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We explored the microstructure-related scale effect on the tensile failure of coal. Disc specimens with different diameters (25 mm, 38 mm, 50 mm, and 75 mm) were processed for the Brazil split test. The influence of microstructures on fracture initiation and propagation was detected and analysed by the combination of X-ray computed tomography scanning, digital image correlation approach, and acoustic emission monitoring. The investigation indicated that tensile strength decreased with increasing specimen diameter, and the relationship can be described by an exponential equation. Four tensile failure modes were observed in specimens with different diameters, namely central, non-central, central edge, and central multiple pattern. Larger specimens exhibited more complicated failure patterns (central edge and central multiple failure patterns) and a greater percentage of shear failure fractures. Strain concentration and strain reversal were observed, respectively, in mineral—inclusion-rich regions and pre-existing discontinuities regions. This possibly contributed to fracture initiation, propagation, and coalescence. Fractures tended to grow along with the interface of mineral inclusion and coal matrix, and they could have propagated into and connect the pre-existing discontinuities. The greater volume of microstructure in larger specimens resulted in fractures that were more complex and may have increased the number of shear failure cracks and led to failure modes that were more complicated.

Control issue is the key for applying hydraulic hybrid system, especially for common pressure rail (CPR) system which has the huge potential to enhance efficiency. In the paper, the mathematical model of hydraulic cylinder speed control system using new hydraulic transformer is established. Then an adaptive fuzzy sliding mode controller based on Pi-sigma fuzzy neutral network is designed to solve the problem of parameter uncertainty and nonlinearity without establishing the precise model. Furthermore, compared to PID and conventional adaptive fuzzy system, the controller proposed can achieve good control performance and strong robustness in the presence of time-varying uncertainty.

Conventional water-based fracturing treatments may not work well for many shale gas reservoirs. This is due to the fact that shale gas formations are much more sensitive to water because of the significant capillary effects and the potentially high contents of swelling clay, each of which may result in the impairment of productivity. As an alternative to water-based fluids, gaseous stimulants not only avoid this potential impairment in productivity, but also conserve water as a resource and may sequester greenhouse gases underground. However, experimental observations have shown that different fracturing fluids yield variations in the induced fracture. During the hydraulic fracturing process, fracturing fluids will penetrate into the borehole wall, and the evolution of the fracture(s) then results from the coupled phenomena of fluid flow, solid deformation and damage. To represent this, coupled models of rock damage mechanics and fluid flow for both slightly compressible fluids and CO2 are presented. We investigate the fracturing processes driven by pressurization of three kinds of fluids: water, viscous oil and supercritical CO2. Simulation results indicate that SC-CO2-based fracturing indeed has a lower breakdown pressure, as observed in experiments, and may develop fractures with greater complexity than those developed with water-based and oil-based fracturing. We explore the relation between the breakdown pressure to both the dynamic viscosity and the interfacial tension of the fracturing fluids. Modeling demonstrates an increase in the breakdown pressure with an increase both in the dynamic viscosity and in the interfacial tension, consistent with experimental observations.