What are the units used in otpistruct?

Altair Forum User

Hi

 

I am a new user of optistruct and i have some doubts about the units used in optistruct. For example when i want to put a constraint about the deformation on my structure i don't know if the units used are in milimetre or in micrometer. Same for the force in suppose they are in N but i have no way to confirm it.

 

Thanks for your help.

tinh

Hi,

units in solver are often undefined

so you have to make sure that values you input had consistent unit

for example if you draw your structure with unit mm, and you  input force as N

then you have to input pressure as N/mm^2, mass as Ton, density as Ton/mm^3, velocity as mm/s, acceleration as mm/s^2, ...

some sets of consistent units you can refer here

http://www.altairhyperworks.com/(S(3fu2zyrlbyi03xcofiue25jd))/hwhelp/Altair/hw11.0/help/hwsolvers/hwsolvers.htm?unit_consistency.htm

alpha_21885

Depending on type of analysis, users must choose the appropriate unit system. In crash analysis, we often use ms as time unit. But in structural analysis, we often use SECOND as time unit. Also, in structural analysis, we often use Mpa (N/mm^2) for stress. The question is why we use TON as Mass unit in structural analysis. The reason is:

1 N = 1 Kg * m/sec^2 (F = ma) = 1 kg * 1000 mm/sec^2 = 1000 kg * mm/sec ^2 = 1 Ton * mm/sec^2.
 

Altair Forum User

Altair Forum User said:

Hi,

 

units in solver are often undefined

 

 

so you have to make sure that values you input had consistent unit

 

 

for example if you draw your structure with unit mm, and you  input force as N

 

 

then you have to input pressure as N/mm^2, mass as Ton, density as Ton/mm^3, velocity as mm/s, acceleration as mm/s^2, ...

 

 

some sets of consistent units you can refer here

 

 

http://www.altairhyperworks.com/(S(3fu2zyrlbyi03xcofiue25jd))/hwhelp/Altair/hw11.0/help/hwsolvers/hwsolvers.htm?unit_consistency.htm

 

Hi tinh,

 

see what I don't quite understand here is that a N is explicitly a kg*m/s^2. Do I have to make my N consistent with the mm I use for length measurement? So 1N would be 1000 kg*mm/s^2. I'm also a bit muddled up with this. Or are Newtons consistent with mm in HyperMesh - Optistruct?

 

Cheers!

 

Josh

Altair Forum User

Hi,

 

There are  7 base units in SI, of which 3 are commonly used in structural Analysis: Time, length and mass

 

First step is to choose a unit for each of these three quantities (could be s, m, kg)

 

The units for all other quantities are derived from that choice, because any other unit (except the 4 'non-structural' base units) is a composition of time, length and/or mass.

 

In Josh's example:

 

For 1N=kg*m/s^2, you have to check if the right side units are the same as your chosen base units.

 

If every single one is the same, then your force unit is correct with 1N.

 

If not, you have to put in the right orders of magnitude to achieve that your chosen units appear on the right side of the equation.

So if you have chosen mm for length, this unit is not consistent with the length unit used in the equation and you have to put in 1000 on the right hand side to replace m with mm. Then divide the equation by 1000, to eliminate the number from the right hand side. On the left you get N/1000 which equals 1mN. So for a set of base units like s, mm, kg, your force unit will be 1 mN.

 

tinh

Hi,

don't care about it much. you just need input values follow unit in the consistency table

for example:

modeling with unit 'mm'

input material properties with mass (T), density (T/mm3), E (MPa)

input force with unit N, moment N.mm, pressure MPa

velocity mm/s

acceleration mm/s2

g=9810