Base Excitation Absolute Result?

  • Last Post 18 May 2019
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Ghost3507 posted this 18 May 2019

I am solving transient analysis by applying base excitation to a structure there is a function in the acceleration setup that appear when I set it to base excitation called absolute result. This function if I set it to "No" the supports get fixed and the body deform about and the stresses are low as well as the deformation. when I set it to "Yes" the supports start to move from their places and showing very high stresses about 7300 MPa. I checked a guide for what is meant by absolute result, this is what I found (link at end of the post):
"The Absolute Result property is set to Yes by default. As needed, change the value to No if you do not want to include enforced motion."
I want to understand what is meant by absolute result (enforced motion), and why the stresses are so excessive, I supposed to simulate a load coming from a bump that is damped from tires and suspensions then applied to my chassis at the support point. The acceleration object load is resultant of the suspension (tabular data).

peteroznewman posted this 18 May 2019

Was the acceleration load measured by an accelerometer on the structure of a real vehicle going over a bump?  Where on the structure was the accelerometer? You say you are using that data to load the frame.  How are you supporting the frame?  Do you have all the mass of the vehicle attached to the frame? Please reply and insert images of your structure. Indicate which faces are Fixed Supports and if you have a non-zero Enforced Displacement, indicate that face too.

Below is a simple example that explains the difference between Absolute = No and Absolute = Yes.

The frame is represented by a rectangular plate.  It is fixed at end A, which represents where the wheel has an input to the frame. At the other end B is a point mass which represents all the mass of the vehicle excluding the rectangular frame.

A modal analysis is linked into two Transient Structural analyses to look at the effect of changing Absolute results from No to Yes.

The acceleration load in the two Transients is a Base Acceleration in the Y direction on the Fixed Support.
The graph below is in the units of m/s^2.


There are two Directional Displacement plots. One on the Fixed Support A end and the other on the Mass B end.

Absolute = Yes analysis

The graph of the displacement of the Fixed Support A end shows the frame being forced down 11 mm and pulled back up over a 15 ms period, then held at zero for the remainder of the 100 ms duration. This is the acceleration input being integrated twice to get to displacement.

Here is the response of the Mass at the B end. While the other end is forced rapidly down and up, the flexibility of the frame and the substantial mass at the other end allows it to respond more gradually to the disturbance at the "fixed" end.

Both the above plots are relative to ground when the Base is moving according to the acceleration input.

Absolute = No analysis

The graph of the displacement of the Fixed Support A end shows zero displacement.

Here is the response of the Mass at the B end. Relative to the fixed end, the mass goes up 11 mm and down over 15 ms then begins to oscillate.

These graphs are not relative to absolute ground, but measured from the point of view of a ruler on the "fixed" end.

Stress Results from the two analyses

The stress results from the two analyses are identical, because all the Absolute Result Yes/No choice is doing in this model is changing the point of view of the deformations, which has no effect on the stress.

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