If the support is drawn over the critical curve the tip reaches the stability frontier
and does an irreversible jump. For two dimensions the stability frontier equals
the critical curve. In the picture you see the critical curves for an isotropic elasticity matrix.
For a hard bond it consists of a set of stars with four teeth which lay on the maxima of
the potentials (top of the hills). Grid lines which cross the stars are coupled with friciton, grid
lines between the stars are not (a).
For soft bonds the stars grow and finally cross the grid lines. Now there are no more grid
lines without friction (b).