Single-camera Stereo-digital Image Correlation Research Articles

Dual-biprism-based single-camera high-speed 3D-digital image correlation for deformation measurement on sandwich structures under low velocity impact

Abstract Herein, a single-camera stereo digital image correlation (DIC) method is developed for dynamic deformation measurement, and a dual-biprism high-speed stereo camera system is proposed to capture surface images. Then, full-field displacement and strain distributions are obtained after correlation and differential calculation using the stereo-DIC algorithm. After a typical experiment is performed to verify the accuracy of the proposed system, full-field displacement and strain distributions of sandwich panels subjected to low-velocity impacts are determined. These results show that the proposed system is an efficient and convenient tool for dynamic deformation measurement.

Making use of Digital Image Correlation to identify the true character of the applied load

Digital Image Correlation became one of the dominant optical full-field techniques used in experimental mechanics. It does not require sophisticated equipment to evaluate the full displacement vector field of the specimen under test with great accuracy. In this paper, we would like to put the reader's attention to the DIC technique as a solution that may identify the genuine character of the load applied to the object under test. Our experience with testing wind turbine blades using DIC proves that possibility. Full Text: PDF References E.M.C Jones, M.A. Iadicola, "A good practises guide for digital image correlation" International Digital Image Correlation Society, (2018), CrossRef B. Pan, "Digital image correlation for surface deformation measurement: historical developments, recent advances and future goals", Meas. Sci. Technol. 29, 082001 (2018), CrossRef B. Pan, L.P. Yu, Q.B. Zhang, "Review of single-camera stereo-digital image correlation techniques for full-field 3D shape and deformation measurement", Sci. China Technol. Sci. 61, 2 (2018), CrossRef S. Sharma, N. Iniyan Thiruselvam, S.J. Subramanian, G.S. Kumar, "Computation of strains from stereo digital image correlation using principal component analysis", Meas. Sci. Technol. 32, 105201 (2021), CrossRef IEC 61400-5:2020 Wind energy generation systems - Part 5: Wind turbine, DirectLink IEC 61400-23:2014 Wind turbines - Part 23: Full-scale structural testing of rotor blades, DirectLink

Creep Behavior Characterization of Nickel-Based Single-Crystal Superalloy DD6 Thin-Walled Specimens Based on a 3D-DIC Method.

The thickness debit effect of creep behavior has been a focal point of nickel-based single-crystal superalloy research, and there is a need for an advanced creep deformation measurement method. This study developed a novel high-temperature creep test system based on a single-camera stereo digital image correlation (DIC) method with four plane mirrors to conduct creep tests on thin-walled specimens of a nickel-based single-crystal alloy, DD6, with thicknesses of 0.6 mm and 1.2 mm under experimental conditions of 980 °C/250 MPa. The reliability of the single-camera stereo DIC method in measuring long-term deformation at a high temperature was experimentally verified. The experimental results show that the creep life of the thinner specimen was significantly shorter. It was found the lack of coordination in the creep deformation process of the edge and middle section of the thin-walled specimens may be an important factor in the thickness debit effect according to the full-field strain contour. By comparing the local strain curve at the rupture point with the average creep strain curve, it was found that the creep rate at the rupture point was less affected by the specimen thickness during the secondary creep stage, while the average creep rate in the working section significantly increased as the wall thickness decreased. The thicker specimen usually had a higher average rupture strain and higher damage tolerance, which prolonged the rupture time.

Design and validation of a demand-oriented single-camera stereo-DIC system with a four-mirror adapter

Four-mirror-assisted single-camera stereo-digital image correlation (stereo-DIC) system overperforms conventional two-camera stereo-DIC system in terms of system compactness, cost investment and easy implementation. However, how to design a suitable four-mirror adaptor orienting a specific demand has not been clearly described in previous researches. To address this issue and satisfy practical requirements encountered in real applications, we proposed a design strategy for determining the optimal structure of a four-mirror adaptor. Specifically, to optimize the structural parameters of a four-mirror adaptor, the proposed design strategy comprehensively considers various factors, including the desired field of view (FOV), working distance, measurement accuracy, system compactness as well as the parameters of available devices (camera and lens). After implementing the algorithm based on the degrees of freedom (DOF) of the imaging system, an optimal solution can be determined from all the solutions that can satisfy the identical requirements. The designed four-mirror adapter is then printed by a 3D printer, and can be directly attached to the lens for single-camera pseudo-stereo imaging. Further, two specific design paradigms demonstrated the correctness and effectiveness of the proposed design strategy. It is expected that the proposed method can help the researchers to construct a demand-satisfied yet compact four-mirror-assisted single-camera stereo-DIC system.

High-temperature stereo-digital image correlation using a single polarization camera.

We propose a new, to the best of our knowledge, full-frame, single-camera stereo-digital image correlation (stereo-DIC) technique that is capable of measuring surface profile and deformation over a wide temperature range. This technique uses a single polarization camera and a polarizing beam splitter to synchronously capture two full-frame sub-images with orthogonal polarization directions. By further adopting the idea of active imaging that combines monochromatic blue-light active lighting and narrow bandpass filter imaging, this system can capture stable and high-contrast speckle images of specimens at extremely high temperatures. From the captured sub-image pairs, profile and deformation fields can be retrieved using a regular stereo-DIC algorithm. Compared with the existing full-frame single-camera stereo-DIC systems implemented on a color camera, the established system offers wider applicability, since it is insensitive to serious variations in ambient light in non-laboratory environments and to the thermal radiation of hot objects in extreme high-temperature environments. The effectiveness and accuracy of the proposed technique are validated by room-temperature tests and high-temperature tests at up to 1000°C. The advantages of a simple configuration, free synchronization, and full-frame high-temperature deformation measurement ability offer the proposed technique potential in both high-speed and high-temperature 3D measurement applications.

A Simple and Practical Single-Camera Stereo-Digital Image Correlation Using a Color Camera and X-Cube Prism.

A simple and practical full-frame single-camera stereo-digital image correlation (stereo-DIC) technique for three-dimensional (3D) shape, displacement, and deformation measurements is proposed. The technique uses a compact X-cube prism-based color separation device and a color camera to capture images of blue and red colors from different optical paths, and then extracts the surface 3D shape and deformation information of a test sample by processing the captured two sub-channel color images using regular stereo-DIC algorithm. Compared with the existing full-frame single-camera stereo-DICs, the proposed one eliminates the need for a beam splitter and two bandpass filters to capture images, and offers more simple, compact, and easy-to-use optical arrangement. This novel single-camera stereo-DIC technique was validated by a series of baseline experiments involving 3D surface reconstructions, translation tests, and full-field deformation measurements, which provide a new flexible and practical avenue for measuring surface 3D shape and deformation, particularly in microscopic and high-speed applications.

Calibration-free single camera stereo-digital image correlation for small-scale underwater deformation measurement

Stereo-digital image correlation (stereo-DIC) has been routinely used as a practical and powerful optical technique for surface 3D full-field shape and deformation measurements in various scenarios. However, it is challenging to perform accurate stereo-DIC measurements for submerged objects due to the significant refraction presented at the interfaces of air and water. In this paper, a novel underwater full-field 3D profile and deformation measurements method using the single camera stereo-DIC technique that combines single bilateral telecentric lens imaging and bi-prism-assisted pseudo stereovision is proposed. In using this technique, an immersed surface projects through the (semi-) submerged bi-prism and the bilateral telecentric lens, forming two virtual images on left and right parts of the camera sensor. Matching the virtual left and right images using DIC and substituting the matched image points into a set of newly derived linear equations, accurate 3D profiles and further 3D deformation fields can be readily obtained. The effectiveness and accuracy of the proposed method are successfully validated by a set of real experiments including underwater 3D shape reconstruction, in-plane and out-of-plane translation, and membrane inflation experiments. Because of the distinctive advantages of simple and compact optical configuration, without the need of stereo calibration, and strong robustness against water fluctuation and ambient light variation, the proposed method is expected to be a simple yet effective method for many underwater applications like in vitro biological tissues deformation measurements and submerged materials characterization.

Low-frame-rate single camera system for 3D full-field high-frequency vibration measurements

Abstract A single-camera stereo-digital image correlation (stereo-DIC) system to obtain 3D full-field vibration measurements is proposed. The optical setup is composed of two planar mirrors and a single low frame rate camera, thus resulting in a compact and low-cost equipment. The two mirrors are used to create pseudo-stereo images of a target surface on the camera sensor, which are then correlated by using stereo-DIC. The image acquisition process is carried out at low frame rates and the Nyquist-Shannon frequency limitation is overcome by adopting a down-sampling approach under the hypothesis that the vibration signal is characterized by a single known frequency component. The developed pseudo-stereo DIC system allows to obtain 3D full-field vibration measurements in a frequency range up to 4 kHz even with an available frame rate (at full resolution) of 178 fps. The effectiveness of the described approach has been verified by performing vibration measurements on a cantilever plate and a turbine blade.

Experimental study of tensile properties and deformation evolutions of 2D and 2.5D woven SiO2f/SiO2 composites using single-camera stereo-digital image correlation

SiO2f/SiO2 composites have become a promising candidate material for thermal protection systems due to its unique combination of various properties such as low density, high thermal shock resistance, favorable electrical insulating properties and enhanced mechanical properties. The increasing interests in these materials lead to a pressing requirement on comprehensively understanding their mechanical behaviors and load bearing mechanisms under external loads. The present work focuses on experimental study and comparison of the mechanical properties and deformation evolutions of 2D and 2.5D woven SiO2f/SiO2 composites using single-camera stereo-digital image correlation (stereo-DIC) technique. Based on the full-field displacements and strains retrieved by single-camera stereo-DIC, the mechanical properties of 2D and 2.5D woven SiO2f/SiO2 composites can be determined. It is found that 2D woven SiO2f/SiO2 composites present a lower yield strength and elastic modulus than 2.5D SiO2f/SiO2 composites. Also, remarkable differences in full-field displacement and strain distributions of the 2D and 2.5D SiO2f/SiO2 composites are observed, indicating a strong influence of the weave architecture on the deformation evolution and load bearing mechanism of SiO2f/SiO2 composites. Finally, the failure mechanisms of these two composites are discussed.

Low-cost, portable, robust and high-resolution single-camera stereo-DIC system and its application in high-temperature deformation measurements

A low-cost, portable, robust and high-resolution single-camera stereo-digital image correlation (stereo-DIC) system for accurate surface three-dimensional (3D) shape and deformation measurements is described. This system adopts a single consumer-grade high-resolution digital Single Lens Reflex (SLR) camera and a four-mirror adaptor, rather than two synchronized industrial digital cameras, for stereo image acquisition. In addition, monochromatic blue light illumination and coupled bandpass filter imaging are integrated to ensure the robustness of the system against ambient light variations. In contrast to conventional binocular stereo-DIC systems, the developed pseudo-stereo-DIC system offers the advantages of low cost, portability, robustness against ambient light variations, and high resolution. The accuracy and precision of the developed single SLR camera-based stereo-DIC system were validated by measuring the 3D shape of a stationary sphere along with in-plane and out-of-plane displacements of a translated planar plate. Application of the established system to thermal deformation measurement of an alumina ceramic plate and a stainless-steel plate subjected to radiation heating was also demonstrated.

Review of single-camera stereo-digital image correlation techniques for full-field 3D shape and deformation measurement

Single-camera stereo-digital image correlation (stereo-DIC) techniques have gained increasing attentions and demonstrated excellent prospects in the experimental mechanics community owing to their prominent advantages of cost-effectiveness, compactness, and the avoidance of the complicated camera synchronization. Using additional optical devices, e.g. a diffraction grating, a bi-prism or a set of planar mirrors, pseudo stereo images of a test sample surface can be recorded with a single camera. By correlating these stereo images using DIC, full-field three-dimensional (3D) shape and deformation can be retrieved. This review comprehensively summarizes the historical development, methodologies, strengths and weaknesses of the diffraction grating-based, prism-based, four-mirror-adaptor-based single-camera stereo-DIC techniques, and the recently proposed novel full-frame single color camera-based stereo-DIC technique for full-field 3D shape and deformation measurement. The optical arrangements, principles and calibration procedures of these single-camera stereo-DIC techniques are described in detail. Since high-speed deformation measurement is efficiently achieved by combining the single-camera stereo-DIC with one high-speed camera, single-camera stereo-DIC techniques show great potential in impact engineering, vibration and other dynamic tests.

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.

Single-camera stereo-digital image correlation with a four-mirror adapter: optimized design and validation

A low-cost, easy-to-implement but practical single-camera stereo-digital image correlation (DIC) system using a four-mirror adapter is established for accurate shape and three-dimensional (3D) deformation measurements. The mirrors assisted pseudo-stereo imaging system can convert a single camera into two virtual cameras, which view a specimen from different angles and record the surface images of the test object onto two halves of the camera sensor. To enable deformation measurement in non-laboratory conditions or extreme high temperature environments, an active imaging optical design, combining an actively illuminated monochromatic source with a coupled band-pass optical filter, is compactly integrated to the pseudo-stereo DIC system. The optical design, basic principles and implementation procedures of the established system for 3D profile and deformation measurements are described in detail. The effectiveness and accuracy of the established system are verified by measuring the profile of a regular cylinder surface and displacements of a translated planar plate. As an application example, the established system is used to determine the tensile strains and Poisson׳s ratio of a composite solid propellant specimen during stress relaxation test. Since the established single-camera stereo-DIC system only needs a single camera and presents strong robustness against variations in ambient light or the thermal radiation of a hot object, it demonstrates great potential in determining transient deformation in non-laboratory or high-temperature environments with the aid of a single high-speed camera.

Three-dimensional microscopic deformation measurements on cellular solids

The increasing interest in small-scale problems demands novel experimental protocols providing dense sets of 3D deformation data of complex shaped microstructures. Obtaining such information is particularly significant for the study of natural and engineered cellular solids for which experimental data collected at macro scale and describing the global mechanical response provide only limited information on their function/structure relationship. Cellular solids, in fact, due their superior mechanical performances to a unique arrangement of the bulk material properties (i.e. anisotropy and heterogeneity) and cell structural features (i.e. pores shape, size and distribution) at the micro- and nano-scales.To address the need for full-field experimental data down to the cell level, this paper proposes a single-camera stereo-Digital Image Correlation (DIC) system that makes use of a wedge prism in series to a telecentric lens for performing surface shape and deformation measurements on microstructures in three dimensions. Although the system possesses a limited measurement volume (FOV~2.8×4.3mm2, error-free DOF ~1mm), large surface areas of cellular samples can be accurately covered by employing a sequential image capturing scheme followed by an optimization-based mosaicing procedure. The basic principles of the proposed method together with the results of the benchmarking of its metrological performances and error analysis are here reported and discussed in detail. Finally, the potential utility of this method is illustrated with micro-resolution three-dimensional measurements on a 3D printed honeycomb and on a block sample of a Luffa sponge under compression.