Characterisation of Indentation-Induced Pattern Using Full-Field Strain Measurement J.-C. Kuo*, S.-H. Tung † , M.-H. Shih ‡ and Y.-Y. Lu* *Department of Materials Science and Engineering, National Cheng-Kung University, Tainan 701, Taiwan † Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan ‡ Department of Construction Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung 824, Taiwan ABSTRACT: In this study, digital image correlation (DIC)-based strain analysis software was suc- cessfully developed. Its strain resolution lies in the order of 2.3 · 10 )4 –3.1 · 10 )4 . Full-strain field measurement was used to study indentation-induced plastic patterns around the spherical indenter for a polycrystal and a single crystal of pure aluminium. During indentation, the pure aluminium specimen of the single crystal revealed a symmetric indentation pattern of von Mises strain. The piling-up around the residual impression was successfully and directly characterised by examining the sign of strain e X and e Y in the X and Y directions. However, the inward, out-of-plane movement results in an error in calculating in-plane strain referred to as a ‘distortion strain’ using two- dimensional DIC. KEY WORDS: anisotropy, full-field strain, indentation, pile-up Introduction Various available methods, such as digital image correlation (DIC) [1–9], electronic speckle-pattern interferometry (ESPI) [10–13], photoelasticity and thermoelasticity, are capable of evaluating the strain field. In the last decade, optical digital image tech- niques for strain measurement, due to the availability of cheap and powerful digital imaging and data-pro- cessing hardware, have advanced significantly. There were some investigations on applying the optical digital image technique to characterise the localisa- tion mechanical behaviour in the field of materials science. The localisation deformation within small zones of the specimen will cause failure and damage. Localisation effects are considered to be very impor- tant in understanding materials failure. A local deformation pattern can be visualised with the help of the optical digital image technique for strain measurement, which is incorporated with DIC in this study. Digital image correlation has been increasingly used in recent years for full-field strain mapping applications in many materials and mechanics research laboratories. This technique often called digital image correlation ‘DIC’ has been developed and improved over years by Bruck et al. [1], Sutton et al. [9] and Vendroux and Knauss [8]. Because of the optimisation of many factors including the quality of images acquired and the algorithms and parameters adapted in image correlation processing, good reli- ability and accuracy of the strain mapping results can now be achieved routinely. For strain mapping of planar deformation and two-dimensional rigid body motion of a flat object, the errors in global average strain and in local strain variations are limited to 10 )4 –10 )5 or less [14, 15]. The application of the digital image measurement system to evaluate the plastic zone on a surface after indentation was demonstrated in this study. A simple analysis approach was proposed to reveal the aniso- tropic pattern of piling-up around the spherical indenter for the aluminium single crystal by analy- sing the components of the strain tensor. In the fol- lowing section, we will first describe the principle of the DIC technique. Digital Image Correlation and Strain Estimation Digital image correlation is a technique in non-con- tact strain measurement and surface profilometry by tracking the grey value pattern in small regions, which are called subsets. DIC is based on examining the pixel intensity of the two subsets in speckle images before and after deformation. The corre- sponding positions of the two images are related by a Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Publishing Ltd j Strain (2010) 46, 277–282 277