  The differential evolution indicators, recently introduced for imaging
mechanical evolution in highly scattering solids, is examined in a laboratory
setting with the focus on spatiotemporal tracking of an advancing damage zone
in an elastic specimen. To this end, a prismatic slab of charcoal granite is
quasi-statically fractured in the three-point-bending (3PB) configuration,
while ultrasonic shear waves are periodically generated in the sample at
certain time steps $t_\kappa$, $\kappa = \circ, 1, 2, ..., 4$. The interaction
of probing waves with the propagating damage give rise to transient velocity
responses measured on the plate's boundary by a 3D scanning laser Doppler
vibrometer. Thus obtained sensory data are then carefully processed to retrieve
the associated spectra of scattered displacement fields $v^\kappa$ at every
$t_\kappa$. On deploying consecutive pairs of multi-frequency data $(v^\kappa,
v^{\kappa+1})$, the differential indicators are computed exposing the progress
of 3PB-induced damage in the specimen. Verified with in-situ observations, each
indicator map successfully reconstructs (a) the support of newborn fractures,
and (b) the loci of discontinuities in the process zone that undergo
interfacial evolution in the designated timeframe $[t_\kappa \,\,\,
t_{\kappa+1}]$. Further, it is shown that the evolution indicators help better
understand the damage mechanism e.g., by shining light on the fragmented nature
of induced cracks and their coalescence. For completeness, data inversion via
reduced and partial-aperture data is investigated, including the one-sided
reconstruction.
