Although transitions from dimple to cleavage fracture with decreasing temperature in steel Charpy specimens are often associated with the crack initiation stage, similar transitions occur at a constant temperature during crack growth. Furthermore, in Charpy specimens there is often a reverse transition from cleavage back to dimple fracture. A model of such behavior is derived from a stress-based deterministic cleavage criterion, along with quasi-static plane strain, slip-line fracture mechanics. In practice, there is also concern in the growth stage for the statistics of desirably rare dimple-to-cleavage transitions. A prior statistical model for a constant stress field is re-examined and simplified. It has a high coefficient of variation, as has sometimes been observed. A way is proposed to extrapolate the probabilities of transitions from tests on specimens at low temperatures to estimate the minimum allowable temperatures for very rare cleavage transition in large structures.
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