## General Mechanical

#### Three point bending displacement control (false)

• Oelot
Subscriber

Hi, I need to compute and study a three point bending under displacement control

So I modelise it like this :

it is a simple beam ( box ) of 100mm length and the cross section 16*16 mm

I fixed one cross section with fixed support and the other one (the one with symetry) I put the displacement control :

but my problem is I had too much stress :

Thanks you

• peteroznewman
Subscriber

1) You have not created a Symmetry Region, but you can create the effect with the displacement boundary condition on the face at the center of the full length of the beam.  That would be to set X = 0, not Free.  I assume the beam lies along the X axis.

2) You have not created a roller support at the end of the beam, but a cantilever support by using a Fixed Support boundary condition.  Delete the Fixed Support and instead select that face and create a Remote Displacement.  Set Y = 0,  Z = 0 and Rotation X = 0. This assumes you have created a beam geometry that is 100 mm long for the half length.

3) Optionally, you could slice the beam in half again, and have an 8 mm wide beam, and apply a Z = 0 Displacement boundary condition on that center face to represent the symmetry plane.

• Oelot
Subscriber

Thanks you, I just did what you told ( except the 3. ), I have still too much stresses

• peteroznewman
Subscriber

To get accurate stress results, you should have more than 2 elements through the thickness top to bottom.
Try using 4 to 10 elements.

What stress did you want?
You can reduce the stress by reducing the Young's Modulus of the material.

• Oelot
Subscriber

I want to see if with a displacement of 30 mm i will reach the yield strength or not, the yield stregth is about 420 Mpa ...

• peteroznewman
Subscriber

You have your answer, the maximum stress is > 420 MPa, so this beam has exceeded yield strength at 30 mm of displacement.

• Oelot
Subscriber

Yes don't you think 16 000 Mpa is too huge ? at this stress I think the beam would have break long time before

I meshed with smaller element size, but I still have this tremendous stress

• peteroznewman
Subscriber