Optical Inspection Techniques for Vibration Analysis and Defect Indication in Railway

The proper fabrication of concrete-embedded tracks is a prerequisite for their long-term durability and their economic efficiency. Frequent defects which are caused either during fabrication or during operation are (partially) debonding between the pre-fabricated sleepers and the site-mixed concrete. This paper deals with the Pulse-ESPI-Technique as a measuring method which enables to detect these defects as well as further imperfections within the base plate. Introduction: Increasing speed and axle loads which are characteristics for modern railway traffic induce further development of road systems as well. Different solutions are under investigation: these are either further developments of the proven road bed or rigid track systems, where the road bed is replaced by a concrete plate which redistributes the applied loading [3]. In order to assess various track systems and different types of construction not only technical characteristics as acoustics emission and vibration propagation are important. Even more economic parameters as production and maintenance costs, availability and life time are under consideration. The production of rigid track system requires significant higher costs compared to conventional road beds. However their benefits arte reduced maintenance effort and costs as well as increased lifetime. Therefore the overall costs are reduced compared to the conventional techniques assuming that the production and installation is free of defects. Adequate inspection procedures useful in order to assess defects during installation and maintenance should resolve defects for different types of road bed, should be mobile for on-site inspection and should allow easy and fast data assessment. A typical construction specific defect is a poor interconnection between ready-mixed concrete structures and the local concrete. Due to the fact that currently no proper inspection technique is available to detect these defects, production failures become obvious later during maintenance when sleepers get loose or dust is exciting from the interconnection gap. A feasibility study has investigated different non destructive methods for the quality assessment of rigid road beds, comparing their overall performance and the limits for hybrid techniques using ultrasonic inspection, impact echo and impulse radar for he same specimen [4]. The Bundesanstalt für Materialforschung und -prüfung (BAM) provided a segment of the rigid road bed system Rheda 2000 with defect interconnection gaps which was used for a comparative study in a laboratory [2]. The measurements presented here include the detection of gaps in the failured interconnections of rigid road beds. For this purpose the BAM provided a sample (Type Reha 2000) Test mounting: For the measurements a test sample segment (1:1) (dimension 2,0m x 2,7m x 0,24m) of a rigid road bed was provided by the Bundesanstalt für Materialforschung und –prüfung (BAM) (rf. Fig. 1) Fig. 1: Rigid road bed segment as test sample for measurements of the interconnection joint of sleepers and local concrete. Fig. 2: Reinforcement of the test sample [2]. Fig. 2: Reinforcement of the test sample [2]. The sample consists of 6 half-sleepers which are linked by means of loose reinforcements (rf. fig. 2). During manufacturing of rigid road beds, the space between the sleepers are filled with concrete. Therefore a single half-sleeper has interconnection joints with the surrounding concrete at 5 faces. In order to provide a meaningful sample with predefined defects to be detected, single sleepers have been especially prepared by gluing foils or apply a layer of vaseline. This has been done for 4 of the total 6 sleepers. An overview about the individual sleepers is given in tab. 1. Tabl. 1: Overview about the various gaps in the interconnection joint [2]. Sleeper according to fig. 2 Type of gap Sleeper 1 Loose Sleeper 2 Fuly coupled Sleeper 3 Lateral loose Sleeper 4 Fully coupled Sleeper 5 Bottom loose Sleeper 6 Lateral loose (vaseline) The test sample was integrated into a test bench at the Instituts für Mechanik at the HelmutSchmidt-Universität, Universität der Bundeswehr Hamburg [6], [7], [8]. For these measurements no tracks have been integrated in order to provide fully optical access to the concrete. Laser-Speckle-Interferometry The use of ESPI techniques (Electronic Speckle-Pattern Interferometry) have been widely spread for the study of full field displacement fields. These methods operate non-contact and without any feedback to the sample under test. Recent application have been in the fields of mechanical construction of safety related structures and for vibration analysis [1] and [5]. ESPI techniques could be applied to any object geometry and surface quality and its accuracy is down to a few percent of the wavelength of the light (typ. 30 nm). The ESPI techniques is based on the presence of the so called speckles. These speckles, statistically interference patterns, are generated, when a technical surface (rough) is illuminated with a coherent laser beam. The reflected light is imaged onto a CCD camera (object beam). A second beam (reference beam) which is split out of the main beam using a beam splitter is directly imaged onto the CCD. The superposition of the object light and reference light allows the calculation of changes of the object surface. ig. 3: ESPI principle and laboratory setup e ESPI technique uses the images of two different states of the object’s surface, e.g. unloaded est realisation For the experiments the optical setup was prepared in the way that two neighboured half-sleepers could be inspected in parallel. This restriction is due to the limited laser energy which of course should be avoided for field studies. Messrichtung