  Griffith's energetic criterion, or `energy balance', has for a century formed
the basis for fracture mechanics; the energy flowing into a crack front is
precisely balanced by the dissipation (fracture energy) at the front. If the
crack front structure is not properly accounted for, energy balance will either
appear to fail or lead to unrealistic results. Here, we study the influence of
the secondary structure of low-speed crack propagation in hydrogels under
tensile loading conditions. We first show that these cracks are bistable;
either simple (cracks having no secondary structure) or faceted crack states
(formed by steps propagating along crack fronts) can be generated under
identical loading conditions. The selection of either crack state is determined
by the form of the initial `seed' crack; perfect seed cracks generate simple
cracks while a small local mode~III component generates crack fronts having
multiple steps. Step coarsening eventually leads to single steps that propagate
along crack fronts. As they evolve, steps locally change the instantaneous
structure and motion of the crack front, breaking transverse translational
invariance. In contrast to simple cracks, faceted cracks can, therefore, no
longer be considered as existing in a quasi-2D system. For both simple and
faceted cracks we simultaneously measure the energy flux and local dissipation
along these crack fronts over velocities, $v$, spanning $0<v<0.2c_R$ ($c_R$ is
the Rayleigh wave speed). We find that, in the presence of secondary structure
within the crack front, the implementation of energy balance must be
generalized for 3D systems; faceted cracks reveal energy balance, only when we
account for the local dynamic dissipation at each point along the crack front.
