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The development of a nondestructive, full-field, quantitative optical technique, and its feasibility to study dynamic deformations of opaque and diffusively reflecting solids under transient loads, are discussed. The technique involves recording a sequence of dynamically changing two-beam speckle interference patterns (also called holographic speckle patterns) of a rapidly deforming body which is doubly illuminated by a laser light source. The time sequence of speckle patterns is recorded by means of a high-speed camera on an ultra-sensitive 35-mm film. The developed negatives are then digitized by a CCD camera into an image processing system. An initial speckle pattern corresponding to the undeformed state of the object is taken as the reference, and subsequent speckle patterns are digitally subtracted (reconstructed) from it to produce time-varying fringe patterns corresponding to the relative deformation of the test object. In order to gain confidence that the technique can be used to record truly transient deformation, it is tested here on a vibrating plate at resonance, thereby obtaining the evolution of the fringe pattern during 1/2 cycle of deformation corresponding to 160 µs.

1 Introduction

Optical techniques such as holographic and speckle interferometry provide means by which one can obtain full-field, quantitative measurements of deformations of objects under different loading conditions. The nondestructive nature of such techniques makes them suitable for use in hostile environments where other types of probes may be damaged, or may interfere with the implementation of the experiment, rendering them ineffective.

Speckle patterns are random intensity distributions, produced by a coherent light source, such as a laser, as it illuminates a diffusely reflecting body (Dainty, 1975). There are two types of fundamentally different speckle patterns; holographic speckle patterns which contain both phase and amplitude information simultaneously; and photographic speckle patterns which contain information pertaining only to the amplitude of the lightwave. In the latter case, the speckle patterns are obtained when a diffuse surface of an object is illuminated by a single coherent beam and the resulting scattered wavefront is recorded on a photographic emulsion medium. A double exposure of such speckle photographs, one exposure taken before and another taken after deformation of the object, is then illuminated by laser beams at discrete points to generate the corresponding Young's fringes, used to quantify the displacement components. Considerable effort has been devoted to this type of speckle-pattern photography, and detailed studies can be found in Francon (1979). Holographic speckle patterns, on the other hand, are obtained when the scattered wavefront emanating from the diffuse surface of the object is recorded simultaneously with a reference wavefront on the photographic emulsion medium. The speckle pattern recorded in the original experiment of Gabor's in-line holography (Gabor, 1948), leading to the hologram of the object, is an example of a holographic speckle pattern. Holographic interferometry is, in essence, the interference of two recorded holographic speckle patterns. Two such speckle patterns, corresponding to the undeformed and deformed state of the object, are recorded on the same photographic emulsion medium, hence involving double-exposure holography. The addition of these intensity variations produces fringe patterns that are directly related to the displacement of the object (Solid, 1969). Cartesian components of the displacement vector can be separated and individually recorded by holographic interferometric techniques (Sciammarella and Gilbert, 1976). These methods have been complemented by powerful image processing techniques and, during the past decade, considerable research has been devoted toward facilitating computer-based holographic interferometric techniques for quantitative measurements of displacement and strain fields of deforming bodies, making them an effective tool for experimental stress analysis (Sciammarella and Ahmadshahi, 1984, 1986, 1988). Further advances in this field have led to what is known as electronic speckle-pattern interferometry, sometimes referred to as electronic holography (Stetson and Brohinsky, 1985).

Image

Speckle-pattern interferometry, be it holographic or photographic, has until now been limited to the study of static deformations, or at most, periodic deformations which occur when an object is under steady-state vibration. In the latter case, temporal modulation of the amplitude and/or phase of the illuminating lightwave (Aleksoff, 1971; and Ahmadshahi, 1988) has made full-field quantitative measurements of displacement and strain of vibrating bodies possible. However, in view of many technical problems, it has been stated as recently as 1989 that "dynamic speckle-pattern interferometry is not possible" (see Jones and Wykes, 1989). Although there are in principle no inherent limitations preventing the use of such techniques to record transient deformation, a number of technical difficulties must be overcome before dynamic speckle interferometry becomes a viable technique. Among these, intensity, resolution, and perhaps most importantly, image registration, have accounted for the preclusion of speckle techniques in the study of transient deformations until now.

In this paper, we discuss the development of dynamic holographic-electronic speckle-pattern interferometry and its application to analyze the deformations of diffusely reflecting bodies under transient loads. The objective of this research is to show that holographic speckle interferometry is applicable to dynamic problems, and more importantly, to demonstrate how this technique can be realized. Other optical methods, such as photoelasticity (Dally, Riley, and Durelli, 1959; Dally, Durelli and Riley, 1960; Durelli and Riley, 1961) and moire (Riley and Durelli, 1962; Huntley and Field, 1989), have been employed with considerable success for dynamic stress and displacement analysis. However, they have many limitations. Michelson interferometry and some shearing interferometers (Krishnaswamy, Tippur, and Rosakis, 1992) require that the material be either transparent or, when it is opaque, be highly polishable. Photoelastic studies can only be performed on transparent, birefringent materials. It is further limited because auxiliary information is needed for the separation of the principal stresses. Moiré methods utilize gratings that need to be ruled, etched, or embedded onto the specimen and thus do not qualify as nondestructive testing techniques. Even in those circumstances where gratings with minimal surface intrusion are used, elevated temperature may compromise the integrity of the grating.

The technique described in this paper is complementary to the existing optical techniques in that it makes possible fullfield, nondestructive, time-sequence measurements of components of the surface displacements of deforming bodies which are opaque and whose surfaces may not be polishable or which may degrade with temperature. The proposed method of dynamic holographic-electronic speckle-pattern interferometry can therefore be a valuable tool in the analysis of structural and/or material behavior over a relatively wide range of deformation rates and temperatures.

In the following sections, the principles of holographic speckle interferometry and the requirements of speckle correlation are first briefly outlined to lay the basis for the subsequent description of the proposed technique.

2 Holographic Speckle-Pattern Interferometry

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Michael M. Ahmadshahi Ph.D., Esq. is among the best and most thorough Intellectual Property attorneys. He has over 20 years of first-hand experience researching, writing, and filing patents. His firm has facilitated over 150 worldwide patents, in the U.S., Canada, Great Britain, Germany, France, Switzerland, Sweden, Belgium, Netherlands, Austria, Spain, Italy, Greece, Turkey, and Australia. Mr. Ahmadshahi has experience in the prosecution of patent applications in the fields of electrical, optical, mechanical, electro-mechanical, opto-electronic devices, circuits, nanotechnology, business methods, and computer programs.

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