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Nov29-06, 02:53 PM
Sci Advisor
HW Helper
P: 7,111
No, you don't add the number of cycles together, Miner's law says you add the amounts of life used up in one flight cycle. Think about it - if the component sees more fatigue (from the other two stress conditions) the fatigue life must be less than the 9600 cycles from the first condition, not more.

In the S = 130 case, the fraction of the life used up in 1 flight cycle was 1/9600 = 1.04e-4 (Hence the total life of 9600 flights).

Work out what the corresponding fraction of the life is for the other parts and add up the three. If the sum came to 4.0e-4 (that's just an example - I haven't done the sums for the other parts!) that means (according to Miner) you used up 4.0e-4 of the life in one flight cycle, so the number of flight cycles would be 1/4.0e-4 = 2500.