  The morphology of fracture surfaces encodes the various complex damage and
fracture processes occurring at the microstructure scale that have lead to the
failure of a given heterogeneous material. Understanding how to decipher this
morphology is therefore of fundamental interest. This has been extensively
investigated over these two last decades. It has been established that 1D
profiles of these fracture surfaces exhibit properties of scaling invariance.
In this paper, we present deeper analysis and investigate the 2D scaling
properties of these fracture surfaces. We showed that the properties of scaling
invariance are anisotropic and evidenced the existence of two peculiar
directions on the post-mortem fracture surface caracterized by two different
scaling exponents: the direction of the crack growth and the direction of the
crack front. These two exponents were found to be universal, independent of the
crack growth velocity, in both silica glass and aluminum alloy, archetype of
brittle and ductile material respectively. Moreover, the 2D structure function
that fully characterizes the scaling properties of the fracture surface was
shown to take a peculiar form similar to the one predicted by some models
issued from out-of-equilibrium statistical physics. This suggest some promising
analogies between dynamic phase transition models and the stability of a crack
front pinned/unpinned by the heterogenities of the material.
