As a practical and effective tool for quantitative in-plane deformation measurement of a planar object surface, two-dimensional digital image correlation (2D DIC) is now widely accepted and commonly used in the field of experimental mechanics. It directly provides full-field displacements to sub-pixel accuracy and full-field strains by comparing the digital images of a test object surface acquired before and after deformation. In this review, methodologies of the 2D DIC technique for displacement field measurement and strain field estimation are systematically reviewed and discussed. Detailed analyses of the measurement accuracy considering the influences of both experimental conditions and algorithm details are provided. Measures for achieving high accuracy deformation measurement using the 2D DIC technique are also recommended. Since microscale and nanoscale deformation measurement can easily be realized by combining the 2D DIC technique with high-spatial-resolution microscopes, the 2D DIC technique should find more applications in broad areas.

https://iopscience.iop.org/article/10.1088/0957-0233/20/6/062001/pdf

Two-dimensional digital image correlation for in-plane displacement and strain measurement: a review

Digital Image Correlation (DIC) is an important and widely used non-contact technique for measuring material deformation. Considerable progress has been made in recent decades in both developing new experimental DIC techniques and in enhancing the performance of the relevant computational algorithms. Despite this progress, there is a distinct lack of a freely available, high-quality, flexible DIC software. This paper documents a new DIC software package Ncorr that is meant to fill that crucial gap. Ncorr is an open-source subset-based 2D DIC package that amalgamates modern DIC algorithms proposed in the literature with additional enhancements. Several applications of Ncorr that both validate it and showcase its capabilities are discussed.

http://ncorr.com/download/publications/blaberncorr.pdf

Ncorr: Open-Source 2D Digital Image Correlation Matlab Software

Role of {10–12} twinning characteristics in the deformation behavior of a polycrystalline magnesium alloy

Digital Image Correlation (DIC) has been established as a flexible and effective technique to measure the displacements on specimen surface by matching the reference subsets in the undeformed image with the target subsets in the deformed image. With the existing DIC techniques, the user must rely on experience and intuition to manually define the size of the reference subset, which is found to be critical to the accuracy of measured displacements. In this paper, the problem of subset size selection in the DIC technique is investigated. Based on the Sum of Squared Differences (SSD) correlation criterion as well as the assumption that the gray intensity gradients of image noise are much lower than that of speckle image, a theoretical model of the displacement measurement accuracy of DIC is derived. The theoretical model indicates that the displacement measurement accuracy of DIC can be accurately predicted based on the variance of image noise and Sum of Square of Subset Intensity Gradients (SSSIG). The model further leads to a simple criterion for choosing a proper subset size for the DIC analysis. Numerical experiments have been performed to validate the proposed concepts, and the calculated results show good agreements with the theoretical predictions.

Study on subset size selection in digital image correlation for speckle patterns.

Digital image correlation for surface deformation measurement: historical developments, recent advances and future goals

The performance of four digital image correlation criteria widely used in strain mapping applications has been critically examined using three sets of digital images with various whole-field deformation characteristics. The deformed images in these image sets are digitally modified to simulate the less-than-ideal image acquisition conditions in an actual experiment, such as variable brightness, contrast, uneven local lighting and blurring. The relative robustness, computational cost and reliability of each criterion are assessed for precision strain mapping applications. Recommen- dations are given for selecting a proper image correlation criterion to efficiently extract reliable deformation data from a given set of digital images.

http://lyle.smu.edu/~wtong/papers/Tong2005Strainv41n4p167_DIC.pdf

An Evaluation of Digital Image Correlation Criteria for Strain Mapping Applications

https://engineering.purdue.edu/~zavattie/papers/IJP09_TWIP.pdf

Spatio-temporal characteristics of the Portevin-Le Châtelier effect in austenitic steel with twinning induced plasticity

An Mg AZ31 alloy with modified microstructures was investigated to determine microstructural effects on room temperature mechanical properties acquired from low strain rate (∼10 −3  s −1 ) tensile testing to failure. Three distinct microstructures were generated via heat treatment, viz.: (1) single phase, fine equiaxed grains; (2) single phase, coarse grains with twins; (3) fine, equiaxed grains decorated with Mg 17 (Al,Zn) 12 grain-boundary precipitates. Each microstructure was separately characterized with optical microscopy, X-ray crystallography, fractography, and hardness measurement prior to tensile testing. Tensile coupons fabricated from each microstructure were then elongated to failure in a miniature stage, and true stress–true strain curves were computed with the digital image correlation (DIC) technique. Initial yield point, ultimate tensile strength and maximum elongation were also computed and examined within the context of key features of each microstructure to infer the mechanism of plastic deformation in tension and whether or not the potential for improved room temperature formability exists.

https://lyle.smu.edu/~wtong/papers/Marya_etal2006MaterSciEngA418p341_MgAlloyAZ31.pdf

Microstructural effects of AZ31 magnesium alloy on its tensile deformation and failure behaviors

Plastic deformation of miniature tensile bars generated from dual-phase steel weld microstructures (i.e., fusion zone, heat-affected zone, and base material) was investigated up to final rupture failure. Uniaxial tensile true stress-strain curves beyond diffuse necking were obtained with a novel strain-mapping technique based on digital image correlation (DIC). Key microstructural features (including defects) in each of these three metallurgical zones were examined to explore the material influence on the plastic deformation and failure behavior. For weld fusion zones with minimal defects, diffuse necking was found to begin at 6 pct strain and continue up to 55 to 80 pct strain. The flow stresses of the weld fusion zones were at least twice those of the base material, and fracture strains exceeded 100 pct for both materials. The heat-affected zones exhibited a range of complex deformation behaviors, as expected from their microstructural variety. Only those fusion zones with substantial defects (e.g., shrinkage voids, cracks, and contaminants) failed prematurely by edge cracking, as signaled by their highly irregular strain maps.

http://lyle.smu.edu/~wtong/papers/Tong2005MetMatTransAv36p2651_SpotWeldedDualPhaseSteel.pdf

Deformation and fracture of miniature tensile bars with resistance-spot-weld microstructures

http://lyle.smu.edu/~wtong/papers/Tongetal2005ScriptaMatv53n1p87_PLCHighSpeedDigitalImaging.pdf

Wei Tong

Papers

An Evaluation of Digital Image Correlation Criteria for Strain Mapping Applications

Spatio-temporal characteristics of the Portevin-Le Châtelier effect in austenitic steel with twinning induced plasticity

Microstructural effects of AZ31 magnesium alloy on its tensile deformation and failure behaviors

Deformation and fracture of miniature tensile bars with resistance-spot-weld microstructures

Time-resolved strain mapping measurements of individual Portevin¿Le Chatelier deformation bands