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Fatigue Study when Stress in the model exceed S-N value

Question asked by Pk M on Sep 5, 2014
Latest reply on Sep 9, 2014 by Jared Conway

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Hi,

I am trying to run a fatigue study to estimate life of the product. Static study showed that results are well within in yield value for most of the body with few edges & spots where stress value was much more than yield value. I still need to investigate if those higher stress values are because of stress concentration.Before spending any more time on that, I just wanted to run a fatigue study with existing study to see how results are. When I run the study, I got message which says that stress value in the model is more than that of S-N curve. It is asking me if 1. either to update the S-N curve and continue study or 2. run the study with smallest number of cycles from S-N curve.

Just in case, if those stress values are because of stress concentration, is there any way to manually specify stress intensity (P1-P3) to run the fatigue study?

I attached actual message I got.

thank you,

pk pavv

SW 2014 SP 2.0

Win 7 X64

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Fatigue-message.jpg94.4 KB

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Jared Conway Sep 5, 2014 12:56 PM

"Just in case, if those stress values are because of stress concentration, is there any way to manually specify stress intensity (P1-P3) to run the fatigue study?"

what do you mean by manually specify the stress intensity? as in tell it to ignore the high stress?

i believe yo uwill just have to extend the curve to get around this message but would agree it would be nice to have it report "nill" or something similar if the curve doesn't cover cases like this one

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Shaun Densberger @ Jared Conway on Sep 5, 2014 1:42 PM

Jared,

Is it possible to have the fatigue study compare the stress reading of a sensor to the S-N chart? If so, then I'd think a work around would to be place a sensor at the highest non-singular stress point.

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Jared Conway @ Shaun Densberger on Sep 5, 2014 3:01 PM

nope, does all nodes

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Pk M @ Jared Conway on Sep 5, 2014 5:17 PM

Hi Jared,

thank you for quick response. Yes I mean to tell the program to ignore high stress value beyond certain value.

How to extend the curve to get around this?

Another thing I am trying to understand is - we are expecting 2000 cycle life for this. When I got that message, I let it finish the study for smallest number of cycles.I attached default life plot. It shows as if it can get 5000 cycle life (pls. see modified chart clearly shows this).

Can I take that this box works for what I need (by comparing with minimum no. of cycles from S-N curve) ?

Thanks,

pavv

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box-results.zip1.7 MB

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Jared Conway @ Pk M on Sep 5, 2014 11:02 PM

number of cycles input is for calculation of damage

otherwise the software just takes the alternating stress and compares against the sn curve to output the fatigue life

as for high stress values, if you have stress that goes beyond your curve, the software asks that you extend the curve manually, nothing gets ignored

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Jared Conway @ Jared Conway on Sep 5, 2014 11:04 PM

actually sorry, if you don't extend the curve, it uses whatever the first number (lowest number of cycles) is on your curve. so yes, you could say it ignores reporting.

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Pk M @ Jared Conway on Sep 8, 2014 11:23 PM

Hi Jared,

Could you tell me how it actually ignores? Please take a look at plots I attached. For P1 plot, max P1 is 34860 psi. Default SW SN curve (screen shot attached) shows 318 MPa ( 46122 psi) for 7000 cycles.

1.How to extend curve?

2. Please see total life plot. Can results be trusted (at least for total life plot) when study run study for minimum cycles?

thank you very much in advance,

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Jared Conway @ Pk M on Sep 9, 2014 1:11 AM

what do you mean by exactly? the message is pretty clear, it takes the last points on the curve and reports them

extending the curve, use excel and extend your curve. but the better answer is to get a curve that covers a wider range.

total life plot, I would recommend you take a look at the definitions for damage and life plots. life plots report what the results are on the sn curve, damage is based on the input number of cycles. so if your streses are being ignored and your curve range isn't large enough, life plots and damage plots have to be taken with a grain of salt.

my recommendation would be to do the fatigue study manually and then compare to simulation.

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8 Replies

Jared Conway Sep 5, 2014 12:56 PM
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"Just in case, if those stress values are because of stress concentration, is there any way to manually specify stress intensity (P1-P3) to run the fatigue study?"

what do you mean by manually specify the stress intensity? as in tell it to ignore the high stress?

i believe yo uwill just have to extend the curve to get around this message but would agree it would be nice to have it report "nill" or something similar if the curve doesn't cover cases like this one
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- Shaun Densberger @ Jared Conway on Sep 5, 2014 1:42 PM
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  Jared,
  
  Is it possible to have the fatigue study compare the stress reading of a sensor to the S-N chart? If so, then I'd think a work around would to be place a sensor at the highest non-singular stress point.
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  - Jared Conway @ Shaun Densberger on Sep 5, 2014 3:01 PM
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    nope, does all nodes
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- Pk M @ Jared Conway on Sep 5, 2014 5:17 PM
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  Correct Answer
  Hi Jared,
  
  thank you for quick response. Yes I mean to tell the program to ignore high stress value beyond certain value.
  How to extend the curve to get around this?
  Another thing I am trying to understand is - we are expecting 2000 cycle life for this. When I got that message, I let it finish the study for smallest number of cycles.I attached default life plot. It shows as if it can get 5000 cycle life (pls. see modified chart clearly shows this).
  Can I take that this box works for what I need (by comparing with minimum no. of cycles from S-N curve) ?
  
  Thanks,
  pavv
  Attachments
  - box-results.zip1.7 MB
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  - Jared Conway @ Pk M on Sep 5, 2014 11:02 PM
    Mark Correct
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    number of cycles input is for calculation of damage
    otherwise the software just takes the alternating stress and compares against the sn curve to output the fatigue life
    as for high stress values, if you have stress that goes beyond your curve, the software asks that you extend the curve manually, nothing gets ignored
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    - Jared Conway @ Jared Conway on Sep 5, 2014 11:04 PM
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      actually sorry, if you don't extend the curve, it uses whatever the first number (lowest number of cycles) is on your curve. so yes, you could say it ignores reporting.
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      - Pk M @ Jared Conway on Sep 8, 2014 11:23 PM
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        Hi Jared,
        Could you tell me how it actually ignores? Please take a look at plots I attached. For P1 plot, max P1 is 34860 psi. Default SW SN curve (screen shot attached) shows 318 MPa ( 46122 psi) for 7000 cycles.
        1.How to extend curve?
        2. Please see total life plot. Can results be trusted (at least for total life plot) when study run study for minimum cycles?
        thank you very much in advance,
        pk
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        Jared Conway @ Pk M on Sep 9, 2014 1:11 AM
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        what do you mean by exactly? the message is pretty clear, it takes the last points on the curve and reports them
        
        extending the curve, use excel and extend your curve. but the better answer is to get a curve that covers a wider range.
        
        total life plot, I would recommend you take a look at the definitions for damage and life plots. life plots report what the results are on the sn curve, damage is based on the input number of cycles. so if your streses are being ignored and your curve range isn't large enough, life plots and damage plots have to be taken with a grain of salt.
        
        my recommendation would be to do the fatigue study manually and then compare to simulation.
        Like • Show 0 Likes0
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Fatigue Study when Stress in the model exceed S-N value

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