Quantitative verification of laser-modulated phase-stepping digital shearography for monofrequency vibration measurement

An offspring of holographic and shearing interferometry, digital shearography is a full-field, optical technique. Its full-field nature allows examination of approximately a one square foot area, requiring no scanning. In its usual implementation for vibration studies, time-averaged speckle correlation fringes are formed. While sufficient for observing mode shapes, these fringe patterns carry several disadvantages when quantitative interpretation is attempted. Digital shearography, combined with stroboscopic laser illumination and optical phase stepping, overcomes these limitations. The differential displacement field during monofrequency vibration is calculated independently at each point, and is no longer dependent upon fringe interpretation. The system, a new combination of several well-established techniques, has not been validated in either the optics or acoustics literature. The goal of the current work is to verify the technique via side-by-side measurements made with shearography and laser vibrometry. A simple cantilever beam is vibrated at several resonance and antiresonance frequencies and examined by both methods. Analysis techniques for comparing velocity output of the vibrometer to differential displacement output from shearography will be discussed. Agreement between the two systems is excellent, demonstrating the potential usefulness of this new method. [Work supported by the Penn State University Applied Research Lab.]