  To clarify effects of crack speed and cross-link density on fracture energy
of acrylamide gels, we evaluated the roughness of the fracture surface and
measured the fracture energy taking into account the roughness. The fracture
energy increases linearly with crack speed $V$ in a fast crack speed region,
and the increasing rate of fracture energy with $V$ decreases with increasing
cross link density in the gels. In a slow crack speed region the fracture
energy depends on crack speed more strongly than in the fast crack speed
region. This indicates that a qualitative change exists in fracture process of
the gels.
