How to constrain the surface to be traction free?

[louisa]

Hi Everyone,

I want to specify certain stress components to be zero at some surfaces. So I tried Loads -> Surface traction -> Magnitude=0 in the Edit Load dialog box. I got the error message "Load must be created with a non-zero magnitudes unless utilizing a user subroutine".

Is there any easy way to make certain stress components at the surface remain zero throughout the anaysis? Thanks!

 

[xerf]

A surface is traction free as long as you do not apply external tractions on it.

If you check the stress components, with respect to the surface, (extrapolated) at the nodes they should be very close to zero if the material is elastic.

If the material undergoes permanent deformation near the surface then these stresses might have a small value (but higher than for the elastic material) in general due to high stress gradient in the elements near the traction free surface.

 

[louisa]

Thank you, xerf.

This is a composite material and I applied uniform deformation boundary condition. Therefore unwanted stress components develop at the surface due to the underlying heterogeneous structure. I want to constrain these unwanted stress components to be zero so that I can use Hook's law to calculate average elastic properties of the composite.

I want to specify a certain stress component to be zero all the time during the analysis. But I don't know how to do this.

 

[johnhors]

In a 3D model at a free surface with no applied loads or supports the only zero stress component is the one normal to the surface. Since static FE analysis is basically the solution of F=k.x , the user can only have control of the force applied (F) and displacement (x).

 

[xerf]

if a surface is traction free and one decompose the zero traction vector into the normal component and the shear component , and the normal component is the only zero component then....
....how about the shear component on the surface ?

 

[johnhors]

3D solid elements in general produce 3 principal stresses at their nodes. At a free surface one of these principal stresses is zero (or at least very close to zero compared to the other two) in the direction normal to the surface. The system of stresses then simplifies to a 2D stress system. Depending on the geometry and the nature of the loading one of the in plane principal stresses may also be zero.