Infinite of finite life for a shaft

[2slowtogofast]

I am having trouble finding the method to figure out if a shaft has finite or infinite life if it is subjected to a fluctuating load. I know how to solve for n(factor of safety) in these types of problems. Is there a value that if n is above it has infinite life.

 

[Brian_C]

That depends entirely on the material. I believe that steel has an unlimited life below a certain stress amplitude, but other materials may be different.

Rather than assuming that the lifetime is "infinite" for a certain stress amplitude, you could use damage tolerance principles to calculate the cycles to failure. You basically assume that the part has cracks, and set the initial crack length equal to the minimum size that can be resolved through non-destructive testing. You then determine the critical crack length for failure, and then apply the appropriate crack growth law to calculate the cycles to failure.

 

[xxChrisxx]

Fatigue limit
http://en.wikipedia.org/wiki/Fatigue_limit

 

[Danger]

Does 'infinite' mean something different in engineering than it does in reality? Nothing has an infinite life; even black holes will eventually be gone.

 

[Q_Goest]

Hi Danger,

Does 'infinite' mean something different in engineering than it does in reality? Nothings has an infinite life; even black holes will eventually be gone.

Yea, infinite life has a specific meaning in engineering when the term is used in relation to stresses in materials. Imagine putting stress on a beam by supporting the two ends and putting a weight in the middle. Imagine the stress being much less than yield so when you remove the weight, the beam springs back to where it originally was. Now imagine putting that weight on and taking it off millions of time. You might think the beam would bend a little and spring back just like it did originally, and that it would do that no matter how many times you applied the weight. But that isn't what necessarily happens. If the weight puts more stress on the beam than a given amount, microscopic cracks can develop, and after many millions of cycles, the beam could break all the way through. If the weight isn't enough to cause this microscopic cracking, you can essentially apply the weight as many times as you want and it won't break (ie: it has infinite life).

 

[Danger]

Hi Danger,

If the weight puts more stress on the beam than a given amount, microscopic cracks can develop, and after many millions of cycles, the beam could break all the way through.

Ahh... so it's an issue of what I think of as 'fatigue'. Thanks for the clarification, Q.