Abstract
Cleavage fracture in steels obeys a critical tensile stress criterion; fracture occurs when a microstructurally determined tensile stress is locally exceeded. As a first approximation, the cleavage of a steel can be regarded as the propagation of a Griffith defect, this defect being provided by a cracked carbide particle. Dimensional considerations show that while being a necessary condition, a critical tensile stress is not sufficient to describe cleavage fracture occurring at the tip of a sharp crack. Ritchie, Knott, and Rice (hereafter referred to as RKR) postulated that the fracture stress must be exceeded over a microstructural ‘characteristic distance’ ahead of the crack tip before a steel will cleave. This model of cleavage fracture was used to predict the temperature dependence of the fracture toughness of mild steel, the predictions being in good agreement with experimental results when the characteristic distance was taken as being equal to two grain diameters. Although originally proposed to explain the cleavage toughness of mild steel, the RKR model has been shown by Parks to be applicable to low-alloy steels as well.
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