Studies on Digital Shearography for Testing of Aircraft Composite Structures and Honeycomb-Based Specimen
This paper reviews shearography and its applications for testing of aircraft composite structures and honeycomb-based specimen. Shearography is a laser-based interferometry in conjunction with the digital imaging processing technique for full-field measurement of surface deformation. It reveals defects in an object by looking for defect-induced deformation anomalies. It does not require special vibration isolation, and with the development of a small and mobile measuring device (portable inspection system), it can be employed easily in field/factory environments.
Speckle-Shearing Interferometric Camera -- A Tool For Measurement Of Derivatives Of Surface-Displacement
A camera having a lens with two apertures and equipped with a shearing mechanism is employed to image a specimen which is illuminated by coherent light. With this arrangement, the speckle from one point on the surface is allowed to interfere with the speckle from a neighboring point so that a speckle-grid pattern may be detected in the image plane. By double-exposure or real-time techniques, the speckle-grid pattern corresponding to the deformed state of the specimen is added to the speckle-grid pattern corresponding to the undeformed state of the specimen. The resulting mechanical interference produces a moire-fringe pattern which depicts derivatives of surface-displacements. The camera will be referred to as "Speckle-shearing Interferometric Camera". Three applications of the speckle-shearing interferometric camera are described: (1) mea-surement of slopes of structural deflections, (2) in-plane strain measurement, (3) vibration analy-sis. Experimental demonstrations for each case of the applications are presented. Although the speckle-shearing interferometric camera is based on interferometric principles, it overcomes several of the limitations associated with holographic and speckle interferometries, namely: (1) the setup is simple and does not re-quire laborous alignment of optical components, (2) it does not require stringent mechanical and ambient atmospheric stabilities, (3) coherent requirements of light are greatly relaxed, (4) the sensitivity is greatly reduced, and (5) it does not require differentiation to obtain strains.
Shearography: A novel and practical approach for nondestructive inspection
This paper presents an optical technique referred to as shearography for nondestructive inspection. Shearography is an interferometric method which allows full-field observation of surface displacement derivatives. It reveals flaws in materials by looking for flaw-induced strain anomalies. Both surface and interior flaws can be detected. Because of its numerous advantages, shearography is more practical than holography. The simplicity and the alleviation of vibration isolation requirement make shearography a practical approach for nondestructive testing in field/plant environments.