Final Report Summary - HTCSENSOR (Development of A Portable 3D Deformation Sensor for High Temperature Creep Measurement)
Creep is one of the main damage mechanisms of components operated at high temperature. Currently, more than 70% of European power plants are more than 20 years old, 30% are more than 30 years old. The life extension of these aging plants requires inspection techniques that can detect creep damage in service.
Digital Image Correlation (DIC) is a non-contact technique for full field strain measurement that has been widely used in material characterization and structural integrity assessment. It also has the potential for creep measurement at elevated temperature. However, applying DIC for high temperature creep (HTC) measurement faces two major challenges: long term stability of the speckle pattern and the accuracy of the measurement system. This project aims to address these challenges by developing a HTC sensor and associated algorithms for creep strain measurement and remaining life estimation of the component.
Firstly, a HTC sensor has been developed which consists of two parts, a protective mechanism and a measurement setup. Since the main obstacle in long-term creep strain measurement at high temperature is oxidation of materials, inert gas such as argon is used to fill the sealed space so as to protect the materials from oxidation and other contaminations. Micro indentations are produced on the inspection area of the steam pipe, which act as the speckle pattern for strain measurement by DIC. As the inspection area is fully protected by inert gas via a mechanical sealing, the speckle pattern maintains its stability throughout the service of the sensor.
To ensure that the mechanical sealing works over a prolonged period of time, a curved silver ring (as opposed to commonly used copper O-ring) has been developed. Such a silver seal will not only resist oxidation at high temperature up to 620°C, but also can survive the numerous temperature cycles between ambient and operating temperatures.
A rapid camera deployment mechanism is also developed which has a three leg positioning device. The three legs correspond to three positioning holes on the mechanical sealing device. As a result, the camera can be put in place quickly during image acquisition. This enables stability of the measurement system without the need to keep the camera in place at all time. To further minimize the measurement error due to positioning discrepancy, a telecentric lens is used. This ensures constant image magnification within the working range. Experimental results demonstrated the high accuracy of the alignment system.
Secondly, remaining life assessment algorithms that are based on continuous mechanical damage models have been developed. The remaining life is calculated from both the instant creep strain rate and the accumulated strain. Once the strain rate is beyond the constant-creep-rate or linear region, or the cumulated strain is beyond the failure strain of the component, creep failure is expected to occur. With the inert gas based HTC sensor, more regular creep strain measurement will be possible. These results will be fed into the remaining life assessment module, enabling the residual life of the components at high temperature to be evaluated.
Thirdly, a comprehensive DIC software package has been developed which incorporated advanced algorithms for incremental digital image correlation for large deformation measurement, automated fast initial estimation, error prediction of the matching results in the area of interest, interpolation using GPU B-spline, multi-view matching for stereo-based measurement, distortion correction algorithms, camera calibration, 3D shape and deformation measurement. These advanced features represent the state-of-the-art of digital image correlation, which has improved the accuracy, robustness, and speed of the algorithm for strain measurement using digital image correlation. The DIC software and algorithms are intended to be used for calculating the displacement and deformation of 2D specimens under loading. It can also be adapted for stereo-matching of different images captured from different orientations.
Since creep is a long-term deformation at high temperature, the full demonstration of the capability of the developed HTC sensor and system will require multiple years of testing, hopefully in a power plant environment. Nevertheless, preliminary high temperature creep tests already showed that the HTC sensor and system is promising to meet the requirements for the pipeline creep measurement at high temperature environment. It solves the fundamental problems of oxidation in high temperature creep measurement using DIC in a power plant, thus having great potential for commercial application in power generation sector or other chemical processing plants. A medium long term (6 months) test is on-going, and even longer term (2-year duration) is under planning.
Digital Image Correlation (DIC) is a non-contact technique for full field strain measurement that has been widely used in material characterization and structural integrity assessment. It also has the potential for creep measurement at elevated temperature. However, applying DIC for high temperature creep (HTC) measurement faces two major challenges: long term stability of the speckle pattern and the accuracy of the measurement system. This project aims to address these challenges by developing a HTC sensor and associated algorithms for creep strain measurement and remaining life estimation of the component.
Firstly, a HTC sensor has been developed which consists of two parts, a protective mechanism and a measurement setup. Since the main obstacle in long-term creep strain measurement at high temperature is oxidation of materials, inert gas such as argon is used to fill the sealed space so as to protect the materials from oxidation and other contaminations. Micro indentations are produced on the inspection area of the steam pipe, which act as the speckle pattern for strain measurement by DIC. As the inspection area is fully protected by inert gas via a mechanical sealing, the speckle pattern maintains its stability throughout the service of the sensor.
To ensure that the mechanical sealing works over a prolonged period of time, a curved silver ring (as opposed to commonly used copper O-ring) has been developed. Such a silver seal will not only resist oxidation at high temperature up to 620°C, but also can survive the numerous temperature cycles between ambient and operating temperatures.
A rapid camera deployment mechanism is also developed which has a three leg positioning device. The three legs correspond to three positioning holes on the mechanical sealing device. As a result, the camera can be put in place quickly during image acquisition. This enables stability of the measurement system without the need to keep the camera in place at all time. To further minimize the measurement error due to positioning discrepancy, a telecentric lens is used. This ensures constant image magnification within the working range. Experimental results demonstrated the high accuracy of the alignment system.
Secondly, remaining life assessment algorithms that are based on continuous mechanical damage models have been developed. The remaining life is calculated from both the instant creep strain rate and the accumulated strain. Once the strain rate is beyond the constant-creep-rate or linear region, or the cumulated strain is beyond the failure strain of the component, creep failure is expected to occur. With the inert gas based HTC sensor, more regular creep strain measurement will be possible. These results will be fed into the remaining life assessment module, enabling the residual life of the components at high temperature to be evaluated.
Thirdly, a comprehensive DIC software package has been developed which incorporated advanced algorithms for incremental digital image correlation for large deformation measurement, automated fast initial estimation, error prediction of the matching results in the area of interest, interpolation using GPU B-spline, multi-view matching for stereo-based measurement, distortion correction algorithms, camera calibration, 3D shape and deformation measurement. These advanced features represent the state-of-the-art of digital image correlation, which has improved the accuracy, robustness, and speed of the algorithm for strain measurement using digital image correlation. The DIC software and algorithms are intended to be used for calculating the displacement and deformation of 2D specimens under loading. It can also be adapted for stereo-matching of different images captured from different orientations.
Since creep is a long-term deformation at high temperature, the full demonstration of the capability of the developed HTC sensor and system will require multiple years of testing, hopefully in a power plant environment. Nevertheless, preliminary high temperature creep tests already showed that the HTC sensor and system is promising to meet the requirements for the pipeline creep measurement at high temperature environment. It solves the fundamental problems of oxidation in high temperature creep measurement using DIC in a power plant, thus having great potential for commercial application in power generation sector or other chemical processing plants. A medium long term (6 months) test is on-going, and even longer term (2-year duration) is under planning.