Measurement Principles of Shearography

image of shearography overview

General Principle

The basic concept of Shearography is to examine the test surface of the specimen with a shearography camera. The camera records one interferometric photo of the surface. This photo can be thought of as a unique footprint of this surface at this unloaded state, including surface roughness and shape.

We now stress this material with a small amount of load, for example with heat. The material wants to expand when heated up, and if it has weak spots, it will be allowed to expand more. At the loaded state one more interferometric photo is taken. Now we also have a interferometric footprint of the area at the deformed state.

To extract information about the difference between the two states, we subtract the two images and a shearogram is created.

This shearogram can be thought of as a topography of the surface, but only gradients (slopes) are measured, not absolute heights of the hills. Defects will be seen as “hills” popping out of the plane. You can quantify the size of the defects (in plane size) by measuring how large your hills are.

Speckle and Shearing

When a surface is illuminated with a coherent laser light source, a stochastic interference pattern is created. This “speckle” pattern is projected on a camera’s CCD chip. In contrast to ESPI (Electronic Speckle Pattern Interferometry) where the speckle is compared with a reference light path, Shearography uses a reference created by shearing the image of the test object to create a double image. This makes the method much less sensitive to external vibrations and noise, and you can measure without any need for a vibration damped table, making Shearography ideal for field usage.

image of shearography fundamentals
Image 1: Shearography fundamentals; a intereferometric footprint is created from the surface at two states – unloaded test object, and loaded test object. In-structure defects deformations under load will show up when comparing the two foot prints.
image of a primitive shearography setup
Image 2: A primitive Shearography setup; Two physical points on test object will be projected on to one point on the CCD chip to record an interferometic footprint. The tested surface is illuminated with a monochromatic light, typical 650nm.

Image Subtraction

The basic Shearography principle is to subtract two images (interferometric footprints) of a test object, before and after load. The intensity information from those images is then subtracted, and the surface deformation can be displayed due to the speckle information. Surface roughness will be neglected ad-hoc in the subtraction process.

image of the primitive shearography principle
Image 3: The primitive Shearography principle. A shearography image is recorded at unloaded state and one image is recorded in the loaded state. Thereafter they are subtracted and in the result defects can be detected.

Phase Shift Technology

To increase the sensitivity of the technique, a real-time phase shift process is used in the sensor. This uses a stepping mirror that shifts the reference beam and enhances the results with directional information included with the deformation.

image of a phase stepping shearography setup
Image 4: A phase stepping Shearography setup; Two physical points on test object will be projected onto one point on the CCD chip to record a interferometic footprint. A modified Michelson cube is used here with a double breaking mirror as a beam splitter. One mirror is for adjustment of shear properties and the other one is the phase stepper.
image of phase stepper
Image 5: The phase stepper moves through its four positions with an internal difference of 1/4 Wavelength, at each position an image is recorded and sent through the software processor to evaluate the phase relationship with a best fit algorithm.

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