A Single Shot Shearography Device for Simultaneous Measurement in Three Shearing Directions

Shearography is a quite robust optical measurement technique frequently used for in-field testing. Defect inspections with shearography on fiber-reinforced plastics parts, widely used in aircrafts, ships, chemical and oil and gas industries, are often performed outside of the lab. However, environmental disturbances can make this task difficult or even impossible. Mechanical vibrations, air and thermal instabilities are among the most disturbing agents in harsh environments that can destroy the interference signal or, at least, dramatically reduce the interference signal quality, what raises the measurement uncertainty to unacceptable levels. For a successful testing, it is crucial to neutralize the influence of those agents. Compactness, high stiffness, robust mechanical design and an effective clamping system are very important design considerations to minimize the influence of those agents. However, the most effective solution is using a single-shot measurement technique; namely, acquiring a single image instead of a series of images. The exposure time is kept short enough to "freeze" any relative motion between the parts or drift of the measurement signal. This paper presents, describes and explores two configurations of multiple aperture single-shot shearography used by the authors for testing fiber-reinforced plastics used in the oil and gas industry outside the lab. The first one is a two-aperture setup variation based in an already existing configuration that produces carrier fringes on the speckle pattern and allows fringe processing in the 2D Fourier plane. The second one is a new three-aperture configuration that allows acquiring three fringe patterns simultaneously form three different shearing directions for each given loading state. A set of three wedges are used in combination with the three apertures to produce simultaneous shearing interferograms in three different orientations. One single shot image is acquired using a single high-resolution camera before a loading is applied to the specimen and another single shot image for each different loading levels. Since the resulting fringe patterns have carrier fringes in different orientations they are easily separated in the Fourier plane. Once the device has no movable parts and a single shot image is acquired, it is very compact, robust and can be successfully used outside of the optical bench. The paper presents applications of both devices. They have a great potential to expand the use of shearography for testing in harsh environments.