ACOUSTIC EMISSION SIGNALS GENERATED BY MONOPOLE ( PENCIL-LEAD BREAK ) VERSUS DIPOLE SOURCES : FINITE ELEMENT MODELING AND EXPERIMENTS

Acoustic emission (AE) practitioners routinely use surface pencil-lead breaks (monopoles) to observe expected AE signal characteristics. In contrast, stress-generated AE sources are almost universally composed of dipoles. Thus, understanding the primary differences between the signals generated by these two different source classes is of key importance. This research had the goal of analyzing and contrasting the AE signals generated by monopole and dipole sources. A finite-element-modeled (FEM) database of AE signals provided an ideal means to study these two source types. The AE signals represented the top-surface out-of-plane displacement versus time from point sources inside an aluminum plate of 4.7-mm thickness. In addition, monopole sources both on the plate top surface and the edge surface were included in the database. The AE signals were obtained from both in-plane and out-of-plane monopole and dipole sources. Results were analyzed with both a bandpass filter of 100 to 300 kHz and a 40-kHz high-pass filter. The wide-plate specimen domain effectively eliminated edge reflections from interfering with the direct signal arrivals. To supplement and compare with the FEM results, experiments with pencil-lead breaks were carried out on the top surface and the edge of a large aluminum alloy plate of 3.1-mm thickness.