Today's industry demands high-performance components meeting toughest mechanical features and ultimate safety standards. Especially in automotive and aircraft industry the development focuses on tailor-made design and solutions according to customer specifications. To reconcile economy, light-weight construction has become a key issue. Many companies are looking for new advanced strain/stress analysis techniques to improve cost efficiency and the limitations of classical methods. Detection of weak points and fatigue tests are carried out mainly with strain gauges which need careful application and experience. ESPI (electronic peckle pattern interferometry) allows a rapid, full field and 3D-measurement without contact. This paper presents the principle and application of a new miniaturized laser optical sensor combining contour and deformation measurement. In it's basic employment ESPI is an interferometric method measuring deformations at modern working materials with high accuracy. Here also a module for contouring was developed and integrated into a single interferometer. Therefore even at complex components it is possible to measure and display strain-fields and -gradients with respect to the underlying contour. The new sensor is a unique device for flexible strain-analysis at weldedmaterials, extrusions, engines, car-bodies, etc. Without preparation and due to the full field and 3D-measurement 'hot spots' are shown, reducing the testing procedure and increasing the reliability of complex component testing significantly.In this paper the recent development of a miniaturized ESPI-interferometer for strain and stress measurement is described. Advanced features according to classical techniques are specified and new applications in material and component testing are presented.
In its classical application Electronic Speckle-Pattern Interferometry (ESPI) is used to measure deformations with high resolution'. Additionally, this method is also able of measuring the 3D topography of technical surfaces even with discontinuities2'3'4'5. If adequate set-ups are used, combined measurements of shape and deformation can also be carried out6. Especially in production of machine sand other metal parts, the determination of the stress and strain e.g. of welding points is a very important issue for evaluating about the quality of the specimen under test7. Here, the use of wire resistance strain gauges is state-of-the-art for measuring length variations of parts under mechanical load8. It is very time-consuming to prepare the corresponding measurement environment for strain gauges. Moreover, only very limited information about the strain can be measured by this means because all information is integrated over the whole area covered by the strain gauges without lateral resolution. In order to extend this kind of metrology to a matrix of some thousands points even including sensitivity in the out-of-plane-direction, ESPI-methods can be used. As described in this paper, it is therefore necessary to perform both, the shape and the deformation measurement to obtain the necessary information. Of course, the directions of sensitivity depend on the contour of the test specimen and can be determined due to the previously measured topography.In this paper current work on the field of stress and strain measurements with ESPI is described. The experimental result of several technical applications is shown and it is compared to measurements with conventional strain gauges. Further possible technical applications are discussed and the prototype of an ESPI stress sensor is presented.
Diode laser based speckle interferometry for dimensional measurementsIn Electronic Speckle-Pattern Interferometry (ESPI) the utilization of tuneable diode lasers enables us to measure the 3D topography of technical freeform surfaces. The resolution of this so-called Two Wavelength ESPI (TWESPI) depends decisively on the tuneable wavelength range of the used laser source. With a Distributed-Bragg-Reflector Laser (DBR-Laser) shape measurements were carried out successfully. In this paper new methods for the extension of the measurement range, for phase-shifting and for calibration purposes will be presented. Moreover a set-up for the combined measurement of deformation and shape will be introduced and the obtained results will be shown.
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