SUBJECT: Ph.D. Proposal Presentation
BY: Alejandro Barrios Santos
TIME: Thursday, April 25, 2019, 8:00 a.m.
PLACE: Love Building, 210
TITLE: Fatigue Mechanisms in Nickel Microbeams under Extreme Stress Gradients
COMMITTEE: Dr. Olivier Pierron, Chair (ME)
Dr. Richard Neu (ME)
Dr. Shuman Xia (ME)
Dr. Josh Kacher (MSE)
Dr. Gustavo Castelluccio (Cranfield University)


Understanding the fatigue properties of small-scale materials has become of great importance due to the widespread use of metallic films and micrometer-scale structures in applications such as flexible electronics, and micro electromechanical systems (MEMS). New techniques are required to characterize the fatigue damage and its size effects in metallic microcomponents under loading conditions relevant to their applications. The goal of this work is to characterize the nanoscale crack nucleation and propagation mechanisms in Ni microbeams under extreme stress gradients by utilizing two small-scale fatigue testing techniques. The microbeams are loaded under bending and subjected to in situ High Cycle Fatigue (HCF) and Low Cycle Fatigue (LCF) testing.

The in situ HCF tests reveal strong environmental effects on fatigue lives that are three orders of magnitude longer in a vacuum than in air. They also highlight crack propagation rates that are extremely low, indicating that the fatigue mechanism does not follow the common crack tip stress intensification. Instead, crack nucleation and propagation are caused by the formation of voids that nucleate from the condensation of vacancies. The LCF tests indicate that the testing frequency has no effect on the fatigue life of these microbeams. Fractography images show that at high stress amplitudes the physical mechanisms of crack nucleation and propagation are dominated by the large cyclic plastic deformation at the crack tip. Further characterization using Electron Backscatter Diffraction (EBSD), Transmission Electron Microscopy (TEM) and Crystal Plasticity modeling will be performed to achieve a more thorough understanding of the microstructural variation along the crack path in the microbeam. These results are expected to provide a better understanding of the fatigue behavior in micro and nano scale devices and therefore allow for a more robust design of these devices.