ACOUSTIC EMISSION EVOLUTION IN INDENTATION FRACTURE OF ROCKS UNDER DIFFERENT LATERAL STRESS-FREE BOUNDARIES
CHEN Lihsien1，HUANG Kuochung2，CHEN Yaochung2
(1. Department of Civil Engineering，Taipei University of Technology，Taipei，Taiwan 10608，China；2. Department of Construction Engineering，Taiwan University of Science and Technology，Taipei，Taiwan 10672，China)
Abstract：Automatic efficiency and engineering safety are both enhanced by using mechanical excavation in tunnel construction；nevertheless，the mechanism for excavation by mechanical cutter is still uncertain. The interaction between the cutter and the position of weak plane during cutting is important for studying the behavior of rock fragmentation，so quasi-brittle materials were used to investigate the fracture mechanism of wedge indentation in order to provide references for the construction in practice. Equipment of a normal wedge indentation was established；and non-destructive acoustic emission(AE) technique was applied for the tests. It simulates discontinuous weak plane of the open joint under the lateral stress-free boundary for indentation at various positions by a wedge with a sharp-shaped tip at angle of 90°. In order to obtain a complete loading history，crack mouth opening displacement(CMOD) control was adopted to avoid unstable fracture after the peak level. From the experimental results，features of geometrical similarity were observed for the crack paths at different indentation positions. When positions approach to the lateral stress-free boundary，the maximum indentation force and the maximum indentation depth both decrease；however，the impact for nominal indentation pressure is not significant. Moreover，it is figured out that microseismic events received by the AE technique can stand for the initiation of the microcracks. Outcomes of AE locations are verified to be very precise by comparing the microseismic events with the macroscopic initial fracture and its development. Localization takes place earlier as indentation approaches to the lateral stress-free boundary；and the radius of elastoplastic interface becomes smaller.
