The very day of the emergency landing in Philadelphia by Southwest Airlines Flight 1380, investigators were blaming a type of failure that spells alarm for materials engineers everywhere: metal fatigue.

That means one of the engine's titanium fan blades, after vibrating or otherwise flexing many thousands of times over its lifetime, developed a series of microscopic cracks that suddenly "propagated" on Tuesday — tearing through the metal with violent speed.

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The fateful question is: Why?

Certain metals, including titanium, theoretically can flex back and forth an infinite number of times without developing fatigue, so long as the stress on the material remains below a certain limit, said David Brookstein, a Temple University professor of mechanical engineering.

But if the metal contains defects, fatigue will occur at lower levels of stress and after fewer "cycles" of loading, said Brookstein, the senior associate dean for undergraduate studies at Temple's College of Engineering.

Such defects could include damage from flying debris, which can be where cracks initiate, said Derek Warner, an associate professor of civil and environmental engineering at Cornell University.

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"Even if you have a millimeter-size nick in one of these blades, that really affects the fatigue life drastically," said Warner, director of the Cornell Fracture Group.

This type of damage could be detected with a visual inspection during routine maintenance. Inspectors also can test fan blades for microscopic fatigue cracks with ultrasound and other high-tech equipment — as had been recommended for the Southwest jet's type of engine by its manufacturer, CFM International, after a previous fan-blade failure on another Southwest plane in 2016.

But a broken fan blade alone should not have spelled danger for the Southwest jet that landed in Philadelphia, Brookstein added. Once it broke off, the blade should have been stopped by a surrounding containment ring — a band of durable material such as Kevlar, the material used to make bulletproof vests, Brookstein said.

"It's the bulletproof vest of the engine," Brookstein said.

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In photos of the doomed Southwest engine, the lemon-yellow containment ring is exposed, and appears to have partly torn, he said.

Aircraft makers have been worrying about fatigue since the 1953 crash of a British-made de Havilland Comet that killed 43. Investigators later determined that fatigue cracks began to spread through the plane's metal skin because stress was concentrated at the corners of its squarish windows. That is why aircraft windows now are designed with a rounded shape.

Fan blades also have been known to fail due to fatigue, though such events are uncommon. The blades are designed to spin at tremendous speed, typically reaching 2,000 rotations per minute at takeoff and drawing in 2,500 pounds of air per second into the engine, according to a 2013 review in Progress in Aerospace Sciences.

"To put this in perspective, this is enough to vacuum the air from a four-bedroom house in less than half a second," wrote author Leye M. Amoo, then an engineer with engine-maker Pratt & Whitney.