Structural Health Monitoring Of Power Plant Structures Using Ultrasonic Guided Waves

In nuclear and coal power plants, various structures (such as, steam pipelines, heat exchangers, steam turbines, etc.) are working in harsh environments (such as, high temperature, high pressure, corrosion, radiation, etc.). In the life time of these structures, micro damage might be generated, gradually increase to macro damage, and finally cause the structural failure, due to continuous corrosion, creep, fatigue, stressing or other effects. The failure of power plant structures may cause safety and environmental issues. Therefore, continuous damage detection and monitoring of power plant structures are needed for the safety and environment concern.Structural health monitoring (SHM) assesses structural health and predicts structural remaining life through appropriate diagnosis and prognosis technologies. An SHM system behaves like the human nervous system with the particular purpose to monitor the structural behavior in quasi-real time, indicate the health state of structure, and "trigger" the safety and remedial actions. Among various SHM technologies, guided ultrasonic waves have shown great potential for fast and large area SHM, due to their sensitivity to small defects and capability to propagate long distances. Recent advances in guided wave based SHM technologies have demonstrated their feasibility of detecting damage in aerospace structures such as thin aluminum and composite plates.Although the SHM in aerospace structures has been studied by many researchers, the SHM for power plants is less investigated. The objective of this dissertation research is to develop an SHM system for power plant structures. In this research, the concept and framework of the power plant SHM system are established. Moreover, the difficulties and issues of establishing the power plant SHM system are discussed. Last but not least, this dissertation research is to explore guided wave based damage detection methods for power plant structures.With these objectives, this dissertation research focuses on the following four parts:(1) Guided wave fundamentals:The guided wave fundamentals, such as dispersion curves and mode shapes, are investigated. Especially, this study focuses on guided waves in three main types of power plant structures, thick-wall structures, pipes and water loaded structures.(2) Analytical and numerical simulations:Numerical methods, such as finite integration method and finite element method, are investigated. Guided waves in a plate with one side in water are simulated by using the finite integration method. The finite element method is used to investigate guided waves in a complex geometry honeycomb sandwich structure. Moreover, beside numerical simulations, a fast analytical method for simulating Lamb waves is established based on the closed-form solutions of Lamb wave transfer functions. Last but not the least, a scanning laser vibrometry approach is established to validate simulations in both time and space.(3) Guided wave array imaging:Two types of commonly used arrays, sparse arrays and phased arrays, are investigated. Sparse arrays use sensors spatially distributed around the hot spot for damage detection. In phased arrays, the sensors are distributed in a compact format. By properly controlling the phase delay of each array element, the phased array can steer the generated waves to any desired directions. Moreover, comparative studies between these two arrays are conducted, from different perspectives, such as array configurations, design concepts, imaging algorithms. Last but not the least, the time domain and the frequency domain array beamforming methods are compared.(4) Guided wave based crack imaging:This part investigates guided wave interactions with cracks of different sizes (depths, widths and lengths) in plates and pipes. Moreover, this part develops guided wave based array imaging methods for crack detection and quantification in plates and pipes.This dissertation research establishes the fundamentals for developing a power plant SHM system. Moreover, this dissertation research brings guided wave based damage detection methods to power plant structures. We envision that one day the power plant SHM system will be established and been used in all the power plants for their safety.