Grid Method for Microscale Discontinuous Deformation Measurement H. Jin & S. Haldar & H.A. Bruck & W.-Y. Lu Received: 8 April 2010 / Accepted: 13 December 2010 / Published online: 27 January 2011 # Society for Experimental Mechanics 2011 Abstract The objective of this paper is to explore both grid method and Digital Image Correlation (DIC) technique for microscale and discontinuous displacement measurements, such as those associated with crack tips. First, the principle of the grid method is revisited. The grid method and DIC technique are then applied to computer generated images to calculate the displacement field around crack tips. Finally, the grid method is applied to actual experimental images of fracture tests which are conducted inside a Scanning Electron Microscope (SEM) chamber. A new technique is developed to generate microscale pattern that is suitable for both grid method and DIC technique. The displacement fields calculated from grid method are compared with those from DIC technique to identify the strengths and weak- nesses of each technique for the microscale and discontinuous displacement measurements. It has been determined that grid method can obtain data closer to the discontinuity than DIC; however, DIC produces smoother displacement fields at the far field. Using this new pattern generation technique, both grid method and DIC technique can be applied to the fracture test at the microscale to complement with each other to achieve the best experiment results. Keywords Grid method . Digital Image Correlation . Microscale . Discontinuous Introduction The grid method is one of the oldest methods for obtaining full-field deformation measurements [1–9]. A grid usually consists of lines, circles, dots or other shapes assembled in regular patterns. The grid is first applied to specimen surface before conducting tests. Images of the grid are then obtained before and while the specimen is undergoing loading. By comparing the grid of the deformed specimen to that of the undeformed reference state, the displacements and strains which are developed in the specimen during the loading can be quantified. There is a variety of approaches used to perform the image analysis, such as spot centroid tracking [5, 6], cross-grid tracking [9], the spectral method [10, 11] and Fourier transformation of the grid patterns [12]. The grid method has been successfully applied to obtain the displacement and strain mapping in various materials and structures, such as perforated aluminum strips [13], cracks in lamellar TiAl [14], three dimensional surface deformations of metals [15], necking zone of a tensile specimen [16] and small animal bones [17]. The advantage of the grid method, compared to other full-field deformation measurement techniques such as Fourier transformation or Digital Image Correlation (DIC), is the less intense computational requirements for image analysis. However, the steady advancement of modern technology to perform powerful computation has made the computation intensity less of an issue. DIC, which uses random gray scale patterns, has therefore become more popular technique for measuring surface displacement and strain. It was originally proposed and used by researchers at the University of South Carolina [18–22]. It is now well- accepted in the experimental mechanics community for a wide range of applications, not only with wide strain ranges from sub-millistrain up to 500%, but also with various sizes H. Jin (*, SEM member) : W.-Y. Lu (SEM member) Mechanics of Materials, Sandia National Laboratories California, Livermore, CA, USA e-mail: hjin@sandia.gov S. Haldar : H.A. Bruck (SEM member) Department of Mechanical Engineering, University of Maryland, College Park, MD, USA Experimental Mechanics (2011) 51:565–574 DOI 10.1007/s11340-010-9459-7