Bing Pan

Full-field strain measurement using a two-dimensional Savitzky-Golay digital differentiator in digital image correlation

Many published research works regarding digital image correlation (DIC) have been focused on the improvements of the accuracy of displacement estimation. However, the original displacement fields calculated at discrete locations using DIC are unavoidably contaminated by noises. If the strain fields are directly computed by differentiating the original displacement fields, the noises will be amplified even at a higher level, and the resulting strain fields are untrustworthy. Based on the principle of local least-square fitting using two-dimensional (2D) polynomials, a 2D Savitzky-Golay (SG) digital differentiator is deduced and used to calculate strain fields from the original displacement fields obtained by DIC. The calculation process can be easily implemented by convolving the SG digital differentiator with the estimated displacement fields. Both homogeneous and inhomogeneous deformation images are employed to verify the proposed technique. The calculated strain fields clearly demonstrate that the proposed technique is simple and effective.

Experimental study of moiré method in laser scanning confocal microscopy

By integrating moire method with laser scanning confocal microscopy (LSCM), a novel class of moire patterns, i.e., LSCM moire patterns, is presented and investigated by experiment in this article. The moire patterns are formed by the interference of a 1200line∕mm cross-line holographic grating and the scanning lines of LSCM. The principles and conditions of forming LSCM moire are described in detail. The measured parameters of moire fringes from experimental images agree well with theoretical analysis. The experimental results verify the feasibility and reality of the proposed moire method to be another effective and high accuracy technique for measuring and observing in-plane microdeformation at micrometer scale.

A new mark shearing technique for strain measurement using digital image correlation method

The digital image correlation (DIC) method is a noncontact, full-field surface deformation measuring method, which is widely used in experimental mechanics. Although the DIC method has a high precision in displacement measurement, its precision in strain determination is a bit low. In this paper, a new mark shearing technique is proposed to improve the accuracy of the strain measurement of the DIC method. A wedge mirror is used to introduce a shearing distance of the marks, which are made on the specimen to calculate the strain. The measurement principle of the mark shearing technique is described in detail. From the analysis, it can be concluded that this method is suitable for both large-scaled and small-scaled specimens, and thus has a wider adaptability than the common DIC method. Using this method, the maximum gauge length is 80 mm, and the accuracy of strain measurement can reach 4 microm strains. A tensile experiment with aluminum sample was conducted, and the successful results demonstrated the feasibility of this method.

The Technique for Fabricating Submicron Moiré Grating Using FIB Milling

In this study, focused gallium ion (Ga+) beam is utilised to fabricate micro/submicron spacing gratings on specimen surface. The grating types include: parallel, cross, and hybrid (grating with double-frequency). Several hybrid gratings with double frequency were produced in combination mode or superposition mode, which have a good potential to measure deformation within different range. Techniques for producing different type of gratings are discussed in detail. As an application, a 5000 lines/mm grating was fabricated on an amorphous SIC MEMS cantilever and was successfully used to measure its virtual strain with aid of digital moiré. The experimental results verify the feasibility of fabricating high frequency grating on metal or non-metal surface using the FIB milling and the resultant grating can be used to measure micro-deformation.

Determining thermal and mechanical properties of polyimide using the DIC method

Polyimide (PI) is widely used in the aerospace and microelectronics industry and engineering due to its excellent electric and mechanical properties; however, the high thermal expansion of the PI causes the problem on the thermal expansion mismatching. A new type of PI with P-Phenylene diamine as diamine monomer is proposed to reduce the thermal expansion. The coefficient of thermal expansion (CTE) and the Youngs module of this PI films are studied in this paper using DIC method. The CTE of the new type PI films is about 21x10-6/°C, which is greatly reduced. The successful results demonstrated the feasibility of DIC method to measure the thermal and mechanical properties of films.

Displacement Smoothing and Strain Estimation Using Savitaky-Golay Filters in Digital Image Correlation

Recently, many research works were concentrated on how to improve the accuracy of displacement fields in digital image correlation (DIC). However, the original displacement fields calculated at discrete locations using DIC are unavoidably contaminated by noises. If the strain fields are directly computed by differentiating the original displacement fields, the noises will be amplified even at a higher level and the resulting strain fields are untrustworthy. To acquire reliably estimation of strain fields, in this paper, Savitaky-Golay (SG) filters are introduced to obtain smoothed displacement fields and reasonable strain estimation. The principle of two-dimensional SG filters is described in detail first. Then images of uniaxial tensile and three-point bending experiments were used to verify the proposed approach. The resulting smoothed displacement fields and strain fields clearly show that the proposed method is simple and effective.

Automated initial guess in digital image correlation aided by Fourier–Mellin transform

Abstract. The state-of-the-art digital image correlation (DIC) method using iterative spatial-domain cross correlation, e.g., the inverse-compositional Gauss–Newton algorithm, for full-field displacement mapping requires an initial guess of deformation, which should be sufficiently close to the true value to ensure a rapid and accurate convergence. Although various initial guess approaches have been proposed, automated, robust, and fast initial guess remains to be a challenging task, especially when large rotation occurs to the deformed images. An integrated scheme, which combines the Fourier–Mellin transform-based cross correlation (FMT-CC) for seed point initiation with a reliability-guided displacement tracking (RGDT) strategy for the remaining points, is proposed to provide accurate initial guess for DIC calculation, even in the presence of large rotations. By using FMT-CC algorithm, the initial guess of the seed point can be automatically and accurately determined between pairs of interrogation subsets with up to ±180  deg of rotation even in the presence of large translation. Then the initial guess of the rest of the calculation points can be accurately predicted by the robust RGDT scheme. The robustness and effectiveness of the present initial guess approach are verified by numerical simulation tests and real experiment.

Study of Mechanical Properties of the Spider Silk Using a Micro-Tensile System and DIC Methods

The spider silk is considered as a new type of biomaterials with its excellent mechanical properties. The mechanical properties of the spider silk are crucial to their applications. In this study the mechanical properties of spider silk were studied with a micro-tensile system driven by magnet-coil force actuator, which is very effective to measure the properties of low dimensional materials. The Young’s modulus of the spider silk is obtained, the relationship between the mechanical properties of spider silk and time is also acquired.

Experimental and Numerical Simulation of the Mechanical Behavior of Stratospheric Balloons Materials

The school of aerospace of Tsinghua University has started a project to develop new kind of stratospheric balloons for earth observation. These balloons will be designed to stand in the same position during a few days at 20 kilometers of altitude. To reach this goal, the first step is to select adapted materials for the balloon envelope. The materials for stratospheric balloons application should have specific properties adapted to the environment such as mechanical, thermo-optical, and permeability properties. Thus, we have asked a material manufacture company to develop and manufacture a material adapted to our application. In this paper, we will present the first results concerning the mechanical properties of the material. Experimental facilities have been developed to test the mechanical behavior of the materials at room and low temperature. The future development of the project will be to identify behavior law adapted to our material that fits with the experimental results.

Development of the Advanced Moiré Methods With High Resolution Microscopy at Fml

The mechanical behaviours of engineering materials in micro/nanoscopic range have aroused general concern in recent years. To understand the rule of these behaviours, the experimental technique with high sensitivity and spatial resolution are required. However, most of the existing methods cannot meet such requirements. Some novel micro/nano-moire methods have been developed recently at failure mechanics lab. (FML), Tsinghua University, China. This paper offers an introduction of these new methods, which can be realized under the focus ion beam (FIB), atomic force microscope (AFM), scanning tunnelling microscope (STM), laser scanning confocal microscope (LSCM) as well as the transmission electron microscope (TEM). These micro/nano-moire methods are able to offer quantitative analysis to micro/nano-deformation of the sample surface. The measurement principles and experimental techniques of these methods are described in detail. Some applications of these methods are given. The successful experimental results demonstrate the feasibility of these methods and also verify that the methods can offer a high sensitivity for displacement measurement with mico/nano-meter spatial resolution, and it will find a wide application in micro/ nano-mechanics research.