Mechanism of Stress Corrosion Cracking of Towing Beam of Coupler in Subway

Cracks were found in the towing beam of coupler in a subway of aluminum alloy 7020, the failure analysis indicated it was caused by stress corrosion cracking(SCC). The mechanism of SCC behavior of high strength aluminum alloy 7020 was studied by means of double cantilever beam specimen (DCB) experiment. The results showed that the aluminum alloy 7020 was sensitive to SCC in environment containing Cl-, although the time of crack initiation was relatively long and the crack growth was relatively slow at the “Z-X” and “Z-Y” direction. The cracks were horizontal extended, some step-like discontinuous expansion and clustering phenomenon were found on the surface, the results of SEM showed that the cracks were intergranular and all the characteristics agreed with the cracked beam.