Theoretical analysis of acoustic emission signal propagation in fluid-filled pipes

Abstract

The theoretical investigation of acoustical wave propagation in cylindrical layered media is the main interest of our research. The propagation of wire break or slip related acoustical signal in the buried water-filled Prestressed Concrete Cylinder Pipe (PCCP) is taken as a specific application. The PCCPs are widely used for potable-and waste-water distribution and transmission systems, which are generally located below the surface ground. Therefore, it is difficult to inspect or detect the damage caused by the wire-break or slip related events in the pipeline. In current practice, the acoustic emission (AE) monitoring system is used for random examination of prestressing wires by excavating or internal inspecting of the pipe walls, which is based on field data analysis. This gives only the localized knowledge of wire break or slip, which can be misleading, underestimated of the extent of corroded areas, deterioration of wire failure, due to the system resonance, acoustoelasic effect, loading effect, etc. There is no systematic theoretical analysis from the acoustic signal generation to propagation related to these effects, and hence, a common problem in AD technology is to extract the physical features of the ideal events, so as to detect the similar signals. The theoretical analysis is important to understand how the AE signal is generated by the leak, wire break or slip related events and how the path characteristics, excitation frequency, and modes of propagation physically affect the signal propagation. For this purpose, and acoustical model is developed from the Navier's equation of motion. This can simulate vibrating AE signal propagation through the fluid-filled PCCP. The interaction of this propagation with the pipe structure is modeled by using Newton's law of motion in equilibrium. The principle of virtual work is used to develop the fluid-structure interaction. In this work, the impact of the path on the spectral profiles of the vibrating AE signals in different locations throughout the pipes were investigated for low and high frequency excitation signals. At low frequency, there is only plane wave propagation, therefore the stoneley or tube mode analysis is used for this purpose. The tube wave effects on the acoustical wave propagation were observed from this analysis. At high frequencies, there also exist rayleigh or shear modes which exhibit oscillatory amplitudes in the fluid and a decaying amplitude in the pipe and the surrounding medium. The eigenfrequency and the modal analysis is used in this case. From the analyses, the phase velocity, group velocity, tube wave velocity, system resonance frequencies, cut-off frequencies were observed. The high frequency analysis has some special advantage over low frequency signal. This can provide an earlier indication of incipient faults, which is important to detect the AE event in early stage of pipe deterioration. Moreover, it was established that the frequency of propagating AE signal in the pressurizing fluid medium ranges up to 30kHz. Therefore, it is important to investigate the wave propagation of AE signal propagation through the fluid column inside the pipe within the range of sonic/ultrasonic frequency. The acoustic wave propagation in fluid-filled PCCP of various radius, stiffness and thickness of the pipe as well as different types of surrounding medium, is obtained by applying a numerical Finite Element Method (FEM). Finally, the results are compared with available analytical solutions. The proposed model is independent of sources, dimensions and medium characteristics. Therefore, it can be used for the analysis of acoustic wave propagation through any type of cylindrical shells immersed or surrounded by different types of medium. The current analysis, therefore, has fundamental importance in many applications.