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AERO 3465 AEROSPACE DESIGN
FATIGUE AND DAMAGE TOLERANCE ASSIGNMENT #3
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1 F111 WCTB
The Wing Carry Through Box (WCTB) on the F111 is made from D6ac steel. Material properties are:
Ftu = 230 KSI
KIC = 70 KSI√in (Room temp)
KIC = 50 KSI√in (-40 C)
Assume that there is a small semicircular surface crack, with β = 0.75, at a location where the stress
at ultimate load is 175 KSI:
(a) What size crack will reduce strength to limit load at room temperature?
(b) A cold proof test is done by applying Limit Load at -40 C. What load, as a percentage of limit load,
can be carried at room temperature?
2 Crack growth
A 3 in wide x 0.1 in thick strip of 7075-T6 aluminium has an alternating load which varies between 0
and 5000 lb.
Given an initial centre crack with a = 0.05 in, what is the unfactored life of the strip?
Assume:
β = √(sec(πa/w))
da/dn = C * (ΔK)m (Paris equation), defined by Table below.
ΔK da/dn
2.0 1.0e-9
39 1.0e-3
Table 1 Data for Paris Equation
Kc = 49 KSI√in
For integrating the number of cycles, calculate the number of cycles for given increments of crack
length, taking the average length to calculate β. Use a geometric progression of crack lengths.
Hint: Determine Paris Equation constants by taking logs of both sides and solving simultaneously.
3 Airliner inspection interval
The crack growth curve for a location on a wing is shown in Figure 1, and the residual strength in
Figure 2. The structure is multiple load path. Stress at limit load is 25 KSI. Detectable crack for the
chosen inspection method is a = 0.75 in. Assumed crack length at manufacture is 0.05 in.
What are the numbers of flights for:
(a) Initial inspection
(b) Ongoing inspection interval
AERO 3465 AEROSPACE DESIGN 2 2023
FATIGUE AND DAMAGE TOLERANCE ASSIGNMENT #3
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Figure 1: Crack Growth
Figure 2: Residual Strength
Problem 5 (Crack Length vs. Flights)
Residual strength
AERO 3465 AEROSPACE DESIGN 2 2023
FATIGUE AND DAMAGE TOLERANCE ASSIGNMENT #3
3
4 DATA
References:
An excellent book on fracture mechanics is:
Broek, D, "The Practical Use of Fracture Mechanics"
Software: AFGROW crack growth program from the US Air Force is widely used and accepted.
Current versions are expensive, but older versions could be downloaded free from the link below.
They may still be available.