Relations between stress drops and acoustic emission measured during mechanical loading

Avalanche events occur during mechanical loading of many material systems and are characterized by stress drops and acoustic emission (AE). Stress drops are directly related to the macroscopic response of the investigated material, but their detection capability is restricted to relatively large and slow events. AE measurements can detect events with smaller amplitude and shorter duration, but their energy and duration are not directly related to the change of the system. In this paper, we present simultaneous measurements of stress drops and AE during mechanically induced twin boundary motion in Ni-Mn-Ga. We found that the probability of finding an AE event during a stress drop is ∼100 times higher than between stress drops. Analysis of the relations between mechanical energy drops ΔUm and acoustic emitted energy EAE, on the level of individual events, reveals the existence of a lower bound for EAE, which is approximately proportional to ΔUm. These results imply that the macroscopic stress changes generate acoustic waves, which contribute a well-defined amount of energy that is equal to the lower bound function. Furthermore, smaller scale events that are related to microscopic subprocesses by which the twin boundary moves generate additional AE energy. The latter contribution displays a power-law distribution, which implies that these processes are close to criticality.