Investigation
National Transportation Safety Board officials were on scene within hours of the accident. As the investigation unfolded, it became apparent that the entire fan disk and blade assembly from engine number 2, a component approximately 8 feet (2.4 m) in diameter and made of titanium alloy, was missing from the accident scene. Despite an extensive search in the weeks following the crash, the missing disk and blade assembly could not be located. Realizing that that disk potentially held the key to understanding the reasons for the engine failure, the engine's manufacturer, General Electric, offered a $50,000 reward to whoever could locate the disk, and $1,000 for each fan blade. On October 10, 1989, three months after the crash, Janice Sorenson, a farmer harvesting corn near Alta, Iowa, felt resistance on her combine, and after getting out to see what was causing it, discovered most of the fan disk with a number of blades still attached partially buried in her cornfield. The rest of the fan disk and most of the additional blades were located later in the harvest.
Examination of the fan disk did indeed solve the mystery. Investigators discovered an impurity and fatigue crack in the disk, and traced this defect all the way back to the initial ingot formation in 1971. Titanium when melted reacts with air which creates impurities; to prevent this, the ingot which would become the fan disk was formed using a "double vacuum" process: the raw materials were melted together in a vacuum, allowed to cool and solidify, then melted in a vacuum once more. Afterwards, the ingot was shaped into a billet, a sausage-like form about 16 inches in diameter, and tested using ultrasound to look for defects. Defects were located and the ingot was further processed to remove them. Unfortunately, some contamination remained.
The contamination caused what is known as a hard alpha inclusion, a brittle part of the metal, which cracked during forging and then fell out during final machining. This formed a cavity with microscopic cracks at the edges. For the next 18 years, the crack grew slightly each time the engine was powered up and brought to operating temperature. Eventually the crack grew large enough to cause structural failure of the disk.
Read more about this topic: United Airlines Flight 232