利用光学干涉条纹图相位分析测量纳米位移

Optical methods such as Twyman-Green interferometry, moiré interferometry, holographic interferometry and speckle interferometry are useful to measure displacement and strain in the full-field of structures. Recently phase analysis method of fringe patterns obtained by these optical methods becomes popular, and it provides accurate quantitative results in the full-field. In this paper, in order to measure displacement and strain, real-time or high-speed nano-meter displacement measurement methods developed by the authors are introduced. That is, (1) out-of-plane displacement analysis by Twyman-Green interferometry using integrated phase-shifting method using Fourier transform phase-shifting method, (2) simultaneous two-dimensional in-plane displacement analysis by moiré interferometry and (3) out-of-plane displacement analysis by phase-shifting digital holographic interferometry. The theories and applications are shown.     

The principle and application of sticking film moiré internnferometry

This paper presents a new method for obtaining a high-resolution one-dimensional or two-dimensional moiré pattern on a specimen. The technique uses a holographic film that adheres to the specimen. The film is twice exposed by a virtual holographic grating, then it is removed from the specimen, devellped, fixed, bleached, and illuminated. This process produces clearly visible moiré patterns. The method is used to measure elastic modulus values of some materials, deflection and longitudinal displacement of a beam, stress-intensity factors (SIF), crack-opening displacement (COD,J integral, and a dynamic deformation. This paper explains the principles of the method, derives two displacement equations of two-dimensional moiré interferometry, and discusses the reliability, range of application, and measuring precission of the new method.     

Investigation of Impact Response of Composites with Projection Moiré Enhancement

As a method for measuring full-field out-of-plane displacement, projection moiré provides high measuring quality with simple experimental setup. Based on an image processing program developed by Heredia and Patterson, this article presents the implementation of projection moiré in low-velocity impact testing. Results from projection moiré agree reasonably well with those obtained from the commonly used load cell method. In an attempt to better document composite response to impact loading, the possibility of correlating the external out-of-plane displacement measurement with the internal delamination is discussed. The technique is further applied to composites with various microstructures, including laminated, two-dimensional woven and quasi-three-dimensional (Q3D) woven composites. Projection moiré is able to provide some insights of the delamination propagation of the composites under impact loading.     

Determination of the full-field stress and displacement using photoelasticity and sampling moiré method in a 3D-printed model

The quantitative characterization of the full-field stress and displacement is significant for analyzing the failure and instability of engineering materials. Various optical measurement techniques such as photoelasticity, moiré and digital image correlation methods have been developed to achieve this goal. However, these methods are difficult to incorporate to determine the stress and displacement fields simultaneously because the tested models must contain particles and grating for displacement measurement; however, these elements will disturb the light passing through the tested models using photoelasticity. In this study, by combining photoelasticity and the sampling moiré method, we developed a method to determine the stress and displacement fields simultaneously in a three-dimensional (3D)-printed photoelastic model with orthogonal grating. Then, the full-field stress was determined by analyzing 10 photoelastic patterns, and the displacement fields were calculated using the sampling moiré method. The results indicate that the developed method can simultaneously determine the stress and displacement fields.     

A multilevel treatment of moiré fringe data using finite elements

Finite element procedures are applied to the modeling, analysis and visualization of experimental moiré data. Smoothing elements are introduced and evaluated with respect to data sparseness and error. A one-dimensional smoothing element is uniquely coupled with the method of principal curves to extract moiré fringe centers. A two-dimensional smoothing element is then used to produce a full-field representation given the fringe locations. The moiré technique is applied to the four-point bend experiment, and the surface-modeling technique is used to obtain displacement and gradient (strain) information.     

Differentiation of holographic-moiré patterns

Partial derivatives of displacement are optically determined by superimposing two shifted holographic reconstructions which correspond to single components of displacement. Two different methods are proposed, one of which involves shifting a holographic-moiré pattern on itself. A second method takes advantage of holographic multiplexing techniques to incorporate an initial pattern into the shifting process. The latter facilitates the formation of the moiré corresponding to strain information. This paper demonstrates that either method of obtaining derivatives is feasible by documenting an investigation of a disk subjected to diametral compression.     

In-plane shear testing of graphite-woven fabric composites

The losipescu shear test method was used to determine the in-plane shear response of AS4 and Celion carbon fiber/epoxy fabric composite materials. Several weave architectures were studied: AS4 uniweave, AS4 and Celion plain weaves, Celion 5-harness and 8-harness satin weaves. Specimens were tested using traditional strain gage techniques and full-field moiré interferometry. A full-node localized hybrid analysis is introduced to perform efficient reduction of moiré data, producing whole-field strain distributions in the specimen test section. It was found that the fabric yarn size greatly influenced the uniformity of the shear field in the specimen test section. However, consistent shear moduli still can be obtained using the modified losipescu specimen and Wyoming fixture except for fabrics with large fiber yarns.     

Automated fringe pattern analysis for moiré interferometry

The theory of an automated fringe-pattern analysis using temporal phase stepping method for moiré interferometry is presented. The automated method provides a fast and accurate strain measurement for moiré interferometry. Measurements on a tapered composite specimen with a dropped ply are shown to highlight the capability of this technique to obtain the strain distribution, particularly around the region with a dropped ply.     

An Experimental Technique for the Dynamic Characterization of Soft Complex Materials

We describe an experimental technique to study the dynamic behavior of complex soft materials, based on high-speed microscopic imaging and direct measurements of dynamic forces and deformations. The setup includes high sensitivity dynamic displacement measurements based on geometric moiré interferometry and high-speed imaging for in-situ, full-field visualization of the complex micro-scale dynamic deformations. The method allows extracting dynamic stress-strain profiles both from the moiré interferometry and from the high-speed microscopic imaging. We discuss the advantages of using these two complementing components concurrently. We use this technique to study the dynamic response of vertically aligned carbon nanotube foams subjected to impact loadings at variable deformation rates. The same technique can be used to study other micro-structured materials and complex hierarchical structures.     

Recent research activities in experimental mechanics in China

A brief review is presented of the recent activities in the field of experimental mechanics in the People's Republic of China. The current research work covers the following subjects: (1) photoelastic phenomena, such as the classical three-dimensional photoelasticity, the scattered-light technique, birefringent coatings, birefringent materials; (2) holography, holographic interferometry, speckle interferometry and their applications; (3) moiré method; (4) strain-gage techniques and strain indicators.     

Inverse method to determine elastic constants using a circular disk and moiré interferometry

We propose an inverse method, using a circular disk in diametrical compression, for the simultaneous determination of two elastic constants,E andv, from a single displacement map. Moiré interferometry combined with the phase-shifting technique provides a full-field displacement field. An overdeterministic approach using the least-squares method is implemented to fit the experimentally determined displacements to the theoretical solution. An implementation guideline is provided, considering the effects of accidental rigid-body motions, random noise and imperfect position of the origin. Accuracy and repeatability of the proposed method are verified experimentally.     

红宝石脉冲激光动态光力学方法研究与进展

本文提出了瞬态图像采集技术和电磁脉冲加载装置。应用红宝石脉冲激光光源,介绍了动态全息光弹性、动态散斑干涉、动态云纹干涉、水下爆炸全息干涉、动态电子散斑干涉和动态数字散斑相关等多种方法的测试原理和测试技术。为动态位移场、应变场和应力场的定量分析提供了广泛的应用前景。     

红宝石脉冲激光动态光力学方法研究与进展

本文提出了瞬态图像采集技术和电磁脉冲加载装置。应用红宝石脉冲激光光源,介绍了动态全息光弹性、动态散斑干涉、动态云纹干涉、水下爆炸全息干涉、动态电子散斑干涉和动态数字散斑相关等多种方法的测试原理和测试技术。为动态位移场、应变场和应力场的定量分析提供了广泛的应用前景。     

The use of automated projection interferometry in monitoring aerofoil buckling

Projection moiré is frequently used to examine the out-of-plane displacement of thin-walled shells during buckling. One way of implementing this technique is to use double exposure photography to superimpose the initial and deformed images of a grating projected onto the surface of the specimen. This generates a pattern of fringes representative of points of equal displacement, thus presenting a snapshot of the full-field buckling behavior. This paper outlines a technique to extend this method to provide a computer generated real-time fringe pattern throughout the whole buckling and post-buckling process. This is achieved by using a CCD camera and specially developed processing software to continuously superimpose the initial image of the grating (i.e., the first frame of the captured video) onto subsequent frames in which this grating is deformed due to the displacement. This method produces series of fringes in digital format, which are ideal for further processing.     

A localized hybrid method of stress analysis: A combination of moiré interferometry and FEM

A method of combining moiré interferometry and the finite-element method to effect localized stress analysis is presented. The displacement data from local regions of interest in the optical experiment are used as boundary conditions for the finite-element stress analysis. The stability of the method is examined with data from simple numerical models one of which corresponded to the stress analysis of a pin-loaded plate with friction. These studies show that the method requires the sensivity of moiré interferometry for successful implementation, i.e., displacement data accuracy within 0.1 μm or 4 μin. This localized hybrid method of stress analysis provides a powerful and efficient method for the reduction of experimental data.     

Residual strain measurement in composites using the cure-referencing method

This paper describes the details of a novel procedure called the cure-referencing method (CRM) to measure the strains associated with residual stresses on the surface of composite panels. The CRM involves the replication of diffraction gratings onto the surface of composite specimens during the autoclave during cycle. Residual strains associated with the curing process are measured using moiré interferometry at room temperature after the specimens have been taken out of the autoclave. The procedures for both the grating replication and the moiré interferometry experiment are described in detail. A method of high-temperature moiré interferometry was developed to resolve the residual strains due to thermal expansion from those due to chemical matrix shrinkage and stress relaxation. These procedures are demonstrated on unidirectional and multidirectional laminates and on woven textile composites.     

Comparative holographic interferometry: A nondestructive inspection system for detection of flaws

Comparative holographic interferometry addresses a key problem of nondestructive evaluation: comparison of the resistance to strength in real time of two nominally identical specimens. After outlining the basis of comparative holography in flaw detection, the present paper reports on some new and complementary developments in comparative holographic interferometry. The proposed systems stand out by their ability to store the master displacement field in the interferometric setup. The integration of the storage capacity in the instrument considerably augments its potential in nondestructive routine inspection tasks. The visual display of the fringes contouring the difference in mechanical response is shown to improve considerably with the addition of auxiliary phase difference satisfying certain conditions. Methods for the generation of corresponding fringes are considered and their localization investigated in brief. Particular attention is devoted to the formation of the holographic moiré fringes. The influence of system misalignment on the moiré fringe interpretation is examined. A potential application of comparative holography to the quantitative evaluation of fatigue is described. Experimental evidence supporting the operational feasibility of the technique along with the results obtained in application to flaw detection are finally presented.     

Determining fracture parameters with full-field optical methods

In order to increase the accuracy of stressintensity-factor measurements and to obtain data on additional parameters which may influence fracture behavior (such as crack branching and crack curvature) a technique for fullfield fringe-pattern analysis (referred to as the local collocation method) has been developed. This method removes the restriction of limiting the data analysis region to the near-field region by including additional nonsingular terms in the algorithm. In this paper the theory of the method is developed and sample results using photoelastic, holographic and moiré full-field patterns are provided. Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5–10.     

Measurement of slopes of structural deflections by speckle-shearing interferometry

A specimen illuminated by coherent light is imaged by a camera through a shearing mechanism so that the speckle from one point on the surface can be made to interfere with the speckle from a neighboring point. The resultant speckle pattern is recorded. By mechanically interfering the recorded speckle pattern corresponding to deformed and undeformed states of the specimen, respectively, using double-exposure technique, a speckle-moiré-fringe pattern is generated. These fringes which depict derivatives of deflections of the specimen are made visible by spatial-filtering technique. Speckle-moiré fringes can also be obtained in real time. This method is a new interferometry and will be referred to as “speckle-shearing interferometry”. Speckle-shearing interferometry has the same function as Ligtenberg's technique. However, it does not have the sometimes inconvenient requirement of Ligtenberg's technique that the surface of the specimen must be of mirror quality. The new technique will be particularly useful in studies of flexural deformation such as flexed beams and plates. Although speckle-shearing interferometry is an interferometric method, it overcomes several of the limitations associated with holographic and speckle interferometries, namely: (1) the setup is simple and does not need laborious alignments of optical components, (2) it does not require stringent mechanical and ambient stabilities, (3) coherent requirement of light is greatly relaxed, and (4) the sensitivity is reduced that somehow fills the gap in sensitivity between moiré techniques and holographic or speckle interferometry.