Estimation of plain strain fracture toughness of AISI M2 steel from precracked round-bar specimens

Abstract

The fracture toughness of AISI M2 high speed steel measured in round notched and fatigue precracked tensile specimens is discussed. It was established that the blunting of the fatigue crack tip caused by thermal treatment of specimens had no effect on the fracture toughness. The measured fracture toughness was affected by the eccentricity of the fatigued area and resulted in an underestimated fracture toughness. The accuracy of fracture toughness measurements is improved if the eccentricity is numerically evaluated and the measured values corrected accordingly. It was further established that the scattering of results is also influenced by the position of the main crack nucleation site which, in the case of hard steel, most often does not coincide with the fatigue crack frontline. In such cases, fracture is initiated usually in weak spots, e.g. in larger carbide clusters ahead of the fatigue crack frontline, and the measurement gives an overestimated fracture toughness. A correct value may be obtained if the measured fracture toughness is corrected by a statistical analysis in which the radial distance of the weak spot from the fatigue crack frontline is considered.