Measuring creep in power station components
Digital image correlation (DIC) is a non-contact technique for assessing materials and structural integrity. However, for DIC to be applied to'High temperature creep' (HTC) two problems must be overcome: the long-term stability of the speckle pattern and the accuracy of the measurement system. The HTCSENSOR (Development of a portable 3D deformation sensor for high temperature creep measurement) project addressed these challenges by developing an HTC sensor and associated algorithms for measuring creep strain and estimating the remaining life of the component. Researchers developed an HTC sensor that comprised a protective mechanism and a measurement device. Inert argon gas was used to fill the sealed space to protect the materials from oxidation and other contamination. A silver ring (rather than the commonly used copper O ring) was used to ensure the mechanical sealing worked over a prolonged period of time. Micro indentations were produced on the inspection area of the steam pipe, thereby acting as a speckle pattern for the DIC. The inert gas and the mechanical sealing ensured that the speckle pattern maintained its stability throughout the sensor’s working life. A camera deployment mechanism was also developed, which could be quickly positioned on the mechanical sealing device. This ensured stability of the measurement system, without the need for a camera to be in place all the time. In addition, remaining life assessment algorithms were developed based on mechanical damage models and were calculated from both the instant creep strain rate and the accumulated strain. The results were fed into the remaining life assessment module, thereby enabling scientists to evaluate the remaining life of the components subject to high temperature. Finally, the project developed a DIC software package that incorporated advanced algorithms for digital image correlation for measuring deformation and improving accuracy and speed of measurement. Since creep is characterised over the long term at high temperature it will take several years to fully demonstrate the HTC sensor developed. However, preliminary results have shown that the HTC sensor and system have successfully met the requirements for pipeline creep measurement at high temperature. HTCSENSOR successfully solved the fundamental problem of oxidation in high temperatures creep measurement using DIC in a power station. The system therefore has great potential for application in the power generation sector an in chemical processing plants.
Keywords
Creep, digital image correlation, high temperature creep, speckle pattern, HTCSENSOR, algorithms, argon gas, silver ring
