Lianxiang Yang

Strain measurement by three-dimensional electronic speckle pattern interferometry: potentials, limitations, and applications

Principles and new developments of 3-D electronic speckle pattern interferometry (3-D ESPI) for strain and stress analysis are presented. The potentials and limitations are discussed, and some applications used in industry are shown. The newly developed 3-D ESPI technique allows rapid measurement of both the shape and the 3-D deformation of complex industrial components. The combination of shape and deformation provides all the necessary data for the accurate and quantitative determination of true strain and thus stress on nearly any industrial component. Based on a lightweight compact sensor design and new glass fiber concepts, the new device has been greatly miniaturized and can be easily attached to the components during testing. It is well suited for the harsh environmental conditions often found in the real test world.

Precision measurement and nondestructive testing by means of digital phase shifting speckle pattern and speckle pattern shearing interferometry

Speckle pattern and speckle pattern shearing interferometry, which were developed in the last two decades, are becoming more and more important in the areas of precision measurement and nondestructive testing. The fringe pattern of speckle pattern interferometry depicts loci of displacements, whereas the fringe pattern of speckle pattern shearing interferometry depicts loci of displacement gradients. By applying phase shifting technique they realize not only a direct measurement of the out-of-plane as well as in-plane displacements and strains, but also an automatic evaluation of them. They are both tools well-suited for either precision measurement or for nondestructive testing. This paper presents the recent development of speckle pattern and speckle pattern shearing interferometry in technique and in theory. A few applications are shown in this paper.

High temperature displacement and strain measurement using a monochromatic light illuminated stereo digital image correlation system

This paper describes full-field, three-dimensional, non-contact measurements of displacement and strain under high temperature condition based on digital image correlation (DIC). To conduct DIC measurements at high temperatures, two important factors need to be considered: (1) the ability of the coating to resist heat and withstand deformation without cracking or peeling off; (2) the radiation from the specimens surface at high temperature. This paper proposes a solution to both of the most important issues in high temperature DIC measurement. First, different coating materials were investigated, and a procedure to generate a necessary speckle pattern required by DIC to resist heat and withstand deformation at high temperature has been developed. Second, a monochromatic illumination system in combination with a filter set has been studied to eliminate the radiation effect. A DIC system which enables a high temperature displacement and strain measurement up to 1100??C is presented and demonstrated by experimental measurements.

Unified approach for holography and shearography in surface deformation measurement and nondestructive testing

Holography and shearography are two useful whole-field non- contacting optical tools for nondestructive flaw detection and precision measurements. Holography serves as a displacement transducer since it gives direct measurements on displacements whereas shearography serves as a strain gage since it gives direct measurements on displace- ment gradients. This paper views holography and shearography and their variations as a single optical technique having the same basic mathematical formulation and instrumentation. A key optical component used in both techniques is a doubly-refractive prism that combines two angularly separated laser rays to interfere at near collinearity, thereby permitting the use of a low-resolution CCD camera for recording the interference pattern. Shearography uses a doubly-refractive prism with small image shearing so that two neighboring points on the test surface are brought to interfere at the image plane of the camera, whereas ho- lography, on the other hand, uses a doubly-refractive prism with large image shearing so that light scattered from two different objects—a test object and a reference surface (serving as a reference beam)—are brought to interfere at the image plane of the camera. Hence, testing and measurements made using holography may also be made using shearography, and vice versa.

Review of electronic speckle pattern interferometry (ESPI) for three dimensional displacement measurement

Three dimensional(3D) displacements, which can be translated further into 3D strain, are key parameters for design, manufacturing and quality control. Using different optical setups, phase-shift methods, and algorithms, several different 3D electronic speckle pattern interferometry(ESPI) systems for displacement and strain measurements have been achieved and commercialized. This paper provides a review of the recent developments in ESPI systems for 3D displacement and strain measurement. After an overview of the fundamentals of ESPI theory, temporal phase-shift, and spatial phase-shift techniques, 3D deformation measurements by the temporal phase-shift ESPI system, which is suited well for static measurement, and by the spatial phase-shift ESPI system, which is particularly useful for dynamic measurement, are discussed. For each method, the basic theory, a brief derivation and different optical layouts are presented. The state of art application, potential and limitation of the ESPI systems are shown and demonstrated.

Strain analysis by means of digital shearography : Potential, limitations and demonstration

Abstract Shearography, also called speckle pattern shearing interferometry, is a coherent optical method which measures displacement derivatives directly. It is suited well for localization of strain concentrations and has been used as an industrial tool for non-destructive testing (NDT) in the last few years. However, its application for strain measurement has not been widely adopted in industry, because, in general, shearography can measure only out-of-plane displacement derivatives ∂w/∂x and ∂w/∂y. This paper presents recent developments of shearography for strain measurement. With the support of digital image processing the automatic and quantitative evaluation of the shearogram becomes possible. Not only flexural strains [∂2w/∂x2, ∂2w/∂y2 and ∂2w/(∂y)] but also in-plane strains (∂u/∂x, ∂v/∂y, ∂u/∂y and ∂v/∂x) can be determined by the shearographic measuring method. The potentials and limitations for strain measurement are discussed. Some applications are shown.

Michelson interferometer based spatial phase shift shearography

This paper presents a simple spatial phase shift shearography based on the Michelson interferometer. The Michelson interferometer based shearographic system has been widely utilized in industry as a practical nondestructive test tool. In the system, the Michelson interferometer is used as a shearing device to generate a shearing distance by tilting a small angle in one of the two mirrors. In fact, tilting the mirror in the Michelson interferometer also generates spatial frequency shift. Based on this feature, we introduce a simple Michelson interferometer based spatial phase shift shearography. The Fourier transform (FT) method is applied to separate the spectrum on the spatial frequency domain. The phase change due to the loading can be evaluated using a properly selected windowed inverse-FT. This system can generate a phase map of shearography by using only a single image. The effects of shearing angle, spatial resolution of couple charge device camera, and filter methods are discussed in detail. The theory and the experimental results are presented.

Digital laser microinterferometer and its applications

A universal digital laser microinterferometer is presented, which can measure the shape, displacement, and strain and stress of microelements and microelectromechanical systems (MEMSs). This device can measure microelements with either a smooth or a rough surface. The method is based on laser interferometry (for smooth surfaces) and laser speckle interferometry (for rough surfaces), incorporating a diode laser, a long-distance microscope, a CCD camera, and a high-precision phase-shifting technique. The measuring system can be utilized to measure shape and out-of-plane displacement for samples with smooth surfaces, and with minor modification it can be applied to investigate out-of-plane and in-plane displacements for samples with rough surfaces. The theory and methodology of the universal digital laser microinterferometer are described. In particular, calibration and error compensation are introduced. The usefulness of the microinterferometer is demonstrated by examples of shape and displacement measurement for different MEMSs and microelements.

Enlarging the angle of view in Michelson-interferometer-based shearography by embedding a 4f system.

Digital shearography based on Michelson interferometers suffers from the disadvantage of a small angle of view due to the structure. We demonstrate a novel digital shearography system with a large angle of view. In the optical arrangement, the imaging lens is in front of the Michelson interferometer rather than behind it as in traditional digital shearography. Thus, the angle of view is no longer limited by the Michelson interferometer. The images transmitting between the separate lens and camera are accomplished by a 4f system in the new style of shearography. The influences of the 4f system on shearography are also discussed.

Application of shearography to quality assurance

Holographic interferometry is a typical optical method for the measurement of displacement. However, it is the first or second derivative of displacement in many cases that is of interest, rather than the displacement information. Shearography which is developed in last dozen of years permits full-field, noncontacting measurement of the first derivative of displacement. It yields directly the strain information of the object and therefore it is suited very well for nondestructive testing and quality assurance system. This paper describes the relevant theory and its recent development. Its applications in nondestructive testing are presented.