In Altair SimSolid, all model geometry is stored in the units defined by the CAD system or, in the case of STL, the units defined when reading the file. For consistency, all measurements or other references to the geometry will be done in this input length units basis.
All other unit references are stored in a neutral format and will be translated on the fly to the units specified by the user on both input or output operations. All unit consistency is automatically managed by Altair SimSolid.
One place where this is very convenient is material properties specification. Most material properties are given in SI units. Even so, a user working in the IPS system of units can select any material property definition. Altair SimSolid will manage all unit conversion.
Default units are specified in the Project tree. To access the dialog, use the right mouse button (RMB) menu, Edit, or simply double-click in the units item in the tree. Units are saved with the project but default settings for future models can be set here as well. Separate unit defaults can be set for both input and output parameters.
Even after default units are defined, the user is free to specify alternate units on most data input dialogs. Just look for the relevant units pulldown menus. For example, on this pressure dialog, even though the default units are specified in pascals, you may enter pressure in other SI or IPS units. This makes it very easy to do data input and reduces common unit translation errors.
In a similar manner, units may be changed during results processing. The default output units are displayed on the contour legend. As desired, this can be changed (for the current plot only) by selecting the Change Units button as shown.
Special Considerations for Density
One common area of confusion is how to specify material density properly using IPS units. Density is defined as mass/volume. In SI units, mass is given in kilograms and volume in cubic-meters so density is in kg/m^3 – easy. But in IPS units, mass is given in something called a Slug. What the heck is a Slug?
It turns out that the Slug is the unit of mass in the US common system of units, where the pound is the unit of force (lbf). The pound is therefore the unit of weight since weight is defined as the force of gravity on an object. Weight is related to mass by the well-known equation w=m*g where w is weight, m is mass and g is the acceleration of gravity. So, something with a mass of 1 slug at the earth’s surface (where g=32.174 ft/s^2) will weigh 32.174 lbf. Also, if you invert this equation, m=w/g so 1 slug = 1 lbf*s^2/ft.
Where this gets confusing is that density is often specified in many references in units of lbs/ft^3 – it should be in slugs/ft^3. Altair SimSolid uses the proper mass/volume definition. In the IPS (inch-pound-second) system of units, this inch equivalent density is given as snails/in^3 where a snail (NOTE 1) is 12 slugs. Since 1 snail =lbf*s^2/in, Altair SimSolid specifies density in IPS in units of lbf*s^2/in^4. Wow, confusing isn’t it?
So to make it easy, we recommend you enter material properties in SI units. Altair SimSolid will convert them on-the-fly for those that insist on using IPS.
If you insist on entering material properties in IPS, be sure to enter density in lbf*s^2/in^4. To convert density given as lb/in^3 (weight/volume) to lbf*s^2/in^4 (mass/volume) just divide by the acceleration of gravity (which is 386.0886 in/s^2).
NOTE 1 – a “snail” is also referred to as a “slinch” (combination of slug and inch).
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