Optical surface profile tracking for high-resolution strain measurement

Characterization of the mechanical behaviour of materials often requires accurate determination of specimen strain. This paper presents an optical method whereby the surface displacements of a cylindrical specimen undergoing uniaxial loading are measured by tracking the profile of its surface texture. These profiles are obtained through sub-pixel edge detection from a digital image. High-resolution digital imaging with sub-pixel edge detection is used to measure the radial position of edges in images of the specimen. Edges are compared in consecutive images to track the positions of zero-crossings and local maxima of the surface roughness. A method for dealing with cases in which the mean wavelength of the surface texture is shorter than the displacement between frames is presented. False matches can still occur and are removed by modal average proximity filtering in sub-regions spanning short axial-lengths and several frames. The new method of finding specimen displacement is found to yield high data densities (in our case 32 independent measurements per mm). The data compare favourably with measurements made using a laser extensometer: surface roughness tracking produces a drop in accuracy of ±25 μm whilst increasing the spatial data density. A further advantage of the technique is that no specimen preparation is required. The data are also used to calculate strain, values of which are validated against strains inferred from the change in cross-sectional area. © 2011 IOP Publishing Ltd.