Bulk Metallic Glasses (BMGs) are a class of newly emerging materials with many attractive properties like high strength, stiffness and good corrosion resistance. They have many potential engineering applications like in biomedical devices, sporting goods and defence equipment. However, they may exhibit poor fracture resistance which can impede their usage in structural components. Furthermore, their deformation and fracture response has several intriguing features such as heterogeneous plastic flow through multiple shear bands, wide variation in fracture toughness values and enhancement in toughness with Poisson’s ratio. In this lecture, recent finite element and experimental studies aimed at understanding the above issues will be described.
A continuum elastic-plastic material model, based on the Mohr-Coulomb yield condition, which incorporates several important aspects characteristic of BMGs such as pressure and normal stress dependence of yielding and dilatation induced softening is employed in the simulations. Detailed finite element investigation of the mode I and mixed mode (I and II) stationary crack tip fields under plane strain, small scale yielding conditions has been carried out to examine the effects of internal friction parameter, Poisson’s ratio and strain softening. The results show that under mode I loading, a higher friction parameter leads to a larger normalized plastic zone size and higher plastic strain level near the notch tip, but causes a substantial decrease in the opening stress. The brittle crack trajectories and shear band patterns around the notch are also simulated. An increase in Poisson’s ratio reduces the extent of plastic zone and plastic strain levels in front of the notch tip. The possible variations of fracture toughness Jc with mode mixity predicted by employing two simple fracture criteria are examined. Finally, mixed mode (I and II) fracture experiments on a Zr-based bulk metallic glass are performed. It is found that the fracture toughness increases as the loading changes from mode II to mode I. The operative failure mechanism and fracture process zone size are discerned based on observations of incipient crack growth and SEM fractographs. Lastly, a fracture criterion is proposed which predicts the experimentally observed variation of fracture toughness with mode mixity. |
Professor 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. His research interests include Computational Mechanics and Materials Science, Fracture Behaviour of Materials and Indentation Mechanics. He has published more than seventy five articles in refereed international journals and guided the thesis dissertation of about twenty research students. He is a Fellow of Indian Academy of Sciences and Indian National Academy of Engineering. He has served on the editorial board of many journals such as Current Science, Engineering Fracture Mechanics and International Journal of Fracture.
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