Brittle Fracture of Rocks

Abstract

Most rocks specimens fail violently and uncontrollably at their peak strength when tested under unconfined stress conditions in conventional hydraulic loading systems. At failure their resitance to carry load rapidly drops to zero. This behaviour is commonly known as the “brittle” behaviour of rocks. Only recently it was recognized that such a behaviour is largely due to the rapid release of stored strain energy from the specimen-machine system (COOK 1965; HUGES and CHAPMAN 1966; BIENIAWSKI 1967). The understanding of this problem made possible a considerable progress in studying the fracture processes in rocks in laboratory testing. WAWERSIK (1968) showed in one of the most comprehensive fracture studies up to date by using a “stiff” loading system that fracture of rock may be considered as a continuous progressive and controllable breakdown process occurring over the entire deformation range, from initial loading to the complete disintegration. The concept of the application of such “stiff” loading systems related to rock testing is shown in Fig. 1 for a soft (K₁), a relatively stiff (K₂) and a very stiff (K₃) system (K are the stiffnesses, defined as the ratio between the drop in force, ΔF, and the corresponding platen displacement).