Metal Fatigue
While the concept of metal fatigue was not completely unknown in the 1800s by the scientific and engineering communities, it was a concept that was neither thoroughly observed nor investigated until the twentieth century. Metal fatigue is the term used to describe the weakened condition of metal parts used in machinery, aircraft, vehicles, etc. after extensive continuous use. The need for information on metal fatigue became increasingly important as aviation technology advanced. The metal parts of early aircraft sustained heavy damages--such as snapped-off wings and shattered propellers--now attributable to metal fatigue.
In 1914 Ambrose Swasey (1846-1937), a machine-tool builder living in Cleveland, Ohio, founded the Engineering Foundation to research metal fatigue common to aircraft specifically and found in other machinery as well. Hoping to determine stress limits, the foundation conducted thousands of tests and experiments on steel, aluminum, copper, bronze, brass, and all other metals used in the production of aircraft and machinery. Swasey also worked to ensure that the foundation's findings were disseminated to circles within the general public, beyond those of private industry.
Until the early 1950s, the potential problems caused by metal fatigue were largely ignored. In 1954 a Comet jet airliner (a plane manufactured by the English firm de Havilland Aircraft Company) exploded in mid-air and plunged into the Mediterranean Sea; there were no survivors. In April of that year, as the wreckage was still being recovered and reassembled in England for further investigation, a second crash occurred, this time off the coast of Italy. Prior to these two incidents, another crash involving a Comet had occurred; structural stress due to severe weather was determined to be the cause. However, investigators of the 1954 crashes had another theory--metal fatigue.
To test their theory, the fuselage of one of the remaining recently grounded Comets was placed into a tank filled with water. After simulating the pressurization that occurs during a typical flight--3,000 times--the investigators drained the tank and discovered a split in the fuselage starting in the corner of an escape hatch window and extending for 8 ft. (2.4m), going through a window frame in its path. Apparently each of the Comets had developed these metal fatigue cracks, which resulted in the mid-air explosions.
The detailed investigation of these tragedies yielded invaluable data on the causes and dangers of metal fatigue, as well as on what preventative measures should be taken to prevent the condition. In the airline industry for example, the engineering design of the airplane was vastly redrawn: a thicker aluminum skin was used; "tear stoppers" were welded at frequent intervals to the inside of the skin; and the fuselage was put through 50,000 pressurization (taking off and landing) cycles.
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