Deformations in wide, center-notched, thin panels, part I: three-dimensional shape and deformation measurements by computer vision

The response of wide, thin, center-notched, 2024-T3 aluminum panels undergoing far-field tensile load is investigated. Three panels with a notch length to panel width of 0.33 and widths of 305, 610, and 1016 mm were subjected to far-field tensile loading. As part of the experimental program, two pairs of cameras were configured into separate stereovision systems and used to simultaneously capture both the global response of the sheet and the local response near a notch tip. Global areas, ranging in size from 250x 250 mm to 550x 550 mm, were imaged for each panel. A second stereovison system recorded images of a small area, 10×20 mm, ahead of one notch tip. Postprocessing of the stereovision measurement data from global and local systems using three-dimensional digital image correlation was used to obtain the complete displacement field at each point in the region of interest. In general, results demonstrate that the combination of stereovision and three-dimensional digital image correlation is capable of accurately measuring true, three-dimensional structural deformations in regions undergoing both large out-of-plane displacements and large displacement gradients. Furthermore, 3-D measurements on the panel specimen near the grip location are shown to provide an independent assessment of the true boundary conditions, with specimen slippage clearly noted in the 1016-mm specimen. Results from the extensive notched, wide panel experimental program demonstrate that (a) each panel has an initial shape that deviates up to 3 mm from planarity, with the greatest deviations occurring at the center of the notch, (b) the global load-displacement response is essentially linear for load levels that are well beyond the onset of large, out-of-plane displacements in the notch region, and (c) increasing the size of the notched, thin panel specimen results in distinctly different surface deformations and deformed shapes, with three separate maxima/minima in the out-of-plane component of the largest panel. The region where tensile opening strains are above 2% extends several millimeters ahead of the hole, while compressive strains parallel to the notch direction are contained within a few millimeters of the hole. The in-plane shear strains are concentrated along circular lobes at +/-45 deg from the horizontal direction, a trend which is generally consistent with plane stress conditions.