THE VARIABLE STRESS RATIO FATIGUE STRENGTH CURVE

This paper presents methods and procedures for utilizing variable stress ratio fatigue strength curves for establishing the fatigue life of helicopter structural components. This method is particularly useful when the flight test data has a significant variation in stress ratio, but when the stress ratio varies in a systematic manner rather than a random manner, A simplified fatigue analysis of a typical helicopter main rotor blade section is used as an example of this method. For a helicopter rotor blade, as the cyclic loads increase, the steady loads usually increase at a lower rate, resulting in a systematic variation of stress ratio with cyclic load. The paper demonstrates how certain pitfalls in fatigue analysis, which can result in either conservative or unconservative fatigue life determination, can be avoided by the use of a variable stress ratio fatigue strength curve. The major advantage of this method is to be able to obtain satisfactory accuracy in fatigue life determination with a considerable savings in time.