Working with inches and pounds in Abaqus

[Etbraz]

Hi all,

I'm doing the FEA of a concrete canoe model in Abaqus and I can't figure out what's wrong with my units. I am using inches as my distance unit and pounds as my units of mass. In my FEA model I'm tring to simulate the canoe sitting on two stands. The only load acting on the canoe is its self-weight, which I have applied as a gravity load. The model runs alright but the values for the max/min stresses seem quite absurd. I was wondering if anyone would know of the correct units to use when one is working with inches and pounds. I've been told to use slinches as the unit of mass but I'm also not sure if that's the right way to do this. I have listed the parameters I used in the model with the units I assumed Abaqus would be using in the computation. Let me know if more information is required for you to understand this.

Parameters used for the model:
- Unit weight of concrete: 0.0348 pci (60.1 pcf)
- Young's modulus: 571,000 psi (571ksi)
- Poisson's ratio: 0.17
- Thickness: 0.75 in
- Gravity: 386.4in/s^2 (=32.2ft/s^2)

For comparison:
Compressive strenght of concrete cylinders: 2078psi

From the model:
- Max compressive stress: 11460psi (=165.03 x 10^4psf)
- Min tensile stress: 1.062 x 10^-27psi (152.93 x 10^-27psf)

Thank you for your time

 

[DanStro]

I am assuming that the canoe is modeled in inches and not feet.

Do you have contact conditions present? If so, are the extremes at the edges of the contact areas?

Han primo incensus

 

[Etbraz]

Yes, I am modeling the canoe in inches. The canoe is 17 feet long an I've put the the contact conditions - boundary conditions - at 2.5 feet from the extremes.

 

[DanStro]

Are the extreme stress values just outside of the areas in contact?

Han primo incensus

 

[Etbraz]

Yes! And I'm not getting any tensile stress in the middle, where the maximum moment and tensile stress are located.

 

[DanStro]

Then you may be singularities due to the contact definition. Take a look at just the elements in the middle and see if they are close to your results. If they are close to your expected results your model is probably fine and you can either ignore the unrealistic values or try playing around with the mesh near the edges of contact.

Han primo incensus

 

[TGS4]

Imperial units are an issue in general. If you use the mass unit of pounds, then gravity should be expressed as 1 lbf/lbm. If you insist on using 386.4in/s^2, then your unit of mass should be the slug, not the pound.