## General Mechanical

#### Fatigue Random Vibration Analysis : error with stress life curve

• pierj88
Subscriber

Hi all, I'm Piergiorgio, an italian mechanical engineer that is starting to learn Ansys Workbench. I just registered on this forum (a nice forum , I have read a lot of old discussion in the past weeks) and I need an help for a random vibration analysis with fatigue.

The steps I've followed are:

- Define material properties, including Tensile Ultimate Strenght and Alternating Stress Mean Stress Curve.

-Import the model, reducing all to mid plane surfaces

-Meshing

-Do a MODAL ANALISYS, not damped!

-Do a RANDOM VIBRATION ANALYSIS, with constant damping ratio of 0,08. Number of modes to use: all. Output control>Calculate velocity and acceleration: yes.

-I entered 3 PSD G Acceleration, because I've 16 hours PSD acceleration in x-axis, then 16 hours in y-axis, then 16 hours in z-axis.

-In Solution>Fatigue Tools I've set: Method=Steinberg. Stress component=Von-Mises. Exposure duration=57600 s (because the sollecitation is for 16 hours in every direction)

-To obtain the total fatigue damage, I entered a User Define Result with Expression=d1+d2+d3.

Now, after all, I get and error:  An error occurred inside the FATIGUE module: Invalid or missing stress life curve.

But the curve is set with Alternating Stress Mean Stress Curve or Bilinear S-N curve (both in attachment). There is something I'm missing?

NOTE: Modal and Random Vibration have the green mark. Solution have the red flash symbol.

And another question: to find the fatigue damage, with the stress that I said before (16hr for every axis), the procedure is correct, using User Define Result d1+d2+d3?

Sorry for this long post, but I've done a lot of try and nothing worked!

Piergiorgio

• peteroznewman
Subscriber

ANSYS staff are not permitted to open attachments. In future posts please use the Insert Image button. I have done that for you below.

What version of ANSYS are you using?

I will study this problem and return with more information, but maybe someone else will comment too.

• pierj88
Subscriber

Thanks a lot, Peter. I'm using Ansys Workbench 18.0.

• Wenlong
Ansys Employee

Hi,

Can you check the material properties displayed in Mechanical? Does the S-N curve pass in successfully? Or does it make sense?

Regards,

Wenlong

• pierj88
Subscriber

Solved using the stock "Structural Steel" and replacing only the properties I needed different

Thanks all for the support!!!