Acoustic emission monitoring of pipes; combining finite element simulation and experiment for advanced source location and identification

Abstract

Impact is a common source of damage in pipes and pipeline systems, detecting the location and nature of damage is vital for reliability and safety of these systems. This work sets out to assess the capacity of Acoustic Emission (AE) to monitor pipes and pipelines for externally applied mechanical damage. AE is a non-destructive testing and monitoring technique that relies on the propagation of elastic (stress) waves generated by impulsive events such as particle impingement, cracking or fluid flow. These waves are recorded at one or more sensors mounted on the surface of the object to be monitored. The key scientific question was to determine the extent to which the structure of a non-impulsive event could be reconstructed using sensors located on the external surface of a pipe. The aim was to combine Finite Element simulations with a series of experiments in order that the relationship between the generating event (source) and the resulting stress-time history at a given point on the surface could be elucidated. Experiments and simulations were carried out with impulsive sources (pencil-lead breaks) and dropped objects, the latter being used to represent a non-impulsive event with a reproducible structure lasting around one second. The AE resulting from these sources was recorded over a period of around 2 seconds for both experiments and simulations. Two test objects, a solid cylindrical steel block of diameter 307mm and length 166mm and various lengths of pipe of diameter 100mm and wall thickness 10mm were used, the former to provide a relatively simple and well-studied platform to examine a number of essential principles. The work on the solid cylinder first validated the simulation of the stress wave from an impulsive source and identified the main modes present, by comparing with analytical solutions. Then it was possible to identify the part of the experimental time series record at a given sensor which is uncontaminated by reflections from the edges and surfaces of the cylinder. The dropped object measurements on the solid cylinder provided clear records of the first and subsequent impacts as the dropped steel balls recoiled and returned back to the surface. There was a clear relationship between the measured AE energy and the estimated incident energy of the dropped objects at a range of timescales irrespective of contamination by reflections. The work on the pipe sections formed the main series of systematic experiments. First it was established that an unloading time in the simulations of around 10-8 seconds gave a reasonable representation of the frequency structure of experimentally observed stress waves. It was also observed from both experiments and simulations that a low amplitude wave travelling at around 5500ms-1 was the first to arrive at any surface sensor. The structure thereafter was complex, probably involving reflections from the inner wall of the cylinder and geometric interference as the wave spreads around the circumference of the pipe. The key finding of this aspect of the work is that the AE line structure of an impulsive source can be reproduced by simulation for short times, for longer times, the damping associated with reflections would require to be measured and introduced into the simulations in order to fully represent the real practical simulation. The degree of damping is important in making a cumulative assessment of multiple impulsive sources. The dropped objects on the pipe confirmed that a mechanical disturbance which is extended in time can be identified from its energy-time imprint carried on the stress wave. The analysis was carried out at three different timescales; short (initial interactions free of reflections), medium (first contact including recoil) and long (involving several bounces). Generally, for medium and short timescales, the AE energy varied with drop height and mass consistently with existing models for balls on plate. For multiple bounces, the behaviour was more erratic probably due to the imprecise control of ball contact point. The simulations of AE worked well at medium and long timescales, providing an idealised framework unto which could be added effects of restitution and damping. At the short timescale, the twin challenges of time and spatial resolution meant that a solution could not be obtained within the limitations of the computing power available. It is generally concluded that AE monitoring can be used to identify the nature of a mechanical disturbance on the surface of a pipe. Suggestions for future work include improvements to the simulations to include attenuation and to better simulate the dynamics of mechanical interactions at the surface, and extensions to the experiments to cover the effect of internal and external pipe environment and the use of mechanical sources which involve actual pipe damage.