MIE Seminar: Breakdown of elasticity: from nano-indentation in metals to rigidity loss in soft-particle suspensions.
The Department of Mechanical and Industrial Engineering
Professor CRAIG E. MALONEY, Carnegie Mellon University
Topic: Breakdown of elasticity: from nano-indentation in metals to rigidity loss in soft-particle suspensions.
Date: Monday, February 24, 2014
Time: 10:30am to 12:00pm (Refreshments from 10:00am to 10:30am)
Location: 333 Curry Student Center
Abstract: I will discuss two very different classes of material systems where elasticity fails in complex ways. The first of these is nanoindentation of thin films of single crystals of fcc metals. Linear elastic contact mechanics provides a surprisingly good description of the loading curves at up to a large fraction of the ultimate load. Eventually, however, the crystal becomes unstable, the load on the indenter tip drops catastrophically, and dislocations are nucleated sub-surface, below the indenter. Despite intense work over the last decade, a quantitative and predictive description of this process remains elusive. Using atomistic computer simulations, we demonstrate scaling laws for the size and location of the incipient dislocation loop responsible for elastic breakdown and initiation of plasticity that are surprisingly robust with respect to variations in the form of the interatomic interaction potential-- from embedded atom potentials for Aluminum to simple linear springs -- and crystallographic orientation.
The second class of materials I will discuss is assemblies of confined, soft, deformable particles such as micro-gel suspensions or micro-emulsions. These particle assemblies can behave like conventional solids and exhibit a finite low frequency shear modulus; but only when they are confined at finite pressure. As the confining pressure is relaxed to zero, they lose rigidity and fall apart. Near this rigidity loss, conventional approaches to estimate the elastic moduli (like effective medium theory) completely break down, and it is not clear whether one may use conventional elasticity theory to describe the mechanical response. We examine the elastic response in computer simulations of model micro-emulsions via imposing both homogeneous deformation and point loads. We show that there are two distinct characteristic lengthscales -- one governing the longitudinal modes and the other governing transverse modes -- that diverge in different ways as the pressure goes to zero. It is only beyond these lengthscales that conventional linear elasticity theory provides a good description for the mechanical response. We will finish with a speculative discussion of how this breakdown of elasticity at zero confining pressure might impact the rheological response of these materials in steady and large-amplitdue-oscillatory shear flow.
Bio: Craig Maloney received his PhD in physics from UC Santa Barbara in 2005 where he worked with Jim Langer on the mechanical properties of metallic glasses. He then did a post-doc with Mark Robbins at Johns Hopkins from 2005-2007. He started as an Assistant Professor in Civil and Environmental Engineering at CMU in 2007 and is currently Associate Professor. His work focuses on a broad range of topics in computational materials science and soft matter / complex fluids. Maloney received the NSF CAREER award in 2011, and his work is currently supported by the NSF through DMR, CMMI, and CBET, and by the DOE through the National Energy Technology Laboratory's Regional University Alliance.
Monday, February 24, 2014 at 10:30am