Low-Cost Non-Destructive Inspection by Simplified Digital Speckle Interferometry

The paper demonstrates a low-cost approach based on speckle interferometry (SI) for the nondestructive inspection (NDI) of metallic and composite plates. Speckle interferometry, and optical methods in general, provide a clear picture of the inspected area, where anomalies are immediately detected, and can yield results in real-time. They are often expensive though, and sensitive to environmental disturbances. Relevant literature in the area of nondestructive inspection is reviewed aiming to find the most suitable configuration for a low-cost system. The chosen setup is a simplified digital speckle interferometer (SDSI), which includes only three main components: a He-Ne laser, a standard CCD camera and a light-scattering window. This simplified configuration removes the requirement of mechanical stability by using a configuration close to the common-path interferometer. The SDSI device is here tested for the detection of thinning defects in Aluminum plates, and delamination in composite plates. Among the various stressing techniques, localized heating was found to be the simplest, yet most effective approach. Results show that the presence of defects creates unique features in the live correlation fringes, from which it is possible to identify the location and size of the defects. It is in conclusion demonstrated that the SDSI device is a valuable tool for fast, low-cost, and non-contact nondestructive inspection.