Full-field Digital Image Correlation Research Articles

Assessing pseudo-ductile behavior of woven thermoplastic composites under tension and bending

Most fiber-reinforced composites are inherently brittle and fail suddenly at low strains without yielding and energy-absorbing capability. Still, under some conditions, they can demonstrate ductile like response known as pseudo-ductility. To investigate such a response, experimental analysis of carbon- and glass-fabric reinforced thermoplastic polymer (C/GFRP) composites was performed in on- and off-axis orientations under service loading conditions of tension and bending. Tensile tests of off-axis specimens were conducted with a full-field strain-measurement digital image correlation (DIC) technique. Cyclic bending tests of on- and off-axis C/GFRP specimens were performed to assess their ductility and damage behavior. The tests revealed that on-axis CFRP laminates failed due to fracture of brittle carbon fibers under tension, monotonic and cyclic bending. The on-axis GFRP samples demonstrated a linear-elastic brittle response under tension but a visco-elasto-plastic nonlinear behavior under monotonic and cyclic bending with hysteresis and energy absorption. This nonlinearity could be due to stress stiffening of thin laminates under large-deflection bending and mesoscopic effects in the woven fabrics. The off-axis C/GFRP specimens exhibited ductile behavior akin to metals, enduring high strains with permanent deformation before ultimate failure, and absorbing substantial amounts of energy. The pseudo-ductile response of the off-axis specimens is attributed to plasticity due to matrix cracking, fiber-matrix debonding as well as fiber trellising, whereas in the on-axis GFRP specimens, it is primarily due to visco-elasto-plastic behavior of glass fibers and the TPU matrix. It is concluded that material's response can be tailored for stiffness, strength and ductility for specific applications.

Experimental investigation of the air blast performance of hybrid composite skinned sandwich panels with X-ray micro-CT damage assessment

This research investigates the performance of interlaminar hybrid composites as the skins of composite sandwich panels under blast loading with the aim of promoting delamination between dissimilar plies for energy absorption. The deformation of the composite panels was captured using high-speed digital image correlation (DIC). High-speed full-field DIC enables failure to be captured at the moment it occurs across the entire panel. X-ray micro-CT imaging was used to assess the post-blast damage sustained by particular areas of interest from each panel, which were selected based on DIC results. The combination of full-field DIC and detailed X-ray micro-CT scanning enabled a unique comparison of both the global and localised blast resilience of hybrid and conventional composite sandwich panels to be performed.Following a single blast load, the extent of damage to the Hybrid-3B skinned sandwich panel was found to lie between that of GFRP and CFRP skinned sandwich panels. X-ray micro-CT scanning of these panels reveals that there is no continuous damage path through the skin thickness of Hybrid-3B, whereas the GFRP and CFRP panels sustain damage in every ply.Following repeat blast loading, the Hybrid-4 skinned sandwich panel suffered from a front skin crack spanning the length of the panel. Post-blast compressive strength testing reveals that this skin crack and resulting core crack acted as a stress relief, limiting the damage sustained elsewhere in the panel.It was concluded that Hybrid-3B results in a good trade-off between strength and stiffness and is advantageous over conventional CFRP and GFRP panels under a single blast load. Under repeated loading Hybrid-4 offers advantages over Hybrid-3B. Finally, the design of the support structure can significantly aid in blast resilience, and, a holistic approach considering both panels and support should be taken when designing for blast resilience.

Mesoscale shock structure in particulate composites

Multiscale experiments in heterogeneous materials and the knowledge of their physics under shock compression are limited. This study examines the multiscale shock response of particulate composites comprised of soda-lime glass particles in a PMMA matrix using full-field high speed digital image correlation (DIC) for the first time. Normal plate impact experiments, and complementary numerical simulations, are conducted at stresses ranging from 1.1−3.1GPa to elucidate the mesoscale mechanisms responsible for the distinct shock structure observed in particulate composites. The particle velocity from the macroscopic measurement at continuum scale shows a relatively smooth velocity profile, with shock thickness decreasing with an increase in shock stress, and the composite exhibits strain rate scaling as the second power of the shock stress. In contrast, the mesoscopic response was highly heterogeneous, which led to a rough shock front and the formation of a train of weak shocks traveling at different velocities. Additionally, the normal shock was seen to diffuse the momentum in the transverse direction, affecting the shock rise and the rounding-off observed at the continuum scale measurements. The numerical simulations indicate that the reflections at the interfaces, wave scattering, and interference of these reflected waves are the primary mechanisms for the observed rough shock fronts.

Investigation of the Dynamic Strain Aging Effect in Austenitic Weld Metals by 3D-DIC

Austenitic stainless steels similar to type AISI 316L are widely used structural materials in current and future nuclear reactors. Careful development and characterization of these materials and their welds is needed to verify the structural integrity of large-scale multicomponent structures. Understanding the local deformation behavior in heterogeneous materials and the mechanisms involved is key to further improve the performance and reliability of the materials at the global scale and can help in developing more accurate models and design rules. The full-field 3D digital image correlation (3D-DIC) technique was used to characterize two 316L multi-pass welds, based on cylindrical uniaxial tensile tests at room temperature, 350 °C, and 450 °C. The results were compared to solution annealed 316L material. The inhomogeneous character and dynamic behavior of the 316L base and weld materials were successfully characterized using 3D-DIC data, yielding high-quality and accurate local strain calculations for geometrically challenging conditions. The difference in character of the dynamic strain aging (DSA) effect present in base and weld materials was identified, where local inhomogeneous straining in weld material resulted in discontinuous type A Portevin–Le Châtelier (PLC) bands. This technique characterized the difference between local and global material behavior, whereas standard mechanical tests could not.

Characterizing the Tensile Behavior of Double Wire-Feed Electron Beam Additive Manufactured “Copper–Steel” Using Digital Image Correlation

The paper presents the results of the evaluation of the mechanical characteristics of samples of multi-metal “copper-steel” structures fabricated by additive double wire electron beam method. The global and local mechanical characteristics were evaluated using uniaxial tensile tests and full-field two-dimensional digital image correlation (DIC) method. DIC revealed the peculiarities of the fracture stages: at the first stage (0.02<ε≤0.08) the formation of V-shaped shear lines occurs; at the second stage (0.08<ε≤0.15) transverse shear lines lead to the formation of a block structure; at the third stage (0.15<ε≤0.21) the plasticity resource ends in the central part of the two necks cracks are formed, and the main crack is the cause of the fracture of the joint. It is found that shear lines are formed first in copper and then propagate to steel. Electron microscopy proves that uniformly distributed iron particles could always be found in the “Fe-Cu” and “Cu-Fe” interfaces. Additionally, the evolution of average strain rates and standard deviations were measured (calculated) in the regions of necks in copper and steel regions. New shear approach shows that the most of angles for parallel shears components are ±45°, rupture angles are about 0°, and combined account of these two types of shears provides us additional discrete angles.

Peridynamic material model calibration based on digital image correlation experimental measurements

The non-local theory of peridynamics, based on integral equation of motion, offers a computation alternative for materials with discontinuities. In this study, the peridynamic material model parameters are calibrated using full field Digital Image Correlation measurements applied to simple tensile test experiments, circumventing thus the need for more challenging test setups/methods for the determination of the peridynamic material parameters (especially the critical stretch failure parameter). A two stage algorithm in the optimization software LS-Opt together with a MATLAB stage for performing the peridynamic simulation and an Excel stage for defining the outputs of the simulation are implemented to minimize the differences between the peridynamic model and experimental results. The material model parameters determined based on the simple test setup and the calibration procedure proposed here can then be used for modeling more challenging geometries and load cases in peridynamics.

Determination of Mechanical Properties of Epoxy Composite Materials Reinforced with Silicate Nanofillers Using Digital Image Correlation (DIC).

In this study, silicate nanofillers; dicalcium silicate, magnesium silicate, tricalcium silicate, and wollastonite; were synthesized using four different methods and incorporated into the epoxy resin to improve its mechanical properties. Characterization of the newly synthesized nanofillers was performed using Fourier-transformation infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The purpose of this study was to analyze newly developed composite materials reinforced with silicate nanoparticles utilizing tensile testing and a full-field non-contact 3D Digital Image Correlation (DIC) method. Analysis of deformation and displacement fields gives precise material behavior during testing. Testing results allowed a more reliable assessment of the structural integrity of epoxy composite materials reinforced using different silicate nanofillers. It was concluded that the addition of 3% of dicalcium silicate, magnesium silicate, tricalcium silicate, and wollastonite lead to the increasement of tensile strength up to 31.5%, 29.0%, 27.5%, and 23.5% in comparison with neat epoxy, respectively. In order to offer more trustworthy information about the viscoelastic behavior of neat epoxy and composites, a dynamic mechanical analysis (DMA) was also performed and rheological measurements of uncured epoxy matrix and epoxy suspensions were obtained.

Integrating visual sensing and structural identification using 3D-digital image correlation and topology optimization to detect and reconstruct the 3D geometry of structural damage

This paper describes a novel technique for detecting internal or unseen damage in structural steel members by combining measurements from full-field three-dimensional digital image correlation (3D-DIC) with a topology optimization framework. Unlike the majority of conventional methods that rely on specialized forms of surface-penetrating waves or radiation imaging, this work employs optical cameras to measure surface strains and deformations using the 3D-DIC technique followed by an optimization approach to detect the existing damage. This data-rich representation of the structural component’s behavior is then used to reconstruct the underlying subsurface abnormalities via an inverse mechanical problem. The inverse problem is solved using a topology optimization formulation that iteratively adjusts a fine-tuned finite element model (FEM) of the structure to reveal irregularities within it. Having recently demonstrated the feasibility of detecting and reconstructing defects in small-scale structural components, this paper expands on the authors' previous work to demonstrate the feasibility and performance of the proposed method through an experimental program in which a set of large-scale structural steel beams with and without buried defects tested using a full-field 3D DIC sensing approach. The structure’s initial FEM is first created to discretize the member into elements whose constitutive properties are treated as unknowns in the optimization problem. The goal of the optimization is to minimize the discrepancies between the observed full-field response measured experimentally using DIC and that computed numerically using the model. To that end, an objective function is first computed as the sum of residuals by mapping both responses onto a common grid, which is then pushed to a minimum via the method of moving asymptotes (MMA) as the optimization algorithm. This study demonstrates that the proposed approach can identify unseen damage with an average accuracy (ACC) score of 96.80% on the defined configurations, with relatively minimal false identifications.

An investigation into the characterization of the hardening response of sheet metals using tensile and shear tests with surface strain measurement

The adoption of full-field digital image correlation (DIC) strain measurement in recent years has fundamentally transformed conventional analysis procedures for the mechanical characterization of sheet metals. The wealth of local strain data that is now available until fracture has enabled experimentalists to propose new analysis techniques for tensile and shear tests to obtain the hardening response to large strain levels. Although the tests show great potential, significant gaps remain surrounding their applicability. The choice of test geometry and suitability of the analysis across a broad range of sheet metals with diverse hardening and anisotropic behaviour remain open questions. Critically, the assumptions within the tensile and shear methodologies in applying surface strain measurements to large strains while omitting through-thickness gradients have not been verified. The objective of the present study is to perform a critical evaluation of constitutive characterization techniques for sheet metals under quasi-static, ambient conditions. A series of virtual experiments were developed to numerically replicate the testing and analysis procedures followed in lab-scale experiments to extract the hardening response. A simple shear and two standard tensile test geometries were evaluated across a broad range of plasticity characteristics including different hardening rates and anisotropic behaviour. For each case, the input hardening curve in the virtual experiment is treated as the exact solution and compared to the hardening response obtained from each analysis method. The so-called area reduction method (ARM) used for tensile specimens was found to be relatively insensitive to the specimen geometry, yield exponent, and plastic anisotropy but overestimated the flow stress. The performance of the ARM method hinges upon the aspect ratio of the cross-section to avoid shear band formation and requires empirical correlations to improve its predictions at large strains. Despite their simplicity, tensile analysis methods that use small extensometers or local DIC point data are shown to systematically overestimate the hardening behaviour. Simple shear tests can provide accurate estimations of the hardening response, but the choice of geometry is material dependant. The study concludes by considering a strongly anisotropic mild steel to evaluate the tensile and shear characterization strategies with the results of a hydraulic bulge analysis.

Strain accuracy enhancement of stereo digital image correlation for object deformation with large rotations

We propose a full-field stereo digital image correlation (DIC) strain measurement method to overcome the poor accuracy while measuring the deformation under large rotations. Such a drawback comes from the failure to consider rotation movements of the deformed objects when calculating their strain values. To address this, we first used a DIC matching algorithm combined with a rotated subset and feature point detection to obtain displacement fields. By employing a singular value decomposition method, we can then calculate rotation matrices of the strain windows before and after deformations. Finally, to eliminate the strain errors caused by rotation, we introduced the rotation matrices into the classical pointwise least squares DIC strain calculation method. Both numerical simulations and experiments are performed, and the accuracy and effectiveness of the proposed method are confirmed by the experimental results.

Combined approach for fatigue crack characterisation in metals

In this work a comprehensive characterisation of the fatigue crack growth of an Al 2024 alloy is presented. A powerful Christopher-James-Patterson (CJP) model is used for multi-parameter characterisation of the crack state at a few different locations through the fatigue crack growth. The CJP model of crack tip displacements and stress fields was proposed in order to better capture the influences on the applied elastic stress field of the plastic enclave that is generated around a growing fatigue crack. The model does this through a set of elastic stresses applied at a notional elastic-plastic boundary, and it has been shown to accurately model plastic zone shape and size, whilst its ability to predict the effective range of stress intensity factor during a fatigue cycle has been verified. Thus, the model can predict the driving force component of the crack and additional components that account for different shielding mechanisms. CJP model is combined with full-field non-contact Digital Image Correlation (DIC) so that the model can be fed at any moment with real experimental information. The results are then correlated with Paris law data for Al 2024 alloy through Scanning Electron Microscopy (SEM) observations of the fracture surface. In this work the effect of crack length and load level are thoroughly studied and validated with fatigue crack growth rate measurements.

Refined extraction of crack characteristics in large-scale concrete experiments based on digital image correlation

The accurate extraction of the crack patterns and measurements of crack kinematics are essential for understanding the mechanical behaviour in experiments on structural concrete as well as in the validation and further development of sound mechanical models. This paper presents important refinements of the authors' recently published automatic crack detection and measurement procedure (ACDM) based on surface displacement measurements obtained with digital image correlation (DIC). The proposed refinements are crucial for reliably assessing the crack behaviour in large-scale experiments with complex crack patterns, since the original methods of ACDM may fail or result in biased measurements at locations with closely spaced cracks, crack intersections or cracks with high morphological curvature. The main refinements are (i) a Canny edge-based crack detector, which is applied on the DIC major principal strain field and (ii) enhancements in the crack kinematic measurement to assess the reliability of the results. The latter includes the automatic selection of optimum reference points used in the crack kinematic measurement to increase its reliability and remove uncertain results. The refined ACDM procedure is validated using several large-scale 2.0 × 2.0 m shear panel experiments with highly complex crack patterns. Compared to the original ACDM, significantly thinner cracks can be detected with a much higher reliability of crack locations and crack kinematic measurements, particularly close to crack intersections and at closely spaced cracks. Additionally, two approaches for the statistical consolidation of the large amount of gathered data into characteristic crack properties in large-scale homogeneous concrete element experiments are proposed and compared. The results show that the statistical consolidation of the ACDM data using a 95%-quantile match well with the direct extraction of the best-fit homogeneous crack properties from the full-field DIC displacements. The consolidated data provides highly valuable insight into the mechanical behaviour, especially regarding crack phenomena.

Measurement and identification of the nonlinear dynamics of a jointed structure using full-field data, Part I: Measurement of nonlinear dynamics

Jointed structures are ubiquitous constituents of engineering systems; however, their dynamic properties (e.g., natural frequencies and damping ratios) are challenging to identify correctly due to the complex, nonlinear nature of interfaces. This research seeks to extend the efficacy of traditional experimental methods for linear system identification (such as impact testing, shaker ringdown testing, random excitation, and force or amplitude-control stepped sine testing) on nonlinear jointed systems, e.g., the half Brake–Reußbeam, by augmenting them with full-field data collected by high-speed videography. The full-field response is acquired using high-speed cameras combined with Digital Image Correlation (DIC), which enables studying the spatial–temporal dynamic characteristics of the system. As this is a video-based experiment, additional constraints are attached to the beam at the node points to remove the rigid body motion, which ensures that the beam is in the view of the camera during the entire test. The use of a video-based method introduces new sources of experimental error, such as noise from the high-speed camera’s fan and electrical noise, and so the measurement accuracy of DIC is validated using accelerometer data. After validating the DIC data, the measurements are recorded for several types of excitation, including hammer testing, shaker ringdown testing, fixed sine testing, and stepped sine testing. Using the DIC data to augment standard nonlinear system identification techniques, modal coupling and the mode shapes’ evolution are investigated. The suitability of videography methods for nonlinear system identification of nonlinear beams is explored for the first time in this paper, and recommendations for techniques to facilitate this process are made. This article focuses on developing an accurate data collection methodology as well as recommendations for nonlinear testing with DIC, which paves the way for video-based investigation of nonlinear system identification. In Part-II (Jin et al., 2021) of this work, the same data set is used for a rigorous assessment of nonlinear system identification with full-field DIC data.

Direct observation of plastic shear strain concentration in the thick GLARE laminates under bending loading

The results of the digital image correlation (DIC) analysis for the three-point bending tests with GLARE1-9/8 laminates (9 aluminum and 8 unidirectional GFRP layers) are presented in this paper. The findings show that, in samples with a small span-to-thickness ratio, an intensive concentration of the plastic shear strain arises inside the GFRP layers, which significantly affects the load-bearing capacity and apparent interlaminar shear strength of GLARE. Previously, such effects were predicted based on numerical simulations. In the present paper, we validate these theoretical predictions based on the full-field DIC analysis of the shear strain distribution across the GLARE thickness. For this analysis, samples with longitudinal and transverse orientation of glass fibers in the GFRP layers were studied and compared. For the transverse samples, we also found that the matrix cracking in the GFRP layers becomes an additional source for the overall non-linear behavior of GLARE, such that the pure failure mode related to the interlaminar shear cannot be achieved.

In-plane lateral cyclic behaviour of lime-mortar and clay-brick masonry walls in dry and wet conditions

This paper presents an experimental investigation into the structural and material response of ambient-dry and wet clay-brick/lime-mortar masonry elements. In addition to cyclic tests on four large-scale masonry walls subjected to lateral in-plane displacement and co-existing compressive gravity load, the study also includes complementary tests on square masonry panels under diagonal compression and cylindrical masonry cores in compression. After describing the specimen details, wetting method and testing arrangements, the main results and observations are provided and discussed. The results obtained from full-field digital image correlation measurements enable a detailed assessment of the material shear-compression strength envelope, and permit a direct comparison with the strength characteristics of structural walls. The full load-deformation behaviour of the large-scale walls is also evaluated, including their ductility and failure modes, and compared with the predictions of available assessment models. It is shown that moisture has a notable effect on the main material properties, including the shear and compression strengths, brick–mortar interaction parameters, and the elastic and shear moduli. The extent of the moisture effects is a function of the governing behaviour and material characteristics as well as the interaction between shear and precompression stresses, and can lead to a loss of more than a third of the stiffness and strength. For the large scale wall specimens subjected to lateral loading and co-existing compression, the wet-to-dry reduction was found to be up to 20% and 11% in terms of stiffness and lateral strength, respectively, whilst the ductility ratio diminished by up to 12%. Overall, provided that the key moisture-dependent material properties are appropriately evaluated, it is shown that analytical assessment methods can be reliably adapted for predicting the response, in terms of the lateral stiffness, strength and overall load-deformation, for both dry and wet masonry walls.

Strain monitoring of wind turbines using a semi-autonomous drone

In this work, an approach is proposed that can perform a nondestructive evaluation of wind turbine structures using a non-contact, three-dimensional full-field optical digital image correlation (DIC) technique. This approach can quantify the level of strain and loading conditions that rotating structures such as wind turbines experience during operation. The optical technique does not interfere with the structural functionality of the wind turbine. Moreover, the use of Unmanned Aerial Vehicles (UAVs) for remote inspection enables robust measurements for periodic inspection. The strain obtained using the proposed approach is validated using strain gauges mounted on the blades. A control algorithm is designed for the UAV to stabilized and obtain the desired field of interest and working distance based on the turbine size. Blending the benefits of the remote accessibility of UAVs and full-field dynamic evaluation of structures using strain data obtained with DIC is a novel method of monitoring wind turbines.

Characterization of 304L laser welds using digital image correlation and x-ray computed tomography

304L stainless steel partial-penetration laser welds with varying penetration depths are needed for a variety of national security applications. Their mechanical performance depends on many parameters including weld schedule, penetration depth, and weld porosity. Unfortunately, these parameters are not easily divorced from one another as each has varying impacts upon the others. Therefore, an understanding of the interplay between these parameters as well as their impact on weld performance is rather useful. In this work, partial penetration laser welds are produced with four varying penetration depths at two different weld schedules. Mechanical testing is conducted to characterize the mechanical performance while full-field digital image correlation (DIC) is applied during testing to obtain the displacement field down to the micron-scale to better understand both global and local deformation behavior. Porosity of these welds is also investigated using x-ray computed tomography (XCT) prior to mechanical testing. The integration of DIC and XCT techniques with mechanical testing provides an expanded experimental dataset to evaluate the variations in mechanical response as a function of penetration depth, weld schedule and porosity. These experimental techniques also provide a promising approach for future study of laser performance.

Digital image correlation for continuous mapping of fatigue crack initiation sites on corroded surface from offshore mooring chain

The full-field digital image correlation (DIC) technique was used to identify crack initiation and early crack growth on irregular, corroded surfaces from offshore mooring chain. Finite element analysis was paired with the DIC measurements, which made it possible to correlate initiation events with local stress concentrations due to corrosion pits. An initiation S-N curve was established based on this correlation. It included 14 crack initiation events from 4 specimens, in addition to statistical treatment of regions where no initiation occurred. The first cracks initiated at 13–24% of the total lifetimes of the specimens.

Coupled Thermomechanical Response Measurement of Deformation of Nickel-Based Superalloys Using Full-Field Digital Image Correlation and Infrared Thermography.

The paper is devoted to highlighting the potential application of the quantitative imaging technique through results associated with work hardening, strain rate and heat generated during elastic and plastic deformation. The aim of the research presented in this article is to determine the relationship between deformation in the uniaxial tensile test of samples made of 1-mm-thick nickel-based superalloys and their change in temperature during deformation. The relationship between yield stress and the Taylor–Quinney coefficient and their change with the strain rate were determined. The research material was 1-mm-thick sheets of three grades of Inconel alloys: 625 HX and 718. The Aramis (GOM GmbH, a company of the ZEISS Group) measurement system and high-sensitivity infrared thermal imaging camera were used for the tests. The uniaxial tensile tests were carried out at three different strain rates. A clear tendency to increase the sample temperature with an increase in the strain rate was observed. This conclusion applies to all materials and directions of sample cutting investigated with respect to the sheet-rolling direction. An almost linear correlation was found between the percent strain and the value of the maximum surface temperature of the specimens. The method used is helpful in assessing the extent of homogeneity of the strain and the material effort during its deformation based on the measurement of the surface temperature.

Thermomechanical Behavior of Steels in Tension Studied with Synchronized Full-Field Deformation and Temperature Measurements

Despite tensile testing being commonly used for investigating the mechanical behavior of materials, the occurrence of heterogeneous strain and increasing temperature at high strain rates make the experiment much more complex. This work presents a method integrating synchronous full-field stereo Digital Image Correlation (DIC) and Infrared Thermography (IRT). This method enabled high resolution investigations of the development of local temperatures and strains of the specimen during tensile loading of four steels at strain rates ranging from 2.5·10−4 to 900 s−1. The tests were monitored by a stereo setup of optical cameras and an infrared camera. Data acquisition was synchronized, and a pinhole camera model was used to translate the images from all cameras to the same three-dimensional space. The displacement vector fields from DIC were subtracted from the IRT images to represent the temperature maps in a Lagrangian coordinate system. The overall thermomechanical response of the materials was shown as 3D waterfall plots, which represent localized strain and temperature as a function of position and engineering strain. The results show that temperature increased homogeneously during uniform deformation at higher strain rates (10−2-900 s−1) and increased markedly with the onset of necking on the region of localized strain. At these strain rates, the localized increase of strain and temperature during necking were observed at the same global engineering strain and position, evidencing the spatial and temporal synchronization. The described method was used to accurately investigate the evolution of localized strain and temperature in both low and high strain rate regime.