Metal fatigue
The phenomenon in which repeated cycles of stress far below a material's yield strength grow microscopic cracks until the part finally fractures.
The world's first jet airliner, the de Havilland Comet, had square windows that caused disaster. Pressurization cycles concentrated stress at the corners, causing metal fatigue cracks; in 1954 two Comets broke apart mid-air, killing dozens. Every airliner window since has been oval.
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