Huaixi Wang

Error evaluation technique for three-dimensional digital image correlation

Three-dimensional (3D) digital image correlation (DIC) is one of the most popular techniques used in engineering for strain and deformation measurement. However, the error analysis of 3D DIC, especially which kind of parameters dominates the error of 3D coordinate reconstruction in any kind of configuration, is still under study. In this paper, a technique that can be used for error determination of reconstruction is presented. The influence from the system calibration and image correlation to the error is theoretically analyzed. From numerical experiments of one-dimensional line and two-dimensional plane, the evaluation procedure is validated to be flexible. A typical test with standard objects is also conducted. With this technique, once a 3D DIC system is set up and images of objects with speckles and calibration boards are recorded, the error of the configuration can be immediately evaluated. The standard deviation of every point in the world coordinate can be determined by statistical analysis.

Fabrication of micro-scale speckle pattern and its applications for deformation measurement

A novel and effective method of fabricating a micro-scale speckle pattern applied to metallic and polymeric materials is described. It can be used for measuring deformation under an optical microscope or other high magnification imaging systems. In this paper, the mean intensity gradient (MIG) is used to evaluate the quality of speckle pattern under different fabrication conditions, such as proportion of compound, spinning rates and centrifugal solidifying time, and the optimal fabrication parameters are proposed. Finally, deformation of a PET thin film and interface of an optical fiber device are measured with fabricated micro-scale speckle pattern. Further, it is proved that the speckle pattern produced by the novel method can be used with digital image correlation to measure deformation at micro-length scale.

Residual stresses measurement by using ring-core method and 3D digital image correlation technique

Ring-core method/three-dimensional digital image correlation (3D DIC) residual stresses measurement is proposed. Ring-core cutting is a mechanical stress relief method, and combining with 3D DIC system the deformation of the specimen surface can be measured. An optimization iteration method is proposed to obtain the residual stress and rigid-body motion. The method has the ability to cut an annular trench at a different location out of the field of view. A compression test is carried out to demonstrate how residual stress is determined by using 3D DIC system and outfield measurement. The results determined by the approach are in good agreement with the theoretical value. Ring-core/3D DIC has shown its robustness to determine residual stress and can be extended to application in the engineering field.

The geometric phase analysis method based on the local high resolution discrete Fourier transform for deformation measurement

The geometric phase analysis (GPA) method based on the local high resolution discrete Fourier transform (LHR-DFT) for deformation measurement, defined as LHR-DFT GPA, is proposed to improve the measurement accuracy. In the general GPA method, the fundamental frequency of the image plays a crucial role. However, the fast Fourier transform, which is generally employed in the general GPA method, could make it difficult to locate the fundamental frequency accurately when the fundamental frequency is not located at an integer pixel position in the Fourier spectrum. This study focuses on this issue and presents a LHR-DFT algorithm that can locate the fundamental frequency with sub-pixel precision in a specific frequency region for the GPA method. An error analysis is offered and simulation is conducted to verify the effectiveness of the proposed method; both results show that the LHR-DFT algorithm can accurately locate the fundamental frequency and improve the measurement accuracy of the GPA method. Furthermore, typical tensile and bending tests are carried out and the experimental results verify the effectiveness of the proposed method.

Deformation grating fabrication technique based on the solvent-assisted microcontact molding

A deformation grating fabrication technique based on solvent-assisted microcontact molding (SAMIM) is reported in this paper. The fabrication process can be divided into three steps: imprinting a grating on a medium polymer substrate (MPS) by SAMIM, coating a thin metal film on the MPS, and transferring the film to the measured surface. In order to increase the stiffness of the elastic mold without decreasing its conformal contact formation ability, a re-useable, glass-embedded polydimethylsiloxane (PDMS) mold is used. In addition, a characterization method based on the Fourier transform and phase analysis is proposed to check the quality of the fabricated grating. Verified by experiment, the proposed fabrication technique can fabricate a high-frequency large-area grating on different specimens, which can be a qualified deformation sensor for the moiré method.

Fabrication of a DIC sensor for in-plane deformation measurement

A thin epoxy film with a micro speckle pattern was developed as a deformation sensor, and replicated on the surface of a sample. In order to improve the toughness of the thin epoxy film, a flexibilizer was added into the epoxy compound and the mechanical property of the thin epoxy film was analyzed. The measurement results show that the toughness of the thin film has been improved greatly, which enlarges the range of application of deformation measurement. Finally, the feasibility of application of the thin film digital image correlation (DIC) sensor was verified using a standard tensile experiment. And the interface deformation of the signal lap joint was measured by using fabrication and replication of the micro speckle pattern and micro digital image correlation. The failure mode of a single lap joint at micro-scale lengths was determined by analyzing the captured images of the micro speckle pattern.

Elastic constants characterization on graphite at 500°C by the virtual fields method

In this paper the elastic constants of graphite at elevated temperature were experimentally investigated by using the virtual fields method (VFM). A new method was presented for the characterization of mechanical properties at elevated temperature. The three-point bending tests were performed on graphite materials by an universal testing machine equipped with heating furnace. Based on the heterogeneous deformation fields measured by the digital image correlation (DIC) technique, the elastic constants were then extracted by using VFM. The measurement results of the elastic constants at 500°C were obtained. The effect on the experimental results was also analyzed. The successful results verify the feasibility of using the proposed method to measure the properties of graphite at high temperature, and the proposed method is believed to have a good potential for further applications.

c-axis textured La2CuO4 thin films prepared by NaClO oxidation: I. Superconducting and structural properties

Thin films of lanthanum cuprate with c-axis texture were grown on gold covered SrTiO3 substrates by pulsed laser deposition. Superconductivity has been introduced into these films through the process of post-deposition chemical oxidation using NaClO solution. The systematic change in resistance of the oxidized films suggests that the films undergo a change from an underdoped state to an overdoped state as the oxidation time increases. The absence of the time effect found in the chemical oxidation of La2CuO4 epitaxial thin films evidences that grain orientations have a strong effect on the oxidation behaviour of these films. Raman analyses reveal that the thickness of the oxidized film decreases with increasing oxidation time. These results indicate that c-axis textured LCO thin films resemble powder samples rather than epitaxial thin films, in the light of their chemical oxidation behaviours.

Study on the high temperature deformation measurement using the mark shearing technique

The mark shearing technique has been successfully utilized for room-temperature strain measurement in solid mechanics research. However, when the method is applied to deformation measurement at high temperature, some new issues will occur. From our experience, we have found that an inevitable thermal radiation will emit from the oven in the measurement and may lead to a distortion of the measuring system. As a result, an undesired measurement error will arise; furthermore, the oxidation resistance marker is crucial to obtain an ideal result. In order to solve the abovementioned issues, the mark shearing techniques for high-temperature deformation measurement are studied in this work, and a novel method for fabricating the marker pattern is proposed, which can be utilized to fabricate a deformation carrier for high temperature on a different specimen surface. In combination with this marker with our self-developed mark shearing system, the high-temperature (1000 °C) mechanical properties of the high-temperature nickel alloy (GH4037) was measured.In order to characterize the thermal deformation caused by the oven during the high temperature experiment, the surface temperature distribution of the mark shearing system is measured by an infrared camera. On the basis of this result, the corresponding thermal deformation is analyzed by the finite element analysis (FEM) method. From the analysis result, we can conclude that the virtual strain of the system incurred by thermal radiation cannot be ignored for a long-time measurement.

Thermodynamic and Kinetic Modelling of a Bond Coat Alloy: Oxidation Induced Depletion and Its Effect on Microstructure and Material Behavior

In this paper oxidation induced micro structural changes of a Co based bond coat alloy are discussed. Micro structure of Co32Ni21Cr(4–12)A13ReY was studied at 1000 °C and modelled using thermo-calc. The effect of varying aluminum content on precipitating phases was quantified and experimental results were compared with modelled microstructure to access modelling capabilities of thermo-calc. Dictra was used to predict microstructural changes due to Al depletion and the influence of alloy‘s microstructure on material‘s properties was also considered. Microstructural changes across oxidized alloy were related to spatially varying alloy‘s hardness. Experimental findings proved thermodynamic modelling and kinetic simulations produced excellent qualitative and quantitative phase information to represent the system under oxidation at high temperature.