Thermit Welding (GB) Ltd.  Link , Vortok and two gentlemen from Portugal were on site today for a demonstration of non-destructive testing of locked in stress measurement in laid rail. Using the the section of CWR just north of Shottle Station at MP 137 a section of rail was unclipped and raised. Support pieces were inserted between concrete sleeper and rail at precise distances either side of the measurement point.
                             

The rig used to jack up the rail and take measurements of applied load and rail deflection was first assembled at the centre of the raised rail and then basic details of location by line number and distance, track number and rail number were punched in on the keyboard of the data logger.
     

The next step was to start applying load and taking measurements. Three parameters were measured.
(i) Temperature by means of a thermal sensor clipped to the rail
(ii) Applied load by means of a loadcell between the lifting beam cable and the rail attachment clamp.
(iii) Vertical deflection by means of a Linear Variable Displacement Transducer (LVDT)
The load was applied using a hand operated hydraulic jack in a series of regular strokes of the handle until a load of 1 ton ( I think) was applied. During this process a sequence of measurement are automatically taken and recorded by the data logger. The measurements of load and displacement are by analogue transducers and their outputs converted to digital in the equipment attached. These features can be seen in the next series of photos.
                 

This loading procedure is usually done three times and the mean values would normally be used in analysis. The acquired data can be viewed on the hand-held unit for preliminary validation. The data will subsequently be copied to a computer, on this occassion a lap top PC belonging to the Engineer from Vortok. This computer has an analysis program on it which, using existing algorithms, can determine the stress, compressive or tensile, in the rail at the precise temperature at the time.
     

The process was repeated on the other rail. A 'normalised' temperature of 27 degsC is taken for UK and the stress locked in shall be tensile. There is a tolerance band for acceptance and this is biased in favour of the safer option, tensile, wheras compressive loads with elevated temperatures can cause buckling and potential derailment. Rails which fall outside the tolerance have to be cut and retensioned and welded. (This is where we came in).
Bad news WyvernRail, we measured compressive!

A most interesting diversion from normal tasks and my thanks for their co-operation from the representatives from TWS and Vortok who were most helpful with information. If I've got any of it wrong please correct me by email to webmaster@evra.org.uk.