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Free-Free modal results using glued contacts

N/A

Hi,
I have run a test model using glued contacts to connect the components together.
The model ran for the modal and static load cases, but for the modal case, using
no constraints, the first modes were not at nominally zero frequency, i.e. the rigid
body modes were not present. Is this normal? or does it show some error in the analysis?
HAs anyone else had much experience of using the glued contacts for static and dynamic
analysis?
regards,
Carlton.
3 REPLIES

Re: Free-Free modal results using glued contacts

N/A

Dear Carlton,
Using the Eigenvalue Nastran card with default values you will get the 10 first
eigenvalues, with the six rigid modes, see below (open your *.dat file):
EIGRL 100 10 0 7 MASS
See F06 output list:
MODE EXTRACTION EIGENVALUE RADIANS CYCLES
GENERALIZED GENERALIZED
NO. ORDER
MASS STIFFNESS
1 1 -4.518684E-04 2.125720E-02 3.383188E-03
1.000000E+00 -4.518684E-04
2 2 -3.269156E-04 1.808081E-02 2.877650E-03
1.000000E+00 -3.269156E-04
3 3 -3.137998E-05 5.601784E-03 8.915516E-04
1.000000E+00 -3.137998E-05
4 4 -8.101575E-06 2.846327E-03 4.530069E-04
1.000000E+00 -8.101575E-06
5 5 2.887184E-04 1.699172E-02 2.704316E-03
1.000000E+00 2.887184E-04
6 6 9.026909E-04 3.004481E-02 4.781781E-03
1.000000E+00 9.026909E-04
7 7 7.407238E+07 8.606531E+03 1.369772E+03
1.000000E+00 7.407238E+07
8 8 2.603905E+08 1.613662E+04 2.568223E+03
1.000000E+00 2.603905E+08
9 9 5.177361E+08 2.275381E+04 3.621382E+03
1.000000E+00 5.177361E+08
10 10 5.336419E+08 2.310069E+04 3.676589E+03
1.000000E+00 5.336419E+08
In summary, I do not see any difference using or not Glue Contact, may sure to run
the last version of NX NASTRAN 6.1.
Best regards,
Blas.

"Carlton Matthew" wrote:
>
>Hi,
>I have run a test model using glued contacts to connect the components together.
>The model ran for the modal and static load cases, but for the modal case,
>using
>no constraints, the first modes were not at nominally zero frequency, i.e.
>the rigid
>body modes were not present. Is this normal? or does it show some error in
>the analysis?
>
>HAs anyone else had much experience of using the glued contacts for static
>and dynamic
>analysis?
>regards,
>Carlton.

Re: Free-Free modal results using glued contacts

N/A

"Carlton Matthew" wrote:
>
>Hi,
>I have run a test model using glued contacts to connect the components together.
>The model ran for the modal and static load cases, but for the modal case,
>using
>no constraints, the first modes were not at nominally zero frequency, i.e.
>the rigid
>body modes were not present. Is this normal? or does it show some error in
>the analysis?
>
>HAs anyone else had much experience of using the glued contacts for static
>and dynamic
>analysis?
>regards,
>Carlton.

Carlton,
This can be an issue using the original glue formulation (penalty stiffness) if
the glue surfaces are not coincident. This type of glue is similar to using a CELAS
spring between non-coincident nodes.
NX Nastran 6 has a new glue formulation (which is the default) that uses a connection
method more like a cweld, so it can transfer moment between non-coincident glue
surfaces. using this new method, you should get 6 clean rigid body modes on a glued
free-free structure.
Regards,
Jim

Re: Free-Free modal results using glued contacts

N/A

"Jim Bernard" wrote:
>
>"Carlton Matthew" wrote:
>>
>>Hi,
>>I have run a test model using glued contacts to connect the components together.
>>The model ran for the modal and static load cases, but for the modal case,
>>using
>>no constraints, the first modes were not at nominally zero frequency, i.e.
>>the rigid
>>body modes were not present. Is this normal? or does it show some error in
>>the analysis?
>>
>>HAs anyone else had much experience of using the glued contacts for static
>>and dynamic
>>analysis?
>>regards,
>>Carlton.

>
>Carlton,
>
>This can be an issue using the original glue formulation (penalty stiffness)
>if
>the glue surfaces are not coincident. This type of glue is similar to using
>a CELAS
>spring between non-coincident nodes.
>
>NX Nastran 6 has a new glue formulation (which is the default) that uses a
>connection
>method more like a cweld, so it can transfer moment between non-coincident
>glue
>surfaces. using this new method, you should get 6 clean rigid body modes on
>a glued
>free-free structure.
>
>Regards,
>Jim
>
Minor correction: the new "weld like" glue formulation was introduced in NX Nastran
6.1 (not 6.0)