SUBJECT: Ph.D. Dissertation Defense
BY: Farzad Sadeghi-Tohidi
TIME: Monday, December 14, 2015, 2:00 p.m.
PLACE: Love Building, 210
TITLE: Investigation of the Effects of Extreme Stress Gradients on Fatigue Behavior of Nickel Microbeams
COMMITTEE: Dr. Olivier Pierron, Chair (ME)
Dr. Samuel Graham (ME)
Dr. Hamid Garmestani (MSE)
Dr. Richard W. Neu (ME)
Dr. Antonia Antoniou (ME)


Thin films technologies continue to play a key role in the development of stretchable electronics, flexible displays, and a wide range of microelectromechanical systems (MEMS) applications. Movable components can be exposed to cyclic loading in these applications, which can result in fatigue failure. Hence the investigation of fatigue degradation mechanisms of thin films is required to address some of these reliability concerns. Particularly, extreme stress gradients can occur for notched micro-components or micro-components under bending. In this research, a microresonator-based technique is presented to investigate the effect of extreme stress gradient (normalized stress gradients of 17 and 36%/micrometer) on the fatigue properties of 20-micrometer-thick electroplated Ni microbeams in both mild (30C, 50% RH) and harsh (80C, 90% RH) environments. The technique relies on the measured evolution of the resonance frequency throughout the fatigue test and finite element models to calculate a fatigue life corresponding to the nucleation and growth of a ~2-micrometer-long crack. In addition, the growth rates of these microstructurally small cracks were estimated also based on the measured resonance frequency evolution. It is shown that the fatigue life is dominated by the ultraslow growth of microstructurally small cracks, which is a strong function of the applied stress gradient and results in stress-life (S-N) fatigue curves with low Basquin exponents (in absolute value). As a result, the fatigue endurance limit increases from 35 to 50% of the tensile strength for stress gradients increasing from 17 to 36%/micrometer. The environmental effects are also discussed. The results of this research bring significant insight regarding the reliability concerns of metallic microbeams.