Metal Fatigue Everyday Danger – Small Failures Have Big Consequences

A door was torn off a San Francisco passenger train last week as it sped through an underground tunnel. Metal fatigue was cited as the root cause of the incident, as a broken metal stop allowed the door to come off its track while the train was operating.
Accidents like this one typically result from a combination of events:

• Years of operating strain produced fatigue cracks in a small metal stop, causing it to break off and loosening the door.
• As the train entered an underground tunnel, “The air and the wind in the tunnel made the movement of the door a little more pronounced,” said San Francisco Municipal Transportation Authority (SFMTA) Director of Transit John Haley.
• With the loose door agitated and off its track, it clipped a passing signal post and was ripped from the train.

Fortunately, no one was seated by the door and no injuries were reported, but the incident highlights the serious consequences of metal fatigue and critical component failures.
Local rider James Chang shared photos of the incident:

Door flew off the muni train in the tunnel between Castro and church just now #muni #sfmuni #sf @sfmta_muni @SFGate @SFist @Hoodline pic.twitter.com/f0WSW5uQD8

— James Chang (@jameschangtweet) May 3, 2017

Many of the San Francisco Muni trains have been operating day and night for over twenty years. Decades of cyclic loading and vibrations put repeated tensile strain on vulnerable metal parts.
When fatigue cracks propagate beneath a component’s surface, they can be difficult to detect via inspections and the component becomes susceptible to unexpected failure. Component failures in the public transportation industry are particularly dangerous as they put crew and passenger lives at risk.
Laser peening inhibits crack propagation and enhances metal fatigue strength, helping prevent cracking and fatigue failures that can lead to catastrophic accidents. It is routinely applied to commercial aircraft components to prevent the kind of unexpected failures that struck the San Francisco Muni train last week.
Any metal component subjected to cyclic stress loading can benefit from surface enhancement.  Laser peening is a powerful method for improving the safety and reliability of critical parts.
Contact LSPT to learn more.