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Identification of short transient signals within impact-echo data using the Hilbert-Huang-Transform
(2005)
Impact-Echo (IE) measurements on concrete structures with compact
dimensions are influenced by geometry effects, i.e. reflections especially of the
surface waves at the boundaries of the test object, which in fact can lead to
misinterpretation of the results. In contrast to the normal reflection arrangement, in
which the excitation and the measuring sensor are placed on the same side of the
specimen, measurements were carried out in a transmission arrangement, where the
impactor and the measuring sensor are placed opposite to each other. This serves to
reduce the influence of surface waves, and to investigate further their responsibility
for the existence of geometry effects. Results obtained from laboratory specimens
with smooth surfaces were still seriously affected by geometry effects. Numerical
simulations led to the conclusion that surface waves travel around the edges of the
specimen, thus creating geometry effects even on the opposite side. Measurements
on a specimen, which was cut out from a bridge structure and had irregular
connecting planes between the excitation and the measurement side, proved to be
remarkably less affected.
In order to analyze and visualize the surface wave propagation an arrangement is
used, in which the excitation is fixed at a certain point on one plane of the specimen.
A transducer scans all planes of the specimen in phase with the excitation. Time
slices show the instantaneous wave field on all planes of the specimen. These are
compared with numerical simulations. In particular, it can be proved that surface
waves indeed travel around the edges of a specimen with smooth connecting planes,
thus creating geometry effects even in the transmission arrangement.
Am Deutschen Dom wurden Mikrowellenabsorptionsmessungen, Radar und Infarot- Thermographie als moderne zerstörungsfreie Prüfverfahren der Feuchtemessung in Mauerwerk erprobt. Ergänzende numerische Simulationsrechnungen des gekoppelten Wärme- und Feuchtetransports sollen bei der Auswahl von Trockenlegungskonzepten helfen.
Abstract
In the recent years the application of infrared thermography and especially its active approaches lock-in thermography, impulse themography and pulse phase thermography (PPT) has gained more importance for non-destructive testing (NDT). In this paper it is demonstrated that PPT is very well suited for NDT in civil engineering. The method is based on pulse heating of the investigated specimen and on the observation of the cooling down process on its surface with an infrared camera. The transient behaviour is afterwards analysed with Fast Fourier Transformation, enabling the creation of phase and amplitude images with enhanced visualisation of the defects. Results of European and National (i.e. DFG) funded projects are presented, for example the detection of voids and inhomogeneities close to the surface and the localisation of delaminations on plastered concrete and masonry specimen.
ZUSAMMENFASSUNG
Im Rahmen eines von der Deutschen Forschungsgemeinschaft geförderten Vorhabens zur zerstörungsfreien Prüfung im Bauwesen wird in der Bundesanstalt für Materialforschung und -prüfung (BAM) in Zusammenarbeit mit der Technischen Universität Berlin (TUB) die Puls-Phasen-Thermografie (PPT) weiterentwickelt, bewertet und eingesetzt. Die durch die Impuls-Thermografie (IT) aufgenommenen Daten werden mittels Fast Fourier Transformation (FFT) bearbeitet. Die durch die FFT gewonnenen Phasenbilder reduzieren die Störeinflüsse von Oberflächeninhomogenitäten und ungleichmäßiger Erwärmung.
Die dargestellten Ergebnisse zeigen deutlich, dass sich die PPT sehr gut zur zerstörungsfreien Prüfung im Bauwesen eignet. Insbesondere Inhomogenitäten im oberflächennahen Bereich, wie Ablösungen und Hohlstellen hinter Beschichtungen (Putz, CFK-Laminate etc.), aber auch Defekte in größeren Tiefen können zuverlässig geortet und bildgebend dargestellt werden.