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Acoustic emission (AE) is a common nondestructive evaluation tool that has been used to monitor fracture in materials and structures. The direct connection between AE events and their source, however, is difficult because of material, geometry and sensor contributions to the recorded signals. Moreover, the recorded AE activity is affected by several noise sources which further complicate the identification process. This article uses a combination of in situ experiments inside the scanning electron microscope to observe fracture in an aluminum alloy at the time and scale it occurs and a novel AE signal processing framework to identify characteristics that correlate with fracture events. Specifically, a signal processing method is designed to cluster AE activity based on the selection of a subset of features objectively identified by examining their correlation and variance. The identified clusters are then compared to both mechanical and in situ observed microstructural damage. Results from a set of nanoindentation tests as well as a carefully designed computational model are also presented to validate the conclusions drawn from signal processing.

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Efforts to understand fatigue in materials generally rely on defining relationships between the material response to fatigue loading and the evolving remaining material life portion. Typical measurements and data associated with such relationships include load (stress), displacement (strain), and associated properties (eg, mean stress and strain, energy dissipation, and residual stiffness). In addition to obtaining such measurements, efforts have been made to leverage nondestructive evaluation (NDE) methods to obtain additional information and enhance the description of the material fatigue response, especially its evolution due to interacting material‐microstructure‐property relationships. Given the range of available NDE methods and the number of materials they can be applied to, a critical overview of related investigations is presented in this article. Specifically, the state of the art of applying optical, thermal, acoustic, electromagnetic, X‐ray, and other diffraction NDE methods in fatigue investigations is provided and discussed. The methods are individually described for background and defining their relationship to fatigue, followed by descriptions of their current uses and contributions to enhancing the understanding of fatigue, and their limitations and ways that improvements could be made.