Failure Analysis of a Large Wind Tunnel Compressor Blade

A failure analysis was performed to establish the cause and prevent the recurrence of a 2014-T6 aluminum compressor blade failure in a large NASA-Ames Research Center wind tunnel. A metallurgical failure analysis showed that a 0.13 mm (0.005 in.) deep scratch in the shank of the blade had acted as an initiation site for a fatigue crack. The crack subsequently grew by Stage II fatigue across most of the blade's 22.9-cm (9-in.) base transition region before final fracture occurred by unstable crack growth. Extensive fractographic characterization of both the blade and laboratory specimens was performed using a scanning electron microscope. Surface morphology, including fatigue striations, was used to estimate the vibration stress levels, the time to grow the crack, and the magnitude of the local mean stress present.

A structural failure investigation was conducted to determine the resonance vibration condition which made the blade sensitive to a scratch. Studies of the blade natural frequencies were made to determine the mode responsible for the vibrations which grew the crack. The effect of the crack on modal frequency was measured and computed. Tunnel measurements were used to determine the vibration resonance tunnel speed, magnitude, and source. These data were used to compare with the crack growth data from the fractographic studies and to make recommendations to avoid future blade failures.