Strength, Toughness, Damage And Fatigue Of Rock

Assessment of rock mechanical properties depends on sample size and testing methodologies. Even for samples cored from the same rock outcrop the difference in properties appears to be sensitive to the local thermal and stress histories of the rock structure. Variations in the fracture toughness, unconfined compressive strength and tensile strength of a suite of granite samples, when tested using different procedures, are discussed in terms of experimental errors of the loading system as well as the thermal history

The Brazilian disc test under a non-uniform contact pressure along its thickness

Using a three-dimensional analytical approach, the sensitivity of the Brazilian test to its standard testing recommendations was investigated. It was concluded that the tensile stress induced in a Brazilian disc is significantly affected by the distribution of the applied load along its thickness rather than its circumferential condition. Under a non-uniform contact pressure along the BTS thickness, it was evident that both the numerical value and the location of the maximum tensile stress varied as a function of the geometrical aspect ratio of the disc specimen. For test conditions in which load distribution in the contact region along the thickness does not follow the standards or the uncertainty of its exact nature is large, e.g. in testing of super hard materials with relatively high stiffness and hardness greater than the contact testing platens, great care should be taken in regard to the interpretation of the Brazilian test result

On stability of time marching in numerical solutions of rayleigh-plesset equation for ultrasonic cavitation

Ultrasonic irradiation approach has become one of the most popular methods applied in chemical processing including lignocellulosic biomass pretreatment and industrial cleansing. The phenomenon of ultrasonic cavitation can be indeed delineated via the Rayleigh-Plesset equation (RPE), which governs the transient radius of the bubble. Nonetheless, the time marching in the numerical solutions for RPE is highly unstable, which cannot be assured using von Neumann analysis. High sensitivity of RPE to time step may lead to extremely long computational time. The lack of numerical investigation into the time stepping issue of RPE has hindered in-depth simulation of ultrasonic cavitation. Therefore, the purpose of this paper is to investigate the stability criterion of time stepping for RPE in different time progression schemes, namely Euler explicit, 2nd order Taylor’s method, 4th order Runge-Kutta, Runge-Kutta Fehlberg and Cash-Karp Runge-Kutta method. A simple modified adaptive time step method and a independence study has been introduced in this paper for fast, stable and accurate computation of RPE. Compared with the traditional constant time marching method, the new model is able to improve the computational cost significantly without affecting the time marching stability and resolution of the results. Among the investigated method, Runge-Kutta family solvers have higher computational accuracy, with the cost of higher critical a value. The model is also applied to compute the pressure and temperature hike during bubble collapse due to different sonication power. The simulation results show that the ultrasonic irradiation with higher sonication power could produce a higher energy to break the lignocellulose wall

Modelling cohesive, frictional and viscoplastic materials

Most materials in mining and civil engineering construction are not only viscoplastic, but also cohesive frictional. Fresh concrete, fly ash and mining slurries are all granular-frictional-visco-plastic fluids, although solid concrete is normally considered as a cohesive frictional material. As an introductory mathematical framework, this paper presents formulation for flow rate as a function of pressure and pressure gradient in pipes and discs

High-impact engineering education: using the LTI to influence knowledge and skills for sustainable economy

Australia needs more creative graduates. Over the past decade, the Australian economy has become increasingly dependent on the mining and goods/services sectors. Growth in these areas has largely been at the expense of the manufacturing sector, which was previously the largest sector in the economy. The country’s education system has also progressed disproportionately towards the dominant service sector, mostly at the expense of a progressively-designed, innovative Engineering education system. Traditional Engineering curriculum has inappropriately progressed towards abstractness, rather than the necessary interactive, integrated experience based on practical learning. There is a need to re-evaluate the current educational infrastructure to ensure that emerging professionals are equipped to function and excel in both service and manufacturing capacities

Heat transfer during cavitation bubble collapse

Cavitation phenomenon has found various industrial applications. The collapse process of a cavitation bubble is extremely violent in its final stage and the gas within the bubble can become extraordinarily hot. This paper introduces a heat transfer model to calculate accurately the temperature change and heat transfer during a bubble collapse based on the Rayleigh–Plesset (RP) equation and CFD modelling. To demonstrate the variations of pressure, temperature and velocity distribution in the liquid and bubble, a two-phase compressible CFD model developed to simulate the process of the bubble collapse. Results from the RP equation – modified with conduction and radiation effects – match the numerical CFD results very well. Further investigations were carried out on the bubble collapse and temperature increase, heat transfer rate by conduction and radiation, and accumulative heat transfer of bubbles with different bubble sizes. When a cavitation bubble with initial maximum radius of 2 mm collapses, the maximum temperature of the air can rise up over 0.02 mega degrees Kelvin (MK) and the transferred heat by radiation and conduction accumulated in the first cycle of collapse can reach 40 micro-joules (μJ). The solution to the modified RP equation provides a practical method for the estimation of heat transfer and temperature increase in cavitation equipment

Stress analysis of longwall top coal caving

Longwall top coal caving (LTCC) is a relatively new method of mining thick coal seams that is currently achieving high productivity and efficiency in application, particularly in China. The technique is similar to traditional longwall mining in that a cutting head slices coal from the lower section of the coal seam onto a conveyor belt installed in front of the hydraulic support near the cutting face. In modern LTCC an additional rear conveyor belt is located behind the support, to which the flow of the caved coal from the upper part of the seam can be controlled by a moveable flipper attached to the canopy of the support. The mining method relies on the fracturing of the top coal by the front abutment pressure to achieve satisfactory caving into the rear conveyor. This paper develops a yield and caveability criterion based on in situ conditions in the top coal in advance of the mining face (yield) and behind the supports (caveability). Yielding and caving effects are combined into one single number called caving number (CN), which is the multiplication result of caving factor (CF) and yield factor (YF). Analytical derivations are based on in situ stress conditions, Mohr-Coulomb and/or Hoek-Brown rock failure criteria and a non-associated elastoplastic strain softening material behaviour. The yield and caveability criteria are in agreement with results from both numerical studies and mine data. The caving number is normalised to mining conditions of a reference Chinese mine (LMX mine) and is used to assess LTCC performance at fourteen other Chinese working longwalls that have had varying success with the LTCC technology. The caving number is found to be in good agreement with observations from working LTCC mines. As a predictive model, results of this analytical/numerical study are useful to assess the potential success of caving in new LTCC operations and in different mining conditions. © 2009 Elsevier Ltd

Viscous, cohesive, non-Newtonian, depositing, slurry pipe flow

Backfilling and injection of granular materials into the mining induced voids, separated beddings and cracks, as either diluted slurry or concrete paste, is widely used to control coal mine subsidence. As a viable environmental solution, mine waste and rejected materials from underground coal seams are used in both backfilling and injection mine operations. During longwall mining the grout slurry is pumped into the separated beds of the rock mass through a central vertical borehole, which is drilled deep into the inter-burden rock strata above the coal seam. Either as dilute slurry or thick paste or cake, the fill material normally needs to travel a significant distance in a long pipeline. An undesirable blockage can occur in the pipeline when the slurry velocity falls below a certain critical threshold velocity, indicating a material phase change from cohesive-viscous to cohesive-frictional. In a previous study of radial flow through disks, complete analytical solutions of the required pump pressure versus fluid volume rate were presented for such slurries, categorised as frictional Bingham - Herschel - Bulkley fluids. This paper is an extension to the theory of fluid mechanics to this type of flow in uniform circular pipes. General analytical solutions have been developed for complex fluids in terms of velocity and pressure gradients and velocity and pressure, as a function of pipe length, from which special and familiar equations for simpler fluids are derivable by mathematical reduction of the general equations. This study differs from the previous research in several distinct aspects, namely, consideration of the variable shear parameters rather than fixed values, inclusion of total nonlinear behaviour, and implementation of a friction function to mimic behaviour of the depositing and consolidating stiff slurry, which can cause a significant pressure rise as a result of the increased shear resistance

Backfill grouting for mining subsidence prevention

Mining subsidence has been a major hazard in most underground coal mines, particularly those designs and practices based on the wrong assumption of fixed, permanent and nondeteriorating coal pillars. Mining induced subsidence significantly affects mining costs where major surface structures and natural environment need to be protected. Remedial measures to manage damage caused by subsidence can often be very costly with potentially damaging impacts and irreversible consequences. Backfilling and injection of granular materials into the mining induced voids, separated beddings and cracks, as either diluted granular slurry or concrete paste, is widely used to control mine subsidence. Granular grouts and slurries made of mine and power plant wastes and rejects are viable environmental backfill solutions to both ground stability and mine waste management problems. Like concrete paste, the flowing slurry can be categorised as a generally nonlinear frictional viscous cohesive (Bingham Herschel-Bulkley) fluid. The general frictional viscous, cohesive, non-Newtonian fluid model has been applied to concrete flowability problems such as L-box and slump tests. While slump test is used in shallow foundations, L-box test is used in difficult deep foundations. It is designed to measure workability and flowability of tremie pipe concrete as an indirect index measure of concrete viscosity and plastic yield. Tremie pipes are used to control concrete flow rate and minimise bleeding and dilution when placed into deep submerged excavations. Mathematical and experimental models have been developed to not only solve the flow velocity along the L-box channel length as a function of time and distance, but also simulate the flow of the backfill material and demonstrate the detailed process of filling the voids to minimise any further subsidence