Study of impact excited elastic waves in concrete by scanning laser vibrometry

Laser vibrometer based contactless measurements of elastic wave fields at the surface of a concrete block excited by a mechanical impact are presented. Numerical simulations of the same situation provide wavefront snapshots, which show very good agreement in nearly all of the essential features of the measured wave modes. The simulation results are displayed additionally along cross sections through the volume which helps to clarify the origin of displacement fields at the surfaces. Only shear and Rayleigh wave modes could be identified both in experiment and simulation. Obviously the longitudinal wave modes are too weak to be visible in the present snapshot images with the intensity scales used. This dominance of shear and Rayleigh waves is one of the reasons for a great deal of difficulties the impact echo method suffers from.