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Subject: Re: Units and Material Lib.
Author: Chris Rogers
Date: 1999-01-06 04:08:00

If you want to use a really useful and as someone asked "pretty" set of
units, use SI.

Length m
Mass kg
Time s

Gravity 9.81 m/s^2

Leads to

Force N
Stress N/m^2
etc.

SI units are also consistent dynamic units, so no messing about with Tonnes
and other fabricated sub divisions.

This doesn't help if the length units really MUST be mm, although being
cheeky its not too difficult to divide by 1000 for length and 10^6 for
stress. :-) Even if you are importing models from a CAD system, ANSYS will
scale nodal or keypoint co-ordinates, so the scaling can happen immediately
after the data has been read.

Except in special circumstances, I have only used SI for 15 years or so with
no need to think about units when switching models between static and
dynamic applications.

==+==+==+==+==+==+[ http://www.cr-engineering.co.uk ]+==+==+==+==+==+==+==
Eur Ing Chris Rogers - Technical Director
CREA Consultants Limited Engineering Solution Providers
cr-ea@c... Committed To Excellence

-----Original Message-----
To: 'xansys@o...'

>From: Philippe Vidori

>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-----
>From: Scott Reiss [SMTP:sreiss@p...]
>Sent: Tuesday, January 05, 1999 1:33 PM
>To: xansys@o...; xansys@o...
>Subject: [xansys] Re: Units and Material Lib.

>From: "Scott Reiss"

>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" >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 #129AuthorDateScore
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