Electronic speckle pattern interferometry

Abstract

Nondestructive Testing of various kinds is one of the cornerstones in Structural-Health-Monitoring concepts, maintenance plans, and quality management. Increasing automation and speed of production processes is motivating research on appropriately instrumented Nondestructive Testing methods that also show potential for automation. Furthermore, an increasing use of advanced engineering materials and the simultaneously occurring challenges in detection and assessment of damage is motivating research into suitable additional and complementary inspection methods. Commercial off-the-shelf measurement systems, which can be easily adapted to a specific task, are of special interest in industry. This research work investigates the feasibility and applicability of visual defect detection with a commercial double-pulse Electronic Speckle Pattern Interferometry (ESPI) system, based on impact-excited elastic waves, on methods with forced vibrations, and on methods with thermoelastic deformations. In the case of forced vibrations, methods are adopted that show an increased potential of defect-selectivity. Elements of civil infrastructure (made of reinforced concrete-a slab track segment, block sleepers, sleepers) are investigated for defects with respect to instrumented NDT in quality management. The traditional single-exciter "mode studies" method in use with doublepulse ESPI is extended with multi-exciter methods adapted from vibration testing. The potential of successful application can be concluded for all of the applied excitation methods. Samples of advanced engineering materials (CFRP laminates, composite honeycomb) are investigated for artificial defects with elastic-wave and vibration-based methods and with thermoelastic deformations. The traditional "mode studies" method is complemented with "chirp" and band-limited "narrow noise" excitation. The potential of successful application and increased defect-selectivity can be demonstrated for the vibration methods. Further advance in elastic-wave and thermoelastic-deformation-based defect detection in CFRP laminates will become possible by gaining skill and experience in defect-pattern recognition. A full-scale skin-stringer structure (an elevator flap of a Breguet 1150 aircraft) is tested with elastic waves and forced vibrations. First insights are gained into the "full-field" appearance of related phenomena. This complements on the one hand the analysis of such structures to date and, on the other hand, points out the challenges confronting visual defect detection. In all cases under consideration, the encouraging results obtained justify continuing research and proposals for future work are put forward. THIS PAGE INTENTIONALLY LEFT BLANK List of Acronyms and Abbreviations xix * Symbols/quantities in common use, auxiliary variables. † Dimensions, geometric quantities. ‡ Complex and phasor notation. § Propagation of light and speckle interferometry. Continuum mechanics and elastic waves. # Wave propagation in periodic structures. * "live", † double/multi pulsed, ‡ strob...

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“…Speckle fringe patterns were obtained from a ''superposition'' of the speckle patterns before and after application of the dead weight loads according to conventional methods of fringe processing. 6 A typical speckle fringe pattern obtained on the surface of a bovine specimen is shown in Fig. 3(a).…”

Optical Nondestructive Inspection for Turbofan Engines' Abradable Liners

“…The digital‐speckle technique is based on the ability to measure the differing intensity of a laser speckle reflected off the surface of an object. This experiment used a simplified digital speckle interferometry (SDSI) setup that has been shown in previous experiments to give reliable results with relatively inexpensive equipment that display out‐of‐plane displacements of a sample subjected to stress 7 . Most objects have a relatively coarse surface structure when viewed at the microscopic level and produce sufficient speckle patterns to be utilized in this fashion.…”

“…This experiment used a simplified digital speckle interferometry (SDSI) setup that has been shown in previous experiments to give reliable results with relatively inexpensive equipment that display out-of-plane displacements of a sample subjected to stress. 7 Most objects have a relatively coarse surface structure when viewed at the microscopic level and produce sufficient speckle patterns to be utilized in this fashion. Using a CCD camera, a reference beam and the beam reflected from the sample surface are used to capture digital images and record changes within the intensity captured at the CCD.…”

Speckle Interferometry Made Simple and Cheap

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“…There are other ways of creating the reference beam while maintaining the advantage inherent in common path configurations. 10 In the configuration presented above, only correlation fringes can be created. The system can be converted to a phase-shifting interferometer by using a driver to move the scattering plate.…”