Abstract:

Digital shearography is a full-field speckle interferometric technique similar to electronic holography. Traditionally, shearography and holography have been used for vibration measurement with either time averaged or stroboscopic techniques. Time averaging images sinusoidal motion of an object's surface over many periods of vibration, resulting in a fringe pattern representative of the magnitude of the displacement mode shape. Stroboscopic illumination synchronizes short bursts, or strokes, of laser light with extrema of vibration. This essentially freezes motion of the vibrating surface, allowing it to be studied with techniques normally reserved for static deformation. In particular, phase stepping, an established technique for capturing a series of images and calculating the exact displacement at every point, can be applied to vibration measurement. If displacement amplitudes calculated from laser strobing and phase stepping are to be used for measurement of peak vibration response, corrections are necessary to compensate for errors due to the finite duration of the laser strokes as well as their improper timing due to system impedance. In this presentation, effects of stroke duration will be expressed mathematically and trends demonstrated experimentally. Potential solutions for stroke offset correction will be discussed and experimental validation provided. [Work sponsored by the PSU Applied Research Laboratory.]