NDE with Lockin-Interferometry: Principle and Applications

Abstract

Speckle-interferometric imaging is a well known method where the­difference between two deformation states of an object (with and without applied load) is displayed as a fringe pattern. The standard procedure for finding hidden defects is to compare the observed fringe pattern either to the pattern that one expects or to one measured on a reference component. Observed differences are then attributed to a defect. We show how the method can be improved in terms of signal to noise ratio and resolution.

The idea is to apply the modulation principle to speckle-interferometry that makes lockin-thermography superior to conventional thermography: The inspected object is periodically illuminated to induce a thermal wave and a corresponding modulation of thermal expansion while fringe images are recorded with electronic speckle pattern interferometry (ESPI) or shearography. Then, the time-dependent content of each pixel is Fourier transformed at the excitation frequency. This way the information content is compressed into local amplitude and phase of the response at this frequency. In the phase image, defects reveal themselves as deviations from a constant background and can therefore be identified easily. The signal to noise ratio is improved by up to an order of magnitude. Depth range can be adjusted by variation of the modulation frequency. This paper shows results of model samples as well as of automotive and aerospace parts.