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Fatigue - Defining load cycle

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sigurd.naess
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Fatigue - Defining load cycle

Hi! 

I'm trying to define a rather long load cycle (slightly new to fatigue analysis).

 

My model is a simple cylider in Super Duplex (with non-linear stress/strain curve defined -> not sure if this will have an effect)

 

My load history:
from 0 to P and then cycle betwen P and 0.95*P for 50,000 cycles before returning to 0 as illustrated below

 

These cycles represents the assumed pressure variations my cylinder can experience pr. day   

Graph.PNG

 

When I paste the load cycle in the load history table I get the following error when trying to "Show XY plot" (before inventor crashes):

error.PNG

 

Are there any limits to numer of load cycles I can input?

 

Also, the SN curve is wrong when viewing the XY plot, I noticed a couple of forum threads about this issue. Is this only a graphing issue? 

 

To find the "life" of the cylinder, is it correct to simply add the  max "solid damage" until i get to 1?
i.e. the number of additions would in my case be number of days (Miner's rule?)

 

Regards 
Sigurd 

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Message 2 of 2
John_Holtz
in reply to: sigurd.naess

Hi Sigurd,

 

There may be a different limit for the number of data points entered in the table in Inventor and what Nastran can handle. Let me ask this question: if you do not try to view the XY plot for your 50000 cycle input, does it still give the error message?

 

I am not that familiar with the manual calculation for the Miner rule (and the "rainflow counting method) which are used for this type of "complex" method. I think the key question is how much of an effect does the 50000 cycles between P and 0.95P have compared to going from 0 to P to 0. Maybe you could run the analysis with 10 cycles between P and 0.95P, and compare that to an analysis with 20 cycles, compared to an analysis with 30 cycles. Those 3 results may show how much of the damage is due to 1 cycles of 0 to P to 0 versus N cycles between P and 0.95P. (Or maybe use 100, 200, and 300 cycles.) Once you know that, you should be able to manually calculate how many cycles of each load are required to reach a total damage of 1.

 

The answers to some of your other comments are as follows:

  • Are nonlinear materials taken into account? No, the fatigue analysis is linear and uses the rule of superposition. (The result due to multiple loads equals the sum of the result due to each load. This is not even true in a linear analysis with separation contact.)
  • The incorrect XY Plot of the fatigue material properties is just a graphical issue in the interface. The Nastran analysis uses the correct graph.

 

 

 



John Holtz, P.E.

Global Product Support
Autodesk, Inc.


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