Feipeng Zhu

Enhancement of strain measurement accuracy using optical extensometer by application of dual-reflector imaging

At present, the accuracy of strain measurement using a common optical extensometer with 2D digital image correlation is not sufficient for experimental applications due to the effect of out-of-plane motion. Therefore, this paper proposes a dual-reflector imaging method to improve the accuracy of strain measurement when using a common optical extensometer, with which the front and rear surfaces of a specimen can be simultaneously recorded in the sensor plane of a digital camera. By averaging the strain in two optical extensometers formed on the front and rear surfaces of a specimen, the effect of any slight out-of-plane motion can be eliminated and therefore the strain measurement accuracy can also be improved. Uniaxial tensile tests with an Al-alloy specimen, including static loading and continuous loading, were conducted to validate the feasibility and reliability of the proposed method. The strain measurement results obtained with the proposed method and those obtained with an electrical-resistance strain gauge were found to be in good agreement. The average errors of the proposed method for the two continuous loading tests were found to be 8 ± 10 μe and −6 ± 8 μe. Given that no correction sheet or compensation specimen is required, the proposed method is easy to implement and thus especially suitable for determining the mechanical properties of brittle materials due to the high level of accuracy with which strain can be measured.

Measurement system of dynamic microscopic deformation for membrane based on fringe projection

Abstract. The bulge test has been utilized extensively in determining the mechanical properties of thin film materials. We develop a bulge-test system that consists of three main components, including an optical system, a loading system, and a control system. A telecentric lens is adopted as an imaging camera in an optical system to provide high imaging quality along with a constant angle of view across the entire field of view. The out-of-plane deformation is obtained by projecting a sinusoidal fringe pattern onto a specimen surface and analyzing recorded images with a digital image correlation algorithm. A least-squares polynomial fitting is presented to solve the problem that the out-of-plane calibration coefficient is not a constant. Experiments were performed to validate the availability and reliability of this proposed bulge-test system in the measurement of a dynamic microscopic deformation of membrane. The measuring range of deformation is from several microns to hundreds of microns with an accuracy of 0.6 μm.

Nonlinear calibration for generalized fringe projection profilometry under large measuring depth range

A mathematical description of the absolute surface height distribution in generalized fringe projection profilometry under large measuring depth range is presented. Based on least-squares polynomial fitting, a nonlinear calibration to determine the mapping between phase change and surface height is proposed by considering the unequal height arrangement of the projector and the camera. To solve surface height from phase change, an iteration method is brought forward. Experiments are implemented to demonstrate the validity of the proposed calibration and an accuracy of 0.3 mm for surface profile under 300 mm measuring depth can be achieved.

Finite element analysis of a vibration test bed frame

Abstract The stability and reliability of a vibration test bed are significantly influenced by the mechanical property of its structure. In order to reveal the mechanical property of the structure and its effect on the strength and stiffness, a finite element model of the damper vibration test bed frame has been created, the loads and constraints have been imposed, and the static solution has been carried out, from which the stress states and the maximum deformation of the test bed frame have been obtained. Moreover, according to the determined stress distribution, the strength and stiffness of the test bed frame have been extracted. The simulation results show that the maximum deformation, which is 0.0717 mm, is within the safety range and meets the stiffness safety condition. The maximum stress is 59.6 MPa and meets also the strength security condition. Finally, the minimum section stress has been compared to the theoretical prediction, and the results agree very well.

Normal deformation measurement of free surfaces on rocks under biaxial compression

Abstract The 3D digital image correlation technique has been employed to measure the 3D profile and normal deformation of the marble free surface of Jinping hydropower station under biaxial compression to ensure the safety of the engineering structure. The 3D profile is consistent with the pattern of rock cutting, which shows that the digital image correlation method shows a high precision of engineering measurement. From the diagram of the displacement gradient and the load, it is concluded that the displacement gradient increases sharply before the destruction of the specimen, which proves that the destruction and the displacement gradient has close correlation. Due to the fact that the displacement contour is a relatively dense area which is more prone to produce tension effects than the maximum normal displacement, it is easier to destroy the cracks. Based on the above conclusions, the distribution of free surface contours of the displacement field can determine the area which is more likely to show free surface damage cracks.

Experimental Study on Dynamic Strength Improvement Mechanism of Concrete Materials

This research mainly investigates the dynamic strength improvement mechanism of concrete materials based on two aspects: the time lapse of strain and the crack development modes. Based on the contrastive experiments on axial compression and flexural-tensile tests, the load-deflection curves under quasi-static load and dynamic load were obtained. Consequently, the stress-strain curves of concrete materials were available. Therefore, the fracture energy of the specimen was calculated through numerical integration of the load-deflection curves. The strain response under dynamic load is significantly less than that under quasi-static load at the same stress level. The time lapse of strain exists and is more significant as the stress level increases. The analysis of micropattern morphology on crack development under dynamic load shows that there is not enough time for microcracks to independently select the next weakest part for further propagation. Therefore, some cracks go through the aggregate, and more energy is required for the crack development, thus leading to an improvement of dynamic failure strength.