Terminal density of granular materials due to thermal cycling

Granular materials, widespread in natural and engineered systems, are constantly subjected to cyclic temperature variations. Experimental and numerical evidence has demonstrated the existence of ‘terminal states’ in granular materials subjected to cyclic mechanical, hydraulic and chemical loading. In such cases, granular materials deform until reaching a terminal void ratio, after which their density remains unchanged under further cycling. However, the existence of terminal conditions and densities has seemingly never been investigated in relation to thermal cycling. This study addresses this knowledge gap using discrete-element modelling. Specifically, it examines the influence of temperature amplitude and initial relative density on the structural evolution and bulk deformation of granular materials subjected to thermal cycles. The results reveal that thermal cycling can also drive granular materials towards a distinct terminal density. Yet, thermal cycling drives granular materials towards a unique terminal density determined solely by the cyclic temperature amplitude. This behaviour contrasts with cyclic mechanical loading, which leads to different terminal densities depending on the initial void ratio and applied stress amplitude. While the number of thermal cycles required to reach this state is strongly influenced by the initial void ratio, the final terminal state is independent of it.