某7020铝合金地铁列车在检修过程中发现车钩牵引梁存在开裂现象, 失效分析结果表明该裂纹性质为应力腐蚀开裂(SCC)。采用断裂力学双悬臂梁(DCB)试样, 对高强度7020铝合金的应力腐蚀开裂特征进行了研究。结果表明: 7020铝合金在含氯离子的水溶液环境中对SCC敏感。板料的Z-X和Z-Y方向有SCC裂纹萌生, 裂纹开始萌生的时间较长, 扩展速度较慢, SCC裂纹的特征为水平扩展, 表面有呈台级式不连续扩展和群集的现象, 开裂面为沿晶扩展, 和车钩牵引梁的裂纹特征相同。

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.