Shear interferometry

Shearography is an optical holography technique that monitors differential motion of a damaged part that is examined before and after a small stress is applied. Dual exposure holograms of the interference patterns from a laser-light are acquired before and after the load is applied. A speckled pattern is formed on the stressed surface which is observed by a video camera through an optical shearing device. A computer is used to identify differences in the two speckled pattern. The use of the shearing device gives additional stability from vibration of the part. Shearography has the advantage that it can be used in the field on large surfaces. It is more accurate for thinner laminates because of the need to stress the part. Its accuracy also diminishes the deeper the defect is within the part. It has particular advantages for detecting damage in large sandwich structures, which ultrasonic methods cannot penetrate.