ABSTRACT
Stress and energy are discussed first in the context of the individualAEevent.The AE wave redistributes the elastic energy stored in the structure. The propagation of thewave restores equilibrium to a systemwhose equilibriumhas beenmomentarily disturbed. On a longer timescale, the occurrence of multiples acoustic emission events during loading and load holds is also indicative of the material and structure seeking equilibrium in response to stress challenges. The response of the material takes time. AE data from FRP-wrapped air flasks is presented to illustrate the AE behaviour of damaged and undamaged composite material with respect to global stress, local stress and time. During high-pressure hold periods, damaged material produces much more AE than undamaged material, but theAE decays more rapidly. This is explained in terms of the higher stresses that are present in damaged regions. Information on cascades (the equivalent of earthquake foreshocks and aftershocks) during delamination processes is also presented. There is evidence that damagedmaterial ismore likely to produce cascades than undamagedmaterial, and an explanation is offered. The time intervals between the events in the cascades in this case are on the order of 10−2 s.
