×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a partâ€™s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!