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# units for density in FE nastran

## units for density in FE nastran

(OP)
Hello All,

Question 1: I have a general question regarding the units consistency for linear static runs. I have always been told to use consistent units i.e. in my case its typically N, mm, MPa, tonne, tonne/mm^3,mm/s^2. I always stick to these units but could someone tell me if kg (mass), kg/mm^3 (density) and m/s^2 (gravity) could be used as an alternative. Again this is for linear static runs only. I query this because using the above still gives the same results for F=ma and I believe its because the kg,m/s^2 or tonne,mm/s^2 essentially give the same value. I have done some trial runs using the above rational and the results are identical, the forces due to the applied loads and mass inertia also tie up with hand calcs. Am I missing something here?

Question2:
What are the units for material density when running SOL103 for a free-free check? For units of mm, MPa will the density be tonne/mm^3 as per the typical recommended unit system? And how is mass accounted for in a free free modal check despite no use of gravity cards?

### RE: units for density in FE nastran

#### Quote:

Since N /= kg, you're not using consistent units even in this discussion.

He's multiplying the mass by g to get weight, and using the appropriate length unit for g (mm or m), so the initial units are consistent.

In answer to the question, yes kg (mass), kg/mm^3 (density) and m/s^2 (gravity) could be used as an alternative in conjunction with N for force and MPa for stress. The mass values will be 1000 x greater, and the acceleration 1000 x less, so the force will be the same value, but using different length units for density and acceleration doesn't seem like a good idea to me.

Personally I prefer tonne, m, kN, kPa, tonne/m3, m2/s units, because that results in values that are the same order of magnitude as the values usually quoted in specifications, geotech reports etc (other than structural material strength, which is usually given in MPa, but you cant have everything).

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: units for density in FE nastran

In reply to question 2, I have no idea what SOL103 and a free/free check are, but if you can provide more details we can probably work out what the appropriate units would be.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: units for density in FE nastran

"He's multiplying the mass by g to get weight, and using the appropriate length unit for g (mm or m), so the initial units are consistent"

The fact that kg*m = tonne*mm seemed to be beyond the OP's comprehension led to the question.

TTFN (ta ta for now)
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### RE: units for density in FE nastran

(OP)
Thank you all for the feedback.
'IDS' sol103 is nastran terminology and refers to normal modes analysis. And free-free is referring to the 6 rigid body modes with no constraints/loads. I want to know how units 0f density impact the frequencies.

### RE: units for density in FE nastran

As long as the mass and force units are consistent, and assuming that time is always in seconds, the units of density should not affect the frequencies. If they do, there is something wrong.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: units for density in FE nastran

If you google "fea consistent units" you will get several examples.

Personally I always use SI units. kg, m, N (kg * m/s^2), even if it isn't obviouis there are sometimes units hidden inside units. So I try to avoid any "inventions" unless I an sure.

Thomas

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