Digital Image Correlation Approach Research Articles

Peridynamics-Based Digital Image Correlation Algorithm Suitable for Cracks and Other Discontinuities

AbstractThis work presents a novel approach to circumventing the difficulties associated with using digital image correlation (DIC) for problems that involve discontinuities like fracture and fragmentation. The proposed method provides a simple means to combine traditional DIC approaches with peridynamics to achieve full-field displacements, even in the vicinity of cracks. In regions where DIC is accurate and effective, the displacement field is determined using a traditional approach. In discontinuous regions, the displacement field is solved for using peridynamics. This study shows that this method can be used successfully for investigating damage in materials such as nuclear graphite and fiber-RC (FRC). It also shows that the proposed method alleviates nonphysical strains in the vicinity of cracks, common to alternative approaches, which detrimentally degrade the quality of postprocessed quantities of interest such as fracture toughness.

Crack initiation and propagation in epoxy asphalt concrete in the three-point bending test

To determine the micro-cracking mechanism in epoxy asphalt concrete (EAC) so as to guide the crack rehabilitation strategy, the process of cracking initiation and propagation in the EAC were measured using a digital image correlation approach, and was simulated based on an extended finite element method. Firstly, the three-point bending test was conducted during which cracking initiation and propagation in EAC were measured and fracture parameters of mastic cement and aggregate were determined. Then numerical simulation was performed based on an EAC microstructure model. Simulation results of cracking match well with the micro-crack initiation and propagation process in EAC measured in the laboratory at a low temperature. The specimen fracture process and the cracking propagating route are discussed based on the analysis results.

Sequential-Digital Image Correlation for Mapping Human Posterior Sclera and Optic Nerve Head Deformation

Optic nerve head (ONH) deformations may be involved in the onset or further development of glaucoma, including in patients with relatively normal intraocular pressures (IOPs). Characterizing posterior scleral deformations over physiological pressures may provide a better understanding of how changes in IOP lead to changes in the mechanical environment of the ONH and possibly retinal ganglion cell death. Pressure inflation measurement test protocols are commonly used to measure deformation of the peripapillary sclera with full-field noncontact optical methods. The purpose of this work was to develop and validate a new sequential 3D digital image correlation (S-DIC) approach for quantification of posterior scleral pressure induced deformation that improves z (in-depth) resolution of the DIC measurement without losing in-plane sensitivity, while also being able to contour and map deformations of the complex-shaped ONH. Our approach combines two orthogonal axes of parallax with standard 3D DIC methods using a single high-resolution camera. The enhanced capabilities of S-DIC with respect to standard 3D DIC has been demonstrated by carrying out a complete benchmark for shape, deformation, and strain measurement on an object of known complex geometry. Our S-DIC method provided a reconstruction accuracy of 0.17% and an uncertainty in z-position measurement of 8 μm. The developed methodology has also been applied to a human posterior scleral shell, including the full peripapillary sclera and optic nerve. The relatively inexpensive S-DIC approach may provide new information on the biomechanical deformations of the optic nerve head and, thus, the death of retinal ganglion cells in primary open angle glaucoma.

Pixel-level robust digital image correlation

Digital Image Correlation (DIC) is a well-established non-contact optical metrology method. It employs digital image analysis to extract the full-field displacements and strains that occur in objects subjected to external stresses. Despite recent DIC progress, many problematic areas which greatly affect accuracy and that can seldomly be avoided, received very little attention. Problems posed by the presence of sharp displacement discontinuities, reflections, object borders or edges can be linked to the analysed object's properties and deformation. Other problematic areas, such as image noise, localized reflections or shadows are related more to the image acquisition process. This paper proposes a new subset-based pixel-level robust DIC method for in-plane displacement measurement which addresses all of these problems in a straightforward and unified approach, significantly improving DIC measurement accuracy compared to classic approaches. The proposed approach minimizes a robust energy functional which adaptively weighs pixel differences in the motion estimation process. The aim is to limit the negative influence of pixels that present erroneous or inconsistent motions by enforcing local motion consistency. The proposed method is compared to the classic Newton-Raphson DIC method in terms of displacement accuracy in three experiments. The first experiment is numerical and presents three combined problems: sharp displacement discontinuities, missing image information and image noise. The second experiment is a real experiment in which a plastic specimen is developing a lateral crack due to the application of uniaxial stress. The region around the crack presents both reflections that saturate the image intensity levels leading to missing image information, as well as sharp motion discontinuities due to the plastic film rupturing. The third experiment compares the proposed and classic DIC approaches with generic computer vision optical flow methods using images from the popular Middlebury optical flow evaluation dataset. Results in all experiments clearly show the proposed method's improved measurement accuracy with respect to the classic approach considering the challenging conditions. Furthermore, in image areas where the classic approach completely fails to recover motion due to severe image de-correlation, the proposed method provides reliable results.

Experiments and modeling of high-crystalline polyethylene yielding under different stress states

The mechanical response of high-density polyethylene (HDPE) was examined under different stress states. The biaxial yielding of HDPE material was investigated from a series of biaxial shear/tension and shear/compression tests using butterfly-shaped specimens deformed with an Arcan apparatus equipped with a digital image correlation (DIC) system for local strain measurements. In order to investigate a wider range of stress states, notched round bar specimens with different curvature radii were also tested using a video-controlled tensile testing apparatus. More conventional mechanical loading paths (uniaxial tension/compression and simple shear tests) were also examined to provide better insights on the stress state effects. The present investigation is more particularly focused on the yield envelope determination of HDPE material. A combined DIC and analytical approach was proposed to measure the yield strengths of butterfly-shaped specimens in the region where the yielding occurs. The relevance of classical yield criteria, exhibiting dependence on both the deviatoric and hydrostatic stresses, is verified. Considering HDPE as a heterogeneous medium consisting of a percolated crystalline matrix and a discrete amorphous phase, a micromechanics-based yield locus is tested. The experimental biaxial yield data are found to support this theoretical yield criterion and thus the suggested morphological representation for high-crystalline polymers.

Damage evolution study of sandstone by cyclic uniaxial test and digital image correlation

Abstract The damage evolution process and crack development of rocks under cyclic uniaxial compression were studied. In this work, sandstone samples subjected to axially cyclic loading with different amplitudes were tested and optical images of specimen surface in loading–unloading periods are presented for digital image correlation (DIC) analysis. The experimental results obtained are then used to determine apparent strains of the sandstones by means of DIC approach. The apparent strains are in turn employed to describe the process of damage evolution, crack nucleation and propagation until failure. Axial load–displacement curves show that the residual displacement increases slightly with an increase in cycles and the dissipated energy decreases rapidly after the first cycle then tends to remain constant. It is found from the full-field apparent strains that when the amplitude of the cyclic load exceeds a certain value, the value of cycles has a significant effect on the apparent strain localization and damage evolution.

A novel speckle pattern—Adaptive digital image correlation approach with robust strain calculation

Digital image correlation (DIC) has seen widespread acceptance and usage as a non-contact method for the determination of full-field displacements and strains in experimental mechanics. The advances of imaging hardware in the last decades led to high resolution and speed cameras being more affordable than in the past making large amounts of data image available for typical DIC experimental scenarios. The work presented in this paper is aimed at maximizing both the accuracy and speed of DIC methods when employed with such images. A low-level framework for speckle image partitioning which replaces regularly shaped blocks with image-adaptive cells in the displacement calculation is introduced. The Newton–Raphson DIC method is modified to use the image pixels of the cells and to perform adaptive regularization to increase the spatial consistency of the displacements. Furthermore, a novel robust framework for strain calculation based also on the Newton–Raphson algorithm is introduced. The proposed methods are evaluated in five experimental scenarios, out of which four use numerically deformed images and one uses real experimental data. Results indicate that, as the desired strain density increases, significant computational gains can be obtained while maintaining or improving accuracy and rigid-body rotation sensitivity.

Identification of a crack propagation law by digital image correlation

It is proposed to use digital image correlation (DIC) to identify parameters governing crack propagation of commercially pure titanium. To achieve this goal, crack tip location, stress intensity factor, T-stress and plastic zone size are sought. Most of the DIC approaches are based upon local analyzes of displacements, and their subsequent projection onto a set of mechanically relevant fields. It is proposed to perform these two sequential steps in a unique (and integrated) way, and to compare the results with a global approach to DIC with subsequent post-processing. A priori performances of two global approaches are compared, and a propagation law is identified form the series of raw images of a fatigue test on commercially pure titanium with the integrated approach that yields better results.

Digital image correlation approach to cracking and decohesion in a brittle coating/ductile substrate system

By using a digital image correlation technique, the full/local field strain in a brittle coating/ductile substrate system during tension has been successfully monitored. One of the most important experimental results indicates that the distribution of interfacial shear stress in the segmented coating is antisymmetric about the center, which clarifies several controversial assumptions introduced in theoretical models. Two key mechanical properties of thermal barrier coatings, fracture strength in coating and interfacial adhesion strength, were determined as 35.0 ± 4.6 and 14.1 ± 3.2 MPa, respectively, which are consistent with available experimental data.

On the Identification and Validation of an Anisotropic Damage Model Using Full-field Measurements

Two different mechanical tests are performed on a laminated composite coupon to induce an anisotropic damage affecting essentially shear modulus softening. The first test is a uniaxial tension loading on a straight coupon, which is used to evaluate the damage law using a conventional approach, while the second contains a notch that enhances dramatically the strain (and hence damage) heterogeneity. A global digital image correlation approach is used to quantify the kinematic fields all along the loading path of the second experiment. Displacement fields are hence evaluated based on a finite element type discretization. A further exploitation based on the reconditioned equilibrium gap method (and without any further information) gives access to a quantitative measurement of the damage law. The latter approach makes use of a finite element model based on the very same mesh and element shape function. This full-field-based identification method compares very well with traditional techniques, up to the stage where macroscopic localization prevents their subsequent exploitations. Moreover, it is shown that neither the type of mechanical test, nor the discretization of the displacement field, affects the identification of the damage law.

On the use of the digital image correlation method for heterogeneous deformation measurement of porous solids

In this paper, several crucial issues arising from the application of the digital image correlation (DIC) method to the measurement of heterogeneous deformation of porous solids are discussed. To handle samples with complex geometry, the performance of the two commonly employed DIC methods, namely the subset-based DIC and the finite-element based DIC methods are first evaluated and compared. A combined DIC approach and an adaptive DIC approach suitable for samples with discontinuities/holes are then proposed. Aluminum plates with circular holes subject to compressive loading are employed to evaluate the accuracy of the proposed methods. It has been found that in addition to other factors such as the number of pixels and speckle size, the orientation of the camera lens also plays an important role on the measurement accuracy. A calibration method for the adjustment of camera orientation is proposed, which leads to a good agreement between the experimentally measured displacements and finite element simulation results. Another finding of the presented work is that for relatively stiff specimens, the deformation of the loading system itself must be considered in order to obtain an accurate displacement.

Fracture mechanisms in soft rock: Identification and quantification of evolving displacement discontinuities by extended digital image correlation

Deformation of rocks at shallow depths (i.e., at low confining pressures) is primarily driven by the evolution of new fractures or the propagation and coalescence of pre-existing ones. In this paper, fracture evolution from inclined flaws (cuts) in a soft rock deformed under plane-strain uniaxial compression is analyzed using high-resolution digital photographs and Digital Image Correlation (DIC). DIC provides a full-field measurement of the kinematics and thus allows the analysis of non-homogeneous deformation. However, standard DIC approaches do not handle correctly fractures, as they are displacement discontinuities. Therefore, an extended DIC method is developed that allows automatic tracing of discontinuities and their quantification in terms of the displacement jumps along their length. The analysis of spatial and temporal fracture evolution using this method enhances the understanding of the phenomena in that it provides experimental quantification of fracture mode (opening/closing and shearing).

3D digital image correlation methods for full-field vibration measurement

In the area of modal test/analysis/correlation, significant effort has been expended over the past twenty years in order to make reduced models and to expand test data for correlation and eventual updating of the finite element models. This has been restricted by vibration measurements which are traditionally limited to the location of relatively few applied sensors. Advances in computers and digital imaging technology have allowed 3D digital image correlation (DIC) methods to measure the shape and deformation of a vibrating structure. This technique allows for full-field measurement of structural response, thus providing a wealth of simultaneous test data. This paper presents some preliminary results for the test/analysis/correlation of data measured using the DIC approach along with traditional accelerometers and a scanning laser vibrometer for comparison to a finite element model. The results indicate that all three approaches correlated well with the finite element model and provide validation for the DIC approach for full-field vibration measurement. Some of the advantages and limitations of the technique are presented and discussed.

Simultaneous velocity and diffusivity mapping in the case of 3-D transient heat diffusion: heat pulse thermography and IR image sequence analysis

A non uniform heat pulse is used as a non intrusive marker on opaque liquids or highly deformable solid media, in order to estimate both the full velocity and thermal diffusivity fields from thermal image processing. The difficult issue of identifying both the local transport and diffusion properties is addressed by adopting two different inverse approaches. First, a new specific version of the Total Least Squares (TLS) method is implemented, yielding both sensitivity analysis and confidence estimation, with no prior knowledge of the measurement noise variance. A second strategy is proposed from an iterative Digital Image Correlation (DIC) approach, yielding an hybrid DIC/TLS method. Both methods are first validated on simulated data,and calibrated with homogenous materials under uniform displacement. Two different grids are used in order to analyze the effect of the spatial period of the heat pulse on the retrieved parameters. Then, the effective velocity and thermal diffusivity fields are retrieved from an experimental bench in the case of a moving Thermisorel® sample. The Fine grid is found to yield better estimation results than the coarse grid. Thermal diffusivity mapping obtained with the TLS method is more trusted.