To promote the application of sampling moiré method, in this study, the determination of the moiré spacing in sampling moiré method is studied in details. The ordinary equation for determining the sampling moiré spacing is found to be inapplicable in some cases, then a new equation is derived based on the generation mechanism of sampling moiré. It is found that the ordinary equation is a particular form of the new equation in a specific range. Besides, simulation tests demonstrate that the new equation has a wide application range.
Additive manufacturing (AM) is a rapidly developing technique which presents both substantial opportunities and challenges for manufacturing industry. However, the internal defects will influence the mechanical behavior of AM structures, and thus characterization of the defects of AM materials becomes an interesting issue. In this study, a non-destructive testing technique for AM structures is proposed based on the reflective coherent gradient sensing (CGS) method. Using this method, the slope of a reflective surface can be measured accurately. In the study, the measured structure is firstly prepared a reflective film using replication technique, and then is exerted an external load. The full filed slope of the structure can be measured using CGS and will show a turbulence which can be used to determine the location of the defects of structure (pores or cracks). Using this technique, the spatial resolution of defect size can reach 200μm. From the experimental results, the method shows the merits of full-field, non-contact and real-time measurement. The successful experimental results show that reflective CGS method is effective for detecting the internal defects of AM structures, and will have a good prospect of further application.
Deformation measurement at macro/microscale can be fulfilled by a variety of methods with the development of experimental measuring techniques. However, the measurement of free-standing thin films is still difficult, especially for full-field measurement. The difficulty is the fabrication of the deformation markers (such as gratings or speckles) on the free-standing thin films. A grating fabrication technique based on focused ion beam (FIB) deposition is introduced in our previous works, and the superiority of this technique is minimal damage to the surface of the specimen. We adopted this FIB deposition technique to fabricate a frequency of 2000 line/mm gratings on the free-standing metallic films with a thickness of 2 μm. Notches of 10 μm length and 2 μm width are prepared by the FIB etching at the edge of the films. An in situ tensile test is operated in the scanning electronic microscope system. At the crack initiation stage, the displacement field is measured from the deformation of the gratings. The fracture parameter J integral is characterized for the 2-μm metallic films. The results demonstrate that the full-field measurement of the free-standing thin films at microscale can be realized successfully by this method.
A phase shifting method was developed for Coherent Gradient Sensing (CGS) using a three-step phase shifting method. Three different inteferograms were obtained by changing the distance between two gratings. The phase filed can be calculated accurately from the three inteferograms. The interference fringes (phase field) in reflection mode represent the gradient contours of the out-of-plane displacement of a surface. The curvatures and shape of the surface both can be calculated by numerical methods using the fringe patterns. The measurement principle and experimental setup were introduced in detail. As an application, a standard specimen with a curvature radius of 5 m was measured. From the analysis of the experimental results, we find that the relative error of the curvature using this method was about 0.78%. The method has good potentials for measuring the slopes, curvatures and shapes of thin film/substrate systems.
In recent years, inverse moiré methods have been developed to reconstruct micro/nano-scale planar periodic structures with a larger field of view than those constructed using conventional methods. In these methods, moiré fringes generated by superposition of the periodic structure and a reference grating are analyzed to reconstruct the periodic structure. There are two approaches to inverse moiré methods: the fringe-centerlines method and the phase-shifting method. The former has lower accuracy and is difficult to automate, while the latter requires at least three moiré images with complicated processing. A reconstruction method for planar periodic structures using Fourier analysis is proposed. This method can be used to characterize the micro/nano periodic structure from a single microscope moiré pattern. At the same time, when combined with a linewidth characterization method, the period and linewidth of the microstructure can be obtained simultaneously. As practical examples, the period and linewidth of a scanning electron microscopy raster are calibrated. Then the microstructures of a micro-electroformed grating and a butterfly wing are reconstructed using the calibrated system. The proposed method provides a tool for the characterization of large area micro/nano periodic structures. Further, this is a promising approach to detect defects in periodic structures.
The rapid development of micro-electronics and micro-nano material engineering make it an urgent task to characterize the mechanical properties of micro-device and micro-nano material accurately. Due to the advantages of high precision, high sensitivity and full field measurement, moiré method has been applied in the micro-deformation measurement widely. Since the grating is the indispensable deformation sensor of moiré method, how to fabricate high frequency grating with high quality is the key problem to solve for moiré method. In this paper, some fabrication techniques developed recently with their applications will be summarized, including holographic photolithography, electron beam lithography (EBL), focused ion beam (FIB) and nano-imprint lithography(NIL), aiming to popularize the applications of moiré method in the micro-deformation measurement and provide some valuable guidelines on how to choose a proper fabrication technique.
KEYWORDS: Calibration, Microscopes, Fourier transforms, Scanning electron microscopy, Image processing, Optical engineering, Mechanics, Electron microscopes, Optical microscopes, Chemical species
Microscopes are widely applied in characterizing feature sizes at the micro-/nanoscale, and magnification calibration plays a key role in achieving precise measurements. However, it is difficult to obtain accurate results by using the general magnification calibration method if comparing the displayed size of a test-piece under microscope and its original one. In this study, a high-accuracy and automatic magnification calibration method that could be applied to different types of microscopes is proposed. A standard grating is employed as the reference, and a high-resolution discrete Fourier transform is used to analyze the images captured under various magnifications in this method. With utilization of the high-order harmonic component in the Fourier spectrum, the proposed method is capable of performing the calibration over a wide range of magnifications while maintaining identical precision. The relative error of the proposed method can be theoretically limited to 0.01%; moreover, the image noise can be tolerated. Furthermore, the validation and extensive adaptability of this method are demonstrated by calibrating the magnification of a scanning electron microscope and an optical microscope.
Elastic constant both on along-fiber (AF) and cross-fiber (CF) direction of two kinds of bamboo charcoal are measured
using digital speckle correlation method (DSCM). Fracture toughness of two kinds of SiC which consist of bamboo
charcoal are analyzed by using DSCM and conventional Loading method. The experiment shows that the results
measured by DSCM are in accordance with the results measured by using conventional Loading method. Moreover, we
can conclude from the results that bamboo charcoal is a kind of anisotropic material, SiC remains the anisotropic
property of bamboo charcoal.
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.
On the basis of the mechanically scratching using the Atomic Force Microscope, this paper proposes a new method for manufacturing high frequency grating. The grating was fabricated on a polycarbonate compact disc with a silicon AFM tip under the contact mode. The fabrication technique and the optimization of parameters for the technique are discussed in detail. From the experiment, the minimum spacing of the grating can reach 30 nm. The digital nano-moire patterns verify that the grating has good potential to be applied to the nano-deformation measurement.
One effect of spatially sampling of an image is averaging the image intensity over each pixel. In this paper, this spatial average effect on phase-shifting detection of fringe pattern is investigated. The phase error due to pixel size and sampling rate is formulated. It is shown that lower sampling frequency, for example, 1/4 fringes per pixel for 1-D fringe signal, 1/6 fringes per pixel for 2-D signal, will cause larger measurement error.
In this paper, some novel micro/nano- moire grating fabricating techniques are introduced. The gratings are produced by the SPM lithography, FIB lithography, and molecular beam epitaxy (MBE) method. The moire patterns formed with these gratings are also introduced. The gratings are successfully to be used to measure the residual deformation in the surface around a step edge of the Al/Si artificial nanocluster with the moire methods. The successful experimental results verify the feasibility of these methods.
Fringe detection technique is discussed with information theory and signal processing approaches. Uncertainty principle for fringe signal detection is set up. Its application on the effect of signal length, phase measurement, and strain or slope measurement is discussed. The relationship between the accuracy and spatial resolution for measurand is investigated.
Two novel micro/nano moire method, SEM scanning moiré and AFM scanning moire techniques are discussed in this paper. The principle and applications of two scanning moire methods are described in detail. The residual deformation in a polysilicon MEMS cantilever structure with a 5000 lines/mm grating after removing the SiO2 sacrificial layer is accurately measured by SEM scanning moire method. While AFM scanning moire method is used to detect thermal deformation of electronic package components, and formation of nano-moire on a freshly cleaved mica crystal. Experimental results demonstrate the feasibility of these two moire methods, and also show they are effective methods to measure the deformation from micron to nano-scales.
A new moire interferometer, known as multifunction 3D function interferometry system is developed by authors, which combines the advantages of four-beam moire interferometer with the Twyman/Green interference system. The system can measure in-plane and out-of-plane deformation synchronously with high accuracy and high sensitivity. A compact phase shifter is used to improve the measurement sensitivity. The optical theory, components, key techniques, unique ability, and specifications of the system are described in detail in this paper. Some typical experiments using this system have been done and gained good results.
This paper introduces how to in situ observe fracture behavior aroung a crack tip in ferroelectric ceramics under combined electromechancial loading by use of a moire interferometry technique. The deformation field induced by electric field and stress concentration near the crack tip in three-points bending experiments was measured. By analyzing the moire interferometry images it is found that under a constant mechanical load, an electical field has no effect on crack extension in the case that the directions of the poling, electric field and crack extension are perpendicular to each other. When the poling direction is parallel to the crack extension direction and perpendicular to the electric field, strain decreases faster than values predicted by the theoretical analysis as the distance away from the crack tip increases. In addition, as the electric field raises the strain near the crack tip increase, and the strain concentration phenomena becomes more significant.
A new focused ion beam (FIB) miore method is proposed to measure the in-plane deformation of object in a micrometer scale. The FIB moire is generated by the interference bewteen a prepared specimen grating and FIB raster scan lines. The principle of the FIB moire is described. Several specimen gratings with 0.14 and 0.20 micron spacing are used to generate FIB miore patterns. The FIB moire method is successfully used to measure the residual deformation in a MEMS structure after removing the SiO2 sacrificial layer with a 5000 lines/mm grating. The results demonstrate the feasibility of this method.
A new phase shifting SEM scanning moire method is proposed in this paper. The phase shifting technique is realized in four steps from 0 to 2 π by shifting electron beam in y-axis direction controlled by SEM system. It is successfully applied to measure the virtual strain of a MEMS structure with grating of 5000 lines/mm. The experiments prove the technique can be widely used in meso-deformation measurement, and also show the sensitivity of experiments is highly improved after phase shifting technique. It provides a new way for disposal of fringes patterns in sub-micro moire method.
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