Tao Hua

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.

Study of the performance of different subpixel image correlation methods in 3D digital image correlation

The three-dimensional digital image correlation (3D-DIC) method is rapidly developing and is being widely applied to engineering and manufacturing. Despite its extensive use, the error caused by different image matching algorithms is seldom discussed. An algorithm for 3D speckle image generation is proposed, and the performances of different subpixel correlation algorithms are studied. The advantage is that there is no interpolation bias of texture in the simulation before and after deformation, and the error from the interpolation of speckle can be omitted in this algorithm. An error criterion for 3D reconstruction is proposed. 3D speckle images were simulated, and the performance of four subpixel algorithms is addressed. Based on the research results of different subpixel algorithms, a first-order Newton-Raphson iteration method and gradient-based method are recommended for 3D-DIC measurement.

A 2D mark shearing technique for characterizing the mechanical properties of material

The digital image correlation (DIC) method has been widely used in mechanical experiments due to their non-contact, full-field feature and high accuracy in displacement measurement. Yet the precision in strain determination is not so desirable. In this paper, a 2D mark shearing technique was proposed to improve the accuracy of the strain measurement in both vertical direction and horizontal direction. A four-prism system was designed and used to introduce a shearing distance of the marks, which were made on the specimen to calculate the two strain components. The measurement principle of the mark shearing technique is described in detail. A tensile experiment using an aluminum sample was conducted, and the Youngs modulus and Poisson ratio were measured. The successful experimental results demonstrated the feasibility of this method.

Measurement of Young's modulus and Poisson's ratio of Human Hair using Optical techniques

Human hair is a complex nanocomposite fiber whose physical appearance and mechanical strength are governed by a variety of factors like ethnicity, cleaning, grooming, chemical treatments and environment. Characterization of mechanical properties of hair is essential to develop better cosmetic products and advance biological and cosmetic science. Hence the behavior of hair under tension is of interest to beauty care science. Human hair fibers experience tensile forces as they are groomed and styled. Previous researches about tensile testing of human hair were seemingly focused on the longitudinal direction, such as elastic modulus, yield strength, breaking strength and strain at break after different treatment. In this research, experiment of evaluating the mechanical properties of human hair, such as Youngs modulus and Poissons ratio, was designed and conducted. The principle of the experimental instrument was presented. The system of testing instrument to evaluate the Youngs modulus and Poissons ratio was introduced. The range of Poissons ratio of the hair from the identical person was evaluated. Experiments were conducted for testing the mechanical properties after acid, aqueous alkali and neutral solution treatment of human hair. Explanation of Youngs modulus and Poissons ratio was conducted base on these results of experiments. These results can be useful to hair treatment and cosmetic product.

A new dynamic device for low-dimensional materials testing

As the geometrical size of low-dimensional materials decreases to micro- or nanoscale, the traditional dynamic loading system cannot be used anymore to measure the dynamic mechanical property. In this study, a new dynamic loading system was developed. A piezoelectric transducer actuator was used for displacement loading, and a mechanical lever was designed to amplify the displacement load. Finite element method simulation and validation experiments were conducted to analyze the strength and function of the mechanical lever. As an application test, a sample from an aluminum film was investigated using the system. The success of the experiment, as shown by the results, demonstrated the feasibility of the system for low-dimensional materials study.

Advanced intensity correlation method for evaluating Poisson’s ratio of fiberlike material

An advanced intensity correlation method for evaluating Poissons ratio of fiberlike materials is presented. The method is based on wave diffraction theory and digital image correlation (DIC) analysis. With the new method, the transverse strain of the fiberlike specimen under stretch can be conveniently and accurately determined by fitting the DIC-measured deformation distributions as a straight line following the theoretical diffraction model. The validity and flexibility of the method have been verified by numerical simulations and an experiment. The method can be readily extended to the characterizations of many fiberlike materials.

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.

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.

Study on the mechanical properties of TiNi shape memory alloy wires using mark shearing system

The Shape memory alloys (SMAs) wires have been used for medical and engineering filed due to the shape memory effect and the superelasticity effect. It is necessary to measure the mechanical properties of the SMA wires accurately. In this paper the mechanical properties of the shape memory alloy wires with different diameters were studied by a self-made testing system. A loading apparatus driven by step motor was used to apply load, and a load cell with the accuracy of 10 mN and the capacity of 90N was used to measure the value of the load. An optical extensometer based on the mark shearing method was used to measure the strain of the SMA wires, which had accuracy with 5 micron-strains in strain measurement. The mechanical properties of the shape memory alloy wires were studied. The nominal stress-strain curves of the SMA wires were acquired. The superelasticity effect was also studied. The successful experimental results also demonstrated the feasibility of the system in silk-like specimen measurement.