New possibilities and applications of Lockin-Speckle-Interferometry for non-destructive testing of polymers

Techniques of optical interferometry (i.e. Shearography or Electronic-Speckle-Pattern-Interferometry-ESPI) are commonly used non-contact methods to measure the deformation of objects. Due to their sensitivity these methods are suited for non-destructive detection of defects and areas of weakness in components. The challenge is to image these faults by applying appropriate techniques of excitation and evaluation. The result of a measurement is typically a fringe pattern (similar to the elevation profile of a hiking map) that has to be interpreted by the user. Since the interpretation can be complex, the evaluation often has to be done by experts. This article demonstrates that the signal-to-noise ratio can be improved significantly by the combination of thermal waves and Lockin technique. Furthermore, one can determine the depth of defects because the depth range of thermal waves depends on their frequency For this purpose, the component under inspection is heated periodically while its deformation is monitored to give a stack of images. Subsequently, a Fourier transformation of this image stack is performed at the excitation frequency (Lock in-frequency), which compresses the information to an amplitude- and a phase image. The phase image can be interpreted easily since only defects contribute to a signal change. This defect selectivity is particularly relevant for applications. The performance of Lockin-Speckle-Interferometry is shown for model samples and for components made from polymers.