ARCMAC Optical Strain Measurement for Creep Monitoring: Recent Improvements in Accuracy and Resolution

Remaining life of power station high pressure steam pipes is heavily dependant upon material creep rates. However, due to the difficulty in monitoring strain in these pipes as a result of the demanding operational conditions, a rugged optical strain gauge system has been developed. The current E.ON UK ARCMAC gauge system has been validated using the UK National Physical Laboratory standard grade extensometer and provides a strain measurement accuracy of 64 micro-strain with an error of <10%. This system uses precision optics, a CCD camera and a light source system to capture images of uniaxial and biaxial optical strain gauges on steam pipes during periodic maintenance. Further developments of the ARCMAC system have included the design, manufacture and validation of an advanced ARCMAC optical measurement system with improved sensor resolution and improved accuracy. Additionally, the methodology of image processing has been studied in order to reduce errors in both the existing and the new ARCMAC systems. Finally, Digital Image Correlation (DIC) has been used alongside ARCMAC gauges to monitor strain fields around welds and defects in steam pipes. Some of these techniques have also been used in a related study into strain monitoring in wind turbine blades.