Two-dimensional Digital Image Correlation Research Articles

Evaluating angular deflections from the digital gradient sensing method with rigid-motion deleted

The digital gradient sensing method is used for measuring small angular deflections of light rays due to local stresses in transparent planar solids. The method is based on two-dimensional (2D) digital image correlation (DIC) to measure the angular deflection of light rays; however, when a specimen is subjected to loading, deformation measurement from DIC is not perfect because of the existence of small in-plane and out-of-plane motions of the test sample surface that occurred after loading. These disadvantages will lead to errors in the measured angular deflections. The influence of unavoidable in-plane and out-of-plane motions was discussed, and a method to eliminate the influence to show the pure stress gradient of polymethy methacrylate is demonstrated.

二维多相机全场数字图像相关变形测量方法

二维多相机全场数字图像相关变形测量方法

Measurement of lacunar bone strains and crack formation during tensile loading by digital volume correlation of second harmonic generation images

The maintenance of healthy bone tissue depends upon the ability of osteocytes to respond to mechanical cues on the cellular level. The combination of digital volume correlation and second harmonic generation microscopy offers the opportunity to investigate the mechanical microenvironment of intact bone on the scale of individual osteocytes. Adult human femurs were imaged under tensile loads of 5 and 15MPa and volumes of approximately 492×429×31μm3 were analyzed, along with an image of a bone microcrack under the same loading conditions. Principal strains were significantly higher in three-dimensional digital volume correlation when compared to two-dimensional digital image correlation. The average maximum principal strain magnitude was 5.06-fold greater than the applied global strain, with peak strains of up to 23.14-fold over global strains measured at the borders of osteocyte lacunae. Finally, a microcrack that initiated at an osteocyte lacunae had its greatest tensile strain magnitudes at the crack expansion front in the direction of a second lacunae, but strain at the crack border was reduced to background strain magnitudes upon breaching the second lacunae. This serveed to demonstrate the role of lacunae in initiating, mediating and terminating microcrack growth.

Uneven intensity change correction of speckle images using morphological Top-Hat transform in digital image correlation

By comparing two digital speckle images recorded before and after deformation, two-dimensional digital image correlation (DIC) method can accurately determine the in-plane displacement fields and strain fields. In a practical measurement, however, the variance of light source intensity, location and direction will cause the random uneven intensity change of the random speckle images and will lead to the obvious measurement error. Numerical simulation experiment is first carried out to analyse the influence of the recorded speckle images undergoing uneven light variation on DIC measurement accuracy. Then, a correction method for speckle images with uneven intensity change is proposed based on morphological Top-Hat transform. In addition, quantitative measurements of both in-plane rotation of a rigid body and three-point bending beam are investigated experimentally by DIC to verify the feasibility of the correction method. Experimental results show that the measurement accuracy of DIC is improved dramatically after the procedure of uneven light variation correction.

Lens distortion correction for improving measurement accuracy of digital image correlation

Digital image correlation is widely applied in the measurement of displacement and strain fields through detecting the laser or sprayed speckle on the specimen surface. Generally, the existing digital image correlation methods often met difficulties of low detection accuracy under the influence of lens distortion. To tackle such difficulties, the piecewise spline function is used to describe the distortion, and a continuous smooth distortion model can be obtained after eliminating the noises by means of the mean value filtering and smoothing spline algorithm. Since the proposed method corrects lens distortion on the pixel plane, it will provide opportunity to obtain unbiased displacement in digital image correlation. Experiment results verify that the proposed method eliminates the effect of lens distortion from displacements measurement by means of two-dimensional digital image correlation.

An experimental method for eliminating effect of rigid out-of-plane motion on 2D-DIC

The out-of-plane motion is one of the most important factors that affect the precision of two-dimensional digital image correlation (2D-DIC). In this paper, a novel solution is presented to improve conventional 2D-DIC by eliminating the effect of out-of-plane motion, including translation and rotation. Firstly, an experimental technique using two projected laser strips is proposed to measure the out-of-plane motion of a planar specimen. A theoretical model is then established to predict the pseudostrains caused by out-of-plane motion based on the pin-hole imaging model. Using the measured out-of-plane displacement, the captured deformed images used in 2D-DIC are amended to eliminate the effect of out-of-plane motion by the theoretical model. Finally, two experiments were conducted to validate the effectiveness of the proposed method. Results indicate that application of the proposed method can effectively eliminate the errors caused by out-of-plane motion.

Strain Profile Measurement for Structural Health Monitoring of Woven Carbon-fiber Reinforced Polymer Composite Bonded joints by Fiber Optic Sensing Using an Optical Backscatter Reflectometer

In order to monitor crack initiation and propagation under static and fatigue loading in adhesively bonded joints, strain profile measurement such as backface strain (BFS) is a very efficient technique. In single lap (SL) joints, crack initiation and propagation in the glue line can be monitored by detecting the negative peak of the strain profile. Therefore, the accuracy of the monitoring system greatly depends on detecting the strain profile correctly and accurately. Previously, an array of electrical strain gages as well as fiber Bragg grating (FBG) sensors had been used successfully to capture the profile of BFS of a SL joint by applying sensors on the backface of an adherend, near the overlap zone. In this work, the backface technique is improved by replacing an array of strain sensors by an ordinary optical fiber (without FBG sensors) connected to an optical backscatter reflectometer. The great advantage of this system over the more conventional arrays of FBG is that the entire length of the fiber can be used for sensing strains, and hence it provides a better spatial resolution. The experimental results are compared with finite element analyses, which were further validated by two-dimensional digital image correlation measurements.

Temperature Dependence of the Anisotropy and Creep in a Single-Crystal Nickel Superalloy

The thermomechanical response of the second-generation single-crystal nickel superalloy (SC180) was obtained for wide range of temperatures (25–1000°C). Uniaxial tension and stress relaxation experiments were performed to study the influence of [100] and [110] crystallographic orientation on stress anisotropy and creep responses. Experiments were conducted using micromechanical testing systems and strains were measured using two-dimensional digital image correlation technique. Results were reported on coefficient of thermal expansion, Young’s modulus (E), yield strength, work hardening (n), and activation energies (Q). The stress relaxation experiments were used to calculate activation energy in [100] and [110] crystallographic directions and found to be 300 kJ/mol and 350 kJ/mol, respectively.

Experimental and numerical investigations into the ductile fracture during the forming of flat-rolled 5083-O aluminum alloy sheet

The present research aimed to investigate the ductile fracture during the forming of 5083-O aluminum alloy sheet and establish a rational criterion to predict it. Seven specimens with different shapes were tension-tested, and the main deformation areas of the specimens were subjected to various stress states ranging from shear to plane strain. Fracture strains, representing the forming limits, were determined by applying a combined experimental–numerical approach. Surface strain fields were measured by using the two-dimensional digital image correlation (DIC) method, while the local stress and strain histories were predicted by finite element (FE) simulations. The stress triaxiality and Lode parameter (characterizing the stress state) of fracture onset were calculated for each specimen. The analysis indicated that the fracture strain was insensitive to the Lode parameter but strongly dependent on the stress triaxiality. The fracture locus in the space of the fracture strain and stress triaxiality fitted in with the Johnson–Cook fracture criterion. The Johnson–Cook fracture criterion was then implemented into the ABAQUS/Explicit code to predict the fracture onsets of three specimens subjected to ball punch deformation. The strain fields of these specimens measured by DIC-3D were in agreement with the numerical results. Their fracture locations and strokes were accurately predicted by using the Johnson–Cook fracture criterion.

Anisotropy effect on the fracture model of DP980 sheets considering the loading path

This paper is concerned with the anisotropy effect of DP980 sheets on the fracture model considering the variation of the loading path. For better application of a material, it is essential to understand its plastic behavior to predict the onset of the fracture in metals accurately. Especially in sheet metal forming, the anisotropy effect on the fracture strain should be taken into account to consider the fracture behavior more properly. In order to construct the fracture model, three different types of tests were carried out to obtain the facture strain from uniaxial tensile tests with classic dog-bone specimens, pure shear specimens and plane strain grooved specimens using Instron 5583. Each specimen was made along three different orientations: the angle of 0°(RD); 45°(DD); 90°(TD) with respect to the rolling direction of sheet metals to evaluate the anisotropy effect on the fracture strain. Strains were measured using the two-dimensional digital image correlation (2D-DIC) method. In order to consider the influence of anisotropy on the fracture strain accurately, the Hill’48 yield function was applied to a Lou–Huh fracture model instead of von Mises yield function. A modified Lou–Huh fracture model was represented in the space of the strain path to use the experiment result directly. The effect of the change of the loading path was also applied to the modified Lou–Huh fracture model to consider damage accumulation in the material.

2D-DIC for the quantitative validation of FE simulations and non-destructive inspection of aft end debonds in solid propellant grains

This paper presents the application of 2D full-field optical displacement measurements for quantitative validation of Finite Element (FE) simulations in solid propellant grains. The experimental validation is done using an instrumented small-scale Structural Test Motor (STM) subjected to thermal load. Displacement field at the aft end of STM has been computed using two-dimensional Digital Image Correlation (2D-DIC) and compared with FE simulation results. The current research aims towards highlighting two important aspects in the application of 2D field measurements with solid propellants. Firstly, the effect of out-of-plane deformation of the propellant face, due to thermal shrinkage, on the experimentally measured displacement field is studied and a compensation is applied to correct for the resulting displacement bias. Secondly, the potential of using full-field displacements for non-destructive testing is demonstrated; from the analysis of observed displacement profile at the aft end, a small debond near the propellant–insulation–metal triple interface is identified and the FE model is then updated based on this information to complete the validation. It is also shown that the complete profile of the debond can be approximated in this manner.

Gaussian pre-filtering for uncertainty minimization in digital image correlation using numerically-designed speckle patterns

This paper discusses the effect of pre-processing image blurring on the uncertainty of two-dimensional digital image correlation (DIC) measurements for the specific case of numerically-designed speckle patterns having particles with well-defined and consistent shape, size and spacing. Such patterns are more suitable for large measurement surfaces on large-scale specimens than traditional spray-painted random patterns without well-defined particles. The methodology consists of numerical simulations where Gaussian digital filters with varying standard deviation are applied to a reference speckle pattern. To simplify the pattern application process for large areas and increase contrast to reduce measurement uncertainty, the speckle shape, mean size and on-center spacing were selected to be representative of numerically-designed patterns that can be applied on large surfaces through different techniques (e.g., spray-painting through stencils). Such “designer patterns” are characterized by well-defined regions of non-zero frequency content and non-zero peaks, and are fundamentally different from typical spray-painted patterns whose frequency content exhibits near-zero peaks. The effect of blurring filters is examined for constant, linear, quadratic and cubic displacement fields. Maximum strains between ±250 and ±20,000µε are simulated, thus covering a relevant range for structural materials subjected to service and ultimate stresses. The robustness of the simulation procedure is verified experimentally using a physical speckle pattern subjected to constant displacements. The stability of the relation between standard deviation of the Gaussian filter and measurement uncertainty is assessed for linear displacement fields at varying image noise levels, subset size, and frequency content of the speckle pattern. It is shown that bias error as well as measurement uncertainty are minimized through Gaussian pre-filtering. This finding does not apply to typical spray-painted patterns without well-defined particles, for which image blurring is only beneficial in reducing bias errors.

Diffraction-Assisted Image Correlation for Three-Dimensional Surface Profiling

The recently developed Diffraction-Assisted Image Correlation (DAIC) method (Xia et al. Exp Mech 53(5):755–765, 2013) provides a simple and accurate means for three-dimensional (3D) full-field deformation measurement. In the DAIC method, a test specimen is viewed through a transmission diffraction grating, resulting in multiple diffracted views of the same specimen that encode 3D geometric information. Here, we extend the original DAIC method to permit quantitative measurement of 3D surface profiles. We show, through a pinhole projection model, that the 3D shape of an object surface can be reconstructed by simply performing two-dimensional digital image correlation (2D-DIC) analysis between the negative and positive first-order diffracted views. Test results on a Barbie doll’s face and a set of well-defined cylindrical, conical and step surfaces are presented to illustrate the implementation and performance of the proposed surface profiling method.

Investigation of Portevin-Le Chatelier effect in 5456 Al-based alloy using digital image correlation

A variety of experimental methods have been proposed for Portevin-Le Chatelier (PLC) effect. They mainly focused on the in-plane deformation. In order to achieve the high-accuracy measurement, three-dimensional digital image correlation (3D-DIC) was employed in this work to investigate the PLC effect in 5456 Al-based alloy. The temporal and spatial evolutions of deformation in the full field of specimen surface were observed. The large deformation of localized necking was determined experimentally. The distributions of out-of-plane displacement over the loading procedure were also obtained. Furthermore, a comparison of measurement accuracy between two-dimensional digital image correlation (2D-DIC) and 3D-DIC was also performed. Due to the theoretical restriction, the measurement accuracy of 2D-DIC decreases with the increase of deformation. A maximum discrepancy of about 20% with 3D-DIC was observed in this work. Therefore, 3D-DIC is actually more essential for the high-accuracy investigation of PLC effect.

Accurate ex situ deformation measurement using an ultra-stable two-dimensional digital image correlation system.

In situ planar deformation measurement using two-dimensional digital image correlation (2D-DIC) and a fixed camera has been fully investigated in many published works and widely used for various applications. However, in certain special cases (e.g., long-term deformation monitoring of engineering structures, or deformation measurement of a specimen subjected to uncommon loading), it is very difficult or impossible to carry out in situ deformation measurement using 2D-DIC, and necessitates ex situ 2D-DIC measurements for a repositioned specimen or using a relocated camera. To achieve accurate measurements, the error sources of ex situ 2D-DIC measurements should be identified and minimized, and the strain accuracy of ex situ 2D-DIC measurements should be quantified. In this work, the potential error sources of ex situ 2D-DIC measurements are first discussed in detail. Then, to mitigate the errors associated with these issues, an ultra-stable 2D-DIC system combining the idea of active imaging and a well-designed bilateral telecentric lens is established, which is invariant to the potential variations in ambient lighting and the possible small out-of-plane motions in object surface and/or image plane. The established ultra-stable 2D-DIC system is first compared with the regular 2D-DIC setup in determining the surface strains of an unstrained sample. Then, ex situ residual stress measurement using the hole-drilling technique and the established 2D-DIC system was conducted and compared with the applied ones. The results of this work confirm that the accuracy of ex situ 2D-DIC deformation measurements using the proposed system is of high fidelity, and can be used for accurate deformation measurement in practical ex situ tests.

In-plane displacement and strain measurements using a camera phone and digital image correlation

In-plane displacement and strain measurements of planar objects by processing the digital images captured by a camera phone using digital image correlation (DIC) are performed in this paper. As a convenient communication tool for everyday use, the principal advantages of a camera phone are its low cost, easy accessibility, and compactness. However, when used as a two-dimensional DIC system for mechanical metrology, the assumed imaging model of a camera phone may be slightly altered during the measurement process due to camera misalignment, imperfect loading, sample deformation, and temperature variations of the camera phone, which can produce appreciable errors in the measured displacements. In order to obtain accurate DIC measurements using a camera phone, the virtual displacements caused by these issues are first identified using an unstrained compensating specimen and then corrected by means of a parametric model. The proposed technique is first verified using in-plane translation and out-of-plane translation tests. Then, it is validated through a determination of the tensile strains and elastic properties of an aluminum specimen. Results of the present study show that accurate DIC measurements can be conducted using a common camera phone provided that an adequate correction is employed.

Integrating fringe projection and digital image correlation for high-quality measurements of shape changes

An approach for the measurement of surface displacement fields in three dimensions is presented based on the combination of two-dimensional digital image correlation with fringe projection. Only a single RGB image is required at each deformation state, thereby allowing real-time data acquisition, which is achieved using red speckle and projected blue fringes that are captured in the single image and separated using a Bayer filter. The approach allows both a perpendicular alignment relative to a flat reference surface and self-calibration, i.e., no calibration object is employed. The minimum measurement uncertainty of such a system is found to be 0.0083±0.00239 and 0.0238±0.0068 mm, respectively, for the in-plane and out-of-plane displacements. The potential of the approach is demonstrated for an elastic membrane undergoing large (5 to 20 mm) applied out-of-plane displacements, and the results show no significant difference (<1%) in the measured in-plane displacement fields compared with a commercially available system for stereoscopic digital image correlation.

Ultraviolet digital image correlation (UV-DIC) for high temperature applications.

A method is presented for extending two-dimensional digital image correlation (DIC) to a higher range of temperatures by using ultraviolet (UV) lights and UV optics to minimize the light emitted by specimens at those temperatures. The method, which we refer to as UV-DIC, is compared against DIC using unfiltered white light and DIC using filtered blue light which in the past have been used for high temperature applications. It is shown that at low temperatures for which sample glowing is not an issue all three methods produce the same results. At higher temperatures in our experiments, the unfiltered white light method showed significant glowing between 500 and 600 °C and the blue light between 800 and 900 °C, while the UV-DIC remained minimally affected until the material began nearing its melting point (about 1260 °C). The three methods were then used to obtain the coefficient of thermal expansion as a function of temperature for the nickel superalloy Hastelloy-X. All three methods give similar coefficients at temperatures below which glowing becomes significant, with the values also being comparable to the manufacturers specifications. Similar results were also seen in uniaxial tension tests.

High-performance hybrid CPU and GPU parallel algorithm for digital volume correlation

We present a hybrid Message Passing Interface (MPI) and graphics processing unit (GPU)-based parallel digital volume correlation (DVC) algorithm for measuring three-dimensional (3D) displacement and strain fields inside a material undergoing motion or deformation. Our algorithm achieves resolution comparable to that achieved in two-dimensional (2D) digital image correlation (DIC), in time that is commensurate with the image acquisition time, in this case, using microcomputed tomography ([Formula: see text] CT) for scanning images. For DVC, the volume of data and number of correlation points both grow cubically with the linear dimensions of the image. We turn to parallel computing to gain sufficient processing power to scale to high resolution, and are able to achieve more than an order-of-magnitude increase in resolution compared with previous efforts that are not based on a parallel framework.

Fracture modelling of DP780 sheets using a hybrid experimental-numerical method and two-dimensional digital image correlation

This paper is concerned with the construction of fracture envelopes of DP780 sheets using two methods: a hybrid experimental-numerical method; two-dimensional digital image correlation (2D-DIC). For the hybrid method, four types of ductile fracture tests were carried out covering a wide range of stress states on specimens: with a central hole; two symmetric circular notches; flat grooved; and diagonally double-notched. Based on the fracture strain and loading paths identified with finite element simulation, a fracture envelope was obtained by employing the three-parameter modified Mohr-Coulomb fracture model. In addition, the fracture surface strain was directly measured using 2D-DIC. Loading histories of each test were extracted from a surface element of a three dimensional finite element model. The comparison of fracture envelopes constructed by the two methods reveals that there is little difference. Thus, it can be concluded that 2D-DIC is applicable to fracture modelling of DP780 sheets despite the assumption of the plane stress condition even after necking.