A problem defining material properties... HELP! 2

[ElMorris]

Hi again everybody.

As usual, the way ANSYS handles units is a good source of confusion. In my model i'm asuming that my dimensions are in meters, so if i have to input a node coordinate in (1x;2y) milimeters, i input (0.001x;0.002y) for being consistent with my units. But when i'm defining the material properties i don't get any results in a modal analysis. I guess maybe it's because the units i'm using at the time i'm defining material properties.

An example of what i'm doing at defining is this:

Preprocessor - Material Models - Material Model Number 1 - Density

Here for aluminium i input in SI: 2700 (kg/m^3)

And again:
Preprocessor - Material Models - Material Model Number 1 - Linear Isotropic - EX

Here, for aluminium, i input in SI: 70000 (MPa)

But the modal analysis doesn't returns any result!

Anyone can tell me what's wrong here???

Thank you very much.

 

[meshparts]

Have you checked the warnings an errors in the output window.

Or better, check the listing of the *.err file in List>Files>Error File.

Second, after defining the material, check list>properties>all materials and see if the density is really defined.

For aluminium you should see:

MATERIAL NUMBER = 1 EVALUATED AT TEMPERATURE OF 0.0000
EX = 0.70000E+11
NUXY = 0.30000
DENS = 2700.0

Regards
Alex

 

[cbrn]

Hi,
the mistake is in the E unit: you mustn't use [MPa] but [Pa], so that the unit of force is [N]:
[pressure] = [force]/[surface] = [force]/([length]*[length] = [N]/[m^2] = [Pa].
Btw: the unit for acceleration is coherently [m/s^2].

Regards

 

[cbrn]

Hi,
sorry for the post in two pieces... I forgot:
also make sure that the size of your model does match the dimensional system you are using: list the keypoints' or nodes' coordinates and see if they are coherent with "meters" (no coordinates such as "12640"...).
If you have volumes defined, check their properties with VSUM.

If your model comes from CAD in [mm], you'd better drop the SI system and use:
native [mm] for length
[N] for force
[MPa] for pressure (and pressure-like units)
[mm/s^2] for acceleration
[tonne] = [10^3 kg] for mass
[tonne/mm^3] for density

Regards

 

[ElMorris]

Thanks mihaiupb and cbrn for your good will. But sadly there's no solution to my problem by the time. I've tried to import my model from a CAD software, make it with ANSYS using different dimmensions, and i've tried too by changing the units at the time i'm defining my material, but there's no results at all.

I don't have any idea of what's happening here. The software only gives results for Aluminium if:

1 - I draw my model being consistent with SI: In this case is a rectangular rod 6mm x 3mm area and 20mm long. I command it this way to ANSYS: 0.006 x 0.003 and extrude 0.02

2 - I define the material properties like this:
EX = 70 (In fact, Elastic modulus for Al is 70GPa - SI))
PRXY = 2,8 (It doesn't changes at all)

Silly that ANSYS only give me results if i'm inconsistent with the units i'm using. So, as i said before, i don't have any idea of what's happening here.

Thanks.

 

[meshparts]

Hello,

Did you really input MP,PRXY,,2.8 ??? Then that is your problem. I don't know any material with such a huge PRXY (Major Poisson's ratio)

Try for aluminium: PRXY=0.3

Alex

 

[meshparts]

Hello again,

for isotropic materials PRXY must be lower then 0.5.

Alex

 

[ElMorris]

Oh no, no. That 2,8 was a mistake of mine while writing my thread. I was trying to write 0,38 but for any reason it slipped. Thanks mihaiupb. By the time i'm thinking that maybe i don't get any result because the loads in my model doesn't left much degrees of freedom... maybe.

By the other hand, i still got the problem with the units... only if i make my aluminium rod become a jelly rod, i can see any result in the results summary.

That issue still is there.

 

[meshparts]

Why don't you post your code? I would take a look at it.