This paper explains the procedure for analysing surface strains and the detection of near-surface defects by means of a novel measurement device. The measurement instrument combines the shearographic measurement principle with an endoscope. In the field of non-destructive testing, the optical measurement technique provides a possibility for the detection of near-surface defects. As a full-field, noncontact, coherent-optical and non-destructive testing and measuring method on surfaces, electronic shearography meets those requirements. Irregularities on body surfaces can be recorded up to a dimension that is within the range of the used laser light wave-length. With shearography, objects as well as whole structures can be checked independently of their material. Shearography measures the strains as well as the gradients of the deformation in the given direction of image-shearing. From the quantitative strain measurement, stresses in the specimen surface can be observed. By a qualitative evaluation of the shearogram, strain concentration and thus imperfections are located. Through the endoscope, the measurement method of shearography is expanded equipment-wise. In this way, shearography enables examinations of cavities in technical units through the smallest holes. The shearographic-imaging endoscope is the Interferoscope. The Interferoscope is used, where conventional shearography fails as the examined location is difficult to access. With the Interferoscope, examinations of objects are just as well possible under application of dynamic excitation. This brings up the advantage, that also deep-seated imperfections are noticeable on the surface and are thus detectable. Through the dynamic measurement excitation the detection of object specific eigenfrequencies becomes as well possible.
In industry non-contacting measurement systems are becoming more and more a necessity. Among non-contacting
techniques, optical method enjoys the virtues of being whole-field and non-contaminating and is getting more and more
acceptance by industries for characterization of microsystems. In this paper, a universal digital microscopic laser speckle
interferometer, which is called "Microferoscope" in the paper, is presented. The system is designed for measuring
microstructures and Micro-Electro-Mechanical Systems (MEMS) with rough surface. The method is based on laser
speckle interferometry incorporating a long distance microscope (LDM), a CCD-Camera and a high precision phase
shifting technique. In this paper, the theory and methodology of the universal digital laser Microferoscope are described.
A special software program "OPTIS" is developed for automatic data acquisition and evaluation. The usefulness of the
Microferoscope is demonstrated by examples of shape and displacement measurement for different samples.
The shearographic interferometry is employed as a nondestructive full field, optical testing and measuring method without contact. Fringes of contant strain (so called isotase, tasis (Greek) equals strain) can be observed in real-time on the surface of the investigated machine parts and structures of any material and are represented by the shearogram. Using shearography two states of deformation are recorded by doubly exposing a Holotest film in an ordinary camera or stored by an electronic image processing system. In the objective of the camera a shearing element is integrated or the lateral Michelson shearing interferometer is used. Rigid body motions of the object are not recorded. Local deformation irregulatrites caused by a defect under or on the surface of the specimen create strain concentrations; the homogeneous surrounding is poorly superimposed by an interference pattern. The shearogram shows dark and bright fringes which are the functions of the displaceemnt derivative. The holographic interferometry measures the out of plane deformations directly. Terms of the out-of-plane strain can be determined by the shearographic method as well as the in-plane strain fringes which are described in this paper.
Holographic interferometry is an optical method for measuring 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 has developed in the last twelve years permits full-field, noncontacting measurement of the first derivative of displacement, and, therefore, it is rapidly gaining acceptance by the industry. This paper presents the recent development of shearography in technique and in theory. A few applications are shown in this paper.
Since speckle pattern shearing interferometry (SPSI) allows derivatives of surface displacement to be measured directly, it is very useful for non-destructive testing and strain analysis. In conventional SPSI, observation of fringes requires an optical processing of double- exposed interferograms and therefore, this technique is not in real time. This paper presents the development of the SPSI, that is, the technique of electronic speckle pattern shearing interferometry, which enables the shearing interferometric fringes to be observed in real time and to be evaluated quantitatively. Its relevant theory is described and a few applications are also shown in this paper.
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