"Correcting" an SN curve for a different % probability of survival?
"Correcting" an SN curve for a different % probability of survival?
(OP)
To all
I am looking for some suggestions / explanation on the following. I came across a simple fatigue estimate in a report. What I don't really understand is the "correction" of the SN curve from the data in MMPDs for 84% probability of survival. MMPDS is giving the SN curve for 50% probability of survival (I believe) and it was somehow corrected for a higher probability of survival. The infinite life is therefore much lower (and so is the life at N=1000cycles)
1. How is this done? Is there a formula for that?
2. why 84%? that's 16% failure I suspect
Any ideas
Thanks
Regards
I am looking for some suggestions / explanation on the following. I came across a simple fatigue estimate in a report. What I don't really understand is the "correction" of the SN curve from the data in MMPDs for 84% probability of survival. MMPDS is giving the SN curve for 50% probability of survival (I believe) and it was somehow corrected for a higher probability of survival. The infinite life is therefore much lower (and so is the life at N=1000cycles)
1. How is this done? Is there a formula for that?
2. why 84%? that's 16% failure I suspect
Any ideas
Thanks
Regards






RE: "Correcting" an SN curve for a different % probability of survival?
to change to an 84% life curve, you'd need to assume a normal (or some other) distribution of the population, and assume a standard deviation (or some other parameter to define the population distribution. Once you've done that I think it's an easy statistics exercise to determine the 84% curve (lower than the 50% curve).
again, seems an "odd" thing to do ...
another day in paradise, or is paradise one day closer ?
RE: "Correcting" an SN curve for a different % probability of survival?
another day in paradise, or is paradise one day closer ?
RE: "Correcting" an SN curve for a different % probability of survival?
RE: "Correcting" an SN curve for a different % probability of survival?
Scorrected = S50%*10^SE*SD
where
SE = Standard Error
SD = Standard Deviation
I am no sure but I think 84% probability of survival corresponds to -1 SD from the mean
Taking MMPDS data as an example, the SN curves are 50% (survivability!) and the doc provides some info on SE & SD. See attached snapshot. Not sure how to read the data though! Any ideas?
RE: "Correcting" an SN curve for a different % probability of survival?
My sense (and I'm no expert on this) would be to calc the mean life (well, log (life)) and factor log(life) (divide by 1.18 to get -1 SD ?) and then life (=10^log(life)) and then safe life=life/4 ??
another day in paradise, or is paradise one day closer ?
RE: "Correcting" an SN curve for a different % probability of survival?
then what does one do with the Standard Error (SE) quoted with the data?
RE: "Correcting" an SN curve for a different % probability of survival?
Maybe what they're saying is they can accept 1 in 6 failing in-service ?
Notice how similar the results are ...
with SF = 4 applied to mean life = 12,500 cycles
applying 84% survivable factoring, life = 9,856 cycles
Maybe you're looking at someone else's calcs, and this is what they did ?? seems overly conservative (to apply the SF on top of the 84% reduction).
The only thing I'd change is "N factored" should be IMHO "N reduced" or "N 84%" or "N 5/6"
another day in paradise, or is paradise one day closer ?
RE: "Correcting" an SN curve for a different % probability of survival?
RE: "Correcting" an SN curve for a different % probability of survival?
new life = mean_life^(1/1.18) ... I think !?
so this'd work into the logNf equation ... by dividing the constant (20.68) and the factor (9.84) by 1.18
more importantly, I wouldn't apply a safe life factor to the resulting 84% life ... I think 84% life equates to safe life, and explains why to do it in the first place !
another day in paradise, or is paradise one day closer ?
RE: "Correcting" an SN curve for a different % probability of survival?
the "shifted" SN curve is log(N)= 20.68-m*Log(S) - Z*SD
where SD is provided (1.18) and Z is the Normal quantile (Z = 1.0 for about 84% survival rate)
for 2 selected lives N0 & N1 get the S0shift & S1shift
did the exercise with N0=1000 & N1=10^8 (with R = -1) and the shift seems reasonable (the slope of the SN curve is conserved)
RE: "Correcting" an SN curve for a different % probability of survival?
another day in paradise, or is paradise one day closer ?
RE: "Correcting" an SN curve for a different % probability of survival?