WTMASS

[trelau]

Does a weight-to-mass conversion factor (WTMASS) trigger anything when HyperSizer reads in the bdf? My original FEM is in weight units and I use WTMASS to convert to mass and then have HyperSizer export properties in weight units.

Thanks,
Trevor

[Phil]

There are a couple ways to approach this. If you enter all of your FEM masses as lb, you can either put in a wtmass parameter card with a value of (1/386.4= 0.002588) OR you can tell HyperSizer that your FEM units are in lb, by checking the "FEM densities and mass elements in WEIGHT units" on the FEM Import/Export options form (see attached figure).  But do not do both.

If you include a WTMASS parameter in the FEM, HyperSizer will read and remember this and translate weights to masses and vice-versa automatically when importing the FEM and also when updating the iteration FEM.

If you use the "FEM densities and mass elements in WEIGHT units" selection, HyperSizer will assume your FEM has masses as weight units (lb) and translate accordingly on import and iteration.

If you do both of these, HyperSizer will double bookkeep and your masses exported to the iteration FEM will not be correct.

[trelau]

Phil,

Perhaps I'm misunderstanding this, but I've noticed something strange when using WTMASS in the bdf file. In my case, my FEM weights are in pounds (force) and I specify a WTMASS parameter in the bdf file (0.00258). In HyperSizer, I choose "FEM densities and mass elements are in MASS units."

When I size the structure, the nonstructural mass on the pm1 card is in the right units (lbs force/sq in), but then when I hit the "Update FEM" button, all the nonstructural masses in the pm1 file are multiplied by the WTMASS parameter. I have only noticed this now because I only use the "Update FEM" button on the first of 3 or 4 runs, so the remaining runs correct the first. I noticed then when using HyperFEA because I'm assuming its updating the FEM each iteration.

Am I misunderstanding the way HyperSizer uses the WTMASS parameters?

Thanks.