1. Oct 27, 2008

### Hydroshock

So currently i'm in school for Mechanical Engineering, though I got a general question.

I could have sworn that I read in my physics book working up to the point I am now reading that as long as a spring isn't pushed past it's stress point that it should last forever. Though now I have had someone trying to tell me that it would last only so many cycles and wear out over time ??? I'd suppose this would be somewhat important to know for sure when designing machines.

If anyone could answer that would be great.

Thanks.

2. Oct 27, 2008

### Topher925

If a structure is known to last at least 10e6 cycles, then its life is assumed infinite. The life of a structure under oscillating stress is generally determined by one of three different criteria, the modified Goodman, Elliptical, or ASME (i think?) criteria.

And yes, this is VERY important for any machine design.

3. Oct 27, 2008

### minger

http://www.mech.uwa.edu.au/DANotes/springs/fatigue/fatigue.html

Summarizing a little, since most springs are made of steel, they have an endurance or fatigue limit. This is a strength that which below, the part can "theoretically" endure infinite cycles. Aluminum materials typically do not exhibit this limit, but since steels do, there is a stress that which below you can assume infinite cycles (give yourself a good safety factor though).

In real life, one would need to account for other factors which may limit a parts life

4. Oct 29, 2008

### ank_gl

Think of it this way, each stress cycle lowers the maximum allowed stress a lil bit. So after many many cycles, maximum permissible stress ll come near to the operating stress, & failure.
This is just a crude explanation. Read mechanism of fatigue loading from metallurgy book to understand it in a better way.