Development of Shearography and Laser Speckle Imaging Technique for Non-Destructive Testing and Measurement Applications

Abstract

The need for rapid, non-destructive measurement methods has driven the development of optical techniques like speckle imaging and speckle interferometry, which have been used for over four decades. Speckle imaging, particularly useful for surface roughness measurement, captures microstructural surface details. However, conventional methods like speckle contrast face limitations in measurement range due to its dependency on the wavelength of the laser used for object illumination. This thesis proposes new image processing techniques for wide range of surface roughness measurement of the machined samples based on objective speckle imaging and defocused speckle imaging systems. Shearography, known for real time testing with high sensitivity, encodes measurement information in the phase of the interferogram. The two-step phase-shifting interferometry has become quite popular for precise on-line phase measurement where low computational complexity with comparable accuracy is highly desirable. The proposed method evaluates the phase over a wide range of phase step. The high computational efficiency with comparable accuracy for varying single-to-noise ratio is reported for simulation and experimental fringe patterns. The last objective underlines the possibility of integrating the speckle imaging technique and interferometry for surface deformation measurement. An experimental study of a circular aluminum plate under mechanical loading demonstrates the effectiveness of speckle contrast techniques. The comparative analysis between the phase and the speckle activity map corresponding to the deformation is presented demonstrating the applicability of the speckle contrast imaging for deformation measurement.