Papers by Author: Jin Long Chen

Abstract: In this paper, two-steps digital image correlation method is well advanced with the sub-pixel reconstruction in sub-image by utilizing the higher precision calculation of bicubic spline interpolation value method, and the accuracy of displacement is extended to better than 0.01 pixel, the strain resolution is limited to less than 0.0002 in micro-region. The experimental results show that the method of two-steps digital image correlation is an up-to-date technique to comprehensive investigations of base metal-coating composition at extremely small size scales. In the meantime, influence of crack in base metal perpendicular to interface on coating is directly observed, and the measurement of the residual compressive strain in base metal is performed according to different thickness of coatings. Lastly, the bond behavior between base metal and coating has been quantitatively analyzed, and the experimental results also prove the Ni-Lan coating is combined with base metal by toughened interface.

Abstract: Extensive research has been done on porous silicon (PS) and its applications in optoelectronics since the discovery of its light emitting properties. Porous silicon technology is also used for silicon micro machining. However, porous films can be seriously strained and this often causes mechanical curling, fracture and device failures. In the present study an optical apparatus based on substrate curvature method was developed for intrinsic stress measurement of thin films, which offered a lot of advantages as overall field, non-contact, high precision, nondestructive, easy operation and quick response. Using the apparatus, the residual stress in porous silicon layers prepared by electrochemical etching was obtained. The residual stresses in the films were determined by measuring the curvature of the Si substrate before and after etching. It is found that the residual tensile stress tends to increase with the porosity increasing and the doping concentration of the silicon wafer increasing. The results show that there is a deep connection between the microstructure PS and the residual stress distribution.

Abstract: The traditional finite element analysis method in conjunction with the atomic simulation technology was applied to study the mechanical properties of nanostructure materials. A phase mixture model in which nanocrystalline material is regarded as a mixture of crystalline phases and intercrystalline phases (grain-boundary, triple line junction and quadratic node) and pores is presented. The Morse potential function was used to simulate the nonlinear constitutive model of grain boundary phase of NC Fe. The effects of grain size and porosity were investigated in the literature. The calculated results are compared with previously published experimental data.

Abstract: The main purpose of this paper is to develop a set of improved micro-metrology system, which consists of a digital image-measuring device and a microscope with a long focal length, for the study of displacement fields, strain fields in the area across the interphases, and also the effect of temperature on the micro-mechanics properties of interphases in thermoplastic composites. Meanwhile, the precision of the micro-metrology system is assessed by using an extra solution within of the framework of infinitesimal strain in particular, the choice of the size of subset in the correlation technique is discussed when the displacements are not infinitesimal. Experimental results show that, under practical service conditions of thermo-mechanical coupled loads, a significant residual thermal stress mismatch due to the difference in coefficients of thermal expansion between the fiber and the matrix is balanced out.

Abstract: In order to solve the problem of web buckling for hot rolled I-beams without membrane subjected to concentrated load, a plastic mechanism analysis that involves consideration of the plastic hinges developing in the flanges and the yield lines forming in the web plate have been made, and a simple theoretical prediction of collapse loads have been developed. At the same time, wide ranges of finite element studies have been made. Based on the finite element results, certain approximations and empirical modifications are introduced because of the complex geometrical nature of the problem and the purpose to make the resulting design method simple enough for routine practical application. The theoretical results have been compared, and show close correlation, with the finite element results.

Abstract: When a sandwich structure is loaded in uniaxial in-plane compression the skin over the debonded region may buckle allowing further propagation of the debond, leading to structural collapse. Hence how to detect the shape and size of debonded defect, and to trace propagation trend of the defect are key factor in the process of damage evaluation of subsurface defect. In the paper, a nondestructive testing system is developed based on liquid crystal cell and Wollaston shearing prism. Nematic liquid crystal has the property of modulating the intensity and phase of light, and the intensity will not change during the phase modulating. According to this property, the phase-shifting of digital shearography are performed successfully. The experimental results prove that we can not only perform detection of the defect size and shape in subsurface of sandwich structure, but also trace the further propagation of debonded defect because of local buckle. This indicates that the method of digital shearogrephy will offer an effective measurement tool to research the buckle behavior of debond defect in subsurface of sandwich under uniaxial in-plane compression.

Abstract: The fracture properties of Plexiglass bright the attentions of the researchers as it is the import material used in aero-planes industry The white speckle technique could obtain displacement fields nondestructively under the normal environment. Compare to the laser speckle method there are no interference light source and the vibration isolation needed. In the paper the principle of the technique is described and the displacement field near crack and SIF are measured. The results show that the technique is very suitable to the application in industry.

Abstract: The behavior of thick SiC gradient coatings on different graphites under high power electron beam irradiation, pulsed high energy laser impact, and HT-7 limiter plasma action was investigated. In the following tests at these facilities, the samples were examined using SIMS, EDAX, X-ray crystallography, profilometer and Auger spectroscopy. Thick SiC gradient coatings showed excellent durability under high heat flux and in real conditions of HT-7 limiter plasma irradiation. The primary results indicated that the use of multi-element doped graphite, GBST1308 (1%B4C, 2.5%Si, 7.5%Ti) with high thermal conductivity, as substrate will improve the durability of thick SiC gradient coatings significantly.