Deformation and fracture lab course
Determination of KIc

Introduction
In this lab course, we are going to determine the critical stress intensity factor〖 K〗_Ic, with regard of three different samples of heat treated steel. On a first part, I will do a short theoretical review about the fracture toughness followed by the description of the microstructure and how it is linked with the toughness. In a second part, I will discuss the experiments of the samples tested according to the standard AFNOR NF EN ISO 12737 and the results. Definitions
Contrary to the ‘standard mechanics’, in fracture mechanics we consider a discontinuity in the bulk such a crack or a defect. It allows us to determine, considering the size of the crack, the load and the intrinsic parameters of the bulk to have the speed of the propagation of the crack and the critical size of a flaw for which we have propagation and then failure. The fracture criterion is then the primary basis of the safe evaluation in cracked materials
If one considers only plane cracks propagates in their own plane, one shows the general state of propagation of a crack is the superposition of three modes of propagation of a crack (Figure 1): Mode I: Opening mode Mode II: Forward shear mode Mode III: transverse shear mode Figure 1: Ways of driving a crack in a body
Since the mode I is the most critical, it explain the numerous studies of this mode. Also, one distinguishes three kinds of cracks: edge cracks, internal cracks and through cracks (figure 2). Figure 2: Kind of cracks Griffith’s theory:
In the 20’s, Griffith A.A developed a fracture theory motivated by one particular problem[1]: why the stress needed for fracture bulk glass is about hundred times lower than the theoretical stress needed for breaking atomic bonds? Griffith suggested that the low fracture strength observed in experiments, as well as the size-dependence of strength, was due to the presence of microscopic flaws in the bulk