Yielding, Relaxation, and Recovery in Amorphous Materials

Abstract

Amorphous materials span a wide range of systems, including foam bubbles, colloidal molecules, and polymer strands. Regardless of their composition, they all share the common feature of structural disorder. This results in universal flow behaviour. The yielding, relaxation and recovery of these systems are relevant to widespread processes, from spreading mayonnaise to flowing magma. Studying their behaviour under rheological shear protocols, by imposing loads and deformations of various forms, reveals fascinating phenomena that challenge the conventional notions of solid and liquid properties. Utilising theoretical mesoscopic models can help in understanding experimentally observed behaviours, and also predict new ones. The work in this thesis consists of three distinct studies into the material responses to imposed shear, and the exploration of their origins and consequences.

The first study researches the stress relaxation of an amorphous material after the imposition of a step strain within two mesoscopic models. The key finding is that a catastrophic shear instability can occur at a long delay time after the initial strain application, under conditions that might intuitively be presumed stable. This failure event is then studied in detail, by analysing its origins in the slow build-up of mesoscopic yield events leading to a shear localisation avalanche, and examining how the delay time before failure occurs depends on the relevant control parameters of the protocol and system.

Following that study, the yielding of an amorphous protein gel under the imposition of a step stress is simulated using a modification, newly introduced in this work, of an established model. More specifically, introducing permanent breaking of the mesoscopic substructures into the model replicates phenomena found experimentally, including the Basquin law of fatigue, Monkman-Grant relation, and three creep regimes. In addition, the study explores the precursors to failure under creep, and how the time of fluidisation can be influenced by the properties of the material and protocol.

The final study investigates the yielding, relaxation, and recovery of an amorphous material under the creep-recovery test protocol through mesoscopic simulation. The primary focus is on recoverable strain, where strain that arises during the stress application can be recovered after the stress is switched off. High levels of strain recovery are predicted, and its dependence on system parameters is explored. An important new discovery is that the recoverable strain in the model system is a result of plastic events, a phenomenon known as reversible plasticity.