Digital image correlation

Digital Image Correlation and Tracking (DIC/DDIT) is an optical method used to measure displacement and strain from digital images.

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Overview

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. Of particular relevance to this thesis, 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 spatial 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.

Optical methods are a logical solution to this litany of challenges. One approach is the interferometric strain/displacement gage (ISDG) developed by Sharpe, a laser-based technique that affords significant advantages over conventional strain measurement methods. ISDG utilizes two markers on the surface of the specimen that provide interference fringes that are analogous to Young’s two slit experiment, although in reflection as opposed to transmission. This technique offers superior resolution (as low as 5 με) and local strain determination, although is limited to 1D measurements and requires some degree of experimental complexity. It also demands the use of markers, which have typically been either Vickers microhardness indents or patterned lines (applied by gas-assisted chemistries in the FIB or physical vapor deposition and photolithography). In the case of thin film mechanical testing where thicknesses are in the submicron range, hardness indents are out of the question and deposited lines can be up to an order of magnitude thicker than the specimen itself, which could significantly alter the apparent intrinsic properties of the material being tested.

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 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 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.

The appeal of these image-based techniques coupled with the lack of flexibility and prohibitive cost of commercial DIC software packages provided the impetus for the author and his colleagues to develop a custom in-house software suite using the mathematical package MATLAB as the engine for calculations. This resulted in an open-source package that was uploaded to the public domain in an effort to provide free tools to users, but also to generate feedback for potential improvements and addition to the code. As such, a brief discussion of the primary features and methodology of this technique, along with some background on DIC and peak tracking will be presented here. DIC for strain measurement constitutes a major field of research and is followed by a healthy, vigorous, and dynamic discussion and discourse, so it is not the author’s intention to provide an exhaustive survey of the field. Instead, a more focused description of the tools required to make accurate measurements that provide insight on the deformation mechanisms that govern plasticity in nc-Al thin films is given.

DIC was first conceived and developed at the University of South Carolina in the early 1980s and has been optimized and improved in recent years. DIC is predicated on the maximization of a correlation coefficient that is determined by examining pixel intensity array subsets on two or more corresponding images and extracting the deformation mapping function that relates the images (Figure 1). An iterative approach is used to minimize the 2D correlation coefficient by using nonlinear optimization techniques. The cross correlation coefficient r_ij is defined as

Here F(x_i ,y_j) is the pixel intensity or the gray scale value at a point (x_i ,y_j) in the undeformed image and G(x_i* ,y_j*) is the gray scale value at a point (x_i* ,y_j*) in the deformed image. and are mean values of the intensity matrices F and G, respectively. The coordinates or grid points (x_i ,y_j) and (x_i^* ,y_j^*) are related by the deformation that occurs between the two images. If the motion is perpendicular to the optical axis of the camera, then the relation between (x_i ,y_j) and (x_i^* ,y_j^*) can be approximated by a 2D affine transformation,

Here u and v are translations of the center of the sub-image in the X and Y directions, respectively. The distances from the center of the sub-image to the point (x, y) are denoted by and . Thus, the correlation coefficient r_ij is a function of displacement components (u, v) and displacement gradients ( , , , ).

DIC has proven to be very effective at mapping deformation in macroscopic mechanical testing, where the application of specular markers (e.g. paint, toner powder) or surface finishes from machining and polishing provide the needed contrast to correlate images well. However, these methods for applying surface contrast do not extend to the application of freestanding thin films for several reasons. First, vapor deposition at normal temperatures on semiconductor grade substrates results in mirror-finish quality films with rms roughnesses that are typically on the order of several nanometers. No subsequent polishing or finishing steps are required, and unless electron imaging techniques are employed that can resolve microstructural features, the films do not possess enough useful surface contrast to adequately correlate images. Typically this challenge can be circumvented by applying paint that results in a random speckle pattern on the surface, although the large and turbulent forces resulting from either spraying or applying paint to the surface of a freestanding thin film are too high and would break the specimens. In addition, the sizes of individual paint particles are on the order of μms, while the film thickness is only several hundred nms, which would be analogous to supporting a large boulder on a thin sheet of paper.

Very recently, advances in pattern application and deposition at reduced length scales have exploited small-scale synthesis methods including nano-scale chemical surface restructuring and photolithography of computer-generated random specular patterns to produce suitable surface contrast for DIC. A different approach was employed in this thesis work, namely the application of very fine powder particles that would electrostatically adhere to the surface of the film and could be digitally tracked. Fine alumina abrasive polishing powder was initially used since the particle sizes are relatively well controlled, although the adhesion to Al films was not very good and the particles tended to agglomerate excessively. The candidate that worked most effectively was a silica powder designed for a high temperature adhesive compound (Aremco, inc.), which was applied through a plastic syringe. A light blanket of powder would coat the gage section of the tensile sample and the larger particles could be blown away gently. The remaining particles would be those with the best adhesion to the surface, and under low-angle grazing illumination conditions, the specimen gage section would appear as shown in Figure 2. While the surface contrast present is not ideal for DIC, the high intensity ratio between the particles and the background provide a unique opportunity to track the particles between consecutive digital images taken during deformation. This can be achieved quite straightforwardly using digital image processing techniques, although the resolution is always limited to a single pixel. To attain tracking with subpixel resolution, a novel image-based tracking algorithm using MATLAB was developed, dubbed Digital Differential Image Tracking (DDIT), and will be discussed here briefly.

Differential Digital Image Tracking (DDIT)

The DDIT method exploits the shape of these powder particles when digitally imaged in the intensity domain as shown in Figure 2. The resemblance of the particles to mathematical functions that are adept at describing peak shapes with precise center locations and broadening (tails) allow them to be fit to a given function and thus tracked.   It is perhaps coincidental that the symmetric normal (Gauss) distribution function proficiently fits the intensity profiles of the particles, although many functions would be suitable as well (e.g. Pearson VII, Cauchy). The Gaussian function can be generally defined as the function f(x), where

and a is the amplitude of the function, xo is the center of the data, σ is the spread or standard deviation, and b is the offset or background level relative to some reference datum. This function can also be described in two dimensions by the following:

The quality of the Gaussian fit to a peak profile is shown in Figure 3.  

The DDIT script works in the following fashion as schematically shown in Figure 4 (alongside, for comparison, the DIC code, see link, that was also developed). A detailed guide that describes the inner workings of both the DDIT and DIC code can be found below. First, images are captured during the course of a mechanical test. Second, a list of image filenames is generated and the image capture times are extracted from the original images in order to synchronize the DDIT data to that of the data acquisition system. The markers are then automatically detected in the first image (after undergoing automatic background subtraction) by an image processing algorithm that labels connected components in a binary image and subsequently information regarding the size and shape of these components are extracted (e.g. area, bounding box, centroid, major axis length, minor axis length, etc.). Particles with properties that do not conform to specifications for “ideal” shapes are thrown out, and the remaining markers in the first image are fit to a Gaussian function (in this thesis work) using a nonlinear least-squares algorithm in both the longitudinal and transverse directions. The normalized residuals of the fit of the peak to the function are calculated for every peak (typically several hundred in an image such as Figure 5) and again, fits deemed “poor” as given by the value of the residual are removed from the analysis. This process now continues for every image in the sequence, and the result includes the position of the peak center, which is then post-processed using a visualization and data analysis script that allows visualization and output of the quantities of interest. Incidentally, the DDIT technique has also been successfully applied to the testing of brittle SiO₂ and ductile Au thin films.

Resolution of DIC/DDIT

The resolution that one can achieve in practice using these image-based techniques depends on a number of factors, including but not limited to camera resolution, lens optical quality, and marker size and quality. To demonstrate the achievable resolution that one can achieve using the setup and techniques described herein using both DIC and DDIT, a tensile test performed on a brittle linear elastic submicron (t ~ 250 nm) freestanding Al-32 at%Mo specimen was analyzed for strain using both methods. This specimen was chosen because it exhibits small strains that are difficult to resolve and also because it is amorphous and thus minimizes any microstructural inhomogeneities. The initial raster grid that was applied for the DIC method and the automatically labeled markers (“good” markers are shown as circles) are shown in Figures 6 and 7, respectively. The representative stress strain response from this film is shown in Figure 8, where both longitudinal (shown in blue) and transverse (shown in green) strains were calculated using DIC with two subset image sizes (15 and 25 pixels) and DDIT. It is apparent from these results that the peak tracking algorithm works quite effectively at resolving the response of this film, even when tracking about 50 times fewer points. The insets of Figure 8 show the typical strain variation that was achieved, where as low as 40 με was observed in the longitudinal direction using DDIT. It was concluded that either of these techniques was effective at measuring the Young’s moduli of these specimens, although the transverse strain resolution obtained using DIC is at the upper limit of what one would desire to measure Poisson’s ratio.

References

• T.C. Chu, W.F. Ranson, M.A. Sutton, W.H. Peters, Exp Mech 25 (1985) 232.
• H.A. Bruck, S.R. McNeill, M.A. Sutton, W.H. Peters III, Exp Mech 29 (1989) 261.
• W.H. Peters, W.F. Ranson, Opt Eng 21 (1982) 427.
• A.S. Kobayashi, Handbook on Experimental Mechanics, Prentice Hall / Society for Experimental Mechanics, Inc., Lebanon, Indiana, 1993.
• M.R. James, W.L. Morris, B.N. Cox, Exp Mech 30 (1990) 60.
• B.W. Smith, X. Li, W. Tong, Exp Tech 22 (1998) 19.
• M.A. Sutton, S.R. McNeill, J.D. Helm, & Y.J. Chao, in: PK Rastogi (Ed.), Photomechanics, Springer-Verlag, Berlin Heidelberg, 2000, pp. 323-372.

See also

• Stress
• Strain
• Displacement vector
• Particle Image Velocimetry