  When brittle hydrogels fail, several mechanisms conspire to alter the state
of stress near the tip of a crack, and it is challenging to identify which
mechanism is dominant. In the fracture of brittle solids, a sufficient
far-field stress results in the complete loss of structural strength as the
material `unzips' at the tip of a crack, where stresses are concentrated.
Direct studies of the so-called small-scale yielding zone, where deformation is
large, are sparing. Using hydrogels as a model brittle solid, we probe the
small-scale yielding region with a combination of microscopy methods that
resolve the kinematics of the deformation. A zone over which most of the energy
is dissipated through the loss of cohesion is identified in the immediate
surroundings of the crack tip. With direct measurements, we determine the scale
and structure of this zone, and identify how the specific loss mechanisms in
this hydrogel material might generalize for brittle material failure.
