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Traditional videometric method can not be used in the measurement of large flexible cable-net structure for its large overall size and small partial size. A videometrics technique was proposed in this work to measure the topography and deformation of a large cable net structure. Tiny spots with high brightness (and large gray gradient) are used to mark the cable net nodes. By arranging the imaging light path properly, the light spot markers can be enlarged and accurately identified in the captured images. The relationship between the imaging parameters and the gray distribution of the light spot markers were derived and verified. And a topographical measurement experiment of a cable net structure was carried out with the proposed videometrics technique. The topography/deformation of the cable net can be measured with tiny-light spot markers, and the effectiveness and robustness of the technique on topography and deformation measurement of large cable-net structures are demonstrated.

As one typical heterogeneous material, the heterogeneity of rock micro parameters has an important effect on its macro mechanical behavior. The study of the heterogeneity of micro parameters is more important to reveal the root cause of deformation and failure. However, as a typical heterogeneous material, the current testing and inversion method is not suitable for micro parameters measurement for the rock. Aiming at obtaining the distribution of micro Young’s modulus and micro Poisson’s ratio of the rock, based on the digital image correlation method (DIC) and finite element method (FEM), this paper proposed a parameter field inversion method, namely the DF-PF inversion method. Its inversion accuracy is verified using numerical simulation and laboratory uniaxial compression test. Considering the influences of heterogeneity, stress state and dimension difference, the average inversion error of Young’s modulus field and Poisson’s ratio field are below 10%, and the proportion of elements with an error of less than 15% accounts for more than 86% in the whole specimen model. Compared with the conventional measuring method, the error of macro Young’s modulus and macro Poisson’s ratio calculated by the DF-PF inversion method is less than 2.8% and 9.07%, respectively. Based on the statistical analysis of Young’s modulus field and Poisson’s ratio field, the parameter homogeneity and quantitative function relation between the micro parameter and the principal strain can also be obtained in laboratory tests. The DF-PF inversion method provides a new effective method of testing Young’s modulus field and Poisson’s ratio field of the rocks under complex stress states.

The energy supply effect caused by the stiffness difference between roofs and sidewalls is an important factor that induces strain coal bursts. In order to quantitatively reveal the energy supply mechanism of strain coal bursts, this paper first establishes a coal burst energy model of the rock–coal system and proposes the calculation formula of coal burst kinetic energy considering supply energy and the stiffness ratio of rock to coal. Then the whole energy evolution law of the rock–coal system with different stiffness ratios is researched by using the numerical simulation method, and the whole process is divided into three stages. With the decrease in the stiffness ratio, the elastic strain energy of the coal changes little, while its kinetic energy is negatively correlated with the stiffness ratio in a power function. Meanwhile, the elastic strain energy and kinetic energy of the rock have power function relations with the stiffness ratio, too. When the rock–coal system is fractured, the kinetic energy of the coal comes from the release of elastic strain energy from the coal and the energy supplied from the rock. The energy supply rate is between 22% and 35% when the stiffness ratio changes from 3.0 to 0.5, and they show a linear relationship, while the supplied energy has a negative power function relationship with the stiffness ratio.

Bayer chromatic cameras are increasingly being used in photogrammetry for high-accuracy deformation measurements. However, studies on the quantitative accuracy evaluation of different photogrammetric methods using Bayer chromatic images are lacking, as the influence mechanism of chromatic images on measurement accuracy is unclear. This work performed substantial tests to investigate the influence of chromatic images on the photogrammetric measurement accuracy. Firstly, the spatial resolution and signal-to-noise ratio (SNR) of chromatic images were quantitatively evaluated. The performance of Bayer chromatic images was found to be systematically worse than the monochromatic counterparts with the same imaging configurations. Different photogrammetric measurements were then carried out using monochromatic and chromatic images. The measurement accuracy from chromatic images was significantly lower than that from monochromatic images, except for the grid method with square-profile fringes. Therefore, it is recommended to use monochromatic cameras for carrying out deformation measurements, unless there is a requirement of colour identification.

The side-wall pressure distribution in static silos is an important and open problem related to storage safety of granular materials. Referring to Lvin's theory [Powder Technology 4, 280 (1971)] and Rodolfo's 2D numerical result [Phys. Rev. E 97.1(2018)], a new theoretical model of side-wall pressure is established by assuming the friction coefficient as well as the ratio of horizontal to vertical stress changing with depth position in granular columns. Furthermore, the transition of stress state between hydrostatic-like and Janssen-like in a cylindrical granular container with flat bottom is systematically investigated by our model and experiment, and finally an empirical expression describing this transition is proposed.

We investigated image distortion induced by the coupling effect between the self-heating of a digital camera and environmental temperature, and then proposed a model to explain the relationship between environmental temperature and thermal-induced image distortion. First, we analyzed the heat transfer between the (self-heating) camera and the environment to elucidate the relationship between the environmental and camera-component temperatures. Next, we analyzed the thermal-induced change of imaging optical path to derive the relationship between the camera-component temperatures and image distortion. Finally, we established and verified a new model for thermal-induced image distortion. Our results indicate that camera-self-heating-induced image distortion forms a special case described by our model.

Most photogrammetry-based object position and attitude measurement systems use area cameras for two-dimensional (2D) image acquisition. However, the large volume of 2D images provided by area cameras limits the image capture speed, thus preventing the application of such methods to some high-speed measurements. Linear camera-based systems cannot be used to obtain real-time position and attitude parameters of non-specially designed objects because of scanning imaging and optimisation requirements. In this work, a novel position and attitude measurement method based on linear cameras was proposed. The position and attitude of objects can be obtained by analysing simultaneously recorded images of a specially designed five-line plane target using two calibrated linear cameras. This method can be used to carry out high-speed measurement of position and attitude parameters because no scanning imaging is required. In the present paper, we introduce the principle of this method, present verifications based on virtual and actual experiments, and analyse the main factors that affect measurement error.

Digital cameras represented by industrial cameras are widely used as image acquisition sensors in the field of image-based mechanics measurement, and their thermal effect inevitably induces thermal-induced errors of the mechanics measurement. To deeply understand the errors, the research for digital camera&rsquo, s thermal effect is necessary. This study systematically investigated the heat transfer processes and temperature characteristics of a working digital camera. Concretely, based on the temperature distribution of a typical working digital camera, the heat transfer of the working digital camera was investigated, and a model describing the temperature variation and distribution was presented and verified experimentally. With this model, the thermal equilibrium time and thermal equilibrium temperature of the camera system were calculated. Then, the influences of thermal parameters of digital camera and environmental temperature on the temperature characteristics of working digital camera were simulated and experimentally investigated. The theory analysis and experimental results demonstrate that the presented model can accurately describe the temperature characteristics and further calculate the thermal equilibrium state of working digital camera, all of which contribute to guiding mechanics measurement and thermal design based on such camera sensors.

Nuclear graphite is widely used as a moderator, reflector, and core structure material in very/high temperature gas-cooled reactors. However, because graphite may experience impacts with medium-low velocities during its transport and installation or under extreme conditions such as severe earthquakes, it is necessary to evaluate the dynamic fracture toughness of graphite to assess the safety of the reactors. The ASTM standard recommends a formula to calculate the fracture toughness; however, this formula includes the crack length parameter, which cannot be measured accurately in a dynamic process. To enable the accurate measurement of the dynamic crack propagation length of graphite and subsequently obtain the fracture toughness, a virtual extensometer method based on digital image correlation was established in this study. Three-point bending experiments were conducted using pre-cracked specimens to obtain the dynamic fracture toughness of graphite IG11 under medium-low impact velocities. The results indicated that the dynamic fracture toughness of graphite IG11 increased with the increase in the impact velocities, and its value was substantially higher than the static fracture toughness in certain cases. The proposed approach involves a simple operation, yields a high accuracy, and can enable automatic calculation. Moreover, this approach can be used to evaluate the dynamic fracture performance of other brittle or quasi-brittle materials.

A new photogrammetry based measurement technique for contact areas in line contact structures is developed. From the data collected by this technique, a method for measuring the contact strength of contact structures in high temperature applications is proposed. The contact strength of a line contact structure of polyimide at different temperatures is measured; the results show that the increase in temperature decreases the contact strength of the structure. The effect of temperature on the contact strength is studied by investigating the occurrence and evolution of yielding core within the structure at different temperatures. The decrease in the yield strength and elastic modulus of the polyimide material at high temperature induces core yielding at lower loads and eases the propagation of core yielding to the surface.

Subset-based digital image correlation (DIC) has limitations in subset size selection for heterogeneous deformation field measurement. To maintain good spatial resolution, selecting sufficiently small subsets is necessary, but the measurement resolution is reduced because of poor matching information, inducing large errors in the correlation matching procedure. Spatial-temporal subset digital image correlation (STS-DIC) incorporates the temporal continuity of displacement in the deformation analysis. This can increase the matching information of subset in the temporal dimension. Here, the advantages of STS-DIC for the measurement of heterogeneous deformation fields were investigated through simulations and experiments. The results demonstrated that the measurement resolution was improved while ensuring a good spatial resolution for heterogeneous deformation field measurement by using STS-DIC. Finally, STS-DIC was applied to calculate the displacement field near a precast crack, and the stress intensity factors were calculated accurately from high accuracy displacement fields.

In a high-speed stereovision system, high-speed cameras (HSCs) are used to record images and time information, and the measurement accuracy depends on the time precision of the cameras. When two HSCs are used in a binocular stereovision system, the image acquisitions of both cameras are unlikely to start simultaneously. The images that are supposed to be captured at the “same time” may record different states of the test object during an experiment, which will cause measurement errors. In this work, a theoretical model was established to correlate the start-up time difference (STD) and the STD-induced measurement errors of a binocular stereovision system. A simulation of the deflection of a cantilever beam verified the accuracy of the theoretical model. A suggestion of the experimental layout was discussed to reduce the STD-induced measurement errors in one direction, and a method was proposed to correct the STD-induced measurement errors. The effectiveness of the correction method was validated by two experiments, that is, the trajectory of a marker point and the out-of-plane deformation of a bubble. The results showed that the absolute errors of the marker point trajectory were almost negligible and the deformation contours of the bubble were more symmetric after the correction. This correction method should be useful for three-dimensional measurements of physical quantities using a binocular stereovision system.

High-speed cameras are widely used in experimental research and industrial measurement. Although multiple cameras are commonly used, whether the cameras are triggered at the same time is typically overlooked. This study measures the startup time difference between two high-speed cameras employing a proposed measuring system. A series of comparative experiments was conducted to consider the complex factors that can lead to a time difference. The system recorded the startup time differences for different combinations of two cameras at different frame rates, and thus acquired the dependence of the time difference on these factors. Suggestions are made on the basis of the experimental results.

Purpose: This study aimed to compare the difference in mechanical behavior between two types of female-part clips of the Stress-Free Implant Bar (SFI-Bar) system during simulation of insertion-removal cycles. Materials and Methods: A total of 10 samples simulating SFI-Bar-attachment-retained implant overdentures were fabricated and randomly divided into two groups (n = 5). One group used E-clips (Elitor alloy) as the female part of the SFI-Bar, and the other used T-clips (all titanium grade IV with red nylon inserts). A total of 14,000 insertion- removal cycles were carried out on each sample. Retentive forces from each cycle were recorded for analysis. Results: Significant differences were found between the two groups (P < .05). Conclusion: The retentive force of E-clips increased as the number of dislodging cycles increased, suggesting that some adjustment may be needed to lower this part's retentive force. T-clips with changeable nylon inserts were deformed after about 4,200 insertion-removal cycles, which interfered with insertion. This indicated that T-clips may need replacement after 2 to 3 years of clinical use.

The yielding formation and propagation of a ductile polycarbonate cylinder subjected to line loading, which corresponds to the global behavior of the structure, was experimentally studied and evaluated by measuring the strain fields on the end of the cylinder using Digital Image Correlation. The global behavior of the structure is expressed by a relationship between the average stress (load divided by contact area) and the equivalent strain (ratio of half width of contact area to radius of the cylinder); the contact area was measured in the same experiment by analyzing the images of the compressed pressure film captured through a transparent plate below the cylinder. A correspondence between the yielding state and the nonlinearity of the global behavior was constructed and, as loading was increased, the cylinder was found to first yield at a specific point after which a yielding core formed and propagated. Before the yielding core propagated to the surface of the cylinder, the global behavior of the structure remained linear, although the cylinder was no longer homogeneous and the material was nonlinear. After the yielding core propagated to the surface of the cylinder, the propagation was accelerated and the global behavior became nonlinear. The correspondence constructed in the paper will be helpful to understand the failure process and to evaluate the carrying capacity of a ductile polycarbonate cylinder subjected to line loading.

Compared to other approaches to improving digital image correlation (DIC) measurement accuracy, improving image quality is becoming increasingly effective. Image noise level, which is the main indicator of image quality, is very sensitive to the configurations of an imaging system and affects DIC measurement accuracy. However, the mechanism by which the imaging configurations affect accuracy is unclear. In this study, the influence of imaging configuration on DIC measurement accuracy was investigated using the common factor of noise. Firstly, the type and levels of noise introduced into actual images by different imaging system configurations were investigated. Next, the effect of noise on DIC measurement accuracy was determined using simulated images. Finally, the effects of imaging configuration on DIC measurement accuracy were experimentally investigated using actual measurements. The results show that both camera-related and lens-related configurations affect DIC measurement accuracy. In particular, lengthening the exposure time and reducing the f-number of the lens achieve similar improvement in DIC measurement accuracy, and a prime lens with high light transmittance is recommended for full-field optical measurements. In contrast, the imaging gain does not affect the accuracy of DIC measurements.

The systematic strain measurement error in Digital Image Correlation (DIC) induced by the environment temperature variation around the digital camera was extensively studied. The temperature variation of different camera components along with the changes of the environment temperature is experimentally studied and the motions of different components are then analyzed. The strain error in DIC is then analyzed according to a physical model to express the imaging geometry changes. Finally, the DIC measurement error caused by environment temperature variation was experimentally verified.

The systematic error for photomechanic methods caused by self-heating induced image expansion when using a digital camera was systematically studied, and a new physical model to explain the mechanism has been proposed and verified. The experimental results showed that the thermal expansion of the camera outer case and lens mount, instead of mechanical components within the camera, were the main reason for image expansion. The corresponding systematic error for both image analysis and fringe analysis based photomechanic methods were analyzed and measured, then error compensation techniques were proposed and verified.

A simple uniaxial oedometric system is developed to test the elastic modulus of granular materials. The stress–strain relationship is first measured under conditions of uniaxial compression with additional lateral stress and strain, then the elastic modulus of the material is determined by the linear fitting method. It is found that the modulus is positively correlated to the grain size and negatively correlated to the container size. Arching and dragging are revealed to be the mechanism of such correlations by using the digital image correlation method and the pressure film technology based on the statistical method.

Binocular stereo vision system has been widely used in 3D shape and deformation measurement. Many experimental systems and instruments have been developed based on this method. To realize the stereo vision measurement, the measured surface must carry some image "structures", such as points, fringes, or even random speckles, so that the parallax of the dual camera could be obtained by matching the "structures". However, for on-line inspection of black surface, the general projection methods are impossible to produce the appropriate "structures". In this paper a new binocular stereo vision system based on the laser dot matrix projection is presented. The laser projector is made up of two pieces of orthogonal fine grating and laser diode, when the laser beam was projected to the grating, a laser dot matrix will be formed and this high contrast dot matrix could be imaged on any surfaces, including the black surfaces. Furthermore, the image quality is greatly improved by mounting a red color filter (if a red laser) on the camera lens. Finally, a 3D shape measurement of a black cylinder is completed to verify the validity of the system.

In this work, a pre-cracked semi-circular shaped explosive simulant was loaded using a split Hopkinson pressure bar (SHPB). A high-speed camera was used to capture the deformation and fracture process of the specimen in situ. The digital images were processed using the digital image correlation (DIC) method. Next, full displacement and strain fields were obtained. The displacement vector field shows that the specimen fractured under tensile stress action. The strain field can be used to predict the crack propagation. Results show that the method of combined DIC and SHPB is effective to study the dynamic deformation and fracture behavior of explosive simulants. In addition, the specimen was loaded using a drop weight. The fracture toughness of the specimen was preliminary measured.

A simple optical system for small rotation angle measurement is constructed in this paper. A laser diode is fixed on the rotation part of the mechanism and the laser beam is received by a screen. The position of the spot could be registered from the image captured by CCD camera using the gray centroid algorithm. Then rotation could be obtained from the spot movement according to geometrical relationship. In order to verify the system and evaluate its accuracy, a deflection test of a low carbon steel cantilever beam is performed. It shows that the maximum relative error of experiment results and theoretical value is less than 1.5%, the measuring accuracy and stability is satisfied. The system in this paper has many advantages, such as high resolution, low cost, great convenience and could be used to measure small rotation angles of very complex mechanisms under non-contact request.

This paper focuses on the effect of operating temperature of uncooled CCD on the strain measurement error when the CCD is used to measure strain in an optical extensometer. A special experiment is designed in which an optical extensometer is used to measure the strain and a thermocouple is used to measure the temperature of CCD. The results show that the measured temperature of CCD has a similar tendency with the change of the measured strain. For different types of uncooled CCD, the temperature varies about 5~10 degree from the beginning of the experiment to the end, and the error of the measured strain induced by the temperature was about 120~180μe. It is considered that the error is related to the inherent noise of the electronic components of the CCD whose operating mode is sensitive to the temperature change. When using the optical extensometer, it is suggested to preheat the CCD 1~2 hours before the experiment especially in doing high accuracy measurement. Otherwise, the strain induced by the operating temperature should be eliminated from the measurement results through conducting temperature compensation method mentioned in this paper.

Coherent gradient sensing (CGS), a shear interferometry method, is developed to measure the full-field curvatures of a film/substrate system at high temperature. We obtain the relationship between an interferogram phase and specimen topography, accounting for temperature effect. The self-interference of CGS combined with designed setup can reduce the air effect. The full-field phases can be extracted by fast Fourier transform. Both nonuniform thin-film stresses and interfacial stresses are obtained by the extended Stoney's formula. The evolution of thermo-stresses verifies the feasibility of the proposed interferometry method and implies the "nonlocal" effect featured by the experimental results.