Hi all,
I use ccx to do modal vibration analysis in 3D. I’d like to understand why ccx compute a widely different total effective mass if I am using density or lumped masses.
Here is a simple example. I computed the total effective mass of a cantilever beam (1,2m long, rectangular bar section 0,05x0,20m, “density 7860kg/m3” or “density 1kg/m3 + lumped mass total 94kg”).
The expected results are :
- the mass of the cantilever beam is 94kg
- the effective modal mass should be 8/9*94=84kg since there is 1 fixed node and 9 nodes in total in this example (at least when I use lumped masses)
- the first eigen mode is 28Hz.
Example1 : If I compute the modes using a material density (7860kg/m3) for the bar and no lumped mass, the dat file show :
- the good frequency (28Hz)
- a total effective mass of 818 → it is 10 times the effective mass computed below with lumped masses
- see inp file example 1 below
Example2 : If I compute the modes using lumped mass (total lumped mass of 94kg=9 nodes x10.444kg/nodes) instead of density (1kg/m3 instead of 7860kg/m3 to neutralize density), the dat file show :
- a relatively good frequency (27Hz) → OK I use a very coarse mesh in this example
- a total effective mass of 84 → OK
- see inp file example 2 below
My final aim is not to compute the total effective mass mtot, but to compute the effective modal mass mi of each mode. Since I cannot understand CCX results for mtot for now, I am not sure if the values mi are correct or not.
Does anybody know why using density or lumped mass give so different results (factor 10) regarding modal mass ?
I initially asked the question on the Mecway forum (which use ccx as solver), but I guess the answer lies in ccx and not in Mecway (my previous message is : CCX modal mass with density / with lumped masses - Forum). I reproduce below the inp file of both examples generated by Mecway. Victor of Mecway suggested the answer might be related to the units mass*length^2 given in ccx manual for modal mass. But even so, why the widely different total masses whatever the units are ?
Thanks for your help
Example 1 with density only
** Generated by Mecway 13.0
*NODE
1,0,0,0
2,1.2,0,0
3,0.6,0,0
4,0.3,0,0
5,0.9,0,0
6,0.15,0,0
7,0.45,0,0
8,0.75,0,0
9,1.05,0,0
*ELEMENT,TYPE=B31
1,1,6
2,6,4
3,4,7
4,7,3
5,3,8
6,8,5
7,5,9
8,9,2
*NSET,NSET=fixed_support_nodes
1
*ELSET,ELSET=Default
1
2
3
4
5
6
7
8
*MATERIAL,NAME=Material
*ELASTIC,TYPE=ISOTROPIC
200000000000,0.3
*DENSITY
7860
*BEAM SECTION,ELSET=Default,MATERIAL=Material,SECTION=RECT
0.2,0.05
0,1,0
*BOUNDARY
1,1,0
1,2,0
1,3,0
1,4,0
1,5,0
1,6,0
*STEP
*FREQUENCY
4
*NODE FILE,GLOBAL=YES
U
*END STEP
Example 2 with lumped masses and very low density
** Generated by Mecway 13.0
*NODE
1,0,0,0
2,1.2,0,0
3,0.6,0,0
4,0.3,0,0
5,0.9,0,0
6,0.15,0,0
7,0.45,0,0
8,0.75,0,0
9,1.05,0,0
*ELEMENT,TYPE=B31
1,2,9
2,9,5
3,5,8
4,8,3
5,3,7
6,7,4
7,4,6
8,6,1
*ELEMENT,TYPE=MASS
9,1
10,6
11,4
12,7
13,3
14,8
15,5
16,9
17,2
*NSET,NSET=fixed_support_nodes
1
*NSET,NSET=mass_nodes
1
2
3
4
5
6
7
8
9
*ELSET,ELSET=Default
1
2
3
4
5
6
7
8
*ELSET,ELSET=Mass_1
9
10
11
12
13
14
15
16
17
*MATERIAL,NAME=Material
*ELASTIC,TYPE=ISOTROPIC
200000000000,0.3
*DENSITY
1
*BEAM SECTION,ELSET=Default,MATERIAL=Material,SECTION=RECT
0.2,0.05
0,-1,0
*BOUNDARY
1,1,0
1,2,0
1,3,0
1,4,0
1,5,0
1,6,0
*MASS,ELSET=Mass_1
10.44444444444
*STEP
*FREQUENCY
4
*NODE FILE,GLOBAL=YES
U
*END STEP
View of cantilever bar with lumped masses (example 2)
