Deflectometry: a tool to measure full-field slopes

The most well-known full-field deformation technique in mechanical engineering is "Digital Image Correlation" (DIC) which is based on imaging the deformation of a contrasted pattern attached to the test specimen surface. By combining two cameras in a stereo-imaging configuration, the three components of the displacement vector at the surface of the tested sample can be obtained. One of the limitations however is that the resolution is too low to image small surface strains caused by vibrations and therefore, only deformed shapes can generally be obtained (see contribution of Alex Nila to the forum). Deflectometry is an alternative to DIC that allows for the measurement of surface slopes from images of a target (grid or speckle pattern) at the surface of a flat and specularly-reflective (mirror-like) test specimen. The main advantages of the technique is that it measures slopes directly, which for thin plates in bending, is only one differentiation away from curvatures which are proportional to surface strains; and the fact that the technique is extremely sensitive while quite inexpensive. Microradians can be resolved, which lead to surface curvatures of the order of km^-1. The main limitations are that it requires a specularly reflective surface (though surface coatings can be applied) and is currently only applicable to flat surfaces, though we are currently working on extending it to moderately curved surfaces. The presentation will focus on the basic principle of the method and show some examples in vibration at both low and high (ultrasonic range) frequencies.