Huimin Xie

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

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.

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

Digital Image Correlation (DIC) is a flexible and effective technique to measure the displacements on specimen surfaces 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 an optimal 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.

Equivalence of digital image correlation criteria for pattern matching.

In digital image correlation (DIC), to obtain the displacements of each point of interest, a correlation criterion must be predefined to evaluate the similarity between the reference subset and the target subset. The correlation criterion is of fundamental importance in DIC, and various correlation criteria have been designed and used in literature. However, little research has been carried out to investigate their relations. In this paper, we first provide a comprehensive overview of various correlation criteria used in DIC. Then we focus on three robust and most widely used correlation criteria, i.e., a zero-mean normalized cross-correlation (ZNCC) criterion, a zero-mean normalized sum of squared difference (ZNSSD) criterion, and a parametric sum of squared difference (PSSD(ab)) criterion with two additional unknown parameters, since they are insensitive to the scale and offset changes of the target subset intensity and have been highly recommended for practical use in literature. The three correlation criteria are analyzed to establish their transversal relationships, and the theoretical analyses clearly indicate that the three correlation criteria are actually equivalent, which elegantly unifies these correlation criteria for pattern matching. Finally, the equivalence of these correlation criteria is further validated by numerical simulation and actual experiment.

Full-field strain measurement using a two-dimensional Savitzky-Golay digital differentiator in digital image correlation

Many published research works regarding digital image correlation (DIC) have been focused on the improvements of the accuracy of displacement estimation. However, the original displacement fields calculated at discrete locations using DIC are unavoidably contaminated by noises. If the strain fields are directly computed by differentiating the original displacement fields, the noises will be amplified even at a higher level, and the resulting strain fields are untrustworthy. Based on the principle of local least-square fitting using two-dimensional (2D) polynomials, a 2D Savitzky-Golay (SG) digital differentiator is deduced and used to calculate strain fields from the original displacement fields obtained by DIC. The calculation process can be easily implemented by convolving the SG digital differentiator with the estimated displacement fields. Both homogeneous and inhomogeneous deformation images are employed to verify the proposed technique. The calculated strain fields clearly demonstrate that the proposed technique is simple and effective.

In-plane deformation measurement using the atomic force microscope moiré method

In this paper, a new scanning moire method is developed to measure the in-plane deformation of mica using an atomic force microscope (AFM). Moire patterns are formed by the scanning line of the CRT in the AFM system, and the atomic lattice of the mica or high-orientated pyrolytic graphite (HOPG). The measurement principle and the techniques employed for grating preparation are described in detail. This new method is used to measure the residual deformation of a mica plate after irradiation by a Nd-YAG laser, and to determine the residual strain of HOPG under a tensile load. Some interesting results are obtained. The successful results verify the feasibility of this method for measuring deformation in the nanometre range using the lattice of the material as the model grid.

Three-dimensional shape measurement with a fast and accurate approach

A noncontact, fast, accurate, low-cost, broad-range, full-field, easy-to-implement three-dimensional (3D) shape measurement technique is presented. The technique is based on a generalized fringe projection profilometry setup that allows each system component to be arbitrarily positioned. It employs random phase-shifting, multifrequency projection fringes, ultrafast direct phase unwrapping, and inverse self-calibration schemes to perform 3D shape determination with enhanced accuracy in a fast manner. The relative measurement accuracy can reach 1/10,000 or higher, and the acquisition speed is faster than two 3D views per second. The validity and practicability of the proposed technique have been verified by experiments. Because of its superior capability, the proposed 3D shape measurement technique is suitable for numerous applications in a variety of fields.

Study of the performance of different subpixel image correlation methods in 3D digital image correlation

The three-dimensional digital image correlation (3D-DIC) method is rapidly developing and is being widely applied to engineering and manufacturing. Despite its extensive use, the error caused by different image matching algorithms is seldom discussed. An algorithm for 3D speckle image generation is proposed, and the performances of different subpixel correlation algorithms are studied. The advantage is that there is no interpolation bias of texture in the simulation before and after deformation, and the error from the interpolation of speckle can be omitted in this algorithm. An error criterion for 3D reconstruction is proposed. 3D speckle images were simulated, and the performance of four subpixel algorithms is addressed. Based on the research results of different subpixel algorithms, a first-order Newton-Raphson iteration method and gradient-based method are recommended for 3D-DIC measurement.

Error evaluation technique for three-dimensional digital image correlation

Three-dimensional (3D) digital image correlation (DIC) is one of the most popular techniques used in engineering for strain and deformation measurement. However, the error analysis of 3D DIC, especially which kind of parameters dominates the error of 3D coordinate reconstruction in any kind of configuration, is still under study. In this paper, a technique that can be used for error determination of reconstruction is presented. The influence from the system calibration and image correlation to the error is theoretically analyzed. From numerical experiments of one-dimensional line and two-dimensional plane, the evaluation procedure is validated to be flexible. A typical test with standard objects is also conducted. With this technique, once a 3D DIC system is set up and images of objects with speckles and calibration boards are recorded, the error of the configuration can be immediately evaluated. The standard deviation of every point in the world coordinate can be determined by statistical analysis.

A study on the digital nano-moire method and its phase shifting technique

A novel digital nano-moire method is proposed to measure the in-plane nanoscopic deformation of an object. In the measurement, the periodic lattice of a single-crystal material acts as a specimen grating while a digital reference grating (DRG) is prepared by computer software. These two gratings overlap to generate a moire fringe pattern. The preparation of the grating, the formation principle of digital nano-moire fringes and its relative phase shifting technique are described in detail. A typical experiment was conducted with a highly oriented pyrolytic graphite (HOPG) sample. The residual deformation of the irradiated HOPG sample was measured using this method. The experiment result verifies the feasibility of this method, and demonstrates the potential for further applications.

High resolution AFM scanning Moiré method and its application to the micro-deformation in the BGA electronic package

Abstract The formation mechanism of atomic force microscope (AFM) Moire is explained using the transmittance function. The technique for preparing the AFM Moire specimen grating is described. The sensitivity and accuracy of this method is analyzed. AFM Moire method is used to measure the thermal deformation ball grid array (BGA) electronic package. The shear strain at the different solders in the BGA package is measured. The result is compared with that from electron beam Moire method. The consistent comparison result verifies the AFM Moire method is reliable and effective in the micro-deformation measurement in the electronic package.