Electronic Speckle Pattern Interferometry: A Powerful On Line Optoelectronic NDE Tool for Aerospace Components

The recently evolved optoelectronic technique namely Electronic Speckle Pattern Interferometry (ESPI) is a fast developing whole field optical technique widely used for measuring displacement components, their derivatives, surface roughness, shape, surface contours, etc. Due to the non-contact nature and high sensitivity, this technique has been used as a powerful on line inspection tool for Non-Destructive Evaluation (NDE) of materials in industrial environment. The salient feature of ESPI is its capability to display the correlation fringes in real-time on a monitor without the need of photographic processing or optical filtering. In ESPI, the speckle interferometry data of the object under different loading conditions are stored as digital images and then electronically processed to produce correlation fringes which are contours of constant phase changes corresponding to the object movement. The anomaly in the interferogram indicates change in surface displacement which in turn relates to the strain variation due to localized defects in the specimen. These anomalies are interpreted in terms of the size and type of the defects in the object. The present paper deals with experimental and theoretical studies conducted on ESPI for NDE of low modulus materials used for aerospace applications. The Speckle Non-Destructive Testing (SNDT) using mechanical and thermal loading techniques for low modulus materials has been evolved as a comprehensive practical methodology. The theoretical basis and the computer programming for the analysis of interferometry fringe pattern have been developed. They are effectively utilized for the defect detection from the ESPI fringe pattern of the test object. The present experimental and theoretical data have been generated for some selected low modulus materials of high damping characteristics, used as thermal insulator in solid rocket motors.

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