Experimental Investigations on Nondestructive Testing Methods for Defect Detection with Double-Pulse 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.

Qualitätssicherung, Strukturüberwachung (Structural-Health-Monitoring), fortschreitende Automatisierung und Beschleunigung der Produktion sowie zunehmender Gebrauch von modernen Ingenieurwerkstoffen motivieren die Forschung an weiteren zerstörungsfreien Prüfmethoden und -verfahren, die automatisierbar und in den Produktionsprozess integrierbar sind. Von besonderem Interesse sind in der Industrie handelsübliche, bewährte Messsysteme und -verfahren, die leicht an spezifische Aufgaben angepasst werden können. --- Die vorliegende Arbeit untersucht Machbarkeit und Anwendbarkeit der Schadensdetektion mit einem kommerziellen Doppelpuls Electronic Speckle Pattern Interferometry (ESPI)-System, einem optischen und daher berührungslos und rückwirkungsfrei arbeitenden Messverfahren. --- Bauteile aus Stahl- und Spannbeton (Feste Fahrbahn, Blockschwellen, Schwellen), Probekörper aus modernen Ingenieurwerkstoffen (Laminate und Wabenkernstruktur aus faserverstärktem Kunststoff) und ein Flugzeugbauteil in Stringerbauweise werden exemplarisch untersucht. Zur Anwendung kommen verschiedene Anregungsmethoden - punktuell monofrequent bzw. disloziert mit unterschiedlichen Frequenzen, multifrequente Chirpsignale und bandbegrenztes Rauschen, stoßerregte elastische Wellen und (wo anwendbar) thermoelastische Deformationen. --- Die erzielten Ergebnisse zeigen überzeugend die Einsetzbarkeit der Doppelpuls ESPI Technik in der Qualitätssicherung und zur Strukturüberwachung. Die Arbeit schließt mit Vorschlägen für die Fortsetzung der Untersuchungen.