Study of Effect of Strain Rate on Critical Damage In Granite By Ultrasonic Imaging

摘要

Mechanical properties and fracture behaviour of rock mass is highly dependent on strain rate. Fracture or failure of rock occurs by nucleation, extension, interaction and coalescence of microcracks forming a macroscopic crack. To study the effect of strain rate, laboratory experiments were carred out at strain rates 10~-5, 10~-6 and 10~-7 S~-1 on dray granite. Different phases of microcrack development in terms of crack initiation, growth, and coalescence were invesitagted using Ultrasonic C-scan imaging technique. Ultrasonic images taken at different stress levels, were identified as micro(A,B,C & D type) and macro crack damage (E type). With the increases of stress there is a transformation from one type of damage into the other occurring in the sequence, A->B->C->D->E. Microcrack damage increases with the increase of stress and also with the decrease of strain rate. Critical damage occurs at lower stress levels with the decrease of strain rate, with the result that strength decreases with the decrease of strain rate.