An interferometric-type optical arrangement, as described in Figure 1, provides the display. Optical interference is possible when a luminous granular pattern called speckle can be seen by illuminating a surface using a laser beam. This granular pattern will change when the object distorts.
Average power lasers are used, between 50 and 200 milliwatts depending on the surface to be inspected in a single operation.
The images will be captured at various stages of loading the object inspected. Their combinations can visualise any area distorting differently from the rest of the surface because defective, as seen in image 3 in Figure 2. A phase offset analysis of resulting images may also be obtained by a more complex combination of images.
(Note, definition of the word speckle: Appearance of graininess, blemishes or speckling presented by an image and caused either by observing a target with irregularities at the scale of the wavelength using a coherent beam or by the propagation of the coherent beam in an atmosphere characterised by random variations in the refractive index.
Reference: CNES 1985, link (translation of the French definition): http://gdt.oqlf.gouv.qc.ca/ficheOqlf.aspx?Id_Fiche=8420619)
The full image taking process takes between 4 and 45 seconds respectively for detection of surface or deep defects.
The sensitivity level of shearography is directly linked to the type and level of stress that will be applied naturally during use or simulated. As an example, a 5°C increase in temperature can constitute sufficient stress level to detect delamination in a composite material.
2. Test method
Implementation is very easy. The optical system is included in a box called “shearing interferometric camera” and is positioned so that the area to be tested can be observed. This can be achieved by sending the image via successive mirrors, as for example in the tyre industry, where a single camera records the whole of the entire tyre cylinder in one operation. The operator simply has to adjust the focus and luminosity received by the detector.
Like any NDT method, a calibration part is necessary to check that the system is working properly and confirm that the sensitivity level is reached. It must include natural defects and not artificial defects as used for ultrasounds; even if they can be detected, they show far less distortion and are therefore not representative.
The operator will launch the loading process and image capture sequence predefined in the test procedure.
This technology is used to test all types of material, except for liquids or amorphous substances, and complex assemblies. Its only limitation to date is the quantification of defects in the thickness, such as the porosity level in a composite.
Defects that are flat, three-dimensional, parallel and perpendicular to the surface inspected can be detected. Depending on the loading method, the detachment of elements perpendicular to the surface inspected can also be localised, even if they are still in contact.
4. Advantages of the method
This method requires no contact with the surface to be tested. Defects can be found up to 50 cm in sandwich structures.
It is faster than traditional methods and can test very large complex structures, such as rocket components.
It can achieve a level of probability of detection and reliability at least as effective and even better than the traditional methods, as demonstrated by the SANDIA test centre for the aeronautical sector in the United States.
5. Related standard
ASTM E 2581– Standard Practice for Shearography of Polymer Matrix Composites, Sandwich Core Materials and Filament-Wound Pressure Vessels in Aerospace Applications
Text prepared by COFREND in conjunction with Marie-Anne de Smet (Airbus).