Splicing 1765.4 Effects on Bolted Joint Properties 1795.4.1 Delaminations at the Holes 1805.4.2 Oversize Holes 1825.4.3 Over-Torqued Fasteners 1825.4.4 Porosity Near Fasteners 1845.5 Effects on Bonded Joint Properties 1845.5.1 Assessment of Defects in Design of Bonded Joints 1855.6 Statistical Considerations 1855.6.1 Mean versus Design Values 1895.6.2 Simpson’s Paradox 1895.6.3 Design of Experiments 1895.7 Suggested Approach for Evaluation of Defects 1915.8 Evaluation of Scaling and Multiple Defects 192References 1946 Selected Case Studies in Effects of Defects 2016.1 Introduction 2016.2 Case Study 1: The Ohio Timber Road IIWind Turbine Failure Due to Wrinkles 2016.2.1 Event 2016.2.2 Background 2016.2.3 Investigation 2026.2.4 Lessons Learned 2026.3 Case Study 2: Faulty Repairs of Sandwich Core Structure 2036.3.1 Event 2036.3.2 Lessons Learned 2036.4 Case Study 3: Bonded Repair Failure 2056.4.1 Event 2056.4.2 Investigation 2056.4.3 Lessons Learned 2056.5 Case Study 4: Air Transat 961 Sandwich-Composite Failure 2066.5.1 Event 2066.5.2 Investigation 2066.5.3 Lessons Learned 2076.6 Case Study 5: Debonding Failure of a Sandwich-Composite Cryogenic Fuel Tank 2086.6.1 Event 2086.6.2 Investigation 2086.6.3 Lessons Learned 208References 209Glossary 211Index 215