STUDY OF EFFECT OF STRAIN RATE ON CRITICAL DAMAGE IN GRANITE BY ULTRASONIC IMAGING
G. M. Nagaraja Rao, C. R. L. Murthy* and N. M. Raju National Institute of Rock Mechanics, Kolar Gold Fields 563 117, India *Indian Institute of Science, Bangalore 560 012, India
Strain may change in seconds during blasting and earthquakes, or over many years as mining proceeds, thus possible strain rates may cover a range of over orders of magnitude. 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 (fault). So to study the effect of strain rate, laboratory experiments were carried out with strain rate varying from 10-5 S-1 to 10-7 S-1 on dry granite. And, different phases of microcrack development in terms of crack initiation, growth, and coalescence leading to final fault formation were investigated using Ultrasonic C-Scan imaging technique. To understand the microcrack development ultrasonic images were taken at different stress levels. And, these images have shown, (a) microcrack damage which has been designated as types A, B, C and D and (b) macrocrack damage named as E type. With the increase of stress there is a transformation from one type of damage in to the other occurring in the sequence
A-> B-> C-> D-> E->
Ultrasonic images obtained from the above experiments show microcrack damage in the form of clusters. Coalescence of clusters forming a network leading to macrocrack formation occurs for a strain rate of 10-5 at 98% of failure strength, for strain rate of 10-6 at 82% of the failure strength and for strain rate of 10-7 at 77% of the failure strength. From these images, the critical damage beyond which the macrocracks initiate fault formation is found to be about 62%. Critical damage occurs at lower stress level with the decrease of strain rate with the result that the strength decreases with the decrease of strain rate.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 2, pages 791 - 794 Publisher:Ashgate Publishing Company
NDTnet
