[CIG-SHORT] Normal Force and shear force on an inclined plane

[Brad Aagaard]

Qian,

In PyLith the shear and normal tractions on a fault are available via 
the output. Most other finite-element codes that model faults also 
include this feature. These tractions are calculated using the fault 
implementation, not from calculating the stresses within the volume 
elements. Alternatively, you can calculate the stress tensor on the face 
of a volume element and use the face normal to get a traction. However, 
in lower order finite-elements the stresses are not continuous across 
element boundaries (the displacement field is continuous but the strains 
and stresses are not).

In most studies that I am aware of the Mohr-Coulomb criterion is applied 
locally using the shear and normal tractions. This accounts for 
potential spatial variation of the tractions due to loading, 
heterogeneous material properties, etc that cause failure on a subset of 
the fault.

Regards,
Brad


On 10/21/2011 03:41 PM, Qian Gao wrote:
> Hi, Brad,
>
> I am sorry I did not present my question clear.
>
> I want to determine the total shear force and normal force on an inclined
> plane, not shear stress or normal stress at a single points or multiple
> points, because I want to use Mohr Coulomb criterion to evaluate the
> stability state of a fault (an inclined plane in the model).
>
> Based on finite element results, six stress components for each element are
> known. I think it is not correct to transfer the six stress components into
> shear and normal stress on an inclined fault plane, because the six stress
> component usually represent the stress state for a single element, not for
> one face of the element, which is a portion of the fault plane.
>
> I want to find a reliable way to calculate shear force and normal force on
> an inclined fault plane, based on finite element calculation.
>
> Thank you vary much!
>
> Best Regards!
> Qian Gao
>