It is usual to describe AE in terms of sound. But most of fracture processes are accompanied by AE in inaudible frequencies and therefore it is difficult for newcomers to imaging and understand the phenomenon. This amazing movie (Farid F. Abraham, D. Brodbeck, R. A. Rafey, W. E. Rudge) with simulation of a shear crack propagation effectively visualizes AE phenomena. Propagating crack causes a fast redistribution of stresses at the crack tip. This results in a emission of stress (AE) waves which propagates along the surface of the specimen.

Reference: Farid F. Abraham, D. Brodbeck, R. A. Rafey, W. E. Rudge, Instability Dynamics of Fracture: A Computer Simulation Investigation, IBM Research Division, Almaden Research Center

Plastic deformation development is one of the main sources of AE in metals. In common industrial metals over 80% of energy expended on fracture goes to development of plastic deformation. The next movie done by Professor Hideharu Nakashima (ageigz@mbox.nc.kyushu-u.ac.jp) of Kyushu University, Japan, shows the movement of dislocations in ferritic steels  at elevated temperature. The images are taken in a hot-stage in a transmission electron microscope. This movie demonstrates that the plastic deformation development is a dynamic process of deformation effectively causing AE.