Trent Kaplan, age 14, of Camden, N.J., for his question:

WHAT CAUSES METAL FATIGUE?

Fatigue in metals is a progressive deterioration that ultimately results in the breaking of the metal. Fatigue is caused by repeated application of stress to the metal and the deformation of a material or object as a result of the stress is known as creep.

The fatigue strength of a typical steel alloy is about 50 percent of the ultimate strength and 75 percent of the elastic strength but may be considerably lower, particularly for the strongest heat treated steels.

If the elastic strength of a steel beam is 100,000 pounds, it could withstand a continuous  s tress of 90,000 pounds for centuries with no measurable yielding. A stress of 80,000 pounds alternately applied and withdrawn, however, would probably cause fatigue failure after a few million applications.

Fatigue is not important in civil engineering structures, in which stress is generally continuous, but in an engine turning at 3,000 revolutions per minute, any stress to which an engine past is subject will be applied 1 million times within six hours of operation.