UBC Theses and Dissertations
Improvements in electronic speckle pattern interferometry for residual stress measurements Cavusoglu, Mehmet Cagdas
Electronic Speckle Pattern Interferometry (ESPI) is an optical technique used for measuring surface displacements in the order of a wavelength of light by comparing interference patterns taken before and after surface deformation. Residual stress measurement is one of the applications where ESPI is useful. The technique is attractive because it provides very detailed information on deformation field and low per-measurement cost. However, ESPI data possess high noise content due to its high sensitivity to disturbances. In this research, factors that affect the quality of ESPI data were studied. The most important ones were found to be the specimen surface quality, illumination level and speckle size. The image quality was greatly improved by surface preparation. Good and faulty data were separated by evaluating their modulation level and identifying the saturated pixels. Mathematical methods were proposed to improve the data quality by either replacing the faulty data with good data or smoothing the data by filtering. Two common-path arrangements with single and double mirrors, which provide in-plane sensitive measurements, were designed to eliminate the separate and delicate optical paths. They improved the stability of ESPI measurements and greatly reduced the pixel drift that was a problem in the existing arrangement. The double mirror arrangement provided the measurement of full stress field. The single mirror method was confirmed by stress measurement, whereas the double mirror method could not be validated due to low data quality. Recommendations were made for an enhanced future design of this method.
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