Digital Image Correlation Strain Research Articles

Distortion Correction Protocol for Digital Image Correlation after Scanning Electron Microscopy: Emphasis on Long Duration and Ex-Situ Experiments

Recently, scanning electron microscopy (SEM) has been used for digital image correlation (DIC), as micrographs can be acquired with high magnification, providing improved resolution to quantify strain heterogeneities. However, it has been shown by researchers that SEM images can be problematic due to inherent electromagnetic distortions that are not present in optical images. Drift, spatial distortions, and magnification uncertainties are the main issues that can seriously affect the accuracy of localized strain measurements. The present work focuses on long duration experiments, for which images are taken days or weeks apart. We have proposed a systematic procedure to reduce drift, correct spatial distortion, and account for magnification variations between pairs of acquired images. Additionally, SEM parameters are discussed and chosen to increase the signal-to-noise ratio and improve the accuracy of the DIC measurements. The spatial distortion correction increases the repeatability of the correlated values and the precision of the measurements. The implementation for this type of correction is done by applying the measured distortion gradient of a certified grid onto the DIC strain field. The magnification adjustment increases the reliability of the strain maps, ensuring the measurements are in agreement with the actual strain induced during the experiment. We have presented a systematic protocol for ex-situ DIC experiments within the SEM and some basic cross-check procedures that can be performed to evaluate the reliability of the reference grid and the precision of the final strain map.

Failure parameter identification and validation for a dual-phase 780 steel sheet

A hybrid experimental-numerical procedure was implemented to determine the failure surface of a dual-phase 780 steel sheet as a function of the effective plastic strain, triaxiality, and Lode parameter using butterfly specimens with in situ digital image correlation strain measurement and supporting finite element calculations. A butterfly-type test specimen was employed to experimentally obtain stress states ranging from simple shear to plane strain tension including mixed tensile and shear loading. The numerically-derived failure surface was implemented into the phenomenological GISSMO damage model in the commercial finite element code LS-DYNA and the accuracy of the failure surface was determined using finite element predictions of the characterization experiments. A series of independent validation experiments related to sheet metal forming were performed including a hole tension test, a conical and flat punch hole expansion test, and a hemispherical punch test. The finite element models utilizing the damage model were able to accurately reproduce the load–displacement and surface strains of the sheet material for both the characterization and validation experiments. Prediction of the failure orientation and location compared favorably to each of the validation tests.

Investigation into the shear stress, localization and fracture behaviour of DP600 and AA5182-O sheet metal alloys under elevated strain rates

Shear tests were performed at strain rates ranging from quasi-static (0.01 s−1) to elevated rates (600 s−1) considering DP600 steel and AA5182-O aluminum alloy sheet at room temperature. The shear specimen due to Piers et al. [J. Peirs, P. Verleysen, J. Degrieck, Experimental Mechanics, 52 (7), pp. 729–741, 2012] was scaled (reduced in size) to perform high strain rate shear testing. In situ digital image correlation (DIC) techniques were employed to measure the strains in the experiments and methods are proposed for characterizing the local strain within shear bands. Using the DIC strain measurements along with finite-strain theory and the logarithmic objective stress rate, a simple methodology was developed to obtain the work hardening response to large strain levels using only shear and tensile experiments. At lower strains, the DP600 shows positive rate sensitivity while the AA5182 exhibited limited sensitivity as strain rate increases. At equivalent strains greater than approximately 20%, the DP600 and AA5182 alloys demonstrated a reduced work hardening rate at elevated strain rates. For both alloys, the strain to localization (using the Zener-Holloman criterion) and subsequent fracture strain, measured using the DIC technique, decreased with strain rate in shear loading, but increased under uniaxial tensile loading. Microscopic assessment of fractured DP600 specimens was also performed using measurement of grain boundary rotation to determine local strains at fracture corresponding to length scales below the DIC measurements. The local strains at final failure are much higher and reveal an increase in the shear fracture strain with strain rate, in contrast to the trends based on the DIC analysis.

Contribution of the Imaging System Components in the Overall Error of the Two-Dimensional Digital Image Correlation Technique

Abstract Digital image correlation (DIC) is one of the most widely used non-invasive methods for measuring full-field surface strains in a wide variety of applications. The DIC method has been used by numerous researchers for measuring strains during the plastic range of deformation where the strains are relatively large. The estimation of the amount of background strain error in the measurements is of prime importance for determining the applicability of this method for measuring small strains (such as the elastic strains in metals, ceramics, bone samples, etc.). In this study, the strain errors in 2D-DIC measurements associated with different types of imaging systems were investigated. In-plane rigid-body-translation, experiments were used to estimate the overall amount of error in DIC displacement and strain measurements. Different types of cameras having different types of sensors and different spatial resolutions were used in the study. Also, for the same type of camera, different types of lenses were used. Results show that the DIC measurement accuracy depends on the magnitude of image displacement and that different error estimation parameters can be used for quantifying the accuracy of the measurements. Also, the effect of the lens on measurement accuracy is more pronounced than that of the camera. Furthermore, imaging conditions such as image sharpness and camera gain also affect the accuracy. Further still, the measurement accuracy was found to be influenced by the direction of translation. The results indicate that measurement error can be reduced by orienting the camera such that the major displacement direction is parallel to the width direction of the image. The experimental approach used in this study can be used for quantitatively assessing the quality of the different types of cameras and lenses and to determine their suitability for use in experimental techniques that depend on image analysis such as DIC and particle image velocimetry (PIV).

Comparison of strain measurement in the mouse forearm using subject-specific finite element models, strain gaging, and digital image correlation.

Mechanical loading in bone leads to the activation of bone-forming pathways that are most likely associated with a minimum strain threshold being experienced by the osteocyte. To investigate the correlation between cellular response and mechanical stimuli, researchers must develop accurate ways to measure/compute strain both externally on the bone surface and internally at the osteocyte level. This study investigates the use of finite element (FE) models to compute bone surface strains on the mouse forearm. Strains from three FE models were compared to data collected experimentally through strain gaging and digital image correlation (DIC). Each FE model was assigned subject-specific bone properties and consisted of one-dimensional springs representing the interosseous membrane. After three-point bending was performed on the ulnae and radii, moment of inertia was determined from microCT analysis of the bone region between the supports and then used along with standard beam analyses to calculate the Young's modulus. Non-contact strain measurements from DIC were determined to be more suitable for validating numerical results than experimental data obtained through conventional strain gaging. When comparing strain responses in the three ulnae, we observed a 3-14% difference between numerical and DIC strains while the strain gage values were 37-56% lower than numerical values. This study demonstrates a computational approach for capturing bone surface strains in the mouse forearm. Ultimately, strains from these macroscale models can be used as inputs for microscale and nanoscale FE models designed to analyze strains directly in the osteocyte lacunae.

High-temperature delamination mechanisms of thermal barrier coatings: In-situ digital image correlation and finite element analyses

Through in-situ high-temperature digital image correlation (DIC) and finite element simulation, we unveil new, critical delamination and fracture mechanisms of dense vertically cracked thermal barrier coatings (DVC TBCs). Full-field DIC strain maps of cross-sectioned TBCs during thermal cycling up to 1200 °C reveal the coupled formation mechanisms of vertical and horizontal cracks, which showed that horizontal cracks were formed by vertical crack deflection/branching. Horizontal crack growth was characterized by crack bridging, crack shielding, and crack deflection. Internal defects and weak interfaces such as pre-existing voids and inter-splat boundaries promoted crack formation. Finite-element simulations revealed that the interaction between the vertical and horizontal cracks produced favorable strain energy release rates (ERR) for horizontal crack initiation and propagation on the interior of the coating compared to along the bond coat interface, which contrasts with conventional delamination mechanisms. These results indicate that high-temperature DIC is a powerful tool for in-situ measurement of microstructure-processing-deformation-failure relationships that are otherwise unobtainable.

Image-based correlation between the meso-scale structure and deformation of closed-cell foam

In the correlation between structural parameters and compressive behaviour of cellular materials, previous studies have mostly focused on averaged structural parameters and bulk material properties for different samples. This study focuses on the meso-scale correlation between structure and deformation in a 2D foam sample generated from a computed tomography slice of Alporas™ foam, for which quasi-static compression was simulated using 2D image-based finite element modelling. First, a comprehensive meso-scale structural characterisation of the 2D foam was carried out to determine the size, aspect ratio, orientation and anisotropy of individual cells, as well as the length, straightness, inclination and thickness of individual cell walls. Measurements were then conducted to obtain the axial distributions of local structural parameters averaged laterally to compression axis. Second, the meso-scale deformation was characterised by cell-wall strain, cell area ratio, digital image correlation strain and local compressive engineering strain. According to the results, the through-width sub-regions over an axial length between the average (lower bound) and the maximum (upper bound) of cell size should be used to characterise the meso-scale heterogeneity of the cell structure and deformation. It was found that the first crush band forms in a sub-region where the ratio of cell-wall thickness to cell-wall length is a minimum, in which the collapse deformation is dominated by the plastic bending and buckling of cell walls. Other morphological parameters have secondary effect on the initiation of crush band in the 2D foam. The finding of this study suggests that the measurement of local structural properties is crucial for the identification of the “weakest” region which determines the initiation of collapse and hence the corresponding collapse load of a heterogeneous cellular material.

Strain concentrations in BCC micro lattices obtained by AM

The micro-lattices produced by additive manufacturing process (AM) represent a recent important advancement for engineering structural applications, in particular for weight-saving purposes. The design of the components manufactured with these meta-materials generally refers to the idealized structures. In reality, the geometry obtained by the AM process profoundly differs from the original one, in particular local geometrical irregularities were found to produce local stress and strain localizations which are difficult to be a-priori predicted by the analyses on the idealized structures. These geometrical defects may have a significant role for the structural integrity of the component and it is important to quantify their effect on the local stress and strain fields. In this study, we present an experimental investigation of a typical AlSi10Mg micro-lattice, namely the BCC cell. 3D tomography was used to reconstruct the original geometry and, successively, full-field digital image correlation strain measurements were performed to capture the localization of strains which are considered the precursor of the micro-lattice damage. The local strain measurements were used to calculate and classify the strain concentration factors arising from the geometrical irregularities. These results were compared with the finite element results obtained for the idealized and the real micro-lattice geometries providing important considerations for the structural integrity assessment of the components produced with the AM micro-lattices.

Quantifying the three-dimensional damage and stress redistribution mechanisms of braided SiC/SiC composites by in situ volumetric digital image correlation

Coupled in situ micro-X-ray computed tomography and volumetric digital image correlation (V-DIC) strain measurements of expanding plug tests revealed the three-dimensional, microstructure-dependent mechanisms behind strain localization, damage initiation and stress redistribution in braided SiC/SiC composites. Hoop strain varied significantly through the composite thickness and was highest at regions of tow crossover; at higher loads, tow fracture initiated at these locations, and sample rupture propagated axially by connecting points of tow overlap. Finally, strain measurements after the failure of a tow on the interior surface quantified the three-dimensional stress redistribution mechanisms and damage tolerance of the SiC/SiC composite.

Macro- and micromechanical behaviors of poly(lactic acid)–hydroxyapatite electrospun composite scaffolds

It was demonstrated that the macro- and micromechanical behaviors of electrospun composite scaffolds based on PLA and HA can be simultaneously studied using 3D digital image correlation strain measurements coupled with an axial tensile test. It was proven that the addition of a small quantity (2%) of HA particles into PLA electrospun scaffolds decreased their mechanical properties by approximately 40 and 60% at the micro- and macroscales, respectively; nevertheless, when this amount was increased beyond 2% of HA, the composites recovered their stiffness, showing Young’s moduli ranging between 400 and 600 MPa. Hence, the mechanical responses of PLA–4% HA and PLA–6% HA electrospun scaffolds were conveniently enhanced with the addition of hydroxyapatite nanoparticles. The micromechanical measurements were able to capture the microstrain mechanism between electrospun nanofibers and the effect of HA nanoparticles on their mechanical response. This methodology could be a powerful tool for developing scaffolds for specific applications in tissue engineering.

Analogue modelling of inverted domino‐style basement fault systems

AbstractInversion of pre‐existing extensional fault systems is common in rift systems, back‐arc basins and passive margins. It can significantly influence the development of structural traps in hydrocarbon basins. The analogue models of domino‐style basement fault systems shown in this paper produced, on extension, characteristic hangingwall growth stratal wedges that, when contracted and inverted, formed classic inversion harpoon geometries and asymmetric hangingwall contractional fault‐propagation folds. Segmented footwall shortcut faults formed as the basement faults were progressively back‐rotated and steepened. The pre‐existing extensional fault architectures, basement fault geometries and the relative hangingwall and footwall block rotations exerted fundamental controls on the inversion styles. Digital image correlation (DIC) strain monitoring illustrated complex vertical fault segmentation and linkage during inversion as the major faults were reactivated and strain was progressively transferred onto footwall shortcut faults. Hangingwall deformation during inversion was dominated by significant back‐rotation as the inversion progressed. The mechanical stratigraphy of the cover sequences strongly influenced the fold and fault evolution of the reactivated fault systems. The implications of the experimental results for the interpretation and analysis of inversion structures are discussed and are compared with natural examples of inverted basement‐involved extensional faults observed in seismic datasets.

Feasibility of component state awareness of high strain rate events using fiber Bragg grating sensors

Strategically located Fiber Bragg Grating (FBG) Sensors have been proposed as an in situ method to increase the signal to noise ratio (SNR) for metallic and composite components. This paper presents a systematic study that investigates the viability of FBG Sensors under high strain rate loading by initially measuring 1D-strains in a compression Hopkinson bar experiment, followed by 2D full-field strain-tensor in impact and blast experiments on plates. Specifically, high strain rates from commercialized FBG Sensors are compared to traditional resistive and semi-conductor based strain gages under various levels of 1D high strain rate loading. In the projectile-plate impact experiments, full-field back-surface strain measured using FBG Sensor arrays are compared with that measured from 3D surface Digital Image Correlation (3D-sDIC) strain measuring technique. Finally, strains in welded steel plates subjected to high explosive discharge are monitored with mounted FBG Sensors on the back surface. From this study, potential improvements in the SNR of FBG Sensors are recommended, and the survivability of these sensors under more complex, dynamic loading is evaluated.

Investigating the Potential of Using Off-Axis 3D Woven Composites in Composite Joints\u2019 Applications

The effect of circular notch has been evaluated for three different architectures of three-dimensional (3D) carbon fibre woven composites (orthogonal, ORT; layer-to-layer, LTL; angle interlock, AI) through open-hole quasi-static tension and double-lap bearing strength tests in the off-axis (45°) direction. Damage characterisation is monitored using Digital Image correlation (DIC) for open-hole testing and X-ray Computed Tomography (CT) for double-lap bearing strength test. The off-axis notched 3D woven composites exhibits minor reduction (less than 10 %) of the notched strength compared to the un-notched strength. DIC strain contour clearly show stress/strain localisation regions around the hole periphery and stress/strain redistribution away from the whole due to the z-binder existence, especially for ORT architecture. Up to 50 % bearing strain, no significant difference in the bearing stress/bearing strain response is observed. However when ORT architecture was loaded up to failure, it demonstrates higher strain to failure (~140 %) followed by AI (~105 %) and lastly LTL (~85 %). X-ray CT scans reveal the effect of the z-binder architecture on damage evolution and delamination resistance. The study suggests that off-axis loaded 3D woven composites, especially ORT architecture, has a great potential of overcoming the current challenges facing composite laminates when used in composite joints’ applications.

Fracture Characterization of Rolled Sheet Alloys in Shear Loading: Studies of Specimen Geometry, Anisotropy, and Rate Sensitivity

Two different shear sample geometries were employed to investigate the failure behaviour of two automotive alloy rolled sheets; a highly anisotropic magnesium alloy (ZEK100) and a relatively isotropic dual phase steel (DP780) at room temperature. The performance of the butterfly type specimen (Mohr and Henn Exp Mech 47:805–820, 16; Dunand and Mohr Eng Fract Mech 78:2919-2934, 17) was evaluated at quasi-static conditions along with that of the shear geometry of Peirs et al Exp Mech 52:729-741, (27) using in situ digital image correlation (DIC) strain measurement techniques. It was shown that both test geometries resulted in similar strain-paths; however, the fracture strains obtained using the butterfly specimen were lower for both alloys. It is demonstrated that ZEK100 exhibits strong anisotropy in terms of failure strain. In addition, the strain rate sensitivity of fracture for ZEK100 was studied in shear tests with strain rates from quasi-static (0.01 s−1) to elevated strain rates of 10 and 100 s−1, for which a reduction in fracture strain was observed with increasing strain rate.

Experimental and numerical analysis of the Lüders phenomenon in simple shear

The Lüders phenomenon is investigated in a low carbon ferritic steel under simple shear loading. Tensile and shear experiments are carried out associated with digital image correlation (DIC) measurements of the local strain field. An elastoviscoplastic constitutive law is identified with special attention paid to the choice of the equivalent stress measure in the yield function. It is then used to simulate the shear experiment using finite element analyses. Several mesh types and sizes are used to illustrate some mesh sensitivity observed in the finite element results. A regularized model based on strain gradient plasticity is then proposed to ensure fully mesh insensitive simulations and properly describe the finite thickness of the Lüders band front. The additional internal length introduced in the regularized model is identified from the DIC strain measurements. Finally, the boundary conditions best–suited for an accurate description of the shear experiment are discussed.

Experiments and Constitutive Model for Deep and Superficial Fascia. Digital Image Correlation and Finite Element Validation

AbstractFascia is a highly organized collagenous tissue that is intimately connected with muscles. The mechanical properties of fascia strongly affect muscular actions and development of pathologies. The objective of this paper is to determine the mechanical properties of the deep and superficial fascia by uniaxial and pure shear tests and to propose and address the feasibility of a material hyperelastic constitutive model using combined Digital Image Correlation strain measurements and Finite Element (FE) computations. Experiments on deep and superficial fascia samples from the hindlimbs of six adult sheep along or perpendicular to collagen fibres and FE simulations of each experiment were compared. An anisotropic strain energy function was proposed to reproduce the mechanical behaviour of deep and superficial fascia and the material parameters were fitted using an optimization method and included in the FE simulations.The mechanical response of the deep and superficial fascia shows typical behaviour of soft connective tissues. It is shown that the samples in longitudinal direction are stiffer than the samples in transversal direction for both kinds of tissues. When considering the deep and superficial samples, it is evident a greater stiffness of deep samples for both longitudinal and transversal directions with respect to superficial samples. Overall good predictions were obtained with the model proposed which present relatively low ε values, ε = 0.1189 and ε = 0.0722 for deep and superficial fascia respectively. The experimental and FE results were compared. The simulated curves slightly underestimated or overestimated the load for uniaxial and pure shear tests, respectively, but very accurately captured the stiffness and the overall response. These results demonstrate that the proposed model and the fitted material properties applied have a good capability of reproducing the experimental conditions.

Experimental and computational investigation of the effect of phase transformation on fracture parameters of an SMA

A comprehensive, multi-method experimental characterization of fracture is conducted on shape memory alloy NiTi that exhibits superelasticity due to austenite-to-martensite stress induced phase transformation. This characterization includes (i) load-based measurement of critical stress intensity factor (Kmax) using ASTM standard E399, (ii) measurement of crack tip opening displacement (CTOD) per ASTM standard E1290, (iii) the digital image correlation (DIC) characterization of the transformation zone as well as the displacement-field based measurement of Kmax from the DIC data. Samples have also been tested at T = 100 °C to suppress the martensitic transformation to investigate transformation toughening. The experimental investigation is complemented with finite element (FE) analysis that uses Auricchio–Taylor–Lubliner constitutive model. A direct observation with DIC revealed a small scale transformation (K-dominance). Kmax of the transforming material is higher than that of the transformation-suppressed material tested at 100 °C, suggesting transformation toughening. At 100 °C, the material becomes quite brittle with a very small crack-tip plastic zone when the transformation mechanism is blocked. By measures of critical CTOD, the gap widens even more between the superelastic and transformation-suppressed cases, particularly because of the side effect that, in this very interesting material, material modulus increases with temperature. Evaluating the transformation zone from the DIC strains with reference to the uniaxial stress–strain curve, an equivalent strain form is proposed in conjunction with the plane stress FE prediction.

Comparison of different filtering strategies to reduce noise in strain measurement with digital image correlation

The main limitation of digital image correlation is the remarkable noise affecting the digital image correlation–computed strain distributions. Neither manufacturers of digital image correlation systems nor the literature provide guidelines for optimal filtering of digital image correlation strain distributions. However, filtering is also associated with loss of information (smoothing of the strain gradients). We systematically explored different filtering strategies to reduce noise while minimizing the loss of information in the digital image correlation–computed strain distributions. The first filtering strategy was directly applied to the acquired images that were then fed to the digital image correlation software. Median adaptive low-pass filters and notch filters were used to eliminate noise: both strategies increased (rather than reducing) the noise in the digital image correlation–computed strain distributions. The second strategy explored was a Gaussian low-pass filtering of the strain distributions. When the optimal cutoff frequency was selected, the noise was remarkably reduced (by 70%) without excessive loss of information. At the same time, when non-optimal cutoff frequencies were used, the residual noise and/or loss of information seriously compromised the results. Finally, image combination techniques were applied both to the input images and to the strain distributions. This strategy was extremely time-consuming but not very effective (noise reduction <10%). In conclusion, the only truly effective noise reduction strategy, if measurements are carried out using commercial closed software, consists in filtering the strain distribution.

Physical assessment of essential work of fracture parameters based on m-LLDPE blown films

A new experimental approach based on essential work of fracture (EWF) method and an in-situ digital image correlation strain mapping analysis has been developed to quantify different aspects of fracture energy on model metallocene linear low-density polyethylene (m-LLDPE) blown films. Three distinctive fracture process zones have been identified from the photoelastic observation of the model m-LLDPE blown films. These three zones include the essential work (We) zone due to necking and crack propagation, the non-essential plastic deformation zone (Wp), and a newly proposed recoverable viscoelastic deformation zone (Wv), which is found to be nontrivial. Furthermore, in contrast to the assumption involved in the EWF theory, We is found to be proportional to the volume of the materials involved in the inner fracture process zone. The current approach allows for exact physical interpretation of the EWF parameters and their correlation to material characteristics. Usefulness of the present work for development of stronger polymeric blown films is discussed.

Enhancement of mechanical properties for quenched and partitioned steels via intragranular austenite

Mechanical properties of quenched and partitioned steels were dramatically enhanced by two types of austenite, including intragranular austenite in ferrite grains and allotriomorphic austenite. The formation mechanism of intragranular austenite as well as its effect on ductility was studied. Digital image correlation strain analyses combined with microstructural observations indicated that most intragranular austenite experienced higher plastic strain than allotriomorphic austenite upon loading, however, intragranular austenite behaved more stable than allotriomorphic austenite, even if both characteristics were very similar. Intragranular austenite in ferrite played a positive role during deformation, like reconciliation of strain, grain refinement as well as transformation-induced plasticity effect. Further adjustment for the amount and stability of intragranular austenite is expected to enhance the ductility of transformation induced plasticity-assisted steels.