How to define history data in fatigue tool for one cycle?

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  • Last Post 13 June 2018
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keyurckp posted this 12 June 2018

  • how can i define above load so that it is applied for one complete cycle.
  • Note- If i choose cycles as option for unit name it shows 1 block as one cycle is this the first block or the second one 

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peteroznewman posted this 12 June 2018

The six points in your graph connected by five lines the load-time history. The y axis is the scale factor of any load in your model and those six points define 1 block of fatigue. Let's say the 6 points in your graph go from 0 to 60 seconds and that is 1 block.

If I have single axis accelerometer data in units of G that represents 60 seconds of vibration, I would apply a 1 G load (standard earth gravity) to the Static Structural model.  If the fatigue life prediction says the minimum life is 60,000 blocks, that is 360,000 seconds or 1,000 hours.

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keyurckp posted this 12 June 2018

  • Now sir i would like to go one step deep what if my above loading intervals are different then as shown in the image how would i apply it then

keyurckp posted this 12 June 2018

Also sir i am very much interested to learn frequency based fatigue is there any online way to learn it please share the link or i am at my own for learning?

peteroznewman posted this 12 June 2018

I trimmed your figure to take out the white space.

You show an 80 minute block. So if the fatigue tool says minimum life is 7,500 cycles then that is 600,000 minutes or 10,000 hours.

There are many tutorials on YouTube just search for mechanical fatigue failure and there are lots of hits.

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keyurckp posted this 12 June 2018

Actually my basic question is how does ansys knows that loads that i input has to be at the length of different interval as an example i will attach a link for my dat file. In that file we only have to fill first column and nothing else so how does software recognizes the length of loading interval.https://drive.google.com/open?id=1VuqtTpxBpwmgUYS6whsSeZFRS32SEaq3

peteroznewman posted this 13 June 2018

ANSYS doesn't care show long the block takes to complete.
(You can make it care if you are working with a creep material model).
Fatigue calculations are based on changes in load amplitude.

Here is your load column:

330
250
250
330
250

That is equivalent to 

330
250
330
250

All ANSYS cares about is the change in load. You can keep track of the block period. ANSYS doesn't care if the block takes seconds, minutes or hours for one block.  That is only for your benefit.

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keyurckp posted this 13 June 2018

Thank you sir for your help now let us consider that i have to use a creep material model then i will introduce it at the time of defining material data itself then how does ansys take that into account as you just said that " you can make it care if you are using a creep material model". I am really curious to know how do we take care of block length and how does it affects material differently during creep. 

Thank you

 

peteroznewman posted this 13 June 2018

Read this this for a Creep analysis.  Here is another discussion on creep.

Creep analysis has a different objective than a fatigue analysis and a different failure mode. 

Do you have a concern that creep would be an issue in your case?

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