Abstract
Phenomena of crack overlaps are very common and essential in tunnel constructions by rock blasting. In this study, in conjunction with the high-speed camera, directional fracture controlled blasting experiments are performed on three sets of PMMA specimens using the optical caustics method. The lenticular fragments formed as two cracks approach each other are measured and compared, as well as other fracture parameters, such as crack velocity, mode I and II dynamic stress intensity factors and curvature of crack propagation path. It is found that both mode I stress intensity factor and crack velocity exhibit an abrupt rise upon crack interaction. Further, mode II stress intensity factors show more complicated variations in comparison with mode I, divided into three loading regimes, i.e. crack initiation, blast stress wave loading and crack overlap. And an appropriate ratio of crack offset distance/horizontal distance (h/s) between two boreholes for apparent mixed mode failure exists.
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