Stress Corrosion Cracking of Tunnel Ventilation Fan Blades: A Case Study

Installations in road tunnels, such as jet fans, are exposed to a harsh environment. After two failures of rotor blades, made of aluminum cast, a failure analysis was initiated. The main goal of this study was to determine the causes, e.g., overloading, material defect, corrosion, and design deficiencies, and to define actions to avoid such events in the future. The fractographical analysis showed corrosion attack and a cleavage type of fracture. These findings pointed to two damage mechanisms acting in both cases: stress corrosion cracking due to hydrogen embrittlement, causing a brittle static failure. The failure analysis did not lead to a full understanding of the damage mechanisms: Service loads were far too low to explain the high stresses needed for a brittle static failure. A wedging effect of corrosion products in the gap between the blade and the mounting was proposed as the cause of high static stresses in the fractured cross section. Alternative materials as well as different corrosion surface protections were evaluated with respect to corrosion and corrosion fatigue resistance. Finally, an appropriate surface protection was proposed as the solution for inhibiting corrosion and therefore the risk of the reproduction of such failures.