Mechanical and Civil Engineering Seminar

Cavitation induced failure in brittle bulk metallic glasses

Professor R. Narasimhan

Department of Mechanical Engineering

Indian Institute of Science

Bangalore, India

Abstract

    Bulk Metallic Glasses (BMGs) have many attractive properties like high strength, stiffness and good corrosion resistance. They have many potential engineering applications like in biomedical devices, sporting goods and defense equipment. However, they may exhibit poor fracture resistance which can impede their usage in structural components. A detailed understanding of the physical mechanisms that govern fracture in BMGs remains elusive. This lecture will focus on a cavitation induced failure mechanism which has been observed in brittle BMGs.

   Mode I fracture experiments were recently conducted using annealed Vit-1 specimens. It was found that brittle fracture occurs and the fracture surface exhibits several interesting features. While HRSEM pictures show some evidence of nano-scale void nucleation close to the original notch front, further away, periodic nano-scale corrugations are noticed. Such fractographic features have also been reported by other investigators for inherently brittle (like Mg-based) BMGs. Atomistic simulations show shear band mediated plastic flow and intense crack blunting  in a ductile (Cu-Zr) BMG, whereas brittle fracture by a cavitation mechanism is noticed for a brittle (Fe-P) BMG. The latter exhibits several intriguing features such as a low cavitation stress to yield stress ratio and void-insensitive cavitation behavior. These are attributed to nano-scale fluctuations in atomic density that are  present in the brittle BMG which give rise to weak regions with low yield strength.

Motivated by the above observations, 2D plane strain continuum finite element analysis of cavitation in a heterogeneous plastic solid containing a distribution of weak zones is performed to understand all features pertaining to nucleation of cavities in brittle BMGs. It is found that the presence of weak zones can significantly reduce the critical hydrostatic stress corresponding to onset of cavitation which is controlled uniquely by the local yield properties of the weak zones and the prevailing stress state. The volume fraction of weak zones and stress state influence the nature of cavitation bifurcation. The results also show that snap-cavitation may occur in heterogeneous plastic solids, giving rise to sudden formation of voids with finite size, which does not happen in homogeneous plastic solids. Continuum simulations of crack initiation under mode-I plane strain, small scale yielding conditions in a heterogeneous plastic solid are also performed and various steps involved in cavitation-induced fracture are delineated.

Brief Background of the Speaker

Narasimhan Ramarathinam received his Ph.D. in Applied Mechanics from California Institute of Technology in 1986. He worked at Indian Institute of Technology, Mumbai, for four years before joining the Mechanical Engineering Department at Indian Institute of Science, Bangalore, in 1991, where he is currently J.C.Bose National Fellow. His research interests include Computational Mechanics and Materials Science, Fracture Behaviour of Materials and Indentation Mechanics. He is a Fellow of Indian Academy of Sciences, Indian National Science Academy and Indian National Academy of Engineering. He has served on the editorial board of many journals such as Current Science, Engineering Fracture Mechanics, International Journal of Fracture and Journal of the Mechanics and Physics of Solids.