Evaluation of Acoustic Emission Source Location in Long Steel Pipes for Continuous and Semi-continuous Sources

Acoustic emission (AE) is a commercially-available technique used for structural health monitoring in large structures such as pipeline systems. It is a passive technique that relies on AE and can be generated in continuous and/or burst form (semi-continuous), recorded at one or more sensors with known positions. In truly continuous emission, the source has little or no temporal structure, making it difficult to identify the departure time of a source. In this research, the focus of the analysis was to evaluate distortions of AE continuous and semi-continuous sources in time and frequency domains and to examine how attenuation compares with a discontinuous on long steel pipe. Two different methods have generated artificial, relatively white, sources; continuous AE from a compressed air jet, and semi-continuous signals using a solenoid valve to modulate the air jet. Several arrays have been used to study axial and circumferential propagation, and a combination of time (energy-based) and frequency (time-based) domain processing has been used to develop generic approaches for source location. It is concluded that an energy-based technique combining a digital filter applied to the more heavily attenuated spectral component is more effective in location continuous emission, while semi-continuous signals are more effectively dealt with using cross-correlation applied to a less heavily attenuated spectral component to determine the arrival time of a recognizable time segment. Moreover, it has been observed that the distinction between axial and circumferential propagation of AE is less significant in long pipes.