Damage Detection by Wave Propagation Observation with Laser SpeckleInterferometry

Alexander Schmidt∗1, Rolf Lammering 1, and Michael Plenge 2

1 Department of Mechanical Engineering, Helmut-Schmidt-University - University of the Federal Armed Forces Hamburg,Holstenhofweg 85, 22043 Hamburg, Germany

2 Department of Mechanical Engineering and Production, University of Applied Sciences Hamburg, Berliner Tor 21, 20099Hamburg, Germany

For quality insurance and fault detection, one and three dimensional ESPI (Electronic Speckle Pattern Interferometry) tech-nique is applied to various structures. These experiments are an extension of previous experiments with Holographic Inter-ferometry [4], a related measurement technique. ESPI is a non contact, high resolution Laser measurement technique. Itfacilitates transient full field vibration measurements that deliver absolute deflections, either one dimensional out-of-plane orthree dimensional in and out-of-plane.

Experiments have been performed with a full scale concrete embedded track segment. Different known artificial defectsrepresent various flaws and voids due to the fabrication process and operation. Wave propagation due to impact is observedand allows for damage detection. ESPI measurements are in good accordance with expectations. An in-situ measurementsetup for quality management of concrete embedded railway tracks has been built.

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

First of all the main features of the employed pulse Laser ESPI technique are presented.Measurements are taken of two different object states (double exposure) as follows. Two consecutive Laser pulses are

generated, they illuminate the object surface and interfere with reference Laser light. The resulting interference patterns(Speckle patterns) of both exposures are recorded. Both Speckle patterns are compared and phase information are obtained.From those phase images displacements are computed. By simultaneously employing three different measurement directionsit is possible to compute not only out-of-plane displacements but also in-plane displacements. Hence ESPI is a non contactand therefore non destructive full field measurement technique. It is capable of measuring transient effects of e.g. wavepropagation.

One and three dimensional ESPI experiments on wave propagation have been performed with the test specimen that can beseen on the left in Figure 1. The specimen is shown in there without the rails. This full scale 2000 × 2700 × 240mm is madeof prefabricated sleepers that are embedded in a mix-in-situ concrete structure.

Fig. 1 Test specimen (left) and observed area (right), by courtesy of the Bundesanstalt fur Materialforschung und -prufung,BAM.

Several known artificial flaws and voids are implemented. These are in the case of the area observed (Figure 1, on theright): a completely debonded sleeper 1, sleeper 5 debonded at the base. Sleepers 2 and 4 were intended to be perfectlycoupled with the mix-in-situ concrete structure.

Three dimensional experimental results shown in Figures 2, 3 are each part of series of measurements that track thepropagation of a wave front through the concrete structure. In either figure is presented on the left just one of the threephase images of a measurement. On the right can be seen the corresponding displacement field. The in-plane displacementcomponents are displayed as vector plot of small black arrows. The out-of-plane component is displayed in coloured levelcurves.

The propagating pulse was induced at the right side due to impact of a heft dropped from a known height. The measurementswere triggered by the dropping heft passing a light barrier.

∗ Corresponding author: e-mail: schmidt.alexander@hsu-hh.de, Phone: +49 40 6541 2304, Fax: +49 40 6541 2034

PAMM · Proc. Appl. Math. Mech. 5, 361–362 (2005) / DOI 10.1002/pamm.200510157

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Fig. 2 Phase image (left) and displacement field (right) of a passing wave front.

Figure 2 shows the wave front passing sleepers 1 and 4. From the phase image one can get a better impression of shape andstate of propagation. A distortion in symmetry can be recognized comparing the locations of sleepers 1, 2 (top) and sleepers4, 5 (bottom). However the displacement field offers insight into the motion. While there is in-plane motion at the area nextto the passing wave front, none seems to be present in the wake. Phase image as well as displacement field indicate clearlysleeper 5 as imperfectly connected. Slight distortions at sleeper positions 2, 4 may result from mass density differences.

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Fig. 3 Phase image (left) and displacement field (right) after the wave front passed.

In the phase image of Figure 3 there appear to be some reflexion effects on the left. A striking behaviour of sleeper 1in displacement field as well as phase image indicates its imperfect connection. Also sleeper 5 shows a significant differentin-plane motion. The cause of the upper right corner distortion in displacements is not clearly identified yet.

As conclusion one can say that different types of known defects in a concrete embedded track segment were detectedwith pulse ESPI observation of wave propagation phenomena. Promising results show the usability for quality insurance anddamage detection purposes. An in-situ measurement setup for quality management of concrete embedded railway tracks hasbeen built ([1],[2],[3]).

More pulse ESPI experiments are intended with different other structures for further studies of wave propagation phenom-ena for damage and fault detection and characterization.

References

[1] M. Plenge et al., in: European Slab Track - Feste Fahrbahn Europa, Berlin 2005, Proceedings edited by Interdisiplinrer Forschungsver-bund Bahntechnik e.V., Berlin.

[2] R. Lammering et al., Qualitatsicherung von Massivbaukonstruktionen mit Hilfe der Puls-ESPI Technik am Beispiel Fester Fahrbahn-systeme, Eisenbahningenieur 79, 528–533 (2004).

[3] R. Lammering et al., Qualitatsicherung Fester Fahrbahnsysteme mit Hilfe der Puls-ESPI Technik, VDI Berichte 1825, 177–189 (2004).[4] M. Plenge et al., in: System Dynamics and Long Term Behavior of Railway Vehicles, Track and Subgrade , edited by K. Popp and W.

Schiehlen, Lecture Notes in Applied Mechanics Vol.6 (Springer, Berlin, 2003), p. 295.

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