Numerical modelling of fracture processes in thermal shock weakened rock

This paper presents some preliminary results of a research project aiming at the simulation of thermal shock assisted percussive drilling. In the present study, a numerical model for transient thermal shock induced damage in rock is presented. This model includes a rock mesostructure description accounting for different mineral properties and a thermo-mechanical constitutive model based on embedded discontinuity finite elements. In the numerical simulations, the thermal shock induced damge process is first simulated. Then the uniaxial compression test on thermally affected numerical rock samples is carried out. The effect of thermal shock is demonstrated by comparison to uniaxial compression test simulation on intact rock. The results show that the thermal-shock assisted rock breakage is a feasible idea to be extended to percussive drilling as well.Peer reviewe

Comparative Numerical Study on the Weakening Effects of Microwave Irradiation and Surface Flux Heating Pretreatments in Comminution of Granite

Thermal pretreatments of rock, such as conventional heating and microwave irradiation, have received considerable attention recently as a viable method of improving the energy efficiency of mining processes that involve rock fracturing. This study presents a numerical analysis of the effects of thermal shock and microwave heating on the mechanical properties of hard, granite-like rock. More specifically, the aim is to numerically assess the reduction of uniaxial compressive strength of thermally pretreated specimens compared to intact ones. We also compare the performance of these two pretreatments (conventional heating and microwave irradiation) in terms of consumed energy and induced damage. Rock fracture is modelled by a damage-viscoplasticity model, with separate damage variables in tension and compression. A global solution strategy is developed for solving the thermo-mechanical problem (conventional heating) and the electromagnetic–thermo-mechanical problem (microwave heating). The electromagnetic part of the microwave heating problem is solved in COMSOL Multiphysics software Version 6.1 first. The electromagnetic solution is used as an input for the thermo-mechanical problem, which is finally solved by means of a staggered explicit solution method. Due to the predominance of the external thermal sources, the thermal and the mechanical parts of the problem in both cases are considered as uncoupled. Three-dimensional finite element simulations are utilized to study the damage-viscoplasticity model. An ore-shaped three-mineral numerical rock specimen is used in uniaxial compression tests.publishedVersionPeer reviewe

Numerical modelling of microwave irradiated rock fracture

Rock fracturing through microwave irradiation has received significant attention recently as a viable pretreatment for improving the energy efficiency of comminution processes. This study presents a numerical analysis on the effects of microwave heating on the mechanical properties of hard rock. In particular, the reduction of the uniaxial compressive and tensile strength of granite-like rock due to microwave irradiation induced damage is numerically assessed. Rock fracture is modelled by a damage-viscoplasticity model, with separate damage variables for tension and compression types of failure. A global solution strategy is developed where first the electromagnetic problem is solved in COMSOL multiphysics software, then its solution is used as an input for the thermomechanical problem, which is finally solved by means of a staggered explicit solution method. Due to the preeminence of the thermal radiation, the thermal and the mechanical parts of the problem are considered as uncoupled. The model behaviour is tested in 3D finite element simulations of three-mineral numerical rock specimens, with mesostructures explicitly defined, pretreated first in a microwave oven and then subjected to uniaxial compression and tension tests. The results show that the compressive and tensile strength of rock can be considerably reduced by the microwave irradiation pretreatment.Peer reviewe

Modelling the effect of concrete cement composition on its strength and failure behavior

Typical concrete is a mixture of Portland cement, water, and aggregates. While aggregates have a substantial effect on the concrete strength and fracture behavior, the focus of the present study is on the hardened cement paste which can be further divided into the unreacted core, inner and outer products. In high strength concrete, water-to-cement ratio is low, and thus the distance between cement particles is small. Also, the amount of unreacted (high strength) core is higher, and the porosity is low. When water-to-cement ratio is higher, both the distance between cement particles and the porosity due to capillary pores increases. In the present study, we develop a numerical model based on the embedded discontinuity finite elements to predict the effect of the water-to-cement ratio on the compressive fracture behavior of concrete. Representative 2D plane strain simulations demonstrate that the present method captures the major features of concrete fracture and, particularly, qualitatively predicts the known effects of the water-to-cement ratio on concrete compressive strength.publishedVersionPeer reviewe

Weakening of Compressive Strength of Granite by Piezoelectric Actuation of Quartz Using High-Frequency and High-Voltage Alternating Current: A 3D Numerical Study

Piezoelectric excitation of quartz mineral phase in granite using high-frequency and high-voltage alternating current (HF-HV-AC) is a potential new weakening pretreatment in comminution of rock. The present study addresses this topic numerically by quantifying the weakening effect on the compressive strength of granite. For this end, a numerical method based on a damage-viscoplasticity model for granite failure under piezoelectric actuation is developed. The rock material is modelled as heterogeneous and isotropic. However, the piezoelectric properties of quartz are anisotropic. The governing global piezoelectro-mechanical problem is solved in a staggered manner explicitly in time. Numerical simulations predict that the weakening effect on compressive strength of granite is 10% with the excitation frequency and voltage of 274.4 kHz and 150 kV of the pretreatment. As the weakening effect takes place at a natural frequency of the numerical rock sample, the quartz content has only a slight effect on the frequency at which maximum weakening occurs. Moreover, the weakening effect depends strongly on the orientation of the quartz crystals. In a more practical application of simulating low-rate compression of a sphere-shaped rock sample, a weakening effect of 8% after the HF-HV-AC pretreatment was predicted.publishedVersio

A numerical analysis of weakening of a granitic rock by piezoelectric excitation of quartz

This work presents a numerical model to simulate intergranular damage in a granitic rock by oscillating piezoelectric excitation of quartz dispersed in the structure. The damage evolution at grain boundaries was assumed to be related to fatigue and it was modelled using cohesive elements and a damage evolution model formulated in terms of discipation of mechanical work. An explicit representation of the granular mesostructure was built, and it was subjected to high-voltage alternating-current exitation. The effect of the fatigue damage on the mechanical properties was quantified by simulated tension and compression tests. The numerical results show that the electrical treatment can potentially cause rock weakening due to fatigue, but the model needs to be calibrated with experimental data for a quantitative analysis.Peer reviewe

Weakening of tensile strength of granitic rock by HV-HF-AC actuation of piezoelectric properties of Quartz : a 3D numerical study

High-voltage and high-frequency alternating current (HV-HF-AC) excitation of piezoelectric properties of Quartz is a potential method to induce cracks in granite. This was recently shown in a numerical feasibility study [6], where cracking was induced on cylindrical rock samples made of granite by sinusoidal AC excitation at the frequency of ~100 kHz and the amplitude of ~10 kV. However, this study did not investigate the weakening effect due to this cracking on the tensile strength of the sample. The present study addresses this topic numerically. For this end, a numerical method based on 3D embedded discontinuity finite elements for rock fracture and an explicit time stepping scheme to solve the coupled piezoelectro-mechanical problem is adopted. Rock heterogeneity and anisotropy are accounted for at the mineral mesotructure level. A preliminary numerical simulation demonstrates that the HV-HF-AC treatment reduces the tensile strength of a cylindrical granite sample by 12 %, making it thus a potential non-conventional pre-treatment method in comminution and excavation of Quartz bearing rocks and ores.publishedVersionPeer reviewe

Progressive Weakening of Granite by Piezoelectric Excitation of Quartz with Alternating Current

A promising solution to reduce energy usage and mitigate the wear of drilling and comminution tools during mining operations involves inducing vibrations within the piezoelectric phases dispersed in the structure of rocks using alternating current (AC). This paper presents experimental evidence of AC-induced weakening of Kuru granite, manifested as improvements in rock drillability and reductions of strength. Sievers’ J-miniature drill tests were used to assess surface drillability. The impact of AC treatment on the quasi-static strength of granite was assessed via three-point bending and indirect tension Brazilian disk tests. The influence of AC treatment on the dynamic tensile strength of the rock was determined using split Hopkinson bar tests, with the fragmentation process captured using in situ ultra-fast synchrotron X-ray phase contrast imaging. The quasi-static tests revealed no reduction in rock strength after the AC treatment. In contrast, reductions of 25% in hardness and 18% in dynamic tensile strength were observed. Fragmentation patterns differed between treated and non-treated rocks, with treated specimens exhibiting reduced macrocrack formation during loading.Peer reviewe

Numerical modelling of rock materials with polygonal finite elements

This article presents some preliminary results on numerical modeling of rock materials with polygonal finite elements. A method to describe the rock microstructure based on Voronoi diagrams, representing the rock grain texture, is sketched. In this method, the minerals constituting the rock are represented as Voronoi cells which themselves are polygonal finite elements. A three-point bending problem under plane stress linear elasticity condition is solved in order to compare the performance of polygonal elements to ordinary finite elements. Moreover, it is demonstrated by solving the stress state in uni-axial compression that the heterogeneity described with the present method results in short-range tensile stresses which could initiate mode-I cracks.Peer reviewe

Numerical modelling of bit-rock interaction in percussive drilling

publishedVersionNon peer reviewe