Practical implementation of the double linear damage rule and damage curve approach for treating cumulative fatigue damage

Simple procedures are presented for treating cumulative fatigue damage under complex loading history using either the Damage Curve concept or the Double Linear Damage Rule. A single equation is provided for use with the Damage Curve approach; each loading event providing a fraction of damage until failure is presumed to occur when the damage sum becomes unity. For the Double Linear Damage Rule, analytical expressions are provided for determining the two phases of life. The procedure involves two steps, each similar to the conventional application of the commonly used Linear Damage Rule. When the sum of cycle ratios based on Phase I lives reaches unity, Phase I is presumed complete, and further loadings are summed as cycle ratios based on Phase II lives. When the Phase II sum reaches unity, failure is presumed to occur. No other physical properties or material constants than those normally used in a conventional Linear Damage Rule analysis are required for application of either of the two cumulative damage methods described. Illustrations and comparisons of both methods are discussed.