Environmental Engineering Reference
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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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