Stress Calculations

[rentien]

Hi, I am very new to Mechanica. I am trying to do a simple stress calculation on a cantilevered beam, so that I can verify the stress calculations that Mechanica does, with my hand calcs. The beam is a 1in*1in*100in beam.

I have constrained the back face of the beam so there is no rotation nor translation in any of the three possible directions, and on the furthest face (the tip), i have a force (300in*lbm/s^2) along the top edge going downwards.

The stress calculations seem to very from my hand calcs by as much as 32%.

My calcs max stress = 180,000 lbm*in/in^2*s

Mechanica: max_stress_prin = 2.214994e5
Max_stress_zz=-2.37e5

Does anyone know what threads i can look in for more information on this, or what could be wrong? Thanks.

 

[rentien]

This is what mechanica outputs to me.





------------------------------------------------------------
Mechanica Structure Version K-03-39:spg
Summary for Design Study "MyAnalysis"
Fri Mar 20, 2009 14:32:29
------------------------------------------------------------

Run Settings
Memory allocation for block solver: 128.0

Parallel Processing Status
Parallel task limit for current run: 4
Parallel task limit for current platform: 64
Number of processors detected automatically: 4
Checking the model before creating elements...
These checks take into account the fact that AutoGEM will
automatically create elements in volumes with material
properties, on surfaces with shell properties, and on curves
with beam section properties.

Generate elements automatically.
Checking the model after creating elements...

Excluded elements may be required near one or more
loads due to concentrated stresses.
No errors were found in the model.

Mechanica Structure Model Summary

Principal System of Units: Inch lbm Second (Pro/E Default)

Length: in
Mass: lbm
Time: sec
Temperature: F

Model Type: Three Dimensional

Points: 12
Edges: 33
Faces: 34

Springs: 0
Masses: 0
Beams: 0
Shells: 0
Solids: 12

Elements: 12

------------------------------------------------------------

Standard Design Study

Analyses:
MyAnalysis
static_mpa

Analysis MyAnalysis requires input data from
analysis static_mpa.

Static Analysis "static_mpa":

Convergence Method: Single-Pass Adaptive
Plotting Grid: 4

Convergence Loop Log: (14:32:30)

>> Pass 1 <<
Calculating Element Equations (14:32:30)
Total Number of Equations: 288
Maximum Edge Order: 3
Solving Equations (14:32:30)
Post-Processing Solution (14:32:30)
Checking Convergence (14:32:30)
Resource Check (14:32:30)
Elapsed Time (sec): 0.73
CPU Time (sec): 0.45
Memory Usage (kb): 180156
Wrk Dir Dsk Usage (kb): 0

>> Pass 2 <<
Calculating Element Equations (14:32:30)
Total Number of Equations: 567
Maximum Edge Order: 5
Solving Equations (14:32:30)
Post-Processing Solution (14:32:30)
Checking Convergence (14:32:30)
Calculating Disp and Stress Results (14:32:30)

RMS Stress Error Estimates:

Load Set Stress Error % of Max Prin Str
---------------- ------------ -----------------
MyLoadSet 3.20e+03 1.3% of 2.37e+05

Resource Check (14:32:30)
Elapsed Time (sec): 0.95
CPU Time (sec): 0.58
Memory Usage (kb): 182260
Wrk Dir Dsk Usage (kb): 0

Total Mass of Model: 9.791206e+00

Total Cost of Model: 0.000000e+00

Mass Moments of Inertia about WCS Origin:

Ixx: 3.26382e+04
Ixy: -7.11237e-17 Iyy: 3.26382e+04
Ixz: -6.21725e-15 Iyz: 7.10543e-15 Izz: 1.63187e+00

Principal MMOI and Principal Axes Relative to WCS Origin:

Max Prin Mid Prin Min Prin
3.26382e+04 3.26382e+04 1.63187e+00

WCS X: 0.00000e+00 1.00000e+00 0.00000e+00
WCS Y: 1.00000e+00 0.00000e+00 0.00000e+00
WCS Z: 0.00000e+00 0.00000e+00 1.00000e+00

Center of Mass Location Relative to WCS Origin:
(-3.54343e-18, -2.26780e-17, 5.00000e+01)

Mass Moments of Inertia about the Center of Mass:

Ixx: 8.16015e+03
Ixy: -7.11237e-17 Iyy: 8.16015e+03
Ixz: -7.95197e-15 Iyz: -3.99680e-15 Izz: 1.63187e+00

Principal MMOI and Principal Axes Relative to COM:

Max Prin Mid Prin Min Prin
8.16015e+03 8.16015e+03 1.63187e+00

WCS X: 0.00000e+00 1.00000e+00 0.00000e+00
WCS Y: 1.00000e+00 0.00000e+00 0.00000e+00
WCS Z: 0.00000e+00 0.00000e+00 1.00000e+00

Constraint Set: MyConstraintSet

Load Set: MyLoadSet

Resultant Load on Model:
in global X direction: -7.083754e-06
in global Y direction: -3.000000e+02
in global Z direction: -7.049525e-09

Measures:

max_beam_bending: 0.000000e+00
max_beam_tensile: 0.000000e+00
max_beam_torsion: 0.000000e+00
max_beam_total: 0.000000e+00
max_disp_mag: 3.028639e-01
max_disp_x: 2.798528e-05
max_disp_y: -3.028552e-01
max_disp_z: 2.292079e-03
max_prin_mag: 2.371181e+05
max_rot_mag: 0.000000e+00
max_rot_x: 0.000000e+00
max_rot_y: 0.000000e+00
max_rot_z: 0.000000e+00
max_stress_prin: 2.371181e+05
max_stress_vm: 1.650229e+05
max_stress_xx: 1.098862e+05
max_stress_xy: -7.696971e+03
max_stress_xz: -2.665295e+03
max_stress_yy: 1.045842e+05
max_stress_yz: 2.954562e+03
max_stress_zz: 2.370292e+05
min_stress_prin: -2.214972e+05
strain_energy: 4.542822e+01

Analysis "static_mpa" Completed (14:32:30)

Buckling Analysis "MyAnalysis":

Convergence Method: Single-Pass Adaptive
Plotting Grid: 4

Convergence Loop Log: (14:32:30)

>> Pass 1 <<
Calculating Element Equations (14:32:30)
Total Number of Equations: 288
Maximum Edge Order: 3
Solving Equations (14:32:30)
Post-Processing Solution (14:32:30)
Checking Convergence (14:32:30)
Resource Check (14:32:30)
Elapsed Time (sec): 1.25
CPU Time (sec): 0.73
Memory Usage (kb): 187316
Wrk Dir Dsk Usage (kb): 0

>> Pass 2 <<
Calculating Element Equations (14:32:30)
Total Number of Equations: 381
Maximum Edge Order: 4
Solving Equations (14:32:30)
Post-Processing Solution (14:32:30)
Checking Convergence (14:32:30)
Calculating Disp and Stress Results (14:32:31)

RMS Stress Error Estimates:

Mode Stress Error (% of Max Modal Stress)
---- --------------
1 19.6%

Resource Check (14:32:31)
Elapsed Time (sec): 1.49
CPU Time (sec): 0.86
Memory Usage (kb): 188404
Wrk Dir Dsk Usage (kb): 0

Total Mass of Model: 9.791206e+00

Total Cost of Model: 0.000000e+00

Mass Moments of Inertia about WCS Origin:

Ixx: 3.26382e+04
Ixy: -7.11237e-17 Iyy: 3.26382e+04
Ixz: -6.21725e-15 Iyz: 7.10543e-15 Izz: 1.63187e+00

Principal MMOI and Principal Axes Relative to WCS Origin:

Max Prin Mid Prin Min Prin
3.26382e+04 3.26382e+04 1.63187e+00

WCS X: 0.00000e+00 1.00000e+00 0.00000e+00
WCS Y: 1.00000e+00 0.00000e+00 0.00000e+00
WCS Z: 0.00000e+00 0.00000e+00 1.00000e+00

Center of Mass Location Relative to WCS Origin:
(-3.54343e-18, -2.26780e-17, 5.00000e+01)

Mass Moments of Inertia about the Center of Mass:

Ixx: 8.16015e+03
Ixy: -7.11237e-17 Iyy: 8.16015e+03
Ixz: -7.95197e-15 Iyz: -3.99680e-15 Izz: 1.63187e+00

Principal MMOI and Principal Axes Relative to COM:

Max Prin Mid Prin Min Prin
8.16015e+03 8.16015e+03 1.63187e+00

WCS X: 0.00000e+00 1.00000e+00 0.00000e+00
WCS Y: 1.00000e+00 0.00000e+00 0.00000e+00
WCS Z: 0.00000e+00 0.00000e+00 1.00000e+00

Previous Static Analysis: static_mpa
Load Set: MyLoadSet

Constraint Set: MyConstraintSet

Number of Modes: 1

Mode B. L. F.
---- ------------
1 1.151762e+02

Analysis "MyAnalysis" Completed (14:32:31)

------------------------------------------------------------

Memory and Disk Usage:

Machine Type: Windows NT/x86
RAM Allocation for Solver (megabytes): 128.0

Total Elapsed Time (seconds): 1.54
Total CPU Time (seconds): 0.89
Maximum Memory Usage (kilobytes): 188404
Working Directory Disk Usage (kilobytes): 0

Results Directory Size (kilobytes):
460 .\MyAnalysis

------------------------------------------------------------
Run Completed
Fri Mar 20, 2009 14:32:31
------------------------------------------------------------

 

[Twoballcane]

Hi rentien

Well first how are you doing your hand calculations for stress? Are you calculating the Von Mises stress, because this will take in consideration of all of the principal stress and max shear and this is what you see in your plot?



Tobalcane
"If you avoid failure, you also avoid success."

 

[Twoballcane]

Oh, also you should do your ProM in units of Inch Pound Seconds instead of Inch Mass Secounds, makes life easier.

Tobalcane
"If you avoid failure, you also avoid success."

 

[mohankrishna87]

Hi.. I am trying to do an optimization study for a motorbike frame. I want to know the various load's that are acting on the frame.

Thanks
Mohan

 

[Twoballcane]

Hi Mohan,

Are you using ProM? And it would be a good idea to start your own thread on this topic.

Tobalcane
"If you avoid failure, you also avoid success."