Digital Speckle Correlation Research Articles

Crack and crack growth behavior analysis of asphalt mixtures based on the digital speckle correlation method

Cracking is one of the main reasons for asphalt pavement damage. Because of the anisotropism of asphalt mixtures, traditional experimental methods cannot analyze displacement and strain fields accurately. A full-field, noncontact measurement technology—the digital speckle correlation method (DSCM)—is a good method to analyze the deformation characteristics of asphalt mixtures. By using DSCM and a semi-circular bending (SCB) test, the crack initiation and propagation rules can be tested experimentally; the full-field displacement and strain can be analyzed by vic-3D software. The results show that the strain field (Exx) and the rate of change displacement field (dU/dt) are well suited to study the properties of crack and fracture characteristics of asphalt mixtures. The interface between asphalt binder and stone is the weakest part of the entire structure, and comparing the crack growth rate and load displacement curve of a crumb-rubber (CR) modified asphalt mixtures and a styrene-butadienestyrene (SBS) modified asphalt mixtures, it is found that the carrying capacity of a CR modified asphalt mixtures is stronger than that of a SBS modified asphalt mixtures.

Experimental and Numerical Investigation on the Bearing and Failure Mechanism of Multiple Pillars Under Overburden

To reveal the mechanical response of a multi-pillar supporting system under external loads, compressive tests were carried out on single-pillar and double-pillar specimens. The digital speckle correlation method and acoustic emission technique were applied to record and analyse information of the deformation and failure processes. Numerical simulations with the software programme PFC2D were also conducted. In the compressive process of the double-pillar system, if both individual pillars have the same mechanical properties, each pillar deforms similarly and reaches the critical stable state almost simultaneously by sharing equal loads. If the two individual pillars have different mechanical properties, the pillar with higher elastic modulus or lower strength would be damaged and lose its bearing capacity firstly. The load would then be transferred to the other pillar under a load redistribution process. When the pillar with higher strength is strong enough, the load carried by the pillar system would increase again. However, the maximum bearing load of the double-pillar system is smaller than the sum of peak load of individual pillars. The study also indicates that the strength, elastic modulus, and load state of pillars all influence the supporting capacity of the pillar system. In underground space engineering, the appropriate choice of pillar dimensions and layout may play a great role in preventing the occurrence of cascading pillar failure.

Deformation-phase measurement by digital speckle correlation method

A novel algorithm which extracts the out-of-plane component of deformation phase from two continuous fringe patterns is proposed. The velocity field between two consecutive frames is estimated by digital speckle correlation method (DSCM). After that, according to the optical flow constrained equation, the whole-field deformation-phase map is obtained by the estimations of the velocity field and the local frequency of the original image. The operation of the proposed method is simple compared with other phase demodulation methods. Moreover, the new method works perfectly at the areas with dense fringes. In this paper, the proposed algorithm is introduced. Meanwhile, in order to verify the effectiveness, the new algorithm is applied to simulated interferogram and real fringe pattern with a centrally loaded and edge-clamped plate. The results of simulation and experiment show that the new method can demodulate the out-of-plane component of deformation phase from the visible in-plane velocity field without unwrapping process. Further, dynamic deformation-phase extraction will be realized when we know the time interval of two continuous images. The proposed algorithm provides a new approach for whole-field deformation-phase measurement and dynamic deformation measurement.

Mechanical and optical characterization of a tissue surrogate polymer gel

The mechanical and optical characteristics of a transparent tissue surrogate ballistic polymer gel were measured at different loading rates. This included measurement of tension and shear responses of the gel. Due to the compliant nature of the material, special strategies had to be developed for sample preparation, specimen gripping and deformation measurement. The digital speckle correlation method was used for measuring 2D strains in uniaxial tension tests. Subsequently, the Mooney-Rivlin description was used to extract the material characteristics from measured stress-strain data. Using a newly introduced optical method called Digital Gradient Sensing (DGS), the elasto-optical constants for the material were evaluated at different loading rates.

Displacement measurement using digital speckle multi-frequency harmonic wave correlation method

In the displacement measurement using digital speckle correlation method, significant measuring error due to the multi-peak nature of the correlation coefficient function is a critical issue as sometimes the secondary peak can be wrongly determined as the maximum instead of the highest peak. In the multi-frequency harmonic wave correlation method (MHCM) as proposed in this work, for speckle patterns before and after the displacement, correlation calculations were performed on corresponding sets of data matrices assembled by amplitudes/intensities obtained from harmonic waves with different frequencies. The calculated maximum correlation coefficients were compared with each other to locate the maximum in order to measure the displacement. Both ultrasonic speckle MHCM and laser speckle MHCM were applied to the displacement measurement respectively. Measured results indicated that MHCM effectively helps to ensure the correct determination of the maximum correlation coefficient, to avoid random errors caused by other factors and improves the measurement accuracy in the meanwhile.

Effect of impact damage on the curved beam interlaminar strength of carbon/epoxy laminates

The bending interlaminar strength and bending interlaminar strength after impact of carbon/epoxy-laminated curved beams were studied experimentally using four-point bending test and low velocity impact. First, the post- impact damage of the laminated curved beams with different radii was analyzed based on ultrasonic C-scan images. Then, the effect of impact damage on both the interlaminar strength and the maximum interlaminar radial stresses of the laminated curved beams were investigated. Finally, the full-field displacement distributions of the laminated curved beams were obtained using digital speckle correlation method. Four-point bending experimental results play a significant role for interlaminar strength in evaluating the laminated curved beams with and without impact damage.

Biaxial tensile testing of cruciform slim superalloy at elevated temperatures

The stress–strain curves and yield loci of metallic materials under biaxial tensile tests can be precisely predicted using a yield criterion. In this study, biaxial tensile testing equipment with high-accuracy displacement, temperature control, and digital speckle correlation method of non-contact optical strain measurement is developed to study the thermo-mechanical deformation behavior of a GH738 nickel-based superalloy, which is widely used for important components of aeronautic engines, to exploit its excellent mechanical properties at high temperatures. Additionally, the used specimen shape is discussed. The results of this study show that the special sample shape size solves the problem in which an extremely thin sheet metal bulges when subjected to tension. The equivalent stress–strain curves and yield surfaces, which were fitted according to the Hill'48 yield criterion, exhibit deformation characteristics that are more precise than those according to the von Mises criterion. Eventually, the efficacy of the related test methods is corroborated.

The Influence of the Radiation Source Parameters on the Accuracy of Digital Speckle Correlation Method

The article presents the impact of the laser radiation divergence and the wavelength on the harmonic distortion detection, as well as the influence of the distance between the optical circuit elements on the harmonic distortion and the sensitivity of amplitude. The research was made by recording the speckle images from the vibrating surface of the object with its digital processing later.

Coupled modeling of anisotropy variation and damage evolution for high strength steel tubular materials

High strength stainless steel tube (HSST) presents promising applications in many clusters as one of key lightweight materials. While, over thinning and further crack in plastic deformation are prone to occur due to limited strain hardening and high yield-strength ratio. To avoid this phenomenon, the accurate prediction of forming limit of HSST needs to be achieved considering uneven deformation induced fracture. Via the digital speckle correlation method (DSCM) based tension, both the diffuse necking limited hardening and the variation of Lankford coefficient R along tube deformation are studied, modeled and coupled into the Hill׳48 anisotropic yield framework; then by replacing the Mises effective stress with the extended Hill׳s anisotropic one and using a stepwise inverse method for damage parameter calibration, both the GTN and Lemaitre ductile fracture criteria (DFCs) coupled with anisotropy evolution are established and numerically implemented; thus, regarding several indexes in cases of uniaxial tension, flaring and mandrel bending of HSST, four individual anisotropic plasticity models and two coupled models are compared and evaluated. Due to considering the interplay between inhomogeneous deformation and damage evolution, the coupling model with the improved anisotropic plasticity provides the most accurate prediction of overall performance in all cases. The significance of the coupling DFCs with the anisotropic plasticity on overall simulation of complex forming processes of tubular materials is thus recognized.

Strain and mechanical properties of the VCM multilayer sheet and their composites using the digital speckle correlation method.

The digital speckle correlation method (DSCM) is introduced to solve the challenging problems in the related geometric measurement. Theoretical calculations of strain are deduced using the DSCM. Corresponding strains along x and y directions are obtained from uniaxial tension experiments and digital speckle measurements, using the VCM nondeep drawing multilayer sheet, the VCM deep-drawing multilayer sheet, clad films, nondeep drawing substrate, and deep-drawing substrate sheet as the targeted experimental objects. The results show that the maximum strains along the x direction of the VCM nondeep drawing multilayer sheet, the VCM deep-drawing multilayer sheet, clad film, nondeep drawing substrate, and deep-drawing substrate sheet are 68.473%, 48.632%, 91.632%, 50.784% and 40.068%, respectively, while the maximum strains along the y direction are -2.657%, -15.381%, 2.826%, -9.780% and -7.783%, respectively. The mechanical properties of the VCM multilayer sheet are between those of the substrate and clad film, while mechanical properties of the VCM deep-drawing multilayer sheet are superior to those of the VCM nondeep drawing multi-layer sheet.

Deformation tests and failure process analysis of an anchorage structure

In order to study the failure process of an anchorage structure and the evolution law of the body’s deformation field, anchor push-out tests were carried out based on digital speckle correlation methods (DSCM). The stress distribution of the anchorage interface was investigated using the particle flow numerical simulation method. The results indicate that there are three stages in the deformation and failure process of an anchorage structure: elastic bonding stage, a de-bonding stage and a failure stage. The stress distribution in the interface controls the stability of the structure. In the elastic bonding stage, the shear stress peak point of the interface is close to the loading end, and the displacement field gradually develops into a “V” shape. In the de-bonding stage, there is a shear stress plateau in the center of the anchorage section, and shear strain localization begins to form in the deformation field. In the failure stage, the bonding of the interface fails rapidly and the shear stress peak point moves to the anchorage free end. The anchorage structure moves integrally along the macro-crack. The de-bonding stage is a research focus in the deformation and failure process of an anchorage structure, and plays an important guiding role in roadway support design and prediction of the stability of the surrounding rock.

Measurement of short shaft power based on a digital speckle correlation method

This paper proposes a new method of using a camera to monitor changes in short shaft power. First, the camera is used to collect a sequence of images of the shaft surface without torque in advance as reference images. Then strain patterns of the shaft surface under the action of torque are collected and displacements between two adjacent frames are calculated to get the rotation speed by a digital speckle correlation method; a gradient algorithm is used to measure sub-pixel displacements between two ends of the strain images, and the torque is obtained. Finally, shaft power is obtained from rotational speed and torque. To verify the accuracy of measurement, bench experiments using different rotational speeds and torques are done, which are compared with standard sensor measurement. The measurement errors of speed, torque and shaft power are 0.1%–0.11%, 7.79%–10.91% and 7.7%–10.82%, respectively. Results show that the measurement system of shaft power based on a digital industrial camera has high precision. In comparison to the state-of-the-art methods, the proposed method has overcome the shortcomings of traditional methods whose installation is complex and whose signals easily interfere. It is characterized by simple construction, and applied to monitor dynamic short shaft power online.

Santa Maria di Collemaggio Church (L’Aquila, Italy): Historical Reconstruction by Non-Destructive Testing Techniques

The main goal of this work was the non-destructive testing (NDT) of an ancient fresco (15th century) preserved in the Santa Maria di Collemaggio Church (L’Aquila, Italy) and damaged after the 2009 earthquake. Active infrared thermography (IRT), near-infrared (NIR) reflectography and ultraviolet imaging (UV) were used. In addition, the state of the fresco prior to the earthquake was analyzed by electronic speckle pattern interferometry (ESPI), digital speckle correlation (DSC), raking light, tap, and chemical NDT techniques. The use of these techniques was important for the monitoring of new damages and for a comparison between the results over the years. Square heating thermography (SHT) data were processed using principal component thermography (PCT) and pulsed phase thermography (PPT) algorithms, in order to improve the defects’ signature and to reduce the impact of non-uniform heating and emissivity variations due to the painting’s pigments. A multi-analysis approach, segmentation operators and a specific data correlation method emphasize the overall study of the fresco. Furthermore, the facade and the high altar area were inspected by passive thermography and ground-penetrating radar (GPR), respectively. In the present case, the combined use of NDT techniques was useful to fill in the gaps in the construction history of the building.

应用数字散斑相关的运动轴面位置匹配

应用数字散斑相关的运动轴面位置匹配

二维数字散斑相关技术测量中斜光轴拍摄的误差修正

二维数字散斑相关技术测量中斜光轴拍摄的误差修正

Study on fracture behavior of polypropylene fiber reinforced concrete with bending beam test and digital speckle method

Portland cement concrete, which has higher strength and stiffness than asphalt concrete, has been widely applied on pavements. However, the brittle fracture characteristic of cement concrete restricts its application in highway pavement construction. Since the polypropylene fiber can improve the fracture toughness of cement concrete, Polypropylene Fiber-Reinforced Concrete (PFRC) is attracting more and more attention in civil engineering. In order to study the effect of polypropylene fiber on the generation and evolution process of the local deformation band in concrete, a series of three-point bending tests were performed using the new technology of the digital speckle correlation method for FRC notched beams with different volumetric contents of polypropylene fiber. The modified Double-K model was utilized for the first time to calculate the stress intensity factors of instability and crack initiation of fiber-reinforced concrete beams. The results indicate that the polypropylene fiber can enhance the fracture toughness. Based on the modified Double-K fracture theory, the maximum fracture energy of concrete with 3.2% fiber (in volume) is 47 times higher than the plain concrete. No effort of fiber content on the strength of the concrete was found. Meanwhile to balance the strength and resistant fracture toughness, concrete with 1.6% fiber is recommended to be applied in pavement construction.

Variation of contractile strain ratio of Ti–3Al–2.5V tubes and its effects in tubes numerical control bending process

The rapid development of high-technology fields, such as aviation and aerospace engineering, leads to the increased demand for lightweight and high-strength bent tubes due to their desirable properties. However, these tubes are of clear anisotropy. The anisotropy of tubes is usually described in terms of the contractile strain ratio (CSR). The current method to determine CSR can only be applied to cases of small uniform plastic deformation. This makes it difficult to obtain the variation in CSR during the plastic forming processes in which large plastic deformation usually occurs. Considering that the circumferential strain and axial strain can be directly and continuously obtained using the Digital Speckle Correlation Method (DSCM), this study proposed a new method based on the DSCM for determining CSR values over a large deformation range. Using this method, small gage lengths can be flexibly adopted to obtain a uniform plastic deformation as large as possible. To improve the precision of the method, elastic deformation was compensated theoretically, and varied values of Young's modulus with deformation were adopted. Using this method, the CSR of Ti–3Al–2.5V (ASTM Gr. 9) titanium alloy seamless tubes, which are typical anisotropic tubes, was observed varying with plastic deformation in that it decreased rapidly in the initial stage, then decreased slowly until stabilizing in the final stage. The variation can be expressed approximately as a second-order decay equation. The results of this study also show that the variation of CSR is sensitive to the values of Young's modulus and Poisson's ratio, therefore, varied values of Young's modulus and reliable Poisson's ratio should be adopted to improve precision. By embedding the variation in CSR with deformation into the finite element (FE) simulation for the numerical control (NC) bending of Ti–3Al–2.5V seamless tubes, the predication accuracy for the thinning degree of wall thickness, the flattening degree of the cross-section, and the springback angle can all be improved.

Calculation Principle and Test Applications for Digital Speckle Correlation Technology

With SSD algorithm and calculus process in detail computing, the basic calculation principles and the solution process from digital image to displacement of digital speckle correlation technology were described. Then the digital speckle correlation technology were used to the tensile test of CFRP plate embedded reinforcement beams, obtained the displacement cloud image and destruction rules of the specimens. The results show that the calculation results analyzed by the digital image correlation technology are in good agreement with the experimental phenomena. And the technology has the characteristic of full field measurement, simple operation, strong adaptability and high precision, so it has the widespread application prospect, especially suitable for application in the mechanical properties of materials under complex conditions, a large section of the destruction process monitoring and other fields.

Digital image correlation for large deformation applied in Ti alloy compression and tension test

In this paper, digital speckle correlation is used in the measurement of Ti alloy compression and tension test. The key technologies applied in the measurement are discussed in detail, including camera calibration with telephoto lens and digital image correlation in large deformation. Single camera self-calibration algorithm based on photogrammetry is proposed. In the algorithm, the interior parameters of camera are estimated without calibrated object, using the bundle adjustment technique, so the 3-D coordinates of calibration target points are not needed in advance to get a reliable camera calibration result. An updating reference image scheme could be employed to deal with large deformation situation. A large deformation measurement scheme, updating reference image scheme, is proposed in this paper. The un-deformed image is used as reference in correlation at first. Only for extremely large deformation field, in which iteration of correlation is not convergent, the reference image is updated to the image of previous deformed stage. Using this method, not only extremely large deformation can be measured successfully but also the accumulated error could be controlled. The 75mm lens is calibrated in the measurement and compared the result with extensometer and un-calibrated image. Experimental results show that up to 150% tensile deformation and 50% compression deformation can be measured successfully.

Nonuniform Current Distributions in YBa2Cu3O7−x Coated Conductor Caused by Fatigue Damage with Digital Speckle Correlation Analysis

Digital speckle correlation method (DSCM) is used to investigate fatigue evolution in the YBa2Cu3O7−x coated conductors (YBCO CCs) at room temperature. Damage analysis based on the DSCM has been performed to detect the damaged areas on the experimental samples. Two important parameters including stiffness variation (E) and dissipated energy per cycle (H) have been calculated for every fatigue test. Moreover, the superconducting specimens are subdivided into five sections with a distance of 1 cm between each of them, and the local critical currents (77 K and self-field) of fatigue and fatigue free are measured. The nonuniform current distributions are observed and can be interpreted qualitatively.