Digital Image Correlation Data Research Articles

A novel energetic approach for predicting fatigue crack growth in pseudoelastic NiTi alloys

A novel strain energy-based method for analyzing fatigue crack growth (FCG) in pseudoelastic Nickel-Titanium (NiTi) -based shape memory alloys (SMAs) is proposed. It uses the dissipated energy to predict the FCG. This parameter can be computed either by volume integration of near-crack-tip strain energy or by the force–displacement hysteresis measured at the loading pins of the sample. Eccentrically-loaded single Edge Crack tension samples (ESE(T)) were used for FCG tests. High resolution Digital Image Correlation (DIC) was used to capture near-crack-tip displacement and strain fields. DIC data were also used to estimate the effective stress intensity range by regressing the William’s expansion series. Furthermore, a constitutive model for SMAs was used for FE simulations of ESE(T) samples, and the evolution of the dissipated energy during FCG tests was computed. Experimental FCG rate, plotted against the cyclic dissipated energy, are well fitted by a power law equation (R2 = 0.99). This result would open new perspectives to develop predictive models for FCG in SMAs considering intricate thermomechanical coupling mechanisms and material non-linearities.

Data driven statistical method for the multiscale characterization and modelling of fiber reinforced composites

Multiscale analysis of composite laminates allows for predicting the mechanical response of these materials avoiding cumbersome experimental campaigns. The matrix and fibre material properties and the size of the Representative Volume Element (RVE) are the main parameters affecting the accuracy of multiscale models. This paper proposes a statistical inverse method to calibrate micromechanical material parameters from macroscale experiments and 3D reconstruction. First, glass fiber reinforced epoxy laminates have been analysed with Computer Tomography (CT), then, the material 3D microstructure has been reconstructed and fibre, matrix, and voids were segmented. Tensile tests have been performed on the composite specimen, measuring the surface strains with a Digital Image Correlation (DIC) system. The reconstructed volume, converted to a voxel mesh, has been used to compute the homogenized response of composite by Fast Fourier Transform (FFT) analysis. By comparing the marginal distribution of homogenized material stiffness extracted from DIC data of tensile tests, with the conditioned distribution computed by varying the FFT model parameter, a Stochastic Volume Element (SVE) is finally calibrated. A probabilistic multiscale model based on the SVE that propagates the uncertainty from the microscale to the structure level is presented.

Experimental characterization of forming behavior of 3rd GEN AHSS

Third generation of Advanced High Strength Steels (3rd GEN AHSS) for cold stamping combine superior formability and ductility at higher strengths. Their combination with the Laser Welded Blanks (LWB) technology is one answer to improve safety and weight reduction of vehicules for automakers. In this study, a Nakazima test is used to evaluate the forming behavior of LWB based 3rd GEN AHSS and to estimate their Forming Limit. Tests equipped with a Digital Image Correlation (DIC) system are carried out for several materials combinations in order to understand the influence of the weld on LWB formability. The experimental results are used to determine the critical area during this forming test and a dedicated FLC for each material combination. The location of the onset of failure is investigated, from optical observations, DIC data as well as modeling. It appears that different failure locations are obtained depending on both AHSS combination and strain paths. In order to improve the accuracy of the determination of the onset of the failure, a method is proposed to evaluate the failure risk in base metal and in the vicinity of the weld. The improvement of failure prediction will be very helpful to analyze part formability and provide design recommendations.

Automated slip system identification and strain analysis framework using high-resolution digital image correlation data: Application to a bimodal Ti-6Al-4V alloy

Statistical analysis of slip system identification and quantitative study on local plasticity is crucial to understand the collective deformation by the sub-grain-scale slip activities in polycrystalline metallic materials. In this study, an automated framework for identifying slip system and assessing strain localization of slip bands termed ASSISL (automated Slip System Identification and Strain Localization analysis of slip bands) is introduced, using the results from high resolution digital image correlation (HR-DIC), and is demonstrated on 1591 primary α grains of a bimodal Ti-6Al-4V alloy under tensile loading. The framework includes: (1) alignment of electron backscattered diffraction (EBSD) maps with strain field maps from HR-DIC through treatment of EBSD distortion, (2) slip band orientations identification from strain field map of each grain through a Radon-transform-based algorithm, (3) slip system assignment with combined Schmid factor and critical resolved shear stress analysis, (4) quantification of plasticity by slip activities which provides information on the numbers, positions and mean strain of slip bands in each grain. The confidence of ASSISL is validated through comparing current statistical results with existing literature, as well as examining reasons for grains assigned with wrong slip systems, which accounts for 9.6 percent (153 out of 1591) of the primary α grains. A comparison between ASSISL and other HR-DIC-based slip identification methods is also conducted. This framework provides a method for analyzing slip activities in a large number of grains of polycrystalline metals in a time-saving and automated fashion.

Effect of cyclic loading on the mobilization of unit base resistance of model piles jacked in sand

This paper reports the results of a series of monotonic compressive and cyclic load tests performed on a closed-ended jacked model pile installed in a half-cylindrical calibration chamber with image analysis capabilities. The monotonic compressive load tests were carried out before and after performing a displacement-controlled cyclic load test to determine the impact of cycling on unit base resistance. Digital images of the sand and the model pile were taken during cyclic loading and processed using the digital image correlation (DIC) technique to obtain the cumulative displacement and strain fields in the sand domain. The results show that the ultimate unit base resistance can drop significantly after cycling. The magnitude of the drop in ultimate unit base resistance depends on both the magnitude of the cyclic displacement amplitude and the number of cycles. However, the unit base resistance at plunging increases after large-displacement half-amplitude cycling. The increase in unit base resistance at plunging after cycling is linked to the flow of sand particles to the zone below the conical base, the occurrence of sand particle crushing, and the dilative behavior of the sand outside a bulb of crushed particles formed during cyclic loading. The processed DIC data show that for cyclic displacement half-amplitudes ∆wcyclic less than or equal to 0.25 mm (0.0007B), the cumulative radial and vertical displacements in the soil domain normalized by ∆wcyclic are negligible. Values of ∆wcyclic greater than 0.25 mm produce normalized displacements in the soil domain that increase as the cyclic displacement amplitude increases.

Study on crack propagation of functionally graded ultra-high performance cementitious composite

This study investigates the damage process and crack propagation of functionally graded ultra-high-performance cementitious composite (FGUHPCC) with different layers (two and three layers) and fibers types (steel and PVA) through four-point bending test. The evolution process of surface displacement and strain of the sample was analyzed by digital image correlation (DIC) technology, and the influence of the top, middle and bottom layers of FGUHPCC on the damage pattern are also investigated by RFPA3D program. The results indicate that functionally graded design improves the equivalent flexural strength and deflection capacity of the specimens compared with the UHPCC with single layer design. FGUHPCC with three layers and mixed steel and polyvinyl alcohol fibers exhibited best flexural behavior. According to the recorded DIC data and corresponding analyses, the crack opening displacement (COD) profile of functionally graded design with three layers is mutable and nonlinear, and the maximum crack propagation speed is 0.096 L/s, which is the lowest compared with other tested specimens, and the largest crack area ratios are about 0.04% and 0.147% at LOP and MOR, respectively. Based on RFPA3D results, with the change of fiber content, the cumulative AE energy growth rate (k) of bottom layer is about 2.7 times and 1.5 times of that of the middle layer and the top layer, respectively.

Applying a homogeneous pressure distribution to the upper vertebral endplate: Validation of a new loading system, pilot application to human vertebral bodies, and finite element predictions of DIC measured displacements and strains

Image-based personalized Finite Element Models (pFEM) could detect alterations in physiological deformation of human vertebral bodies, but their accuracy has been seldom reported. Meaningful validation experiments should allow vertebral endplate deformability and ensure well-controlled boundary conditions. This study aimed to (i) validate a new loading system to apply a homogeneous pressure on the vertebral endplate during vertebral body compression regardless of endplate deformation; (ii) perform a pilot study on human vertebral bodies measuring surface displacements and strains with Digital Image Correlation (DIC); (iii) determine the accuracy of pFEM of the vertebral bodies.Homogeneous pressure application was achieved by pressurizing a fluid silicone encased in a rubber silicone film acting on the cranial endplate. The loading system was validated by comparing DIC-measured longitudinal strains and lower-end contact pressures, measured on three homogeneous pseudovertebrae of constant transversal section at 2.0 kN, against theoretically calculated values. Longitudinal strains and contact pressures were rather homogeneous, and their mean values close to theoretical calculations (5% underestimation).DIC measurements of surface longitudinal and circumferential displacements and strains were obtained on three human vertebral bodies at 2.0 kN. Complete displacement and strain maps were achieved for anterolateral aspects with random errors ≤0.2 μm and ≤30 μstrain, respectively. Venous plexus and double curvatures limited the completeness and accuracy of DIC data in posterior aspects.pFEM of vertebral bodies, including cortical bone mapping, were built from computed tomography images. In anterolateral aspects, pFEM accuracy of the three vertebrae was: (i) comparable to literature in terms of longitudinal displacements (R2>0.8); (ii) extended to circumferential displacements (pooled data: R2>0.9) and longitudinal strains (zero median error, 95% error: <27%). Circumferential strains were overestimated (median error: 39%).The new methods presented may permit to study how physiological and pathologic conditions influence the ability of vertebral endplates/bodies to sustain loads.

Analysis of Non-uniform Local Strain in Corroded Reinforcing Bar in Concrete using Digital Image Correlation

Change in the mechanism of post corrosion bond behavior was investigated in a previous experimental study by analyzing the deformations in concrete using digital image correlation (DIC). Using their DIC data, this study focuses on the local strain non-uniformity in reinforcing bar during the uniaxial tensile test due to the effect of rib height reduction and corrosion layer. The results show that, the local strain below the rib tip is higher than that below the flat part between ribs, due to compressive stress resulting from mechanical interaction at the rib tip. Furthermore, the non-uniformity in local strain is less pronounced in highly corroded reinforcing bar because less compressive stress is transferred due to gentler rib slope. In highly corroded reinforcing bar, the appearance of non-uniformity in local strain is delayed since the accumulated corrosion product on the reinforcing bar surface delays the mechanical interaction and stress transfer between the concrete and the reinforcing bar.

Thermal to digital image correlation image to image translation with CycleGAN and Pix2Pix

This paper explores the use of image-to-image translation techniques for translating thermography into digital image correlation data. A Pix2Pix model and a CycleGAN model were trained on a dataset of paired and unpaired images to translate between the two image domains. The results show that while the CycleGAN model exhibited some challenges in correlating images obtained by different optical systems, the Pix2Pix model demonstrates promising potential. The trained Pix2Pix model were also validated on unseen data, providing confidence for future research in this area.

An optimization method for stochastic reconstruction from empirical data - A limestone rock strain fields study-case using digital image correlation data

Stochastic field reconstruction is a crucial technique to improve the accuracy of modern rock simulation. It allows explicit modelling of field conditions, often employed in uncertainty quantification analysis and upsampling and upscaling procedures. This paper presents a case-study of a framework for the stochastic reconstruction of rock’s strain field using experimental data. The proposed framework is applied to a limestone outcrop in which the strain field is measured using Digital Image Correlation (DIC). Assuming that the strain fields of these rocks are well-represented by Gaussian random fields, we capitalize on the algorithms used for training Gaussian processes to estimate the correlation family and the parameters that best represent these fields. Although the spherical and exponential kernels often correspond to the best fit, our results depict that each field shall be analyzed separately and no general rule can be defined. We show that the method is versatile and can be employed in any measurable field reasonably represented by a Gaussian random field. Therefore, the present work aims to highlight the following topics:•A study-case of stochastic strain field reconstruction aims to contribute to uncertainty quantification of rock experimental procedures.•A stochastic minimization algorithm is presented to solve the maximum likelihood estimation to define the most suitable hyper-parameter: correlation length.•The calculated hyper-parameters of a set correlation functions are presented to best reproduce the strain fields of a rock sample.

Behavior and Damage Characterization of Impulsively Loaded Cross-Laminated Timber Panels

This research bridges the gap between the quasi-static and high-strain-rate loading regimes in cross-laminated timber (CLT) by investigating two areas that have remained unstudied or elusive, i.e., rolling shear failure of CLT under impulsive, blast-like loading and intermediate strain rates in CLT. To study the conditions that would promote shear modes of failure, a novel, highly adaptable center-point testing system and methodology were developed that permitted the application of impulsive loading to undamaged CLT panels in a highly controlled and repeatable manner. The loading condition and low span-to-depth ratio (6.40 ≤ L:h ≤ 6.55) CLT were selected to encourage the development of shear modes of failure. Changes to the rotational rigidity at the boundary conditions allowed for the empirical simulation of realistic boundary conditions. Digital Image Correlation (DIC) and load cell data were used to identify failure modes following loss in resistance in the specimens. Overall, the experiment was successful in consistently eliciting shear modes of failure and providing damage characterization in impulsively loaded CLT. Shear modes of failure resulted in the dramatic loss of resistance in all specimens tested. Strain-rate enhancement in the dynamic apparent flexural stiffness of CLT of 1.3 to 7.2 times was observed. Lower levels of damage were observed in specimens with higher levels of boundary-condition rotational rigidity.

Observation of fracture process zone and produced fracture surface roughness in granite under Brazilian splitting tests

In many rock engineering structures, the fracture network's complexity and the generated fractures' roughness are immensely important for understanding the hydraulic conductivity of the rock mass. Despite significant progress, the mechanisms governing nucleation and growth of fracture process zone (FPZ) are still debated. The present work investigates the characteristics of the FPZ and the surface roughness of the produced fracture in granite under tensile loading at two different loading rates. Acoustic emission (AE) and digital image correlation (DIC) techniques were simultaneously employed to monitor the development of the FPZ in disc specimens of granite subjected to Brazilian splitting tests. In addition, several roughness parameters of the produced fractures were measured, and it was tried to link the locations and the magnitudes of the AE events with the height distribution of the fracture surface. Our results indicated that the loading rate significantly influences the extent of the FPZ. The tensile strength of specimens also increased with the loading rate. The FPZ estimated from the AE and DIC data were in good agreement; however, AE provided a slightly wider process zone for all specimens, regardless of the loading rate. It was observed that reducing the loading rate led to a larger FPZ and generated macroscopic fractures with higher roughness characteristics. Moment tensor inversion of the AE signals revealed that in all tested specimens, macroscopic fractures, which are conventionally considered tensile fractures, are composed of three main cracking mechanisms at the microscale, including tensile, shear, and compressive sources. Focal mechanisms distribution showed that the cracks might initiate under tensile, shear, or a combination of both mechanisms. This study’s findings can help in better understanding the rock fracturing processes under tensile loading in granitic rocks for field applications.

Automated identification of slip system activity fields from digital image correlation data

Crystallographic slip system identification methods are widely employed to characterize the fine scale deformation of metals. While powerful, they usually rely on the occurrence of discrete slip bands with clear slip traces and can struggle when complex mechanisms such as cross-slip, curved slip, diffuse slip and/or intersecting slip occur. This paper proposes a novel slip system identification framework, termed SSLIP (for Slip Systems based Local Identification of Plasticity), in which the measured displacement gradient fields (from Digital Image Correlation) are locally matched to the kinematics of one or multiple combined theoretical slip systems, based on the measured crystal orientations. To identify the amount of slip that conforms to the measured kinematics, an optimization problem is solved for every datapoint individually, resulting in a slip activity field for every considered slip system. The identification framework is demonstrated and validated on an HCP virtual experiment, for discrete and diffuse slip, incorporating 24 slip systems. Experimental case studies on FCC and BCC metals show how full-field identification of discrete slip, diffuse slip and cross-slip becomes feasible, even when considering 48 slip systems for BCC. Moreover, the methodology is extended into a dedicated cross-slip identification method, which directly yields the orientation of the local slip plane trace orientation, purely based on the measured kinematics and on one or two chosen slip directions. For even more challenging cases revealing a persistent uncertainty in the slip identification, a two-step identification approach can be employed, as is demonstrated on a highly challenging HCP virtual experiment.

Machine learning-assisted parameter identification for constitutive models based on concatenated loading path sequences

A hybrid strategy for the identification of material parameters of constitutive models is presented. One main challenge in the context of classic optimisation-based parameter identification schemes is the generation of adequate starting values for the multi-objective optimisation procedure. We address this issue by employing an artificial neural network that is trained with data obtained from the material model, more precisely speaking, from different solutions of the direct problem for homogeneous states of deformation. As a result, a solution of the inverse problem of parameter identification can be approximated with the help of a neural network processing experimental data. This approximate solution is used as a starting value for a subsequent, classic optimisation-based parameter identification approach. One key advantage of this strategy is that a network only has to be trained once per material model and can subsequently be applied for different materials. A rather sophisticated material model, incorporating gradient-enhanced damage coupled to plasticity in a geometrically non-linear setting, is used to demonstrate the capabilities of this approach. Two different strategies for the generation of training data are investigated and compared, and the neural network’s hyperparameters are optimised for improved prediction capabilities. In view of the calibration of material parameters related to the non-locality of the model, the obtained set of parameters is again considered as a starting value for a final optimisation step, where we consider inhomogeneous states of deformation and digital image correlation data.

A Pre-Process Enhanced Digital Image Correlation Approach for Smart Structure Monitoring

This research provides a practical guideline for Digital Image Correlation (DIC) data variations minimization in structural engineering through simple image processing techniques. The main objective of this research is to investigate the Pixel Averaging (P.A.) effect on the differential strain Diff(εx) variations. Three concrete arches were tested with three-point bending using the DIC technique for strain measurements. The measured strains are obtained through two virtual horizontal extensometers in the middle of each arch. The Diff(εx) was selected to avoid other 2D-DIC issues, such as the sample-camera out-of-plane movement. Three image cases, namely, one, ten, and twenty averaged images, were used for DIC analysis of each arch. The conditions of each image case are assessed by computing the Diff(εx) variance and the linear least square criterion (R2) between the two extensometers. The second objective is to examine the speckles’ dilation effects on the speckle pattern density and surface component quality utilizing the Image Erode (I.E.) technique. The (P.A.) technique provided consistent differential strain Diff(εx) values with a variance reduction of up to (90%) when averaged images were used. The (R2) has considerably increased (from 0.46, 0.66, 0.91 to 0.90, 0.96, 0.99), respectively, for the three samples. Moreover, the (I.E.) technique provided qualitatively denser speckles with a highly consistent DIC surface component.

In situ high-resolution ultrasonic visualization of damage evolution in the volume of quasi-isotropic CFRP laminates under tension

The work is devoted to novel experimental equipment for in situ visualization of damage development in the volume of CFRP under tensile. The acoustic microscope with long-focus probe beam of high-frequency ultrasound (50–100 MHz) was combined with mechanical testing machine. The experimental equipment provides visualization in dynamics of the processes of appearance, growth and transformation of microstructural damages in the volume of the specimen under mechanical loading. The method was validated by 4- and 7-ply quasi-isotropic CFRP laminates with symmetrical round notches. We explore the possibilities of ultrasound imaging for detection of matrix crack opening, and the mechanism of delamination formation with various combination of fiber orientation. According to the ultrasound imaging of the volumetric microstructure, we depicted several mechanisms of damage appearance and evolution under tension. For calculating the level of local deformation in the region with round notches, we applied the digital image correlation (DIC) of ultrasonic scans obtained during the experiment. The DIC data provides more correct values of deformations occurring in the specimen zone with round notches. It was shown that the values in the central area differ significantly from the deformation calculation along the whole length of the specimen, especially under loads close to the critical ones.

Experimental and Theoretical Investigation of the Mechanical Properties of PHBH Biopolymer Parts Produced by Fused Deposition Modeling

AbstractThe mechanical response of printed dog‐bone specimens of poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate) (PHBH) in linear and nonlinear regimes is investigated. Fused filament fabrication (FFF), also regarded as fused deposition modeling (FDM), is adopted to manufacture the PHBH elements. A compressible Mooney–Rivlin constitutive law is back fitted onto digital image correlation (DIC) data, as well as uni‐axial tensile test data. Effective mechanical properties are given, which may be used to design and optimize the response of printed structures. Strain values at the verge of failure in tension are also experimentally obtained. Results are especially useful for modeling mechanical response of PHBH, which is an emerging biopolymer with promising application fields in biomedical engineering, through additive manufacturing techniques.

Fatigue crack characterisation in 2024-T351 aluminium alloy through SEM observation combined with the CJP model

This work characterises crack growth in AA2024-T315 by combining different methods to further increase the reliability of the results. The Christopher-James-Patterson (CJP) model was fitted to experimental data obtained by digital image correlation (DIC). The effective value of the CJP stress intensity factors were successfully correlated with the ΔK-da/dN curve as obtained with Scanning Electron Microscopy measurements of the depth of striations on the fracture surface. This approach based on fitting the CJP model with DIC data and SEM observations allowed estimation of opening and closure loads and allowed the propagation rate and fracture mode to be effectively characterised.

Application of optical measurement techniques during various stages of pregnancy

Optical measurement techniques have evolved since the mid-2000's due to the advancement in computer processing and digital camera sensors. These techniques allow for the digitization of full three-dimensional (3D) surfaces and provide time-domain surface information data with high details not previously possible with more traditional measurement techniques. A variety of industries are using these technologies with a focus on high accuracy industrial applications including aerospace and automotive. This paper introduces optical measurements techniques used to provide quantifiable data and visual information of a woman's abdomen during various stages of pregnancy. Two non-invasive techniques are used for this study including structured light (SL) scanning and digital image correlation (DIC). An overview of these techniques, the process for data collection, results of the data, and opportunities for future studies are presented in this paper. The structured light scanning was used to capture a 3D surface of the woman's abdomen at four to six week intervals starting at week 15 of maternity and ending six days prior to birth. The results quantify growth during each stage of pregnancy along with full surface data in addition to postnatal results. The DIC data was collected during week 30 when the baby was very active in the womb and external deformations on the woman's abdomen were observed visually. The results from these studies were not intended to be a formal medical study but to provide feasibility of the SL and DIC process during the stages of pregnancy and encourage unique uses of otherwise industrial measurements.

Transverse tensile mechanical experimental method and behavior of ceramic matrix mini-composites

Transverse tensile mechanical experiments of ceramic matrix mini-composites (CMMC) were realized in this paper by overcoming the difficulties from material preparation, specimen design, gripping method, loading strategy, deformation measurement, transverse tensile stress calculation, etc. The transverse tensile stress-strain responses were obtained based on the proposed method. The stress-strain response was strongly non-linear, and its elastic modulus was positively correlated to the matrix volume fraction. The digital image correlation (DIC) was utilized in the experiment. The staged transverse tensile strain field evolution process was analyzed. The crack initiation and propagation could be clearly captured by the strain field contour. In addition, a novel DIC data analysis strategy was applied to investigate the uniformity variation of the transverse tensile strain field. This paper found that the strain field uniformity has a strong correlation with the damage extent and can be used to reflect the damage evolution of the material sensitively. The relationship between local damage and overall mechanical responses, and the localization and non-uniformity characteristics of damage evolution were also discussed. The impact of microstructure on transverse tensile elastic modulus was analyzed, and a model that considered the microstructure was built to predict CMMC’s transverse tensile elastic modulus.