C. Eberl, D. S. Gianola, R. Thompson (in alphabetic order)

With this contribution we would like to point to a free MATLAB tool which uses digital image correlation and tracking techniques to measure strain from a series of digital images. The code can be found on the ‘MATLAB central file exchange’ as well as the documentation, example images and some slides. We use the code on a daily basis for micro- and nanoscale measurements and present it here to be used and further developed by the community. Since it was posted at the end of september the code is now ranked place one or two in google and has been downloaded about 1000 times.

The accurate measurement of displacement and strains during deformation of advanced materials and devices endures as a primary challenge to designers and experimental mechanicians.  The increasing complexity of technological devices with stringent space requirements leads to imperfect boundary conditions that have to be properly accounted for.  The push toward miniaturizing devices down to nanometer length scales imparts additional difficulties in measuring strains as the application of conventional extensometers and resistance foil gages are cumbersome, damaging, or even impossible.  Compounding this problem is also the fact that compliance of small-scale testing machines precludes the use of the displacement of external actuators for estimating specimen strain.  As a consequence, a technique with the following features is extremely desirable: i) no contact with the specimen required, ii) sufficient strain resolution to measure locally at the region of interest, iii) the ability to capture non-uniform full-field deformations, and iv) a direct measurement that does not require recourse to a numerical or analytical model.

Digital image correlation (DIC) techniques have been increasing in popularity, especially in micro- and nano-scale mechanical testing applications due to its relative ease of implementation and use.  Advances in digital imaging and increasing computational resources have been the enabling technology for this method and while white-light optics has been the predominate approach, DIC has recently been extended to SEM/FIB and AFM.  Above and beyond the ability of image-based methods to provide a “box-seat” to the events that are occurring during deformation, these techniques were applied to the testing of freestanding thin films for this thesis work also because it offers a full-field description and is relatively robust at tracking a wide range of “markers” and varying surface contrast.

For these reasons this code was written together with Rob Thompson and Dan Gianola during my stay in the group of Kevin J. Hemker at the Johns Hopkins University in Baltimore, MD, USA.  This code is not meant to be a direct competitor to commercial code since our intention was not to develop ‘slick’ user interfaces, but rather useful code with the advantages of being ‘free’, ‘flexible’ and ‘scalable’.  ‘Free’ in terms of free access even though we would like to ask you to cite our code if you use it and ‘free’ again although you need MATLAB together with some toolboxes.  Since most research institutions have access to this important tool I think we still can call it ‘free’.  ‘Flexible’ in terms of the relative ease in which you can enhance this MATLAB code as a script language where you can add either other toolboxes or your own code to flex it around your application.  We would appreciate it if you as a user could share your own code with all of us out here so the community can learn from your creativity. Finally, ‘scalable’ since you can easily start several sessions to process your images on more than one processor (core) and because there is a good chance that we will be able to use Graphic Processing Units (GPU = the graphics processor on a graphics card) or other add-on boards to enhance processing speed in the next few years.

It is also important to emphasize that this tool is scale-free, since it only requires digital images, formed by AFM, SEM, TEM or white-light optics, etc.  Deformation of structures and materials at all length scales, from bridges and buildings to nanometer scales can be analyzed.

We also would like to acknowledge K. J. Hemker and W. N. Sharpe for their support and help.