Why displacement is different between Femap & Solidwork

[poorguy]

Hi! I am a newbie in Femap. Please help me understand why there is different displacement betwwen Femap and Solidwork.
For example, I ran statis analysis on a beam that has cross section of 1.0in x 1.0in and length of 10.0in.
The material is AISI 4340 taken from Solidwork.
Yield strength: 470 MPa
Elastic modulus: 2.05E+11
Mass density: 7850
The load is 10,000N.
The result was that much differed:

In Solidwork, displacement is 0.0192mm.
In Femap, it is 0.134mm

Please let me know if I did something wrong.

 

[ThomasH]

Hi

What result would you get if you did the calculation "by hand"?

Regards

Thomas

 

[rb1957]

watch the mixed units (mm and in) ... what units for density ? how do different codes work with denisty (mass or weight) ?

like thomas above ... have you done a hand calc ? obviously something is amiss (getting answers that differ by a factor of 8 is a clue).

if this is a simply supported beam, i'd've expected a displacement of 0.5mm (=0.02") or 0.02mm if it's a tension rod (which looks like your solidworks answer).

 

[rb1957]

your density is in kg/mm^3

your model needs to be in mm units (length = 254mm, section = 25.4*25.4)

 

[poorguy]

Thank you for your response.
Here is my hand calculation in tension analysis:

Stress = Force/Cross section = (10,000N)/(0.254)*(0.254)
= 15.5E+6 Pa
Strain = Stress/Modulus = (15.5E+6)/(205E+9) = 0.0756E-3
Displacement = Strain*length = (0.0756E-3)*(0.254)
= 1.92E-5 m = 0.0192 mmm

My objective was to create a simple analysis to ensure that my procedure in Femap was correct.
Please advise what wrong I did as Femap was so confussed.

Is "translation" in Femap equivalent to "displacement" in other FEA? The attachment would help you to help me.

Thanks in advance for your help.

 

[poorguy]

Sorry!!! Here is the correct hand calculation

Stress = Force/Cross section = (10,000N)/(0.0254m)*(0.0254m)
= 15.5E+6 Pa
Strain = Stress/Modulus = (15.5E+6)/(205E+9) = 0.0756E-3
Displacement = Strain*length = (0.0756E-3)*(0.254m)
= 1.92E-5 m = 0.0192 mmm

 

[rb1957]

hand cacl looks right, so there is something wrong in the FeMap model. yes, "translation" = "displacement".

i suspect you are applying a lot more load than you mean to, check the "freebody display" ... you should be able to sum the reactions to see the load the model is applying. i assume you applied 10,000N as a surface load over the end ?

check carefully the density.

 

[poorguy]

Thank you for your response.
Yes! I applied 10,000N to the surface of one end. Another end is constrained as fixed.

Also, I copied material both in SI and English from Solidwork to Femap library. The result was the same to both SI and English unit.

I believed that something must be wrong that I don't know yet. If you can get this result in Femap, please share and help me to do it correctly.

 

[ThomasH]

Make sure that you have consistent units everywhere. Why not create a model in Femap from scratch to make sure that nothing unwanted gets imported.

Also, how do the stresses compare?

Regards

Thomas

 

[rb1957]

sorry, but i think you missed the point of my post ... i know you think you applied 10,000N, but what reaction does the model have. thomas (again) makes a good point ... what are the stresses in the models like ? (i doubt they are the same)

 

[poorguy]

After spending a couple hours practicing over the weekends, I finally made my first analysis on Femap and found out the below rues:

1. Regardsless of what model it is imported into, all INPUT unit MUST be consistent.
2. For SI, input unit must be "N" and output will be "Pa" for stress and "m" for displacement.
3. For English, input must be "psi or ksi" and output will be "psi" for stress and "in" for displacement.

Femap did not have "unit preset" option and would cause trouble for any mixed units.

Please advise if it is correct as I stated and let me know if your recommendations/suggestions.

Thank you for your response, hints, and recommendation as I will be exercising a little deeper in Femap and will have more questions for you to help me familiar with this Femap.

Regards,

 

[rb1957]

yes ! but always watch the units of "g" ...

g = 32.17 ft/sec^2 = 386 in/sec^2

you can go with N and mm units (and MPa) ... just watch g !

and watch density (always mass not weight)

start with simple problems, the simple tension rod you have looked into, a simply supported beam ...

i think you've gone straight to 3D elements ... look into the others (1D and 2D).

simple problems alway you to explore the different element types, understand how they work, where they're good and, just as important, where they're no good.

 

[ThomasH]

Like you said, Femap does not have any units. It's up to you as the the user to make that choise. 1 mm/inch/meter/foot/???, femap doesn't care.

And unfortunately it's a very common error. Especially if you normally work in SI and have to use some other system or vise versa.

So take care and good luck

/Thomas

 

[poorguy]

Thanh you for your input.
I have tried another simple analysis of sheet solid element (2D) and made it straight good. I've never done anything with 1D element. Please help me with another question as I am evaluating this Femap under a certain timeframe.

There was a windows below the main working window showing the number of nodes and elements after meshing and the status of the model. For some reasons, I closed it and next time launching the program, it is NOT there anymore>

Please help and let me know how I can have it back.
Thank you in advance.

Regards,

 

[ThomasH]

Could it be the messages window you are missing?

Set your mousepointer at the field just below the to border (that says Femap). Right click and select panes in the menu.

If you click at the far left option of the now appearing toolbox that might solve it. At least it I understand your problem correctly.

Regards

Thomas