Optical video extensometer has been widely used in the field of strain measurement because of its features of non-contact, large range, high precision, being suitable for special environments and materials. A new optical video extensometer with the large field of view and high accuracy was designed in this paper. The hardware includes dual telecentric lens, CCD camera, ring light source, tripod, slide rail and some supporting fixtures. This integrated hardware has the advantages of low cost and high imaging quality. Also, the supporting software is developed. By sub-pixel tracking of selected markers, it can automatically calculate the data of transversal and longitudinal strain values. Poisson's ratio of the material can also be obtained. The measured results of the video extensometer are proven to be correct and accurate by experimental verification. Compared with strain gauges, the maximum strain error of the optical video extensometer is about 15με, and the standard deviation error is 8.38με. This optical video extensometer can be used for high-precision strain measurement of large size samples of metal, polymer, film and large deformation materials.
The deflection response characteristics of Ionic Polymer-Metal Composite (IPMC) cantilever actuator under Alternating Current (AC) excitations are investigated in the present work. A program based on numerical calculation platform MATLAB for Digital Image Correlation (DIC) analysis with multiple Regions of Interest (ROI) is developed. The program can be used to study the micro-scale dynamic lateral movement processes of the fabricated IPMC sample with Pt electrodes, which are recorded by a digital microscope. The results indicate that the deflection amplitude increases linearly with the increase of excitation voltage and from the clamped side of the sample to the free side and has approximately a negative exponent relation with the excitation frequency. Moreover, the vibration center of vibrating IPMC sample is not fixed. The program has been proved to obtain deflections of multiple ROIs simultaneously, thus is very useful for mechanical measurements.
Displacement measurement method provides quantitative information for structural safety evaluation and maintenance purposes. To overcome the difficulties in the outdoor measurement of large-span bridges, a high-resolution camera on an Unmanned Aerial Vehicle (UAV) was used to implement the deflect detection. First of all, the position of the UAV (including three displacements and three angles) was calibrated based on the speeded up robust features (SURF) algorithm. The SURF algorithm was used to extract the position information of the static areas in the image, and the calibration motion of the UAV was obtained by comparing the position changes of the static areas. The calibration used several self-designed marks with dots and circles to improve the accuracy. Then, the actual vibration of target was extracted from the raw data by subtracting the calibrated motion of the UAV. The numerical simulation indicates that the maximum error of displacement measurement can be controlled within 0.2 pixels. The calibrated experiments in the lab demonstrate that the overall correction accuracy of the displacement measurement is about 1.3 pixels. However, the maximum error of displacement measurement can be decreased to about 0.3 pixels if the rotation of UAV can be restrained by using damping mechanical devices.
The deflection that can reflect the vertical stiffness of a bridge plays an important role in the structural evaluation and health monitoring of bridges. In the past 20 years, the bridge deflection measurement methods based on computer vision and photogrammetry have been gradually applied to the field measurement due to the advantages of noncontact measurement, simple experimental setup, and easy installation. The technical research progress of vision-based bridge deflection measurement is reported from four aspects: basic principles, measurement methods, influencing factors, and applications. Basic principles mainly include camera calibration, three-dimensional (3D) stereo vision, photogrammetry, feature detection, and matching. For measurement methods, the single-camera two-dimensional measurement, the dual-camera 3D measurement, the quasistatic measurement based on photogrammetry, the multipoint dynamic measurement based on the displacement-relay videometrics and the deflection measurement based on UAV platform are introduced, respectively. In the section of influencing factors, this part summarizes the work of many researchers on the effects of camera imaging factors, calibration factors, algorithm factors, and environmental factors on measurement results. The field measurement results at different measurement distances and measurement accuracy based on these are presented in terms of applications. Finally, the future development trends of vision-based bridge deflection measurement are expected.
Digital image correlation (DIC) combined with an optical microscope has been used to realize the micro scale deformation measurement; micro-scale speckle pattern film was fabricated by spinning an epoxy resin and powder, and transferred to the surface of a test specimen. Generally, measurement accuracy of a 2D DIC will be affected by the small out-of-plane displacement of the test specimen. However, a telecentric lens is not convenient to be used in micro-scale digital image correlation to minimize the measurement displacement error. Thus, measurement error due to geometrical lens aberration, or lens distortion, should be corrected to overcome this problem. Camera calibration including lens distortion is considered in this paper. A corresponding distortion model was constructed and the distortion coefficient was determined by a rigid-body translation experiment using the fabricated micro speckle patterns. The distortion image was calibrated using a first-order aberration function. The results show that the relative error of the rigid-body translations was reduced 50%. Therefore, the proposed method can effectively correct the lens distortion under the large-magnification microscope.
Ionic polymer–metal composite (IPMC) cantilever actuator demonstrates significant bending deformation upon application of excitation voltage across electrodes without external load. In the present work, the non-contact digital image correlation (DIC) and a digital microscope were used to investigate the micro-scale displacement and strain distributions on the cross section of the actuator under excitation voltages, according to the low mass and film properties of IPMC material. The target surface of the fabricated IPMC sample with Pt electrodes was roughened with fine sandpapers to prepare an appropriate speckled surface. The experimental results indicate that longitudinal normal strain is linearly distributed along the thickness direction and strain gradient of longitudinal normal strain varies linearly with electric field. The longitudinal and transverse normal strains decrease with the increase of the frequency of the excitation voltage. Moreover, due to water loss of the sample in air, the IPMC actuator demonstrates contractive deformation when exposed in the air. The micro scale DIC technique has been proved to have excellent accuracy over a large range of strains, thus is very powerful for mechanical analysis of IPMC materials.
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