Fatigue Analysis Using Rainflow Counting & S-N Curves

[Dutchero]

Hi,

For a fatigue analysis assignment I am working with rainflow counting and S-N curves.The given data is a plot of the stresses in a wingbox during a single flight. I have already derived the S-N curves, did the rainflow analysis and stored the (half) cycles (including the residu) in a Markov matrix.

So what i have now is the matrix, a plot of the residu after (four-point) rainflow analysis and the S-N curve. The assignment tells me to assume that the residu cycles can be represented by load cycles at Sm = 0 and the half cycles in the
matrix are characterised by R=Smin/Smax=0 -> Smin =0. Now I have to calculate how many flights can be performed given the data.

I have no idea how to do this. Do I have to use the Miner rule, and how? Or how else I should approach this problem?

Thanks in advance.

 

[Navin A S]

To calculate the number of flights that can be performed, you will need to use the Miner's Rule. This rule states that the total number of cycles (or flights) that can be withstood before failure is equal to the sum of all the half cycles in the Markov matrix divided by the stress range (Smax - Smin). In your case, since Smin = 0, the calculation would look like this: Number of flights = Sum of half cycles / Smax Once you have calculated the number of flights, you can then compare it to the fatigue life predicted using the S-N curve. If the number of flights calculated using the Miner's Rule is lower than the fatigue life predicted by the S-N curve, then you know that the wingbox will fail before the predicted number of flights.