Abstract
The paper presents methods for assessing the onset of local critical states for the initiation of cleavage and ductile fracture in engineering materials with spheroidized structures. These methods are based on statistical analyses of the microstructure using the "weakest link" approach. This yields the probability of formation of local critical state as a function of the distance from the tip of the main crack and of the temperature. These evaluation methods have been applied to a commercial grade of low-carbon microalloyed steel after its quenching and a spheroidizing annealing. A diagram of the quantified probability, that a local critical state for microcrack formation arises, was found to be affected by two competing influences: the growth of homogeneously stressed elements of volume and the decline of local stresses with increasing distance from the main crack tip. Consequently, this diagram displays a peak at a certain distance from the tip of the main crack. The probability that a local critical state results from the coalescence of voids was found to diminish uniformly with increasing distance from the main crack tip.

Key words
engineering materials, spheroidized structures, critical states, initiation of cleavage and ductile fracture

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