Edinburgh Napier University

  Stress waves propagate through a structure when it experiences rapid changes in loading, which can come about from a number of phenomena of interest in mechanical and process engineering. One class of applications is in structural health monitoring, where the challenge is to deduce the characteristics of the generating source from one or more signal recorded at one or more sensor, often located at some distance from the source on a large or long structural element. Several methods have been proposed to analyse these signals and relate their characteristics (energy, frequency etc.) to the state of the structure, particularly in the structural health monitoring of pipes as well as civil engineering structures such as bridges and dams. In practical situations, a key aspect of source identification is the loading rate, which can potentially distinguish between, say, an impact and a leak. Current AE research techniques tend to rely mainly on experiments which can be costly and difficult to carry out and in which it is difficult to control the nature of the source, it being common to use step-unload sources, such as the Hsu-Nielson pencil lead break source. In this work, the effect of unloading rate at a source on the surface stress at a sensor is investigated using a finite element simulation. A range of different pipe sizes were modelled with a fixed source position, unloading from a fixed force at rates which varied over three orders of magnitude. The resulting stress wave versus time was “recorded” at various locations along the pipe and the characteristics of the recorded AE were determined. It is shown that arrival times are consistent with longitudinal stress waves and the frequency structure is broadly as would be expected in practically-recorded AE from pencil lead breaks. Some preliminary analysis is carried out on putative reflections as a preparation for a more systematic study of the effect of source temporal structure on AE recorded in practical situations.