Modal analysis of acoustic emission signals from artificial and fatigue crack sources in aerospace grade steel

Current analysis of Acoustic Emission (AE) signals is dominated by waveform parameter assessment, such as amplitude, rise-time, absolute energy, initiation frequency etc. whilst the actual waveform collected is often ignored. This paper explores the dispersive behaviour of Lamb Wave propagation as a means of estimating source to sensor distance and source orientation from collected waveforms. The presence of the two dominant wave modes, symmetric (S-0) and asymmetric (A(0)), in digitised AE signals is observed in laboratory tests from artificial sources and fatigue crack data. Artificial sources in plane and normal to the sensor face plane were collected using the pencil lead fracture technique whilst signals from fatigue crack growth were collected from a four point bend test. Aerospace grade steel was used for all experiments. The frequency characteristics of the two modes are compared and by examining the arrival time of the modes a source to sensor distance is estimated. Furthermore by comparing the amplitude ratio of the two wave modes it is demonstrated that it is possible to determine the orientation of a source compared with a sensor position. The work presented in this paper forms part of the research undertaken by Messier-Dowty Ltd. and Cardiff University to develop a Structural Health Monitoring (SHM) system for landing gear modules during certification tests.