<p> Small scale tests were utilized in order to obtain characteristic fracture mechanics parameters such as the crack opening displacement (C.O.D.) and the J-integral. Two main types of steels were used (H.S.L.A. and AISI 4340) in obtaining data over a wide range of yield strengths and ductilities. Tests were done to see the effect of both notch geometry and sample geometry·in one of the H.S.L.A. steels, and it is verified that the minimum value of C.O.D., at crack initiation in plane strain, is independent of the geometry and plastic zone size. Further the development of a simple single specimen J-integral test method is done during this work. In terms of the data obtained both the minimum C.O.D.i values and the Jlc values increase with increasing ductility of the materials tested. In the attempt to relate the magnitude of the fracture toughness with microstructural parameters and the limiting processes occuring at the crack tip prior to fracture, the concept of the process zone is discussed. For this study a simple plain carbon steel spheroidized with different carbon contents was examined in addition to the H.S.L.A. and 4340 steels. From the data obtained the minimum C.O.D.i at crack initiation was found to be approximately equal to the product of the materials plain strain ductility and a characteristic distance scaling with the spacing between large non metallic inclusions or the spacing between the bands of the sulphides. </p> / Thesis / Master of Engineering (MEngr)