Alternating Stress Formula 2

[parkerdude]

I have been looking for the formula for alternating stress. It's the formula for fatigue analysis whereby "R" is the variable and ranges between R = -1 (full reversible stress; Fmin = -Fmax) and R = 1 (Fmin = Fmax... static load). The formula is some function of Fmin and Fmax but it is NOT (I don't think...) R = Fmin/Fmax since both Fmin and Fmax can be zero. If that IS the formula, then I need more understanding of the definition of Fmax and Fmin. Thanks.

 

[rb1957]

Falt = (Fmax-Fmin)/2
Fmean = (Fmax+Fmin)/2

 

[TVP]

The following is a good description of R ratio and other mean stress definitions:

http://books.google.com/books?id=9E...Xwh6tAi&sig=vVbm0Y1jFrgzrns4Ywx-DZvURqQ&hl=en

I don't know the answer to the question "what is the R ratio when the maximum stress is 0 and there is a divide by 0 error?".

 

[rb1957]

R = Fmin/Fmax ... which is mathematically undefined if Fmax = 0, but then a stress cycle with Fmax = 0 wouldn't cause fatigue damge (simplistically ... i know there is some fatigue due to compression cycles, usually as a spectrum with tension cycles).

 

[nveneno]

There are different ways to relate the fatigue life with the alternating stress levels.

Some are the Goodman, Soderberg and Gerber formulations wish use the mean and alternating stress in their relationships.

Those equations can be easily transformed to R factor relations replacing it's definition on the formulae.

take a look at this link:

http://www.utm.edu/departments/engin/lemaster/Machine Design/Lecture 10.pdf

regards.