In situ monitoring of particle fracture in aluminium alloys

Abstract

Material damage at the microscale involves both initiation and interaction effects that are typically activated long before the appearance of macroscopically observable failure events. These early appearances of damage have been broadly classified as microstructure‐sensitive damage precursors or indicators. A particular class of such precursors which is of importance to aluminium and other precipitate‐hardened alloys is the focus of this article. Specifically, the hard, intermetallic particles in aluminium alloys fracture before significant failure occur in the surrounding matrix. In this investigation, an effort to directly assess particle fracture activity at the time and scale that it occurs is made by coupling mechanical testing inside a scanning electron microscope with nondestructive evaluation techniques including digital image correlation as well as real‐time acoustic emission monitoring. The use of a surface measurement technique along with a volumetric monitoring method at the microscope scale provides a way for coupling of fracture information at locations which are directly related to the particle activity. In this article, Al2024‐T3 specimens in the as‐received condition were subjected to tension as well as to tension‐tension cyclic loading. The obtained in situ results demonstrate, for the first time to the best knowledge of the authors, that particle fracture occurs early in the damage process which justifies its characterization as a material damage precursor. The overall approach provides datasets capable to detect particle fracture initiation, which could become useful in future structural health monitoring applications.

The use of nondestructive evaluation methods in fatigue: A review

“…The use of these optical NDE methods at the microscale and nanoscale leverages imaging provided by scanning and transmission electron microscopes (SEM and TEM) which is achieved by the application of surface patterns that have sufficient electrical contrast to be used by the DIC method . Measurements obtained at such length scales have provided unprecedented insights into the driving forces of fatigue damage initiation and evolution, as described next.…”

“…Figure shows typical experimental setups to perform optical metrology using the DIC method at the (a) microscale and (b) laboratory‐scale. The setup at the microscale (Figure A) relies on using a SEM retrofitted with a mechanical “stage,” ie, an apparatus capable of applying fatigue loading, while allowing the use of the available detectors (eg, the backscattered electron detector, BSE) of the SEM . The “AE Connection” in Figure A is associated with further modifications of the SEM door to allow the extraction of the information (both loading and NDE‐related) recorded inside the chamber of the SEM, which is further explained in the acoustic methods section of this article.…”

“…Typical digital image correlation (DIC) setups for A, the microscale using a scanning electron microscope and B, the laboratory scale using stereo (3D) DIC for crack growth investigations [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Furthermore, DIC has been proven useful in investigating microstructural effects related to fatigue damage incubation and initiation. For example, particle fracture was observed by Payne et al 57 using in situ DIC based on images captured during mechanical loading in the SEM microscope . By coupling in situ SEM observations with appropriately scaled DIC patterns, it is possible to define linkages between microstructural changes, such as particle fracture in alloys .…”

Multiscale deformation measurements using multispectral optical metrology

“…In parallel, the digital image correlation (DIC) method has been used extensively in applications ranging from the micro‐ to the structural scale . In fact, DIC has been used in experimental testing to measure deformation.…”