Integration of Fringe Projection and 2D Digital Image Correlation for the measurement of 3D displacements and strains

Abstract

In many engineering applications, the measurement of displacements maps in all three spatial directions over the surface of a loaded object is often required, such an example is, structural testing or manufacturing quality control. At present, for this proposes, typically 3-dimensional digital image correlation (3D-DIC) is often used. This technique employs two cameras acquiring images from different viewing angles of the object while it is deformed. The cameras have to be perfectly synchronized and calibrated for both: 3D reconstruction and for tracking each surface element while the deformation occurs. The calibration process of the technique requires acquiring a sequence of several images of a calibration object and a high amount of computational resources. Alternative techniques have been proposed to obtain 3D displacement maps by combining Fringe Projection (FP) with two-dimensional DIC (2D-DIC). The hybrid technique only requires one camera and a fringe projector. In this thesis it is presented a novel device that allows obtaining the maps of displacements in X-, Y- and Z- direction at the surface of an object during deformation. The system is based on a method that combines 2D-DIC and FP to obtain the in- and out-of-plane components of displacement during deformation. The device operates by acquiring only one image of the studied object at each deformation state desired to analyze during the total test, thereby allowing real time data acquisition. The goal of the presented work is that the device allows both: a precise perpendicular alignment respect to a flat reference surface, and a self-calibration (i.e. no calibration object is employed). Thus, a fringe calibration constant is estimated for each pixel as well as all the required parameters for the in-plane displacement correction. To illustrate the potential of the proposed device, a set of static and dynamic experiments have been conducted using hyperelastic materials. The maximum out-of-plane displacement achieved was 20 mm with an uncertainty of 0,023 mm and an in-plane displacement uncertainty of 0,0083 mm. Results have been compared with those obtained using a commercial three dimensional digital image correlation system showing a very high level of agreement.