title: A micromechanical investigation on the mechanism of energy dissipation in granular materials
Creators: name: Essayah, Abdurrahim
Publisher: name: University of Glasgow
Year: 2023

description: Energy dissipation is crucial for understanding the mechanics of granular materials. Discrete element modelling has been carried out to investigate the mechanism of energy dissipation in granular materials. Firstly, the relationship between energy dissipation and contact networks is investigated. Using the discrete element method, a series of triaxial compression tests with different stress paths have been simulated. The energy dissipation was analysed using two existing contact force network partitioning approaches, one based on the magnitude of average contact force and the other one based on the contribution of contact forces to the global deviator stress. The results indicate that for both techniques, neither the strong nor the weak contact networks exhibit negligible energy dissipation. When the average contact force partitioning approach is used, more than 70% of the energy dissipation occurs in the weak contact network, although the dissipation per sliding contact is greater in the strong contact network because the tangential contact force is greater. When the contact network is partitioned depending on the contribution of contact forces to global deviator stress, the strong contact network dissipates around 60% of the total energy. A new normal contact force threshold is found for partitioning the contact force based on the contribution to energy dissipation. Almost 93% of energy dissipation occurs at contacts with a normal contact force that is less than two times the average normal contact force. As a result of little particle sliding, interactions with a greater normal contact force result in a negligible amount of energy loss. Secondly, a quantitative investigation of stored plastic work and energy dissipation in granular materials has been studied using discrete element modelling. Drained triaxial compression tests on samples with different particle size distributions (PSD) have been simulated. The elastic stiffness is measured using stress probe tests and then used to calculate the elastic and plastic strain in the samples. The total work input is decomposed into two parts, including the elastic free energy and plastic work, which are dependent on the elastic and plastic strain, respectively. The plastic work is further decomposed into stored plastic work and dissipated energy. There is little elastic free energy in the material due to the small elastic strain. The stored plastic work is much smaller than the dissipated energy for samples with all particle size distributions. This could be due to a lack of interlocking among spherical particles, which is expected to ‘freeze’ plastic work in the material. Finally, DEM simulation was also carried out to analyse some of the existing energy dissipation and stored plastic work equations. These energy dissipation functions showed inaccuracy in predicting the amount of energy dissipation. A new modified dissipation function was produced. This new function was able to offer a reasonable prediction for samples with different particle size distributions. Furthermore, a new stored plastic work function is developed based on the DEM results. As the quantity of stored plastic is small, a suitable function should be depending on the pressure increment. The new function can provide a good prediction for samples with different particle size distributions.