Three-dimensional digital image correlation technique using single high-speed camera for measuring large out-of-plane displacements at high framing rates

Measuring 3D shape and deformation in the presence of extremely strong ambient light and thermal radiation with a single time-gated camera

Monochromatic light-illuminated active-imaging stereo-digital image correlation (stereo-DIC) has been extensively used for measuring the surface deformation of materials and structures at elevated temperatures. Despite the improvements in the image acquisition techniques or devices, it is still challenging to measure the 3D deformation of materials and structures in the presence of strong, time-varying ambient light and thermal radiation. In this study, we present what we believe to be a novel dual-filtering single-camera stereo-DIC technique for full-field 3D high-temperature deformation measurement, even in the case of extremely intense ambient light and thermal radiation. In contrast to conventional active-imaging stereo-DIC that only suppresses the thermal radiations in the spectral domain, the proposed technique utilized a dual-filtering strategy (i.e., narrow bandpass optical filtering and ultrashort exposing) to suppress the strong ambient light and thermal radiation in both time and spectral domains. Besides, a four-mirror adapter is adopted to realize 3D shape and deformation measurement using a compact single time-gated camera. Experimental verifications, including assessments with laboratory experiments and validations on real thermal deformation tests under transient aerodynamic heating and direct ohmic heating, convincingly demonstrated that the proposed single-camera dual-filtering stereo-DIC method can achieve accurate 3D shape, motion and deformation measurement, even with strong light and thermal radiation from the quartz lamps and the heated sample.

Single-Camera Three-Dimensional Digital Image Correlation with Enhanced Accuracy Based on Four-View Imaging

Owing to the advantages of cost-effectiveness, compactness, and the avoidance of complicated camera synchronization, single-camera three-dimensional (3D) digital image correlation (DIC) techniques have gained increasing attention for deformation measurement of materials and structures. In the traditional single-camera 3D-DIC system, the left and right view images can be recorded by a single camera using diffraction grating, a bi-prism, or a set of planar mirrors. To further improve the measurement accuracy of single-camera 3D-DIC, this paper introduces a single-camera four-view imaging technique by installing a pyramidal prism in front of the camera. The 3D reconstruction of the measured points before and after deformation is realized with eight governing equations induced by four views, and the strong geometric constraints of four views can help to improve the measurement accuracy. A static experiment, a rigid body translation experiment, and a four-point bending experiment show that the proposed single-camera 3D-DIC method can achieve higher measurement accuracy than the dual-view single-camera 3D-DIC techniques and that the single-camera 3D-DIC method has advantages in reducing both random error and systematic error.

Three-dimensional full-field velocity measurements in shock compression experiments using stereo digital image correlation

Shock compression plate impact experiments conventionally rely on point-wise velocimetry measurements based on laser-based interferometric techniques. This study presents an experimental methodology to measure the free surface full-field particle velocity in shock compression experiments using high-speed imaging and three-dimensional (3D) digital image correlation (DIC). The experimental setup has a temporal resolution of 100 ns with a spatial resolution varying from 90 to 200 μm/pixel. Experiments were conducted under three different plate impact configurations to measure spatially resolved free surface velocity and validate the experimental technique. First, a normal impact experiment was conducted on polycarbonate to measure the macroscopic full-field normal free surface velocity. Second, an isentropic compression experiment on Y-cut quartz-tungsten carbide assembly is performed to measure the particle velocity for experiments involving ramp compression waves. To explore the capability of the technique in multiaxial loading conditions, a pressure shear plate impact experiment was conducted to measure both the normal and transverse free surface velocities under combined normal and shear loading. The velocities measured in the experiments using digital image correlation are validated against previous data obtained from laser interferometry. Numerical simulations were also performed using established material models to compare and validate the experimental velocity profiles for these different impact configurations. The novel ability of the employed experimental setup to measure full-field free surface velocities with high spatial resolutions in shock compression experiments is demonstrated for the first time in this work.

Semi-Global Matching Assisted Absolute Phase Unwrapping

Measuring speed is a critical factor to reduce motion artifacts for dynamic scene capture. Phase-shifting methods have the advantage of providing high-accuracy and dense 3D point clouds, but the phase unwrapping process affects the measurement speed. This paper presents an absolute phase unwrapping method capable of using only three speckle-embedded phase-shifted patterns for high-speed three-dimensional (3D) shape measurement on a single-camera, single-projector structured light system. The proposed method obtains the wrapped phase of the object from the speckle-embedded three-step phase-shifted patterns. Next, it utilizes the Semi-Global Matching (SGM) algorithm to establish the coarse correspondence between the image of the object with the embedded speckle pattern and the pre-obtained image of a flat surface with the same embedded speckle pattern. Then, a computational framework uses the coarse correspondence information to determine the fringe order pixel by pixel. The experimental results demonstrated that the proposed method can achieve high-speed and high-quality 3D measurements of complex scenes.

Digital image correlation assisted absolute phase unwrapping

This paper presents an absolute phase unwrapping method for high-speed three-dimensional (3D) shape measurement. This method uses three phase-shifted patterns and one binary random pattern on a single-camera, single-projector structured light system. We calculate the wrapped phase from phase-shifted images and determine the coarse correspondence through the digital image correlation (DIC) between the captured binary random pattern of the object and the pre-captured binary random pattern of a flat surface. We then developed a computational framework to determine fringe order number pixel by pixel using the coarse correspondence information. Since only one additional pattern is used, the proposed method can be used for high-speed 3D shape measurement. Experimental results successfully demonstrated that the proposed method can achieve high-speed and high-quality measurement of complex scenes.

Evaluation of Field Applicability of High-Speed 3D Digital Image Correlation for Shock Vibration Measurement in Underground Mining

When combined with high-speed photography technology, the digital image correlation (DIC) method provides an excellent photographic image processing capability that can be used to convert the evolving full-field surface properties of structures to sets of two-dimensional (2D) or three-dimensional (3D) coordinate values. In this study, the applicability of the DIC method and high-speed cameras in underground mining was investigated by measuring the displacement and vibration of rock pillars caused by blasting. This technique is used extensively in engineering and is increasingly being applied to new fields. As a result of comparing the DIC results for blast vibration with the measured values of the contact sensor through field tests, the maximum displacement and vibration speed were found to be 86% and 93% accurate, respectively. In addition, the obtained values appeared similar to those predicted through numerical analysis. Field test results indicate that both methods yielded similar results. Therefore, it is concluded that it is feasible to use the DIC and high-speed camera to measure ground displacements and vibrations from blasting in underground mining. In addition, the system conditions required for blast vibration measurement were considered by comparing the accuracy with the existing measurement methods.

Three-dimensional shape and deformation measurement on complex structure parts

Stereo digital image correlation technique (stereo-DIC or 3D-DIC) has been widely used in three-dimensional (3D) shape and deformation measurement due to its high accuracy and flexibility. But it is a tough task for it to deal with complex structure components because of the severe perspective distortion in two views. This paper seeks to resolve this issue using a single-camera system based on DIC-assisted fringe projection profilometry (FPP). A pixel-wise and complete 3D geometry of complex structures can be reconstructed using the robust and efficient Gray-coded method based on a FPP system. And then, DIC is just used to perform the temporal matching and complete full-field pixel-to-pixel tracking. The in- and out-of-plane deformation are obtained at the same time by directly comparing the accurate and complete 3D data of each corresponding pixel. Speckle pattern design and fringe denoising methods are carefully compared and chosen to simultaneously guarantee the measuring accuracy of 3D shape and deformation. Experimental results demonstrate the proposed method is an effective means to achieve full-field 3D shape and deformation measurement on complex parts, such as honeycomb structure and braided composite tube, which are challenging and even impossible for the traditional stereo-DIC method.

High-accuracy optical extensometer realized by two parallel cameras and two-dimensional digital image correlation

A conventional optical extensometer realized by a single common camera and two-dimensional digital image correlation (2D-DIC) often provides unsatisfactory strain results owing to the out-of-plane motion of the specimen. In this work, we propose an improved optical extensometer based on two parallel cameras and 2D-DIC. In the proposed extensometer, the gauge points are selected at the image centers of two cameras, which are negligibly affected by the out-of-plane translation and rotation, leading to higher accuracy of strain measurement as compared with the conventional optical extensometer. A rigid out-of-plane translation experiment and four repeated uniaxial tensile tests were conducted to verify the feasibility, reliability, and accuracy of the proposed method. Experimental results indicate that the proposed method has a strong ability to resist the effect of out-of-plane motion and experimental vibrations. Moreover, the strain measurement results obtained with the proposed method were found to be in excellent agreement with those obtained with a strain gauge, and the strain errors between them were only a few microstrains. Given that no compensation method is required, the proposed method is easy to implement with 2D-DIC and can be used for specimens of different sizes by adjusting the distance between the two cameras.

Accuracy analysis of an orthogonally arranged four-camera 3D digital image correlation system

To reduce the uncertainty region of a three-dimensional (3D) position, a four-camera 3D digital image correlation (3D-DIC) system was built by orthogonally arranging two sets of two-camera DIC systems. The theoretical model proposed herein revealed the relationship between 3D coordinates and system parameters and the propagation of the matching error to the position error. Numerical simulation and experiment were conducted to verify the theory. The simulation and experimental results indicated that the 3D position error of the four-camera system was smaller than that of the two-camera DIC system. The present contribution proves the feasibility of using four-camera DIC systems to improve measurement accuracy.

A Cross-Dichroic-Prism-Based Multi-Perspective Digital Image Correlation System

A robust three-perspective digital image correlation (DIC) system based on a cross dichroic prism and single three charge-coupled device (3CCD) color cameras is proposed in this study. Images from three different perspectives are captured by a 3CCD camera using the cross dichroic prism and two planar mirrors. These images are then separated by different CCD channels to perform correlation calculation with an existing multi-camera DIC algorithm. The proposed system is considerably more compact than the conventional multi-camera DIC system. In addition, the proposed system has no loss of spatial resolution compared with the traditional single-camera DIC system. The principle and experimental setup of the proposed system is described in detail, and a series of tests is performed to validate the system. Experimental results show that the proposed system performs well in displacement, morphology, and strain measurement.

Accurate 3D Shape, Displacement and Deformation Measurement Using a Smartphone

The stereo-digital image correlation technique using two synchronized industrial-grade cameras has been extensively used for full-field 3D shape, displacement and deformation measurements. However, its use in resource-limited institutions and field settings is inhibited by the need for relatively expensive, bulky and complicated experimental set-ups. To mitigate this problem, we established a cost-effective and ultra-portable smartphone-based stereo-digital image correlation system, which only uses a smartphone and an optical attachment. This optical attachment is composed of four planar mirrors and a 3D-printed mirror support, and can split the incoming scene into two sub-images, simulating a stereovision system using two virtual smartphones. Although such a mirror-based system has already been used for stereo-image correlation, this is the first time it has been combined with a commercial smartphone. This publication explores the potential and limitations of such a configuration. We first verified the effectiveness and accuracy of this system in 3D shape and displacement measurement through shape measurement and in-plane and out-of-plane translation tests. Severe thermal-induced virtual strains (up to 15,000 με) were found in the measured results due to the smartphone heating. The mechanism for the generation of the temperature-dependent errors in this system was clearly and reasonably explained. After a simple preheating process, the smartphone-based system was demonstrated to be accurate in measuring the strain on the surface of a loaded composite specimen, with comparable accuracy to a strain gauge. Measurements of 3D deformation are illustrated by tracking the deformation on the surface of a deflating ball. This cost-effective and ultra-portable smartphone-based system not only greatly decreases the hardware investment in the system construction, but also increases convenience and efficiency of 3D deformation measurements, thus demonstrating a large potential in resource-limited and field settings.

High-speed stereo-digital image correlation using a single color high-speed camera

A simple and practical high-speed stereo-digital image correlation (stereo-DIC) technique using a single off-the-shelf high-speed color CMOS camera is described in this work. By using the high-speed color CMOS camera and suitable optical filters, the recorded color images can be directly separated into red and blue channel sub-images with negligible color cross-talk between sub-channel images, which offers evident efficiency and accuracy advantages over the existing technique we proposed recently [Opt. Lasers Eng.95, 17 (2017)OLENDN0143-816610.1016/j.optlaseng.2017.03.009]. These separated sub-channel images can then be processed by regular stereo-DIC to retrieve the desired kinematic fields on the test object surface. The accuracy and precision of the established high-speed stereo-DIC system were characterized by measuring the displacements of a stationary object, and the results show good agreement with theoretical predications. To show the broad utility and practicality of the proposed method, three typical experiments, involving (i) transient displacement and velocity measurement of a rotating fan; (ii) full-field vibration measurement of a rectangular aluminum panel; and (iii) transient 3D surface shape, displacement, and strain fields measurement of a balloon during the whole explosion procedure, were carried out. The results show that, by using a proper high-speed color camera, high-speed 3D shape, displacement and deformation measurements can be realized in a cost-effective and easy-to-implement manner. The proposed technique demonstrates great potential in impact engineering, explosion, and vibration tests.

A Coupled Magnetoelastic Strain Sensor Array for Guiding and Monitoring Hernia Repairs

OBJECTIVE

Ventral hernia repairs using mesh prosthetics suffer from high recurrence rates, with 10%-20% of repairs failing within three years. Uneven distribution of stress within the implanted mesh prosthetic is thought to contribute to the high recurrence rate. We propose a method for providing quantitative guidance and monitoring of hernia repairs using an array of magnetoelastic strain sensors.

METHODS

The magnetoelastic strain sensors presented here are based on a coupled design to achieve measurements with higher signal-to-noise ratio (SNR). A first magnetoelastic element (the transducer) is bonded to the mesh prosthetic and is characterized by a strain-dependent magnetic field. The resonance frequency of a second magnetoelastic element (the resonator) encased in a rigid casing is biased by the transducer element's magneticity and can be measured noninvasively using an external interrogation coil. The coupled magnetoelastic strain sensors are assembled using a combination of photochemical machining, patterning, and heat sealing.

RESULTS

The dynamic range of the coupled sensors can be tuned by altering the transducer geometry. Additional spring elements are integrated onto the transducer element to achieve high dynamic range measurements saturating at 74 millistrains.

CONCLUSION

A coupled magnetoelastic strain sensor combines a transducer with an encased resonator element to measure strain with high SNR on an implantable flexible hernia mesh substrate.

SIGNIFICANCE

This study provides surgeons and researchers with a clinically relevant tool to quantify the strain distributions within implanted mesh prosthetics, with the ultimate goal of reducing the recurrence rate of ventral hernia repairs.

Integrating coupled magnetoelastic sensors onto a flexible hernia mesh for high dynamic range strain measurements

Despite better performance over primary repairs, tension-free ventral hernia repairs with mesh still suffer from a high recurrence rate. High stress gradients in the mesh are thought to contribute to hernia recurrence. We propose a postoperative monitoring system based on a coupled pair of magnetoelastic strain sensors to enable patients and physicians to non-invasively measure and track the strain distribution across the hernia mesh. Our design combines an encased resonator with a spring-loaded transducer to achieve high signal amplitude with a wide dynamic range. We also demonstrate a fabrication protocol to integrate the resonant strain sensors with a commercial polypropylene mesh. The packaged sensor is capable of detecting up to 37.5 millistrain, an order of magnitude greater than previously demonstrated.

High-speed triangular pattern phase-shifting 3D measurement based on the motion blur method

Recent advancements in 3D measurement technologies have increased the urgency of requiring high-speed 3D measurement in many fields. This study presents a novel four-step triangular pattern phase-shifting 3D measurement using the motion blur method, which combines the advantages of phase-shifting methods. To comply with the high speed requirement, binary coded triangular patterns are projected and could dither vertically. Therefore, the image captured by the camera is blurred into grayscale-intensity triangular patterns, which can be used for phase unwrapping and 3D reconstruction. The proposed method decreased the projection time compared with sinusoidal patterns using a DMD (digital micromirror device) projector. Furthermore, this study presents a four-step triangular phase-shifting unwrapping algorithm. The experiments indicate that the proposed method can achieve high-speed 3D measurement and reconstruction.

Self-calibration single-lens 3D video extensometer for high-accuracy and real-time strain measurement

The accuracy of strain measurement using a common optical extensometer with two-dimensional (2D) digital image correlation (DIC) is not sufficient for experimental applications due to the effect of out-of-plane motion. Although three-dimensional (3D) DIC can measure all three components of displacement without introducing in-plane displacement errors, 3D-DIC requires the stringent synchronization between two digital cameras and requires complicated system calibration of binocular stereovision, which makes the measurement rather inconvenient. To solve the problems described above, this paper proposes a self-calibration single-lens 3D video extensometer for non-contact, non-destructive and high-accuracy strain measurement. In the established video extensometer, a single-lens 3D imaging system with a prism and two mirrors is constructed to acquire stereo images of the test sample surface, so the problems of synchronization and out-of-plane displacement can be solved easily. Moreover, a speckle-based self-calibration method which calibrates the single-lens stereo system using the reference speckle image of the specimen instead of the calibration targets is proposed, which will make the system more convenient to be used without complicated calibration. Furthermore, an efficient and robust inverse compositional Gauss-Newton algorithm combined with a robust stereo matching stage is employed to achieve high-accuracy and real-time subset-based stereo matching. Tensile tests of an Al-alloy specimen were performed to demonstrate the feasibility and effectiveness of the proposed self-calibration single-lens 3D video extensometer.

Three-dimensional displacement measurement based on the combination of digital image correlation and optical flow

This paper proposes what we believe is a novel simultaneous three-dimensional (3D) displacement measurement technique based on a combination of digital image correlation (DIC) and optical flow (OF). In this method, both the in-plane and out-of-plane displacements can be accurately extracted from only two continuous interferograms. DIC estimates the velocity field between two consecutive frames. According to the optical flow constrained equation, we can then obtain the whole-field out-of-plane displacement map by the estimations of the in-plane displacement components and the local frequency of the original image. The proposed method's operation is simple compared with other phase demodulation methods. Moreover, the new method works perfectly in areas with dense fringes. To verify its effectiveness, we applied a new algorithm to simulated and experimental interferograms. The results of our simulation and experiment show that the new method can demodulate the out-of-plane component of the deformation-phase from the visible in-plane velocity field without an unwrapping process. Further, the proposed algorithm provides a new approach to measure whole-field 3D displacement and dynamic deformation.

Structure parameter analysis and uncertainty evaluation for single-camera stereo-digital image correlation with a four-mirror adapter

Increasing interest in the use of single-camera stereo-digital image correlation (stereo-DIC) with a four-mirror adapter for full-field three-dimensional shape, motion, and deformation measurements has led to an ongoing need for both the optimal design of an optical structure and predictions of the measurement uncertainties. In this study, to get a clear knowledge of the optical model of the four-mirror adapter-assisted single-camera stereo-DIC system and further facilitate an optimal design of the optical structure for specific measurement objects, comprehensive analyses of the structure parameters, e.g., the baseline distance and valid field of view of the virtual stereo-DIC system, are conducted. Based on the law of propagation of uncertainties, the influence of the structure parameters on the measurement uncertainty is assessed theoretically. The effectiveness and accuracy of these analyses are verified by calibrating the single-camera stereo-DIC system and measuring the displacements of a stationary planar plate.

A comparison of 2D and 3D digital image correlation for a membrane under inflation

Three-dimensional (3D) digital image correlation (DIC) is becoming widely used to characterize the behavior of structures undergoing 3D deformations. However, the use of 3D-DIC can be challenging under certain conditions, such as high magnification, and therefore small depth of field, or a highly controlled environment with limited access for two-angled cameras. The purpose of this study is to compare 2D-DIC and 3D-DIC for the same inflation experiment and evaluate whether 2D-DIC can be used when conditions discourage the use of a stereo-vision system. A latex membrane was inflated vertically to 5.41 kPa (reference pressure), then to 7.87 kPa (deformed pressure). A two-camera stereo-vision system acquired top-down images of the membrane, while a single camera system simultaneously recorded images of the membrane in profile. 2D-DIC and 3D-DIC were used to calculate horizontal (in the membrane plane) and vertical (out of the membrane plane) displacements, and meridional strain. Under static conditions, the baseline uncertainty in horizontal displacement and strain were smaller for 3D-DIC than 2D-DIC. However, the opposite was observed for the vertical displacement, for which 2D-DIC had a smaller baseline uncertainty. The baseline absolute error in vertical displacement and strain were similar for both DIC methods, but it was larger for 2D-DIC than 3D-DIC for the horizontal displacement. Under inflation, the variability in the measurements were larger than under static conditions for both DIC methods. 2D-DIC showed a smaller variability in displacements than 3D-DIC, especially for the vertical displacement, but a similar strain uncertainty. The absolute difference in the average displacements and strain between 3D-DIC and 2D-DIC were in the range of the 3D-DIC variability. Those findings suggest that 2D-DIC might be used as an alternative to 3D-DIC to study the inflation response of materials under certain conditions.

Towards a full-field strain sensor for guiding hernia repairs

Each year, approximately 400,000 ventral hernia repairs are performed in the United States [1], [2]. Large ventral hernias (hernias that occur in the abdominal wall) are typically treated by suturing in a surgical mesh to cover and overlap the hernia defect. However, in 10-20% of patients, the hernia repair fails, resulting in recurrence of the hernia, along with other complications including infection and intestinal obstruction [3], [4]. One potential cause of hernia recurrence is the unequal distribution of stress across the mesh resulting in high stress concentrations at the tissue-mesh interface, particularly at the site of mesh fixation to the abdominal wall muscles[5], [6]. Strain across the mesh can be used as an indicator for how evenly stress is distributed across the surface of the mesh. To this end, we have built a full-field, 3D strain measurement system to enable physicians to actively identify and address areas of high strain during the surgery, thus decreasing the rate of hernia recurrence. The strain sensor uses an optical technique, called the grid method, in conjunction with the defocused particle image velocimetry (DPIV) technique to measure the 3D strain distribution across the mesh. The system can achieve a limit of detection down to 0.4% strain and across a 50 cm range z-axis displacement using a Canon EOS 7D camera with a pinhole aperture mask.

Effect of camera temperature variations on stereo-digital image correlation measurements

In laboratory and especially non-laboratory stereo-digital image correlation (stereo-DIC) applications, the extrinsic and intrinsic parameters of the cameras used in the system may change slightly due to the camera warm-up effect and possible variations in ambient temperature. Because these camera parameters are generally calibrated once prior to measurements and considered to be unaltered during the whole measurement period, the changes in these parameters unavoidably induce displacement/strain errors. In this study, the effect of temperature variations on stereo-DIC measurements is investigated experimentally. To quantify the errors associated with camera or ambient temperature changes, surface displacements and strains of a stationary optical quartz glass plate with near-zero thermal expansion were continuously measured using a regular stereo-DIC system. The results confirm that (1) temperature variations in the cameras and ambient environment have a considerable influence on the displacements and strains measured by stereo-DIC due to the slightly altered extrinsic and intrinsic camera parameters; and (2) the corresponding displacement and strain errors correlate with temperature changes. For the specific stereo-DIC configuration used in this work, the temperature-induced strain errors were estimated to be approximately 30-50 με/°C. To minimize the adverse effect of camera temperature variations on stereo-DIC measurements, two simple but effective solutions are suggested.