Original Date: 04/26/1999
Revision Date: 01/18/2007
Best Practice : Electronic Shearography
The spaceflight industry is rapidly changing and introducing stronger and lighter composites into vehicle designs. As a result, Marshall Space Flight Center’s (MSFC’s) inspection teams needed a more flexible and portable system for detecting defects beneath insulation, paint, and laminated composites. Electronic shearography is a video inspection method used to detect debonds or separations in a test specimen. With the Center’s state-of-the-art equipment, images can be collected in real time and viewed on a charge coupled device camera. In the past, traditional film photography was used to document and enhance material inspections. This method was time consuming and often involved cumbersome lasers and laborious manual processes.
In an electronic shearography system, a laser is passed through a beam expander which breaks up the beam and strikes the test specimen with divergent beams. The beams are then reflected back through a telephoto lens and gathered in a Michelson interferometer to provide variable image shear. The system records two images: (1) the reference image and (2) the sheared image. These images will interfere with one another, resulting in a recorded image of a laser speckle pattern indicative of the original test specimen’s surface slope. The system can also detect surface imperfections by recording the slope of the surface. If a defect exists below the surface, it will eventually distort the surface profile as the deformation reaches the surface.
Electronic shearography has proven to be an effective means of inspecting a variety of materials for defects in multiple environments. Since implementing this system, MSFC’s engineering staff can quickly adapt and integrate new inspection technologies. The system provides greater sensitivity, repeatable and uniform testing, and the ability to detect defects near inner surfaces of a closed-body structure such as fuel tanks. MSFC also uses electronic shearography to ensure product conformance and flight safety.
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