  The long-ranged elastic model, which is believed to describe the evolution of
a self-affine rough crack-front, is analyzed to linear and non-linear orders.
It is shown that the nonlinear terms, while important in changing the front
dynamics, are not changing the scaling exponent which characterizes the
roughness of the front. The scaling exponent thus predicted by the model is
much smaller than the one observed experimentally. The inevitable conclusion is
that the gap between the results of experiments and the model that is supposed
to describe them is too large, and some new physics has to be invoked for
another model.
