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
Outline
2


Introduction
Research Questions
Probability of Failure (PF)
 Predictable Range of Risk
 Risk Mitigation


Damage Tolerance Risk Assessment (DTRA)
Comparison
 Proposed Framework



Validation Example
Conclusion
Introduction
3
Research
Question
DTRA
Validation
Conclusion
Purpose

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
Determine how cracks in a component grow with variation of parameters
Obtain an optimal range for inspection and refurbishment
Introduction
4
Research
Question
DTRA
Validation
Conclusion
What is Fatigue?


Fatigue is the degradation of materials due to
repeated loads
Degradation occurs due to
 Mechanically
 Load
induced loads
rate
 Caustic environmental effects
Introduction
5
Research
Question
DTRA
Validation
Conclusion
Examples of Fatigue Degradation

Caustic Environment
3X Less Life
Sump Tank
Lab Air
Introduction
6
Research
Question
DTRA
Validation
Conclusion
Examples of Fatigue Degradation

Load Rate
Wind only conditions
provide more than twice
the life compared to
Ground-Air-Ground
Introduction
7
Research
Question
DTRA
Validation
Conclusion
Sources of Fatigue?

Material in-homogeneity (voids, inclusions, etc.)

Damage (scratches, stress concentration)
Stress Risers
Manufacturing quality is
essential for good fatigue life!
Introduction
8
Research
Question
DTRA
Validation
Conclusion
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
9
Research
Question
DTRA
Validation
Conclusion
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
10
Research
Question
DTRA
Validation
Conclusion
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
11
Research
Question
DTRA
Validation
Conclusion
Research Question #1
Environmental Input
Flight Design Case
Probability
Will see this load
level every flight
Probability
Residual Strength
Extreme Rare Occurrence
Residual Strength
Distributions
Strength
Strength
Interference = Probability of Failure
Interference = Probability of Failure
Introduction
12
Research
Question
DTRA
Validation
Conclusion
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
13
Research
Question
DTRA
Validation
Conclusion
Research Question #2
Slow Crack
Growth
Predictable
(Paris) Area
Fast Crack
Growth
Introduction
14
Research
Question
DTRA
Validation
Conclusion
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
15
Research
Question
DTRA
Validation
Conclusion
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
16
Research
Question
DTRA
Validation
Conclusion
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
17
Research
Question
DTRA
Validation
Conclusion
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

Research
Question
Introduction
18
DTRA
Validation
Conclusion
Proposed Framework

Step 1: Specify Geometry, Loading Conditions and
Material Statistic Properties (Crack Growth)
 Obtain

Step 2: Discontinuity Check
 Obtain

Residual Strength based on distribution
more clear data
Step 3: Obtain Probability of Failures, Probability
of Detection
 Setup
the DTRA
Introduction
19
Research
Question
DTRA
Validation
Conclusion
Proposed Framework – Step 1
Assume Initial Flaw Size
Grow Flaw Until Critical Crack
Length
Final Crack
Initial Flaw
Obtain residual
strength PDF based
on variability of
fracture toughness
Lays foundation for
residual strength
distribution needed for PF
Introduction
20
Research
Question
DTRA
Validation
Conclusion
Proposed Framework – Step 2
Phantom
Distribution
Obtain residual
strength PDF based
on variability of
fracture toughness
Yes
Discontinuities?
No
Create ‘Phantom’
Distribution
Self-check to increase resolution
Residual strength due only to local loading
Introduction
21
Research
Question
DTRA
Validation
Conclusion
Proposed Framework – Step 3
Discontinuities?
No
Intersect Flight Design with
Residual Strength Case
RQ #1
Answer
Plot Probability of Failures
for each crack interval
RQ #3
Answer
Insert Probability of
Detection for crack size
Flight Design
Introduction
DTRA
Validation
Conclusion
Proposed Framework – Setup DTRA
Probability of
Detection
99%
10-7
Probability of Failure
22
Research
Question
Probability of
Failure at Each
Crack Length
Conservative
Optimal
High Risk
10-50
Time
Introduction
23
Research
Question
DTRA
Validation
Conclusion
Summary
Assume Initial Flaw Size
Grow Flaw Until Critical Crack Length
Obtain residual strength PDF based on
variability of fracture toughness
Discontinuities?
No
Intersect Flight Design with Residual Strength Case
RQ #1 Answer
Plot Probability of Failures for each crack interval
Insert Probability of Detection for crack size
in plot
RQ #2 & #3 Answer
Optimal Area determined based on DTRA, PD
and FAA minimum allowable
Yes
Create ‘Phantom’ Distribution
Introduction
24
Research
Question
DTRA
Validation
Conclusion
Validation: Engine Nacelle Inlet
External Loads (Aerodynamic Loads)
When is the optimum time to inspect the
nacelle inlet for fatigue cracks?
Internal Loading
(Engine Noise)
Step 1
Introduction
25
Research
Question
DTRA
Validation
Conclusion
Internal Loads
Need to determine most pertinent loading mode: longitudinal vs. circumferential
Loading is assumed only to act in hoop direction,
thus circumferential natural frequency examined
Step 1
Introduction
26
Research
Question
DTRA
Validation
Conclusion
Internal Loads
FEM & Hand Method
Engine Specification
Internal stresses derived using standard static
techniques for hoop load conditions
Internal Pressure
Step 1
Introduction
27
Research
Question
DTRA
Validation
Conclusion
Crack Growth

Assume manufacturing flaw
 Flaw
is two dimensional
 Use previous internal loading

Determine Residual Strength at some crack length
 Assume
normal material distribution
Step 1
Introduction
28
Research
Question
DTRA
Validation
Conclusion
Crack Growth
Critical crack
Length
This progression only accounts for internal loads
Initial crack
Length
Step 1
Introduction
29
Research
Question
DTRA
Validation
Conclusion
Determine POF
Critical Crack
Flight Design
Case
Failure accounts for internal and external loads
Phantom
Distribution
Each failure accounts for crack growth iteration
Failure Region
Steps 2
&3
Introduction
30
Research
Question
DTRA
Validation
Conclusion
Risk Mitigation
There is a
90%
chance…
Each crack length is
associated with a
flight time
…this crack length
can be found
Step 3
Introduction
31
Research
Question
DTRA
Validation
Conclusion
Damage Tolerance Risk Assessment
FAA Minimum
90% Certainty of Flaw
Detection
Probabilities of Failure
The optimal inspection range
Step 3
Introduction
32
Research
Question
DTRA
Validation
Conclusion
Contributions

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
33
Research
Question
DTRA
Validation
Conclusion
Further Research

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
Acknowledgements
34
Advisor:
 Dr. Seung-Kyum Choi
Reading Committee:
 Dr. Roger Jiao
 Dr. David Scott
Funding:
 Gulfstream Aerospace
References
35
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http://www.tamarackhti.com/tools/FADT_capabilities.asp
http://avstop.com/maint/corrosion/ch5.html
http://www.vgblogger.com/tom-clancys-hawx-briefing-extrememaneuvers-and-enhanced-reality-system-explained/4329/
http://matdl.org/failurecases/images/thumb/9/91/SchenectadyShi
p.png/500px-SchenectadyShip.png
http://pressurevesseltech.asmedigitalcollection.asme.org/data/Jour
nals/JPVTAS/926532/pvt_134_6_061213_f002.png
http://wwwold.me.gatech.edu/jonathan.colton/me4210/castdefect.pdf
Fatigue and Damage Tolerance Assessment of Aircraft Structure
Under Uncertainty, Goksel, L