How to specify modal damping using MP,DAMP?


Below is an ANSYS Release 10.0 input file for a simple damped modal example (short cantilever). The appropriate material damping for the first mode is approximately 1e-4. If the damping is too low, the modes are not affected. If the damping is too high, the frequency is zero.

For each mode, two real and two imaginary terms are printed to the output file. The two real terms should be identical. The real terms are "stability" terms. For a stable solution those terms should be negative. The two imaginary terms should be opposite and equal. The first imaginary term should be positive and the second imaginary term should be negative. The first imaginary term is the damped frequency.

The four terms are used to solve the complex conjugate to calculate the damped mode shape. If the imaginary terms (damped frequency) are zero, it is an indication that the model is overdamped. You can find the critical damping by increasing the damping until, the damped frequency equals zero.

/prep7
et,1,182
mp,ex,1,10e6
mp,prxy,1,.3
mp,dens,1,1e-4
rect,0,10,0,1
esize,.5
amesh,all
nsel,s,loc,x,0
d,all,all,0
alls

/solu
antype,modal
modopt,lanb,5
mxpand,all
solve ! no damping

/post1
set,list

/prep7
mp,damp,1,1e-4

/solu
antype,modal
modopt,damp,5
mxpand,all
solve ! with damping

/post1
set,list





Show Form
No comments yet. Be the first to add a comment!