  A phase field model of a crack front propagating in a three dimensional
brittle material is used to study the fractographic patterns induced by the
branching instability. The numerical results of this model give rise to crack
surfaces that are similar to those obtained in various experimental situations.
Depending on applied loading configurations and initial conditions, we show
that the branching instability is either restricted to a portion of the crack
front or revealed through quasi two dimensional branches. For the former, the
crack front leaves on the main broken surface either aligned or disordered
parabolic marks. For the latter, fractography reveals the so called {\em
\'echelons} cracks showing that branching instability can also induce crack
front fragmentation.
