In-plane displacements in front of notched and precracked concrete specimen were monitored using single laser beam speckle interferametry. Exposures of specimen surface at unstressed and stressed states were recorded on the same film plate to generate speckle correlation fringe patterns. The presence and absence of fringe patterns at several locations in front of the running crack revealed the extent of microcracking zone.
"Application Of Laser Speckle Interferometry To Fracture Of Concrete", Proc. SPIE 0540, Southwest Conf on Optics '85, (20 November 1985); doi: 10.1117/12.976159; https://doi.org/10.1117/12.976159