MSE Colloquium: Dr. Jaafar El-Awady, Dislocation Patterns, Internal Stresses, and Damage Accumulation in Fatigue: Insights from Meso-Scale Modeling and Micro-Scale in situ Experiments
A fundamental understanding of the deformation mechanisms during cyclic loading is essential to predict the usable life of engineering components in aerospace applications. Over the years, many experimental, theoretical, and computational studies have led to significant understanding of plasticity in cyclic loading of metals, however, many open questions remain regarding the evolution of the dislocation microstructure and crack initiation. To address this, here we present a unique approach combining high-frequency fully reversible cyclic uniaxial tension/compression loading in situ scanning electron microscopy experiments and large scale three-dimensional discrete dislocation dynamics (DDD) simulations. The simulations and experiments on Ni single crystal microcrystals are utilized to quantify the effect of crystal size on the formation of persistent slip bands (PSBs) and crack initiation. Experimentally, the cyclic loading is imposed using the high frequency actuator dynamics of a nanoindenters. The changes in the dislocation microstructure, and crack initiation and propagation in the microcrystals are monitored by observing changes in the microcrystal dynamic stiffness and SEM imaging. In the simulations, partially developed PSB structures are simulated under fully reversible loading conditions. The maximum stress of the hysteresis loops and the local dislocation density in the channels/walls are shown to increase with increasing loading cycle. In addition, the spatio-temporal point defect (vacancies and interstitials) generation and evolution is quantified as a function of the dislocation density in the PSB channels and walls. The results are discussed in view of a point defect diffusion model to study their migration rates to the surface.
Prof. El-Awady is currently an Associate Professor of Mechanical Engineering and Materials Science and Engineering at The Johns Hopkins University (JHU). He is also the Chair of the Engineering for Professionals Mechanical Engineering Program. After receiving His PhD in Aerospace Engineering from the University of California Los Angles (UCLA) in 2008, he joined the Air Force Research Laboratory as a vesting scientist for two years. He then joined the faculty of Mechanical Engineering at JHU in 2010, where he established the Computational and Experimental Materials Engineering Laboratory. His expertise is primarily in the field of mechanics of materials for extreme environments, with particular focus on developing: advanced multiscale simulation techniques (from atoms to continuum); and high temperature bulk- and micro-scale experiments, to predict the mechanical properties, underlying deformation mechanisms, damage evolution, and failure in materials. Prof. El-Awady is the recipient of multiple research awards including: the DARPA Young Investigator Program in 2012, the ASME Orr Early Career Award in 2014, the National Science Foundation CAREER Award in 2015, and the Johns Hopkins University Catalyst award in 2018.