[CIG-LONG] factor in MohrCoulomb

[Walter Landry]

js at cp.dias.ie wrote:
> Hi Walter,
> why is there an additional factor term in _MohrCoulomb_GetYieldCriterion?
> ie
> theta = 0.5 * atan( 1.0 / effectiveFrictionCoefficient);
> 
> factor = sin( 2 * theta );
> 
> frictionalStrength = effectiveFrictionCoefficient * pressure * factor +
> effectiveCohesion * factor;

The factor arises because the normal and tangential stresses are not
exactly the pressure and strain rate invariant, but linear
combinations of them that depend on the friction angle.

> and don't 2 and 0.5 cancel?

Yes, they do.  Theta is a number that comes out naturally during the
derivation.  I will be fully documenting it this week.

Cheers,
Walter Landry
walter at geodynamics.org