Automated fine grid technique for measurement of large-strain deformation maps

The fine grid technique has been a standard engineering tool for measuring large strains for many years. The sample surface is marked with a grid, and the deformation of this grid allows the deformation of the sample to be monitored. However, it has never been easy quantitatively to analyse the strain across the whole of a specimen's surface. We describe here an automated approach in which digitised images of a sample prepared with a grid are analysed by the Fourier transform method. This provides phase maps which, when unwrapped, yield planes representing the two in-plane specimen coordinates. An iterative technique follows these deforming planes from one frame to the next as the specimen deforms, allowing displacement fields to be calculated. Numerical differentiation gives strains across the specimen surface. Gerchberg iteration is used to provide immunity to errors resulting from holes or tears in the specimen surface. The method is demonstrated on a propellant simulant containing burn holes (a cylinder of diameter 10 mm; grid PITCH = 76 μm), loaded in compression across a diameter. All in-plane components of strain are calculated up to strains of approximately one-third. Displacement accuracy is of order 1 μm.     

Investigation on ultrasonic volume effects: Stress superposition,acoustic softening and dynamic impact

Conventional high power ultrasonic vibration has been widely used to improve manufacturing processes like surface treatment and metal forming. Ultrasonic vibration affects material properties, leading to a flow stress reduction, which is called ultrasonic volume effect. The volume effect contains multi-mechanisms such as stress superposition due to oscillatory stress, acoustic softening by easier dislocation motion and dynamic impact leading to extra surface plastic deformation. However, most researches ignored the stress superposition for the convenience of measurement, and few studies considered ultrasonic dynamic impact since the relatively low ultrasonic energy in macro scale. The purpose of this study is to investigate the characteristics and mechanisms of different ultrasonic volume effects in micro-forming. A 60 kHz longitudinal ultrasonic-assisted compression test system was developed and a series of ultrasonic-assisted compression tests at different amplitudes on commercially pure aluminum A1100 in micro-scale were carried out combining the surface analysis by SEM, EDX and micro-hardness test. Three different ultrasonic volume effects, stress superposition, acoustic softening and dynamic impact, were confirmed in the ultrasonic-assisted compression tests. In order to quantitatively predict stress superposition, a hybrid model for stress superposition is developed considering the elastic deformation of experimental apparatus in practice, the evolution of the modeling results fitted well with the experimental results. With low ultrasonic amplitude, stress superposition and acoustic softening occurred because vibrated punch contacted with the specimen all the time during compression. However, with higher amplitude, due to the extra surface plastic deformation by larger ultrasonic energy, forming stress was further reduced by the ultrasonic dynamic impact. A possible method to distinguish the effects of dynamic impact and acoustic softening is to analyze the waveform of the oscillatory stress in the process. In the case of ultrasonic dynamic impact effect, a higher amount of oxidation was observed on the specimen surface, which could be the result of local heating by surface plastic deformation and surface friction when the vibrated punch detached from the specimen. The findings of this study provide an instructive understanding of the underlying mechanisms of volume effects in ultrasonic-assisted micro-forming.     

Camera orientation and 3D-deformation measurement by use of cross gratings

The orientation of stereo cameras in a close range environment is derived by a modified bundle block adjustment, using the cross coordinates of a shifted high quality grating. By means of the HOUGH-transform, approximate grating coordinates are determined, which are used to calculate the final coordinates fully automatically with a correlation filter technique. The stereo setup is applied to measure the 3D-deformation on the surface of a cold welded Cu---Al tension specimen supplied with a titanium dioxide cross grating.     

Speckle interferometry for measurement of continuous deformations

Improvements of a method for measurement of continuous displacements and deformations with digital phase shifting speckle pattern interferometry are presented. The method is based on an algorithm that, with the knowledge of the initial phase, only needs one image at a time to evaluate continuos phase changes due to object deformations. In the improved method, the initial random phase of the speckle pattern is evaluated using a number of phase-shifted images before the deformation under study. This is used for increasing the accuracy of the initial phase estimation and reducing influences from image noise and other measurement disturbances. The phase-shifted speckle patterns are used as references for comparison with the speckle patterns of the deformed object, thereby increasing the reliability and accuracy of the phase estimations of the deformed patterns. The technique can be used for measuring deformations such as transients and other dynamic events, heat expansion as well as other phenomena where it is difficult to accomplish phase shifting during deformation.     

Computed speckle decorrelation (CSD) for the study of fatigue damage

A video-based laser speckle technique has been developed for noncontact analysis of fatigue in situ and at speeds approaching video frame rates. This technique, computed speckle decorrelation (CSD), makes use of the speckle decorrelation associated with surface deformation. It is a method of full field inspection which both locates fatigue damage sites and measures damage severity. In its current application, CSD has been used to study the fatigue deformation progression in reverse bending fatigue of a cylindrically notched aluminum specimen. However, film based studies have shown the usefulness of laser speckle decorrelation to analyze fatigue deformation of thick graphite/epoxy composite materials as well. With the development of the CSD method it will now be possible to examine in greater detail the progression of fatigue damage in these materials, allowing a much faster and more quantitative analysis than was previously available.     

Interferometric grating method and its application in Fe-base shape memory alloy structure design

This paper presents an interferometric grating method used in measuring strain fields on a curved surface. This method can be used to determine the small and large strains with high sensitivity and has been applied successfully in Fe-base shape memory alloy (FSMA) structure design. In this study, five diffracted beam from the specimen surface produce the interferometric gratings through an optical system. Using image processing technique (fast Fourier transform with special interpolation and phase shifting technique), we have obtained the strain fields of outer surface of FSMA joint and contact pressure distribution on its inside surface which has mechanical deformation and transformation.     

A modified digital phase-shift moiré technique for impact deformation measurements

Due to the short loading times and high deformation rates inherent to impact experiments, measurement of the occurring deformations is not straightforward. Presented in this paper is a technique to obtain the displacements and deformations of a specimen subjected to a uni-axial impact load. During the experiment the deformation of a line grating attached to the specimen is captured using a streak camera. From the recorded deforming grating the specimen displacements are automatically derived using an advanced numerical algorithm, based on the interference between the specimen grating and a virtual reference grating. Numerical interference is considered because it allows that the pitch of the reference grating is adapted to the changing amplitude of the deformation. Indeed, at each moment of the deformation process the pitch of the reference grating is chosen such that the highest possible accuracy and sensitivity is guaranteed. Because of this, large changes in deformation amplitude are allowed, and the technique is applicable to a wide range of materials. Eventual imperfections of the specimen grating and temperature effects are taken into account. Specimen displacements are extracted automatically by means of a phase-shifting technique.

The non-contact measurement technique yields high resolution, quantitative information on the specimen deformation, along the entire length of the specimen and during the full duration of the experiment. Interaction by the operator is excluded. Results are presented of a high strain rate tensile test on a steel sheet specimen showing local deformations up to about 170%.     

耐高温光刻胶光栅模板的制作与转移技术及其应用

作为试件变形传感元件的云纹光栅的制作是云纹干涉法的关键技术。本文提出了在１２０～２００℃高温下使用的光刻胶光栅模板的制作及在此温度下试件栅的复制工艺，并利用此技术研究了新型微电子封装组件在１２５～２０℃温度载荷下的热变形。实验结果表明：采用该方法可以获得高质量的试件栅，云纹条纹质量好，可用于试件微小区域内热变形的精确测量     

NDT&E using shearography with impulsive thermal stressing and clustering phase extraction

Shearography has been widely adopted for nondestructive testing and evaluation of various materials, especially in the rubber industry and aerospace industry. It detects flaws and defects by identifying deformation anomalies when the specimen is stressed by a certain means. Conventional stressing methods for shearography include pressurization, mechanical or acoustic loading and vibration excitation. These stressing techniques are favorably applied in various applications. In this study, we propose a novel impulsive thermal stressing method using high-power flash lamps for convenient nondestructive testing and evaluation in both laboratory and industrial environment. The proposed technique employs a high-energy heat flux to excite the specimen and detects the thermal deformation anomaly using a shearographic setup. By incorporating a novel clustering phase extraction method, the movement of the continuously deforming object is obtained using only one single deformed speckle image at each deformed stage, thus enabling both qualitative and quantitative measurement. Experiments conducted on various samples with cracks and debonds demonstrate the practicability of proposed technique for both laboratory and industrial applications.     

Phase shifting nano-moiré method with scanning tunneling microscope

In this paper, a new nano-moiré method using scanning tunneling microscope (STM) is proposed. This method is capable of measuring nanoscopic deformation of matter. The formation mechanism of the STM moiré fringe and the phase shifting technique used in STM moiré fringes are explained in details. Typical experiments are conducted with the crystal lattices of freshly cleaved highly oriented pyrolytic graphite, are used as specimen grating, to generate STM moiré fringe patterns. Phase shifting is realized in four steps from 0 to 2π by controlling the PZT in the STM system to shift the specimen in the vertical direction. This method provides a new way for disposal of moiré fringes pattern in the nano-moiré measurement.     

基于摄影测量校正的斜光轴数字图像相关方法

提出一种针对单相机、斜光轴测量条件下,基于摄影测量校正的二维数字图像相关法(DIC).变形前,在被测物表面粘贴不少于3个控制点(圆形标志点),通过摄影测量技术得到控制点的三维坐标,利用控制点的三维坐标及其在图像上对应的二维坐标,通过空间后方交会法得到斜光轴相机的三维空间方位,拟合控制点的三维坐标得到控制面.变形过程中,...     

Analysis of coherent gradient sensing (CGS) by fourier optics

A coherent laser beam reflected or transmitted from a deforming plate specimen acquires an optical path difference (phase change) which is related to the deformation and stress field. The optical method of Coherent Gradient Sensing (CGS) uses two parallel grating plates to displace (shear) and recombine the distorted light beam emerging from the specimen. Fringes are produced on the image plane by the interference of the shifted and unshifted beams. The fringe pattern is related to the spatial difference of the phase in the shearing direction. If the shearing distance is small enough, the spatial difference of the phase approximates to its gradient. Each fringe in the image represents a locus of equal gradient component of the phase in the shearing direction. The technique is interpreted by means of wave optics, using the Fraunhofer approximation and the paraxial theory of lenses. The assumptions made in earlier analyses have been removed here. A more precise analysis based on Fourier optics is presented. The simplicity of the optical setup and variable resolution of the technique have led to its frequent use in the area of solid mechanics, including fracture mechanics. Some examples are discussed in the second half of this paper.     

Dynamic characterization of MEMS diaphragm using time averaged in-line digital holography

In this paper dynamic characterization of a MEMS diaphragm is investigated using lens-less time averaged in-line digital holography. The analysis and capability of the numerically reconstructed amplitude and phase information from in-line time averaged holograms as applied to MEMS vibration are presented. Particularly the effect of mean static state on the phase in time averaged digital holography is explored. A novel double exposure method is also demonstrated using a diverging object wave suitable for dynamic characterization of small size objects. A phase jump in the static deformation fringes in the vibrating regions is observed and described and can be used for precise analysis of vibration mode shape under simultaneous presence of mean static deformation. A simple and robust tool for dynamic optical metrology of MEMS devices and micro-objects using time averaged in-line digital holography is thus proposed.     

Numerical correction of optical vortex using a wrapped phase map analysis algorithm

We demonstrate experimentally a technique for the numerical correction of an optical vortex with a unitary topological charge. A developed algorithm based on the axial behavior of a reconstructed wavefront is used in the detection of the optical vortex. Optimizations of the number of axial phase maps and the window size used in the algorithm yield the precise coordinates of the vortex eye. The obtained coordinates and vortex handedness are used in designing a proper filter, facilitating numerical correction of the vortex phase map. The developed algorithm can be applied to absolute phase and phase difference maps obtained through any reconstruction method.     

Visualization and interferometry for cylindrical and spherical sample tomography and profilometry

A technique combining image processing and laser interferometry for visualizing and detecting the deformation of transparent cylindrical and spherical sample is proposed. This deformation includes geometric deformation such as volume transition in profilometry and physical deformation such as refractive index change in tomography. Phase contour lines are used for quantitative analysis and graphical representation of the deformation. This method allows us to visually detect the spatial variation of the deformation field and to evaluate the test quality such as misalignment of optical system. A theoretical analysis using phase contour map to characterize the deformation field is described in detail. A method using phase contour map to qualify the interferometric test is proposed. Analysis of test examples is carried out. Suggestions on using phase contour line method to ameliorate test system design are finally discussed.     

A novel technique for measuring 3D deformation of adhesively bonded single lap joint

An easy-to-implement yet practical single-camera microscopic stereo-digital image correlation(stereo-DIC) technique is proposed for surface three-dimensional(3D) deformation measurement of singe lap joint(SLJ) samples subjected to mechanical loads. The basic principles, optical configurations and implementation procedures of the proposed technique are described in detail. Compared with existing single-camera 2D-DIC technique, which has been regularly used for in-plane deformation measurement of a SLJ specimen, the proposed technique offers the special merit of simultaneously determining all the three displacement components by simply adding two additional optical elements to existing single-camera 2D-DIC systems. The accuracy and effectiveness of the proposed technique is demonstrated by measuring the 3D deformation of a SLJ specimen subjected to quasi-static tensile loads.     

基于激光技术的压痕应变花及测试应变的原理

介绍了一种用于测试面内三个应变分量的应变测量技术,在试件表面预先压制三个压痕,通过激光照射压痕,经衍射后干涉,产生干涉图。通过分析变形前后干涉斑点的光强值的变化来对待测位置进行应变测量。实验表明该方法对待测试件表面损伤小,且压痕应变花尺寸小,是一种新的光学应变测试技术,不仅可以用来进行普通应变测量,尤其对解决小空间应变测试问题有其独特的优点。     

薄板弯曲大变形高阶非线性偏微分方程推导与优化算法研究

针对薄板弯曲大变形问题, 运用变分原理, 建立了薄板弯曲大变形问题的高阶非线性偏微分方程. 运用有限差分法和动态设计变量优化算法原理, 以离散坐标点的上未知挠度为设计变量, 以离散坐标点的差分方程组构建目标函数, 提出了薄板弯曲大变形挠度求解的动态设计变量优化算法, 编制了相应的优化求解程序. 分析了具有固定边界、均布载荷下的矩形薄板挠度的典型算例. 通过与有限元的结果对比, 表明了本文求解算法的有效性和精确性, 提供了直接求解实际工程问题的基础.     

In-situ neutron diffraction study of phase stress evolutions in a Ni-based porous anode solid oxide fuel cells under uniaxial load

Ni/YSZ porous composite is used widely as anode for solid oxide fuel cells. In this study, neutron diffraction patterns were recorded in-situ while a bulk porous Ni–NiO–YSZ anode was loaded under uniaxial compression. Single peak refinement was used to calculate the lattice strains of each phase in the composite, and the local stress state of each phase was derived from the measured lattice strains and the corresponding diffraction elastic constants. An internal triaxial stress state was observed to develop in the bulk of the specimen under plastic deformation, specifically in the Ni phase. Meanwhile, the NiO and YSZ phases are deforming elastically even in the macroscopically plastic regime of deformation. The von Mises equivalent stress was used to quantify the phase stress evolution. A significant stress concentration induced by the presence of pores becomes manifest in all three phase components. The reduction of stress concentration factor in Ni above the yield point of the composite can be attributed to a gradual change of the grains–pores morphology during the plastic deformation.