Non-destructive testing with double-pulsed laser ESPI

Abstract

Electronic Speckle Pattern Interferometry (ESPI) is an attractive method for non-destructive testing. There are two ESPI working modes: addition ESPI mode and subtraction ESPI mode. Overcoming the disadvantage of the subtraction ESPI mode, which strictly requires that there is no movement among the optical parts in an ESPI system, in the addition ESPI mode, the movement of a measured material is 'frozen' with a short-pulsed laser. As a result, the addition ESPI mode is believed to be feasible in adverse conditions where the subtraction ESPI mode is inapplicable. Based on the theory of double-pulsed laser ESPI, the interference fringe contrast and a simulation method are introduced in this paper. In the simulation, a compound material with a flaw is interrogated. Due to the fact that a result can only be obtained when the interference fringe passes through a special electronic filter, the filter applied to the experiment needs delicate fabrication. Its quality and central frequency influence the final interference fringe result. Therefore, a series of the filters are designed such as band-pass and high-pass filters, with a specific method applied. Subsequently, the result of the simulation for NDT with double-pulsed ESPI method is acquired and analysed.