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08-16-2017 09:14 AM

HI All,

I am about to solve Spring Fatigue life. A compression spring of

coil wire Dia = 0.3 mm

No of turns = 45

total Height = 21 mm

It is subjected to continuous Loading on bottom side and fixed on top side. I need to calculate the Fatigue or the Life span of the spring. Loading is 20 kg and material is Stainless steel 304.

Is there any chance of calculating Life cycles in Nastran. Or please tell me the procedure. it will be useful for futher steps.

thanks a lot.

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3 REPLIES

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08-17-2017 10:26 AM

Hi Blitz,

If the loading is continuous (no variation like +20N, 0N, -20N, 0N, +20N etc...) and stay at 20N or any other value, there is no fatigue phenomena on the spring. Life span is infinite, you only need to make a static analysis and verify there is no yield.

If there is variation of the charge, then lets discuss fatigue. The easy way is to use NX durability (automatic way).

Naguib.

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08-17-2017 10:52 AM

Thanks for your reply . pardon me, the Spring gets downward 20kg/ 200N then it comes to rest say 0 or no load, then again 20 kg - 0kg like cylic order.

Pls consider like a weigh machine, we place something on it then we release, then we load again. like that.

thanks a lot,

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08-17-2017 11:34 AM

There is two way for your case. The first one is to do things "manualy". In Fatigue analysis, the main thing is the amplitude of the changing load, here it is 200N.

Then you need to know the amplitude of stress in the spring during one cycling load. For that, just perform a static analysis in the normal condition (bottom fixed and a compressive load at the top of 200N). You will get stress results, the **maximum principal stress will be used**.

Get the maximum value of the maximum principal stress on the spring for one cycle.

Then you need to compare this value with the Wohler curve (or stress live diagram) of the Stainless steel 304. You can find it almost everywhere since it a common steel. It looks like one of the curve I joined.

The choice of the curve depends of the design detail (in this example, it is extracted by the Eurocode regelmentation) but since there is no particular detail in a spring, we can take the maximum detail (160). Then you only have to report on the graph the stress value you obtained before and read the correponding number of cycle.

(you can easily create the curve with excell on a logarithme scale)

For example is you get 100 MPa (on the maximum principal stresss), the curve telles you that the strucure can endure a less more than 1.4E+7 cycles. If the stress is lower than 20MPa, life span is infinite...

This is the best way to do fatigue because it does not depend of any solver. (Reglementation gives further details about the calculus).

If you have NX durability licence I can show tell you how to do this automatically.

You tell me.

Naguib

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