A study of optimised network flows for prediction of force transmission and crack propagation in bonded granular media

Abstract

This thesis focuses on study bonded granular materials. We mainly analyse discrete element method simulation data for unconfined concrete specimens subjected to uniaxial tension and compression. In these systems, the contacts can support compressive, tensile and shear forces. Thus, under applied loads, a member grain can transmit tensile and/or compressive forces to its neighbours resulting in a highly heterogeneous contact force network.

The objective of this thesis is two-fold. The first objective of this thesis is to develop algorithms for the identification and characterisation of two classes of force transmission patterns in these systems: (a) force chains, (b) force (energy) bottlenecks. The former comprises a subgroup of grains that transmit the majority of the load through the sample, while the latter comprises a subgroup of contacts that are prone to force congestion and damage. These two classes are related and coevolve as loading history proceeds. Here this coevolution is characterised quantitatively to gain new insights into the interdependence between force transmission and failure in bonded grain assemblies.

The second objective of this thesis is to establish the extent to which the ultimate (dominant) crack location can be predicted early in the prefailure regime for disordered and heterogeneous bonded granular media based on known microstructural features. To achieve this, a new data-driven model is developed within the framework of Network Flow Theory which takes as input data on contact network and contact strengths. We tested this model for a range of samples undergoing quasibrittle failure subject to various loading conditions (i.e., uniaxial tension, uniaxial compression) as well as field-scale data for an open-pit mine. In all cases, the location of the ultimate (primary) macrocrack/failure zone is predicted early in the prefailure regime as well as those of other secondary cracks.

We uncovered an optimised force transmission and damage propagation in the prefailure regime, especially by using data from uniaxial tension tests on concrete samples. Tensile force chains emerged in routes that can transmit the global transmission capacity of the contact network through the shortest transmission pathways. Macrocracks developed along with force/energy bottlenecks. We brought some of the commonly used optimisation based fracture criteria into a single framework and showed how heterogeneity and disorder in the contact network affect the prediction.