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Subject: Re: Units and Material Lib.
Author: Philippe Vidori
Date: 1999-01-05 14:04:00

For a structural analysis with gravity effects, I usually use:

Force = Newtons,
Length = mm,
Stress and Young's modulus = N/mm^2 or MPa
Time = second,
Mass = kg,
Density = kg/mm^3
Acceleration = (9800 mm/s^2)/1000 or 9.80 m/s^2

It's very convenient for most metric engineering applications, however this
system will not work for a dynamic analysis; for instance, a modal
analysis with this set of units will yield a first mode value (1000)^(1/2)
off target for a cantilever beam !

If you require inertia effects, you can use the following system for both
types of analysis:

Force = milliNewtons,
Length = mm,
Stress and Young's modulus = 10e-3 N/mm^2 = kPa
Time = second,
Mass = kg,
Density = kg/mm^3
Acceleration = 9800 mm/s^2

Hope this helps !

Philippe Vidori
Mechanical Dynamics Ltd. (ANSYS ASD)
pvido@a...

-----Original Message-----
Sent: Tuesday, January 05, 1999 1:33 PM
To: xansys@o...; xansys@o...

I generally use the followiong units. I think this will work for you, but
verify for yourself to be sure this is correct;

Force = Newtons
Distance = Millimeters
Time = Seconds
Mass = Tonnes (metric tonnes equivalent to 1000 kg)
Density = tonnes/mm^3
Pressure = Mega Pascals (MPa)

Original Text
From: "Dipl.-Ing. Markus Lengauer"

Brower Brent (DTUS/CENGDE) wrote:

> Hello,

> I am working on putting together a material library. Millimeters are
> our standard units, therefore, all imported models are in mm. For
> most cases I can use units for pressure as N/mm^2 (structural analysis
> only). This is fine as long as the model doesn't include gravity
> effects. If gravity is involved I have to convert everything to base
> units I.E.: pressure as [(Kg-mm/s^2) * (1/mm^2)]. This isn't that
> difficult, however, I would eventually like to have everyone who uses
> Ansys at our facility to use one material database, and it would be
> nice if the units in the database could be "pretty". I would like to
> have pressures entered as MPa, forces as N, and still have the model
> geometry be in mm. Has anyone dealt with this problem before? Any
> suggestions would be helpful.

> Brent Brower
> Diesel Technology Co.
> phone: 616.554.6997
> Fax: 616.554.5100
> Email: brent.brower@p...

Hi Brent,

i calculate in mm, MPa and N. When I have to do with accelerations, I
just have to handle the density in a certain way:

density = mass (kg) / volume (mm3) (I)

mass = force (N)/ acceleration (mm/s2) (II)

(II) in (I) gives:

density = force (N) / ( volume (mm3) * acceleration (mm/s2) )

so unit of density must be: N*s2/mm4

1 kg/cm3 equals 1E-9 N*s2/mm4

e.g. steel: instead of 7.85E3 kg/m3 in the kg-m-s system
you have to put 7.85E-9 in the N-mm-s system

Hope evbdy understands my explanation and it helps.

Regards, Markus

Posts possibly associated with message #124AuthorDateScore
119Re: Units and Material Lib.Brent Brower1999/01/055
120Re: Units and Material Lib.Markus Lengauer1999/01/05 
123Re: Units and Material Lib.Scott Reiss1970/01/01 
124Re: Units and Material Lib.Philippe Vidori1999/01/05 
125Re: Units and Material Lib.Christopher Wright1970/01/01 
127Re: Units and Material Lib.Alex Ng Siu Wai1999/01/05 
129Re: Units and Material Lib.Chris Rogers1999/01/06