Cumulative damage in fatigue

Cumulative damage in fatigue is a problem which in recent years has become of interest to designers in almost all branches of structural engineering. To the aircraft designer however — especially under present-day conditions of operation — it is of paramount importance. During its lifetime, an aeroplane is subjected to repetitions of loads of various magnitudes and frequencies, and operating conditions involve take-off and landing at high loads, and flying at high speeds, aircraft which are large and flexible and which, in the attempt to achieve high structural efficiency, have been designed to an absolute practical minimum of weight. The aircraft designer today therefore is faced with the necessity of estimating not only the strength of a structure, but also its life — a task with which he was not confronted before.