Environmental Engineering Reference
In-Depth Information
f
k
c
m
c
f
r
d
¼
ð
9
:
30
Þ
where f
k
is the ultimate material strength of the component, taken here to be the
yield stress, c
f
is the load case partial safety factor as determined in Table
9.7
and
c
m
is the relevant partial safety factor for the material from Table
9.8
. In other
words, the two partial safety factors are multiplied to give the final safety factor.
For the design of a particular component to be deemed safe, then the equivalent
component stress level from Table
9.6
must be lower than the allowable material
stress limit, i.e.:
r
eq
\r
d
ð
9
:
31
Þ
9.3.4 Fatigue Failure Analysis
Load case A in the SLM, requires assessing the fatigue damage. IEC 61400-2
states that this is to be done using Miner's rule. The damage calculation is as
follows
32
:
Damage
¼
X
i
n
i
N
cycles
c
f
c
m
s
i
1
ð
9
:
32
Þ
where n
i
is the number of fatigue cycles in bin i of the characteristic load spectrum,
s
i
is the stress level of the fatigue cycles including effects from both mean and
cyclic stress levels, N
cycles
, is the number of cycles to failure as a function of the
stress, which in turn is calculated using the same combined safety factor c
m
c
f
for
the loads and materials as was used in (
9.30
). The term in parenthesis in the
denominator is called the ''associated stress level''. For the SLM, only case A
considers fatigue and there is only one ''bin'', whose number of fatigue cycles is to
calculated as
33
n
¼
N X
design
T
d
60
ð
9
:
33
Þ
and T
d
is the design life of the turbine in seconds.
By Miner's rule, a component will fail if the damage over the component's
lifetime reaches unity.
Finally, the standard requires the designer to undertake a ''critical deflection
analysis'' to ensure that no component deflection under load will compromise
32
(IEC 48).
33
(IEC 49).
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