A vacuum shearography hood and data display unit is applied to a composite structure to check for defects. An indication of a possible defect appears as the ripple pattern on the bottom right of the data screen.
Shearography or Speckle pattern shearing interferometry is a measuring and testing method similar to holographic interferometry. It uses coherent light or coherentsoundwaves[1] to provide information about the quality of different materials in nondestructive testing, strain measurement, and vibration analysis. Shearography is extensively used in production and development in aerospace,[2] wind rotor blades, automotive, and materials research areas.[3] Advantages of shearography are the large area testing capabilities (up to 1 m2 per minute), non-contact properties, its relative insensitivity to environmental disturbances, and its good performance on honeycomb materials, which is a big challenge for traditional nondestructive testing methods.
Shearing function
When a surface area is illuminated with a highly coherentlaser light, a stochastical interference pattern is created. This interference pattern is called a speckle, and is projected on a rigid camera's CCD chip. Analogous with Electronic speckle pattern interferometry (ESPI), to obtain results from the speckle we need to compare it with a known reference light. Shearography uses the test object itself as the known reference; it shears the image so a double image is created. The superposition of the two images, a shear image, represents the surface of the test object at this unloaded state. This makes the method much less sensitive to external vibrations and noise. By applying a small load, the material will deform. A nonuniform material quality will generate a nonuniform movement of the surface of the test object. A new shearing image is recorded at the loaded state and is compared with the sheared image before load. If a flaw is present, it will be seen.[4]
Phase-shift technology
To increase the sensitivity of the measurement method, a real-time phase shift process is used in the sensor. This contains a stepping mirror that shifts the reference beam, which is then processed with a best fit-algorithm and presents the information in real time.
Applications
The main applications are in composite nondestructive testing, where typical flaws are:
Disbonds,
Delaminations,
Wrinkles,
Porosity,
Foreign objects, and
Impact damages.
Industries where Shearography is used are:
Aerospace,
Space,
Boats,
Wind power,
Automotive,
Tires, and
Art conservation.[5]
Inspection standards
The methodology of shearography is standardized by ASTM International:
ASTM E2581-07, "Standard Practice for Shearography on Polymer Matrix Composites, Sandwich Core Materials and Filament Wound Pressure Vessel’s in Aerospace Applications"
The following NDT personnel certification documents contain references to shearography:
