Bing Pan

As a practical and effective tool for quantitative in-plane deformation measurement of a planar object surface, two-dimensional digital image correlation (2D DIC) is now widely accepted and commonly used in the field of experimental mechanics. It directly provides full-field displacements to sub-pixel accuracy and full-field strains by comparing the digital images of a test object surface acquired before and after deformation. In this review, methodologies of the 2D DIC technique for displacement field measurement and strain field estimation are systematically reviewed and discussed. Detailed analyses of the measurement accuracy considering the influences of both experimental conditions and algorithm details are provided. Measures for achieving high accuracy deformation measurement using the 2D DIC technique are also recommended. Since microscale and nanoscale deformation measurement can easily be realized by combining the 2D DIC technique with high-spatial-resolution microscopes, the 2D DIC technique should find more applications in broad areas.

Digital Image Correlation (DIC) is a flexible and effective technique to measure the displacements on specimen surfaces by matching the reference subsets in the undeformed image with the target subsets in the deformed image. With the existing DIC techniques, the user must rely on experience and intuition to manually define the size of the reference subset, which is found to be critical to the accuracy of measured displacements. In this paper, the problem of subset size selection in the DIC technique is investigated. Based on the Sum of Squared Differences (SSD) correlation criterion as well as the assumption that the gray intensity gradients of image noise are much lower than that of speckle image, a theoretical model of the displacement measurement accuracy of DIC is derived. The theoretical model indicates that the displacement measurement accuracy of DIC can be accurately predicted based on the variance of image noise and Sum of Square of Subset Intensity Gradients (SSSIG). The model further leads to a simple criterion for choosing an optimal subset size for the DIC analysis. Numerical experiments have been performed to validate the proposed concepts, and the calculated results show good agreements with the theoretical predictions.

This article is a personal review of the historical developments of digital image correlation (DIC) techniques, together with recent important advances and future goals. The historical developments of DIC techniques over the past 35 years are divided into a foundation-laying phase (1982–1999) and a boom phase (2000 to the present), and are traced by describing some of the milestones that have enabled new and/or better DIC measurements to be made. Important advances made to DIC since 2010 are reviewed, with an emphasis on new insights into the 2D-DIC system, new improvements to the correlation algorithm, and new developments in stereo-DIC systems. A summary of the current state-of-the-art DIC techniques is provided. Some further improvements that are needed and the future goals in the field are also envisioned.

Developments in digital image correlation in the last two decades have made it a popular and effective tool for full-field displacement and strain measurements in experimental mechanics. In digital image correlation, the use of the sub-pixel registration algorithm is regarded as the key technique to improve accuracy. Different types of sub-pixel registration algorithms have been developed. However, little quantitative research has been carried out to compare their performances. This paper investigates three types of the most commonly used sub-pixel displacement registration algorithms in terms of the registration accuracy and the computational efficiency using computer-simulated speckle images. A detailed examination of the performances of each algorithm reveals that the iterative spatial domain cross-correlation algorithm (Newton–Raphson method) is more accurate, but much slower than other algorithms, and is recommended for use in these applications.

Digital image correlation (DIC) is an image-based optical metrology for full-field deformation measurement. In DIC technique, the test object surface must be covered with a random speckle pattern, which deforms together with the object surface as a carrier of deformation information. In practice, the speckle patterns may show distinctly different intensity distribution characteristics and have an important influence on DIC measurements. How to assess the overall quality of different speckle patterns with a simple yet effective parameter is an interesting but confusing problem, and is also helpful to the optimal use of the technique. In this paper, a novel, simple, easy-to-calculate yet effective global parameter, called mean intensity gradient, is proposed for quality assessment of the speckle patterns used in DIC. To verify the correctness and effectiveness of the new concept, five different speckle patterns are numerically translated, and the displacements measured with DIC are compared with the exact ones. The errors are evaluated in terms of mean bias error and standard deviation error. It is shown that both mean bias error and standard deviation of the measured displacement are closely related to the mean intensity gradient of the speckle pattern used, and a so-called good speckle pattern should be of large mean intensity gradient.

Digital image correlation (DIC) method using iterative least squares algorithm (ILS) for displacement field measurement and pointwise least squares algorithm (PLS) for strain field measurement is proposed in this paper. A more general and practical intensity change model is employed with consideration of the linear intensity change of the deformed image, followed by an iterative least squares algorithm for calculating displacement field with sub-pixel accuracy. The concept of correlation function is not used in the ILS method, even though we prove that the algorithm is actually equivalent to the optimization of the sum of squared difference correlation function using improved Newton–Raphson method. Besides, different from the conventional strain estimation approaches based on smoothing the displacement fields first and followed by differentiation of the smoothed displacement fields, a simple yet effective PLS algorithm is proposed for extracting strain fields from the computed displacement fields. The effectiveness and accuracy of the proposed techniques is verified through numerical simulation experiments. A practical application of the algorithms to residual plastic deformation field measurement of GH4169 alloy subjected to tensile fatigue is also presented.

In digital image correlation (DIC), to obtain the displacements of each point of interest, a correlation criterion must be predefined to evaluate the similarity between the reference subset and the target subset. The correlation criterion is of fundamental importance in DIC, and various correlation criteria have been designed and used in literature. However, little research has been carried out to investigate their relations. In this paper, we first provide a comprehensive overview of various correlation criteria used in DIC. Then we focus on three robust and most widely used correlation criteria, i.e., a zero-mean normalized cross-correlation (ZNCC) criterion, a zero-mean normalized sum of squared difference (ZNSSD) criterion, and a parametric sum of squared difference (PSSD(ab)) criterion with two additional unknown parameters, since they are insensitive to the scale and offset changes of the target subset intensity and have been highly recommended for practical use in literature. The three correlation criteria are analyzed to establish their transversal relationships, and the theoretical analyses clearly indicate that the three correlation criteria are actually equivalent, which elegantly unifies these correlation criteria for pattern matching. Finally, the equivalence of these correlation criteria is further validated by numerical simulation and actual experiment.

The nonlinear intensity response of a digital fringe projection profilometry (FPP) system causes the captured fringe patterns to be nonsinusoidal waveforms and leads to an additional phase measurement error for commonly used three- and four-step phase-shifting algorithms. We perform theoretical analysis of the phase error owing to the nonsinusoidal waveforms. Based on the derived theoretical model, a novel and simple iterative phase compensation algorithm is proposed to compensate the nonsinusoidal phase error. Experiments show that the proposed algorithm can be used for effective phase error compensation in practical phase-shifting FPP.

Fringe projection profilometry is widely used for three-dimensional (3-D) surface shape measurement using phase-shifting (PS) methods with multiple images or transform methods with single projected fringe pattern. In this paper, phase extraction methods from a single fringe pattern using different transform methods are compared using both simulations and experiments. The principles of Fourier transform (FT), windowed Fourier transform (WFT) and wavelet transform (WT) methods for fringe pattern processing are introduced. Implementation of 1-D and 2-D algorithms and phase compensation are discussed. Noisy and non-sinusoidal waveforms are involved into this comparison. The merits and limitations of each of these processing methods are indicated.

Many published research works regarding digital image correlation (DIC) have been focused on the improvements of the accuracy of displacement estimation. However, the original displacement fields calculated at discrete locations using DIC are unavoidably contaminated by noises. If the strain fields are directly computed by differentiating the original displacement fields, the noises will be amplified even at a higher level, and the resulting strain fields are untrustworthy. Based on the principle of local least-square fitting using two-dimensional (2D) polynomials, a 2D Savitzky-Golay (SG) digital differentiator is deduced and used to calculate strain fields from the original displacement fields obtained by DIC. The calculation process can be easily implemented by convolving the SG digital differentiator with the estimated displacement fields. Both homogeneous and inhomogeneous deformation images are employed to verify the proposed technique. The calculated strain fields clearly demonstrate that the proposed technique is simple and effective.

Fast and high-accuracy deformation analysis using digital image correlation (DIC) has been increasingly important and highly demanded in recent years. In literature, the DIC method using the Newton–Rapshon (NR) algorithm has been considered as a gold standard for accurate sub-pixel displacement tracking, as it is insensitive to the relative deformation and rotation of the target subset and thus provides highest sub-pixel registration accuracy and widest applicability. A significant drawback of conventional NR-algorithm-based DIC method, however, is its extremely huge computational expense. In this paper, a fast DIC method is proposed deformation measurement by effectively eliminating the repeating redundant calculations involved in the conventional NR-algorithm-based DIC method. Specifically, a reliability-guided displacement scanning strategy is employed to avoid time-consuming integer–pixel displacement searching for each calculation point, and a pre-computed global interpolation coefficient look-up table is utilized to entirely eliminate repetitive interpolation calculation at sub-pixel locations. With these two approaches, the proposed fast DIC method substantially increases the calculation efficiency of the traditional NR-algorithm-based DIC method. The performance of proposed fast DIC method is carefully tested on real experimental images using various calculation parameters. Results reveal that the computational speed of the present fast DIC is about 120–200 times faster than that of the traditional method, without any loss of its measurement accuracy

An advanced video deflectometer using off-axis digital image correlation is proposed for real-time, non-contact and targetless measurement of vertical deflection of bridges. To achieve real-time displacement tracking with sub-pixel accuracy, an efficient inverse compositional Gauss–Newton algorithm is employed. The detected image displacements in pixels are converted to physical displacements in millimeters using an easy-to-implement yet accurate calibration technique with the aid of a laser rangefinder. Real translation tests with precisely controlled motions were performed to examine the accuracy of the proposed technique. Real-time deflection monitoring of a railway bridge subjected to train pass-by is also demonstrated to show the practicality, accuracy and application potential of the proposed technique.

Rapid decrease in antibacterial efficacy of existing active packages is difficult to promisingly prevent microbial infection during the storage of perishable products. Here, we pioneered an advanced ZnO-doped hollow carbon-encapsulated curcumin (ZHC-Cur)-chitosan (CS) slow-release film (ZHC-Cur-CS) with “nano-barricade” structure through demand-oriented tailoring of the structure and components of zeolitic imidazolate framework-8 (ZIF-8) carrier. Such an exquisite structure realized the effective sustained release of Curcumin through the dual complexity of diffusion pathway by the disordered hierarchical pore structure and steric hindrance. Prepared ZHC-Cur-CS film exhibited boosting bactericidal and antioxidant abilities by virtue of the functional synergy between curcumin and ZnO. Thus, ZHC-Cur-CS film demonstrated excellent preservation performance by significantly prolonging the shelf life of Citrus (∼2.4 times). Furthermore, the upgraded mechanical strength, improved barrier ability, and proven safety laid the foundation for its practical application. These satisfactory properties underscore the applicability of ZHC-Cur-CS film for the efficient preservation of perishable products.

Laser speckles have served as an alternative to artificial speckles for digital image correlation (DIC) measurement in some cases due to the advantages of non-invasive measurement, simplicity and high efficiency in preparation. Regrettably, in using laser speckle DIC, it unavoidably encounters the notorious decorrelation problem when the object surface undergoes a relatively large motion and deformation, which will degrade the accuracy of DIC matching or even destroy the measurement. To realize large deformation measurements, the approach of updating reference image (also termed as incremental DIC) is usually adopted to mitigate the decorrelation problem of laser speckle. However, once the reference image is updated frequently during the incremental DIC calculation, additional errors due to intensity interpolation at the subpixel positions in updated reference images will accumulate over the updating times. To address this issue, an improved calculation scheme composed of incremental DIC with the nearest integer pixel offset and Gaussian pre-filtering is proposed for laser speckle DIC measurement. This involves processing the speckle images with a Gaussian low-pass filter before correlation analysis, and shifting the reference subsets in updated reference images to the nearest integer pixel positions instead of interpolating at subpixel positions during DIC calculation. Simulation tests have confirmed that this scheme effectively mitigates accumulated interpolation errors. Several real tests, including in-plane translation, uniaxial tensile, and high-temperature tests, also demonstrated the effectiveness of this scheme in eliminating the decorrelation issue in laser speckle DIC.

A universally applicable reliability-guided digital image correlation (DIC) method is proposed for reliable image deformation measurement. The zero-mean normalized cross correlation (ZNCC) coefficient is used to identify the reliability of the point computed. The correlation calculation begins with a seed point and is then guided by the ZNCC coefficient. That means the neighbors of the point with the highest ZNCC coefficient in a queue for computed points will be processed first. Thus the calculation path is always along the most reliable direction, and possible error propagation of the conventional DIC method can be avoided. The proposed novel DIC method is universally applicable to the images with shadows, discontinuous areas, and deformation discontinuity. Two image pairs were used to evaluate the performance of the proposed technique, and the successful results clearly demonstrate its robustness and effectiveness.

A simple, easy-to-implement yet effective high-temperature digital image correlation (DIC) method is established for non-contact full-field deformation measurement at elevated temperatures. The technique employs a bandpass optical filter to eliminate the influence of black-body radiation of high-temperature objects on the intensity of captured images. With the bandpass filter, high-quality digital images of an object at high temperatures up to 1200 °C can be easily acquired and directly compared with the reference image recorded at room temperature using the DIC technique to extract full-field deformation information with high fidelity. To verify the performance of the proposed technique, a chromium-nickel austenite stainless steel sample was heated from room temperature to 1200 °C using an infrared heating device, and the surface images at various temperatures were captured using the bandpass filter imaging system. Afterwards, full-field thermal deformation and coefficient of thermal expansion of the sample were determined using the DIC technique. Experimental results indicate that the proposed high-temperature DIC method is easy to implement and can be applied to practical full-field high-temperature deformation measurement with high accuracy.

Fringe-projection profilometry is one of the most commonly used noncontact methods for acquiring the three-dimensional (3D) shape information of objects. In practice, the luminance nonlinearity caused by the gamma effect of a digital projector and a digital camera yields undesired fringe intensity changes, which substantially reduce the measurement accuracy. In this Letter, we present a robust and simple scheme to eliminate the intensity nonlinearity induced by the gamma effect by combining a universal phase-shifting algorithm with a gamma correction method. First, by using three-step and large-step phase-shifting techniques, the gamma value involved in the measurement system can be detected. Then, a gamma pre-encoding process is applied to the system for actual 3D shape measurements. With the proposed technique, high accuracy of measurement can be achieved with the conventional small-step phase-shifting algorithm. The validity of the technique is verified by experiments.

In digital image correlation (DIC), the iterative spatial domain cross-correlation algorithm using high-order B-spline interpolation algorithms has been strongly recommended for accurate sub-pixel displacement measurement. However, the magnitude of the position-dependent bias error increases with the increase of noise level, which dramatically reduces the registration accuracy of DIC for real experimental images. In this paper, a simple method, based on pre-smoothing the speckle images with a 5×5 pixels Gaussian low-pass filter prior to correlation analysis, is proposed for reducing the bias error in measured displacements. Both numerical simulations and real experiments reveal that the proposed technique is capable of reducing the bias error in measured displacement to a negligible degree for both noisy and noiseless images, even though a simple bicubic interpolation is used.

Conventional digital image correlation (DIC) technique using a fixed reference image provides high-accuracy measurements but normally fails when serious decorrelation effect occurs in the deformed images due to large deformation, serious illumination fluctuations or other reasons. In this paper, an incremental reliability-guided digital image correlation (RG-DIC) technique, by combining the recently developed RG-DIC technique and an automatic reference image updating scheme, is proposed for large deformation measurement. In the incremental RG-DIC technique, a seed point is defined in the original reference image and searched in the deformed images, if the estimated correlation coefficient is larger than a preset threshold, which means no serious decorrelation effect exists in the deformed image, the RG-DIC technique is used to continue correlation analysis to obtain full-field displacements. Otherwise, the image recorded just before the current deformed image is chosen as an updated reference image to proceed with correlation analysis. Afterwards, the incremental displacements extracted by comparing the current deformed image and the updated reference image can be cumulated to determine the overall deformation. The effectiveness of the proposed technique is demonstrated by retrieving the full-field deformation of a foam sample subjected to large compressive deformation.

Applications of the digital image correlation method (DIC) for the determination the coefficient of thermal expansion (CTE) of films is investigated in this paper. A heating chamber was designed for applying thermal load and DIC provides the full-field thermal deformation fields of the test film sample due to temperature changes. The average normal strains in the x and y direction from the region of interest are then extracted for the determination of CTE. The influence of unavoidable small rigid body rotation is discussed and a method to eliminate it to show the pure thermal expansion of the test film is demonstrated. For validation, the CTE of a pure copper sample is determined and compared with the textbook value, confirming the effectiveness and accuracy of the proposed technique. Finally, the CTE of Polyimide (PI) composite film in the temperature range of 20–140 °C is measured. The results reveal that the DIC is a practical and effective tool for full-field thermal deformation and CTE measurement of films.

Lens distortion practically presents in a real optical imaging system causing non-uniform geometric distortion in the recorded images, and gives rise to additional errors in the displacement and strain results measured by two-dimensional digital image correlation (2D-DIC). In this work, the systematic errors in the displacement and strain results measured by 2D-DIC due to lens distortion are investigated theoretically using the radial lens distortion model and experimentally through easy-to-implement rigid body, in-plane translation tests. Theoretical analysis shows that the displacement and strain errors at an interrogated image point are not only in linear proportion to the distortion coefficient of the camera lens used, but also depend on its distance relative to distortion center and its magnitude of displacement. To eliminate the systematic errors caused by lens distortion, a simple linear least-squares algorithm is proposed to estimate the distortion coefficient from the distorted displacement results of rigid body, in-plane translation tests, which can be used to correct the distorted displacement fields to obtain unbiased displacement and strain fields. Experimental results verify the correctness of the theoretical derivation and the effectiveness of the proposed lens distortion correction method.

Geomaterials (i.e. rock, sand, soil and concrete) are increasingly being encountered and used in extreme environments, in terms of the pressure magnitude and the loading rate. Advancing the understanding of the mechanical response of materials to impact loading relies heavily on having suitable high-speed diagnostics. One such diagnostic is high-speed photography, which combined with a variety of digital optical measurement techniques can provide detailed insights into phenomena including fracture, impact, fragmentation and penetration in geological materials. This review begins with a brief history of high-speed imaging. Section 2 discusses of the current state of the art of high-speed cameras, which includes a comparison between charge-coupled device and complementary metal-oxide semiconductor sensors. The application of high-speed photography to geomechanical experiments is summarized in Sect. 3. Section 4 is concerned with digital optical measurement techniques including photoelastic coating, Moiré, caustics, holographic interferometry, particle image velocimetry, digital image correlation and infrared thermography, in combination with high-speed photography to capture transient phenomena. The last section provides a brief summary and discussion of future directions in the field.

A low-cost, easy-to-implement single-camera high-speed stereo-digital image correlation (SCHS stereo-DIC) method using a four-mirror adapter is proposed for full-field 3D vibration measurement. With the aid of the four-mirror adapter, surface images of calibration target and test objects can be separately imaged onto two halves of the camera sensor through two different optical paths. These images can be further processed to retrieve the vibration responses on the specimen surface. To validate the effectiveness and accuracy of the proposed approach, dynamic parameters including natural frequencies, damping ratios and mode shapes of a rectangular cantilever plate were extracted from the directly measured vibration responses using the established system. The results reveal that the SCHS stereo-DIC is a simple, practical and effective technique for vibration measurements and dynamic parameters identification.

A novel smelting reduction process based on FeO–SiO2–Al2O3 slag system for spent lithium ion batteries with Al cans was developed, while using copper slag as the only slag former. The feasibility of the process and the mechanism of copper loss in slag were investigated. 98.83% Co, 98.39% Ni and 93.57% Cu were recovered under the optimum conditions of slag former/battery mass ratio of 4.0:1, smelting temperature of 1723 K, and smelting mass ratio of time of 30 min. The FeO–SiO2–Al2O3 slag system for the smelting process is appropriate under the conditions of m(FeO):m(SiO2)=0.58:1–1.03:1, and 17.19%–21.52% Al2O3 content. The obtained alloy was mainly composed of Fe–Co–Cu–Ni solid solution including small amounts of matte. The obtained slag mainly consisted of fayalite and hercynite. Meanwhile, the mechanism of copper loss is the mechanical entrainment from strip-like fayalite particles in the main form of copper sulfide and metallic copper.

Plastics, especially polyethylene terephthalate (PET) plastic bottles, played an indispensable role in people's lives, yet the plastic crisis was one of the greatest challenges for the planet and humanity today. Processing recycled PET (rPET) into functional materials could achieve both environmental and economic significance. In this work, using rPET and carbon black as raw materials, a low cost 3D seawater evaporator was successfully prepared by electrospinning and folding. By examining the concentration of carbon black (1, 3, 5, 6 wt%) on the photothermal conversion performance of the as-spun films, the optimal concentration 6 wt% was determined. The prepared 6 wt% evaporator showed water evaporation rate about 1.457 kg m−2 h−1 and a conversion efficiency of 93.91% under one solar intensity (1 kW/m2) with good performance stability after 11 cycles for seawater desalination. The desalinated water could meet the WHO and EPA drinking water standards. Meanwhile, the materials cost of the prepared evaporator was about 1.19 USD/m2. Compared to the photothermal conversion efficiency, a photothermal quality factor was calculated, and was 3.81 for the prepared rPET based evaporator, which was higher than most of reported materials. These results suggested the electrospun rPET film might have great potential in seawater desalination and point out a new way to for cleaner reuse of rPET.

To mitigate the decay of fruits and vegetables caused by food-borne microorganisms, this contribution proposed for the first time a novel mild temperature-assisted nanozyme-based antibacterial packaging system with self-activating behavior (MnHC-CS films) through a demand-oriented tailoring process that involved the construction of hollow carbon-supported Mn3O4 fillers (MnHC) and its incorporation in chitosan (CS) matrix. In this system, the weak acidity of CS film triggers the self-activation of MnHC nanozyme. Leveraging the synergistic effects of the excellent oxidase-like activity and photothermal properties of MnHC, in conjunction with the inherent bacteria trapping and killing capabilities of CS, the MnHC0.20-CS film (The 0.20 represents the amount of MnHC in the film-forming solution) demonstrates remarkable antibacterial and antifungal performances, with high bactericidal rate, short action time, and mild heating temperature. Therefore, the MnHC0.20-CS film showcases excellent preservation capabilities, significantly extending the shelf life of kumquats by approximately 2.3 times. Additionally, this film also boasts enhanced mechanical strength, improved barrier properties, and sufficient safety, paving the way for its practical application. These exceptional features underscore the good potential of MnHC0.20-CS film in effectively safeguarding perishable products.

Stereo Digital Image Correlation (Stereo-DIC) has become a mainstream optical metrology technique for quantitatively analyzing full-field 3D shape, displacement, or deformation of materials and structures. Whether it is to measure 3D profile or deformation, stereo matching is essential for Stereo-DIC to reconstruct 3D point clouds from stereo images. Traditional feature-based (e.g., SIFT) methods provide initial 2D displacements for stereo matching with the aid of extracting massive features, but at the cost of expensive computational overhead. In addition, these methods preclude precise measurement of objects with steep and ridged surfaces or undergoing large rotation and/or deformation due to low feature matching accuracy of complex regions caused by perspective differences. In this paper, we propose a fast and robust stereo matching method using semi-global matching with geometric constraints (GC-SGM) for initializing and accelerating Stereo-DIC computation. For GC-SGM, an optimized semi-global matching (SGM) algorithm based on GPU acceleration is first utilized to quickly estimate dense and reliable disparity maps between the rectified stereo images. The global pixel-wise 2D correspondence between raw stereo images can be established inversely using epipolar constraints and 1D disparity information, and then converted to accurate and initial second-order deformation parameters for 2D-DIC-based sub-pixel refinement by least-squares-based surface fitting. Experimental results prove that the proposed GC-SGM enhances the matching correctness and robustness for complex objects while improving the processing speed on GPU by 3∼10 times compared with SIFT-based methods, enabling high-precision and computationally efficient 3D shape and deformation measurement.

To determine the full-field high-temperature thermal deformation of the structural materials used in high-speed aerospace flight vehicles, a novel non-contact high-temperature deformation measurement system is established by combining transient aerodynamic heating simulation device with the reliability-guided digital image correlation (RG-DIC). The test planar sample with size varying from several mm2 to several hundreds mm2 can be heated from room temperature to 1100 °C rapidly and accurately using the infrared radiator of the transient aerodynamic heating simulation system. The digital images of the test sample surface at various temperatures are recorded using an ordinary optical imaging system. To cope with the possible local decorrelated regions caused by black-body radiation within the deformed images at the temperatures over 450 °C, the RG-DIC technique is used to extract full-field in-plane thermal deformation from the recorded images. In validation test, the thermal deformation fields and the values of coefficient of thermal expansion (CTEs) of a chromiumnickel austenite stainless steel sample from room temperature to 550 °C is measured and compared with the well-established handbook value, confirming the effectiveness and accuracy of the proposed technique. The experimental results reveal that the present system using an ordinary optical imaging system, is able to accurately measure full-field thermal deformation of metals and alloys at temperatures not exceeding 600 °C.

Digital image correlation (DIC) is an easy-to-implement yet powerful optical metrology for deformation measurement. The technique measures the displacement of a point of interest by matching the subsets surrounding the same point located in the reference image and the deformed image. Although the technique is simple in principle, the existing DIC technique has several deficiencies. For example, for the points located near or at the boundaries of a specified region of interest (ROI), the selected square subsets surrounding these points may contain unwanted or foreign pixels from background image or other regions. In the existing DIC method, these points are either intentionally excluded from calculation or automatically removed after calculation, and leads to the absence of deformation information for the boundary points. Besides, existing DIC technique is prone to yield erroneous measurement for specimen with geometric discontinuities. In this paper, two approaches are developed to overcome the deficiencies of existing DIC technique. First, a modified Zero-mean Normalized Sum of Squared Differences (ZNSSD) criterion is defined for the correlation analysis of subsets surrounding the boundary points. Second, considering the possible complex shape of the ROI, a scanning strategy guided by the correlation coefficients of computed points is proposed to ensure reliable computation between consecutive points. With these two measures, the deformation of all the points including those located near or at the ROI boundaries can be automatically, reliably, and accurately determined. The improved DIC technique is universally applicable to the genuine full-field deformation measurement of objects with complex or arbitrary shapes. Two typical experimental image pairs are processed to evaluate the performance of the proposed method, and the results successfully demonstrate its effectiveness and practicality.

An optimized 3D digital image correlation (3D-DIC) system using active optical imaging is developed for accurate shape and 3D deformation measurements in nonlaboratory conditions or extreme high-temperature environments. In contrast to a conventional 3D-DIC system using white or natural light illumination, the proposed active imaging 3D-DIC system is based on a combination of monochromatic lighting and bandpass filter imaging. Because the bandpass filter attached before the imaging lenses allows only the actively illuminated monochromatic light to pass through and blocks all light outside of its bandpass range, the active imaging 3D-DIC system is therefore insensitive to serious variations in ambient light in nonlaboratory environments and to the thermal radiation of hot objects in extreme high-temperature environments. Two challenging experiments that cannot be performed by a conventional 3D-DIC system were carried out to verify the robustness and accuracy of the developed active imaging 3D-DIC system. Because a much wider application range can be achieved with relatively simple and easy-to-implement improvements, the proposed active imaging 3D-DIC system is highly recommended for practical use instead of the conventional 3D-DIC system.

Fringe-projection-based (FPB) three-dimensional (3D) imaging technique has become one of the most prevalent methods for 3D shape measurement and 3D image acquisition, and an essential component of the technique is the calibration process. This paper presents a framework for hyper-accurate system calibration with flexible setup and inexpensive hardware. Owing to the crucial improvement in the camera calibration technique, an enhanced governing equation for 3D shape determination, and an advanced flexible system calibration technique as well as some practical considerations on accurate fringe phase retrieval, the novel FPB 3D imaging technique can achieve a relative measurement accuracy of 0.010%. The validity and practicality are verified by both simulation and experiments.

Varying ambient light may cause serious decorrelation effect in the images recorded using an ordinary optical imaging device, which prevent digital image correlation (DIC) from out-of-laboratory use. In this paper, we describe an easy-to-implement yet effective monochromatic light illuminated active imaging DIC method for obtaining high-quality images suitable for high fidelity deformation measurement. Experiments reveal that the active imaging DIC method is able to provide reliable and accurate measurements even though the ambient light has been seriously changed. The active imaging DIC method is promising for developing flexible and robust in situ deformation measurement systems for use in both laboratory and non-laboratory environment, and should therefore have more potential engineering applications.

In digital image correlation, the iterative spatial domain cross-correlation algorithm is considered as a gold standard for matching the corresponding points in two images, but requires an accurate initial guess of the deformation parameters to converge correctly and rapidly. In this work, we present a fully automated method to accurately initialize all points of interest for the deformed images in the presence of large rotation and/or heterogeneous deformation. First, a robust computer vision technique is adopted to match feature points detected in reference and deformed images. The deformation parameters of the seed point are initialized from the affine transform, which is fitted to the matched feature points around it. Subsequently, the refined parameters are automatically transferred to adjacent points using a modified quality-guided initial guess propagation scheme. The proposed method not only ensures a rapid and correct convergence of the nonlinear optimization algorithm by providing a complete and accurate initial guess of deformation for each measurement point, but also effectively deals with deformed images with relatively large rotation and/or heterogeneous deformation. Tests on both simulated speckle images and real-world foam compression experiment verify the effectiveness and robustness of the proposed method.

As a novel tool for quantitative 3D internal deformation measurement throughout the interior of a material or tissue, digital volume correlation (DVC) has increasingly gained attention and application in the fields of experimental mechanics, material research and biomedical engineering. However, the practical implementation of DVC involves important challenges such as implementation complexity, calculation accuracy and computational efficiency. In this paper, a least-squares framework is presented for 3D internal displacement and strain field measurement using DVC. The proposed DVC combines a practical linear-intensity-change model with an easy-to-implement iterative least-squares (ILS) algorithm to retrieve 3D internal displacement vector field with sub-voxel accuracy. Because the linear-intensity-change model is capable of accounting for both the possible intensity changes and the relative geometric transform of the target subvolume, the presented DVC thus provides the highest sub-voxel registration accuracy and widest applicability. Furthermore, as the ILS algorithm uses only first-order spatial derivatives of the deformed volumetric image, the developed DVC thus significantly reduces computational complexity. To further extract 3D strain distributions from the 3D discrete displacement vectors obtained by the ILS algorithm, the presented DVC employs a pointwise least-squares algorithm to estimate the strain components for each measurement point. Computer-simulated volume images with controlled displacements are employed to investigate the performance of the proposed DVC method in terms of mean bias error and standard deviation error. Results reveal that the present technique is capable of providing accurate measurements in an easy-to-implement manner, and can be applied to practical 3D internal displacement and strain calculation.

An advanced video deflectometer using actively illuminated LED targets is proposed for remote, real-time measurement of bridge deflection. The system configuration, fundamental principles, and measuring procedures of the video deflectometer are first described. To address the challenge of remote and accurate deflection measurement of large engineering structures without being affected by ambient light, the novel idea of active imaging, which combines high-brightness monochromatic LED targets with coupled bandpass filter imaging, is introduced. Then, to examine the measurement accuracy of the proposed advanced video deflectometer in outdoor environments, vertical motions of an LED target with precisely-controlled translations were measured and compared with prescribed values. Finally, by tracking six LED targets mounted on the bridge, the developed video deflectometer was applied for field, remote, and multipoint deflection measurement of the Wuhan Yangtze River Bridge, one of the most prestigious and most publicized constructions in China, during its routine safety evaluation tests. Since the proposed video deflectometer using actively illuminated LED targets offers prominent merits of remote, contactless, real-time, and multipoint deflection measurement with strong robustness against ambient light changes, it has great potential in the routine safety evaluation of various bridges and other large-scale engineering structures.

Full-frame, high-speed 3D shape and deformation measurement using stereo-digital image correlation (stereo-DIC) technique and a single high-speed color camera is proposed. With the aid of a skillfully designed pseudo stereo-imaging apparatus, color images of a test object surface, composed of blue and red channel images from two different optical paths, are recorded by a high-speed color CMOS camera. The recorded color images can be separated into red and blue channel sub-images using a simple but effective color crosstalk correction method. These separated blue and red channel sub-images are processed by regular stereo-DIC method to retrieve full-field 3D shape and deformation on the test object surface. Compared with existing two-camera high-speed stereo-DIC or four-mirror-adapter-assisted singe-camera high-speed stereo-DIC, the proposed single-camera high-speed stereo-DIC technique offers prominent advantages of full-frame measurements using a single high-speed camera but without sacrificing its spatial resolution. Two real experiments, including shape measurement of a curved surface and vibration measurement of a Chinese double-side drum, demonstrated the effectiveness and accuracy of the proposed technique.

Tobacco bacterial wilt (TBW) caused by Ralstonia solanacearum (RS) is one of the most serious tobacco diseases worldwide and no effective control measures are available to date. This study investigated the potential of Trichoderma harzianum SQR-T037 amended bioorganic fertilizer (BOF) and the arbuscular mycorrhizal fungi (AMF) Glomus mosseae 171 (Gm) on the control of TBW and promotion of plant growth in pot experiments. The results showed that the disease incidence in plants treated with integrated application of G. mosseae 171 and T. harzianum SQR-T037 amended bioorganic fertilizer (Gm + BOF) was the lowest, with a control efficacy of 68.2%, which is greater than that of BOF or Gm alone (26.8% and 14.7%, respectively). The application of BOF or Gm alone significantly reduced the abundance of RS in rhizosphere soil, but the integrated treatment (Gm + BOF) showed the strongest inhibitory effect (with a 21.3% increase in inhibition). The root colonization of G. mosseae 171 in samples treated with Gm + BOF was higher than that in samples with solely Gm treatment, indicating that the BOF significantly promoted G. mosseae mycorrhizal colonization. The results showed that the G. mosseae also had a positive effect on SQR-T037 rhizospheric colonization. Denaturing gradient gel electrophoresis (DGGE) results showed that application of BOF and Gm alone or in combination changed the diversity of the rhizospheric microbial community. The integrated application of Gm + BOF to tobacco plants significantly increases the activity of polyphenol oxidase (PPO), phenylalanine ammonia lyase (PAL), and peroxidase (POD), enzymes associated to systemic resistance. Additionally, the integrated application of Gm with BOF increased the tobacco plant height, shoot dry weight, and root dry weight. In conclusion, a synergistic biological approach integrating application of Gm and BOF for TBW protection seems promising.

Active vibration control of a flexible beam with piezoelectric pieces on the surface is investigated experimentally using the independent modal space control method, which is able to control the first three modes of the beam independently. A comparison between the responses of the beam before and after control indicates that the modal damping of the flexible beam is greatly improved and the effects of vibration suppression are very remarkable. The dynamic equation of the beam is deduced by Hamilton's principles, and numerical simulation of the active vibration control of the first three modes of the beam is also conducted in this paper. The simulation results match the experimental results very well. Both the experimental and numerical results indicate that by using piezo-patches as actuators the independent mode control method is a very effective approach to realize vibration suppression, and has promising applications in the aerospace field.

The ideal pinhole imaging model commonly assumed for an ordinary two-dimensional digital image correlation (2D-DIC) system is neither perfect nor stable because of the existence of small out-of-plane motion of the test sample surface that occurred after loading, small out-of-plane motion of the sensor target due to temperature variation of a camera and unavoidable geometric distortion of an imaging lens. In certain cases, these disadvantages can lead to significant errors in the measured displacements and strains. Although a high-quality bilateral telecentric lens has been strongly recommended to be used in the 2D-DIC system as an essential optical component to achieve high-accuracy measurement, it is not generally applicable due to its fixed field of view, limited depth of focus and high cost. To minimize the errors associated with the imperfectness and instability of a common 2D-DIC system using a low-cost imaging lens, a generalized compensation method using a non-deformable reference sample is proposed in this work. With the proposed method, the displacement of the reference sample rigidly attached behind the test sample is first measured using 2D-DIC, and then it is fitted using a parametric model. The fitted parametric model is then used to correct the displacements of the deformed sample to remove the influences of these unfavorable factors. The validity of the proposed compensation method is first verified using out-of-plane translation, out-of-plane rotation, in-plane translation tests and their combinations. Uniaxial tensile tests of an aluminum specimen were also performed to quantitatively examine the strain accuracy of the proposed compensation method. Experiments show that the proposed compensation method is an easy-to-implement yet effective technique for achieving high-accuracy deformation measurement using an ordinary 2D-DIC system.

In subset-based digital image correlation (DIC), a proper shape function must be chosen to approximate the underlying displacement field of the target subsets to ensure an accurate subset matching. Shape functions with varying orders of Taylor's expansions (e.g. zero-, first- and second-order) have been proposed. However, since the actual deformation occurring in the deformed subsets cannot be known a priori in most practical measurements, problems of mismatch (undermatched or overmatched) inevitably arise, which lead to additional errors in the measurement displacements. Although systematic errors due to undermatched shape functions have been thoroughly studied, the displacement errors associated with overmatched shape functions are still not sufficiently clear to us. In this paper, the systematic and random errors caused by the use of overmatched shape functions are first examined using numerical translation tests with precisely controlled subpixel motions. The results reveal that overmatched shape functions will not introduce additional systematic error but can induce increased random error, while the latter is negligibly small when a larger subset is used. Thus, it can be made explicit that second-order shape functions, capable of depicting more complex local deformation, can be used for practical DIC applications as a default, because this effectively eliminates the possible systematic error associated with the use of first-order shape functions, while the possibly increased random errors are small enough if using a relatively larger subset. Two sets of images with inhomogeneous deformation are also used to verify the accuracy of DIC measurements using second-order shape functions.

ABSTRACT A fast, robust and accurate digital image correlation (DIC) method, which uses a robust zero‐mean normalized sum of squared difference correlation criterion, a sophisticated reliability‐guided displacement tracking strategy and an efficient inverse compositional Gauss–Newton (IC‐GN) algorithm, was recently proposed for full‐field deformation measurement. As an iterative local optimization algorithm, IC‐GN algorithm iteratively solves for the incremental warp assumed on the reference subset until the preset convergence criteria are satisfied. In the literature, different convergence criteria have been set for iterative optimization algorithms. However, on the one hand, stringent convergence criteria lead to increased number of iterations and lessen the computational efficiency. On the other hand, too loose convergence conditions enhance the computational efficiency but may decrease the registration accuracy. Understanding the impact of prescribed convergence criteria on DIC measurement and how to choose proper convergence criteria are therefore fundamental problems in realizing high‐efficiency yet high‐accuracy DIC analysis. In this paper, the convergence characteristics of IC‐GN algorithm are investigated in terms of convergence speed and radius of convergence using real experimental images. The effect of various convergence criteria on the efficiency and accuracy of IC‐GN algorithm are carefully examined. Recommendations are given to select proper convergence criteria for more efficient implement of IC‐GN algorithm.

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.

Being the two primary approaches for full-field kinematics measurements, both subset-based local digital image correlation (DIC) and finite element-based global DIC have been extensively studied. Nowadays, most commercial DIC systems employ local DIC algorithm because of its advantages of straight forward principle and higher efficiency. However, several researchers argue that global DIC can provide better displacement results due to the displacement continuity constraint among adjacent elements. As such, thoroughly examining the performance of these two different DIC methods seems to be highly necessary. Here, the random errors associated with local DIC and two global DIC methods are theoretically analyzed at first. Subsequently, based on the same algorithmic details and parameters during analyses of numerical and real experiments, the performance of the different DIC approaches is fairly compared. Theoretical and experimental results reveal that local DIC outperforms its global counterpart in terms of both displacement results and computational efficiency when element (subset) size is no less than 11 pixels.

A low-cost, easy-to-implement yet effective panoramic-digital image correlation (panoramic-DIC) system comprising a single binocular stereo-DIC system and two planar mirrors is proposed for full-surface 360-deg shape reconstruction and deformation measurement in material testing. With the aid of two mirrors, the rear portions of the object's full surface can be captured and reconstructed by the same stereo-DIC system that directly captures the front surface of the specimen. Two speckle patterns premade on the mirrors are used to transform all surface portions into a common global coordinate thus allowing a straightforward full-surface 360-deg profile and deformation measurements. For method validation, real experiments including full-surface 360-deg shape and displacement measurements of a cylinder and true stress-strain curves determination of a round low carbon steel specimen were performed. Results demonstrate that the proposed panoramic-DIC technique can provide accurate full-surface 360-deg shape and deformation data in standard material testing with important advantages such as low cost, compactness and easy implementation.

We developed an advanced video extensometer for non-contact, real-time, high-accuracy strain measurement in material testing. In the established video extensometer, a "near perfect and ultra-stable" imaging system, combining the idea of active imaging with a high-quality bilateral telecentric lens, is constructed to acquire high-fidelity video images of the test sample surface, which is invariant to ambient lighting changes and small out-of-plane motions occurred between the object surface and image plane. In addition, an efficient and accurate inverse compositional Gauss-Newton algorithm incorporating a temporal initial guess transfer scheme and a high-accuracy interpolation method is employed to achieve real-time, high-accuracy displacement tracking with negligible bias error. Tensile tests of an aluminum sample and a carbon fiber filament sample were performed to demonstrate the efficiency, repeatability and accuracy of the developed advanced video extensometer. The results indicate that longitudinal and transversal strains can be estimated and plotted at a rate of 117 fps and with a maximum strain error less than 30 microstrains.

Accurate camera calibration is of fundamental importance to various vision-based 3D metrological techniques. Despite camera calibration methods using regular planar calibration targets (e.g., checkerboard or circular pattern) have been widely adopted, their accuracy is less-than-desirable due to the limited number and the low registration accuracy of control points. This work presents an alternative calibration method that uses a calibration target based on a synthetic random speckle pattern. Specifically, a set of regularly distributed control points is first specified on the synthetic speckle pattern displayed on a monitor. These control points are then precisely matched to their counterparts on the captured calibration images using the state-of-the-art digital image correlation (DIC) algorithm. Compared with the existing camera calibration methods, the proposed method possesses the advantages of much more effective control points, higher control point match accuracy, which lead to more accurate and precise estimation of camera parameters. To evaluate the performance of the proposed calibration method, simulated calibration tests using images with varied noise levels and real calibration tests were performed on the planar calibration targets using speckle, circular and checkerboard patterns. The influences of various DIC calculation parameters (i.e. the number of control points and the subset size) on calibration results are also studied using noiseless simulated images. The reprojection errors on synthetic noiseless images and real images are computed as 0.004 and 0.038 pixels, respectively, confirming the high calibration accuracy delivered by the proposed method.

High-speed three-dimensional (3D) shape measurement techniques based on fringe projection profilometry (FPP) have undergone huge advances over the past two decades. However, accurate 3D displacement mapping and deformation analysis of dynamic scenes using FPP remains an unsolved problem. Because fringe patterns are projected rather than attached on the tested surfaces, the full-field point-to-point correspondence cannot be accurately established between any two 3D shape results. To deal with this challenge, a DIC-assisted FPP for high-speed 3D shape, displacement and deformation measurement of textured surfaces is proposed. Firstly, a high-speed 3D shape measurement system is adopted using our recently proposed robust and efficient Gray-coded coding strategy, which can accurately reconstruct full-field shape of discontinuous surfaces with rich texture information. Then, the modulation-based method is proposed to retrieve high-quality texture map from three phase-shifting fringe patterns, which can eliminate the adverse influence of the nonuniform and time-varying ambient light. By matching the retrieved surface texture images at difference states using DIC, accurate point tracking between the measured 3D shape data can be fulfilled, leading to precise 3D displacement and deformation measurement. Experiments have verified that the proposed method can achieve 3D shape, displacement and deformation measurement of dynamic scenes at a frame rate of 542 fps without any extra expense of hardware or calibration for the FPP system. The presented method is reliable and promising for further 3D displacement mapping and deformation analysis of dynamic scenes using FPP.

Mirror-assisted multiple-view digital image correlation (MV-DIC) is a recently developed variant of DIC technique for panoramic or dual-surface shape, motion and deformation measurements. In contrast to the conventional MV-DIC technique requiring four or more synchronized cameras, mirror-assisted MV-DIC involves only a regular binocular stereo-DIC system and two reflective mirrors. The inherent advantages of low-cost setup and simple implementation procedures make it a very practical and cost-efficient replacement of the regular MV-DIC technique for panoramic/dual surface deformation measurement of regular-sized samples. This review comprehensively overviews the advances we made to this technique in our recent works. First, the principle of the method is described. Then, detailed implementation of the key procedures (i.e., the calibration of the reflection transformation of the mirrors) involved in mirror-assisted MV-DIC is reviewed. Finally, typical applications of the mirror-assisted MV-DIC are presented. Instead of contributing in originality, this review aims to overview the advancement of mirror-assisted MV-DIC and encourage its future applications in experimental mechanics.

The sunflower stem pith is a natural, ultra-lightweight cellular material with superior mechanical properties that provides the required strong lodging resistance and high energy absorption efficiency. Here, we show that the sunflower pith exploits anisotropic auxeticity to adapt to changing environments. The characteristic dual-gradient structure of sunflower pith is found to be responsible for its anisotropic auxeticity. We demonstrate that the synergy of pore size gradient and the thickness gradient in sunflower pith’s microstructures leads to an alternative buckling deformation pattern at relatively larger strain, accomplishing both high energy absorption and sufficient high stiffness. Furthermore, the dual-gradient structure-based auxetic mechanism is used as inspiration to develop anisotropic auxetic cellular metamaterials. The design strategy for auxetic metamaterials revealed here can be extended into cellular structures with common convex polygons and provide insight into the design of auxetic cellular metamaterials for a wide range of realistic applications.

A simple, cost-effective but practical microscopic 3D-DIC method using a single camera and a transmission diffraction grating is proposed for surface profile and deformation measurement of small-scale objects. By illuminating a test sample with quasi-monochromatic source, the transmission diffraction grating placed in front of the camera can produce two laterally spaced first-order diffraction views of the sample surface into the two halves of the camera target. The single image comprising negative and positive first-order diffraction views can be used to reconstruct the profile of the test sample, while the two single images acquired before and after deformation can be employed to determine the 3D displacements and strains of the sample surface. The basic principles and implementation procedures of the proposed technique for microscopic 3D profile and deformation measurement are described in detail. The effectiveness and accuracy of the presented microscopic 3D-DIC method is verified by measuring the profile and 3D displacements of a regular cylinder surface.

Owing to its inherent computational complexity, practical implementation of digital volume correlation (DVC) for internal displacement and strain mapping faces important challenges in improving its computational efficiency. In this work, an efficient and accurate 3D displacement tracking strategy is proposed for fast DVC calculation. The efficiency advantage is achieved by using three improvements. First, to eliminate the need of updating Hessian matrix in each iteration, an efficient 3D inverse compositional Gauss–Newton (3D IC-GN) algorithm is introduced to replace existing forward additive algorithms for accurate sub-voxel displacement registration. Second, to ensure the 3D IC-GN algorithm that converges accurately and rapidly and avoid time-consuming integer-voxel displacement searching, a generalized reliability-guided displacement tracking strategy is designed to transfer accurate and complete initial guess of deformation for each calculation point from its computed neighbors. Third, to avoid the repeated computation of sub-voxel intensity interpolation coefficients, an interpolation coefficient lookup table is established for tricubic interpolation. The computational complexity of the proposed fast DVC and the existing typical DVC algorithms are first analyzed quantitatively according to necessary arithmetic operations. Then, numerical tests are performed to verify the performance of the fast DVC algorithm in terms of measurement accuracy and computational efficiency. The experimental results indicate that, compared with the existing DVC algorithm, the presented fast DVC algorithm produces similar precision and slightly higher accuracy at a substantially reduced computational cost.

Applications of an easy-to-implement, compact but practical single-camera high-speed (SCHS) stereo-digital image correlation (DIC) technique for full-field transient 3D deformation measurement during ballistic impact are investigated. The established SCHS stereo-DIC system relies on a four-mirror adapter to convert a single high-speed camera into two virtual high-speed cameras, which view a specimen from different angles and record the surface images of the test object onto two halves of the camera sensor. The system configuration, measurement principles and experimental procedures of the proposed SCHS stereo-DIC technique for full-field transient 3D deformation measurement are described first. Then, the effectiveness and accuracy of the established system were verified by measuring the static deformation of a stationary carbon fiber reinforced polymer (CFRP) panel and transient deformation of an aluminum panel under the impact of a cylindrical foam projectile. Finally, full-field in-plane and out-of-plane deformations of the CFRP panel under rigid steel sphere impact were determined using this system, which shed light on the deformation behavior and failure mechanism of the CFRP panel under transient ballistic impact. The results confirm that the SCHS stereo-DIC is a cost-effective and practical technique for full-field transient 3D deformation measurement.

This paper investigates heat-shielding performance and 3D thermal deformation behavior of two superalloy honeycomb-core sandwich panels in time-varying thermal environment by using a self-developed transient aerodynamic heating simulation system and a novel active imaging stereo-digital image correlation (stereo-DIC) technique. To facilitate deformation measurement of large objects using stereo-DIC, a simple but practical technique is adopted for fabricating high-temperature speckle patterns on large measuring areas. The results indicate that the sandwich panels provide good thermal shielding performance under 900 °C with the thermal insulation effect stabilizing at around 30%. The measured high-temperature deformation fields of the test sandwich panels reveal that an in-plane homogeneous thermal expansion occurs as expected, while the out-of-plane deformation shows evident axisymmetric distributions with the maximum deflections dependent on the temperature differences between the back and front facets. The results provide a foundational understanding on the thermal–mechanical characteristics of superalloy honeycomb-core sandwich panels, which is essential to the structural design of a superalloy honeycomb sandwich thermal protection system.

Existing digital image correlation (DIC) using the robust reliability-guided displacement tracking (RGDT) strategy for full-field displacement measurement is a path-dependent process that can only be executed sequentially. This path-dependent tracking strategy not only limits the potential of DIC for further improvement of its computational efficiency but also wastes the parallel computing power of modern computers with multicore processors. To maintain the robustness of the existing RGDT strategy and to overcome its deficiency, an improved RGDT strategy using a two-section tracking scheme is proposed. In the improved RGDT strategy, the calculated points with correlation coefficients higher than a preset threshold are all taken as reliably computed points and given the same priority to extend the correlation analysis to their neighbors. Thus, DIC calculation is first executed in parallel at multiple points by separate independent threads. Then for the few calculated points with correlation coefficients smaller than the threshold, DIC analysis using existing RGDT strategy is adopted. Benefiting from the improved RGDT strategy and the multithread computing, superfast DIC analysis can be accomplished without sacrificing its robustness and accuracy. Experimental results show that the presented parallel DIC method performed on a common eight-core laptop can achieve about a 7 times speedup.

As the main atmospheric pollutants, the nitrogen oxides (NOx) and dust from industrial exhaust have caused a series of environmental problems. Catalytic membrane is one of the most important technologies for integrated denitration and dust removal, which can achieve the efficient treatment of multiple pollutants. In order to achieve the simultaneous removal of NO and dust in low-temperature exhaust, a novel MnOx/TiO2/SiC catalytic membrane was developed. The using of TiO2 aerogel as the transition layer could increase the loading capacity of MnOx by more than twice, but has little effect on gas permeance of SiC ceramic membrane. The MnOx/TiO2/SiC catalytic membrane also exhibit excellent filtration performance with dust rejection rate exceeding 99.97% and thus preventing adverse effects of dust on the catalyst. In the NO and dust coexistence system, the obtained catalytic membrane exhibits good catalytic activity owing to its high loading capacity and outstanding filtration performance, which lead to the high NO conversion rate above 80% with the temperature range of 120–180 °C. Meanwhile, the catalytic membrane exhibits remarkable stability in simulation environment, and the NO conversion rate could maintain at 90% within 65 h. The novel MnOx/TiO2/SiC catalytic membrane showed great promise for low-temperature exhaust gas purification.

Indoor air pollution, including atmospheric particular matter 2.5 (PM2.5) and chemical gaseous pollutants (HCHO), has been an ongoing concern. Seeking a practical strategy for protection of the indoor air quality (IAQ) remains a challenge. Herein, we demonstrated a hierarchical porous membrane consisting of the birnessite-type MnO2 that was filled in the polystyrene porous nanofibers (MnO2/PS HPNM) fabricated by a versatile electrospinning method. The effective air cleaning of both PM2.5 and HCHO at remarkably low resistance was achieved by dominating the hierarchical porous structure. The HCHO gas could easily permeate through the mesopores of the PS nanofibers and contact MnO2, thereby resulting in an excellent HCHO removal efficiency of 88.2% after the initial run and 74% after five cycles. In addition, the pore-filling MnO2 with the layered structure increased the nanofiber membrane surface area, thus enhancing its PM capture performance (99.77%). Moreover, the macropores from the interwoven nanofibers ensured clean air penetration leading to a low airflow resistance of 82 Pa. The hierarchical multifunction porous structure membrane provided a new approach for the IAQ protection.

Accurate initial guess plays a key role to realize full-automatic digital image correlation (DIC) analysis, especially when large rotational deformation presents in deformed images. In this work, an efficient, robust and full-automatic initial guess approach combining the speeded-up robust features (SURF) algorithm and the reliability-guided displacement tracking (RGDT) strategy is proposed, which can not only automatically select and update seed point, but also can effectively deal with target images with large deformation and rotation. The scale- and rotation-invariant SURF algorithm can extract and match a certain number of feature points from two images even though the significant deformation and rotation present. The Euclidean distance and deformation information (including displacement and major orientation rotation angle) of the best-matched point are used to choose the seed point and determine its initial guess, respectively. Then, the RGDT strategy is then employed to continue the DIC analysis of rest calculation points. Compared with existing path-dependent initial guess using RGDT, the proposed method not only can automatically select seed points without manual intervention, but also can provide accurate initial value estimation for the deformed images in the presence of large rotation and/or deformation. Furthermore, it has evident efficiency advantages over existing path-independent initial guess methods based on SIFT and RANSAC. The robustness and effectiveness of the proposed method are validated by numerical simulation tests and real experiments.

Abstract The optimal design of hydraulic fracturing parameters is the key to commercial exploitation of unconventional reservoirs. Hydraulic fracturing test is one of the main methods for optimizing fracturing parameters. It is known that scale effect exists between laboratory experiments and field treatments of hydraulic fracturing. However, studies on how to eliminate the scale effect are rarely reported. In this work, we conduct sensitivity analysis on rock mechanical parameters and fracturing parameters at different scales by using the dimensionless analysis method. The initiation and propagation process of field hydraulic fracturing is reproduced through laboratory tests, and fracturing parameters are analyzed by using numerical simulation. Our results show that the fracture propagation in the laboratory is inconsistent with that in the field fracturing. The fracture initiation and propagation in the field can be reproduced in experiments by using samples with high modulus and low toughness as well as high-viscosity fracturing fluid. Microcracks are created before the breakdown pressure is reached, and hydraulic fractures extend perpendicular to the direction of the minimum principal stress. The Carter’s leak-off coefficient has little effect on breakdown pressure and propagation pressure, but the injection rate and the horizontal principal stress have significant effects on breakdown pressure. This study provides a theoretical basis and guidance for the design of fracturing parameters both in the laboratory and in the field.

Full-field surface shape and deformation measurements of submerged objects using stereo-digital image correlation (stereo-DIC) remains challenging due to the light ray refraction occurred on the interface of two different transparent media (e.g., air, glass and water). In this work, we proposed an underwater full-field 3D deformation measurement method based on the bi-prism-based single-bilateral-telecentric-camera (SBTC) stereo-DIC, which adopts a parametric model to establish the mathematical relationship between the image points and their corresponding object points by taking the refraction into account in the ray-tracing process. The key parameters (e.g., bi-prism/glass orientation and position) of the parametric model were calibrated by a one-step calibration. To verify the correctness and effectiveness of the proposed method, translation tests with a rotated bi-prism and underwater translation tests were performed. Experimental results indicate that the proposed parametric model can effectively handle the imperfect system configuration and the unavoidable light ray refraction that occurred on the optical path, thus allowing high-accuracy underwater deformation measurement. Compared with the existing underwater deformation measurement approaches, the proposed method can not only deliver displacement measurement with higher accuracy, but also is free of any assumptions on the configuration of the camera system or the refractive interface.

A robust two-dimensional continuous wavelet transform (2D-CWT) technique for interferogram analysis is presented. To cope with the phase determination ambiguity issue encountered in the analysis of complex interferograms, a phase determination rule is proposed according to the phase distribution continuity, and a frequency-guided scheme is employed to obtain the correct phase distribution following a conventional 2D-CWT analysis. The theories are given in details, and the validity of the proposed technique is verified by computer simulation and real experiments.

The state-of-the-art digital image correlation (DIC) method using iterative spatial-domain cross correlation, e.g., the inverse-compositional Gauss–Newton algorithm, for full-field displacement mapping requires an initial guess of deformation, which should be sufficiently close to the true value to ensure a rapid and accurate convergence. Although various initial guess approaches have been proposed, automated, robust, and fast initial guess remains to be a challenging task, especially when large rotation occurs to the deformed images. An integrated scheme, which combines the Fourier–Mellin transform-based cross correlation (FMT-CC) for seed point initiation with a reliability-guided displacement tracking (RGDT) strategy for the remaining points, is proposed to provide accurate initial guess for DIC calculation, even in the presence of large rotations. By using FMT-CC algorithm, the initial guess of the seed point can be automatically and accurately determined between pairs of interrogation subsets with up to ±180 deg of rotation even in the presence of large translation. Then the initial guess of the rest of the calculation points can be accurately predicted by the robust RGDT scheme. The robustness and effectiveness of the present initial guess approach are verified by numerical simulation tests and real experiment.

Cellular microstructures within natural materials enlighten and promote the development of novel materials and structures in the industrial and engineering fields. Characterization of the microstructures and mechanical properties of these natural materials can help to understand the morphology-related mechanical properties and guide the structural optimization in industrial design. Among these natural cellular materials, pomelo peels, having a foam-like hierarchical microstructure, represent an ideal model for developing materials with high energy absorption efficiency. In this work, by combining X-ray tomographic imaging technique and digital volume correlation (DVC), in-situ stepwise uniaxial compression tests were performed to quantify the internal morphological evolution and kinematic responses of pomelo peel samples during compression. Via these experiments, the varying microstructure features and thus diverse resistance to compression from endocarp to exocarp are examined, and the evolution of both bundles bending and large strain domain from endocarp to mesocarp are explored. Based on the experimental results, the microstructure-related mechanical properties of pomelo peels in response to compressive loading that demonstrates nearly linear morphology-mechanics relationship were revealed.

We propose an easy-to-implement yet accurate calibration method for large-scale 3D measurements that makes use of a regular-sized phase target and two planar mirrors. Being insensitive to severe defocus, the phase target is placed as to span a large depth within the field of view (FOV) of each camera for accurate intrinsic calibration. Extrinsic calibration is achieved by placing the phase target in the FOV of a short-range virtual stereo-system generated by the mirrors. Results from 3D shape and deformation measurements demonstrate that the proposed method is capable to operate within a working volume of 3 m × 2 m × 1.8 m with an error < 0.1% of the FOV thus opening to new possibilities for large-scale measurements in mechanical and civil engineering applications.

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.

Over the past years, we have been developing new algorithms and approaches for robust, efficient and accurate internal displacement and strain field measurements using digital volume correlation (DVC). This paper will summarize our recent work on the following four major aspects: (a) advanced three-dimensional inverse-compositional Gauss-Newton (3D IC-GN) algorithm for subvoxel registration with enhanced accuracy and efficiency; (b) self-adaptive DVC algorithm with optimal calculation parameters; (c) practical and effective strategies for DVC analyses of high-resolution volumetric images and deformed volumetric images encoded with large deformation; (d) quantitative analysis and correction of thermal errors in DVC measurements due to self-heating effect of X-ray CT scanners. We hope that this paper can guide the readers to know the state-of-art DVC method.

Panoramic shape and deformation measurements of human skin in vivo may provide important information for biomechanical analysis, exercise guidance and medical diagnosis. This work proposes the application of an advanced mirror-assisted multi-view digital image correlation (DIC) method for dynamic measurements of 360-deg shape and deformation of human body parts in vivo. The main advantage of this method consists in its capabilities to perform full-panoramic non-contact measurements with a single pair of synchronized cameras and two planar mirrors thus representing a lean yet effective alternative to conventional multi-camera DIC systems in ‘surrounding’ configuration. We demonstrate the capabilities of this method by measuring the full-panoramic shape of a plastic human head, the deformation of a woman face and the principal strain distribution over the full-360-deg surface of a forearm during fist clenching. The applications of this method can be the most disparate but, given the possibility to determine the full-field strains and derived information (e.g. skin tension lines), we envisage a great potential for the study of skin biomechanics in vivo.

Video deflectometer based on using off-axis digital image correlation (DIC) has emerged as a robust non-contact optical tool for deflection measurements of bridges. In practice, a video deflectometer often needs to measure the deflections at multiple positions of the bridge. The existing 2D-DIC-based measurement methods usually use a laser rangefinder to measure the distance from each point to the camera to obtain the scale factor for the point. It is only suitable for the deflection measurements of a few points since manually measuring distances for a large number of points is time consuming and impractical. In this paper, a novel method for full-field bridge deflection measurement based on off-axis DIC is proposed. Because the bridge is usually a slender structure and the region of interest on the bridge is often a narrow band, the new approach can determine the scale factors of all the points of interest with a spatial straight-line fitting scheme. Moreover, the proposed technique employs reliability-guided processing and a fast initial parameter estimation strategy for real-time and accurate image-matching analysis. An indoor cantilever beam experiment verified the accuracy of the proposed approach, and a field test of a high-speed railway bridge demonstrated the robustness and practicability of the technique.

We propose an easy-to-implement and accurate calibration method for large-scale stereo-digital image correlation (stereo-DIC). First, the intrinsic parameters of each camera are separately calibrated using a regular-sized phase target. Using phase-shifted circular fringe patterns that feature the advantage of being insensitive to the defocus, the phase target can be placed in the close range of each camera. Then, scale-free extrinsic parameters are computed from the epipolar geometry, which can be easily retrieved from DIC registration of homologous point pairs in the stereo images of a test specimen surface. These intrinsic and scale-free extrinsic parameters are used as the initial guess for further optimizing the calibration results. Further, by measuring objects with known physical size, the scale information of the stereo-DIC system can be determined. The metrological performance of the proposed method is evaluated by large-scale shape reconstruction and deformation measurement tests. The obtained results demonstrate that the proposed method is a viable solution for the large-scale 3D deformation measurement tasks.

Distal cholangiocarcinoma (dCCA), originating from the common bile duct, is greatly associated with a dismal prognosis. A series of different studies based on cancer classification have been developed, aimed to optimize therapy and predict and improve prognosis. In this study, we explored and compared several novel machine learning models that might lead to an improvement in prediction accuracy and treatment options for patients with dCCA.In this study, 169 patients with dCCA were recruited and randomly divided into the training cohort (n = 118) and the validation cohort (n = 51), and their medical records were reviewed, including survival outcomes, laboratory values, treatment strategies, pathological results, and demographic information. Variables identified as independently associated with the primary outcome by least absolute shrinkage and selection operator (LASSO) regression, the random survival forest (RSF) algorithm, and univariate and multivariate Cox regression analyses were introduced to establish the following different machine learning models and canonical regression model: support vector machine (SVM), SurvivalTree, Coxboost, RSF, DeepSurv, and Cox proportional hazards (CoxPH). We measured and compared the performance of models using the receiver operating characteristic (ROC) curve, integrated Brier score (IBS), and concordance index (C-index) following cross-validation. The machine learning model with the best performance was screened out and compared with the TNM Classification using ROC, IBS, and C-index. Finally, patients were stratified based on the model with the best performance to assess whether they benefited from postoperative chemotherapy through the log-rank test.Among medical features, five variables, including tumor differentiation, T-stage, lymph node metastasis (LNM), albumin-to-fibrinogen ratio (AFR), and carbohydrate antigen 19-9 (CA19-9), were used to develop machine learning models. In the training cohort and the validation cohort, C-index achieved 0.763 vs. 0.686 (SVM), 0.749 vs. 0.692 (SurvivalTree), 0.747 vs. 0.690 (Coxboost), 0.745 vs. 0.690 (RSF), 0.746 vs. 0.711 (DeepSurv), and 0.724 vs. 0.701 (CoxPH), respectively. The DeepSurv model (0.823 vs. 0.754) had the highest mean area under the ROC curve (AUC) than other models, including SVM (0.819 vs. 0.736), SurvivalTree (0.814 vs. 0.737), Coxboost (0.816 vs. 0.734), RSF (0.813 vs. 0.730), and CoxPH (0.788 vs. 0.753). The IBS of the DeepSurv model (0.132 vs. 0.147) was lower than that of SurvivalTree (0.135 vs. 0.236), Coxboost (0.141 vs. 0.207), RSF (0.140 vs. 0.225), and CoxPH (0.145 vs. 0.196). Results of the calibration chart and decision curve analysis (DCA) also demonstrated that DeepSurv had a satisfactory predictive performance. In addition, the performance of the DeepSurv model was better than that of the TNM Classification in C-index, mean AUC, and IBS (0.746 vs. 0.598, 0.823 vs. 0.613, and 0.132 vs. 0.186, respectively) in the training cohort. Patients were stratified and divided into high- and low-risk groups based on the DeepSurv model. In the training cohort, patients in the high-risk group would not benefit from postoperative chemotherapy (p = 0.519). In the low-risk group, patients receiving postoperative chemotherapy might have a better prognosis (p = 0.035).In this study, the DeepSurv model was good at predicting prognosis and risk stratification to guide treatment options. AFR level might be a potential prognostic factor for dCCA. For the low-risk group in the DeepSurv model, patients might benefit from postoperative chemotherapy.

A technique for non-contact and full-field high-temperature strain measurement of a sample subjected to radiation heating using active imaging digital image correlation is described in this work. A high-performance quartz lamp heater system was designed to reproduce transient thermal environments experienced by hypersonic vehicles. The digital images of the test sample surface at various temperatures are captured using a novel active imaging optical system based on a combination of monochromatic light illumination and bandpass filter imaging. Subsequently, the captured images are processed by a robust reliability-guided displacement tracking algorithm with an automatic reference image updating scheme to extract full-field thermal deformation. With the improvements made in both the imaging system and correlation algorithm, the de-correlation problem of speckle patterns caused by the thermal radiation and surface oxidation of the heated test object are effectively addressed, enabling reliable deformation measurement in extremely high temperature environments. The performance of the proposed active imaging digital image correlation technique is verified by two experiments: (1) measurement of the uniform thermal strains of a chromium–nickel austenite stainless steel sample which is heated from room temperature to 1300 °C, and (2) measurement of the non-uniform thermal strain fields of a woven C/SiC composite at 1550 °C. The test results show that the active imaging digital image correlation is an easy-to-implement yet effective optical technique for high-temperature strain measurement, and has great potential in characterizing thermo-mechanical behaviour of materials and structures for hypersonic vehicles. Limitations and potential improvements of the present technique are also discussed.

A new simultaneous three-dimensional (3D) displacement measurement technique based on the combination of digital holography (DH) and digital imaging correlation (DIC) is proposed. The current DH-based 3D displacement measurement technique needs three sets of DH setups, and only the phase images are utilized in measurements, with all the intensity images discarded. In contrast, the proposed new technique only adopts a single off-axis DH setup. In the proposed technique, the phase images are used to extract out-of-plane displacements, but the intensity images (instead of being discarded) are processed by an intensity correlation algorithm to retrieve in-plane displacement components. Because the proposed technique fully takes advantage of all the information obtained by an off-axis DH without additional optical arrangements, it is simpler and more practical than the existing DH-based 3D displacement measurement technique. Experiments performed on a United States Air Force (USAF) target demonstrate that both the in-plane and out-of-plane displacements can be accurately determined by the proposed technique.

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.

Digital image/volume correlation (DIC/DVC) rely on the digital images acquired by digital cameras and x-ray CT scanners to extract the motion and deformation of test samples. Regrettably, these imaging devices are unstable optical systems, whose imaging geometry may undergo unavoidable slight and continual changes due to self-heating effect or ambient temperature variations. Changes in imaging geometry lead to both shift and expansion in the recorded 2D or 3D images, and finally manifest as systematic displacement and strain errors in DIC/DVC measurements. Since measurement accuracy is always the most important requirement in various experimental mechanics applications, these thermal-induced errors (referred to as thermal errors) should be given serious consideration in order to achieve high accuracy, reproducible DIC/DVC measurements. In this work, theoretical analyses are first given to understand the origin of thermal errors. Then real experiments are conducted to quantify thermal errors. Three solutions are suggested to mitigate or correct thermal errors. Among these solutions, a reference sample compensation approach is highly recommended because of its easy implementation, high accuracy and in-situ error correction capability. Most of the work has appeared in our previously published papers, thus its originality is not claimed. Instead, this paper aims to give a comprehensive overview and more insights of our work on thermal error analysis and compensation for DIC/DVC measurements.

In digital image correlation (DIC), the widely used forward-additive Newton–Raphson (FA-NR) algorithm and the recently introduced equivalent but more efficient inverse-compositional Gauss–Newton (IC-GN) algorithm are capable of providing both displacements and displacement gradients (strains) for each calculation point. However, the obtained displacement gradients are seriously corrupted by various noises, and for this reason these directly computed strains are usually considered as useless information and therefore discarded. To extract strain distributions more accurately, much research efforts have been dedicated to how to smooth and differentiate the noisy displacement fields using appropriate numerical approaches. In this contribution, contrary to these existing strain estimation approaches, a novel and alternative strain estimation approach, based on denoising the noisy strain fields obtained by FA-NR or IC-GN algorithm using a regularized cost-function, is proposed. The effectiveness and practicality of the proposed strain estimation technique is carefully examined using both computer-simulated images with imposed homogeneous and inhomogeneous deformation, and experimentally obtained images. Experimental results reveal that the strains obtained by the proposed method are comparable to those determined by post-processing of the displacement fields using conventional pointwise least squares strain estimation approach.

In recent years, with the requirement of high-resolution and real-time measurement, the computation speed of digital image correlation (DIC) has become increasingly important. At present, the DIC algorithms based on the iterative spatial domain cross-correlation algorithm are widely recognized as the most robust and rapid. In this paper, the integral image technique is extended to handle the complex items in the equations of the DIC algorithm in order to accelerate the calculation process. The influence of the interpolation method on the performance of the DIC algorithm is also investigated. In addition, the analysis of computational complexity and numerical experiment results are presented to illustrate the effectiveness of this method. The results successfully verify that the proposed method can improve the computation speed of the DIC algorithm greatly, and the improvement is more notable when the fast interpolation method is utilized.

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.

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.

In thermal-structural testing of hypersonic materials and structures, deformation measurement on the front surface of an object directly heated by quartz lamps is highly necessary and very challenging. This work describes a novel front-surface high-temperature deformation measurement technique, which adopts ultraviolet 3D digital image correlation (UV 3D-DIC) to observe and measure the high-temperature deformation fields on front surfaces directly heated by quartz lamps. Compared with existing blue light DIC techniques, the established UV 3D-DIC, which combines UV CCD camera, active UV illumination and bandpass filter imaging, can effectively suppress the strong disturbing light emitted by the quartz lamps and the heated sample itself during heating process. Two experiments were carried out to verify the robustness and accuracy of the developed technique: (1) direct observation of front surfaces of a hypersonic thermal structure sample heated from room temperature to 1050 °C, and (2) front-surface thermal stain and coefficient of thermal expansion (CTE) measurement of an Inconel 718 sample up to 800 °C. The well matched strain and CTE results with literature data show that UV 3D-DIC system is an effective technique for front-surface deformation measurement and has great potential in characterizing deformation response of hypersonic materials and structures subjected to transient aerodynamic heating.

We proposed a novel time-gated active imaging digital image correlation (DIC) method based on bandpass filtering and gated single-photon imaging techniques for high-temperature deformation measurement. The proposed method uses a powerful pulsed laser for illumination and then records the reflected pulsed laser light using a gated single-photon camera with an ultrashort exposure time. As a result, the received radiation light from the heated object and the ambient light can be significantly reduced to a negligible level compared with the illumination light intensity from the pulsed laser. Experiments including the varying ambient light test and thermal deformation measurement of a Ni-based alloy at around 700 °C verified the performance of the proposed method. It is observed that the established time-gated active imaging DIC system is dramatically superior to existing blue-light DIC and ultraviolet-DIC (UV-DIC) systems in preserving the image quality at high temperatures, thus showing promising application prospects in extremely challenging scenarios, such as ultra-high-temperature material testing, or thermomechanical testing of hypersonic materials and structures subjected to transient aerodynamic heating simulated by quartz lamps or an arc-heated wind tunnel.

An improved speckle projection profilometry that combines the projection of computer generated random speckle patterns using an ordinary LCD projector and the two-dimensional digital image correlation technique for in-plane displacements measurement is proposed for accurate out-of-plane shape and displacement measurements. The improved technique employs a simple yet effective calibration technique to determine the linear relationship between the out-of-plane height and the measured in-plane displacements. In addition, the iterative spatial domain cross-correlation algorithm, i.e., the improved Newton-Raphson algorithm using the zero-normalized sum of squared differences correlation criterion and the second-order shape function was employed in image correlation analysis for in-plane displacement determination of the projected speckle patterns, which provides more reliable and accurate matching with a higher correlation coefficient. Experimental results of both a regular cylinder and a human hand demonstrate that the proposed technique is easy to implement and can be applied to a practical out-of-plane shape and displacement measurement with high accuracy.

The metallic honeycomb core structure has great value in engineering applications in the areas of aeronautics and astronautics because of its lightweight structure, heat insulation performance and strong resistance to deformation. Through the self-developed aerodynamic heating experiment simulation system, the heat transfer characteristics of a metallic honeycomb core panel were tested from 200 °C to 900 °C. The heat insulation effects at various temperatures were also obtained experimentally. A three-dimensional (3-D) finite element model was used to numerically compute the heat-transfer properties of the metallic honeycomb core panel, and all of the internal radiation of the honeycomb core panel, the heat conduction of the metal structure, and the heat transfer of the air within the honeycomb core cavities were considered in the numerical simulation. Overall, the experimental results agreed well with the numerical simulations. The equivalent thermal conductivity of the metallic honeycomb core panel varied from 0.447 W/(m °C) to 1.52 W/(m °C) when the front surface temperature increased from 200 °C to 900 °C. The findings in this study provide an important foundation for the safety design of high-speed aircraft.

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.

In-situ three-dimensional (3D) ablation shape and recession measurements of ablative materials of hypersonic vehicles in high-temperature wind tunnel tests are essential to understand the ablation performance of these materials, which can provide necessary information for better thermal protection system (TPS) design. In this work, a novel ultraviolet stereo-digital image correlation (UV stereo-DIC) technique is proposed for in-situ measurements of 3D ablation shapes and recession in ablation tests. Specifically, a self-developed UV stereo-DIC system, combing a monochromatic UV illumination, two bandpass filters and two UV cameras, is first established. Using the UV stereo-DIC system, strong thermal radiation emitted from the heated sample and the heating device can be effectively suppressed. Natural textures formed from illuminating the rough sample surface by UV light are used as speckle patterns in stereo-correlation. In-situ 3D ablation shapes of a blunt cone subjected to arc heating were measured with a temperature range of 1000 °C to 1868 °C, and the UV stereo-DIC measurements showed good agreement with post-test measurement of the recession of the test articles. Based on the experimentally measured 3D shapes, several key parameters, such as full-field recession, dynamic recession rate, were calculated. The results demonstrate the efficacy and potential of UV stereo-DIC for 3D shape and recession measurements in ablation tests.

The mechanism of bolt support is an important topic in mining engineering and slope treatment. The artificial material and loading system were self-developed to study the influence of bedding cohesion and bolt number on the anchoring behavior of bedded rock mass. The results show that, both peak strength and elasticity modulus increase gradually with the increase of bedding cohesion and bolt number. The axial stress–strain curve of bedded rock mass under the reinforcement of bolts presents the features of strain-softening and secondary strengthening. Finally, anchoring behavior of bedded rock mass with different bolt numbers was simulated by using FLAC3D numerical program and the results were compared with the experimental results. This study can provide certain bases to the stability control and support design of bedded rock mass in roadway.

Abstract Recent studies on flexible/stretchable medical electronics are almost all focusing on passive monitoring of physiological signs for medical diagnostics, but rare reports have been found on those for active therapeutic purposes. Here, a novel infrared skin‐like active stretchable electronics (ISASE) is introduced with the organic–inorganic composite design for promotion of cutaneous wound healing, which can be conformally mounted anywhere on the human body due to its excellent flexibility and stretchability. Comprehensive experiments, including the proliferation and migration of human skin fibroblasts and rat dermal fibroblasts in vitro, and various tests on wound healing of rats, show favorable effects of the ISASE on promoting the fibroblast migration and proliferation, reducing the inflammatory phase, stimulating the angiogenesis, and shorting the wound healing period. A typical ISASE significantly shortens the cure time for a wounded rat by at least two days compared with the normal treatment with 12 days cure. The mechanical tests confirm the validity of the ISASE when used in extreme large deformations (e.g., stretched by up to 80% strain), which enables the conformability with the human body. The present portable and wearable ISASE demonstrates great potential for cutaneous wound healing, which constitutes an important complement to the current applications in the healthcare field.

Tensile properties of fibers are of high significance for composite materials, thus making their testing methods also highly concerned. In this work, an advanced dual-reflector-assisted video extensometer is established for accurate determination of tensile properties of carbon fiber multifilament. To validate the practicality and effectiveness of the video extensometer, three different types of carbon fiber multifilament specimens manufactured by Toray were tested. The stress-strain curves of these specimens were firstly measured for different gauge lengths, verifying that the test results are not affected by the gauge length of the video extensometer. Then, the dual-reflector-assisted and regular video extensometers were simultaneously applied to measure the tensile strains of the specimens, confirming the higher accuracy and better stability of the dual-reflector-assisted video extensometer. Moreover, the measured tensile modulus and elongation at break show good agreement with the reference values, further validating the practicality and accuracy of the technique. Due to its prominent advantages of easy-implementation, non-contact, high-accuracy, and real-time strain measurements, the proposed dual-reflector-assisted video extensometer demonstrates broad prospects in material testing of various materials.

Video extensometer assesses mechanical properties of materials by measuring their length changes under different loadings with combined use of video recording and image processing. In this Letter, an ultrasensitive video extensometer implemented on a single-camera dual field-of-view (dual FOV) telecentric imaging system is proposed. The method integrates the idea of FOV separation and telecentric imaging, which can increase the strain resolution with an order of magnitude and enhance the strain accuracy significantly. Validation experiments indicate that the root mean square error of a video extensometer was effectively decreased from 25.2 to 4.4 microstrains after employing the proposed method. Time-varying thermal strains of Al alloy and alumina ceramic samples were also precisely measured by the established ultrasensitive video extensometer, confirming its accuracy and practicality.

We propose a novel mirror-assisted multi-view digital image correlation (DIC) method for dual-surface 3D shape and deformation measurements using only a binocular stereo-DIC system and two orthogonal planar mirrors. Through the reflection of the mirrors, virtual surfaces of front and rear surfaces of sheet specimens formed behind the mirrors are simultaneously captured and reconstructed by regular two-camera stereo-DIC. By taking advantage of the speckle patterns premade on the mirrors, reflection transformations of both mirrors are retrieved and used to transform the reconstructed virtual surfaces to their real positions in front of the mirrors. The 3D shapes of both surfaces at each state enable the measurements of full-field 3D deformation on both surfaces and at the third direction (i.e., through-thickness direction). Compared with existing multi-camera DIC methods, the proposed method offers several distinct advantages, such as simple setup, low cost and easy implementation. Real experiments, including dual-surface reconstruction of a coin, in-plane and out-of-plane translation of a plate specimen and full-field deformation measurement of a planar aluminum specimen subjected to uniaxial tensile loading, validated the effectiveness and accuracy of the proposed method. The proposed method opens a new low-cost and convenient avenue for strain measurement in thickness direction, demonstrating great potential in material testing of anisotropic and/or ductile materials.

Based on the recently proposed mirror-assisted multi-view digital image correlation (MV-DIC), we establish a cost-effective and easy-to-implement mirror-assisted multi-view high-speed digital image correlation (MVHS-DIC) method and explore its applications for dual-surface full-field dynamic deformation measurement. In contrast to the general requirement of four expensive high-speed cameras for dual-surface dynamic deformation field measurement, the established mirror-assisted MVHS-DIC halves the cost by involving only two synchronized high-speed cameras and two planar mirrors. The two synchronized high-speed cameras can dynamically measure the front and rear surfaces of a sheet sample simultaneously through the reflection of the two mirrors. The results on the two surfaces are then transformed into the same coordinate system, leading to the required dual-surface 3D dynamical deformation fields. The effectiveness and accuracy of the established system are validated through modal tests of a cantilever aluminum sheet. The vibration measurement of a drum and dual-surface transient deformation measurement of a smartphone in the drop-collision process further prove its practicability. Benefiting from the attractive advantages of multi-view dynamic deformation measurement in a cost-efficient way, the established mirror-assisted MVHS-DIC is expected to encourage more comprehensive dynamic mechanical behavior characterization of regular-sized materials and structures in vibration and impact engineering fields.

Abstract Gray level residual (GLR), also known as correlation residual field or image difference, reflects the intensity differences between reference and deformed images. In finite element-based global digital image correlation (DIC), GLR field can be readily computed and has been successfully used for crack or damage detection. The computation of pixelwise GLR field requires dense displacement results, which can be obtained using mesh interpolation in global DIC. To obtain the GLR field in subset-based local DIC, in which results are obtained at discrete calculation points, a simple, efficient and practical computation method for GLR field is presented, which accounts for the influence of illumination variations unavoidably occurred in real DIC tests. The proposed method can be easily implemented on both central process unit and graphics process unit to realize real-time visualization. Experiments showed that the GLR field can quantitatively and intuitively evaluate the pixelwise matching quality and provide clearer and finer locations for new components or textures (e.g. cracks) presented in deformed images. By fully leveraging the pixelwise matching quality information provided by the GLR field, the capability and robustness of local DIC is expected to be further enhanced.

Three-dimensional digital image correlation (3D-DIC) is a leading optical measurement technique for measuring full-field shape, displacement, and deformation of solid materials and structures. However, compared with the well-developed and well-accepted measurement technique, uncertainty quantification (UQ) of 3D-DIC measurements is less advanced and less widely practiced. There still lacks a simple and effective method for quantifying the uncertainty of displacements measured by a specific 3D-DIC system for a specific speckle pattern. This is because the 3D-DIC practice involves a complex and long measurement chain and the propagation of uncertainty is highly nonlinear, which makes the uncertainty quantitation tricky. This work proposes a Monte Carlo-based method to quantify the uncertainty of 3D-DIC displacement measurements. The method is based on a theoretical analysis of random error in image matching and theoretical estimation of camera calibration uncertainty. It can estimate the uncertainty in 3D-DIC displacement measurements without additional experimental procedures. Validation experiments reveal that the estimated uncertainty agrees well with the actual situation. Based on the proposed method, the effects of the speckle pattern quality, subset size, and camera calibration quality on 3D digital image correlation displacement measurement uncertainties are also examined. The results reveal that, compared with image matching, camera calibration has a more pronounced influence on the uncertainty of displacement measurements. The proposed method can be integrated with existing 3D-DIC software to quantify the metrological performance of 3D-DIC measurements and therefore better interpret the measurement results.