Environmental Engineering Reference
In-Depth Information
Figure 12-21. Fatigue allowable design stresses for the sample problem calculated accord-
ing to the fracture-mechanics method, showing the beneicial effect of overloads.
Margin of Safety
Both the S-N linear damage method and the fracture-mechanics method have been used
successfully in the fatigue design of wind turbines. The end product of the application of
either method is a fatigue allowable stress specified for a given component and sometimes
for a given location in that component. Paired with this fatigue allowable stress is a set of
fatigue design loads, which in our sample cases are the
L
maxmax
loads in Table 12-1. Gener-
ally, a fatigue design is judged to be acceptable if, after the structural engineer adds required
factors of safety
, the
margin of safety
is positive at all critical points in a component. Margin
of safety is calculated as follows:
MOS
=
Fat igue allowable stress
(
Design stress
)
x
(
FOS
)
(12-20)
- 1.0
³
0
where
MOS =
margin of safety; figure of merit for structural integrity
Design stress =
calculated stress in the component under the specified loads (kN/m
2
)
FOS =
factor(s) of safety
For a cost-effective design, margins of safety for design-driving loads should be close to zero.
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