Physicist probes causes of life-shortening 'dwell fatigue' in titanium
"Dwell fatigue" is a phenomenon that can occur in titanium when held under stress and can occur in a jet engine's fan disc during takeoff. The failure mode can cause small cracks that will reduce a component's lifetime.
The most widely used titanium alloy, Ti-6Al-4V, was not believed to exhibit dwell fatigue until the 2017 Air France Flight 066 incident when an Airbus route from Paris to Los Angeles experienced fan disc failure over Greenland that forced an emergency landing.
As a result of this incident and a few other recent concerns, the Federal Aviation Administration and European Union Aviation Safety Agency have coordinated work to determine the root causes of "dwell fatigue.
According to experts, it’s important to make sure that the base metal that you want to cast contains no impurities because the presence of impurities results in cracks in the castings.
Many researchers believe that dwell fatigue can start when slip occurs more unevenly than normally, or is restricted to narrower bands, rather than occurring evenly in three dimensions.
The presence of nanometer-scale intermetallic Ti3Al precipitates promotes the formation of bands, particularly when processing conditions allow for their long-range ordering.
When a contiguous area of soft-oriented grains is exposed to a sudden temperature change, things get dicey.
The resulting deformation, which is caused by the band meeting a hard-oriented grain, which leads to a stress concentration, initiating the cracking process.
Similar to how small fault slippage events can initiate more significant earthquakes, dislocation slip can occur intermittently in bursts or "avalanches".
The initiation of dwell fatigue is strongly influenced by the magnitude and frequency of these slip avalanches.