ENVIRONMENTALLY INDUCED CRACKING IN ALUMINUM

Normal storage conditions can result in spontaneous body fractures in some aluminums. Forged 7049T73 appears to be highly susceptible to this failure mechanism and is the primary subject of this paper. The very unusual failure mode is characterized by critical crack formation and resultant static fracture in the parting plane of the forging. This Environmentally Induced Cracking (EIC) region is intergranular and resembles stress-corrosion cracking except that there are no corrosion products. The failure mode was reproduced by extended exposure to conditions up to 180°F and 90% RH with the resulting data used to predict real life-to-failure. Temperature and stress were proven the most significant factors, and the probable existence of a humidity threshold was indicated. Forged 7075T73 and 7050T74 demonstrated the same failure mode, but with an order of magnitude improvement in EIC initiation time. It is concluded that forged 7049T73 aluminum products with sustained loading transverse to the parting plane should be avoided.