AAE 552 Spring 2011 – A. F. Grandt
1
1
AAE 552:
Nondestructive
Evaluation of Structures
and Materials
A. F. Grandt, Jr.
Professor of Aeronautics and
Astronautics
Purdue University
W. Lafayette, IN 47907
Spring 2011
2
AAE 552
Session 13:
9 February 2011
Last time
•
Finish proof testing
Today:
•
Fatigue crack growth life
•
Inspection intervals/strategies
•
Start visual inspection
3
Assignment
Please read
:
•
Sections 16.5 (Damage Tolerant
Configurations) in textbook
•
Chapter 10 – Visual and Liquid Penetrant
Inspection
Due Dates:
•
Abstract #2 due Monday, 21 Feb
•
Paper #1 due Monday, 28 Feb
Note:
•
some examples of the “hybrid”
(powerpoint notes) format for paper #2 are
posted on blackboard – will discuss this
format in more detail in a later class
4
Advantages – Proof Testing
•
Structure readily screened for gross
defects
•
Previous inspections (ultrasonic, X
ray, etc.) are reliably checked
•
Upper limit for the initial crack size is
readily determined
•
Proof test overload may “retard”
subsequent fatigue crack growth
(see Chapter 7).
5
Disadvantages – Proof Testing
•
May fracture during proof test (i.e. the
method is not “nondestructive”)
•
Can be expensive for large components
•
Difficult to proof all components to same
level, or to accurately determine applied
loads in certain areas
•
Difficult to load structure similar to service
•
Existing cracks may extend subcritically
during proof cycle
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Fatigue Crack Growth Criterion
Objective
:
•
Determine a criterion that specifies cyclic
growth of preexistent cracks
•
Relate cyclic load, crack size, geometry,
material
•
Correlate lab tests with structure
•
Evaluate materials
Approach
:
•
Assume that the cyclic
stress intensity
factor controls fatigue crack growth rate
•
Verify experimentally
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AAE 552 Spring 2011 – A. F. Grandt
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Remote vs Wedge Load
•
Constant amplitude
∆
P fatigue test
•
Measure crack length vs cycles
2a
W
B
P
P
∆
P = constant
P
time
2a
W
B
P
P
a
K
π
σ
∆
=
∆
a
B
P
K
π
∆
=
∆
∆σ
∆σ
=
∆
P/BW
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Measure Crack Growth
da
dN
Crack Length (a)
Number of Cycles (N)
Crack Length (a)
Number of Cycles (N)
da
dN
a*
∆
K
=
∆σ√π
σ√π
σ√π
a
2a
∆σ
∆σ
Remote Load
2a
∆
P
Crack Face Load
=
∆
K
∆
P
B
√π
√π
a
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Correlate Rate da/dN vs
∆
K
2a
2a
Log
∆
K
Log da/dN
∆
K
th
K
c
=
∆
K
∆
P
B
√π
√π
a
∆
K
=
∆σ√π
σ√π
σ√π
a
Crack Length (a)
Number of Cycles (N)
da
dN
a*
da
dN
Crack Length (a)
Number of Cycles (N)
10
da/dN 
∆
K
is Material Property
Log
∆
K
Log da/dN
∆
K
th
K
c
∆
K =
∆σ
(
π
a)
1/2
β
(a)
geometry
stress
crack length
Material
da/dN = F(
∆
K)
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Compute Fatigue Life N
f
•
a
o
, a
f
= initial, final
crack sizes
•
F(K) = function of:
•
cyclic stress:
∆σ
∆σ
, R, . . .
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 Spring '11
 GRANDT
 Crack, Nondestructive testing, crack length, A. F. Grandt

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