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FATIGUE AND DAMAGE TOLERANCE ASSESSMENT OF AIRCRAFT STRUCTURE UNDER UNCERTAINTY Lorens S. Goksel 5/1/2013 Committee Members: Dr. Seung-Kyum Choi, Chair Dr. Roger Jiao Dr. David Scott

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Outline Introduction Research Questions Probability of Failure (PF) Predictable Range of Risk Risk Mitigation Damage Tolerance Risk Assessment (DTRA) Comparison Proposed Framework Validation Example Conclusion 2

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Create a tool for an engineer that assesses the life of a component using cradle-to-grave approach which includes Manufacturing defects Design loading conditions Component failure mitigation approaches Methodology needs to provide economic solutions Introduction Research Question DTRAValidationConclusion Purpose 3 Determine how cracks in a component grow with variation of parameters Obtain an optimal range for inspection and refurbishment

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What is Fatigue? Fatigue is the degradation of materials due to repeated loads Degradation occurs due to Mechanically induced loads Load rate Caustic environmental effects Introduction Research Question DTRAValidationConclusion 4

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Caustic Environment Introduction Research Question DTRAValidationConclusion 3X Less LifeSump Tank Lab Air Examples of Fatigue Degradation 5

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Load Rate Introduction Research Question DTRAValidationConclusion Wind only conditions provide more than twice the life compared to Ground-Air-Ground Examples of Fatigue Degradation 6

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Sources of Fatigue? Material in-homogeneity (voids, inclusions, etc.) Damage (scratches, stress concentration) Introduction Research Question DTRAValidationConclusion Stress Risers Manufacturing quality is essential for good fatigue life! 7

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Damage Tolerance/Crack Growth Crack Growth assumes the material has some initial defect Cracks are two dimensional Failure occurs at critical crack length (fracture) Introduction Research Question DTRAValidationConclusion 8

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How is Risk Related to Fatigue? Each time there is an accumulation of damage, the chance of failure becomes a little higher. Failure is considered the last stage of crack growth i.e. Fracture Usually occurs when critical crack length is reached (potentially catastrophic to system) Introduction Research Question DTRAValidationConclusion 9

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Research Question #1 How does one determine the Probability of Failure for aerospace structures? Hypothesis: When the Fatigue loads exceeds the material strength, failure occurs. Probability of this occurrence depends on the occurrence and size of both the load and material strength. Introduction Research Question DTRAValidationConclusion 10

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Research Question #1 Introduction Research Question DTRAValidationConclusion Probability Strength Residual Strength Interference = Probability of Failure Extreme Rare Occurrence Flight Design Case Will see this load level every flight Probability Strength Environmental Input Residual Strength Distributions Interference = Probability of Failure 11

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Research Question #2 How can one predict risk failure based on a crack growing for aerospace systems? Hypothesis: By knowing the material properties, geometry of crack, and all load conditions, and started from the smallest computational crack size. Introduction Research Question DTRAValidationConclusion 12

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Research Question #2 Introduction Research Question DTRAValidationConclusion Slow Crack Growth Predictable (Paris) Area Fast Crack Growth 13

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Research Question #3 How can one mitigate crack growth risk, economically? Hypothesis: By detecting a crack before it reaches a critical length, but during its predictable growth period. Introduction Research Question DTRAValidationConclusion 14

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Summary Need to understand how crack grows in a part with certain parameters Need for a method that can provide an optimal range for inspections All need to account for: Probability of failure Predict risk associated with failure Minimize Failures Damage Tolerance Risk Assessment incorporates all Introduction Research Question DTRAValidationConclusion 15

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Comparison to Other Methods White [64] proposed risk analysis Includes loading history, material properties and flaw size Indicates gradual increase is fast crack growth area Wang [65] performed risk analysis at bolted connection. Approach: at what crack length can one start inspections based on an acceptable risk level Neglects to provide a range of inspection periods Introduction Research Question DTRAValidationConclusion 16

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Comparison to Other Methods Grooteman [64] Equivalent initial flaw size to and probability of detection curves to determine optimum inspection intervals Computationally arduous Cavallini and Lazzeri [65] Probabilistic Investigation for Safe Aircraft (PISA) Accounts for Initial Flaw Size Material Variability Probability of Detection Computation limitation cannot provide risk associated with small cracks Introduction Research Question DTRAValidationConclusion 17

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Proposed Framework Step 1: Specify Geometry, Loading Conditions and Material Statistic Properties (Crack Growth) Obtain Residual Strength based on distribution Step 2: Discontinuity Check Obtain more clear data Step 3: Obtain Probability of Failures, Probability of Detection Setup the DTRA Introduction Research Question DTRAValidationConclusion 18

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Proposed Framework – Step 1 Introduction Research Question DTRAValidationConclusion Grow Flaw Until Critical Crack Length 19 Initial Flaw Final Crack Assume Initial Flaw Size Obtain residual strength PDF based on variability of fracture toughness Lays foundation for residual strength distribution needed for PF

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Proposed Framework – Step 2 Introduction Research Question DTRAValidationConclusion Obtain residual strength PDF based on variability of fracture toughness 20 Phantom Distribution Discontinuities? Yes Create Phantom Distribution No Self-check to increase resolution Residual strength due only to local loading

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Proposed Framework – Step 3 Introduction Research Question DTRAValidationConclusion 21 Discontinuities? No Intersect Flight Design with Residual Strength Case Flight Design Plot Probability of Failures for each crack interval RQ #1 Answer Insert Probability of Detection for crack size RQ #3 Answer

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Proposed Framework – Setup DTRA Introduction Research Question DTRAValidationConclusion Probability of Failure Time Conservative High Risk 99% Probability of Detection Optimal Probability of Failure at Each Crack Length 22 RQ #2 & #3 Answer

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Summary Introduction Research Question DTRAValidationConclusion Discontinuities? Yes Create Phantom Distribution No Assume Initial Flaw Size Grow Flaw Until Critical Crack Length Obtain residual strength PDF based on variability of fracture toughness Insert Probability of Detection for crack size in plot Intersect Flight Design with Residual Strength Case 23 Plot Probability of Failures for each crack interval RQ #1 Answer Optimal Area determined based on DTRA, PD and FAA minimum allowable RQ #2 & #3 Answer

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Validation: Engine Nacelle Inlet Introduction Research Question DTRAValidationConclusion Internal Loading (Engine Noise) External Loads (Aerodynamic Loads) 24 When is the optimum time to inspect the nacelle inlet for fatigue cracks? Step 1

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Internal Loads Introduction Research Question DTRAValidationConclusion 25 Loading is assumed only to act in hoop direction, thus circumferential natural frequency examined Need to determine most pertinent loading mode: longitudinal vs. circumferential Step 1

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Internal Loads Introduction Research Question DTRAValidationConclusion FEM & Hand MethodEngine SpecificationInternal Pressure Internal stresses derived using standard static techniques for hoop load conditions 26 Step 1

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Crack Growth Assume manufacturing flaw Flaw is two dimensional Use previous internal loading Determine Residual Strength at some crack length Assume normal material distribution Introduction Research Question DTRAValidationConclusion 27 Step 1

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Introduction Research Question DTRAValidationConclusion Critical crack Length Initial crack Length This progression only accounts for internal loads Crack Growth 28 Step 1

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Introduction Research Question DTRAValidationConclusion Failure accounts for internal and external loads Each failure accounts for crack growth iteration Determine POF 29 Steps 2 & 3 Failure Region Flight Design Case Critical Crack Phantom Distribution

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Introduction Research Question DTRAValidationConclusion Risk Mitigation …this crack length can be found There is a 90% chance… Each crack length is associated with a flight time 30 Step 3

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Introduction Research Question DTRAValidationConclusion Damage Tolerance Risk Assessment Probabilities of Failure FAA Minimum90% Certainty of Flaw Detection 31 The optimal inspection range

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Single Visual Aid that accounts for Manufacturing defects as initial flaw size from processes (machining, castings) Material strength variability (fracture toughness assumed to conform under statistical distribution) Aircraft maneuver variability (Passenger vs. fighter jet, extreme value distribution) Flaw detection resolution (Type of material, minimum desired crack detection size, non-destructive techniques) Introduction Research Question DTRAValidationConclusion Contributions 32

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Further Research Account for bulging effects (crack growth more arduous under cylindrical shape) Hammershock Condition (backpressure pulse results in shock during supersonic flight) Statistical range of initial flaws Introduction Research Question DTRAValidationConclusion Further Research 33

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Acknowledgements Advisor: Dr. Seung-Kyum Choi Reading Committee: Dr. Roger Jiao Dr. David Scott Funding: Gulfstream Aerospace 34

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References maneuvers-and-enhanced-reality-system-explained/4329/ p.png/500px-SchenectadyShip.png nals/JPVTAS/926532/pvt_134_6_061213_f002.png old.me.gatech.edu/jonathan.colton/me4210/castdefect.pdf Fatigue and Damage Tolerance Assessment of Aircraft Structure Under Uncertainty, Goksel, L 35

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