Four Point Bend Testing of Shipboard Sensitized Aluminum-Magnesium Alloy to Characterize Effect of Surface Ultrasonic Shot Peening on Stress Corrosion Cracking
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Date
2019-01-01
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Department
Mechanical Engineering
Program
Engineering, Mechanical
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This item may be protected under Title 17 of the U.S. Copyright Law. It is made available by UMBC for non-commercial research and education. For permission to publish or reproduce, please see http://aok.lib.umbc.edu/specoll/repro.php or contact Special Collections at speccoll(at)umbc.edu
This item may be protected under Title 17 of the U.S. Copyright Law. It is made available by UMBC for non-commercial research and education. For permission to publish or reproduce, please see http://aok.lib.umbc.edu/specoll/repro.php or contact Special Collections at speccoll(at)umbc.edu
Abstract
High strength aluminum-magnesium (Al-Mg) alloys are commonly used in naval structural applications but sensitization of their microstructures can lead to stress corrosion cracking (SCC). Sensitized microstructures contain ? phase at the grain boundaries that is anodic to the Al matrix. In order for SCC to occur, a susceptible microstructure, corrosive environment, and stresses must be present. Surface treatment methods that impart compressive stresses are used to combat the harmful tensile stresses that lead to SCC. Ultrasonic shot peening is a repeatable surface treatment method that imparts compressive stresses with a reduced surface roughness in comparison to other surface treatment methods. This study aims to understand the effect of ultrasonic shot peening on the meso- through micro-scales of SCC on a high strength Al-Mg alloy, 5456- H116 Al, under in-service comparable conditions. Shipboard and laboratory sensitized 5456-H116 Al with and without ultrasonic shot peening is tested in static four-point bending in an alternate immersion artificial seawater environment. Ultrasonic shot peened surfaces are characterized for compressive stress using hole drilling, surface texture using confocal microscopy, and grain deformation using electron backscatter diffraction (EBSD). Quantitative data from the bent beam testing differentiates between the time to start cracking and the time to reach full failure. Failed four-point bend specimens are examined using optical microscopy and X-ray computed tomography (X-ray CT) at the meso- and macro-scales to connect the flexural stress state to resulting SCC shape and branching behavior. Three-dimensional (3D) characterization of the micro-scale branching crack using X-ray CT front provides insight into SCC growth mechanisms and microstructure influenced localized corrosion events.