Track & trackside
An important part of the comparison between data sets is by comparison of high resolution imaging. This is carried out by using digital image correlation (DIC). DIC is a well-established technique that has been used for laboratory based measurements, mainly for materials property testing. Work at NPL established that the measurement technique could be applied to large civil engineering structures and used for in- situ measurements. This has been applied to tunnel imagery to make comparisons from run to run.
candidate defects. These could then be displayed using a software tool to allow a comparison between before, and after images both from the high resolution imagery and the shape measurement surface maps.
These could then be viewed by an operator who could make a decision about the classification of the types of changes. Using this methodology greatly reduces the amount of expert examination of tunnel wall that hasn’t changed from run to run and hence doesn’t need defect classification again. Using this methodology means that the computer identifies areas of change, and the human operator identifies the type and classification of tunnel defect indicated by that change in appearance or shape.
Benefits
The main benefits are: • rapid data capture compared to conventional inspection method
• direct inspection run to inspection run comparison highlighting differences to 1mm
• full record of the structure via archived time history of appearance and shape
• more efficient use of time of experienced inspectors
• automated defect report generation • reduction in cost and improvement in workforce safety
• particularly effective for hazardous or difficult-to-access environments
• richer, more detailed 3D spatial data • modular architecture to allow element reuse and adaptation for multiple applications in different sector
•
Fig 3. An area of the tunnel wall at Wansford prior to an introduced defect.
Fig 4. The same area captured in a subsequent measurement run, there is a small change in the coverage of a patch of soot near the middle of the field of view
Trial results
Measurement runs were made at a tunnel used by the Nene Valley Railway at Wansford. Measurements were made throughout the year and data sets captured using the system mounted on the RRV. To investigate system performance, repeated runs were made and defects were introduced at defined locations to investigate the system’s ability to detect changes, [Fig 3-6]. Comparisons were also made between measurements made three months apart to see long- term changes.
After analysis was made, areas of change were identified automatically as
Fig 6. A map of correlation coefficient for the tunnel wall shown in Fig 3 and 4, areas that are darker have poorer correlation
Acknowledgements Supported by the Technology Strategy Board, RSSB and UK National Measurement Office. With thanks to the Nene Valley Railway for access to Wansford Tunnel.
February 2014 Page 77
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