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# nastran units for linear static runs (SOL101)

## nastran units for linear static runs (SOL101)

(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: nastran units for linear static runs (SOL101)

whatever solution you use -always use consistent units: so if length= mm them density= tonne/mm^3
mass = volume * density !

### RE: nastran units for linear static runs (SOL101)

What i believe and follow is consistency means if your model is in mm use mm, kg(or tonn) ,and newton(or milli newto or kilo newton) as units. if the model is in inches use inches , lbf.

correct me if i am wrong .

### RE: nastran units for linear static runs (SOL101)

In NASTRAN (NXNASTRAN) the default set may be:
$* UNITS: mm (milli-newton)$* ... LENGTH : mm
$* ... TIME : sec$* ... MASS : kilogram (kg)
$* ... TEMPERATURE : deg Celsius$* ... FORCE : milli-newton
\$* ... THERMAL ENERGY : mN-mm (micro-joule)

or another set.

If you work in NX, you can "mix" units, since the NX user interface will check the consistency.

I usually use dimensions in mm, density in kg/m², Modulus of elasticity in MPa, pressure in bar, acceleration in g's, forces in N and there seems to be no problem.
There is a converter for units (click the = sign, then "Make Constant" you'll be presented with values in the other available units).

Of course since you double check you calculations with hand calcs, there will be no trouble

But there is a strange thing: you can enter temperature in Kelvin with negative negative values. The consistency is OK (NX will "correctly" convert the temperature to °C) but the validity range is not checked! (PR 1852169 is opened since june 2011...)

### RE: nastran units for linear static runs (SOL101)

Choose 3 "base" units and everything else is a function of those. This is what is meant by being consistent. Temperature is also a base unit, but nothing needs to be "derived" from that, so lets skip it for the following discussion.

For example, you can choose length, force, and time as the base units. Each of these can be any unit. The mass and density are then a FUNCTION of those base units. A common set of base units in the US is IPS (inches, pounds force, seconds). This is why the consistent density is expressed in the relatively awkward unit of lbf-s^2/in^4.

Alternatively, another option is to choose the base units to be length, mass, time. The force and density units would then a function of those base units. Density is then lbm/in^3, but force is an awkward unit. Since force is more commonly used than density, that is what the inches, pounds force, seconds system is usually preferred.

The important aspect to realize is that once the base units are established, the other units are a function of the base units.

Brian
www.espcomposites.com

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