Yilan Kang

Experimental analysis for the effect of dynamic capillarity on stress transformation in porous silicon

This work is funded by the National Natural Science Foundation of China Contract Nos. 10732080 and 10502014.

Experimental Analysis on Deformation Evolution and Crack Propagation of Rock Under Cyclic Indentation

Indentation-induced cracking may have many implications in rock drilling and cutting processes. In the present work, cyclic indentation tests have been conducted on Yunnan sandstones to investigate the deformation and failure process. Deformation fields on the sample surface have been obtained via the digital image correlation (DIC) technique. It has been found that the macroscopic force–indentation curve exhibits hysteresis and the residual displacements increase during the loading–unloading cycles. Furthermore, a median crack has been observed to nucleate, running almost parallel to the loading axis when the indentation force amounts to a critical value. Such a crack always results in the final failure of the sandstone samples. Experimental analysis reveals the deformation and failure mechanism of sandstone during cyclic indentation. By the DIC technique, the displacement distribution along the crack surface can be easily calculated and, hence, the fracture parameters, such as crack opening displacement and fracture toughness.

A damage mechanics model for twisted carbon nanotube fibers

Carbon nanotube fibers can be fabricated by the chemical vapor deposition spinning process. They are promising for a wide range of applications such as the building blocks of high-performance composite materials and micro-electrochemical sensors. Mechanical twisting is an effective means of enhancing the mechanical properties of carbon nanotube fibers during fabrication or by post processing. However, the effects of twisting on the mechanical properties remain an unsolved issue. In this paper, we present a two-scale damage mechanics model to quantitatively investigate the effects of twisting on the mechanical properties of carbon nanotube fibers. The numerical results demonstrate that the developed damage mechanics model can effectively describe the elastic and the plastic-like behaviors of carbon nanotube fibers during the tension process. A definite range of twisting which can effectively enhance the mechanical properties of carbon nanotube fiber is given. The results can be used to guide the mechanical twisting of carbon nanotube fibers to improve their properties and help optimize the mechanical performance of carbon nanotube-based materials.

Inverse Analysis and Modeling for Tunneling Thrust on Shield Machine

With the rapid development of sensor and detection technologies, measured data analysis plays an increasingly important role in the design and control of heavy engineering equipment. The paper proposed a method for inverse analysis and modeling based on mass on-site measured data, in which dimensional analysis and data mining techniques were combined. The method was applied to the modeling of the tunneling thrust on shield machines and an explicit expression for thrust prediction was established. Combined with on-site data from a tunneling project in China, the inverse identification of model coefficients was carried out using the multiple regression method. The model residual was analyzed by statistical methods. By comparing the on-site data and the model predicted results in the other two projects with different tunneling conditions, the feasibility of the model was discussed. The work may provide a scientific basis for the rational design and control of shield tunneling machines and also a new way for mass on-site data analysis of complex engineering systems with nonlinear, multivariable, time-varying characteristics.

Identification of elastic-plastic mechanical properties for bimetallic sheets by hybrid-inverse approach

Analysis, evaluation and interpretation of measured signals become important components in engineering research and practice, especially for material characteristic parameters which can not be obtained directly by experimental measurements. The present paper proposes a hybrid-inverse analysis method for the identification of the nonlinear material parameters of any individual component from the mechanical responses of a global composite. The method couples experimental approach, numerical simulation with inverse search method. The experimental approach is used to provide basic data. Then parameter identification and numerical simulation are utilized to identify elasto-plastic material properties by the experimental data obtained and inverse searching algorithm. A numerical example of a stainless steel clad copper sheet is considered to verify and show the applicability of the proposed hybrid-inverse method. In this example, a set of material parameters in an elasto-plastic constitutive model have been identified by using the obtained experimental data.

Regional identification, partition, and integral phase unwrapping method for processing moiré interferometry images

We present a new method of regional identification, partition, and integral (RIPI) phase unwrapping for processing images, especially those with low quality, obtained from moiré interferometry experiments. By introducing the principle of preorder traversal of a general tree in data structures and then by applying the idea of a regional integral, the proposed method makes regional partition and phase evaluation much easier and more accurate, and it also overcomes the common faults that can occur when conventional approaches, such as line defects, are used. Examples are given to demonstrate the advantage and applicability of the proposed RIPI method when processing experimental images. It is shown that the proposed method works well for global phase distribution, and, at the same time, local mutational information is preserved and limited to its vicinity without affecting other parts.

Strain sensor of carbon nanotubes in microscale: from model to metrology.

A strain sensor composed of carbon nanotubes with Raman spectroscopy can achieve measurement of the three in-plane strain components in microscale. Based on previous work on the mathematic model of carbon nanotube strain sensors, this paper presents a detailed study on the optimization, diversification, and standardization of a CNT strain sensor from the viewpoint of metrology. A new miniaccessory for polarization control is designed, and two different preparing methods for CNT films as sensing media are introduced to provide diversified choices for applications. Then, the standard procedure of creating CNT strain sensors is proposed. Application experiments confirmed the effectiveness of the above improvement, which is helpful in developing this method for convenient metrology.

Modeling Specific Energy for Shield Machine by Non-linear Multiple Regression Method and Mechanical Analysis

The specific energy, defined as the energy consumption to complete the excavation of unit volume of the soil, can well describe the working efficiency of a shield machine. An identification model of the specific energy is established in this paper by introducing the mechanical analysis of the shield excavating process into the nonlinear multiple regression of the on-site data. The mechanical analysis of the shield-soil system helps to decouple the nonlinear multi-parameter problem and the regression process is conducted based on a group of on-site data of subway project in China. Fairly good consistency between the model results and the on-site recorded datum can be achieved. This work provides a useful tool for the analysis of the energy consumption of shield machines.

Application of Digital Speckle Correlation Method to Measure Fracture Toughness of Foil Material

In this paper, the digital speckle correlation method (DSCM) is selected as the experimental means to calculate the J integral of foil material. The deformation fields in the crack tip region are measured and their contour maps are shown. The formula using deformed field to calculate J integral is derived. Through calculating J integral of foil material by the DSCM, it is shown that this experimental method is a useful test technique in studying the mechanical behavior of film or foil materials.

Origin mechanism of residual stresses in porous silicon film

In this article, a metallographic microscopy, an atomic force microscopy and a field emitting scanning electronic microscopy was used to investigate the surface and the cross-sectional morphology of porous silicon films, respectively. Simple micro-structure and micro-mechanical models are established to explain the origin mechanism of residual stresses in the porous silicon. Experimental results reveal that the residual stresses have close relation with the micro-structure of the porous silicon and consist of the lattice mismatch stress, capillary stress, oxidation stress, Van der Walls force and so on. Combining micro-Raman spectroscopy with x-ray diffraction measurements, we get the total residual stress of 900MPa, and its components of the lattice mismatch stress is about of 815.8MPa, the capillary stress of 13.2MPa and the oxidation stress of 71MPa for a chemical etched porous silicon sample with a certain porosity. It can be seen that the lattice mismatch between the porous layer and the Si substrate is a major source (about 91%) for the total residual stress of the porous silicon.