The Estimation of Fatigue Damage in Aircraft Wing Structures

Summary

A simple cumulative fatigue damage law is used to estimate the endurance of a number of typical structural components when subjected to the alternating wing loads encountered by an aircraft in flight. A method is developed to study the fatigue damage in terms of infinitesimally small intervals of alternating load and the corresponding fatigue strength. Recent gust data are used in conjunction with the complete fatigue strength diagrams of 13 simple components. The fatigue damage is studied for the ranges of altitude (i) up to 12,000 ft., and (ii) above 30,000 ft. In each case it is found that the damage rate curve, which gives the intensity of fatigue damage at any gust velocity, takes a characteristic form, with a well-defined range of gust velocities giving the greatest fatigue damage. This leads to a simplification of the whole problem, and it is found, finally, that the life of the structure is governed—in so far as the cumulative law is correct—almost entirely by that gust velocity, which, when applied alone to the structure, gives an endurance of about two million cycles. The results of the survey are compared with the design criteria suggested by Walker and the lives estimated by Williams.