nan Li

Investigation of the mechanical bending and frequency shift induced by adsorption and temperature using micro- and nanocantilever sensors

The marked progress in MEMS/NEMS technology has demanded the development of a fundamental understanding of cantilever-based sensing principles. One of the challenges of cantilever-based detection is identifying and discerning the most influenced parameters responsible for the observed changes in the cantilever response. For example, effects of various force fields such as those induced by atom/molecular adsorption and variations in temperature may occur simultaneously, increasing the number of parameters that need to be concurrently measured to ensure the reliability of sensors. In this paper, we, therefore, systematically investigate the interplay between these two distinctly different mechanisms and attendant mechanical response. To this end, a theory model is proposed to predict the mechanical bending and resonance frequency shift of micro- and nanocantilevers taking into account atom/molecular adsorption and variations in temperature at the same time. The adsorption induced mechanical responses of mic...

Understanding the effects of strain on morphological instabilities of a nanoscale island during heteroepitaxial growth

A comprehensive morphological stability analysis of a nanoscale circular island during heteroepitaxial growth is presented based on continuum elasticity theory. The interplay between kinetic and thermodynamic mechanisms is revealed by including strain-related kinetic processes. In the kinetic regime, the Burton-Cabrera-Frank model is adopted to describe the growth front of the island. Together with kinetic boundary conditions, various kinetic processes including deposition flow, adatom diffusion, attachment-detachment kinetics, and the Ehrlich-Schwoebel barrier can be taken into account at the same time. In the thermodynamic regime, line tension, surface energy, and elastic energy are considered. As the strain relief in the early stages of heteroepitaxy is more complicated than commonly suggested by simple consideration of lattice mismatch, we also investigate the effects of external applied strain and elastic response due to perturbations on the island shape evolution. The analytical expressions for elas...

Buckling propagation of thin films on elastic substrates under alternating load

Thin metal films deposited on elastic substrates are widely used as electrodes and interconnects in Micro-Electro-Mechanical Systems (MEMS) and their fatigue property needs to be sufficiently understood. In this paper, the buckling propagation of nano-thin film relying on a substrate under alternating load has been experimentally studied. The effect of loading condition on buckling propagation is discussed in terms of the frequency, film thickness and number of cycles. In particular, buckling growth rate has been analyzed by means of digital image processing technology. Moreover, two kinds of samples have been designed to compare the influence of different materials.

Application of digital speckle correlation method to investigate a size effect in micro-indentations

An evaluation technique, which digital speckle image correlation method (DSCM) for displacement measure on the surface of the tested object pressed by an indenter, is proposed for the first time in this paper. The size effect in micro-indentation is discussed in detail based on calculating surface plastic deformation modes around indentation by DSCM, comparing hardness measured using three micro-indentation apparatus, and simulating piling-up and sinking-in of 45 carbon steel around conical indentation in finite element. We draw a conclusion that the indentation size effect relates to these factors such as load, test method, geometrical similarity of indentations, geometrical form of indenter tip as well as friction between indenter tip and surface of tested object. The finite element simulation result for 45 carbon steel revealed that the sinking-in occurs at the edge of imprints after unloading, no matter how the magnitude of the load is. In fact, the result obtained based on calculating surface plastic deformation modes around indentation by DSCM indicates that the property of deformation relates to load. When the load decreases from 1 N to 0.5 N, the surface deformation mode around indentation changes along with the load, contrary to the present research which neglects the load when considering deformation around indentation in finite element simulation. DSCM provides the technical support for probing into the essence of the indentation size effects.

Measurements of interlaminar deformation in the composite laminates

In this paper, an experiment is executed to analyze the displacement distribution of 56-ply composite laminates under three-point bending. In the experiment, the displacement fields on the two free edges of the laminated composite are measured by using the moire interferometry method and natural micro-image processing technique based on wavelet transform at the same time. In the first part of this study, a two-beam traditional moire interferometry experiment has been carried out. And in the second part, we present a new image processing method, which combines the theory of correlation and wavelet transform technique.

New method in experimental damage study of AS4/PEEK composite laminates

AS4/PEEK [0/+/- 45/90]7s composite laminates were subjected to a bending moment. We have obtained the displacement fields by moire interferometry with the aid of Long Distance Microscope. Based on wavelet analysis, the digital moire images were processed and analyzed. The special algorithms of unconstrained nonlinear minimization and the scale parameter were used to eliminate the influence of the noise in the digital image processing. Tension tests were carried out on AS4/PEEK [ 0/+/- 45/90 ]2s composites. When AS4/PEEK composite laminates were deformed, the images of natural surface of the AS4/PEEK specimen were recorded to study the damage initiation and growth. The experimental results show that the considerable interlaminal stresses between different plies, and these places were more prone to delaminate. The paper presents the microscopic damages and matrix crack growth observed by an optical microscope and the CCD camera.

Experimental study on surface wrinkling of silicon monoxide film on compliant substrate under thermally induced loads

The wrinkling of a silicon monoxide thin film on a compliant poly(dimethylsiloxane) (PDMS) substrate structure was experimentally investigated in this study. The self-expansion effect of PDMS during film deposition was utilized to impose a pretensile strain on the structure through a specially made fixture. A laser scanning confocal microscope (LSCM) system with an in situ heating stage was employed for the real-time measurement. The Youngs modulus of the silicon monoxide thin film as well as the PDMS substrate was measured on the basis of the elasticity theory. Moreover, the effects of temperature variations on geometric parameters in the postbuckling state, such as wavelength and amplitude, were analyzed. It was proved that wavelength is relatively immune to thermal loads, while amplitude is much more sensitive.

Inflation of stressed cylindrical tubes: an experimental study

The inflation of an initially stressed cylindrical shell provides a good illustration of the phenomenon of the initiation and propagation of an instability, which shares the same mathematical and mechanical features with a variety of other strain localization phenomena in engineering structures and materials. The high speed CCD camera and digital image processing system were used to measure the 3D shape of the inflated cylindrical tube. The localized bulge of a cylindrical tube with closed ends forms when the internal pressure reaches a critical value Pcr. As more air is filled into the tube, the pressure drops but the radius at the centre of the bulge will increase until it reaches a maximum value rmax. With continued inflation, the pressure stays at a constant value Pp. The purpose of this study is to investigate the critical and propagation pressures in the tubes and the profile outside when the shells under axial tension and internal pressure were inflating. We focus on the influence of the axial tension on the critical pressure. In this paper the problem is explored through experimental efforts. A series of experiments were conducted on commercially available natural rubber latex tubes involving different geometries and initial axial tensions, which were regarded as isotropic, homogeneous, incompressible and hyper-elastic materials.

Dimensional coordinate measurements: application in characterizing cervical spine motion

Cervical spine as a complicated part in the human body, the form of its movement is diverse. The movements of the segments of vertebrae are three-dimensional, and it is reflected in the changes of the angle between two joint and the displacement in different directions. Under normal conditions, cervical can flex, extend, lateral flex and rotate. For there is no relative motion between measuring marks fixed on one segment of cervical vertebra, the cervical vertebrae with three marked points can be seen as a body. Body’s motion in space can be decomposed into translational movement and rotational movement around a base point .This study concerns the calculation of dimensional coordinate of the marked points pasted to the human body’s cervical spine by an optical method. Afterward, these measures will allow the calculation of motion parameters for every spine segment. For this study, we choose a three-dimensional measurement method based on binocular stereo vision. The object with marked points is placed in front of the CCD camera. Through each shot, we will get there two parallax images taken from different cameras. According to the principle of binocular vision we can be realized three-dimensional measurements. Cameras are erected parallelly. This paper describes the layout of experimental system and a mathematical model to get the coordinates.

Initiation of nano-film bucklings under low-frequency alternate loads

In this paper, we investigated the initiation of the straight-side buckling of the compressed thin film on flat PMMA substrate under alternate loads, with emphasis on the influences of loading frequency, film thickness and load amplitude on the initiation of the buckling, respectively. A fatigue-loading device based on piezoelectric ceramics has been designed for making the cyclic loadings available. The instability phenomenon has been recorded using a CCD camera and an optical microscope. The aluminum and copper thin films were studied in the experiments, the different loading modes have been involved in detailed experimental analysis. At present, experimental investigations of thin film buckling under cyclic load may be an efficient engineering solution in many practical fields such as micro-electro-mechanical systems(MEMS).