Modal Vibration 3D

Hengre

Hello
I have one question. When doing a modal vibration analysis (example 11.19) you'll get the eigenfrequencies and eigenshapes here y values. Does the hight of the y-numbers have any physically meaning?

Regards
Henning

prop_design

not that I know of. see if it correlates to the display scale factor. there are other analysis types to get true displacement under load. however, matching real life damping is a challenge. usually easier to build and test, for real life vibration performance. modal analysis is good to find what frequencies will resonate and how to strain gauge when testing. once you have test data, you can back out the damping ratio, to get the fea results to match the test data.

JohnM

Modal results are usually Mass Normalized, I think that is the case with Calculix. Look up Mass normalization, you'll find plenty of references. I'm a little rusty on this stuff but I also think you can obtain participation factors that allow you to determine the significance of an eigenvector with respect to the potential loading.

As prop_design says, modal results can help you place strain gauges and also accelerometers. Once you've looked at your modes and know where to place accelerometers and what direction they are active, you can use two methods to determine the damping for that mode. A rap test will allow you to calculate the logarithmic decrement, and a sine sweep will allow you to obtain the amount of damping using the half power points. Look up those terms and I think you'll find the stuff is explained pretty well on the internet.

The transient 3D analysis is not that hard to set up, but the time stepping will often be very small, essentially 1/(10xfrequency) you are trying to capture. You can put in alpha / beta damping using CCX custom commands.

Look at this example for some Mecway tips




Nothing trivial about interpreting this stuff though, this is expert territory :-)

Mishal

The displacements just present the mode shape and have to relevance to any actual displacement. Normalized mode shapes can also be used as in an input to some of the transient solvers.

Calculix does give the mass participations, this is be found in one of the files developed by the solver.