  We study the evolution of the longitudinal release wave that is generated by
induced tensile fracture as it propagates through solid rectangular
Polymethylmethacrylate (PMMA) bars of different constant cross section. High
speed multi-point photoelasticity is used to register the strain wave. In all
cases, oscillations develop at the bottom of the release wave that exhibit the
qualitative features of an undular bore. The pre-strain, post-strain, strain
rate of the release wave and the cross section dimensions determine the
evolution of the oscillations. From the wave speed and strain rate close to the
fracture site, we estimate the strain rate of the release wave as well as the
growth of the amplitude and duration of the leading oscillation away from the
fracture site on using formulae derived from the simple analytical solution [1]
of the linearised Gardner equation (linearised near the pre-strain level at
fracture), developed in our earlier work . Our estimates are then compared to
experimental data, where qualitative and good semi-quantitative agreements are
established.
