A coupled FFM model to interpret fracture toughness values for brittle materials

AbstractAround 1970 quenching AISI 4340 steel from 1200 °C was discovered to lead to much higher fracture toughness, in the as quenched state, than by conventional austenitizing at 870 °C. Further researches have ascertained that the apparent toughness increase is limited to fracture toughness tests, whereas Charpy-V impact tests do not show any betterment due to high temperature austenitizing, in respect to conventional heat-treating. Various explanations of these contradicting results were given on the basis of the then existing theories. The puzzling phenomenon is here interpreted by means of Finite Fracture Mechanics theories, based on the contemporaneous fulfilment of a stress requirement and the energy balance