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Jaafar A. El-Awady
Department of Mechanical Engineering, Whiting School of Engineering
The Johns Hopkins University, Baltimore, MD, USA

A fundamental understanding of deformation mechanisms during cyclic loading is essential to
predict the usable life of engineering components in aerospace applications. While
conventional bulk scale fatigue tests provide a way to quantify the fatigue life of materials,
specific microstructural features that result in failure are difficult to ascertain. One of the
common dislocation microstructures forming during cyclic loading of face centered cubic
metals are persistent slip bands (PSBs). Over the years, many experimental, theoretical, and
computational studies have led to significant understanding of plasticity in PSBs, however,
many open questions remain in the prediction of the formation and evolution of these complex
dislocation structure.

In this talk, we will present a large scale three-dimensional (3D) discrete dislocation dynamics
(DDD) simulations coupled with a novel in situ high-frequency fatigue testing methodology to
investigate PSB formation and evolution in single crystal Ni microcrystal. 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. The dislocation interactions in the
channels and the 3D contours of the local shear stress within the channels as a function of
distance from the PSB walls are characterized to reevaluate the composite model proposed by
H. Mughrabi and the bowing and passing model by L.M. Brown. In addition, the spatiotemporal
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. The effect
of crystal size on the evolution of dislocation microstructures and PSBs is also assessed from
the in situ scanning electron microscopy high-frequency experiments.

Date(s) - Jan 08, 2018
11:00 am - 12:00 pm


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