Mechanical Characterization of Heterogeneous Brittle Materials

Abstract: Comminution is one of the highest energy-consuming processes in the mining and mineral processing industry by consuming around 2% of the global generated energy with an overall efficiency of 1-3%. Different approaches to the optimization of processes have been developed, but there is still room for improvement. The macro events where energy is mostly spent require numerical methods, so an overall optimization of the system is performed by the analysis and optimization of individual subsystems, such as machines and material to be crushed. The challenge when applying numerical analysis lies on the calibration of the models with mechanical parameters inherent to the constitutive laws and physics of the system. It has been seen that mineral material is exposed to a great variety of time dependent forces within the process. A baseline to understanding the interaction of the material with the machines is the analysis of fracture processes under different loading conditions. This thesis focuses on the mechanical characterization of manganese slag core material for the development, calibration and validationof constitutive models via direct and indirect measurements of the strength and fracture behavior. Diametrial and axial compressive tests under quasi-static and dynamic conditions were used by the hand of optical techniques to obtain information about the evolution of damage. Digital image correlation in 2D and 3D was implemented, considering that it is a method virtually independent ofthe geometry, size, material and deformation rate. Quasi-static tests on both Brazilian disc and unconfined axial compression configurations exposed a mechanical behavior of composite-like material where random failure of the components caused high variability of the elastic parameters. Irreversible damage was perceived globally as non-linearities in load-strain curves, while cyclic loading revealed a degradation of the material affecting the elastic modulus where a weakening of the matrix and dominant behavior of the inclusions on the mechanical response is perceived. Dynamic tests were performed in an in-house built Split Hopkinson Pressure Bar which follows the wave propagation theory in the material generated by the impact of a pressure driven projectile. 2D high speed imaging was performed in order to obtain informationabout the crack initiation and fracture process so that a sampling frequency of 380,000 fps and 663,200 fps was obtained for axially and diametrically loaded samples, respectively. Full-field deformations showed a staggered fracture process were on set failure points vary due to the internal events happening in the material. Localized frictional occurrences and inertial effects acting in the pre-cracked matrix have a strong effect on the global mechanical response and, therefore, a great variability of ultimate compressive and tensile strengths was found. The overall strain/loading rate dependency of the material was perceived as a general increase of the UCT and maximum load compared to quasi-static values. In general, the objective of this work was to study the effect of different loading conditions on the mechanical behavior and material parameters of unprocessed slag for the future development of numerical models of large-scale comminution processes.