The height Digital Image Correlation (hDIC) technique for the identification of triaxial surface deformations

Abstract

This paper introduces the height digital image correlation (hDIC) technique for the identification of triaxial deformations. Conventional digital image correlation (DIC) uses pixel image intensity as the basis for the determination of in-plane displacement fields. We demonstrate the advantages of using the out-of-plane surface height and its variation during deformation to extract information about triaxial (in-plane and out-of-plane) displacement fields. Surface height maps can be obtained by optical profilometry or scanning probe microscopies, e.g. stylus profilometry, coordinate measurement machine (CMM), or atomic force microscope (AFM). The changes in height during deformation are sufficiently small to allow efficient correlation of in-plane displacements with sub-pixel accuracy, yet also provide information about out-of-plane displacements. In the present study, the contour map of surface height was created using digital dynamic focus optical microscope. The correlation between the reference and target maps to extract the displacement data was accomplished by a two-step correlation process. Initially, triaxial Cartesian coordinate data of reference and target data sets were cross-correlated at integer-pixel level sensitivity. This was followed by sub-pixel correlation using gradient descent method. As an example of technique application, Al alloy test specimens were subjected to large tensile deformations to failure. Good agreement was found between height digital image correlation (hDIC) analysis of displacements and strains and the reference material properties, with the evolution of both inplane strains and out-of-plane displacements show progressive localisation during postcritical deformation beyond the sample's ultimate tensile strength (UTS).

Polar transformation of 2D X-ray diffraction patterns and the experimental validation of the hDIC technique

“…For this purpose, pixel intensities from height information are defined by a surface function, ( ′ , ′ ), using bi-cubic interpolation scheme which is formulated in Equation 3 that allows calculation of pixel intensity in the target subset at the coordinates defined by ( ′ , ′ ) where , is the coefficient of the pixel defined by the indices of and . Details of the hDIC process and formulation of the gradient descent method can be found in the study that introduces the hDIC technique [5] .…”

“…The developed technique is widely used for analysing the deformations on 2D and 3D surfaces [3] that are difficult to investigate using classical experimental methods [4]. The height Digital Image Correlation (hDIC) technique was developed by Uzun and Korsunsky [5] for the determination of in-plane and out-of-plane deformations. Authors performed tensile test for in situ analysis of the method and showed that the proposed sub-pixel level correlation provides an accuracy of 99.89 % in terms of Pearson coefficient during the elastic deformation.…”

“…Accuracy of this technique was analysed previously [5] by investigating different stages of a tensile test process, but, in this study, it was aimed to create a more complex deformation state that include both axial displacements and rotations. For this purpose, 3-point bent aluminium specimens was prepared.…”

The Use of Surface Topography for the Identification of Discontinuous Displacements Due to Cracks

“…On the other hand, 3D DIC techniques provide depth information with very low resolution and low precision over large areas [11][12][13][14]. The height digital image correlation (hDIC) [15] which is a true full field method and based on focus stacking 2 of 13 optical microscopy (FSOM), fills the gap between the two approaches. Beeck et al [16] also used confocal microscopy for obtaining topography information to perform DIC, but this scanning method is intrinsically slow [8] and requires complex finite element calculations and, accordingly, it is far from being a full field method.…”

“…The hDIC technique [15] was proposed recently for the correlation of optical profilometry data obtained using "infinite focus" microscopy. This allows the use of height data instead of the conventional grey scale colour intensity in typical digital images for the purpose of determination of triaxial surface displacements.…”