Fundamentals of Shearography Metrology

Abstract

The demanding requirements of product quality and reliability have led to the need for a highly efficient nondestructive testing (NDT) method that is real time, full field, and noncontact based.1 Optical methods such as thermography, holography, electronic speckle pattern interferometry (ESPI), and shearography (also called speckle pattern shearing interferometry) are emerging as strong candidates for new industrial NDT tools because of their full-field, noncontacting, and noncontaminating properties. Of the optical techniques identified, ESPI is well known for small deformation and strain measurements. ESPI is capable of providing a full-field deformation measurement of a test object in multiple dimensions with extremely high measurement sensitivity. However, ESPI has strict requirements for the measurement environment, such as vibration isolation and temperature control. These restrictions limit ESPI’s applications to laboratory conditions. To meet the industrial requirements in field applications, a more robust and easily applied optical method is needed. This is where shearographic technology comes into play. Shearography is a coherent optical measurement and test method that is similar to ESPI, but displays many advantages for industrial applications due to its relative insensitivity to environmental disturbances. Shearography measures the gradient of the deformation, not the deformation itself, as ESPI does. Consequently, shearography measures strain information directly. This ability has resulted in shearography becoming increasingly important as an NDT tool in industry.