A study was conducted on the relationships between impact energy, percent fibrosity and test temperature in the Charpy tests for AISI 4340 steel. It is shown that a linear function of percent fibrosity versus energy exists. Also an energy value U sub F derived from this function is shown to be related to basic strainhardening properties. A relationship, based upon the rate of change in U sub F with respect to temperature, was established. It was possible to describe the appearance of the fracture by identifying three regions of behavior, namely, shear lip, center fibrous and flat. The flat region can be considered representative of rapid fracture while the center fibrous would indicate slow ductile fracture, rapid and slow being defined in this study by unstable and stable fracture propagation.