Chapter 8 Mechanical Shape Correlation: A Novel Integrated Digital Image Correlation Approach S.M. Kleinendorst, J.P.M. Hoefnagels, and M.G.D. Geers Abstract Mechanical Shape Correlation (MSC) is a novel integrated digital image correlation technique, used to determine the optimal set of constitutive parameters to describe the experimentally observed mechanical behavior of a test specimen, based on digital images taken during the experiment. In contrast to regular digital image correlation techniques, where grayscale speckle patterns are correlated, the images used in MSC are projections of the sample contour. This enables the analysis of experiments for which this was previously not possible, because of restrictions due to the speckle pattern. For example, analysis becomes impossible if parts of the specimen move or rotate out of view as a result of complex and three-dimensional deformations and if the speckle pattern degrades due to large deformations. When correlating on the sample outline, these problems are overcome. However, it is necessary that the outline is large with respect to the structure volume and that its shape changes significantly upon deformation, to ensure sufficient sensitivity of the images to the model parameters. Virtual experiments concerning stretchable electronic interconnects, which because of their slender wire-like structure satisfy the conditions for MSC, are executed and yield accurate results in the objective model parameters. This is a promising result for the use of the MSC method for tests with stretchable electronics and other (micromechanical) experiments in general. Keywords Mechanical shape correlation • Digital image correlation • Integrated digital image correlation • Stretchable electronics • Parameter identification 8.1 Introduction Mechanical Shape Correlation (MSC) is a novel technique, based on integrated digital image correlation (I-DIC) [1, 2], where a finite element (FE) model is coupled to the image correlation procedure. In such a method the constitutive parameters of the FE model are the unknowns in the correlation procedure, with the objective to obtain a good set of model parameters that describe the experimentally observed behavior of the test specimen correctly. In contrast to images of a speckle pattern applied to the test specimen, as usually used in DIC, in MSC the images used for correlation are projections of the deforming sample shape. This is beneficial in cases where complex three-dimensional deformations occur, such that parts of the specimen rotate out-of-view and other parts rotate into view, or if pattern application is difficult, such as on microscale samples [3]. Usually in DIC approaches correlation is limited to in-plane deformations, or in case of Quasi-3D DIC (also referred to as Digital Height Correlation, DHC) [4–7] or stereo-DIC [8, 9] it is also possible to track the out-ofplane deformation of the surface. However, this surface is required to stay in view and not to move or rotate out of view. By correlating the outline of the specimen this restriction is relaxed and full three-dimensional movement of the sample is allowed. However, for the MSC method the assumption is that the boundary area is large with respect to the volume of the structure and that it changes shape significantly upon deformation, such that deformations are reflected in the specimen edge. A schematic depiction of the method can be seen in Fig. 8.1. During an experiment pictures are taken of the specimen. These images are processed in order to obtain a projection. Also a numerical simulation is executed and similar projections are made. The projections from the experiment and the simulation are compared. If they do not correlate, the model parameters, which are the unknown in the correlation procedure, are updated and a new simulation is performed. This iterative operation is repeated until convergence is reached and the correct set of model parameters to portray the experimental behavior of the structure is achieved. S.M. Kleinendorst ( ) • J.P.M. Hoefnagels • M.G.D. Geers Department of Mechanical Engineering, Eindhoven University of Technology, Gemini-Zuid 4.122, 5600MB, Eindhoven, The Netherlands; e-mail: s.m.kleinendorst@tue.nl; j.p.m.hoefnagels@tue.nl © The Society for Experimental Mechanics, Inc. 2018 L. Lamberti et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-63028-1_8 47
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