×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Extracting real force values from frequency domain vibration analysis

Extracting real force values from frequency domain vibration analysis

Extracting real force values from frequency domain vibration analysis

(OP)
Dear all,
My question is about using Abaqus to predict accurate forces within a vibrating structure. I am carrying out a frequency domain vibration analysis on a tensile test, for my PhD project. Currently I am using the Steady-state Dynamics, Direct Solution procedure as this appeared the best compromise between coping with my boundary conditions and solving time. I wish to know the forces within the structure (and not necessarily the stresses) as I want to compare to a measurement made by a force transducer.

The Abaqus post-processor has a Free-Body Cut tool which, when used with the View Cut manager, can find the free-body force at any planar section. (I had to request NFORC in the Field Outputs to ensure it had the data to work with, so I suspect all it is doing is summing the nodal forces over the nodes nearest the defined plane.)

However, when I use this Free-Body Cut tool a warning pops up telling me:
"Free body computation is based on complex magnitude or phase angle in a steady state dynamics analysis. The result is not physically meaningful."

Which obviously is not a good warning to get when I want to use the simulation to verify the force transducer output (and whether this is a sound thing to do is another question for another day).
I find this puzzling as the kinematic parameters (displacement, velocity, acceleration) are presented as real, physically meaningful values, and the dynamic force experienced by the structure is directly related to the acceleration and the boundary conditions. So why would the force amplitude be any less meaningful? (and of course it is an amplitude, as the analysis is in the frequency domain.)

I should say that I have accurate velocity measurements so I can validate the simulated velocities against those.

So, I guess my questions are:
1. Why can't I take the force (amplitude) values from a Steady-state dynamics analysis as meaningful?; and
2. Should I be using a different Abaqus procedure, and which one? (Should I move to the time domain and Standard Implicit?)

Sorry in advance if this is a trivial question, I just haven't managed to get a straight answer from my colleagues or anyone else I've asked so far. Even though I am surrounded by experts, they all work in slightly different fields (eg exclusively time domain) or have never had to worry about my particlar force problem.

Thanks in advance,
Colin

RE: Extracting real force values from frequency domain vibration analysis

In all steady state dynamic analysis procedures (not only direct but also mode-based and subspace-based) force output is complex number with real and imaginary components. If you want to perform frequency domain analysis there’s no other choice. However instead of using free body cut you can sum nodal forces in selected section. Variable called PRF provides magnitude and phase angle of reaction forces.

RE: Extracting real force values from frequency domain vibration analysis

(OP)
Hello FEA_way,
Thanks for your reply, I looked up PRF in the manual and it looks like that's the sort of thing I'm after, except that I need nodal forces rather than nodal reactions.

I think what I really want is to get traction vector on my cutting surface, time the surface area - but I'm not sure how this translates into FE. Looking at the manual this might be what NFORC is, but I'm not sure. The free body tool seems to use NFORC so I could assume so.

There doesn't seem to be an option for P-NFORC so I suppose I could just ignore the warning; there would be a problem if the individual nodal forces were out of phase, but I assume I have steady-state standing waves and all my parameters are in phase.

Anyway, thanks very much for your help.

Cheers,
Colin

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

White Paper - PLM and ERP: Their Respective Roles in Modern Manufacturing
Leading manufacturers are aligning their people, processes, and tools from initial product ideation through to field service. They do so by providing access to product and enterprise data in the context of each person’s domain expertise. However, it can be complicated and costly to unite engineering with the factory and supply chain. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close