  We derive an analytical expression for the strain field, including fracture
profile, during steady-state crack propagation in viscoelastic solids. This
expression, featuring the hypergeometric function, is obtained by exactly
solving a model that incorporates the standard linear solid (i.e.,
three-element Zener) model. We find three regions in the fracture profile,
distinguished by power-law exponents that evolve with distance from the crack
tip; this explains why crack tips become sharper in rubbers and gels as the
crack-propagation velocity increases. We also provide a solid foundation for de
Gennes' viscoelastic trumpet, previously based only on a scaling argument.
These insights should help prevent catastrophic failure of polymer materials
triggered by velocity jumps.
