< BK21+ BEST Seminar Series Announcement> 

Time and Date : 11:00 ~ 12:00 Thursday 09/27/2018

Place : D603, Engineering Building #4

1) Title : Fracture criteria for nanoscale stress singularity in brittle silicon

Abstract:

Brittle materials such as silicon fail via the crack nucleation and propagation, which is characterized by the singular stress field formed near the crack tip according to fracture mechanics theory. The applicability of the continuum-based theory is, however, uncertain and questionable in a nanoscale system due to its extremely small singular stress field of only a few nanometers. Here, we directly characterize the mechanical behavior of a nanocrack via the in situ nanomechanical testing using a transmission electron microscope as well as quantum-mechanics/molecular-mechanics (QM/MM) simulations and demonstrate that Griffith or fracture mechanics theory can be applied to even 4 nm stress singularity despite their continuum-based concept. We further show that a singular stress field of only several nanometers still governs fracture as successfully as that at the macroscale, whereas both the stress intensity factor and the energy release rate fail to describe fracture below a critically confined singular field of 2–3 nm, i.e., breakdown of fracture mechanics within the framework of the continuum theory. We further propose an energy-based theory that explicitly accounts for the discrete nature of atoms, and demonstrate that our theory not only successfully describes fracture even below the critical size but also seamlessly connects the atomic to macroscales. It thus provides a more universal fracture criterion, and novel atomistic insights into fracture.

2) Title : Size effect on fracture toughness of freestanding copper nano-films

Abstract:

In bulk metals, fracture toughness is insensitive to specimen thickness as long as the specimen thickness is large, and is considered to be a size-independent material constant (i.e. plane strain fracture toughness). However, it is not true for thin film materials. We performed in-situ field emission scanning electron microscopy fracture toughness tests on single-crystalline and polycrystalline Cu films with thicknesses ranging from ~40 nm to ~2700 nm deposited by electron beam evaporation to elucidate the size effect on fracture toughness in the nano- or submicron-scale. In all specimens, the notch root became blunt, and a crack was initiated from the blunted notch root. We evaluated the critical crack tip opening displacement (CTOD) for crack initiation, i, or the fracture toughness on the basis of elasto-plastic fracture mechanics concept. The results indicated a clear thickness effect on fracture toughness, where i decreased with a decrease in the thickness. The critical CTOD normalized by the thickness, i/B, values of the films were similar (i/B = 1.4–1.9), irrespective of the film thickness and microstructure. This suggested that the local fractures of the nano- and submicron-thick Cu films were similar.

Presenter: Prof. Takahiro Shimada, Prof. Hiroyuki Hirakata / Department of Mechanical Engineering and Science, Kyoto University

Host: Prof. Ahn, Jong-Hyun, Yonsei EEE