Understanding the outputs from MLife and creating an SN curve

Wind & Water Testing
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hello all. I am going to try and be more active on these forums rather than the github forums. Sorry it took so long for me to migrate over.

Here are my questions. I have the excel sheet that is produced by MLife. I am trying to understand its outputs.

First, this number in the blue box, what is it and how does it correlate to n_ref and what does it mean by equivalent cycles?

Now, the red box, the L_mf is the fixed mean load, but what units is that? IE 218 what?

Next, equation 2 from the MLife theory manual, what is LRF and where do i find it.

Lastly, i want to use equation 2 from the MLife theory manual but i am not sure what LMF value to use. Notice in the black boxed image, it does not show LMF, do i assume 0? or do i use the LMF from the other boxes such as 218?

I have my whoeler exponent and my L_ULT so i want to create an SN curve. Equation 2 is supposed to help with that but im missing LRF and LMF.

Any information and help would be greatly appreciated.

image

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Dear @Matt.Thomas,

Here are my responses to your MLife questions. When I refer to equation numbers, I’m referring to the MLife Theory Manual: https://www.nrel.gov/wind/nwtc/assets/pdfs/mlife-theory.pdf

I’m not sure what you mean by “n_ref”, but the blue box is n^Life,eq (Eq. (34)).

L_MF will have the same units that you’ve used for the load entered within MLife. So, 218 has the units of whatever you specified for RootFxb1.

L^RF_i in Eq. (2) is the load range where failure would occur at number of cycles N_i. Basically, “L^RF_i” as “S” and “N_i” as “N” in the S/N curve.

L_MF is the fixed mean load about which you are calculating fatigue, if you are applying the Goodman correction (Eq. (3)). If you are not applying the Goodman correction (that is, if you don’t want to consider the effect of mean loads), this value is not used.

Best regards,

3

If you look at the image, the snippet with the LMF, has a n_ref name in the top left with the same number as cycles

Ok, so equation 2 from the manual, if i wanted to get Ni from my current data, i would use a value of 0 for LMF since the excel snippet of “basic time until failure for various sn curves” has no LMF value?

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Dear @Matt.Thomas,

What have you set GoodmanFlag to MLife? This will dictate whether you are using the Goodman correction or not.

Best regards,

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I have it set to (2) so it does both. In the image below, is it using LMF to calculate the time (in seconds) to failure?

image

Now, for equation 2, what value is LRF? I do not see that listed anywhere. or is LRF the 161596703.887162 number of equivalent cycles? IE after 161596703.887162 cycles, it fails

thank you my friend : )

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Dear @Matt.Thomas,

With GoodmanFlag = 2, presumably MLife is calculating the time until failure both with and without the Goodman correction; the former will use L_MF.

I explained what L^RF_i is in my prior post earlier. It is the load range where failure would occur at number of cycles N_i.

Best regards,

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correct, a flag of 2 means it will use both, accordign to the settings file. So in this image, my time to failure, circled in black, is that based off of a 0 for LMF or is it based off of 218?

image

Correct, but i dont know where to FIND that value in the excel sheet. Nothing is labeled LRF. So how do i find out what that actual value is so i can plug it into the equation : )

If i am trying to use equation 2 from the MLife theory manual, then that means i am solving for Ni

So far, this is what i have:

Ni = [(806- |LMF|)/((0.5)(LRF))]^10

LMF and LRF are the only 2 values left i need.

Lastly, Is this number below from the excel sheet, the same as Ni? If not, then what does it represent?

“Number of Equivalent Cycles 161596703.887162”

thanks again : )

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Dear @Matt.Thomas,

With GoodmanFlag = 2, MLife is calculating the time until failure both with and without the Goodman correction; the former will use L_MF. The two calculations should be labeled differently in the MLife output.

There not a single value of N_i or L^RF_i used in the calculations; rather, the rainflow cycle counting algorithm will use many values of N_i and the corresponding values of L^RF_i from Eq. (2) to calculate DELs, damage, and time until failure.

The number of equivalent cycles is defined by Eq. (34), which is not the same as N_i.

Best regards,

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