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Deep hole residual stress measurements using Electronic Speckle Pattern Interferometry Alava, Raymond

Abstract

This research study presents the design, development and testing of a sensor which utilizes the techniques of Electronic Speckle Pattern Interferometry (ESPI) for making deep-hole residual stress measurements. ESPI is optical technique that can be used to determine deformation at the surface of an object by combining interference patterns before and after deformation occurs. Although the technique has previously been used for determining residual stresses close to the surface, it has not yet been applied to measuring stresses deep within the interior of a specimen. The deep-hole method determines interior residual stresses in a specimen by making measurement of a reference hole. Overcoring of the reference hole results in stress relief leading to deformations at the hole surface. Pre- and post-deformation measurements of the reference hole are then used to determine the original stress state. In order to make these measurements a deep hole imaging sensor has been designed. The design focuses light down a long tube to illuminate the reference hole. The hole is then imaged onto a CCD camera and the optical data provides the necessary displacement measurements. A least squares technique utilizing a series of basis functions allows explicit calculation of all six components of the stress tensor from the displacement data. The sensor was subjected to a series of tests to demonstrate its capability to make the desired optical measurements! The basic functionality of the sensor was verified through displacement tests where stable, distinctive ESPI fringe patterns were observed. However, the application of a known stress field produced large disturbances resulting in a de-correlation of the optical data. Based on the observed de-correlation, recommendations are provided for improving the rigidity and overall performance of the design.

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