3D DIC Research Articles

Subsurface impact damage imaging for composite structures using 3D digital image correlation

An integrated system is proposed to visualize subsurface barely visible impact damage (BVID) in composite structures using three-dimensional (3D) digital image correlation (3D DIC). This system uses a pair of digital cameras to record video frames in the field-of-view (FOV) of the structure’s surface, capturing the wavefield generated via chirp excitation in the near-ultrasonic frequency range. Significant pitfalls of previous efforts of damage imaging using two-dimensional DIC have been largely mitigated. First, 3D DIC enables capturing out-of-plane displacements, which are much larger in amplitude versus in-plane displacements that a single camera would be limited to sensing, thus increasing the signal-to-noise ratio. This enhancement in turn increases the sensitivity of the stereo-camera system. Second, a total wave energy (TWE) damage imaging condition is proposed to visualize the local damage region. The monogenic signal obtained via Reisz transform (RT) is employed to compute the instantaneous amplitude, with which the local wave energy can be calculated spatially over time. Since a high displacement amplitude and thus high wave energy will occur in the damage region due to the local resonance, the proposed TWE imaging condition can relax the Nyquist sampling requirement, unlike guided-wave-based structural health monitoring techniques which require fully reconstructing the wavefield and wave modes through sampling that satisfies the Nyquist criterion. As such, a much lower camera frame rate is adequate for the proposed system. Consequently, the maximum spatial resolution of the camera for a given FOV can be achieved at the expense of a reduced frame rate. With the maximized pixel resolution and reduced frame rate for employing the TWE imaging condition, composite structures can be inspected or monitored with a larger FOV. As a result, there is no longer any need to apply signal enhancement techniques, such as sample interleaving, image stitching, or averaging, to increase the effective performance of the camera. Rather than needing thousands of repeated videos for minimizing the incoherent noise, only a single stereo-video with a few seconds of sampling duration is necessary for damage imaging. The use of a powerful piezo-shaker also increases the wave signal amplitude and further enhances sensitivity without permanent adhesion. To demonstrate this stereo-camera concept with the TWE imaging condition, the system was used to image damage in two carbon fiber reinforced polymer composite honeycomb panels, which had been subjected to low-velocity impacts (2 J). For each panel, two excitation configurations were used to verify the robustness of the system. Initial damage maps produced for a 100 × 100-mm FOV using a three-second stereo-video show accurate damage imaging ability that is independent of excitation location and comparable to benchmark damage images computed from laser Doppler vibrometer data and those gathered from ultrasonic and X-ray computerized tomography scans. This efficient and reliable integrated system demonstrated high potential for in-time damage inspection on composite aircraft and other critical structures.

Interfacial fatigue debonding retardation in wrapped composite joints: Experimental and numerical study

Debonding crack propagation at the composite-to-steel interface has been found to be an important failure mechanism for wrapped composite joints under static and fatigue loads. Friction at the interface behind the crack tip may deviate fatigue debonding of the joints from the linear-fracture-mechanics behaviour. This paper presents static and fatigue tests of axial wrapped composite joints. 3D DIC and optical fiber system is employed to monitor displacements and crack propagation. A finite element model is established and validated against static and fatigue test results, where friction is considered at the cracked interface. Through FE modelling, it is proved that the friction at the interface significantly reduce the strain energy release rate (SERR) at the crack tip, leading to retardations of crack growth and stiffness degradation. Parametric study is conducted finally to investigate the influence of friction coefficient, failure modes as well as Paris relationship parameters on the predicted fatigue behaviour of wrapped composite joints.

Experimental characterization of cyclic behaviour of pure lead: Temperature sensitivity and strain-rate effects

Proton beam pulses with an energy of 20 GeV/c collide with a pure-lead based target installed in the neutron Time-Of-Flight facility (n_TOF) at the European Laboratory for Particle Physics (CERN). The interaction between the proton beam and lead produces neutrons via spallation mechanism and results in a rapid temperature increase and propagation of stress waves. To evaluate the material response in such challenging conditions, a reliable thermo-mechanical characterization is necessary for the calibration of an appropriate constitutive model for pure lead that is valid under cyclic plasticity and high temperature. In this work, the experimental bases for the development of such constitutive material description are lied. Starting with metallurgical characterization, the typical grain size of the material was initially investigated as well as any variations in the metallurgical features. The grains appeared to have an equivalent size ranging from 2 to 6 mm. Then, static tensile tests were conducted at room temperature and different strain-rates from 10−1 to 10−4 s−1. The obtained results were crucial for optimizing the specimen geometry and test setup for the subsequent cyclic tests. Tension-compression cyclic tests were performed at different strain amplitudes from 0.1 to 1.5%, and at three different temperatures (room temperature, 90 °C and 150 °C). The strain amplitudes were controlled by an extensometer and the strain field evolution during the test was recorded by means of 2D DIC.

2D DIC from single moving camera by homographies compensation

The use of unmanned aerial vehicles in combination with vision-based measurement techniques has increasing interest in structural health monitoring. When displacement fields must be measured, Digital Image Correlation is one of the most popular techniques for image analysis. However, when images are taken from a single camera mounted on a drone, DIC gives incorrect results, since the camera movement creates apparent displacements and deformations. In the paper a technique for the compensation of these perspective distortions is proposed for planar surface case. The technique uses homographies estimated on a limited region of the measurement target assumed undeformed. Tests on numerically generated images have been performed to validate the proposed algorithm and to study the effect of the non-complete compliance with theoretical assumptions. Model for compensation error prediction is also proposed. Finally, some experimental tests considering real targets have been performed, obtaining good performances for the crack aperture case study.

Mixed-mode fracture of compacted tailing soils. II: Crack properties from full-field displacement fields

This paper deals with the acquisition of the crack evolution and propagation behaviors of the compacted tailing soils from full-field displacement fields by incorporating two-dimensional optical technique-digital image correlation (2D DIC). Meanwhile, the fracture properties, such as mode I+II fracture toughness were also measured and compared to the results obtained in Part I. Cracking properties, such as strain energy rate, cracking initiation angle, and size of critical distance were investigated from the perspective of full-field displacement field and compared to the classic fracture mechanics. It is more intuitive to recognize the mixed mode fracture properties of the compacted tailing soils under asymmetric loading conditions by using optical techniques in full-field displacement fields. This work largely enriched the database of the fracturing properties of compacted tailings soils.

Experimental Evaluation of Hoop Stress–Strain State of 3D-Printed Pipe Ring Tensile Specimens

Data on the strain and stress status of the pipe in the circumferential direction are required for various pipe manufacturing procedures (e.g., in the oil business, the process of manufacturing seamless pipes with a conical shaft). The aim of this study is to develop a procedure to determine the strain and stress behavior of Pipe Ring Tensile Specimens (PRTSs) in the hoop direction, as there are a lack of official standardized methods for testing PRTS. This paper discusses the application of the Digital Image Correlation method for testing plastic PRTSs. PRTSs are tested using a specially designed steel tool with two D blocks. A 3D-printed PRTS is placed over two D-shaped mandrels, which are fixed on a tensile tool and tensile testing machine. The strain evolution in the gage length of the specimens is captured using the three-dimensional Digital Image Correlation (3D DIC) method. To check the geometry of the cross-section of the PRTS after fracture, all the specimens are 3D scanned. For the study, six groups of PRTS are analyzed, consisting of three filling percentages (60, 90, and 100%) and two geometry types (Single and Double PRTS). The results show that the type and percentage of filling, as well as the method of printing, affect the material behavior. However, the approach with the DIC system, 3D printer, and scanner shows that they are effective instruments for mapping complete strain fields in PRTS, and thus are effective in characterizing the mechanical properties of pipes.

A comparison of elasto-plastic parameters of S355 steel obtained in tensile tests using an extensometer, a strain gauge and an ARAMIS 3D DIC system

A comparison of elasto-plastic parameters of S355 steel obtained in tensile tests using an extensometer, a strain gauge and an ARAMIS 3D DIC system

Functionality Study of an Optical Measurement Concept for Local Force Signal Determination in High Strain Rate Tensile Tests

Many mechanical material properties show a dependence on the strain rate, e.g. yield stress or elongation at fracture. The quantitative description of the material behavior under dynamic loading is of major importance for the evaluation of crash safety. This is carried out using numerical methods and requires characteristic values for the materials used. For the standardized determination of dynamic characteristic values in sheet metal materials, tensile tests performed according to the guideline from [1]. A particular challenge in dynamic tensile tests is the force measurement during the test. For this purpose, strain gauges are attached on each specimen, wired to the measuring equipment and calibrated. This is a common way to determine a force signal that is as low in vibration and as free of bending moments as possible. The preparation effort for the used strain gauges are enormous. For these reasons, an optical method to determine the force by strain measurement using DIC is presented. The experiments are carried out on a high speed tensile testing system. In combioantion with a 3D DIC high speed system for optical strain measurement. The elastic deformation of the specimen in the dynamometric section is measured using strain gauges and the optical method. The measured signals are then compared to validate the presented method. The investigations are conducted using the dual phase steel material HCT590X and the aluminum material EN AW-6014 T4. Strain rates of up to 240 s-1 are investigated.

High-speed multi-camera 3D DIC measurement of the deformation of cassette structure with large shaking table

Vision-based deformation measurement methods have been proposed to overcome the disadvantages of laborious, time-consuming measurement data that are difficult to accurately obtain using contact sensors in three directions. However, such methods remain a challenge for large-scale applications owing to difficulties in full-field imaging, stereo calibration, and measurement resolution, among other factors. To the best of our knowledge, there are no reports on the three-dimensional (3D) full-field measurement of a large shaking table structure with multiple cameras. In this paper, a multi-camera 3D DIC method using four high-speed cameras to measure the multi-layer structure model in a shaking table is proposed. To calibrate the stereo cameras with a large field of view, a speckle-based calibration method and a coordinate system unification method are proposed. The implementation of the proposed method is illustrated with a shaking table test for a 1/8 scaled 15-story high-rise cassette concrete structure with several different intensity levels of ground motions. The results of displacement, velocity, and acceleration of the structure measured using the proposed method correspond well with those of a laser displacement meter and accelerometer. The experiment results show that this method can guarantee full-field measurements in the range of tens of square meters and realize a sub-millimeter displacement measurement accuracy. They also indicate that the proposed multi-camera 3D DIC method has high potential for understanding the failure mechanism of structures and improving the structural design.

Experimental and numerical studies of the impact breakage of granite with high ejection velocities.

The impact-induced fragmentation of rock is widely and frequently encountered when natural hazards occur in mountainous areas. This type of fragmentation is an important and complex natural process that should be described. In this study, laboratory impact tests under different impact velocities were first conducted using a novel gas-driven rock impact apparatus. The three-dimensional digital image correlation (3D DIC) technique was used to monitor the dynamic fragmentation process upon impact. Then, coupled 3D finite-discrete element method (FDEM) numerical simulations were performed to numerically investigate the energy and damage evolutions and fragmentation characteristics of the sample under different impact velocities. The laboratory test results show that as the impact velocity increases, the failure pattern of the rock sample gradually changes from shear failure to splitting failure, and the fragmentation intensity increases obviously. The strain localization area gradually increases as the impact velocity increases and as the location gradually deviates away from the impacting face. In the numerical simulation, the proposed model is validated by quasi-static uniaxial compression tests and impact tests. The numerical simulations clearly show the progressive fracture process of the samples, which agrees well with the experimental observations. The evolutions of energy and damage variables were also derived based on the simulation results, which are markedly affected by the impact velocity. The fragment size distributions based on mass and number can be well fitted using a generalized extreme value law. Finally, the distribution of the fragment flying velocity and angle are analyzed.

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.

Spatio-temporal monitoring of humidity induced 3D displacements and strains in mounted and unmounted parchments

The high hygroscopicity and heterogeneity of parchment make it particularly sensitive to changes in environmental conditions. The proper housing of a historical parchment, including mounting should mitigate the impact of the changes on the object. However, finding an appropriate mounting method requires detailed knowledge of the mechanical response of a parchment on the combined influence from environmental changes and forces introduced by the mounting. In this paper we present the capabilities of 3D digital image correlation (3D DIC) method implemented for full-field, spatio-temporal monitoring of displacements and strains in parchment exposed to environmental changes. We combine the measurement concept with the particular, critical task of evaluating the effectiveness of stabilising a single sheet of parchment mounted using elastic polyester strips. 3D DIC is implemented to record in non-invasive and non-contact way the full-field response of parchment mock-ups to fluctuations of the relative humidity (RH) without interfering with the climatic chamber processes. We measure and compare the impact of fluctuating RH on unmounted and mounted parchment samples, through the full-field 3D measurements of dimensional changes at selected humidity levels, and through the local and global analysis of the samples’ response (displacement/strains) in time. We had proven in quantitative and detailed way that the applied mount fully fulfils its task, namely reduce significantly the out-of-plane displacements and strains occurring in the parchment during RH changes in the range from 39 to 80%. Our study also demonstrates the possibilities of using 3D DIC for the efficient support of conservation scientists in the development of storage, mounting, and treatment protocols for historical parchment.

Experimental vibration analysis of large structures using 3D DIC technique with a novel calibration method

Experimental vibration analysis of large structures using 3D DIC technique with a novel calibration method

In-situ temperature-dependent characterization of copper through glass via (TGV)

Glass has been proposed as a promising material for interposer because of its outstanding material properties. The development of through-glass via (TGV) technology is the most challenging process in the fabrication of glass interposers. This study investigates thermomechanical responses of TGVs induced by thermal loading. Optical profilometry was used to measure the protrusion of copper vias during thermal cycling. The TGV was heated from room temperature (RT) 23 °C to 400 °C and then cooled to RT. The height of an irreversible copper protrusion was recorded at different temperatures. In-plane deformation of the glass caused by thermal mismatch is investigated. Two-dimensional digital image correlation (2D DIC) was applied to measure the in-plane deformation of the glass near the copper via during thermal cycling. The in-plane displacement of the glass near copper vias reached its maximum around 250 °C, then started decreasing because of the copper protrusion and the significant creep of copper material at elevated temperatures. The effect of temperature ramp rate (4 °C/min, 15 °C/min, 30 °C/min, 50 °C/min) on the in-plane deformation of the glass was studied. The in-plane deformation of the glass can be reduced be applying slower ramp rate, because more significant creep occurs in the copper with slower ramp rate which alleviates the CTE mismatch-induced thermal stress at the copper-glass interface. Measurements were conducted to study the creep behavior of the copper via at different temperatures with one hour duration. Each of four TGV samples was heated to a peak temperature, 100 °C, 200 °C, 300 °C, and 400 °C, respectively, and the temperatures were held for 1 h. The glass in-plane displacements were measured by a 2D DIC. Numerical simulations were performed to provide an insight into the thermomechanical behavior of the copper vias and the glass during thermal cycling. The equivalent creep strain of the copper and the maximum principal stress of the glass are presented based on the simulation results.

Displacement monitoring of a pedestrian bridge using 3D digital image correlation

Using computer vision methods for monitoring the behaviour of structures in urban environments, especially if it is intended to be a long term operation, requires the vision system and the preparation of the structure to be set up with minimal disruption to the surroundings and the use of the structure itself. With this concept in mind, 3D digital image correlation was used to evaluate the deflection caused by a static load on a pedestrian bridge, using cameras installed by windows in different floors of an adjacent building.The measurements were performed using IP cameras designed for video surveillance and security applications, capturing the bridge’s surface from above. Rather than preparing the surface with a custom speckle pattern, the DIC processing was performed using the pattern appearing naturally due to the irregularities on the surface. A 2D DIC test was performed simultaneously with an industrial camera capturing the same section of the bridge from a horizontal position inside a lower floor of the same building. The results from the 3D and 2D tests were in close agreement with each other and with the expected displacement profile.

On the Fracture Evolution and Instability of Pyrite-Filled Marble Exposed to Freeze-Thaw-Compression Loads

Abstract The preexistence of the geological discontinuities in cold regions is susceptible to freeze-thaw weathering and rock instability, and even the occurrence of geological hazards is strongly impacted by the discontinuities. Knowledge of how natural fracture affects the rock field deformation is crucial to rock stability prediction. This work is aimed at revealing the influences of freeze-thaw on failure process for pyrite-filled marble obtained from an open pit slope. All the tested marbles were selected to roughly have the same initial pyrite band; the full-field displacement and the progressive failure behaviors under uniaxial compression were qualitatively and quantitatively analyzed using 3D digital image correlation (3D DIC) technique. The testing results show that the previous freeze-thaw action weakens the cementation between the rock matrix and pyrite band; the peak stress and strain are obviously impacted by the freeze-thaw treatment. In addition, the stimulation of pyrite bands influences the displacement development and high strain concentration pattern. The stimulation of pyrite band results in the formation of strain concentration zone, and shear sliding occurs until rock failure. Moreover, it is found that the stimulation of pyrite band and its localized strain takes place progressively and develops fast for marble exposed to higher freeze-thaw treatment. It is suggested that the field deformation development depends on the stimulation of the pyrite bands.

Full-Field Deformation Characteristics of Anisotropic Marble under Compression Revealed by 3D Digital Image Correlation

Abstract This work is aimed at revealing the anisotropic full-field displacement and the progressive failure behaviors of interbedded marble under uniaxial compression using a three-dimensional digital image correlation (3D DIC) technique. The effects of the interbed orientation on the field displacement and strain pattern and the crack evolution were analyzed qualitatively and quantitatively. Testing results show that different stress-strain responses can be generated depending on the interbed orientation, and the interbeds influence the localized deformation and high strain concentration pattern. The field displacement evolution curves present different patterns and are impacted by the localized deformation. In addition, the strain localization takes place progressively and develops at a lower rate for rock with 0° and 90° interbed than those of 30° and 60° interbed rock. The quick shear-sliding along the interbed leads to the minimum strength of rock having 30° interbed orientation. It is suggested that rock anisotropic field deformation is structure dependent.

Fatigue behaviour of non-welded wrapped composite joints for steel hollow sections in axial load experiments

Design and execution of fatigue load dominated circular hollow section (CHS) multi-membered structures, such as offshore jacket and floating structures for wind turbines, truss bridges, etc., is hinged on fatigue performance of critical welded joints. An innovative jointing technology of wrapped composite joint connects steel hollow sections by direct bonding through a fibre reinforced polymer composite wrap, which completely avoids welding and therefore has superior fatigue performance than welded joints. In this study first results on fatigue performance of the wrapped composite joints is presented. Tensile cyclic loading tests on wrapped composite X-joint specimens were carried out to characterise their fatigue performance under different constant amplitude load ranges and compare it to equivalent welded joints. In addition, the influence of surface roughness of steel tubes and re-testing (load history) on fatigue performance of wrapped composite joints, as well as the influence of fatigue loading on residual static resistance was investigated in experiments. Preliminary S-N curves of wrapped composite joints are established. The tests results showed that X45 wrapped composite joints exhibited steadier stiffness degradation and 10–100 times longer fatigue life than their welded equivalents. Through 3D DIC surface strain measurements and post-failure microscopic insights to cut specimens, different failure modes including de-bonding at glass fibre composite-to-steel interface, delamination/fracture of the composite layers and fracture of steel brace are distinguished. The relationship between joint stiffness degradation rates, crack propagation rates and nominal stress ranges in the brace are established, based on which a preliminary fatigue life prediction of wrapped composite joints can be made. The re-tested specimens exhibited superior fatigue performance than virgin ones, while specimens with poor steel surface roughness showed worse fatigue performance. After fatigue loading with 40% of stiffness degradation, the specimens were found to still have the potential to sustain its original static resistance.

Perspective error reduction in 2D Digital Image Correlation measurements by combination with Defocused Speckle Imaging

2D Digital Image Correlation (2D DIC) enables full-field in-plane displacement measurement by tracking the physical surface texture in the recorded images. However, because camera magnification is inversely proportional to object distance, points along the imaged surface can have different scales and appear to move at different rates. Surface curvature and out-of-plane motions may thus lead to perspective errors that manifest as apparent strains, limiting the method suitable only for flat objects that move purely in-plane. Defocused Speckle Imaging (DSI) is a related optical method where the movements of a laser-illuminated surface are determined by tracking the scattered interference speckle pattern. When the camera is in focus, the speckles appear to move at the same rate with the physical surface. However, camera defocus causes the speckle motion to deviate from the actual surface motion. Interestingly, the observed speckle motion magnitude increases with positive defocus distance, which is contrary to the perspective effects inherent to 2D DIC. Remarkably, with optimized geometric arrangement, 2D DIC and DSI have perspective errors that are equal in magnitude but with opposite signs. It is thus possible to compensate perspective errors by performing DIC and DSI measurements simultaneously and simply calculating the average of the two signals. This article derives the proposed compensation principle analytically using simple geometric framework. An experimental demonstration shows that the combination of DIC and DSI reduces magnification-dependent perspective errors by 73%, and errors due to object rigid-body rotation by 86%.

Experimental and numerical investigation of thermomechanical cycling of notched NiTi shape memory ribbon using SMA model accounting for plastic deformation

Shape memory alloys (SMAs) are being increasingly applied as thermally driven actuators. The commercially available SMA elements, however, frequently contain stress risers, either due to internal or surface defects or due to the required component shape. Stress risers represent a potential danger for the preliminary fatigue failure of such actuators but its actual mechanism is not very clear. This paper presents a combined experimental (2D DIC analysis of surface strains) and numerical analysis (SMA model with plastic deformation) of the stress, strain and phase fraction fields evolving in a thin NiTi shape memory ribbon with an artificial notch subjected to cyclic cooling-heating through transformation range under constant external force. It appeared that, even if only very low tensile stress is externally applied upon thermal cycling, local tensile stress at the notch tip sharply increases during the first cooling due to the forward martensitic transformation (MT) proceeding heterogeneously in space. This heterogeneity gives rise to plastic deformation at the notch-tip, which gradually accumulates upon thermal cycling and shields the notch tip from tensile overloading. Hence, fatigue performance of thermal NiTi actuator with stress riser depends very much on the plastic deformability of the alloy.