In-Situ Characterization of Stress Corrosion Crack Initiation via X-Ray Synchrotron Tomography

Aluminum alloys in aerospace structures are prone to stress corrosion cracking (SCC), resulting in premature failure. Understanding the key mechanisms initiating SCC is crucial for enhancing resistance. While extensive research exists on SCC, quantifying SCC in 3D volumes is a means to attain a more comprehensive understanding of the driving factors. In-situ x-ray synchrotron tomography offers potential insights unattainable through 2D or surface-level methods, narrowing the characterization scale. This technique has been used to successfully capture SCC in novel ways through discontinuous cracking, through-thickness cracking, hydrogen bubble formation, and secondary phase formation. This has inspired the current effort here to characterize the roles of mechanical stress and hydrogen bubble formation. This study highlights crack propagation and the role of hydrogen bubbles from both mechanical and chemical perspectives, indicating their potential for assessing pre-initiation and initiation stages of SCC. Within 40 minutes, hydrogen bubbles and precipitates were captured where the crack would eventually form, indicating that hydrogen bubbles and precipitates can be used as a way to assess pre-initiation and initiation stages of SCC. By using pixel count in tomographic images as a means to measure crack growth over time, it was found that the trend was nearly logarithmic, which can be attributed to intergranular corrosion. These findings represent a first step in characterizing SCC and understanding their interplay during the initiation stages. The application of in-situ x-ray synchrotron tomography marks progress in comprehending SCC initiation. The insights gained can inform the development of corrosion control strategies and innovative methods for manufacturing SCC-resistant aluminum alloys.