Impact-induced flexural waves in a Timoshenko beam—Shearographic detection and analysis

The propagation of a flexural wave generated by transverse point impact on a beam between clamped supports is observed using pulsed-laser shearography—the rapid double exposure of holographic film to a laser-illuminated object at different instants of its deformation. Sequential images are captured via a camera which has a thin, wedge-shaped prism placed in front of half its lens. This prism produces another image which is sheared in the direction of the wedge, with a net result that two images—sheared with respect to each other—are recorded by the camera. When the structure is deformed and a second exposure is superimposed on the first, a fringe pattern is produced; this becomes visible when observed via high-pass optical Fourier filtering. The fringes represent loci of constant out-of-plane displacement derivatives with respect to the direction of image shear. These are integrated to yield the instantaneous out-of-plane displacement induced by the flexural wave. Adjustment of the delay between the first and second laser pulses produces sequential displacement data on the propagating wave. This is compared with theoretical results based on Timoshenko beam analysis employing the method of characteristics to compute displacement values. The experimental data compare favorably with theoretical predictions, substantiating the use of sheariographic techniques for stress-wave studies.