Evil SolidWorks Factory Default Material Properties
File: PVE-4261, Last Updated: May 18, 2010, By: LB
When a new object is created, SolidWorks specifies that the material is <not specified>. However, if the item is evaluated, it has a factory default material density of 0.04 lb/cuin (1000 kb/m^3) or the density of water. The appropriate material density would be “!Error – material density is not set!”.
An array of cubes is modelled in SolidWorks – each 1″ x 1″ x 1″. The material has been set to 304 stainless steel for all but one which was missed. The bottom faces of the cubes are fixed. Gravity = 386.22 in/sec^2 (1g) is applied.
One material property was missed in SolidWorks. Simulation will wisely not run until it is set. It can be set in two places – in Simulation, or in SolidWorks. Here it is set in Simulation to match the other items.
Time for a quick check – the model is run with gravity only to verify that the correct weight is recorded on the fixed cube faces as a reaction force. A Default gravity for 1g acceleration = 386.22 in/s^2 is applied to all bodies. They are meshed at 1/4″ size and run.
The resultant force in Simulation – the vertical (z) direction = 4.6259 lbf. A quick check back with the SolidWorks Mass Properties tool indicates a problem – the mass is 4.37 lbs vs 4.63 lbs calculated with Simulation. Where does this unacceptable 5.6% error come from?
The material for the one cube that was specified in Simulation did not make it back into SolidWorks. Further, SolidWorks did not declare an Error “Hey bud – you’re trying to weigh an object with the material properties not set – cut it out”. Instead, SolidWorks tried to make the user happy by assuming that all <not specified> materials should have the density of water and produced the wrong answer.
If I could change the Material <not specified> density to 4 * 10 ^ 17 kg/m^3 – the density of a Neutron star – then it would be impossible to be fooled by the default material properties. “Oh yeah my vessel has the same mass as the Earth, I set the material properties in Simulation again!” SolidWorks has lots of factory default values – each one is a hidden error waiting to happen!
Changes made in Simulation do not (usually) show up back in SolidWorks. This is just like a part that is modified in an assembly does not show the changes back in the part file.
It is a good idea to set material properties in SolidWorks instead of Simulation.