Change of the mode of failure by interface friction and width-to-height ratio of coal specimens

Bumps in coal mines have been recognized as a major hazard for many years. These sudden and violent failures around mine openings have compromised safety, ventilation and access to mine workings.Previous studies showed that the violence of coal specimen failure depends on both the interface friction and width-to-height（W/H） ratio of coal specimen. The mode of failure for a uniaxially loaded coal specimen or a coal pillar is a combination of both shear failure along the interface and compressive failure in the coal. The shear failure along the interface triggered the compressive failure in coal. The compressive failure of a coal specimen or a coal pillar can be controlled by changing its W/H ratio. As the W/H ratio increases, the ultimate strength increases. Hence, with a proper combination of interface friction and the W/H ratio of pillar or coal specimen, the mode of failure will change from sudden violent failure which is brittle failure to non-violent failure which is ductile failure. The main objective of this paper is to determine at what W/H ratio and interface friction the mode of failure changes from violent to non-violent. In this research, coal specimens of W/H ratio ranging from 1 to 10 were uniaxially tested under two interface frictions of 0.1 and 0.25, and the results are presented and discussed.

Microstructure, Photoluminescent Properties and Application of ZnO Films Grown on Al Foils

To obtain safety working before long-term early warning, we proposed a process for the preparation of luminescent films on metal substrate to detect the wear life. ZnO films were prepared on aluminum（Al） foils by the magnetron sputtering technique. The microstructure, tribological properties and photoluminescence（PL） spectra of ZnO films before and after the friction test were investigated. The microstructure of ZnO films grown on Al foils exhibited a closely packed hexagonal cone shape. ZnO films were grown along the orientation perpendicular to the substrate. The tribometric tests revealed that the average friction coefficient of ZnO films was lower and more stable than that of the substrate. The results of PL spectra indicated that the effect of Al element on ZnO films led to shifts of the defect related visible band. The luminescent center of ZnO films shifted from the emission peak at 510 nm before the friction to 647 nm after the friction, indicating that the green light shifted into the red light as the friction occurred. The visible light was helpful to understanding the failure characteristics during the friction and wear, and provide an early indicator of the impending failure.