Determination of KIc
In this lab course, we are going to determine the critical stress intensity factor〖 K〗_Ic, with regard of three differentsamples 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 thetoughness. 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.
Contrary to the ‘standard mechanics’, infracture 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 tohave 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 incracked 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 explainthe 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
In the20’s, Griffith A.A developed a fracture theory motivated by one particular problem: why the stress needed for fracture bulk glass is about hundred times lower than the theoretical stress needed forbreaking 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...