SEMINAR: Investigating Anomalous Fracture Mechanisms at the Nanoscale

Abstract:

In conventional macro-scale continuum fracture mechanics, the J-integral is defined as the divergence of the Eshelby energy-momentum tensor and has been widely used to quantify the crack driving force available from thermo-mechanical loading as well as material inhomogeneities. One advantage of using J-integral over other fracture metrics such as strain energy release rate (G) is that the J-integral is applicable even in the presence of significant material nonlinearity. However, in order to extend the concept of the continuum J-integral to the atomistic domain, the following key issues need to be resolved: (a) computation of continuous variables, such as displacement and their derivatives, from discrete atomistic quantities, (b) including nonlocality in J that is inherent in atomistic computations due to long range inter-atomic forces, and (c) incorporating entropic effects due to thermal motion in a atomistic system which is not present in conventional continuum description. Therefore, the objective of this paper is to develop and apply an atomistic J-integral vector incorporating the above attributes as a suitable metric for the evaluation of fracture behavior in materials at the nanoscale. Specifically, the J-integral is employed in this paper to investigate flaw-tolerance at the nanoscale reported by many researchers, as well as to develop a methodology to predict the initiation fracture toughness of the material and the resistance to fracture as a function of crack length. For this purpose, a bond-order based potential (ReaxFF) available in LAMMPS molecular dynamics (MD) software is utilized to accurately pinpoint bond separation. The framework of the atomistic J-integral fully allows for finite deformation thermo-mechanics, nonlocality inherent in MD, and entropic effects due to the thermal motion of atoms at elevated temperature. Predictions obtained using the atomistic J are compared with LEFM predictions to underscore the flaw size effect for the case of a single (zig-zag) graphene sheet with a center crack under tensile loading at elevated temperatures. Significant deviation from linear elastic fracture mechanics (LEFM) is observed. Predicted Jcritical values are compared with fracture toughness data from experiments, and the atomistic R-curve is also used to verify predictions of Quantized Fracture Mechanics (QFM). Work is currently underway to extend the atomistic J-integral concept to amorphous materials, such as polymers.

Biography:

Dr. Samit Roy received his Ph.D. in Engineering Science & Mechanics Virginia Polytechnic Institute & State University Blacksburg, Virginia. He is currently the William D. Jordan Endowed Professor in the Department of Aerospace Engineering and Mechanics at University of Alabama (UA). Dr. Roy’s research interest is directed towards simulation of materials at the nanoscale, multi-scale modeling, and failure prediction of fiber reinforced polymer composites and structural adhesives subjected to aggressive environmental conditions. Dr. Roy has authored one reference book on polymer composite materials, and more than 75 journal publications, 14 book chapters, and 80 conference papers. In December 2004, he was elected Associate Fellow of the American Institute of Aeronautics and Astronautics (AIAA), and elected Fellow of ASME in 2010. He was elected Chairman of the ASME NanoEngineering for Energy and Sustainability (NEES) steering committee in 2014, and Division Chair, Emerging Composite Technologies Technical Division, of the American Society for Composites in 2016. He is one of the contributing editors of the Polymer and Polymer Composites Journal, Mechanics of Advanced Materials and Structures, and section editor of Applied Mechanics Review, and he has been invited to give plenary and keynote lectures at numerous conferences on composites and nanostructured materials.