[en] Linear elastic fracture mechanics can be applied to medium strength steels when they are used in large thicknesses. In this case it is possible to put in evidence the existence of a parameter, Ksub(Ic), characteristic of the material, which depends only on the strain rate and the temperature, and which can be used to predict quantitatively the conditions for fracture of a structure containing a defect. The determination of Ksub(Ic) involves the use of large specimens, and thus of big testing machines. It is then clear that such tests can not be performed systematically on many steels, and certainly not at all in metallurgical quality control. The aim of this study was to check and develop further certain new theories which make it possible, according to their authors, to evaluate Ksub(Ic) with small specimens: correlations between Ksub(Ic) and Charpy V toughness, J-integral, equivalent energy concept. As these techniques lead to values of toughness in plane-strain conditions, we have also studied the significance of R-curves, a concept which has be proposed to characterize the toughness of a material in plane-stress conditions. This report has thus been divided into four parts which correspond to the four approaches just mentionned