Application of ESPI for the NDE of low-modulus materials using mechanical loading

Abstract

Electronic Speckle Pattern Interferometry (ESPI) is a powerful sensitive optical whole field technique for non-destructive evaluation (NDE) as well as structural testing. It permits measurement of deformations in the micron and sub-micron range, which are caused by very small stress applied to the specimen under test. ESPI can be used for the detection of defects even in cryogenic conditions and corrosive environments - a feature that gives it potential for application for the inspection of space vehicle components. In this paper the application of ESPI for defect detection is explored and demonstrated on a low-modulus material which is used as an insulator in solid rocket motor. A comparative speckle interferometry technique with an optical configuration for out-of-plane displacement has been used in the real-time mode. ESPI fringe patterns are generated by digital subtraction of the recorded speckle patterns of the test specimen under different loading conditions. The studies are carried out on specimens with different types of defects under different stress conditions. The defects are revealed as anomalies in the otherwise regular fringe pattern. Fringe analysis techniques used to detect and interpret the fringe anomalies are also presented.