Accurate ex situ deformation measurement using an ultra-stable two-dimensional digital image correlation system.

In situ planar deformation measurement using two-dimensional digital image correlation (2D-DIC) and a fixed camera has been fully investigated in many published works and widely used for various applications. However, in certain special cases (e.g., long-term deformation monitoring of engineering structures, or deformation measurement of a specimen subjected to uncommon loading), it is very difficult or impossible to carry out in situ deformation measurement using 2D-DIC, and necessitates ex situ 2D-DIC measurements for a repositioned specimen or using a relocated camera. To achieve accurate measurements, the error sources of ex situ 2D-DIC measurements should be identified and minimized, and the strain accuracy of ex situ 2D-DIC measurements should be quantified. In this work, the potential error sources of ex situ 2D-DIC measurements are first discussed in detail. Then, to mitigate the errors associated with these issues, an ultra-stable 2D-DIC system combining the idea of active imaging and a well-designed bilateral telecentric lens is established, which is invariant to the potential variations in ambient lighting and the possible small out-of-plane motions in object surface and/or image plane. The established ultra-stable 2D-DIC system is first compared with the regular 2D-DIC setup in determining the surface strains of an unstrained sample. Then, ex situ residual stress measurement using the hole-drilling technique and the established 2D-DIC system was conducted and compared with the applied ones. The results of this work confirm that the accuracy of ex situ 2D-DIC deformation measurements using the proposed system is of high fidelity, and can be used for accurate deformation measurement in practical ex situ tests.