Civil Engineering Reference
In-Depth Information
Fig. 6.11 Stress distribution
for the case of basis unit [
4
]
Calculate the number of cycles n
j
corresponding to each loading phase and
selected resonance frequency. It is calculated from:
n
j
¼
X
f
j
t
j
ð
6
:
6
Þ
where f
j
is the resonance frequency and t
j
is the corresponding time duration of
applied load in each phase. The following empirical stress equation provides an
appropriate approximation for the fatigue behavior data of wrought products made
of 6061-T6 aluminum alloy at room temperature [
7
]:
logN
j
¼
a
1
a
2
log
½
r
max
ð
1
R
Þ
n
a
3
ð
6
:
7
Þ
r
max
: Maximum value of stress in the stress cycle (ksi)
R: stress ratio; R
¼
r
min
=
r
max
a
1
;
a
2
;
a
3
;
and n are empirical constants having the following values:
a
1
= 20.68, a
2
= 9.84, a
3
= 0, n = 0.63.
Figure
6.11
shows the FE results for the maximum mean stress and for the
basis-unit case under quasi-static load = 1g.
Fatigue damage calculations show that basis plate is safe and should not suffer
any cracks. Performing fatigue analysis for the rest of the transportation cases does
not cause enough damage that may cause crack initiation (\0.04). In connection
with the cracks that occurred in the structure during tests in the case of air
transportation, the following decisions were made:
• To improve damaged structural elements;
• To revaluate strength of improves structure;
• To repeat tests for the case of air transportation.
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