input acceleration time history 1

[munozcim]

Hi guys,
We have measured accelerations by attaching the cell phone to a bike frame. Now we want to use such accelerations to feed a dynamic FE model in Abaqus. The problem is that by defining such accelerations in the Boundary Conditions (combined with the corresponding amplitude), gives rise to huge stresses. On top of that, I get larger accelerations than those defined in the Boundary Conditions! The solver I'm using is Dynamic/Implicit.
Thank you advanced for your help.
Best,
David

 

[FEA way]

Exactly how did you define tat acceleration BC with amplitude ? Can you share some screenshots from Abaqus/CAE or paste this fragment of the input file ?

 

[munozcim]

Hi FEA way,

Thank you for your quick reply. I have prepared a simple FE model to show you how Boundary Conditions are defined. As you can see in the attached input file, a vertical acceleration of 9.8m/s2 is suddenly applied at the free end of a cantilever beam. The load is applied during 20 ms. The problem is that it gives rise to very large accelerations, rotations, displacements and stresses.

I have also replaced the acceleration by an "equivalent" force of of 9.6kg x 9.8m/s2 = 94N, where 9.6 kg is the total mass of the beam. This load is also suddenly applied during 20 ms (same amplitude as the previous case). However, this second approach provides much more reasonable results.

I would be very grateful if you could give me any advice.

Thanks,
David

 

[FEA way]

In your case, it might be best to apply gravity load with this amplitude. It should be more realistic than acceleration BC applied to the tip.

 

[munozcim]

Thank you, FEA way. I have tried the Gravity Load and, as you suggested, it works much better, but I wonder why... Shouldn't it be the same?

My second question is the following. Let's suppose we measured accelerations at the top bar of a bike frame by using an accelerometer. Now we want to use the acceleration time history to excite the FE model. Would you apply the gravity load to the whole bike frame or only to the bar where the accelerometer was located?

Many thanks,
David

 

[FEA way]

Acceleration BC, since it's a boundary condition, forces that node to move according to the applied acceleration pattern. If you use gravity load instead, the beam will be able to vibrate freely after the acceleration impulse.

It depends on the simplification level but I would apply the gravity load (which may also serve e.g. as a deceleration load when applied in a different direction and with a different magnitude) to the whole frame. Of course, you could replace some parts of it with point masses.

 

[munozcim]

Thank you for your help. I really appreciate it.
David