Fracture toughness of different types of granite

Abstract

The optimisation of processes such as drilling, cutting, grinding or polishing, demands understanding of the in uence of material related properties in the overall process [1]. For example, the fracture mechanisms involved in material removal (and their dependence on microand macro-defects inevitably present) plays a decisive role in practical rock engineering and fragmentation processes (as those referred above). As a result, a signi®cant number of research studies is currently being undertaken in order to further clarify the in uence of certain fracture parameters (such as the fracture toughness of the stone material) to the fracture mechanisms, as well as to develop accurate methods for their determination [2±4]. Fracture toughness can be de®ned as a measure of the ability of a material to resist the growth of a preexisting crack or aw under stress, and has been extensively used to design fracture safe structures. Using linear elastic fracture mechanics (LEFM) it is possible to establish a relationship between the strength of a cracked structure and the material's fracture toughness, crack size, geometry and location. Fracture toughness determination methods are therefore necessary for safe design and accurate failure analysis. Most of these methods permit evaluation of the lower bound fracture toughness, i.e. the planestrain fracture toughness, which is the crack extension resistance under conditions of crack-tip plane strain. In brittle materials, fracture toughness in pure mode I loading (KIc) is related with the critical value of strain energy release rate (also termed crack extension force), Gc, according to the expression: KIc ˆ  EGc 1y u2† s