Shearography

Shearography ( alternative spellings: . Shearography or English Shearography ) is the short name for the laser speckle shearing interferometry, a coherent optical method based on the laser speckle technique. An optically rough surface coherently illuminated, as in the diagram on the image plane of a granular structure becomes visible, which is referred to as a speckle pattern. The shearography can be used for non-destructive material and component testing (NDT ), the stress and strain measurement and for the qualitative or quantitative vibration measurements in industrial conditions.

Classification of shearografischen measurement method

Operation of digital shearography

The figure below shows the structure and the functioning of simple digital shearography. With an expanded laser beam, the measurement object is illuminated. The surface of the object reflects the laser beam which is incident on the image plane by a digital camera. Shearelement caused by two slightly shifted images of the object in the image plane, whereby an interferogram is created.

Correlating now recorded in two different load states of the object to be measured interferograms, the result is a distinctive fringe pattern, which is referred to as Scherogramm. The correlation in the simplest case of a point-wise subtraction of the gray values ​​of the individual images and subsequent formation amount. The analysis of the Scherogrammes allows conclusions on material inhomogeneities below the component surface, as material defects lead to an inhomogeneous surface deformation.

The strips in Scherogramm can be interpreted as contour lines of the deformation gradients. This differs from the holographic interferometry, the shearography (HI) and of the electronic speckle interferometry ( ESPI ). The HI and ESPI method, the interference pattern represents the deformation itself Holographic interferometry is in the article holography explained.

In contrast to HI and other interferometric techniques Shearography is relatively insensitive to vibration, since the two interfering waves are passed over part of the measured object. Therefore, changes in the optical paths through shocks compensate to a large extent. Therefore, the Shearography is also used in " harsh " environments.

The wedge angle of Shearelementes determines the size of the shear. As Shearelement, an optical wedge or a biprism be used. Alternatively, an optical structure can be used based on a Michelson interferometer. In this case the shearing is achieved by slightly tilting an interferometer mirror. This has the advantage that the size of the shear can be adjusted continuously.

Schematic design of simple digital shearography

Industrial applications

Shearography is temperature and / or vacuum load in the industry, especially in aviation for non-destructive testing of Compositbauteilen used in compact devices for mobile inspection with combined. Here, the application focus is primarily on the time saving and end user-friendly examination of changing or the same tasks.

Swell

  • Thomas Circle: Holographic Interferometry. Principles and Methods. Akademie-Verlag, Berlin 1996, ISBN 3-05-501644-0 ( Akademie Verlag Series in Optical Metrology 1).
  • PK Rastogi (Ed.): Holographic Interferometry. Springer, Berlin et al, 1994, ISBN 3-540-57354-2 ( Springer Series in Optical Sciences 68).
  • Wolfgang stones, Lianxiang Yang: Digital shearography. Theory and application of digital speckle pattern shearing interferometry. SPIE Press, Bellingham WA 2003, ISBN 0-8194-4110-4.
  • U. Schnars, W. Jüptner: Digital Holography. Springer 2005, ISBN 978-3-540-21934-7. springer.com
726415
de