Combined effects of notches and microstructure on the structural performance of medium carbon steel breakaway couplings were discussed. Property troughs commonly associated with tempered martensite embrittlement were characterized as a function of temper for AISI 4130 and 4140 steel couplings. It was observed that a circumferential notch could change the fracture mode from ductile to brittle with no change in tempering condition. Notch-strengthening, a second significant effect, was also observed in similar couplings with an average strength increase of thirty percent. A critical assessment was made of the AASHTO specifications on breakaway couplings used for highway structures and of their interpretation by individual states.

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