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).