Bridge construction spans examination using GALS-1 AE complex.

Topicality of the problem

Bridge construction is a complicated and important structure, consisting of a big number of partial units and node points. The bridge operation reliability touches upon many questions, relating both to chemical composition, properties and structure of materials, fabrication method and to problem of inspection, static and dynamic kinds of bridge constructions testing, evaluation of the construction lifetime in the operating process.
2383 metal spans are used on the Ukrainian railways and 1500 of them are welded. For presence of different damages, 269 metal bridge constructions are considered to be defective.
Fatigue cracks appear almost right after the beginning of an operation. Before occurrence of cracks of sufficient sizes that they could be revealed visually, it passes from four till seven years. In some cases cracks could be observed on the areas with big traffic volume in the first year of the operation. Often such cracks occur in elements and joint welds, where the occurrence was absolutely not expected and consequently they were ignored in the structure strength calculations.
It should be noted, that in the spans of railway bridges, which were projected in 70-80s, owning to a number of design defects, the biggest amount of fatigue cracks was observed. The structural failure happens instantly. Therefore, an early localization of such faults, micro and macro cracks detection during the operation period is a very important task in the testing of the constructions.
 The location of defects during the periodic constructions testing is complicated under certain circumstances. Cracks, appearing in the material are invisible; some bridge constructions are difficult for examination and visual observation. The constructions are covered with paint, affected with different kinds of corrosion and etc. When certain types of cracks detected in constructions, which are meant for present-day and future-proof loading and with an adequate margin of safety, it is impossible to specify the category of the defect risk and conditions of further operating, even with modern methods of the bearing test.
The growing complexity of the reliability control and the durable bridge operation demands new approach to the technical diagnostics.
The acoustical emission method is the most effective method for inspection of especially dangerous objects such as bridges. This is the only method, which allows to follow in real-time the defect formation nature and the defect development in construction material. The method is able to make the source characterization of AE and evaluate the technical state of the construction in whole. Namely, this method was used for the span diagnostics of  single-track bridge over the Vorskla River on the 333km of Kiev – Kharkov line (Fig.1)

Fig.1. Testing members on the bridge.

This metal railway bridge is built under the scheme 3x55, 0 m.  Spans are the metal bolt welded constructions with parallel chords and with bottom-road; they are designed for C14 load and are made of steel 15 HSND by the type plan of Giprotransmost (inv.number   №690). The spans were placed on bearings in 1979.
Already in the first years of operating, cracks of T9, T10 [1] type started to appear in the vertical ribs of stringers of roadway spans along the ends of the joint welds attachment of vertical stiffeners. Now the number of cracks of all spans has reached 70 pieces. Separate cracks stopped their propagation, other continues to develop and there are new cracks.
 In consideration of above-stated, it was decided to make a complex testing of the bridge and to make a repair of spans with use of crack weld up method with following high-frequency mechanical cogging of repair joint welds by IES of Paton developed technology.

Fig.2.Loading locomotive 2TE116.

Inspection
Before the repair works, the diagnostics complex “GLAS-1” was used to carry out inspection of span 2. The aim was to locate the cracks (linear location), to detect the microcrack nucleation and determination of the extent (class) of risk of the defects development in span constructions.
The span 2 was tested with static and dynamic load. A 12-axial diesel locomotive 2TE116 with 16 tons of weight and with axial load of 225, 4 kN was used as a testing load. In static tests it was performed a stepwise loading of the span in order to create a maximum bending moments in panels of roadway and in the whole span. The axial load was sequentially placed on the centre of the stringers and on all intermediate cross-members of the span 2. In dynamic testing the locomotive was passed through the bridge at 50 kilometers per hour.

Fig.3.The AE channels were in close proximity to the probes.

Fig.4. All data were collected in real-time on the PC.

Acoustic emission transducers were set on the vertical walls of the left stringers on panels 7-8, 8-9, 9-10, which had the most number of located cracks, changing over from welds of vertical stiffening rib, fastening to the base metal of the vertical walls at the level of the upper and bottom chords.  It was set 7 probes at all, so the maximum space between the transducers didn’t exceed 3 m.

a)    The first static load.
b)    The second static load.


RESULTS OF AE TESTING AND INTERPRETATION OF THE RESULTS
In the first cycle of loading it was received 9583 AE signals, which helped to locate 406 events. The received signals were processed. Particularly the most usable diagram – signal count vs it’s amplitude – had a classical view (fig. 6), indicating the belonging of received signals exactly to acoustic emission from developing defects (and not signals from hits, friction, electromagnet pickup and that sort of noise sources). A special attention was given to the examination of location pattern, diagram of AE signal amplitude distribution along the location coordinate and to their comparison with previous testing data (pic.7). It is apparent that the results of location confirm the presence of all early detected defects. Indication from beam splice could be observed, and another additional indication near the placement location of sensor №12 could be seen.

Fig.9 Fragments of the graph pic.9
a – Section, typical for registration of the 1 – class sources;
b – Section, typical for registration of the 2 – class sources;
Static reloading showed similar results: it was received 9087 AE signals, which helped to locate 382 events. Location pattern also differs not much (fig.8 a, b). It indicates the presence of AE sources in areas, shown in a location pattern.
GALS-1 system determined in risk level most of the located AE sources relative to the second class and some – to the first. The same conclusion could be drawn when viewing the signal count-time graph on areas, corresponding to the load level stabilization (fig.9). In some cases it could be seen, that curve has soft nonlinearity rate (fig.10) which is typical for sources of the second class. In other cases it goes step-wise to the horizontal line on the graph (fig.10, b) – typically for sources of the first class. After two iterations of static loading, as it was mentioned, AE testing with dynamic load was attempted. There were received 9834 AE signals witch helped to initially locate 841 events. In this case the signal count-amplitude diagram had a shape, which differed from the classic one. It indicates the presence of a significant number of noise signals. After the filtration 7888 signals remained on this graph and 687 events were located. Evidently, this number significantly exceed the quantity of events located while static loading. Here, the location pattern (fig.11) has a fuzzy nature and doesn’t allow location of many sources detected while the static loading. Though, the main regularities still can be seen: small amounts of events in the region of cracks, trussed with superstrong bolts (2 and 3 meters), and beam splices (5.5, 11 and 16.5 meters). In the region of not mended cracks (7th, 9th and 14th meter) there is a burst of activity as well, but they are much concealed with the background noise of different events, while the maximum is shifted one meter to the left. The additional indication is also visible in the region of placement location of the transducer 12, but it is rather weakened.

AE TESTING RESULTS CONFIRMATION USING EDDY CURRENT METHOD
By the results of the AE testing the region of the 12-th transducer placement location was also tested with the eddy current method. VD3-71 flaw detector with MDF0701 probe was used. A region from 12.2 to 13.2 meters on either sides of the beam was scanned. No surface preparation was made – the testing was carried out through a coat of old paint. Two indications were detected; the amplitude corresponded to the detection of a notch with the depth of 0.2 mm on calibration block SOP – 2353.08.  A part the testing report is quoted in the figure 12.

Fig.11 Fragment of the EC report, corresponding to the crack detection.
CONCLUSIONS
The conducted AE tests, results analysis and their selective confirmation allow to make following conclusions:
First: AE inspection using GALS-1 system allows to detect both macro and micro cracks in steel bridge constructions and can be recommended for usage.
Second: The experiments with static and dynamic loading showed that the static loading is preferred since it allows to locate the sources of acoustic emission more effectively.