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The application of pulsed laser Electronic Speckle Pattern Interferometry to the measurement of in-plane strain on high speed rotating components

Swain, R. C. (1993). The application of pulsed laser Electronic Speckle Pattern Interferometry to the measurement of in-plane strain on high speed rotating components. (Unpublished Doctoral thesis, City, University of London)


Increasingly sophisticated rotating system designs demand a need for more advanced experimental measurement systems, providing highly accurate data over a large number of measurement points. Described within is an interferometric measurement system based upon a speckle correlation technique, referred to in the literature as Electronic Speckle Pattern Interferometry (ESPI), TV holography or Digital Speckle Pattern Interferometry (DSPI). The interferometer is configured to measure radial in-plane displacement. A pulsed laser, necessary because of the rotary motion of the specimen, provides the illumination source. The output from the ESPI system is a fringe pattern, which must be further processed to yield useful data. The work is a continuation of an existing project, and focuses on three main aspects:

• To increase the operable speed range and reliability of the system.
• To extract quantitative engineering data from the fringe patterns.
• To develop the existing design into a 'user-friendly' system, capable of transfer into an industrial environment.

A set of two-dimensional Fourier transform programs, running on an 80486 computer, enable accurate phase data to be extracted from the fringes, delivering quantitative measurements of radial strain to subfringe accuracy.

Major advancements include the construction of a new electronic system to control the pulsed laser, enabling interferograms to be obtained at rotational speeds up to 23,842rpm, with a tip speed increase of 2.75 times any previous result (from 132m s'1 to 364m s"1). The new control unit, unlike its predecessor, is designed to be immune to the electromagnetic interference generated by the laser, and allows speckle patterns to be obtained at any rotational speed, up to 24,000rpm, which is the maximum speed of the drive motor. An innovative shaft encoder was also incorporated into the system, producing highly reliable timing data from the rotating specimen. Efforts to increase the reliability of the measurements concentrated on the vibration and windage generated by the disc. A significant increase in the repeatability at all speeds up to approximately 14,000rpm is reported. Above this speed, the vibrations are too severe to enable repeatable results to be obtained. However, high contrast fringes are still obtained, establishing that speckle correlation occurs, and is likely to at even higher speeds.

Results from static specimens indicate that the computer software functions effectively, showing close correlation with measured strains. When used with rotating specimens, the system yields results which are generally higher than the established theory. Investigation into this phenomenon suggests that windage heating of the disc surface induces additional stresses in the specimen, as shown in a series of windage tests. Tests conducted under conditions which minimise the windage heating have good agreement with the analytical theory.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TJ Mechanical engineering and machinery
Departments: School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics
School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
[thumbnail of Swain thesis 1993 PDF-A.pdf]
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