ARCMAC Optical Creep Monitoring: Developments in Image Analysis Techniques and Creep Measurement Validation

Power station steam pipes operate at high temperatures (approximately 570°C) and pressures, and estimation of remaining life is critical in avoiding pipe failure. In estimating remaining life, creep rate for both parent pipe material and weldments is an important metric. A number of optical strain methods have been investigated to measure creep strain, including digital image correlation (DIC) and E.ON’s auto-reference creep management and control (ARCMAC) system. The ARCMAC system measures point to point strain by capturing and analysing images of a pair of inconel gauges. Recently, a modified ARCMAC image capture system has been developed using a DSLR camera, providing higher resolution images and lower costs compared to the existing system. Experimental results from the modified ARCMAC system show reduced accuracy compared to the existing system, likely due to distortion caused by lower quality lens components. However, by calculating and applying a correction for this distortion during image processing, it has been shown that accuracy of the modified system becomes significantly better than the existing system. The processing of ARCMAC images has previously been carried out manually by the user. In order to improve the repeatability and a speed of ARCMAC image processing, a computational code has been developed to carry out the procedure automatically. As well as reducing processing time substantially, the selection of image processing parameters has been standardised, improving accuracy significantly. ARCMAC results from creep tests carried out to simulate in-service conditions and measure creep under controlled conditions are also presented. These creep experiments form part of efforts to develop the ARCMAC system to be used to measure strain on-load rather than during the outage, allowing for more regular readings and more accurate creep rate estimation.