Learning Forum

[vishal295]

Hi Everyone,

I am doing Random vibration analysis in Ansys Mechanical workbench. Where I want simulate shaker table test (6 hours test on structure) with PSD profile given in all three direction (6 hour in X direction, 6 hour in Y direction and 6 hour for Z direction with different PSD profile in each direction).

So as in attachment I have these analysis setting as in attached pictures. and also PSD acceleration. First I want to do it individual and then combined together for all axis.

I have question regarding ''Fatigue tool'' in Random vibration analysis solution.

In Engineering properties I have taken by default SN curve as in attachment.

and in fatigue tool I have specified the time exposure duration 518400 seconds (6 hours) and checked life time of structure. Where I saw that minimum life time was around 1634 seconds at some positions.

Does it mean that its lifetime is 1634 seconds ??? or something else ??

But when I changed Time exposure duration to some random number ''148 seconds'' and checked lifetime of the structure where I saw that minimum life time was same 1634 seconds at same position.

So I guess ''Exposure duration'' what is mentioned in ''Random Analysis'' -> ''Fatigue tool'' is different ?? But I don't know what it is ?? Can anyone please tell me what it is ??

And What I need to change or I have to do in simulation for my case ??

Thank you and Best regards,

Vishal

[danielshaw]

In Engineering properties I have taken by default SN curve as in attachment.
A1: The Mechanical Fatigue Tool (FT) requires a linear or bi-linear SN curve (in log-log space). If you input the SN curve as a table, the FT uses the first and last point of that table to form a power law curve (linear in log-log space). You can verify this behavior by deleting all the intermediate terms in the SN table. The calculated fatigue results will not change.

and in fatigue tool I have specified the time exposure duration 518400 seconds (6 hours) and checked life time of structure. Where I saw that minimum life time was around 1634 seconds at some positions.
Does it mean that its lifetime is 1634 seconds ??? or something else ??
But when I changed Time exposure duration to some random number ''148 seconds'' and checked lifetime of the structure where I saw that minimum life time was same 1634 seconds at same position.
So I guess ''Exposure duration'' what is mentioned in ''Random Analysis'' -> ''Fatigue tool'' is different ?? But I don't know what it is ?? Can anyone please tell me what it is ??
A2: The FT always reports vibration fatigue results in seconds, regardless of the exposure duration. The exposure duration does not affect the calculated life. It does affect the calculated duration. The fatigue damage is reported in terms of exposure duration. It is the amount of damage for that exposure duration. For example, if you double the duration time, the reported fatigue life will not change, but the reported fatigue damage will double.