The undetected propagation of a fatigue crack constitutes a significant cause of aircraft and other structural failures. To raise the structural failure load to a relatively high level, the manufacturer can divide the structure into many small elements, which significantly increases the ability of a structure to tolerate an element failure. This report presents a procedure for calculating the probability that the element has not failed, as a function of the crack propagation time and hence the crack's length. The procedure's form is so simple that computations with a desk calculator can yield reasonably accurate results. To illustrate this, the report uses data that an aircraft manufacturer developed for the structural components/elements that currently limit the service life of an existing transport aircraft.
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