Shearography results show a disbond in a panel caused by a lightning strike.  The honeycomb core is just visible in the background, while the positive and negative slope of the defect result is displayed as a double lobed pattern.  The outer periphery of both lobes of the indication is the actual size, shape and location of the defect.

Theory
Shearography works with coherent laser illumination of the object.  The light from the object is split or sheared into two laterally displaced images, typically of about 1/4 of an inch separation between the images.  The two slightly displaced images are then brought together, into interference, onto a CCD video camera.  During a test, an initial image is stored and the part is stressed to create a strain concentration at the site of the defect.  This result can be displayed in real-time with sub-micron displacement sensitivity.  In a typical test of a honeycomb panel for a skin-to-core disbond, the shearing camera is placed on a tripod or other fixture to view the area of interest.  The area is then excited with a light stress such as slight vacuum, heating or vibration excitation.  A defect area has a reduction in stiffness, causing a slight buckling due to applied load.

Applications
Typical applications are for rapid, wide area, bond-line NDI (disbonds, delaminations, crushed core), from production to field support.  Entire vehicles can be inspected quite rapidly with this method, typically at rates of 100 sq.ft/hr.  Shearography is an ideal composite repair evaluation tool, as it not effected by the non-homogeneity of the repaired structure.  It also is able to detect and easily differentiate between crushed core and disbonding in most cored materials. Kissing disbonds are a particularly difficult problem with composite structures and older NDT techniques, such as ultrasonics and thermography.  Shearography actually applies real stresses to the bondline, and measures the material carrying the load.  "Kissing" disbonds are readily detected.