FATIGUE AND DAMAGE TOLERANCE ASSESSMENT OF AIRCRAFT STRUCTURE UNDER UNCERTAINTY

Lorens S. Goksel

5/1/2013Committee 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

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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

DTRA Validation

Conclusion

Purpose

Determine how cracks in a component grow with variation of parameters

Obtain an optimal range for inspection and refurbishment

4What 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

DTRA Validation

Conclusion

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Caustic Environment

Introduction

Research

Question

DTRA Validation

Conclusion

3X Less Life

Sump Tank

Lab Air

Examples of Fatigue Degradation

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Load Rate

Introduction

Research

Question

DTRA Validation

Conclusion

Wind only conditions provide more than twice the life compared to Ground-Air-Ground

Examples of Fatigue Degradation

7Sources of Fatigue? Material in-homogeneity (voids,

inclusions, etc.)

Damage (scratches, stress concentration)

Introduction

Research

Question

DTRA Validation

Conclusion

Stress Risers

Manufacturing quality is essential for good

fatigue life!

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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

DTRA Validation

Conclusion

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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

DTRA Validation

Conclusion

10Research 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

DTRA Validation

Conclusion

11Research Question #1

Introduction

Research

Question

DTRA Validation

Conclusion

Pro

babili

ty

Strength

Residual Strength

Interference = Probability of Failure

Extreme Rare Occurrence

Flight Design Case

Will see this load level every flight

Pro

babili

ty

Strength

Environmental Input

Residual Strength Distributions

Interference = Probability of Failure

12Research 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

DTRA Validation

Conclusion

13Research Question #2

Introduction

Research

Question

DTRA Validation

Conclusion

Slow Crack

Growth

Predictable (Paris)

Area

Fast Crack

Growth

14Research 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

DTRA Validation

Conclusion

15Summary

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

DTRA Validation

Conclusion

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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

DTRA Validation

Conclusion

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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

DTRA Validation

Conclusion

18Proposed 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

DTRA Validation

Conclusion

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Proposed Framework – Step 1

Introduction

Research

Question

DTRA Validation

Conclusion

Grow Flaw Until Critical Crack Length

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

DTRA Validation

Conclusion

Obtain residual strength PDF

based on variability of

fracture toughness

Phantom Distribution

Discontinuities?

YesCreate

‘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

DTRA Validation

Conclusion

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

DTRA Validation

Conclusion

Pro

babi

lity

of

Fai

lure

Time

10-50

10-7

Conservative High Risk

99%

Probability of Detection

Optimal

Probability of Failure at Each Crack Length

RQ #2 & #3 Answer

23Summary

Introduction

Research

Question

DTRA Validation

Conclusion

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

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

DTRA Validation

Conclusion

Internal Loading (Engine Noise)

External Loads (Aerodynamic Loads)

When is the optimum time to inspect the nacelle inlet for

fatigue cracks?

Step 1

25Internal Loads

Introduction

Research

Question

DTRA Validation

Conclusion

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

26Internal Loads

Introduction

Research

Question

DTRA Validation

Conclusion

FEM & Hand Method Engine Specification Internal Pressure

Internal stresses derived using standard static techniques for hoop

load conditionsStep 1

27Crack 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

DTRA Validation

Conclusion

Step 1

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Introduction

Research

Question

DTRA Validation

Conclusion

Critical crack Length

Initial crack Length

This progression only accounts for internal loads

Crack Growth

Step 1

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Introduction

Research

Question

DTRA Validation

Conclusion

Failure accounts for internal and external loads

Each failure accounts for crack growth iteration

Determine POF

Steps 2 & 3

Failure Region

Flight Design Case

Critical Crack

Phantom Distribution

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Introduction

Research

Question

DTRA Validation

Conclusion

Risk Mitigation

…this crack length can be found

There is a 90% chance…

Each crack length is

associated with a flight time

Step 3

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Step 3

Introduction

Research

Question

DTRA Validation

Conclusion

Damage Tolerance Risk Assessment

Probabilities of Failure

FAA Minimum

90% Certainty of Flaw Detection

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

DTRA Validation

Conclusion

Contributions

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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

DTRA Validation

Conclusion

Further Research

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Acknowledgements

Advisor: Dr. Seung-Kyum ChoiReading Committee: Dr. Roger Jiao Dr. David Scott Funding: Gulfstream Aerospace

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References

http://www.tamarackhti.com/tools/FADT_capabilities.asp http://avstop.com/maint/corrosion/ch5.html http://www.vgblogger.com/tom-clancys-hawx-briefing-

extreme-maneuvers-and-enhanced-reality-system-explained/4329/

http://matdl.org/failurecases/images/thumb/9/91/SchenectadyShip.png/500px-SchenectadyShip.png

http://pressurevesseltech.asmedigitalcollection.asme.org/data/Journals/JPVTAS/926532/pvt_134_6_061213_f002.png

http://www-old.me.gatech.edu/jonathan.colton/me4210/castdefect.pdf

Fatigue and Damage Tolerance Assessment of Aircraft Structure Under Uncertainty, Goksel, L