Alladi Ramakrishnan Hall

Cavitation instabilities in amorphous solids: an athermal study

Umang Dattani

IMSc

[pre-synopsis seminar]

Amorphous solids have a wide-range of applications but they are also known to fail catastrophically. Multiple experiments suggest that the fracture in these solids propagates via coalescence of nano-scale cavities. Understanding these cavities is important not only in understanding the mechanisms of failure but also in designing stronger glasses. The cavitation of the solid can be seen as an instance of gas + solid phase separation in the temperature vs density phase-diagram of amorphous solids. We study various aspects of the cavitation process under athermal conditions, using extensive numerical simulations.

In the first part of our work, we study the consequences of driving the amorphous solid into the phase space of gas + solid coexistence via uniform expansion. We encounter numerous similarities with the shear-response of these amorphous solids. We also elaborate on the mechanism of cavitation from a potential energy landscape point of view.

In the second part of our work, we explore cavitation by combining a secondary drive in the form of cyclic shear with the uniform expansion. We mark the cavitation region in a phase-diagram with amplitude of cyclic shear and the density as the tuning parameters, and demonstrate how these cavitation instabilities couple better to shear than to uniform expansion. Similar findings are also reported for the case when active forcing is used as a secondary drive.

In the third part of our work, we look at the effect of random pinning on cavitation under uniform expansion loading. These randomly pinned sites are a simple model for inclusions that are often seeded in amorphous solid via micro-alloying. We find that even by pinning a very small fraction of particles (2%-5%), the cavitation is delayed and less abrupt. By looking at the eigenmodes near cavitation events, we find that pinning introduces a length scale of plasticity into the system which decreases with increasing the pinning concentration, thereby avoiding avalanches that lead to catastrophic failure of the solid. Thus, pinning resists these cavitation instabilities and also the complete fracture.

To summarise, our work establishes a major chunk of phenomenology in the study of cavitation instabilities in amorphous solids and connects to other well-studied modes of deformation (e.g. simple shear). Our work also explores a way to make stronger and better amorphous solids.